## 2012 TECHNICAL REPORT 

# Revised Wet S tack Design GuideEPRI Project Manager C. Dene

3420 Hillview Avenue Palo Alto, CA 94304

- 1338 USA PO Box 10412 Palo Alto, CA 94303

- 0813 USA 800.313.3774 650.855.2121 askepri@epri.com 

### 1026742

www.epri.com 

### Final Report, December 201 2 

# Revised Wet Stack Design Guide

### DISCLAIMER OF WARRAN TIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WA S PREPARED BY THE OR GANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELE CTRIC POWER RESEARCH INSTITUTE, INC. (EPR I). NEITHER EPRI, ANY MEMBER OF EPRI, ANY CO SPONSOR, THE ORGANIZ ATION(S) BELOW, NOR ANY PERSON ACTING ON BEH ALF OF ANY OF THEM: (A) MAKES ANY WARRAN TY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE O F ANY INFORMATION, A PPARATUS, METHOD, PR OCESS, OR SIMILAR IT EM DISCLOSED IN THIS DO CUMENT, INCLUDING ME RCHANTABILITY AND FITNESS FOR A PAR TICULAR PURPOSE, OR (II) THA T SUCH USE DOES NOT INFRINGE ON OR INTER FERE WITH PRIVATELY OWNED RIGHTS, INCLUDING AN Y PARTY'S INTELLECTU AL PROPERTY, OR (III ) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR US ER'S CIRCUMSTANCE; O R (B) ASSUMES RESPONSI BILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATS OEVER (INCLUDING ANY CONSEQUENTIAL DAMAGE S, EVEN IF EPRI OR A NY EPRI REPRESENTATI VE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING F ROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APP AR ATUS, METHOD, PROCES S, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. REFERENCE HEREIN TO ANY SPECIFIC COMMERC IAL PRODUCT, PROCESS, OR SERVICE BY ITS TRADE NAME, TRADEMARK, MAN UFACTURER, OR OTHERW ISE, DOES NOT NECESS ARILY CONSTITUTE OR IMPLY ITS ENDORSEMEN T, RECOMMENDATION, O R FAVORING BY EPRI. THE FOLLOWING ORGANI ZATIONS , UNDER CONTRACT TO EP RI, PREPARED THIS RE PORT: Alden Research Laboratory, Inc. Comité International des Cheminées Industrielles

NOTE For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or e- mail askepri@epri .com. Electric Power Research Institute, EPRI, and TOGETHER

... SHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright © 2012 Electric Power Research Institute, Inc. All rights reserved

.This publication is a corporate document that should be cited in the literature in the following manner: Revised Wet Stack Design Guide

. EPRI, Palo Alto, CA: 201 2

. 1026 742

.  iii  

# Acknowledgments The following organization, under contract to the Electric Power Research Institute (EPRI), prepared this report: Alden Research Laboratory, Inc. 30 Shrewsbury St. Holden, MA 01520 Principal Investigator s D. Anderson L. Maroti CICIND Comité International des Cheminées Industrielles (International Committee for Industrial Chimneys) Talacker 50, CH -8001 Zurich, Switzerland This report describes research sponsored by EPRI. The authors would like to acknowledge the authors of the original EPRI Wet Stacks Design Guide (EPRI TR

- 107099) upon which this revised guide is based. The authors would also like to acknowledge the program advisory committee for their guidance o n the Revised Wet Stack Design Guide project and for their comments on the draft version of this guide. The committee members are: Victor A. Bochicchio Hamon Custodis, Inc. Steven L. Reid, P. E. Industrial Environmental Systems, Inc. Prof. Gottfried Nonhoff University of Aachen, Germany John C. Sowizal, P. E. Industrial Chimney Engineering Co., Inc. v  

# Product Description For the past 14 years, the design of wet stacks around the world has been guided by the original EPRI Wet Stacks Design Guide (1996)

. Since that time, the number of wet stack installations has grown considerably , and a wealth of practical real

- world operating and maintenance experience has been obtained. The laws of physics have not changed , and most of the information presented in 1996 is just as valid today as it was when originally published. What has changed is the power

- generation industry’s experience in using this information and the day

- to
- day operation of wet stacks. Much ha d been learned over the intervening years about the design and operation of wet stack systems, and it had become clear that some updating of the recommendations made in the original guide were needed. This document, the Revised Wet Stack Design Guide , has been prepared to present this updated informatio n and to provide the power

generation industry with the latest state

- of

- the

- art information for favorable wet stack design and operation. Much of this document will be familiar to those who have read the original design guide. Some sections of the original guide have been reused with only minor changes ; others have been significantly revised ; and new sections discussing the industry’s experience with wet stack operation and maintenance have been added. The outline of the guide has been rearranged to be easier to use and follow, bringing the reader through the entire wet stack design process , from the fundamentals of droplet collection and liquid

- film flow to the stack’s final design, commissioning , and operation. This new document strives to thoughtfully upd ate the original guide and to provide the industry with the definitive reference needed by the engineers and designers responsible for the specification, design, and implementation of effective wet stacks. Background A “ wet stack ” is a chimney , stack, o r flue that exhausts saturated flue gas downstream from a wet

- scrubbing process , such as a wet flue gas desulfurization ( W FGD ) system. All recently designed and constructed W FGD systems have installed wet stacks. Although the technology is relatively mature , there are a number of technical issues that utilities must address to achieve a successful installation. This guide provides answers to these questions , whether the installation is new or retrofit. vi  Objectives

 To provide background information and updates of previously published information

 To summarize current state

- of

- the

- art design

 To list and discuss important parameters and options

 To give specific recommendations for wet stack design Approach Investigators collected the information from a literature survey, the in

- house expertise of contractors, phone contacts with vendors, a utility advisory committee, and a limited number of site visits. They collated and summarized the information to produce the report, which the advisory committee also reviewed. Results The i nformation in the guide covers the design process and operational issues for both new and retrofit wet stack installations. I mportant issues addressed include system design for favorable wet operation, stack liquid discharge, plume downw ash , s tack

- liner geometry, gas velocity in the liner, and liquid

- collection devices and drainage. In addition, the report also provides a guide to developing a we t stack specification. EPRI Perspective Because most new FGD systems include wet stacks, it is impe rative that accurate, reliable information is available. This guide contains the most up

- to
- date information , and it should be useful for personnel responsible for wet stack design , specifications, or operation. Care must be taken to use these recommendati ons with good engineering judgment and consideration for site

- specific installations

. Keywords Air

- emissions c ontrol Flue gas d esulfurization (FGD) Wet s tacks Wet s crubbers SO 2 c ontrol Stack liquid discharge (SLD) vii  

# Abstract In 1996, the Electric Power Research Institute (EPRI), in a tailored collaboration with New York State Electric & Gas (NYSEG), retained Burns & McDonnell and DynaFlow Systems to prepare a design guide for wet stacks. The purpose of this guide was to provide the utility industry with information and recommendations concerning the design and specification of wet stacks

. Since that time, the number of wet stack installations has grown considerably , and a wealth of practical real

- world operating and maintenance experience has been obtained. This document, the Revised Wet Stack Design Guide , has been prepared to present this updated information and to provide the power

- generation industry with the latest state

of

- the

- art information for favorable wet stack design and operation. This new document strives to thoughtfully update the original guide and to provide the industry with the definitive reference needed by the engineers and designers responsible for the specification, design, and implementation of effective wet stacks

. ix  

# Table of Contents

Section 1: Background and Objectives ..................... 1-1 1.1 Preface to the Revised Wet Stack Design Guide

............ 1-1 1.2 Introduction

............................................................... 1-1 1.3 Scenarios for Wet Stack Utilization

.............................. 1-3 1.4 Important Considerations for Wet Stack Design and Operation

....................................................................... 1-3 1.4.1 Mist

- Eliminator Carryover

................................... 1-4 1.4.2 Deposition of Entr ained Liquid Droplets

................ 1-4 1.4.3 Condensation

.................................................... 1-5 1.4.4 Liquid Re

- entrainment

......................................... 1-6 1.4.5 Washing of Wet Fans

........................................ 1-7 1.4.6 Stack Liquid Discharge (SLD)

............................... 1-7 1.4.7 Corrosi o n/Chemical Attack

................................ 1-8 1.4.8 Plume Downwash

.............................................. 1-8 1.4.9 Icing Potential

................................................... 1-9 1.5 Contents of the Design Guide

.................................... 1- 11 1.5.1 Information About Key Issues

............................ 1- 11 1.5.2 Knowledge of the State of the Art

...................... 1- 11 1.5.3 Overview of Project Implementation

................... 1- 12 Section 2: Wet Stack Design Fundamentals .............. 2-1 2.1 Introduction

............................................................... 2-1 2.2 Droplet and Liquid

- Flow Fundamentals

.......................... 2-3 2.2.1 Flow of Droplets Suspended Within a Gas Flow

........................................................................ 2-3 2.2.2 Liquid

- Film Flow on Surfaces and Re

entrain ment Fundamentals

........................................... 2-4 2.2.3 Gas

- Flow in Wet Duct Systems

............................. 2-5 2.2.4 Gas Flows in Wet Stack Systems

.......................... 2-7 2.3 Sources of Liquid in a Duct/Wet Stack System

............ 2- 12 2.3.1 Mist Eliminators and Absorbers

......................... 2- 12 2.3.2 Liner Condensation

.......................................... 2- 20 2.4 Liquid

- Film Behavior on the Liner Wall

........................ 2- 22 2.5 Liner

- Wall Discontinuities

.......................................... 2- 28 2.6 Re

- entrainment

......................................................... 2- 31 2.7 Recommended Liner

- Gas Velocities

............................ 2- 32 2.8 Stack Liquid Discharge

............................................. 2- 33 x  2.9 Plume Downwash and Icing

...................................... 2-35 2.9.1 Plume Downwash on Stack

............................ 2- 35 2.9.2 Stack Top Icing

............................................ 2- 40 Section 3: Practical Design Reco mmendations .......... 3-1 3.1 Introduction

............................................................... 3-1 3.2 New Wet Stack Designs

............................................. 3-2 3.3 Retrofit Wet Stack Designs

.......................................... 3-2 3.4 Wet Duct/Stack Operation

......................................... 3-3 3.5 Absorber Design and Placement

.................................. 3-6 3.6 Wet Ductwork Design

................................................. 3-6 3.6.1 Guidelines for Selection of Geometry for Wet Ducts

......................................................................... 3-8 3.7 Flow

- Control Devices

................................................ 3- 14 3.7.1 Flow

- Control Liquid

- collectors

............................ 3- 14 3.8 Fans

....................................................................... 3- 15 3.8.1 High

- Gas

- Temperature Fans

.............................. 3- 15 3.8.2 Wet Fans and Fan

- Liquid Collectors

................... 3- 15 3.9 Stack

- Li ner Design

.................................................... 3- 16 3.9.1 Liner Materials

................................................ 3- 20 3.9.2 Recommended Liner

- Gas Velocities

.................... 3- 33 3.9.3 Stack

- Entrance Geometry

.................................. 3- 34 3.9.4 Liner

- Diameter Changes

.................................... 3- 40 3.9.5 Internal Structures

............................................ 3- 41 3.9.6 Stack

- Exit Design

............................................. 3- 43 3.9.7 Cyclonic Flow and Continuous

- Emissions Monitors

.................................................................. 3- 44 3.10 Miscellaneous Materials of Construc tion

................... 3- 46 3.10.1 Exterior of Shell

............................................. 3- 46 3.10.2 Exterior of Adjacent Stacks

............................. 3- 46 3.10.3 Interior of Shell for Use with Brick Liners

........... 3- 46 3.10.4 Liner Bands

................................................... 3- 47 3.10.5 Rain Hoods and Roofs

.................................... 3- 47 3.10.6 Lightning

- Protection System

............................. 3- 47 3.10.7 Platforms and Ladders

.................................... 3- 48 3.10.8 Electrical Conduit

.......................................... 3- 48 3.11 Basic Liquid

- Collector

- System Design

........................ 3- 48 3.11.1 Side

- Wall Gutters

.......................................... 3- 50 3.11.2 Ceiling Collectors

.......................................... 3- 51 3.11.3 Floor Gutters

................................................. 3- 53 3.11.4 Flow

- Control Liquid Collectors

......................... 3- 53 3.11.5 Internal Duct Supports and Expansion Joints

...... 3- 54 3.11.6 Stack

- Entrance Collectors – Side

- Entry Breech

.... 3- 56 xi  3.11.7 Stack

- Entrance Collectors – Bottom

- Entry Elbow

...................................................................... 3- 58 3.11.8 Liner Collectors

.............................................. 3- 60 3.11.9 Sloped Liner Floor

.......................................... 3- 63 3.11.10 Stack

- Outlet Collectors

................................ . 3- 63 3.11.11 Drains

........................................................ 3- 65 3.11.12 Post

- Installation Inspections

........................... 3- 67 3.12 Laboratory Flow

- Modeling

....................................... 3- 67 3.12.1 Computer Modeling.

...................................... 3- 69 3.13 Plume Downwash and Icing

.................................... 3- 71 3.13.1 Downwash Modeling

..................................... 3- 71 3.13.2 Windscreen Design

....................................... 3- 72 3.13.3 Single versus Multiple Liners

............................ 3- 72 3.13.4 Methods of Downwash Minimization

............... 3- 73 3.13.5 Stack

- Top Ici ng .............................................. 3- 74 Section 4: Guide to Developing a Specification for Wet Stacks .............................................. 4-1 4.1 Specification Overview

............................................... 4-1 4.1.1 Phase I – Feasibility Study

................................... 4-1 4.1.2 Phase II – Design Process

.................................... 4-3 4.2 Wet Stack Bid

- Preparation Process

............................. 4- 13 4.2.1 Establish Design Criteria

................................... 4- 13 4.2.2 Define Absorber Outlet

- Duct Geometry

............... 4- 14 4.2.3 Def ine Chimney/Liner Geometry

....................... 4- 14 4.2.4 Perform Model Study

........................................ 4- 14 4.2.5 Determine Liquid

- Collection Devices

................... 4- 14 4.2.6 Prepare Bid Specifications

................................ 4- 15 4.2 .7 Prepare Bid Drawings

...................................... 4- 15 4.3 What to Specify in a Bid Document

............................ 4- 16 4.3.1 General Requirements

...................................... 4-16 4.3.2 Site Work

....................................................... 4- 19 4.3.3 Concrete

......................................................... 4- 19 4.3.4 Liner

............................................................... 4- 20 4.3.5 Ductwork and Expansion Joints

......................... 4- 21 4.3.6 Miscellaneous Metals

....................................... 4- 21 4.3.7 Platforms and Ladders

...................................... 4- 21 4.3.8 Access Doors

.................................................. 4- 22 4.3.9 Protective Coatings

.......................................... 4- 22 4.3.10 Special Construction (Tuned Mass

- Damping System)

.................................................................... 4- 23 4.3.11 Personnel Elevator

......................................... 4- 23 4.3.12 Mechanical

................................................... 4- 24 4.3.13 Electrical

....................................................... 4- 24 xii  4.4 What to Ask For in a Bid Document

........................... 4- 25 4.4.1 Warranties/Guarantees

................................... 4- 25 4.4.2 Design Calculations

......................................... 4- 25 4.4.3 Materials Testing

............................................. 4- 27 4.4.4 Construction Procedures

................................... 4- 28 4.4.5 On

- Site Testing and Inspection

.......................... 4- 30 Section 5: Wet Stack Experience .............................. 5-1 5.1 Experience with Wet Stacks

........................................ 5-1 5.1.1 General Operational Experience

......................... 5-1 5.1.2 International Experience with Wet Stacks

............. 5-3 5.2 Before Unit Startup

..................................................... 5-4 5.2 .1 Review of Fabrication/Installation Drawings

......... 5-4 5.2.2 Field

- Installation Inspection

.................................. 5-5 5.3 After Unit Startup

....................................................... 5-5 5.3.1 Liquid

- Drainage Monitoring

................................ 5-6 5 .4 What to do if SLD Occurs

........................................... 5-6 5.4.1 Preliminary Stack Droplet Testing

......................... 5-7 Section 6: References .............................................. 6-1 Appendix A: Glossary .......................................... A-1 A.1 Definitions

................................................................ A-1 A.2 Units and Conversion Factors

..................................... A-8 xiii  

# List of Figures
Figure 2

- 1 Liner Inlet Gas

- flow Patterns – Close

- Coupled Absorber

......................................................................... 2-7 Figure 2 -2 Typical Stack

- Inlet Arrangements

.............................. 2-8 Figure 2 -3 Gas

- flow Patterns Through a Standard 90° Elbow

...... 2-9 Figure 2 -4 Liner

- Inlet Gas

- flow Patterns – Side

- entry Breech

....... 2- 10 Figure 2 -5 Liner

- Inlet Gas

- flow Patterns – 3- Miter Cut Bottom

Entry Elbow

................................................................... 2- 11 Figure 2 -6 Common Mist

- Eliminator Arrangements

................... 2- 14 Figure 2 -7 Comparison of Gas

- Path Lengths Through a Mist Eliminator as a Function of Angle

..................................... 2- 15 Figure 2 -8 Typical Chevron

- Type Mist

- eliminator Pressure Dro p vs. Gas

- Velocity Curve

............................................ 2- 17 Figure 2

- 9 Typical Chevron

- Type Mist

- Eliminator Carryover vs. Gas

- Velocity Curve

.................................................... 2- 17 Figure 2

- 10 Typical Mist

- Eliminator Collection Efficiency vs. Droplet

- Size Curve

......................................................... 2- 18 Figure 2

- 11 Lower Liner Droplet - and Liquid

- Film Flow Patterns – Side

- Entry Breech

............................................. 2- 23 Figure 2

- 12 Liner

- Floor Liquid

- Film Flow Patterns – Side

- Entry Breech

.......................................................................... 2- 24 Figure 2

- 13 Lower

- Liner Dro plet - and Liquid

- Film Flow Patterns – Bottom

- Entry Elbow

.......................................... 2- 25 Figure 2

- 14 Wetting vs. Nonwetting Surfaces

......................... 2- 26 Figure 2

- 15 Liquid

- Film Flow Patterns on a Smooth, Vertical Wetting Surface vs. Vertical Gas Velo city

......................... 2- 28 Figure 2

- 16 Liquid Flow over 1/8

- in (3.1

- mm) Weld Beads

...... 2- 29 Figure 2

- 17 FRP Liner

- Can Joints – Internal

- Taper Requirement

.................................................................. 2- 30 Figure 2

- 18 Plume Downwash

............................................... 2- 36 xiv  Figure 3 -1 Good and Poor Internal Duct

- Truss Arrangements for Wet Operation

......................................................... 3- 12 Figure 3 -2 Internal Duct

- Truss Gusset Plate

.............................. 3- 13 Figure 3

- 3 Chimney with Constant

- Diameter Brick Liner

............ 3- 17 Figure 3 -4 Chimney with Fiberglass

- Reinforced Plastic Liner

...... 3- 18 Figure 3 -5 Chimney with Metal Liner – Typical for Alloy, Glass

- Block, or Protective

- Coating Liner System

.................. 3- 19 Figure 3 -6 Borosilicate Block

– Lined Upper Wind Screen

.......... 3- 30 Figure 3 -7 Sloped Liner

– Floor Arrangements

........................... 3- 37 Figure 3 -8 Miter

- Cut Bottom

- Entry Elbow Arrangements

............ 3- 38 Figure 3 -9 Side

- Wall and Round

- Duct Liquid

- Collection Gutters

.......................................................................... 3- 51 Figure 3

- 10 Ceiling Liquid

- Collection Gutters

.......................... 3- 52 Figure 3

- 11 Horizontal Brace with Liquid

- Re

- entrainment

– Prevention Disk

.............................................................. 3- 55 Figure 3

- 12 Upper and Lower Stack

- liner Aerodynamic Zones

........................................................................... 3- 56 Figure 3

- 13 Typical Side

- Entry Breech Liquid

- Collection System

.......................................................................... 3- 59 Figure 3

- 14 Side

- Entry Breech Wing

- Collector Design

.............. 3- 59 Figure 3

- 15 Bottom

- Entry Elbow Liquid

- Collection System with Liner

- Expansion Joint

................................................ 3- 60 Figure 3

- 16 Liner

- Expansion

- Joint Placement and Incorporation Within Ring Collectors

................................ 3- 61 Figure 3

- 17 Liner

- Expansion

- Joint Liquid

- Collector Concept

....... 3- 61 Figure 3

- 18 Liner Rear

- Wall “V” Diverter

................................ 3- 62 Figure 3

- 19 Stack

- Outlet Liquid Collector

................................ 3- 64 Figure 3

- 20 Alternate Liner

- Outlet Liqu id

- Collector Detail

.......... 3- 65 Figure 3

- 21 Typical Wet Stack Physical

- Flow Models

............... 3- 69 Figure 3

- 22 Typical Plume

- Downwash Study – CFD Model Results

.......................................................................... 3- 71 Figure 3

- 23 Recommended Alignment of a Stack with Two Flues

............................................................................. 3- 73 Figure 5

- 1 Droplet Probe

........................................................ 5-8 xv  Figure 5

- 2 Droplet

- Probe Orientation

....................................... 5-8 Figure 5

- 3 Typical Droplet

- Probe Test Results

............................. 5-9 xvii  

# List of Tables

Table 2 -1 Recommended Stack

- Liner Velocities for Wet Operation

..................................................................... 2- 33 Table 3 -1 Estimated Ranges of Liquid Flows in the Wet Duct/Stack of a Typical 550

- MW Unit

............................... 3-5 Table 3

- 2 Liner Material of Construction

................................ . 3- 21 Table 3 -3 Physical Properties and Chemical Requirements Of Acid

- Resistant Brick1

.................................................. 3- 22 Table 3 -4 Recommended Stack Velocity Range for Stack

liner

– Diameter Sizing

..................................................... 3- 34 Table 4 -1 Phase II Wet Stack Design Process

............................ 4-4 Table 4 -2 Estimated Ranges of Flows in the Wet Duct/Stack of a Typical 550

- MW Plant

............................................... 4-8 1-1  

# Section 1: B ackground and Objectives

1.1 Preface to the Revised Wet Stack Design Guide For the past 14 years, the design of wet stacks around the world has been guided by the original EPRI Wet Stacks Design Guide (1996) [2] . Since that time, the number of wet stack installations has grown considerably , and a wealth of practical real

- world operating and maintenance experience has been obtained. The laws of physics have not changed , and most of the information presented in 1996 is just as valid today as it was when originally published. What has changed is the power

- generation industry’s expe rience in using this information and the day

- to
- day operation of wet stacks. Much ha d been learned over the intervening years about the design and operation of wet stack systems, and it had become clear that some updating of the recommendations made in the original guide were needed. This document, the Revised Wet Stack Design Guide , has been prepared to present this updated information and to provide the power

generation industry with the latest state

- of

- the

- art information for favorable wet stack design a nd operation. Much of this document will be familiar to those who have read the original design guide. Some sections of the original guide have been reused with only minor changes ; others have been significantly revised ; and new sections discussing the ind ustry’s experience with wet stack operation and maintenance have been added. The outline of the guide has been rearranged to be easier to use and follow, bringing the reader through the entire wet stack design process , from the fundamentals of droplet coll ect ion and liquid

- film flow to the duct’s and stack’s final design, commissioning , and operation. This new document strives to thoughtfully update the original guide and to provide the industry with the definitive reference needed by the engineers and desi gners responsible for the specification, design, and implementation of effective wet stacks. 1.2 Introduction A "wet stack" is a chimney, stack, or flue that exhausts saturated, scrubbed flue gas. A wet stack is located downstream from a wet flue gas des ulfurization (WFGD) system. These systems spray slurry ( typically limestone

- based ) into the gas stream, which reduces the sulfur dioxide (SO 2 ) content, saturates the flue gas with water vapor , and reduces the temperature of the flue gas to 115 –130°F (46

– 54 .4°C) for bituminous and hard coals and to 136

– 145°F (57

– 63°C) for lignite and sub

- bituminous coals

. 1-2  The 1990 Clean Air Act Amendments (CAAA) required utilities to reduce emissions of SO 2 . Many utilities have added WFGD systems to comply with the Phase I r equirements , whereas a lesser number have implemented dry

- scrubb er technologies. Phase II of the program went into effect in 2000, further reducing SO 2 emissions and increasing the number of plants affected. As part of their Phase II Compliance Plan , many plants with partial flue gas bypass systems have decided to eliminate the bypass and scrub 100 percent of their flue gas. Many other utilities have installed new or retrofit FGD systems , which typically use wet stack operation because of reduced operating and maintenance costs. The design of ducts and stacks for wet operation must address several issues that were not present in unscrubbed or reheated gas stack designs. Some of the important issues to consider in the design of a wet duct/ stack system include

 Stack liquid discharge (SLD)

 Plume downwash and icing

 Corrosion/chemical attack

 Stack height

 Absorber

- outlet duct arrangement/geometry

 Liner

- breach geometry

 Stack

- liner geometry and material of construction

 Gas velocity in the liner

 Liquid

- collection devi ces and drainage The purpose of this revised guide is to provide the utility industry with information and recommendations concerning the specification and design of wet stacks. However, these recommendations should not be used without applying good engine ering judgment and consideration for site specifics. Operating conditions, design conditions, and economics all play important roles. The goals of the Revised Wet Stack Design Guide are to

 Provide information for the fundamental understanding of wet stack operation
.
 Update previously published information and recommendations

.
 Present the current state

- of

- the

- art wet stack design

.
 Identify the parameters and options that will lead to favorable wet operation

.
 Give specific recommendations regarding wet stack design for new and retrofit installations

.
 Present information related to wet stack startup and maintenance issues

. It is assumed that those who will use this guide have a general familiarity with stack designs and FGD systems. This guide is intended for p ersonnel who are considering wet stacks and/or are responsible for designs, specifications, or operation of wet stacks. 1-3  Section 2 of this guide provides fundamental information on the physical processes and phenomen a important to wet stacks and discusses t he basic objectives for favorable wet stack operation. Section 3 addresses new or retrofit wet stack design and discusses the issues unique to wet stack operation. A guide to develop ing specifications for wet stacks is presented in Section 4. 1.3 Scenario s for Wet Stack Utilization There are two primary scenarios for wet stack utilization:

 A new FGD system incorporating a new wet stack

 A n ew FGD system that will convert an existing dry stack to wet operation The first scenario involves adding a new FGD sys tem that incorporates a wet scrubber and a new wet stack. In such a scenario, the designer is given the most freedom regarding plant layout, liner material, and stack

- geometry selections. A s tate
- of

- the

- art wet stack design should be used that is based on the latest analytical, experimental , and field data available. The goal is to produce an effective wet stack design that minimizes both installation and operating costs. The second scenario involve s converting an existing dry stack to wet operation. In suc h a scenario , the most important design consideration is whether or not the existing liner and ductwork material can withstand the reduced temperature s and wet conditions typical of wet stack operation and whether the liner

- gas velocity will result in drop let re

- entrainment from the stack walls. If not, the existing chimney liner must be modified or a new stack installed. Because of the outage time required to modify an existing stack, it is often more economical to build a new stack for scrubbed gas and us e the existing stack for emergency gas bypass around the scrubber as needed. 1.4 Important Considerations for Wet Stack Design and Operation The liquid inside a stack with a W FGD system exists as droplets from mist

eliminator carryover and as the moisture content of the gas flow entering the stack liner. The vapor content is usually the maximum that can occur when the flue gas is saturated with water vapor at the operating temperature. If there is an induced

- draft (ID) fan between the absorber and the stack, the fan

- temperature rise could result in a lower

- than

- saturation vapor content. What happens to the vapor and liquid content before the gas reaches the top of the liner defines how much liquid there is and in what form it is discharged from the stac k. The major gas /l iquid flow processes that may lead to the SLD are described in the next four subsections —i ncluding the major features of the SLD . A description of the liquid

- flow balance in wet stacks is provided in [1], pp. 4- 3 to 4 -5. Liquid

- flow rates typical for wet ducts and stacks are given in Section 3.4 , “Wet Duct Operation,” Table 3 -1. 1-4  1.4.1 Mist - Eliminator Carryover The gas stream leaving the mist eliminator is saturated with water vapor , and entrained fine liquid droplets are carried by the gas flow under normal design and operating conditions. It is important that the mist eliminators operate properly at the design ed gas velocity and liquid load to minimize the size and rate of droplet carryover. The mist eliminators are designed to reduce carr yover of liquid and slurry to the downstream ductwork. Mist

- eliminator problems can result in increased liquid carryover. Some larger droplets may also be present in the gas stream , which have been re

- entrained from the mist

- eliminator blades, the mist

eli minator support structure, and areas of the mist eliminator containing solids buildup. During the wash cycles, washing the mist eliminator can also increase the amount of liquid carried with the gas flow downstream of the mist eliminator because of the increased localized liquid loading associated with the wash sprays. A field evaluation of the mist

- eliminator performance can be made by performing visual inspections and/or droplet sampling at the discharge face of the mist eliminator. If carryover problems are noticeable, SLD will probabl y occur if effective liquid collectors are not installed in the duct and the stack. M ist

eliminator selection, operation performance , and possible problems are described in [3]. 1.4.2 Deposition of Entrained Liquid Droplets As droplets flow with the flue gas, they will impinge up on internal surfaces because of droplet

- trajectory paths controlled by the earth's gravity, centrifugal forces on the droplets when the gas flow turns, and gas

- drag forces. Large droplets ( 300–6000 μ m) from the gas

- flow path deposit on duct and liner surfaces mo st readily ; medium droplets ( 100–300 μ m) deposit only partially ; and small droplets ( 10–100 μ m) deposit hardly at all. Fine droplets (<10 μ m) follow the gas flow and do not deposit. The higher t he gas velocity and the sharper the turn of the flue gas path, the more likely it will be that significant numbers of droplets will impinge up on walls, turning vanes, baffles, and internal duct components (bracing, gusset plates , and expansion joints). Sma ll - to medium

- sized droplets can also deposit on surfaces as a result of gas flow patterns in separation regions and turbulent deposition. The turbulent deposition of fine droplets generated as a result of adiabatic condensation ( caused by pressure reducti on with elevation change) along the height of the stack is small. The gas velocity, duct geometry, stack geometry, and droplet size govern the amount and location of liquid deposition. The major deposition area in the stack is on the lower liner wall oppos ite the breeching opening . If the liner incorporates a bottom

- entry elbow, droplets will deposit along the elbow’s outer radius. Liquid deposition can be promoted , and the resulting liquid film collected , by properly designed vanes, baffles , and liquid

- col lection devices installed in the duct system. Maximum deposition and collection of the liquid on the duct walls and stack liners must be achieved in order to control SLD . One of the main 1-5  objectives of physical

- flow modeling is to design flow

- control device s and to develop liquid collectors such as ceiling gutters, side

- wall channels, floor guides, ring collectors , and drains to collect deposited liquid before re - entrainment can take place. This process is discussed in Section 3.12, "Laboratory Flow Modeling." Another liquid

- deposition area is on the choke surface in stacks that have chokes. Because of the rapid change in flow direction created by the choke, some of the fine droplets entrained in the gas flow will be deposited on the choke’s inner sur face . T he collected liquid will lead to SLD if the local gas velocities in the choke are high enough to drag the liquid up to the stack outlet and no liquid collector is installed. 1.4.3 Condensation One of the sources of liquid in the wet duct/stack syste m is condensation of water vapor from the saturated gas flow. Two types of condensation take place in wet stacks. Adiabatic Condensation (or bulk condensation) occurs in the bulk of the gas as the pressure of the saturated gas flow decreases as a result of pressure loss and elevation change along the height of the stack. The decrease in absolute pressure of the saturated gas causes water vapor to condense on fine droplets or on small solid particles in the bulk of the gas. Droplets formed by this type of heterogeneous condensation will tend to be very small (<1 μ m) and to be located throughout the volume of the gas flow. Only a very small fraction of this adiabatic condensation reaches the liner surface by the turbulent

- deposition process. The rest is disc harged from the top of the stack , which creates the white plume typical of wet stacks , and which evaporates as the plume mixes with the outside air. Thermal Condensation (or wall condensation) caused by heat transfer is important in wet stacks. Because the saturated flue gas temperature is usually 115–145 °F (46 –63 °C), heat transfer through the liner, the annulus , and the outer concrete shell to the cooler surrounding ambient air causes water vapor to condense on the inside of the stack liner. The rate of co ndensation caused by heat transfer is a function of the liner and shell construction, the type and thickness of liner insulation, the gas

- flow conditions, and the atmospheric conditions (ambient temperature, wind velocity, and barometric pressure). Annulus pressurization of brick

- lined stacks using ambient air increases the rate of condensation on the liner surfaces. Leakage of pressurizing air through the liner in older brick stacks can significantly increase the condensation. Insulation of alloy an d FRP liners can significantly reduce the amount of thermal condensation within the liner. 1-6  1.4.4 Liquid Re - entrainment The liquid on the liner surface produced by deposition and condensation flows in the form of film or rivulets i s governed by gravitational, surface

- tension, and gas

shear forces. If high enough, the gas - shear force can shear the liquid off the surface, causing re - entrainment . This is the most frequent source of SLD and the fallout of liquid droplets in the vicinity of the stack. Re

- entrainment most frequently occurs because of one or more of the following:

 High gas

- velocity shear near rough surfaces with liquid deposits

.
 Instability of the liquid film (waves and ripples) on vertical stack

- liner surfaces. This instability occurs be cause , as gravity is pulling the liquid film or droplet down, the gas

- drag forces are pulling the liquid film or droplet upward. This instability is a function of both the gas

- shear force and the local liquid

- film thickness.

 Surface discontinuities and pr otrusions , such as weld seams, fiberglass

reinforced plastic ( FRP ) joints , and mortar joints in brick liners that disrupt gas and liquid flow locally

.
 Vanes and baffles that cause gas

- flow separation and recirculating liquid

.
 Gas

- flow patterns that drag l iquid along the surface to re - entrainment sites
.
 Strong vortex patterns that can pick water up in the core of the vortex from horizontal or vertical surfaces

. Re

- entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack

- liner surfac es

 Dampers, damper

- blade guides, and trusses

 Thermal

- expansion joints

 Trailing edges of turning vanes

 Duct

- junction corners and edges

 Breeching

- duct area changes

 Stack floor

 Stack

- liner surfaces, particularly in the entrance region and at the intersection with the breeching duct

 Locations of internal solid s buildup or scaling on surfaces

 Stack

- liner contraction or expansion sections

 Stack choke surfaces Liquid re - entrainment at these locations will be in the form of large droplets (300–6000 μ m) ; they will b e discharged at the top of the stack , if not prevented by a combination of favorable gas

- flow path design and an optimized liquid

- 1-7  collector system. Droplets over 100 μ m do not evaporate outside of the stack , and they will probably reach the ground level. 1.4.5 Washing of Wet Fans Occasionally, ID fans will be located between the absorbers and the stack of a utility power plant when retrofitted with an FGD system. These fans usually require periodic or continuous washing to prevent solids buildup on the fan impellers, which can cause fan

- rotor imbalance. All of the liquid sprayed into the fan inlet leaves the fan impeller as droplets. Most of the liquid will be propelled by the high centrifugal

- force field to the fan scroll, where it will deposit. Most of th is liquid will re

- entrain back into the gas flow unless a properly designed high

velocity scroll

- liquid collector is installed. P referably , f ans should be located upstream of the absorber to avoid these problems

. Alternatively, they should be located such that there are a sufficient number of duct turns and/or duct length s between the fan outlet and the stack inlet to provide opportunities for liquid collection. 1.4.6 Stack Liquid Discharge (SLD) The stack liquid discharge (SLD) is also known as rainout o r acid

- mist fallout. It is important to note that all wet stacks have some amount of SLD . However, SLD is only a problem if the droplets are large enough to be detectable at ground level near the stack. The amount of liquid discharged at the top of the stack is a result of gas - and liquid

- flow processes that take place in the ductwork and stack system between the discharge of the absorbers and the top of the stack liner. The source of the liquid discharge can be categorized by the liquid

- flow process and by the droplet sizes as follows:

 Liquid droplets carried from the absorber to the top of the stack liner by the gas stream without depositing along the gas

- flow path. T ypically , t hese are droplets less than 50 μ m in diameter, the majority of which can travel through the ducts without significant deposition. Droplets of this size, when ejected from the stack , will typically evaporate before reaching the ground.

 Droplets re

- entrained from liquid deposition and condensation on duct surfaces that can be discharged. These tend to be larger drops

— greater than 300 μ m in diameter — and are usually the major contributors to the SLD

.
 Droplets formed by adiabatic or bulk condensation. These represent a large liquid

- flow rat e but are very fine droplets (<1 μ m), which have a negligible effect on the SLD detectable at ground level. This condensation forms the visible white plume typical of wet stack operation.

 Liquid deposited and condensed on the liner , which flow s either upward or downward , depending on the local gas velocity. Liquid flows upward in those areas of the stack where the gas velocity is higher than the flow

- reversal velocity for the specific liner material used. This phenomenon can be a significant pro blem in tapered liners , in which the upper portion of the liner 1-8  can experience high gas velocities. The droplet sizes re

- entrained at the top of the liner , where velocities are highest , can be quite large (300 –6000 μ m), depending on the liner

- top geometry. The upper section of a stack choke is usually in this mode of upward liquid flow. The quantity and location of the fallout at a given plant is a function of the droplet

- size distribution of the discharged liquid and the atmospheric conditions

— such as ambi ent temperature, wind, relative humidity, and turbulence level. The very small droplets of the bulk condensation make the gas plume white and visible. The plume is cooled as it mixes with the surrounding air. This cooling results in condensation on the small droplets in the plume. The increase in diameter is only 1 to 10 μ m; therefore, the droplets of the bulk condensation can evaporate before reaching the ground and cannot be detected. The mixing with the drier air away from the stack causes droplet evapor ation. The balance between the cooling and mixing processes defines the length of the visible plume. Therefore, it is a function of the ambient air temperature and the air relative humidity. One of the major objectives in developing an effective wet stack design is to limit SLD to a minim al acceptable level. The goal is to limit the droplet size that will exit the stack. If the droplets are small enough , they will evaporate before hitting the ground. However, if the droplets are large , they will land on nea rby surfaces such as plant structures, equipment, and cars . T he wet stack survey indicates that the fallout of liquid droplets, if noticeable, usually occurs on the downwind side of the stack within a half

- mile radius. 1.4.7 Corrosi o n/Chemical Attack With wet stack operation, corrosion is less severe than with reheat , but it still must be considered. C orrosion can be resisted by means of proper material selection . The floors of the ductwork should be sloped to provide proper drainage of the corrosive liquid after shutdown of the unit. The exposed materials of the ductwork and the stack liner must be resistant to chemical attack. Several types of material are available for both the scrubber

- outlet duct and the stack liner. The materials should be selected acc ording to their corrosion resistance to the chemical properties of the flue gas downstream of the absorber. Refer to Section 3.9.1 , “Materials of Construction,” for more information. 1.4.8 Plume Downwash Designing for a low gas velocity at the stack exit can decrease the amount of SLD . However, this type of design increases the potential for plume downwash at elevated wind velocities. During downwash episodes, the reduced static

pressure zone generated alo ng the downwind side of the stack shell and liner extensions can draw the flue gas into a downwash pattern along the stack shell. In this process, saturated flue gas comes in contact with the liner extension, stack hood , and stack shell. This contact can l ead to deterioration of the stack construction materials because of exposure to acid in the flue gas. It also increases the potential for ice formation on the top of the stack. A cross wind at the top of 1-9  the stack will deflect the plume from its vertical p ath. As the ratio of vertical plume momentum to horizontal wind momentum [( 

### ρ V 2 ) FLUEGAS / ( 

### ρ V 2 ) WIND ] falls below a value of about 2.0 for a single

- flue free

- standing stack, the plume may become partially entrained in the low

- pressure zone that is formed on the downwind side of the stack. This phenomenon is known as plume downwash. To reduce the frequency of downwash, the height of the liner(s) can be increased above the shell , or the momentum ratio can be increased

— either by reducing the liner diameter or by installing a choke at the top of the liner. For a multiple

- flue stack, the equivalent momentum ratios for initiation of plume downwash are higher , and they vary significantly with the wind direction relative to the axis of the liners. The extent of down wash can also be more pronounced , because of the larger size of the shell needed to enclose the multiple liners. The interaction of a plume with a cross wind is a function of the following parameters:

 Liner

- extension diameter

 Shell diameter

 Liner

- extensio n height

 S hape of hood/cap

 Roof geometry

 Number of liners

 Wind direction, velocity, and air density at the top of the stack

 The annual frequency of wind occurrence as a function of wind velocity

 Liner exit

- gas velocity and gas density

 Plume buoyancy due to flue gas temperature The most important fluid

- dynamic parameter is the momentum ratio between the stack

- exit flow and the cross wind. This ratio defines the relative influence of each flow on the other and is determined by the densities and velocities o f the flue gas and the wind. Plume downwash is most likely to occur during reduced unit load operation , in which stack discharge velocities are reduced , and during high wind conditions. Plume downwash is least likely to occur under the opposite conditions of high unit load operation and low wind velocity. For a multiple

- flue stack, the liner

- extension height above the shell is also very important for minimizing plume downwash , as is the axis of the liners relative to the prevailing wind direction. The selection of the stack

- top geometry and stack

- discharge velocity for good plume escape must also consider the stack

- discharge velocity for plume dispersion and stack

- liner velocit y. 1.4.9 Icing Potential Stack

- top icing can occur at below freezing conditions all winter long, every winter, and it creates a potential danger to people and property around the base 1- 10  of the stack. Whether ice forms on the top of the stack depends on the temperature of the stack surface, the temperature of the mixture of saturated flue gas and cold ambient wind, and on whether water vapor will condense out of the mixture. Ice formation is most likely at plants where below

- freezing temperatures are common a nd where they last for extended periods of time. The locations for potential ice formation are discussed below

.

The hood for a single

- flue stack The stack hood is the first element exposed to plume downwash. A hood for a single

- flue stack is usually connected to the stack, liner, and liner extension, which helps to keep it warm (particularly if uninsulated). When ice forms on an outward

- sloping hoo d, it usually forms off the upwind edge and/or in the downwind half, at two locations about 45° off the axis of the wind direction. These ice buildups can be significant and can cause damage when they fall to the ground. Stacks without a liner extension ar e most likely to develop large ice buildups. The liner extension for single or multiple liners The liner extension is the second element exposed to plume downwash. If the liner extension is uninsulated, the surface should always be maintained above freez ing temperature. If so, no ice could form, but some condensation might occur that would then run down to the stack roof. An insulated extension might have some ice formation. The roof of a multiple

- liner stack The stack roof usually does not directly to uch the liners, which permits expansion and movement of the liners. The roof often receives heat from the annulus of the stack, which prevents ice buildup and melts any snow that deposits there. Therefore, accommodations must be made for the drainage of liquid from the roof area. Preferably, drains should be located on the inside of the stack shell to keep them from freezing. Alternatively, they will need to be heat

- traced. Depending on the specific geometry and weather conditions, some icing can occur on the roof, and therefore may require heat

- tracing to eliminate excessive ice buildup. Railings The metal railings at the top of the shell will quickly cool to the temperature of the mixture of flue gas and ambient air, and they will be the first surfaces to become iced; in other words, these railings are the most likely places for ice to form. Platform near top of stack A metal platform on the outside of the stack near the top can also be exposed to low concentrations of flue gas as a result of downwash. Be cause it is metal and is exposed to the ambient and flue gas mixture temperature, it may also experience occasional ice buildup. Such platforms are the second most likely places for ice to form. Shell If ice forms on the shell, it can be expected to be on the downwind side of the stack. This condition occurs when visible downwash can be seen on the downwind side of the stack shell. 1- 11  Icing can often lead to serious buildups that can fall to the ground. However, when the icing is occurring, the platform near the top of the stack, the railings, and possibly the roof , may be slippery. If icing is observed at the top of the stack, care should be taken in the area around the base of the stack because of the possibility that the ice could become dislodged and fall to the ground. The potential for icing can be reduced by employing the following steps: 1. Install a liner extension above the shell to minimize the potential for plume downwash. 2. Select a stack

- liner discharge velocity that minimizes plume downwash over the expected operating range of the unit at the existing local wind conditions and that is consistent with other design objectives. 3. During cold ambient temperature conditions with high winds, run the unit at near full load. (Employing this operating proced ure may be natural under such conditions, because more power is consumed in below

- freezing weather.) 4. Use heated annulus air or heat

- tracing to heat the stack hood, roof, or other areas where ice forms on the top of the st ack above the freezing point. 1.5 C ontents of the Design Guide The main contents of the design guide are intended to provide the industry with information concerning the design and specification of wet stacks. 1.5.1 Information About Key Issues A main objective of the Revised Wet Stack Desi gn Guide is to provide the latest information about key issues. For example, the design guide contains discussion s on gas velocity, materials for liner construction, liquid

- collection d evices and drainage, continuous

- emissions

- monitoring systems, and flow modeling. This guide is intended to provide background information and identify the parameters that need to be considered in the development of an effective wet stack design. The key to the successful design of a wet stack is an understanding of the c hoices that lead to favorable wet stack operation at an early stage in the design process. Often, simple low

- cost design changes can result in significantly improved wet operation. But t hese changes need to be identified early in the design process. This design guide focuses on the options that s hould be considered, identifies existing constraints, and describes the effect s that variations in the parameters have on the design of a wet stack. 1.5.2 Knowledge of the State of t

he Art Another main objective of this Revised Wet Stack Design Guide is to summarize the latest technology available regarding wet stacks. This design guide is a follow

- up to EPRI report CS -2520, Entrainment in Wet Stacks, published in 1982 [1]; EPRI report TR107099, Wet Stacks Design Gu ide, published in 1996 [2]; and EPRI report TR -109380, Guidelines for the Fluid Dynamic Design of 1- 12  Power Plant Ducts, published in 1998 [4]

. The Revised Wet Stack Design Guide contains a presentation of the latest information available, updating what has been learned in the laboratory and from field installations over the past 14 years. 1.5.3 Overview of Project Implementation The Revised Wet Stack Design Guide is intended to provide the wet stack designer with information and recommend ations concerning the design and specification of wet stacks. A systematic process is provided in Section 4 to assist the designer in identifying the important issues that need to be addressed in the design process. The guide is presented in two main secti ons: 1) wet stack fundamentals and 2) the design process. These sections provide the designer with the information and background that lead to a bid specification and identif y what to specify for work scope, what information to provide in the specification s, and what to expect in terms of state

- of

- the

- art operation. 2-1  

# Section 2: Wet Stack Design Fundamentals
2.1 Introduction Stack liquid discharge (SLD) is also known as rainout or acid

- mist fallout. It is found in the form of droplets deposited on the ground and on structures in the vicinity of a wet stack. Typically, these droplets are acidic; they can lead to various forms o f damage in the vicinity of the stack, ranging from damaging ground cover to etching spots on the painted surfaces of automobiles either parked in or passing through the affected area. Damage to vehicle paint is the most common complaint related to SLD. If not mitigated, long

- term exposure can lead to increased levels of metal corrosion on equipment and structures in the vicinity of the stack. It is important to note that all wet stacks will have some amount of SLD. However, SLD is only a problem if the droplets are large enough to be detectable at ground level near the stack. The quantity of liquid discharged from the top of a stack is the result of gas - and liquid

- flow processes that take place in the ductwork and stack system between the discharge of th e absorber(s) and the top of the stack liner. The sources of liquid discharge can be categorized by the type of liquid

- flow process at the location of their origin, and by the resulting droplet sizes, as follows:

 Liquid droplets carried from the absorber to the top of the stack liner by the gas stream without depositing along the gas

- flow path. Only small droplets (10 –100 μ m) travel through the ducts without complete deposition. This means that most conventional scrubber designs, which have horizontal ducts connecting the absorber to the stack, will result in the deposition of most of their larger droplets on the internal duct and stack

- liner surfaces.

 Medium

- sized drops (100 –300 μ m) are typically generated from a mist eliminator that is experiencing perform ance issues. They may or may not collect in the wet duct/stack system, depending on the geometry of the system.

 Droplets re

- entrained from liquid deposition and condensation on duct and liner surfaces that can be discharged if the local gas velocities are high and/or if the local geometry is not favorable for wet operation. These droplets will typically be large (300 –6000 μ m) and are usually the major contributors to the SLD. 2-2   Droplets formed by adiabatic condensation. These represent a large liquid

flow rat e but are made up of very fine droplets (<10 μ m), which have a negligible effect on the SLD detectable at ground level.

 Liquid deposited and condensed on the liner, forming a liquid film that will flow upward in those areas of the stack where the gas velocity is higher than the flow

- reversal velocity for the liner material used (see Section 2.2.2). If this liquid film reaches the stack outlet, it will be re

- entrained back into the gas flow. The droplet sizes re

- entrained at the top of the liner, where veloc ities are highest, can be quite large (300 –6000 μ m), depending on the liner

- top geometry. Droplets of this size will impact ground

- level surfaces in the vicinity of the liner because they will not be able to evaporate before reaching the ground. The upper section of the stack choke, if installed, is usually operating in this mode of upward liquid flow. All plants operating with a WFGD system will have a white plume. This plume is a result of the very small droplets formed both by adiabatic condensation and by additional condensation as the water vapor in the plume is cooled by mixing with the surrounding air. This cooling process results in additional water

- vapor condensation on particulates and on small droplets within the plume. The increase in diameter i s small (only 1

– 10 μ m). At this size, these newly formed droplets will follow the local gas

- flow patterns; they should evaporate before reaching the ground as a result of mixing with the drier air away from the stack. The balance between the plume

- cooling and turbulent

- mixing processes defines the length of the resulting visible plume. Therefore, the visible plume length is primarily a function of the ambient air temperature and the air relative humidity. The quantity and location of the fallout at a given plant is a function of the droplet

- size distribution of the discharged liquid and the local atmospheric conditions, such as ambient temperature, wind, relative humidity, and turbulence level. Large droplets (>300 μ m) discharged from the top of the stack wi ll typically reach the ground under most circumstances. Mid

- sized droplets (100 –300 μ m) can make it to the ground before evaporating, depending on the local weather conditions. For example, droplet deposition is typically greater on hot humid days than on dry days because of the suppressed droplet

- evaporation rates. Small droplets (10 –100 μ m) typically evaporate before hitting the ground. As stated previously, all stacks will have occurrences of SLD. One of the major objectives in developing an effective w et stack design is to limit SLD to a minimal acceptable level through proper system design and the installation of an effective liquid

- collection/drainage system. The goal of this system is to collect and remove deposited liquid from the system before it h as an opportunity to re

entrain back into the gas flow and to limit the size of droplets that will ultimately reach the stack outlet. If the droplets are small enough, they will evaporate before hitting the ground. However, if the droplets are large, they will land on plant structures, equipment, and cars. A survey of plants with wet stacks found that the fallout of liquid droplets, if noticeable, usually occurs within a half

- mile radius of the stack. 2-3  2.2 Droplet and Liquid - Flow Fundamentals 2.2.1 Flow of Droplets Suspended Within a Gas Flow As droplets flow suspended within the flue gas, they will impinge upon and be collected on internal surfaces as a result of droplet

- trajectory paths controlled by the earth's gravity, centrifugal forces on the droplets when the gas flows through a turn, and gas

- drag forces on the droplet. Because of their size and inertia, large droplets (300 –6000 μ m) from the gas

- flow path deposit most readily; medium and small

- sized droplets (10 –300 μ m) deposit only partially; and fin e droplets (<10 μ m) deposit hardly at all because these droplets follow the gas

- flow patterns. The higher the gas velocity and the sharper the turn of the flue gas

– flow path, the more likely it will be that significant numbers of smaller droplets will impi nge upon and be collected on walls, turning vanes, baffles, and internal duct components (bracing, gusset plates, and expansion joints). Large drops are easily separated from the gas flow by both centrifugal and gravitational forces. In addition to gravita tional and centrifugal separation, small to medium

- sized droplets can also deposit on duct and liner surfaces in regions of separated flow and as a result of turbulent deposition. However, because of the generally uniform flow patterns within the upper li ner, the turbulent deposition of fine droplets formed by adiabatic condensation (as a result of the pressure reduction with elevation change in the liner) along the height of the stack is relatively small. Only 2.5

– 5% of the fine droplets formed as a resul t of adiabatic condensation actually contact and collect on the liner wall. Gas velocity, duct geometry, stack geometry, and droplet size govern the amount and location of liquid deposition within the wet stack system. Significant quantities of liquid collect along the outside radius of turns and on the pressure side of turning vanes and baffles. The major droplet

- deposition area in a stack is on the liner opposite the breeching duct for stacks with side

- entry breeches, and along the outside radius of stac ks with bottom

- entry elbows. Liquid deposition and removal from the system can be promoted by installing properly designed collection devices, vanes, and baffles in the wet duct/stack system. In order to minimize the potential for SLD, maximum deposition and collection of the liquid on the duct walls and stack liners must be achieved. Liquid

- collection systems are typically designed by using a physical

- flow model. The main objectives of physical

- flow modeling are 1) to understand the system’s internal depo sition and liquid

- film flow patterns and 2) to design liquid

collectors

— such as ceiling gutters, side

- wall channels, and floor guides

— to collect and channel deposited liquid to a drain before re

- entrainment can take place. This process is discussed in Sect ion 3.11.1, under "Laboratory Flow Modeling." 2-4  2.2.2 Liquid - Film Flow on Surfaces and Re - entrainment Fundamentals The liquid on the duct and liner surfaces is produced by droplet deposition and condensation flows in the form of a film, or as rivulets (small flow streams on the liner surface), governed by the local gravitational, surface

- tension, and gas

- shear forces. The liquid film naturally wants to flow both downward (because of gravitational forces) and in the direction of the gas flow because of the gas

- shear forces). In ducting, these forces will result in liquid flowing toward the duct floor as it moves in the direction of the gas flow near the surfaces. The liquid

- film flow in the vertical stack liner is a function of the gas

- shear and gravitational forces, which are acting in opposite directions to each other. For most liner surfaces, in which gas velocities are below 70 ft/sec (21.3 m/s), gravitational forces dominate, and the liquid film will flow downward. At velocities between 70 and 90 ft/s (21. 3 and 27.4 m/s), the gravitational and shear forces have approximately the same magnitude, and the forces are balanced. In this range, the liquid film in the liner will generally be stagnant on the wall and will not move in either direction. At velocities above 90 ft/s (27.4 m/s), the gas

shear forces dominate, and the liquid film will start to flow vertically toward the stack outlet. This velocity point is called the flow

- reversal velocity. The changes in the liner liquid

- film velocity are gradual. At low gas velocities, gravity dominates, and the liquid film moves quickly downward

— resulting in a thin liquid film. As the flue gas velocity increases, the downward velocity of the liquid film decreases because of increased vertical gas

- shear forces. However, as the liquid deposition and condensation rates are relatively independent of the gas velocity, the total quantity of liquid accumulating on the liner wall remains constant with time. This means that, as the downward velocity of the liquid

- film flow decrea ses, the drainage rate of collected liquid from the liner surface decreases, and the thickness of the film on the liner wall must therefore increase. The surface of thick liquid films is less stable than that of thin ones, and it is easier for waves to fo rm, which can lead to large quantities of re

- entrained droplets. Thick liquid films are the most frequent source of SLD and the resulting fallout of liquid droplets in the vicinity of the stack. Droplets can easily be re

- entrained from the surfaces of thick liquid films because of surface instabilities when they are exposed to a gas flow. Thick liquid films can be generated in three different ways. First, as detailed above, thick liquid films can be generated if the liner surface

- drainage rate is reduced by exposing the film to high gas velocities. Thick liquid films can also be generated if the liquid feed rate to the wall is high as a result of high levels of droplet deposition or condensation. Finally, they can be generated if the stack is too high and al l the collected liquid flows to the bottom of the liner without any intermediate drainage locations. Liquid re

- entrainment most frequently occurs because of one or more of the following:

 High gas

- velocity shear near surfaces covered by a liquid film. 2-5   Inst ability of the liquid film (waves and ripples) on vertical stack

- liner surfaces. This instability occurs because, as gravity is pulling the liquid film downward, the gas

- drag forces are pushing the liquid film or droplet upward. This effect increases drama tically with liquid

- film thickness.

 Surface discontinuities and protrusions that disrupt gas and liquid flow locally, allowing the gas flow to “get under” the liquid.

 Vanes and baffles that cause gas

- flow separation, recirculating liquid flow, and re

- entra inment.

 Gas

- flow patterns that drag liquid along the surface to re

- entrainment sites.

 Strong vortex patterns that can pick water up in the core of the vortex from horizontal or vertical surfaces. Re

- entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack

- liner surfaces

 Dampers, damper

- blade guides, and trusses

 Thermal expansion joints

 Trailing edges of turning vanes

 Duct

- junction corners

 Breeching

- duct area changes

 Stack floor

 Stack

- liner surfaces, particularly in the entrance regi on and at the intersection with the breeching duct

 Solids buildup or scaling on internal surfaces

 Stack

- liner contraction sections

 Stack choke surfaces, because of increasing gas velocity Liquid re

- entrainment at these locations will be in the form of larg e droplets (300–6000 μ m); they will be discharged at the top of the stack, if not prevented by a combination of favorable gas

- flow

- path design and an optimized liquid

collector system. Droplets over 100 μ m do not completely evaporate outside of the stack a nd will probably reach the ground level. 2.2.3 Gas - Flow in Wet Duct Systems With the possible exception of materials, the design of wet ducts is the same as that for dry ducts with respect to their overall design and pressure

- loss considerations. The reader is directed to EPRI Report TR -109380, Guidelines for the Fluid Dynamic Des ign of Power Plant Ducts , for a detailed discussion of this process [3]. Some basic considerations for wet operation are discussed in that report. 2-6  The gasses exiting the absorber will be fully saturated and will contain suspended droplets. These gasses wi ll flow through the ducting, coming in contact with the duct walls and with any internal structures within the duct. Liquid will be continuously condensing from the gas onto these surfaces, forming a liquid film that will flow downward under gravity and in the direction of the local gas flow. Care needs to be taken to ensure that the gas

- flow patterns moving this liquid film are favorable for wet operation, minimizing conditions such as flow separations or recirculation zones

— potentially resulting in the re

- entrainment of liquid back into the gas

- flow. Also, to the extent possible, the gas

- flow patterns should be managed in such a way as to move the resulting liquid films to drainage points for removal from the system. Turning vanes are typically used to optimize gas

- flow patterns, reduce pressure losses, and/or promote liquid collection. An economic assessment must be made with respect to the value of turning vanes. Although desirable from a pressure

loss standpoint, the corrosion

- resistant materials requir ed for installation in a wet environment are very expensive, and the installation cost may not justify the savings gained by the reduced operating cost. An economic study should be performed to quantify the life

- cycle cost or savings of such an installatio n. Some turning vanes may be specifically required to reduce or eliminate SLD. Such a situation might occur if multiple wet ducts combine immediately upstream of the stack entrance. In such a situation, vane installation will be required to ensure that the gas flow entering the stack is favorable for wet operation and, possibly, to minimize swirling gas flow within the liner. Another situation would be one in which the absorber is close

- coupled to a stack, as in Figure 2

- 1. Without vane installation, the g as flow would enter the stack with a high vertical component, minimizing the ability of the stack to generate the lower stack

- liner flow profiles required for favorable wet operation and liquid collection. Although this phenomenon is most prominent in clos e- coupled arrangements utilizing a side

- entry breech, it can also occur in close

- coupled units with bottom

- entry elbows. In such a situation, a turning vane would be required to ensure that the gas flow will enter the stack horizontally. Vanes that are spe cifically required for the minimization of SLD should be installed as recommended. A more detailed discussion of wet duct design is presented in Section 3.6 of this guide. 2-7  Figure 2-1 Liner Inlet Gas

- flow P atterns – Close

- Coupled Absorber 2.2.4 Gas Flows in Wet Stack Systems The gas

- flow profiles within the stack liner are very important with respect to the collection and drainage of liquid from the flow gas. It is important to ensure that these flow profiles are as well defined and robust as possible, given the ductwork arrangement leading to the liner. There are two major stack

- entry design options: a side

- entry breech and a bottom

- entry elbow (Figure 2

- 2). Both designs are similar in terms of fluid dynamic s, given that they both turn the gas flow 90° from horizontal to vertical. However, there are major differences between them with respect to the intensity of the resulting flow patterns and to the way in which these flow patterns interact with their respec tive physical geometries. Droplet Deposition High on Liner Wall

Absorber Outlet Absorber Outlet Droplet Deposition Low on Liner Wall

– Liquid Collection is Enhanced 2-8  Figure 2-2 Typical Stack

- Inlet Arrangements As a uniform gas profile flows through a 90° elbow, the gas flow will want to continue in a straight line. This tendency will result in the formation of a region of high gas velocity along the back wall of the liner for a side

- entry breech arrangement, and along the outside radius of the bend for a bottom

- entry elbow arrangement. The resulting gas velocity along the outer radius of the elb ow can be 50

– 100% higher than the average gas velocity entering the elbow. When the gas hits the back wall of the elbow, it splits in two, and each half flows circumferentially around toward the inside radius of the bend. This flow generates two counter
- ro tating secondary vortices, which form on the downstream side of the turn. If the curvature radius of the elbow is small enough, a flow

- separation zone will also form along the downstream side of the inner radius of the turn. These phenomena are shown in Figure 2

- 3. The strength of the secondary vortices and the size of the separation zone increase with the sharpness of the turn and with the velocity of the gas entering the elbow. 

### Side Entry Breach Bottom Entry Elbow Absorber Outlet Absorber Outlet Liner Liner 2-9  Figure 2-3 Gas

- flow Patterns Through a Standard 90° Elbow For favorable wet operation, strong secondary vortices and a large region of flow separation are desirable. For this reason, the side

- entry breech, defined by a sharp 90° turn, is considered the most favorable design for wet operation. The flow entering the liner through the breech generates strong robust secondary flows, in the form of two large counter

- rotating vortices, in the lower liner. A large gas

recirculation zone is formed just above the breech in the liner as the gas turns from the vertical into the liner. These secondary flows, and the recirculation zone detailed in Figure 2

- 4, are relied upon to move the collected liquid film on the walls of the liner circumferentially out of the main gas flow and into the quiet z ones for drainage from the system. Experience has shown that the secondary flows generated within a liner using a side

- entry breech remain stable and well

defined even at reduced flows, ensuring effective wet operation over a wide range of unit operating l oads. A A

SECTION AA Secondary Flows Higher than Average Axial Velocity Along Outside Radius of Bend

Secondary Flows 2- 10  Figure 2-4 Liner

- Inlet Gas

- flow Patterns – Side

- entry Breech The use of bottom

- entry elbows has become very popular with the increased use of FRP liners. Bottom elbows are very similar to standard duct elbows and, because of their size, are usually fabricated from liner

- can sections spun and assembled on site. Typic ally, bottom

- entry elbows consist of three to four miter cuts, forming a 90° elbow with the center

- line turn radius to internal

- diameter ratio of between 0.8 to 1.5. This design is very clean in terms of fluid dynamics, and it results in low system

- pressur e loss. When flue gas flows through an unvaned bottom

- entry elbow, two counter

- rotating secondary

- flow vortices are generated, as detailed above. Because the change in direction from horizontal to vertical is generally gradual, the intensity of the seconda ry - flow vortices is not as great as that generated in the more abrupt side

- entry breech arrangement. Additionally, because the turn is more gradual, a region of flow separation may or may not be generated along the downstream side of the inside radius of t he elbow. If a region of flow separation is generated, it will be smaller and less intense than that generated in the side

- entry breech arrangement. It has been observed in flow

- model studies of bottom

- entry elbow arrangements that two small

— but very inten se — regions of flow recirculation are generated on both sides of the elbow along the last miter cut (Figure 2

- 5). These recirculation zones 

### C

B

A C

B

A SECTION CC

SECTION BB

SECTION AA Gas Flow Flow Separation High Gas Velocity Along Rear Wall

Secondary Flow Vortices Uniform Vertical Gas Flow

Flow Separation Flow Reattaches

C haotic Flow Along Liner Floor High Gas Velocity Along Rear Wall

Well Centered Stagnation Point Upward Bias to Flow Through Breach 2- 11  are formed because the change in direction from horizontal to vertical is abrupt in these locations, and the incoming horizontal gas flow “over shoots” the inside radius of the turn. Although small, these recirculation zones are problematic, because they can be sites of significant liquid re

- entrainment.

Figure 2-5 Liner

- Inlet Gas

- flow Patterns – 3- Miter Cut Bottom

- Entry Elbow For both liner

- entry arrangements, the counter

- rotating secondary

- flow vortices generated in the entry region dissipate significantly within 3

– 4 liner diameters above the roof of the inlet duct/breech. Above 5

– 6 liner diameters, the flow will be essentially vertical and uniform. For this reason, a liner is typically divided into two regions: The lower liner, where the secondary

- flow vortices are generated and dissipate, is defined as the region in th e liner extending from the liner floor to a point 3

– 4 liner diameters above the roof of the inlet/breech duct; the region above this point is defined as the upper liner, where the gas

- flow profile is essentially uniform and near vertical. It is clear that the secondary

- flow vortices are desirable and necessary for favorable wet stack operation. For this reason, flow controls in the liner are C

B C

B SECTION CC

SECTION BB

SECTION AA A A

Gas Flow Flow Separation High Gas Velocity Along Rear Wall

Secondary Flow Vortices Flow Reattaches

High Intensity Gas Recirculation Along Duct Wall at Seam Uniform Vertical Flow

High Gas Velocity Along Rear Wall Flow Separation 2- 12  discouraged unless absolutely necessary. Flow controls can reduce or eliminate the secondary

- flow vortices and can act as locations for liquid collection and re

entrainment back into the gas flow with little chance for recollection

— both of which can lead to increased levels of SLD. It should be noted that a division wall separating two breech entries into a single liner would not normally be considered to be a flow

- control device as defined above. A properly designed division wall will divide a liner into two separate aerodynamic regions, each of which will be allowed to form its own secondary

- flow vortices and flow

sepa ration zones, resulting in favorable liquid

- collection potential. 2.3 Sources of Liquid in a Duct/Wet Stack System All of the water

- vapor content in the stack

- inlet flue gas flow comes from the absorber outlet where the flue gas is fully saturated. The liq uid content of the stack

- inlet flue gas flow comes from the following sources:

 Mist

- eliminator droplet carryover, which consists of fine drops (<10 μ m), a portion of which never contact a mist

- eliminator blade.

 Medium

- sized droplets (100 –300 μ m ) will re

- entrain from mist

- eliminator vanes at high velocities and from solids buildup during abnormal operation.

 Condensation on the duct surfaces upstream of the stack inlet, which adds liquid onto the duct surfaces.

 Deposition of droplets on duct surf aces, vanes, baffles, and structure, which removes droplets from the gas

- flow stream.

 Re

- entrainment of droplets from duct surfaces, vanes, baffles, and structure, which adds large droplets to the gas

- flow stream.

 Installation of liquid collectors on duct walls and ceilings, vanes, baffles, and structure, which collect liquid from these surfaces and guide the liquid to the duct floor of stack

- bottom drains for removal from the duct/stack system.

 The geometry of the ducts between the absorbers and the stack inlet; the number of vanes, baffles, and liquid collectors in the ducts; and the effectiveness of the mist eliminators and duct liquid collectors will determine the amount of liquid, droplet sizes, and distribution of liquid in the stack breeching duct at the stack inlet. 2 .3.1 Mist Eliminators and Absorbers The gas stream leaving the mist eliminator is saturated with water vapor, and entrained fine liquid droplets are carried by the gas flow under ideal design and operating conditions. Mist

- eliminator carr yover is a function of the gas velocity through the blades and the liquid load in gpm/ft 2 (l/min/m 2 ) at its inlet face. It is important that the mist eliminators operate properly to minimize the size and rate of droplet carryover. The mist eliminators are designed to minimize carryover of liquid and slurry from the WFGD absorber into the downstream ductwork. Absorber problems resulting in a nonuniform gas

- flow profile entering the mist eliminator, or problems such as excessive mist

- eliminator pluggage or fo uling, can result in increased liquid carryover. Some larger droplets may also be 2- 13  present in the gas stream that were re

- entrained from the mist

- eliminator blades, the mist

- eliminator support structure, and areas of solids buildup on the mist eliminator. D uring the wash cycles, washing the mist eliminator can also increase the amount of liquid carried with the gas flow downstream of the mist eliminator because of increased liquid load from the washed section. A field evaluation of the mist

- eliminator perfor mance can be made by performing visual inspections. If carryover problems are noticeable, SLD will probably occur if effective liquid collectors are not installed in the duct and the stack. The mist

- eliminator selection, operation performance, and possible problems are described in [3]. 2.3.1.1 Mist

- Eliminator Design and Arrangement The mist

- eliminator system is the first and most important element of the liquid collectors in a wet duct/stack system. The selection of the best mist eliminators to match the a bsorber and duct design will result in reduced liquid load in the duct/stack system and less elaborate duct and stack liquid

- collection systems. In FGD service, the function of the mist eliminators is to remove entrained scrubbing slurry from the flue gas before it travels out of the absorber into the downstream ductwork and stack. The mist eliminators are located downstream of the absorber spray zones, where the flue gas contacts the scrubbing slurry that removes sulfur dioxide. The slurry content of the l iquid carryover can cause solids buildup in the outlet ductwork and stack, high particulate emissions, and corrosion. The amount of liquid that the flue gas carries into the mist eliminator will depend on many factors, including (but not limited to) the droplet size, the gas velocity, the liquid

- to
- gas ratio, and the washing method. There are many mist

- eliminator designs, available from numerous manufacturers. But the chevron type of collector is almost exclusively used by WFGD systems on coal

- fired units. Mist eliminators are manufactured from many different corrosion

- resistant materials, ranging from alloys to FRP and various extruded plastics. Mist eliminators can be oriented toward any number of gas

- flow directions, from horizontal to vertical (Figure 2

- 6). Most modern WFGD systems utilize a vertical gas

- flow orientation or a singularly supported roof top or diamond design. Older weir

- type scrubbers utilize a horizontal gas

- flow mist

- eliminator design. Depending on the separation efficiency required, a number of stages of mist eliminators in series are used. In utility power

- generation applications, two stages are typically used. In situations in which high liquid loading are experienced, a bulk entrainment separator may be placed in front of the first m ist

- eliminator stage. This device is typically a series of baffles used to limit the liquid loading entering the mist eliminator. 2- 14  Figure 2-6 Common Mist

- Eliminator Arrangements The breakthrough velocity for a mist

- eliminator design is a function of its ability to drain away collected liquid, and it is therefore substantially a function of its orientation relative to vertical. Vertical gas

- flow mist eliminators require collected l iquid to drain back through and against the incoming gas flow. This process will result in higher liquid loading entering the mist eliminator, because the smaller droplets from the drainage flow will be immediately entrained back into the unit after draini ng from the leading edge of the blades. A horizontal gas

- flow mist eliminator will drain its collected liquid perpendicular to the gas flow into a collection box, which typically drains directly back to the absorber’s reaction tank. This design minimizes the liquid entrained within the mist eliminator and results in better performance: either the ability to operate at a higher gas velocity for a given liquid loading or the ability to handle a higher inlet

- liquid loading for a given inlet

- gas velocity. 2 nd ME Stage

1 st ME Stage2ndME Stage1stME Stage

1 st ME Stage 2 nd ME StageGas FlowGas Flow Gas Flow

Drain Box

Drain Box Drainage through Incoming Gas Flow

Drainage through Incoming Gas Flow 

## Vertical Gas Flow Horizontal Gas Flow

Vertical Gas Flow Diamond Arrangement 2- 15  Roof

- top, or diamond

- type, arrangements fall between the two extremes. This arrangement is becoming more popular because of its ability to support what is essentially a two

- stage mist

- eliminator arrangement from a single support elevation within the absorber. In this design, two stages of mist

- eliminator blades are oriented diagonally, forming a parallelogram which has been rotated so that it is supported from two of its corners. The design enhances liquid drainage across most of the collection surface. Higher

- than

- average liquid loadings will still be experienced at the unit’s drainage points because the collected liquid is still required to drain against the incoming gas

- flow at these locations. Another concern with this arrangement is the need to use a mist -e liminator blade design with a more torturous path (increased curvature) to maintain the ability for the blades to inertially separate droplets from the gas flow. For a given blade design, as the angle of the gas passing through it increases from 0° (perpen dicular) to 45° or higher, the path length through the blades increases in the diamond arrangement (Figure 2

- 7). If the curvature of the blade has not changed, the side

- to
- side deflection will be the same for the increased path length through the blades, s o the resulting inertial separation will be lower.

Figure 2-7 Comparison of Gas

- Path Lengths Through a Mist Eliminator as a Function of Angle 

## A

B A

B 135°

137° 90°

95° Direction of Gas Flow Through Mist Eliminator Flow Path Through ME Comparison of Flow Paths Through ME

Perpendicular

ME Side Elevation 2- 16  The amount of liquid

- droplet carryover into the wet duct/stac k system from the mist eliminators can be minimized by 1. Selecting th e most suitable mist

- eliminator elements for the operating conditions of the installation 2. Using two

- stage mist elimination 3. Using a vertical or angled mist

- eliminator orientation 4. Using gas velocities with a safe margin under the critical re

- entrainment velocity for the mist

- eliminator blades selected 5. Achieving a sufficiently uniform velocity profile into the second

- stage mist

eliminator to keep all regions below the critical re

- entrainment v elocity 6. Selecting effective washing techniques and procedures to prevent solids buildup on the mist eliminators, without entraining wash spray in the gas flow 7. Establishing inspection and maintenance procedures for the mist eliminators 8. Maintaining good p H c ontrol to reduce solids precipitation in the mist eliminators EPRI Report GS -6984, FGD Mist Eliminator System Design and Specification Guide [3] describes all essential aspects of mist

- eliminator design, application, operation, and maintenance necessary fo r an effective mist

- elimination system. The measured laboratory performance of several commercially available mist eliminators is also furnished in this report. Information regarding the performance of mist eliminators as a function of gas velocity and inl et
- liquid load is available from the mist

- eliminator manufacturers, and it should be obtained early in the system

- design process. Performance is given as a function of the gas velocity immediately upstream of the mist eliminator. Figures 2

- 8 and 2

- 9 presen t typical vertical gas

- flow chevron

- type mist

- eliminator performance curves for pressure drop and carryover versus gas velocity, respectively. Mist

- eliminator collection efficiency versus droplet size can also be supplied from the manufacturer if desired. This latter performance curve is used to identify the maximum droplet size expected to pass through the mist eliminator without being collected. (See Figure 2

- 10 for an example.) For design and estimating purposes, a typical mist

- eliminator manufacturers c arryover guarantee is 0.02 gr/acf (46 mg/m 3 ) can be used. 2- 17  Figure 2-8 Typical Chevron

- Type Mist

- eliminator Pressure Drop vs. Gas

- Velocity Curve

Figure 2-9 Typical Chevron

- Type Mist

- Eliminator Carryover vs. Gas

- Velocity Curve 0.000 0.050 0.100 0.150 0.200 0.250 0.300

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00Pressure Drop (" W.C.)

Velocity (ft/s) 

### Pressure Drop vs Gas Velocity 2- 18  Figure 2- 10 Typical Mist

- Eliminator Collection Efficiency vs. Droplet

- Size Curve 2.3.1.2 Mist

- Eliminator Average Velocity WFGD absorbers typically operate at an average gas velocity entering (but not within) the mist

- eliminator sections of 10

– 14 ft/s (3.0

– 4.3 m/s) for vertical gas

flow

– oriented mist eliminators and 16

– 20 ft/s (4.9

– 6.1 m/s) for horizontal gas

flow

– oriented mist eliminators. Under normal inlet

- liquid loading conditions

— typically 0.5 gpm/ft 2 (0.0123 l/min/m 2 )— carryover starts to become an issue at velocities above 16 ft/s

– 20 ft/s (4.9

– 6 m/s) for vertical gas

- flow and horizontal gas - mist eliminators, respectively. Locally, hi gher

- than

- average gas velocities can occur through a mist eliminator because of a number of reasons, including

 Mist

- eliminator fouling and/or plugging in localized areas

 The location of the absorber outlet too close to the outlet of the mist eliminators

 A nonuniform gas

- flow distribution into the mist eliminators

 A nonuniform liquid loading from the absorber into the mist eliminators Because local gas velocities in excess of the mist

- eliminator breakthrough velocity will result in increased droplet and slurry carryover into the downstream ducting, every effort must be made to ensure that the mist

- elimination system is clean, well maint ained, and operating correctly. 2- 19  2.3.1.3 Mist

- Eliminator Liquid Carryover If designed and operated properly, a modern mist

- elimination system should experience the guaranteed minimum liquid carryover. Some carryover should be expected during on

- line mist

- eliminator washing. Liquid carryover from the mist

- eliminator will occur if the local liquid loading and/or the local gas velocity are too high. A sudden increase in SLD is often a key indicator of an issue related to mist

- eliminator performance and/or ope ration. As stated previously, information regarding the performance of mist eliminators as a function of gas velocity is available from the mist

- eliminator manufacturers, and it should be obtained early in the system

- design process. To the extent possible , all the liquid carryover from the mist eliminator that deposits on the duct - and stack

- liner surfaces must be collected. A significant portion of this carryover must be collected in the downstream ducting before it reaches the stack liner, making the downstream ducting an important part of the overall liquid

- collection system. For this reason, placing the stack liner directly on top of an absorber should be discouraged. In this arrangement, liquid carryover from the mist eliminator will have very limited deposition and little opportunity for collection before being discharged from the top of the liner as SLD. 2.3.1.4 Mist

- Eliminator Wash

 Mist eliminators have no moving parts. After they are properly installed, they will operate at design conditions without any further attention. Because there are no moving parts, the major potential operating problems are the plugging of the mist eliminator with insoluble solids and/or the corrosion and degradation of the materials of construction. The usual operating

- pressure decrease across the mist eliminator when clean is generally 0.2

– 0.5 in (5.1

– 12.5 mm) of water column per chevron stage.

 If operating conditions remain unchanged, any significant increase in the operating

- pressure drop is undoubtedly attributable to par tial plugging of the mist eliminator. As a general rule, washing frequency is more important than wash

- water pressure and wash

- water volume. However, all three factors must be taken into consideration to ensure a clean, problem

- free mist eliminator. High -q uality water should be used.

 Mist eliminators are typically broken down into multiple smaller segments for on

- line washing. Each segment is washed sequentially in series. A minimum wash frequency for each segment of 1.5 min/h is recommended. More frequent washing is always better. The pressure of the wash

- water nozzles is generally 25

– 45 psig (0.17

– 0.31 MPa). Mist

- eliminator spray nozzles are typically 90

– 120° full

- cone nozzles. Most manufacturers recommend a 150

– 200% overlap of the mist

- eliminator spray

- wash pattern. A nozzle to mist

- eliminator spacing of 2

– 3ft (0.6

– 0.9 m) should be used. 2- 20   Generally, the lower mist eliminator in a two

- stage mist

- elimination system is washed with more liquid than the upper mist eliminator. Most manufacturers recommend washing the bottom of the lower mi st eliminator at 1.5 gpm/ft 2 (61.1 l/min/m 2 ) and the top of the lower mist eliminator at 0.5

– 0.6 gpm/ft 2 (21

– 25 l/min/m 2 ). It is also recommended to wash the bottom of the upper mist eliminator at 0.5

– 0.6 gpm/ft 2 (21

– 25 l/min/m 2 ). Typically, the top of the upper mist eliminator is not washed during absorber operation. During the wash cycle, the liquid within the mist

- eliminator section being washed will be significantly higher, and increased carryover into the downstream ducting from this section will be expected. A properly designed wet duct/stack system will be able to handle this additional carryover without increased potential for SLD. For a detailed discussion on the effectiveness of mist

- eliminator systems, refer to [3]. To maintain effectiveness and t o prevent sludge buildup, it is very important to keep mist eliminators clean by washing them according to the manufacturer’s specifications. 2.3.2 Liner Condensation The most significant source of liquid in a wet duct/stack system is condensation of water vapor from the saturated gas flow exiting the WFGD absorber. This condensation comes from two main sources: 1) thermal condensation on the wall as a result of heat transfer from the flue gas to the outside air through the liner, insulation, annulus air, and concrete shell and 2) adiabatic condensation in the bulk of the gas flow because of the expansion of the saturated gases as a function of the pressure decrease along the height of the stack. Detailed analytical calculations must be completed to predict the quantity of liquid produced by condensation in the absorber outlet duct and in the stack. These computer models are based on the equations presented and discussed in EPRI Report CS -2520, Entrainment in Wet Stacks [1]. The accuracy of the condensation calculation is estimated to be ±10%. The rate of liquid condensation is a function of ambient air temperature, wind velocity, and liner

design variables. Calculations are performed for a selected “worst

- case” atmospheric condition to estimate the maximum quantity of liquid condensation expected in the liner of the subject unit. Determining the expected quantities of liquid condensation in the duct and liner is necessary for the development of an effective liquid

- collection system, because it will affect th e number, sizing, and location of the various liquid

- collection gutters and drains. To perform the condensation calculations, the following information is required:

 Detailed design drawings of the system ductwork from the absorber outlet to the stack inlet, including material specifications and thicknesses

 Information on duct insulation: location, type, thickness, and covering 2- 21   Detailed de sign drawings of the stack shell and liner, including the number and orientation of liners relative to true North, materials of construction, material thickness versus elevation, annulus air

- gap thicknesses versus elevation, location and size of stack

- shel l vents to atmosphere

 Information on the liner insulation: type, thickness, and covering

 Expected flue gas conditions: temperature, flow rate, constituents, water

vapor content

 Plant elevation

 Wind

- rose data for a minimum of 1 year (5 years preferred), presenting air temperatures, barometric pressure, wind speed, and direction 2.3.2.1 Thermal Condensation Thermal condensation (or wall condensation) caused by heat transfer is important in wet stacks. Because the saturated flue gas temperature exiting the ab sorber is usually 120

– 130°F (49

– 54°C), heat transfer through the liner, the annulus, and the concrete shell to the cooler surrounding ambient air causes water vapor to condense on the inside surfaces of the ducts and stack liner. The rate of condensation c aused by heat transfer to the outside environment are functions of the liner and shell construction and its thermal conductivity, the type and quantity of liner insulation, the gas

- flow conditions, and the local atmospheric conditions (ambient temperature, wind velocity, and barometric pressure). Annulus pressurization of brick

- lined stacks using outside air increases the rate of condensation on the liner surfaces. Leakage of pressurizing air through the liner in older brick stacks can significantly increa se the condensation. It should be noted that, with the exception of alloy liners, the use of insulation in wet stacks is not very common. Insulation is typically used in dry stacks to keep the metal temperature of the liner above the acid dew point. And because wet stacks are always below the acid dew point, corrosion

- resistant materials are used that do not require insulation. FRP, borosilicate block, and brick liners have some insulating value and rarely have needed to add insulation. However, because a major source of the liquid generated on the liner surface is thermal condensation, insulation can be used to significantly reduce this phenomenon. Previous condensation studies have shown that the addition of two inches of liner insulation can reduce the quantity of thermal condensation by approximately a factor of four. Additional increases in the insulation thickness will reduce condensation further, but at a diminishing rate of return. 2.3.2.2 Adiabatic Condensation and Stack Effect Adiabatic condensation (or bulk condensation) occurs in the bulk of the gas as the pressure of the saturated gas flow decreases as a result of pressure loss and elevation change along the height of the stack. This type of condensation may generate an appreciable quantity of l iquid. The decrease in absolute pressure of 2- 22  the saturated gas causes water vapor to condense on fine dr oplets or on small solid particles within the bulk of the gas. Droplets formed by this type of heterogeneous condensation will be very small (<10 μ m). Only a very small fraction of this adiabatic condensation will reach and be deposited on the liner surfac e by the turbulent

- deposition process. Estimates of this fraction range from 2.5 to 5%. The expected percentage of droplets coming in contact with the liner wall decreases with increasing liner diameter, because of the greater distance a droplet would need to travel from the center of the liner to the liner wall. Historically (as stated above), estimates of 2.5

– 5% have been used, and these percentages have been anecdotally supported by observations from operating units, which can monitor the liquid

- drainage rate from their liquid

- collection systems. Condensed droplets that do not encounter the liner wall are discharged from the top of the liner, which creates the familiar white plume associated with wet stack operation. These droplets do not represent a prob lem, because their size is very small, and they will evaporate before reaching the ground. The stack effect is the hydrostatic

- pressure difference between the inside and outside of the stack liner. This difference is due to the lower density of the heated gas in the stack compared to the density of the ambient air. Therefore, the stack effect is calculated by multiplying the density difference of the gas in the stack and the ambient air times the height of the stack. Net stack draft is the operating pressur e at the inlet of the stack liner, and it is calculated by subtracting the fluid friction, the inlet losses, and the exit losses from the stack effect. For wet stack design, the worst

- case conditions are the maximum stack gas

- flow rate resulting in the hig hest friction loss, in combination with the hottest and most humid ambient conditions. These conditions can normally be expected to produce a positive gas pressure at the stack inlet. The stack effect at worst

- case conditions is generally only 0.2

– 0.3 in W g (0.05

– 0.07 kPa) for wet stack operation

— a small amount compared to the friction and exit losses at full load. 2.4 Liquid - Film Behavior on the Liner Wall Because the flow characteristics are so different, the discussion of the liquid

- film behavior on the liner wall must be broken down into two sections: the lower liner and the upper liner. The lower liner is defined as that portion of the liner located below a point four liner diameters above the roof of the breech duct or bottom

entry elbow. The upper liner is everything above that location. As discussed previously, the gas

- flow patterns in the lower liner are three

dimensional and complex, and they are used for the majority of the liquid collection in the stack. The flow entering the liner through the breech or bottom

entry elbow generates secondary flows in the form of two large counter

- rotating vortices in the lower liner. These secondary flows are relied upon to drag the liquid film on the walls of the liner circumferentially out of the high

- velocity ga s flow into low

- velocity or separated regions where it can be drained from the system. 2- 23  In a side

- entry breech arrangement, the droplets entrained in the incoming gas flow will typically impact on the rear wall of the liner. The droplets will impact in a z one extending approximately ± 1 liner diameter from a point on the rear wall corresponding to the roof of the breech opening (Figure 2

- 11). Finer droplets will impact in the upper portion of this zone, whereas larger droplets will impact lower in the lower portion. Secondary gas flows generated by the turn into the liner will direct a large portion of the liquid collected on the rear wall of the liner upward, then circumferentially around to the front of the liner above the breech, where it will encounter t he downward flow in the gas

- recirculation zone located above the breech opening on the front wall of the liner. When the liquid reaches this recirculation zone, it will flow downward and be re

- entrained from the top edge of the breech opening. Liquid collected on the liner walls below the droplet

impact zone will either flow downward onto the liner floor or will be pulled circumferentially around the liner to the vertical side edges of the breech, where it will be re

- entrained back into the gas flow. Effect ive liquid collectors can be developed to work with these liquid

- flow patterns to collect and direct these liquid films to drain points for removal from the system.

Figure 2- 11 Lower Liner Droplet - and Liquid

- Film Flow Patterns – Side

- Entry Breech Gas Flow Downward Liquid Film Flow in Flow Separation

Downward Flow Along Breach Opening Uniform Downward Liquid Film Flow From Upper Liner

Liquid Pooling with Chaotic and Vortex Flow On Liner Floor Circumferential Liquid Film Flow

Downward Film Flow Below Impact Zone Droplet Impact Zone Upward Liquid Flow Reentrainment Liquid Film Flow

REAR WALL OF LINER Reentrainment 2- 24  Some liquid will make it to, and collect on, the liner floor. If the liner floor is flat, the collected liquid will pool and slosh around until it finds a drain point. Typically, two vortices can be observed on the flat floor of the liner, their centers located approximately 1/3 of the way across the liner and even with the side walls of the inlet breech duct (Figure 2

- 12). Under some conditions, these vortices can be sufficiently strong to re

- entrain liquid off of the floor and back into the gas flow.

Figure 2- 12 Liner

- Floor Liquid

- Film Flow Patterns – Side

- Entry Breech For a bottom

- entry elbow, the droplets entrained in the incoming gas

- flow will typically impa ct in a zone starting on the outside radius of the elbow at an elevation corresponding approximately to the roof of the inlet duct and extending vertically approximately 1

– 1.5 liner diameters (Figure 2

- 13). Similar to the side

entry breech, finer droplets will impact along the top of the impact zone, and larger droplets will impact along the bottom. Secondary gas flows generated by the 90° elbow into the liner will drag a large portion of the liquid collected on the X/3 X

Vortex Flows on Liner Floor

Liquid Reentrainment from Vortex Flow Gas Flow Gas Flow

Reentrainment Liquid Film Flow Liquid Pooling on Liner Floor

Vortex Flows on Liner Floor 2- 25  rear wall of the liner upward. Then it wi ll be dragged circumferentially around to the inside radius of the inlet elbow, where it will encounter a lower vertical gas velocity. It will then flow downward until it either re

- entrains back into the gas from the top of the duct, or it will flow circum ferentially downward on either side of the inlet duct toward the bottom of the elbow. In this latter case, there is a high probability that the liquid will be shredded off of the wall by recirculation zones located along the walls of the elbow near the inside radius. Liquid that makes it to the floor of the inlet elbow will tend to pool there, because the horizontal gas velocity along the floor is not high enough to drag the liquid back up the outside radius of the elbow.

Figure 2- 13 Lower

- Liner Droplet - and Liquid

- Film Flow Patterns – Bottom

- Entry Elbow Stacks are typically designed so that condensed liquid on the liner wall will flow downward for collection. The stack

- design velocity is the average gas vel ocity in the liner defined as the volume flow rate divided by the area of liner. As discussed above, for both inlet arrangements, the flue gas that enters the liner impacts the wall opposite the inlet and forms a region of locally high gas velocity along t he rear wall of the liner or outer radius of a bottom

- entry elbow. The gas velocity is usually high enough to drag the deposited liquid vertically up the liner wall until the secondary

- flow vortices are strong enough to pull the liquid circumferentially ar ound and out of the high

- velocity region. If the vertical gas velocity is not sufficiently high, the deposited liquid will either move very slowly upward or not

Gas Flow Downward Flow in Flow Separation Upward Film Flow

Circumferential Liquid Film Flow

Droplet Reentrainment from High Intensity Gas Recirculation

Uniform Downward Liquid Film Flow From Upper Liner

Downward Film Flow
Liquid Pooling on Inlet Duct Floor Droplet Deposition

Zone Circumferential Liquid Film Flow Reentrainment Liquid Film Flow

REAR WALL OF LINER 2- 26  move at all. Both of these situations are undesirable, because as more liquid is deposited in t his region, the liquid

- film thickness will increase until the surface becomes unstable, wavelets are formed, and droplets will begin re

- entraining back into the gas flow. There is little chance that many of these droplets can be recollected before exiting the liner as SLD. If this situation occurs, liquid

collection devices in the form of “V” - shaped gutters and other types of liquid collectors will need to be installed in the droplet

- impact region to mechanically help the liquid move circumferentially into the secondary

- flow vortices. These devices will be discussed in more detail in Section 3.11.8. Liquid

- film flows in the upper liner should be better behaved. In this region, the gas - flow profile moving up the liner is more uniform, and the liquid film for med as a result of thermal and adiabatic condensation should flow downward without obstruction. The motion of a vertical liquid film exposed to a gas flow is a function of many variables. The most important of these are the surface tension of the liquid b eing used, the gas velocity near the film, the roughness of the surface, and the liquid

film thickness (which is itself a function of the condensation rate, the film

drainage rate, and the height over which the condensation takes place). To understand liq uid

- film flow on a liner surface, it is best to begin by looking at the ideal case of a smooth wetting surface. A wetting surface is defined as a surface upon which a liquid will form a film, as opposed to beading up after being spread out (Figure 2

- 14). A wetting surface is referred to as being hydrophilic, whereas a nonwetting surface is referred to as being hydrophobic. To be effective, a stack

- liner material must be hydrophilic. Some materials, such as FRP, can transform from one surface characteristic to the other after exposure to the corrosive conditions within the liner.

Figure 2- 14 Wetting vs. Nonwetting Surfaces 

## Wetting Surface Non

- wetting Surface 2- 27  If liquid if deposited on a vertical, smooth wetting surface at a uniform rate without being exposed to a vertical gas flow, the resulting liquid film will flow easily downward under the influence of gravity. If a vertical gas flow is applied to this wetting surface, a new force

— gas - shear

— is added to the downward

- flowing liquid film. This force produces an upward drag on the liquid film, slowing its downward movement. As the gas velocity is increased, the gas

- shear force increases, and more drag is applied to the liquid film

— further slowing it down. Because the liquid is being deposited at a constant rate, the reduction in the downward flow of the film means that the drainage rate is reduced and the inventory of liquid held on the plate is increased. This process results in an increase in the thickness of the liquid film. The thickness is not uniform; it occurs in waves and ripples on the film surface. At gas velocities below 50 ft/s (15.2 m/s), the liquid

- film motion will be downward, and the surface of the film will be smooth. Between 50 and 60 ft/s (15.2

– 18.3 m/s) the liquid

- film motion will still be downward, but waves and ripples will form on the surface of the film. If the gas velocity is increased further

— to 70 ft/s (21.4 m/s)

— the downward flow of the liquid film will be reduced, causing further increases in the film and surface

- wave thickness. The influence of the surface

- tension forces will begin to diminish relative to the gas

- drag forces to the point at which small waves or wavelets will begin to form on the surface of the film. At first these wavelets will be sporadic, and it will look like the surface of the film is standing still or even moving vertically. At 70

– 85 ft/s (21.4

– 25.9 m/s), the magnitude of the gas

- shear force equals that of the gravitational force, and the liquid film stops moving and is stagnant on the wall. At th is point, the thickness of the liquid film continues to increase, until the surface of the film becomes unstable and starts to break down. Waves are formed of sufficient size to allow the gas flow to get under them, forming “white caps” not unlike waves on the ocean. Large quantities of liquid are shed back into the gas flow. If left unchanged, a steady

- state condition will be reached, in which all of the liquid deposited on the plate must leave the plate as re

- entrained droplets. Operating under these cond itions, all of the liquid condensing within a stack liner would leave the liner as SLD. If the gas velocity is increased to 85

– 90 ft/s (25.9

– 27.4 m/s), the gas

- shear forces will begin to dominate the gravitational forces, and the liquid film will begin to move again

— but now vertically up the surface. Significant quantities of droplets will still be re

- entrained off of the surface of the liquid film. Liquid will begin to move toward the top edge of the plate for drainage. If uncollected, this liquid will fl ow over the top of the plate or be re

- entrained back into the gas flow. Additional increases in gas velocity will increase the vertical speed of the liquid film. 2- 28  At velocities above 90 ft/s (24.7 m/s), re

- entrainment off of the liquid

- film surface is si gnificantly reduced (although not eliminated), and the vertical motion of the liquid film is well

- defined. This condition can often exist at the outlet of a wet stack incorporating an outlet choke. The observations described above are presented visually in Figure 2

- 15. A number of factors can impact these ideal observations, particularly with regard to the liquid

- film thickness. The surface of a thick liquid film will become unstable at lower gas velocities than that of a thin liquid film, so film

- thickne ss management must be considered during the design of a wet stack system. Specifically tall stacks

— typically greater than 500 ft (152.5 m)

— without intermediate liquid collectors can experience thick liquid films. Stacks in colder climates can experience th ick liquid films because of high condensation rates within the liner. In these situations, the addition of insulation (or more insulation) is the easiest manner in which to control the resulting liquid

- film thickness.

Figure 2- 15 Liquid

- Film Flow Patterns on a Smooth, Vertical Wetting Surface vs. Vertical Gas Velocity 2.5 Liner - Wall Discontinuities The observations described above apply to the ideal case of a smooth wetting surface. In reality, the surfaces are anything but smooth. On alloy liners, there are horizontal weld beads; on FRP liners, there are joints between adjacent liner cans; and on brick liners, there are horizontal mortar joints every 2

– 4 in (50 –100 mm) up the entire height of the li ner. These disturbances are referred to as liner

wall discontinuities. 0- 50 ft/s (0- 15.2 m/s) 55

- 65 ft/s (16.7

- 19.8 m/s) 70

- 85 ft/s ( 21.3

- 25.9 m/s) >90 ft/s (>27.4 m/s)Smooth Downward Liquid Film FlowNo DropletReentrainmentDownward Liquid Flow with Waves/Rivulets Forming onSurface, Occasional Droplets Reentrained from SurfaceLittle to no Downward Liquid FlowDroplets Reentrained from Entire SurfaceUpward Liquid Film FlowDroplet Reentrainment Reduces withIncreased Gas VelocityGas Flow 2- 29  When the liquid film flows over a horizontal discontinuity, there is a potential for the upward - flowing flue gas to get under the liquid, resulting in the formation of droplets. A portion of these droplets will be re

- entrained back into the gas flow and will exit the liner as SLD. The major areas of concern in an alloy liner are the horizontal weld beads joining the plates together. Vertical weld beads will have little to no impact on droplet re - entrainment. But horizontal weld beads on alloy liners can cause liquid re

entrainment in the stack at gas

- flow velocities as low as 40 ft/s (12 m/s) if not properly made. The height of the horizontal weld beads is critical, and their height should be the minimum possible that does not exceed the standard for good weld quality: 1/8 in (3.1 mm). Grinding all horizontal beads flush to the base material is ideal. However, laboratory studies have demonstrated that, for gas velocities up to 50 ft/s ( 15.2 m/s), a 1/8

- in (3.1

- mm) bead height on a butt joint will result in minimal levels of droplet formation (assuming the bead is smoothly rounded). Slight increases above this velocity —55– 60 ft/s (16.7 –18.3 m/s)

— resulted in liquid holdup along the joint a nd the start of droplet re

entrainment back into the gas flow. At velocities above 65 ft/s (19.8 m/s), droplet re - entrainment becomes more pronounced. Figure 2

- 16 details liquid

- flow patterns over a 1/8

- in (3.1

- mm) weld bead as a function of gas velocity.

Figure 2- 16 Liquid Flow over 1/8

- in (3.1

- mm) Weld Beads The rings of an FRP liner should be joined together with material mostly on the outside of the liner. Only a minimal inward protrusion with smooth tr ansitions is recommended. FRP liner

- can joints are typically lap

- type joints with interior and exterior overlays (Figure 2

- 17). A 6:1 to 10:1 taper should be used from the stiffener ring

— top and bottom

— back to the base

- liner material, with smooth transitio ns.Gas Flow

Gas Velocity < 50 ft/s ( 15.2 m/s) Gas Velocity 55 - 60 ft/s ( 16.8 - 18.3 m/s) Gas Velocity >65 ft/s (19.8 m/s) Wave/Rivulets Forming on Surface Film Thickening Due to Liquid Holdup Droplets Reentrained From Weld Bead Gas gets Under Liquid at Weld Bead and Waves Liquid Flows Over Weld Bead

Liquid Starts to Holdup Over Weld Bead With some Reentrainment Droplets Reentrained From Surface

Weld Bead

Down Flow of Liquid Reduced or Stopped 2- 30  Figure 2- 17 FRP Liner

- Can Joints – Internal

- Taper Requirement Special attention must be paid to brick liners. Every effort must be made to ensure the proper radial alignment of the bricks and to strike the mortar flush with the inside surface of the liner. If the maximum projection or offset between bricks on the interior surface of the liner exceeds 1/8 in (3.1 mm), re

- entrainment may occur from the liner surface. Extruded or depressed mortar joints can be significant sources of liquid re

- entrainment back into the gas flow. The construction should be clean, without mortar smeared on the inside surface of the liner, because this could also create a location for droplet re

- entrainment. For borosilicate -b lock lining systems, such as the PennGuardTM, every effort must be made to ensure the proper radial alignment of the blocks and to strike the adhesive/mastic flush with the inside surface of the liner. The surface of the block should be clean and free from smeared adhesive/mastic material. If the maximum projection or offset between bricks on the interior surface of the liner exceeds 1/8 in (3.1 mm), re

- entrainment may occur from the liner surface. Extruded or depressed adhesive/mastic material can be a sig nificant source of liquid re

- entrainment back into the gas flow. Inside Flue Outside Flue Liner Wall Detail 1

Exterior Reinforcing Band



### Detail 1 Interior Reinforcing Band (Minimum Thickness Possible) Minimum Smooth 1:6 Taper Back To Liner Wall (1:10 Preferred) 2- 31  2.6 Re - entrainment The liquid on the duct and liner surfaces produced by deposition and condensation flows in the form of film or rivulets is governed by gravitational, surface

- tension, an d gas

- shear forces. The gas

- shear force can shear the liquid off of the surface, causing re

- entrainment. This phenomenon is the most frequent source of SLD and fallout of liquid droplets in the vicinity of the stack. Re

- entrainment most frequently occurs b ecause of one or more of the following:

 High gas

- velocity shear near rough surfaces with liquid deposits.

 Instability of the liquid film (waves and ripples) on vertical stack

- liner surfaces. This instability occurs because, as gravity is pulling the liquid film or droplet down, the gas

- drag forces are pulling the liquid film or droplet upward.

 Surface discontinuities and protrusions that disrupt gas and liquid flow locally.

 Vanes and baffles that cause gas

- flow separation, recirculating liquid flow, and re

- entrainment.

 Gas

- flow patterns that drag liquid along the surface to re

- entrainment sites.

 Strong vortex patterns that can pick water up in the core of the vortex from horizontal or vertical surfaces. Re

- entrainment takes place at locations such as

 Duct walls, roofs, floors, and stack

- liner surfaces

 Dampers, damper

- blade guides, and trusses

 Thermal expansion joints

 Trailing edges of turning vanes

 Duct

- junction corners

 Breeching

- duct area changes

 Stack floor

 Stack

- liner surfaces, particularly in the entrance region and at the intersection with the breeching duct

 Internal solids buildup or scaling on surfaces

 Stack

- liner contraction sections

 Stack choke surfaces Liquid re

- entrainment at these locations will be in the form of large droplets (300–6000 μ m); most will be discharged at the top of the stack, if not prevented by a combination of favorable gas

- flow

- path design and an optimized liquid

collector system. Droplets over 100 μ m typically do not completely evaporate outside of the stack before they reach the ground level. 2- 32  2.7 Recommended Liner - Gas Velocities Wet stack designers should select the liner diameter based on liquid

- collection considerations. Selecting the liner diameter so that the gas velocity is below the critical re

- entrainment velocity increases the opportunity for liquid to be collected within the stack rather than being emitted with the gas stream. The critical re

entrainment velocity varies depending on the liner’s surface roughness, material, and quality of construction. Surfaces with high levels of discontinuities and roughness, such as those of a brick liner, will be more likely to re

- entrain liquid, in comparison to smoother surfaces, such as those of an alloy or an FRP liner. Therefore, the liner diameter will depend upon the liner materi al selected. For instance, brick liners will require a larger

- diameter liner than an alloy or FRP liner with the same gas

- flow rate. Although there are many economic drivers for minimization of the stack

- liner diameter, it must be clearly understood that t he primary controlling parameter for effective wet stack operation is that of minimizing the potential for SLD. The stack

- liner velocity recommendations in the original EPRI Wet Stack Design Guide were developed on the basis of laboratory testing of different liner

- material surfaces. These materials were tested in a vertical wind tunnel under ideal conditions, and their surface behaviors were characterized without weld beads or other surface discontinuities. Since this testing was performed, a large amount of valuable field experience has been obtained. On the basis of this experience, the recommended velocities have been slightly reduced to accommodate “practical variabilities” in the quality of the field installation, such as weld

- bead heights on alloy li ners and joints in FRP liners. The currently recommended liner

- gas velocities are presented in Table 2

- 1. These slightly reduced values also provide the plant some margin to account for increases in the flue gas flow rate as a result of changes in fuel sou rce, increases in plant efficiency, and/or future increases in plant output. On the basis of this experience, it is now recommended that alloy and FRP liners should be designed for a maximum velocity of 55 ft/s (16.8 m/s), whereas brick liners (which inherently have a less smooth surface) should be designed for 45 ft/s (13.7 m/s). The maximum recommended liner velocity for borosilicate block is 60 ft/s (18.3 m/s). This recommendation takes into account the significant increase in the effective surface area afforded by the closed

- cell surface structure of the material and the resulting increased surface

- tension forces holding the liquid to the material. 2- 33  Table 2-1 Recommended Stack

- Liner Velocities for Wet Opera tion

*Laboratory testing of coating material is recommended to finalize liner velocity for favorable wet operation. A number of different liner

- coating materials have been suggested for wet stack operation, ranging from gunite to spray
- on polymers. Most of these coatings have not been evaluated for determination of their recommended liner velocity for favorable wet operation. The favorable velocity for coatings is highly dependent upon the smoothness and wetting nature of the m aterial after it has been applied within the liner. Laboratory testing of the coating system should be performed before a liner velocity is finalized for a specific material and application. A velocity of 55 ft/s (16.8 m/s) is recommended for materials tha t result in a smooth, discontinuity

- free surface within the liner. 2.8 Stack Liquid Discharge Stack liquid discharge (SLD) can occur at any time, even from a well

- operating stack. Disturbances in the operating conditions of the plant and/or absorber as wel l as ambient weather conditions can initiate the start of an SLD event. However, SLD is only a problem if the droplets are large enough (>100 μ m) to be detectable at ground level near the stack. The amount of liquid discharged at the top of the stack is a result of gas - and liquid

- flow processes that take place in the ductwork and stack system between the discharge of the absorbers and the top of the stack liner. The source of the liquid discharge can be categorized by the liquid

- flow process and by the dro plet sizes as follows: 2- 34   Liquid droplets carried from the absorber to the top of the stack liner by the gas stream without depositing along the gas

- flow path. Only small droplets (10 –100 μ m) travel through the ducts without complete deposition. This means th at most conventional scrubber designs, with horizontal ducts connecting the absorber to the stack, will result in deposition of most large droplets on the duct surfaces.

 Droplets re

- entrained from liquid deposition and condensation on duct and liner surfac es that can be discharged. These are large drops, and they are usually the major contributors to the SLD.

 Droplets formed by bulk condensation. These represent a large liquid

- flow rate but are very fine droplets (<10 μ m), which have a negligible effect on the SLD detectable at ground level.

 Liquid deposited and condensed on the liner, which flows upward in those areas of the stack where the gas velocity is higher than the flow

- reversal velocity for the liner material. The droplet sizes re

- entrained at the t op of the liner, where velocities are highest, can be quite large (300 –6000 μ m), depending on the liner

- top geometry. The upper section of a stack choke is usually in this mode of upward liquid flow. The quantity and location of the fallout at a given plant is a function of the droplet

- size distribution of the discharged liquid and the local atmospheric conditions

— such as ambient temperature, wind, relative humidity, and turbulence level. The very small droplets of the bulk condensation make the gas plume w hite and visible. The plume is cooled as it mixes with the surrounding air. This cooling results in condensation on the small droplets in the plume. These droplets are typically 1 –10 μ m in diameter and therefore do not fall to the ground and cannot be dete cted. The mixing with the drier air away from the stack causes droplet evaporation. The balance between the cooling and mixing processes defines the length of the visible plume. Therefore, it is a function of the ambient air temperature and the air relativ e humidity. One of the major objectives in developing an effective wet stack design is to limit SLD to a minimum acceptable level. The goal is to limit the droplet size that will exit the stack. If the droplets are small enough, they will evaporate before hitting the ground. However, if the droplets are large, they will land on nearby surfaces such as plant structures, equipment, and cars. The wet stack survey indicated that the fallout of liquid droplets, if noticeable, usually occurs within a half

- mile ra dius of the stack. 

### The most common cause of an unexpected SLD is a problem with the WFGD mist

- eliminator system, such as blade pluggage, fouling, and washing issues. Liquid

- collection system pluggage could also be occurring. It is for this reason that a me thod for monitoring the liquid

- collection drainage flows is often recommended. It is also common for newly commissioned liners to experience some level of SLD for the first few weeks of operation, until the inside surfaces of the liner have been seasoned and become fully wetted. 2- 35  If SLD is experienced, note the time of day, location of the fallout relative to the stack liner, and local weather conditions. If the location of the fallout is known, rolling aluminum foil out on the ground in 2 –6- ft (0.6

– 1.8

- m) squares for a period of time sufficient to collect numerous drops will be extremely helpful. Both the droplet size and the quantities of droplets can be obtained from the foil. This information, along with information on plant operations at the time of th e event, will be critical in evaluating the source and possible cause of the rainout event. If SLD is a continuous problem, some stack sampling may be required. Knowing the sizes of the droplets entrained within the gas flow inside of the liner can be very helpful in identifying the source (or sources) of the discharge. Additionally, an inspection of the liquid

- collection system should be performed as early as possible. The inspection should be performed and photo

- documented before anything inside of the we t ducts and stack are disturbed in any way. Often, clues to the source of the discharge problem can be as simple as a deposit on a wall or marks on the liner surface. 2.9 Plume Downwash and Icing 2.9.1 Plume Downwash on Stack A cross wind at the top of the stack will deflect the plume from its vertical path. As the cross wind increases, the plume may become partially entrained into the low

- pressure region behind the stack windscreen. This phenomenon is known as plume downwash. Designing a liner for favorab le wet operation will decrease the potential for SLD. However, such a design increases the potential for plume downwash at elevated wind velocities (Figure 2

- 18). During downwash episodes, saturated flue gas comes in contact with the liner extension, stack hood, and stack shell. This contact can lead to deterioration of the stack construction materials because of exposure to acid in the flue gas. It also increases the potential for ice formation on the top of the stack. The potential deterioration problem i s a long

- term situation caused by continual exposure to SO 2, which has been evidenced in recent years by the deteriorating conditions of tombstones, stone buildings, and the top of stack shells. The icing problem can occur at below

- freezing conditions all winter long, every winter, and creates a potential danger to people and property. The most important fluid

- dynamic parameter is the momentum ratio between the stack exit flow and the cross wind. This ratio defines the relative influence of each flow on th e other and is determined by the densities and velocities of the flue gas and the wind. This momentum ratio (MR) is defined as:

As the ratio of vertical plume momentum to horizontal wind momentum falls below a value of about 2.0 for a free

- standing single

- flue stack, the plume may become partially entrained in the vortex patterns that are formed on the 2- 36  downwind side of the stack. T o reduce the frequency of downwash, the momentum ratio must be increased

— either by reducing the liner diameter or by installing a choke at the top of the liner.

Figure 2- 18 Plume Downwash The second most im portant ratio is the shell outer diameter to the liner inside diameter at the top of the shell. A larger shell - to liner

- diameter ratio results in increased downwash potential at the same momentum ratio. Plume downwash is most likely to occur during reduced unit load operation, in which stack

- discharge velocities are reduced, and during high wind conditions. Plume downwash is least likely to occur under the opposite conditions of high unit load operation a nd low wind velocity. For a multiple

- flue stack, the equivalent momentum ratios for initiation of plume downwash are higher, and they vary with the relative wind direction. This difference in momentum ratios occurs because the shell - to liner

- diameter rat io is larger for multiple

- flue stacks. The reduced static pressure in the wind vortices generated off the stack shell and liner extensions can draw the flue gas into a downwash pattern along the stack shell. In the process, the saturated flue gases that are drawn into the vortices come into contact with the roof and sides of the stack liner and shell. The interaction of a plume with the cross wind is a function of the following parameters:

 Liner

- extension diameter

 Shell diameter

 Liner

- extension length

 Roof shape

 Number of liners 2- 37   Wind direction, velocity, and air density at the top of the stack

 The annual frequency of wind occurrence as a function of wind velocity

 Liner exit

- gas velocity and gas density

 Plume buoyancy due to flue gas temperature

 Ambient air temperature The momentum ratio (MR) is the most important fluid

- dynamic force ratio that controls plume downwash. The individual parameters within this ratio have the following importance for wet stack plume downwash: 1. Flue gas density ( ρ flue gas) is almost constant for saturated flue gas at about 0.066 lb/ft3 (1.06 kg/m3) at sea

- level ambient pressure. 2. Wind density ( ρ wind) varies with ambient temperature at sea

- level ambient pressure from 0.090 lb/ft3 (1.44 kg/m3) at

- 20°F (

- 29°C) to 0.071 lb/ft3 (1.14 kg/m3) at 100°F (38°C) (an approximately 25% spread). 3. Reduced ambient pressure for plants at high elevations above sea level reduces density by 10

– 20%, but it changes both density values by the same amount. 4. The flu e gas discharge velocity (V flue gas ) is at th e highest stack elevation and is set by the liner or choke discharge diameter and the low

- load to high

- load flue gas volume flow rate. This value can range from 20 to 120 ft/s (6 –37 m/s) for a wet stack plume discharge, including partial

- load operation. Th e wind

- velocity magnitude, direction, and frequency of occurrence varies significantly at different geographical locations, but this information is usually available at weather stations and airports located near existing or new power

plant sites. The infor mation needed is usually referred to as the "wind

- rose" data in graphical or tabular form, and it includes wind velocity and frequency as a function of direction. The quantity of cumulative data is usually supplied on a yearly basis, but the data is also available on a monthly (or more frequent) basis if time of year is important. When reviewing the wind data, a judgment must be made when selecting a worst
- case wind magnitude and cumulative frequency of occurrence (to eliminate unusually high windstorms of short duration that seldom occur). Items 1, 2, 3, and 5 above can usually be set for a specific plant site. Item 4, the flue gas discharge velocity, must then be selected to satisfy the requirements for

 Liquid re

- entrainment from the inside of the liner

 Li quid collection and drainage

 Plume downwash For plume

- downwash considerations, design decisions must be made regarding the extent of allowable downwash and the cumulative amount of time that downwash occurs over the period of a typical year. Factors to be considered are: 2- 38   Unit load schedule versus time

 Wind

- rose data

 Geometry of the top of the stack (liner inner diameter, liner

- extension height, shell outer diameter, roof or cap geometry)

 Extent of downwash allowed:

- None (may not be achievable)

- On liner extension

- On roof or hood*

- On stack shell

 Cumulative time of downwash (per year):

- Zero (may not be achievable)

- Less than 2 days (0.5%)

- 2– 4 days (0.5 –1%)*

- 4– 8 days (1 –2%) *The extent of downwash and the cumulative time of downwash marked are suggested value s at which to start the evaluation of downwash and its effect on stack

- discharge velocity. If the resulting stack

- discharge velocity is unreasonably high, the downwash criteria may have to be relaxed to allow more downwash and longer cumulative time duration per year. A computational fluid dynamic (CFD) flow

- model study of the stack top for a specific plant and geometry is required for an accurate evaluation of the extent of plume downwash and the cumulative time of downwash. Presented below are some approx imate guidelines that can be used for the preliminary evaluation of the expected degree of plume downwash and for the selection of starting geometries of the stack top for CFD model selection. The MR ranges presented below represent a variation from interm ittent downwash, which is nearly continuous for the low MR and intermittent for the larger MR. Below the lower MR for each range, the downwash is continuous. Above the upper MR for each range, the downwash is almost unnoticeable. The following two MR range s are discussed below:

 MR1 – MR range for intermittent downwash to roof or hood

 MR2 – MR range for intermittent downwash to the shell For all stack

- top geometries discussed below, the stack

- shell diameter should be as small as practicable to contain the nu mber of liners. 2- 39  Single

- Flue Stacks 1. For an angled hood and no liner extension:

- MR1 = 3 –5

- MR2 = 2 –3 2. For a flat or angled roof and a short liner extension of 0.5 liner diameter:

- MR1 and MR2 expected to be lower (10 –20%) Two

- Flue Stacks 1. For a flat roof plus a liner extension of about one liner diameter: Best wind direction (prevailing

- wind direction in line with the axis of the two flues)

- MR1 = 1.5

– 2.0

- MR2 = 1.1

– 1.5 Worst wind direction (prevailing

- wind direction perpendicular to the axis of the two flues)

- MR 1 = 3.5 –5

- MR2 = 3

– 4.5 2. For a minimum liner extension of 0.75 liner diameters, the above MR values will remain the same for the best wind direction, but the above MR values for the worst wind direction will rise by 10 –20%. 3. For a taller liner extension of 1.5 extension diameters, the above MR values for the best wind direction will remain the same, but the above MR values for the worst wind direction will drop by 10 –20%. 4. A chamfered stack top (2 sides) will slightly increase (by10

– 20%) the best wind direction MR values, but will reduce (by 30

– 50%) the worst wind direction MR values. Three - and Four

- Flue Stacks

 The best wind direction MR values are expected to be similar to those for single

- flue stacks.

 The worst wind direction MR values are expected to be similar to those for two
- flue stacks. In most locations, there is a prevailing

- wind direction at which the highest wind velocities occur for longer periods of time. For single

- flue stacks, this prevailing

wind direction has no effect on the extent or duration of plume downwash. However, for multiple

- flue stacks, it will be advantageous to orient the multiple

flue arrangement so that the prevailing wind is aligned with the best wind direction, which gives the lowest MR values. This type of alignment will result i n 2- 40  the lowest cumulative time for plume downwash for a specific combination of flue gas

– discharge velocity and stack

- top geometry. These plume

- downwash guidelines can be used to obtain preliminary estimates of the stack

- top design to limit the extent and du ration of plume downwash: 1. Select plume downwash criteria at a specific unit load. 2. Select a basic stack

- top design except for flue gas

– discharge velocity. 3. Obtain the wind

- rose data and select reasonable design wind velocities. 4. Select the appropriate MR

- guideline value that is compatible with steps 1 and 2. 5. Calculate the corresponding flue gas

– discharge velocity, flue gas

– discharge area, and the preliminary stack

- top geometry. 6. Compare the results in Step 5 to values compatible with good plume dispersio n, stack liquid collection and drainage, and construction cost. 7. Repeat steps 1 through 6 until satisfactory results are obtained for all important criteria by making appropriate compromises. The final stack

- top geometry should be optimized by CFD flow

- mode l studies, particularly for multiple

- flue stacks for which downwash

- minimization is important. 2.9.2 Stack Top Icing Whether ice forms on the top of the stack depends on the temperature of the stack surface, the temperature of the mixture of saturated flu e gas and cold ambient wind, and on whether water vapor will condense out of the mixture. Ice formation is most likely at plants where below

- freezing temperatures are common and where they last for extended periods of time. The locations for potential ice formation are discussed below:

The hood for a single

- flue stack The hood is the first element exposed to plume downwash. A hood for a single

- flue stack is usually connected to the stack, liner, and liner extension, which helps to keep it warm. When ice forms on an outward

- sloping hood, it usually forms off the upwind edge and/or in the downwind half at two locations about 45°off the axis of the wind direction. The ice deposits formed are similar to those on the roof valley of a house and can be quite large. The liner extension for single or multiple liners The liner extension is the second element exposed to plume downwash. If the liner extension is uninsulated, the surface should always be maintained above freezing temperature. If so, no ice could form, but some condensation might occur that would then run down to the stack roof. An insulated extension might have some ice formation. 2- 41  The roof of a multiple

- liner stack The stack roof usually does not directly touch the liners, which permits expansion and movement of the liners. The roof can receive heat from the stack annulus. Liquid must be drained from the roof area. Preferably, drains should be located on the inside of the annulus to keep them from freezing. Railings The metal railings at the top of th e shell will quickly cool to the temperature of the mixture of the flue gas and the ambient air, and they will be the first surfaces to become iced. In other words, these railings are the most likely places for ice to form. Platform near top of stack A metal platform on the outside of the stack near the top can also be exposed to low concentrations of flue gas. Because it is metal and is exposed to the ambient - and flue gas

– mixture temperature, it may also experience occasional ice buildup. Such platform s are the second most likely places for ice to form. Shell If ice forms on the shell, it can be expected to be on the downwind side of the stack. This condition occurs when visible downwash can be seen on the downwind side of the stack shell. Icing can result in significant ice buildups along the top of the stack. These ice buildups can dislodge and fall, potentially causing serious damage to people and equipment on the ground. The potential for icing can be reduced by employing the following steps: 1. Select a stack

- liner discharge velocity that minimizes plume downwash over the expected operating range of the unit at the existing local wind conditions and that is consistent with other design objectives. 2. During cold ambient temperature conditions with h igh winds, run the unit at near full load. Employing this operating procedure may be natural under such conditions, because more power is consumed in below

- freezing weather. 3. Use heated annulus air or electrical heating elements to heat the stack hood, roof , or other areas where ice forms on the top of the stack. 4. Do not use insulation on sloped rain hoods constructed of a corrosion

resistant material. 5. Use a parapet wall instead of metal railings around the top of the liner shell. A parapet wall is usually an extension of the concrete shell above the roof. There is normally less icing on a concrete parapet than on a metal railing, and the parapet wall will contain ice that could fall from the liner or liner extension. 3-1  

# Section 3: Practical Design Recommendations
3.1 Introduction Having presented the basic understanding of the fundamentals of gas and liquid flows within wet duct/stack systems in Section 2, we can now turn our attention to the use of these fundamentals for practical applications. Essentially, all new and retrofit WFGD units are typically designed for wet stack operation. During the initial design phases of the project, a feasibility study is normally conducted to determine such factors as layout, materials of construction, economics, and construction time. The Revised Wet Stack Design Guide addresses what needs to be considered during the feasibility

- study phase of a new or retrofit wet stack project. For example: Will the existing stack

- liner exit velocity be acceptable for wet stack operation? Can the existing stack be modified, or will a new stack be required? Wet stacks have been designed and operated effectively at new WFGD installations for many years, thanks largely to the guidance provided to the industry by the original EPRI W et Stacks Design Guide . Although most of the design criteria presented in this original guide is still valid, experience with its practical implementations has shown that some revisions and updates are warranted. In each situation, the rules affecting a system’s favorability for wet operation are the same. However, there are application

- specific issues that need to be addressed. These issues are discussed in detail in the following sections of this guide. The recommendations presented in this revised desi gn guide will both help the designer make decisions that will affect wet operation and provide the necessary background information to understand why the recommendation is being made. Typically, the wet duct/stack liquid

- collection and drainage

- system arr angement is prepared by a flow

- modeling laboratory skilled in the art of wet stack design. It is imperative that these modelers be brought into the design process early enough so that their input on the overall favorability for wet operation can be impleme nted before the system arrangement has been finalized. 3-2  3.2 New Wet Stack Designs WFGD installations incorporating a new wet duct/stack system have the advantage of being designed for favorable wet operation from the start. Using the recommendations presen ted in the following sections of this revised design guide should result in a wet stack system that operates well, with a minimal potential for SLD. New wet stacks can be designed using materials that are acceptable for the wet corrosive environment; can incorporate design characteristics that minimize the potential for droplet re

- entrainment back into the gas flow; and can be designed to operate within the range of gas velocities recommended for the stack

- liner material used. Although the processes control ling the design of an effective wet duct/stack system are well understood, much of the re

- entrainment potential for a particular unit is site

- specific, and a physical

- flow

- model study for each installation is highly recommended. Such a study will evaluate liquid re

- entrainment and liquid collection in the ductwork and liner, based on the design of the actual unit and anticipated gas velocities. 3.3 Retrofit Wet Stack Designs Stacks that operate under reheat, partial bypass, or full bypass generally have be en designed for a higher gas velocity in the liner than new wet stacks. When operating the stack wet, by eliminating reheat, the gas velocity in the liner decreases and the density increases. Wet operation results in system

- pressure - loss decreases of 5 –10% , compared to the system

- pressure loss with a reheat temperature rise of 30

– 60°F (17 –33°C). Operating an existing liner that is converted to wet stack operation can often result in a gas velocity in excess of the recommended liner velocities previously pre sented in Table 2

- 1 for wet liners; this higher gas velocity makes liquid collection within the liner more difficult. Dry stacks were typically designed for higher gas velocities. Existing liners that are converted to wet operation and have velocities tha t exceed these recommendations may require that the majority of the liquid be collected in the absorber top and the absorber

- outlet ductwork. The use of additional and more

- extensive liquid collectors in the absorber top and the absorber

- outlet ductwork will need to be evaluated. Droplets entrained within the gas flow will impinge upon the liner, forming a liquid film. Because of the unfavorably high gas velocity in the liner, a large fraction of the liquid may become re

- entrained back into the gas flow. If the wet

- gas velocity is higher than the flow

- reversal velocity for the liner, all deposited and condensed liquid will flow upward to the top of the liner and be re

- entrained there. If gas velocities in the ductwork are also high, liquid collection in the ductwork may also be difficult; therefore, the gas velocities in the absorber

- outlet ductwork should be evaluated as well. Ductwork for reheated or bypass gas is typically sized for 60 ft/s (18 m/s) or higher. Ductwork for wet stacks should be sized for 50 ft/s (15 m/s) or lower to minimize the potential for liquid re

- entrainment back into 3-3  the gas flow. Critical re

- entrainment velocities in the ductwork are difficult to determine and are not available. If the liner velocity is lower than the range of recom mended maximum values, the liquid re

- entrainment rate is lower and the need for additional liquid collectors is reduced. A single

- stack liner that serves multiple FGD modules may produce large volumes of liquid for drainage. Drainage

- pipe diameters need to be sized accordingly. If the liquid collected from multiple FGD modules is returned to the FGD process, the volume of liquid may be too large to be returned to a single FGD module without significant changes to its operational characteristics. A flow

- mode l study should be performed to evaluate liquid re

- entrainment and liquid collection in the ductwork and liner, based on the anticipated gas velocities. If adequate liquid collection cannot be accomplished at the anticipated velocities, a new liner and/or d uctwork sized for a lower gas velocity should be considered. 3.4 Wet Duct/Stack Operation Initially, water enters the duct and the stack by droplet and water carryover, with the saturated flue gas passing through the mist eliminators of the scrubbers. The liquid accumulates on surfaces by deposition of liquid droplets and by vapor condensation from the saturated flue gas. Mist

- eliminator carryover is usually characterized by fine droplets that can pass through the mist eliminator without being collected and by larger droplets re

entrained from deposits and during washing. The finer droplets will follow the gas flow and will generally not be collected within the system before exiting the stack. However, because these droplets are very small, they will evaporate on the outside of the liner before reaching the ground. The larger droplets (both in size and quantity) re

- entrained from partially plugged mist eliminators and during mist

- eliminator washing, will generally be deposited on the surface within the ductwo rk and the lower liner. Most of the liquid suspended in the gas will be deposited on the walls and internal structures (turning vanes, dampers, trusses) by impingement, caused by the departure of the trajectory of a liquid droplet from the streamline flow in a curved gas

- flow field. Some deposition also occurs because of turbulent deposition, caused by the turbulence

- induced motion of small droplets in a direction perpendicular to the main gas

- flow direction. The liquid on the walls and internal structures will be in the form of films, rivulets, attached droplets, and (in some cases) large pools on horizontal surfaces. The gas

- shear force can drag the liquid through the system (even upwards against gravity and wall friction) if the gas velocities are high e nough; it can also shear off and re

- entrain droplets from the liquid flowing on the walls and internals. Some small droplets may negotiate the gas

- flow path from the scrubber outlet to the top of the stack without contacting a solid surface. 3-4  During normal operation, the largest contributor to the liquid flow on a liner surface is condensation, rather than deposition of the droplet carryover from the mist eliminators. The condensation on the liner cannot be measured in a laboratory

- scale model of the stack. Therefore, analytical calculations are required to define the amount of condensed liquid

- flow rate in the stack. 

### There are two types of condensation mechanisms: 1) condensation on the duct walls and liner surface due to heat transfer in the boundary layer and 2) condensation in the bulk of the gas due to pressure changes and adiabatic expansion from elevation changes. Both of these mechanisms can produce significant liquid by condensation. A basic description of these mechanisms is given in Section 2.3, "Sources of Liquid in Wet Stacks." Liquid condensation occurs continuously on all wet duct and stack

- liner surfaces because the flow gas is saturated and the liner's inside surface temperatures are lower than the gas dew

- point temperature. But the rate of c ondensation per unit surface area can vary significantly, depending on geometry, materials, temperatures, and wind speed. The duct and stack condensation rate cannot be measured in experimental flow models. A 2

- D condensation analysis is satisfactory for c alculating the condensation rate that accounts for stack geometry, mass flow, heat loss, gas psychometric conditions, and the ambient atmospheric

- pressure variation along the height of the stack. Assuming axial symmetry for the stack, all the variables can be described or calculated as a function of vertical height with computer programs developed for this purpose as design tools. The major conclusions drawn from the results of these calculations are as follows:

 The bulk condensation in the liner is a function of stack height.

 For brick liners, the pressurizing

- air leakage through the liner increases the bulk condensation by additional cooling. Thermal condensation also increases because of the colder annulus air used for pressurization.

 The wall condensatio n is directly proportional to the temperature difference between the gas and the ambient air.

 With the exception of borosilicate block (which has an inherently high insulating value), the choice of liner material only marginally affects the wall

condensati on rate, because the thermal resistance of the liner is a small fraction of the total thermal resistance between the flue gas and the ambient air.

 The wall condensation may be reduced by a factor of about four by insulating the outside of alloy or FRP lin ers with a 2

- in (0.05

- m) mineral wool layer. Increasing the insulation thickness from 2 in (0.05 m) to 4 in (0.1 m) will result in a small reduction in condensation rate. Therefore, more than 2 in (0.05 m) of insulation is seldom required. Insulation does not eliminate the liner condensation, but it can reduce it significantly. 3-5   A wind

- speed increase of 20

– 40 mph (9

– 18 m/s) increases the condensation rate by only a few percentage points, because the convective

- heat transfer levels off above 20 mph.

 Having mo re than two liners in a shell increases the total wall

- condensation rate by about 5%. This increase is mainly attributable to a higher condensation rate on the liner extension above the shell. The condensation in the ductwork is primarily thermal wall cond ensation; adiabatic condensation (due to pressure reduction with elevation change) is negligible. Therefore, the rate of condensation here is not as important as in the stack, because most of the condensed liquid and the deposited droplets can be collected and drained out before they enter the stack. The results of the condensation calculation are used to design the liquid collectors and to plan for the liquid

- disposal system. The estimated ranges of the liquid

- flow rates in the wet duct and stack of a typ ical 550

- MW unit are listed in Table 3

- 1. These numerical values help the stack designer estimate the amount of liquid flow to be controlled and accounted for in a wet duct/stack system. Table 3-1 Estimated Ranges of Liquid Flows in the Wet Duct/Stack of a Typical 550

- MW Unit Source of Liquid at the Stack Estimated Flows

Mist

- eliminator carryover:

• The vapor mass

- flow rate in the saturated gas stream is rather large. The total vapor content at saturation temperatures

— 115

– 130°F (46

– 54°C)

— is equivalent to a liquid

- flow rate of 1100

– 1800 gpm (250

– 410 m 3 /h)

• The fine liquid mist (diameters <10 μ m ) with high

- efficiency mist eliminators 0.3

– 1.5 gpm (0.1

– 0.3 m 3 /h)

• Duct

- wall condensation 1– 5 gpm (0.2

– 1.1 m 3 /h)

• Stack

- wall condensation 1– 6 gpm (0.2

– 1.4 m 3 /h)

• Bulk condensation in the stack 6– 8.4 gpm (1.4

– 1.9 m 3 /h)

• Deposition of liquid droplets 1– 12 gpm (0.2

– 2.7 m 3 /h)

• Droplets re

- entrained from the walls (diameters 20

– 2,000 μ m) 0– 10 gpm (0

– 2.3 m 3 /h)

• Liquid from stack drains 2– 12 gpm (0.4

– 2.7 m 3 /h)

• SLD (a function of the duct/stack design and the effectiveness of the liquid

- collectors installed) 0– 15 gpm (0

– 3.4 m 3 /h) 

### All the droplet

- producing sources are controlled primarily by the local gas velocity. The two basic mechanisms for producing liquid droplets are the shear force caused by the local gas velocity and the dynamic pressure of the gas flow at local flow disturbances on the solid surfaces. The particle

- size range for re

- entrained droplets is primarily 300

– 6,000 μm. 3-6  3.5 Absorber Design and Placement There are many types of WFGD absorber designs in use around the world. These inclu de, but are not limited to, open

- spray towers (with and without liquid hold

- up trays), dual

- contact absorbers, jet

- bubbling reactors, venturi scrubbers, and horizontal gas

- flow weir or duct

- type absorbers. All absorbers incorporate mist eliminators at their outlet to ensure that the gas flow exiting into the ducts toward the stack or other downstream equipment has a minimum quantity of suspended droplets. As a matter of economics, most new WFGD installations incorporating a new stack will locate the absorb er and stack close together to minimize the length of connecting ductwork. Some length of ducting is required for effective liquid collection. For example, placing the liner directly on top of the WFGD absorber module is not recommended, because droplets generated off the mist eliminator during upset conditions will not be collected before exiting from the top of the stack. However, in some situations

— most often in those in which a new WFGD absorber is going to use an existing stack

— there may not be sufficient real estate in the vicinity of the existing stack in which to place the new absorber. In such situations, the absorber will be located at a significant distance from the stack, and a long duct run will be required to connect them. After the gasses pass through an absorber’s mist

- eliminator section, they are directed to the absorber outlet, which connects to the ductwork leading to the stack. In the outlet hood, the flue gas is accelerated from the mist

- eliminator outlet velocity to the duct velocity

— ty pically 50 ft/s (15.2 m/s) for wet operation. Absorber outlets can be either horizontal or vertical, and either rectangular or round in shape, depending on the design of the outlet ducting. Absorbers with horizontal outlets tend to be rectangular, whereas absorbers with vertical outlets tend to be round. 3.6 Wet Ductwork Design The proper design of power

- plant ducting is detailed in [4]. The design of a duct that is to operate in a wet environment is basically the same as that for a dry duct. However, some considerations specific to wet operation must be addressed. Properly desi gned ducting with effective mist

- elimination and liquid

- collection systems can result in a system that is very favorable for wet operation. 

### The seven primary droplet

- producing sources in a wet duct are: 1. Mist

- eliminators: The primary source of liquid in st acks is the mist

eliminator carryover from normal operation, partially plugged operation, and washing cycles. If a WFGD retrofit to an existing unit is made with nonideal duct/stack conditions and a wet stack is used, one needs to specify the best possible mist eliminator. The quoted efficiency and carryover from mist eliminators is typically much lower (0.02

– 0.04 gr/acf [46

– 92 mg/m 3 ]) than the limited field data, which may go as high as 0.17

– 2.00 gr/acf (390 –4600 g/m 3 ). The wash cycle of the mist eliminato r can produce short periods of increased liquid

- droplet discharge. EPRI has conducted field tests on mist

- 3-7  eliminator carryover drop sizes and loading levels. EPRI has also conducted tests on many mist

- eliminator modules. 2. Surface discontinuities in duct walls: The liquid films of rivulets flowing in a horizontal, inclined, or vertical passage will be partially entrained at surface discontinuities, such as a step

- up, a step
- down, and sharp bends. The amount of liquid entrained is a strong function of the loca l geometry and the gas - flow disturbance caused by the discontinuity. Liquid

- droplet production at wall discontinuities has been observed in both laboratory models and field wet stack systems, but it has not been measured quantitatively. 3. Thermal

- expansion joints: These duct components represent a special discontinuity because they usually have a capacity to store liquid. Thermal

expansion joints represent a potential liquid re

- entrainment area in the ducts and in the stack, although the expansion joint is o ften covered. Expansion joints can act as liquid collectors on the walls, and they should have drains installed to remove collected liquid to reduce the possibility of re

entrainment. Expansion

- joint drains are only practical at duct pressures near ambient pressure downstream of ID fans. At locations where negative pressure exists in the duct, traps are needed to prevent air leakage into the duct. Other re

- entrainment control methods may also be needed. 4. Internal duct components: Necessary internal component s such as mounting plates, louver

- damper blades, and turning vanes collect liquid by inertial deposition, and re

- entrainment takes place on or downstream of these components. 5. Special flow patterns: Gas

- flow passages of power plants are designed to be funct ional and low

- cost, but they are usually aerodynamically crude. Flow separations, reattachments, and secondary flows are encountered at several locations in most power

- plant duct systems. The liquid droplets and films collect in separated zones, forming a large pool or a heavy stream that entrains more easily than a thin film at lower gas velocity. An increase of gas velocity from a load change, or the addition of an FGD module on

- line, can produce re

- entrainment from unprotected pools. 6. Roughness of surface s: Liquid entrainment by gas streams from rough surfaces is proportional to the gas velocity and the roughness of the surface, starting at some critical threshold velocity that varies with the surface characteristics. Because large surfaces with thousands of square feet are involved, the integrated effect of surface roughness is very important

— particularly in the vertical stack flow. Discontinuities and other sources of surface roughness can be controlled or eliminated by design and careful installation, bu t practical acid

- resistant liner materials have to be accepted and installed as produced. 7. Fan washing: If ID fans are included downstream of wet scrubbers, they may have an intermittent on

- line wash system for cleaning deposits off of fan impellers. When i n use, this system will produce entrained droplets. 3-8  

### 3.6.1 Guidelines for Selection of Geometry for Wet Ducts Because expensive corrosion

- resistant materials must be used to construct or line wet ducts downstream of wet scrubbers, duct sizes will tend to b e smaller than in dry ducts, resulting in higher velocities. Therefore, special care should be given to reducing duct

- component pressure
- loss coefficients by the use of lower

- loss duct

component types and more aerodynamically designed shapes. The following three objectives need to be achieved by the design and the operation of a wet duct and stack system without reheat:

 Minimum re

- entrainment of the deposited liquid by the gas shear and turbulence

 Maximum deposition and separation of the liquid on the duct walls, baffles, turning vanes, and stack liners

 Good collection and protected drainage of the liquid from the duct and stack system In addition, special care is needed if an ID fan located in the wet portion of the system is used. See Section 3.8.2 for gui delines on these components. 

### 3.6.1.1 Duct and Stack Design Velocity Levels The selection of wet duct design

- area average - velocity levels are based on the following considerations:

 The effect of velocity and velocity head on pressure loss and the costs asso ciated with the fan

- energy requirement

 The upper limit of the velocity that starts to entrain liquid droplets from the walls and wall

- related discontinuities

 The movement of liquid films, rivulets, and droplets along the wall because of gravity and gas

- dra g forces

 The incorporation of liquid collectors and drains to prevent entrainment of liquid from surfaces and remove it from the duct system The addition of liquid collectors will add a small amount of additional pressure loss as a result of the contractio n and expansion of the gas flow to get past the liquid collectors. For wet ducts with saturated gas flow downstream of WFGD modules, the duct

design velocity levels are limited by high gas

- velocity droplet re

- entrainment values. Suggested maximum design

- ar ea average - velocity values for different wet duct design situations are presented below. The design

- area average velocities used can be any value less than these. It is necessary that liquid collectors and drains be designed, optimized, and evaluated by an experimental two

- phase laboratory

- model test for each installation. 3-9  1. 65 fps (20 m/s): This maximum velocity level should be used if the following duct features are pres ent:

- Any duct with an upward slant of less than 30°, and in which most of the duct is horizontal or sloped downward. Some sections with vertical, upward, smooth duct slants (60

– 90°) could also be included if the velocity for the stack design is compatible.



### - Duct components are less aerodynamic than those of Item 1 above, but are still reasonably good.

- A few internal trusses, beams, and surface discontinuities are allowed near the stack, but no louver dampers can be near the stack.



### - Ducts have some restrictions on vane extensions to minimize cost, but no restrictions on liquid

- collector design. 2. 50 fps (15 m/s): This maximum velocity level should be used if the following duct features are present:

- Ducts have several or numerous sections that slant upward by 30

– 60°angles from the horizontal.



### - Duct components are aerodynamically crude and include sharp corners.

- Ducts have no restrictions on internal trusses, gussets beams, surface discontinuities, expansion joints, or louver dampers.



### - Ducts have 

### some restrictions on vanes, baffles, and liquid

- collector installation. If higher velocities must be used, then the re

- entrained liquid could still be collected by the installation of more extensive baffles and liquid collectors in the stack bottom. 

### 3.6.1.2 Duc t- Component Selection In the wet sections of a duct system, duct components should be selected to promote droplet impingement, liquid

- film collection, and drainage out of the system. In addition, the duct system should be oriented to encourage liquid films to flow naturally with the gravitational and gas

- drag forces. If these forces are opposed to each other, resulting in liquid flow back against the gas

- drag forces, droplet re

- entrainment may occur. Liquid collectors and drains must be used to intercept th e flowing liquid films and guide the liquid to drain locations where liquid is removed from the duct system. These liquid collectors must be optimized by two

- phase (gas

- liquid) flow experimental

- model tests for each installation to insure satisfactory liqu id

- collection performance 3- 10  The following duct components have desirable features for inclusion in wet duct sections:

 Horizontal large

- angle bends for elbows, junctions, and manifolds with several large vanes or baffles that can act as impingement and coll ection surfaces

 Vertical to horizontal elbows with large vanes that can act as impingement and collection surfaces

 Ducts that are horizontal or sloped downward, in which water will easily flow on surfaces in the direction of the gas flow

 Rounded inner corners on all bends

 The use of a few large vanes in bends that can have vertical or inclined liquid collectors on the outlet edges of the vanes. The vanes may also have extended trailing edges with inclined trailing

- edge collectors for improved collection an d drainage.

 Expansion joints are natural collection regions and must have drains installed to prevent overflow and re

- entrainment. Drains must be designed to prevent flow of air into the flue gas duct. The flexible portion of the expansion joint must not protrude into the duct during plant operation. The following duct components have undesirable features for inclusion in wet duct sections:

 Ducts that slope upwards on the floor or roof, because liquid can flow back against the gas flow and be re

- entrained.

 Horizontal to vertical upward

- vaned and unvaned bends, because the liquid wants to flow downward as a result of gravity against the gas flow. This situation cannot be avoided in the stack bottom. Special liquid collectors are needed in these bends.

 Interna l pipe trusses, gussets perpendicular to the gas flow, louver dampers, and structural beams located near the stack entrance where liquid can impinge and be re

- entrained as larger droplets.

 Steps or discontinuities in the duct walls at flanges, dampers, and expansion joints

— particularly near the stack.

 Sharp corners on elbows, junctions, or manifold entrances where liquid can be re

- entrained.

 Unprotected natural liquid

- pool areas on the floors where gas flow can re

entrain liquid from the pool surfaces. These areas can be protected to prevent re - entrainment.

 Location of dampers close to vaned bends can result in undesirable flow interactions.

 A large number of small vanes in bends, because they would require liquid collectors on each vane. 3- 11  In a field installation, liquid collectors and drains will include some of the following types of devices, fabricated out of corrosion

- resistant materials:

 Gutters on walls and vanes constructed from angles and channels

 Vane extensions and baffles constructed from plat e material

 Protection covers for water films or pools constructed from grids or perforated plates

 Drains made from pipes and properly located 

### To optimize the selection and design of these liquid collectors and drains for a specific installation, experimen tal two

- phase flow

- model tests are recommended. These tests will provide the designer with a high level of confidence for successful operation without significant SLD. 3.6.1.3 Internal Structures Ideally, the inside of the ducting used for wet operation should be as smooth as possible, with a minimum of discontinuities to act as sites for droplet re

entrainment back into the gas flow. However, some structures are often required, and their negative impact on wet operation must be understood and

— to the extent possible

— minimized. To the extent possible, all structural supports should be located on the exterior of ducts used for wet operation. Internal bracing is often installed in rectangular ducts on retrofit units that originally operated dry. Horizontal bracing is also commonly used to support the tall side

- walls of breech openings on FRP liners with side

- entry breeches. These internal support structures are not favorable for wet operation because t hey provide additional impingement surfaces and re

entrainment sites for liquid droplets. Liquid collectors should be designed for these braces, which reduce the re

- entrainment of large droplets. Internal trusses, if necessary, should be designed to direct liquid collected on them to the side

- walls and duct floor (Figure 3

- 1). Trusses that direct collected liquid to the center of the duct should be avoided. Internal truss work is typically fabricated from 3 –6- in (75 –150- mm) piping attached to the duct surf aces using gusset plates. Liquid deposited or condensed on the trusses will flow around to the back or downwind side of the truss and along the length of the truss

— either downward because of gravity, or across the duct if the support is horizontal. 3- 12  Figure 3-1 Good and Poor Internal Duct

- Truss Arrangements for Wet Operation 3.6.1.4 Expansion Joints Expansio n joints are typically required in ducting to allow for thermal expansion of the ducting as well as to isolate duct sections either structurally and/or seismically. These duct components represent a special discontinuity, because they usually have a capaci ty to store liquid. Thermal

- expansion joints represent a potential liquid re

- entrainment area in the ducts. Expansion joints can act as liquid collectors on the walls, and they should have drains installed along the floor to remove collected liquid and red uce the possibility of re

- entrainment. Expansion

- joint drains are only practical at duct pressures near ambient pressure downstream of ID fans. At locations where negative pressure exists in the duct, traps are needed to prevent air leakage into the duct. Other re

- entrainment

control methods may also be needed, such as covering the joint with a sliding plate. There are a number of different types of expansion joints in use in wet power

plant ducting. Duct

- expansion joints can range from 8

– 24 in (0.2

– 0.6 m) in width and are fabricated from a corrosion

- resistant flexible rubber composite or elastomeric material. The most typical joint utilizes a flexible boot, which forms a cavity running around the perimeter of the duct. Some designs incorporate a reverse boo t designed to be nearly flush with the duct

- liner surface. Experience has shown that after a few months of operation, many of these boots have twisted and inverted, with portions protruding into the gas flow

— resulting in increased potential for liquid re -e ntrainment back into the gas flow. LIQUID FILM FLOW TO WALL



### Good Truss Arrangement Poor Truss Arrangement LIQUID FILM

FLOW TO CENTER OF

DUCT

LIQUID FILM

FLOW TO

FLOOR LIQUID FILM FLOW TO

FLOOR DROPLET REENTRAINMENT FROM CENTER OF DUCT DUCT WALL 3- 13  Internal duct

- support trusses are often located at the end of duct sections, and as such, can be located immediately upstream and downstream of an expansion joint. If the truss members are attached to the floor using a gusset plate, care must be taken to ensure that the expansion joint is protected from a gas

- flow separation zone formed downwind of the gusset plate(s) (Figure 3.2). This recirculation zone can be very intense, and it can easily rip water out of the expansi on - joint cavity if the joint is not drained. If such an arrangement exists, it is highly recommended that a 3 –4- ft (0.9

– 1.2

- m)

- wide metal plate be placed over the joint. The plate should be centered immediately behind the gusset plate for protection from t he gas

- recirculation zone. The plate will need to be fabricated from a corrosion

- resistant material and will need to be secured in such a manner that it will not hinder the relative motion between the adjacent duct sections.

Figure 3-2 Internal Duct

- Truss Gusset Plate DUCT FLOOR 3- 4” DIA PIPE

GUSSET PLATE

GAS FLOW

GUSSET PLATE RECIRCULATION ZONE FORMED BEHIND GUSSET PLATE

EXPANSION JOINT 3- 14  3.6.1.5 Man

- Ways and Test Ports Man

- ways and test ports can act as sites for liquid re

- entrainment back into the gas flow if they are encountered by the moving liquid films within the wet duc t systems. To minimize the potential for liquid re

- entrainment, liquid

- collection gutters may be required to direct the moving liquid films around these structures. Because the size and location of these devices is substantially a function of the local gas - and liquid

- film flow patterns, the need for these devices can only be identified

— and their design finalized

— as part of the wet stack physical

- flow

model study required for the overall liquid

- collection

- system design. 3.7 Flow - Control Devices Flow- control devices are often required to optimize the gas

- flow patterns and minimize the pressure losses within duct systems. This requirement is true for wet as well as dry ducts. However, because of the high cost of corrosion

- resistant materials required for use in this environment, the need for flow controls must be weighed against the economics of their installation. For example, the fabrication and installation cost of a turning vane desired for pressure

- loss reduction may be higher than the expected lifetime operating

- cost savings attributable to the reduced system

- pressure loss. Modern WFGD installations with a simple straight run of ductwork between absorbers and stack liner typically do not require flow

- control devices. However, flow controls requ ired for ensuring favorable wet operation should be installed as recommended. For example, a WFGD absorber

- outlet turning vane designed to ensure that the gas flow enters the liner in a way that will maximize liquid collection and the generation of strong and robust secondary flows, which are required for effective in

- stack liquid collection, should be installed if recommended. Flow- control devices such as small vanes, baffles, and diverters are often recommended for the elimination of recirculation and/o r dead zones where liquid films will collect and possibly re

- entrain droplets back into the gas flow. 3.7.1 Flow - Control Liquid - collectors All turning vanes and other flow

- control devices should be evaluated regarding their need for trailing

- edge liquid -c ollectors. These devices are often directly exposed to, and therefore collect the droplets entrained within, the gas flow. These fine droplets collect as a film. They can then re

- entrain back into the gas flow as larger droplets from the downwind or trailing edges of the device. It this potential exists, trailing

- edge collection gutters will need to be designed and installed to direct the collected liquid film off of the device and onto the duct walls or floor for subsequent collection and drainage. 3- 15  3.8 Fan s Occasionally, ID fans will be located between the absorbers and the stack of a utility power plant. In this type of application, the ID fans will operate wet. Only a few existing FGD systems were designed for wet fans, primarily because of the high cost associated with the requirements for corrosion

- resistant materials and the inherent problem of rotor imbalance caused by scale buildup. For most installations, wet fans should be considered only if they can be constructed of corrosion

- resistant alloy and i f a spare fan can be installed to permit a regular maintenance program of cleaning and rebalancing. Wet ID fans will require periodic or continuous washing to prevent solids buildup on the fan impellers, which can cause fan

- rotor imbalance. All of the wash liquid sprayed into the fan inlet leaves the fan impeller as droplets. Most of the liquid will be propelled by the high centrifugal

- force field to the fan scroll, where it will deposit but immediately re

- entrain as a result of the high gas

- stream velocity . Most of the washing liquid will escape the fan discharge as droplets entrained in the gas stream, causing additional liquid load in the ducts and in the stack. The fan

- wash method can be optimized for effective washing with the minimum wash

- flow rate. Tw o- dimensional computer calculations are used to optimize the fan washing. The duct and stack liquid

- collection system must be designed to account for the extra liquid carryover from the wet fan

- wash system. Fans are used with WFGDs for system pressure

– leve l control and for the absorber

- pressure loss. The fans are either ID fans or booster fans added to the ID fan to produce the required pressure rise. Large centrifugal fans are used most frequently, but axial fans are also used. These fans are mostly locate d upstream of the absorber, but sometimes the design places them downstream of the absorbers. 3.8.1 High - Gas - Temperature Fans The fans located upstream of the absorber are operating with high gas temperatures and a lower gas density, which means that a hi gher

- head

- rise fan must be used to produce the pressure loss of the FGD system. They are downstream of the electrostatic precipitators or bag

- houses and encounter only limited fly

- ash erosion. Axial fans are more sensitive to erosion than centrifugal fans. No fan

- liquid collectors are needed. 3.8.2 Wet Fans and Fan - Liquid Collectors Fans located downstream of WFGDs operate with saturated gas at adiabatic temperature level (wet fan). The head rise of the wet fan produces a higher pressure rise with the lower temperature and a higher

- density gas. The gas

temperature rise produced by the fan reduces the relative humidity below saturation in the ducts and the stack. The rate of liquid condensation in the stack is reduced or eliminated completely

— an important ben eficial effect. The carryover from the mist eliminators usually contains water droplets, fly ash, and scrubber sludge, which cause solids buildup on the blades of the fan 3- 16  impellers. Wet fans have to be washed to keep the impellers in balance. The fan

wash spray has to be designed to work effectively with the specific fan

- blade shape and speeds, spraying periodically or continuously. The fan wash is less sensitive to the fan

- blade design of the axial fans. To minimize the SLD, liquid collectors are needed. Fans are good liquid separators, but the liquid has to be captured in the high

- velocity gas flow in the scroll, and in the ducts and the stack downstream of the fan. The centrifugal fans can be equipped with scroll

- liquid collectors especially designed for the high gas velocity in the scroll. The ducts and the stack may require the usual liquid

collection system with wet fans, depending on the temperature rise

— above saturation — remaining in the gas entering the stack. 

### If an ID fan is located in the wet portion of the duct system, the following steps should be taken:

 As much liquid should be collected and drained ahead of the fan as possible.

 An on

- line periodic fan

- wash system should be installed and used to keep the fan impeller clean and in balance. This s ystem needs to be custom

- designed for the particular fan design.

 Liquid collectors should be incorporated into the scroll and the downstream ducts to collect and drain the fan

- wash water. The power input to the fan may provide a 5

– 20°F (2.8

– 11.2°C) reheat of the flue gas if the liquid

- droplet load and its total latent heat of evaporation is minimized. This reheating will help reduce condensation in the downstream ducts and stack liner. 3.9 Stack - Liner Design A variety of factors should be considered in determining the overall geometry configuration of a new wet stack. The stack

- liner geometry should provide the most economical configuration to accommodate the physical and design constraints required. The stack

- liner diameter should be based on the recommended gas velocities to accommodate the liquid downflow on the liners. Platforms, access system, and means of inspection must be considered in the stack

- diameter selection. Stack diameters can be plumb, single -t apered, multiple

tapered, or incorporate stepped diameter changes. Stack

- liner geometry should also be evaluated to minimize the design implications associated with wind and earthquake loads. The major items for the preliminary stack design are discussed i n the following sections

— from the stack floor to the top of the stack. For examples of stack geometry for a brick, FRP, and metal

- lined stack, refer to Figures 3

- 3, 3

- 4, and 3

- 5, respectively. Steel, alloy, and FRP liners are typically, but not necessarily, limited to plumb or constant diameters. Several different support methods are available for these types of liners. For steel and alloy liners, a common arrangement is to support the liner at or near the base or breeching entry. The liner is supported in compression, and bumpers or stay rods are used to provide lateral support. 3- 17  Figure 3-3 Chimney with Constant

- Diameter Brick Liner RAINHOOD

OBSTRUCTION LIGHTS

PLATFORM BRICK LINER

TEST & MONITORING PORTS

LINER BANDS (TYP.)

LIQUID COLLECTION RING

LIQUID COLLECTION GUTTERS

DRAINS EXPANSION JOINT

PRESSURIZATION FAN REINFORCED CONCRETE SHELL

CONCRETE PEDESTAL

FOUNDATION ABSORBER OUTLET DUCT BREECHING DUCT

LINER FLOOR LINER EXTENSION

PLATFORM

OBSTRUCTION LIGHTS 3- 18  Figure 3-4 Chimney with Fiberglass

- Reinforced Plastic Liner

RAINHOOD

OBSTRUCTION LIGHTS

PLATFORM FRP LINER
TEST & MONITORING PORTS

LINER CAN JOINT (TYP.)

LIQUID COLLECTION GUTTERS

DRAINS REINFORCED CONCRETE SHELL

FOUNDATION ABSORBER OUTLET DUCT LINER SUPPORT

LINER BOTTOM ENTRY ELBOW LINER EXTENSION

PLATFORM

OBSTRUCTION LIGHTS LINER SUPPORT

EXPANSION JOINT STAY RODS

STAY RODS

LINER EXPANSION JOINT LIQUID COLLECTION RING

LINER EXPANSION JOINT
LINER SUPPORT 3- 19  Figure 3-5 Chimney with Metal Liner – Typical for Alloy, Glass

- Block, or Protective

- Coating Liner System Tall steel and alloy liners supported in compression would need t o be evaluated for the cost of increased wall

- plate thicknesses, versus providing a second support level near the top of the chimney to support most of the liner in tension. An expansion joint would be required above the lower support to accommodate therma l movements.

RAINHOOD

OBSTRUCTION LIGHTS

PLATFORM ALLOY LINER

TEST & MONITORING PORTS

STIFFENERS (TYP.)

EXPANSION JOINT

LIQUID COLLECTION GUTTERS

DRAINS EXPANSION JOINT REINFORCED CONCRETE SHELL

FOUNDATION ABSORBER OUTLET DUCT BREECHING DUCT

SLOPED LINER FLOOR LINER EXTENSION

PLATFORM

OBSTRUCTION LIGHTS

STAY RODS STAY RODS

LIQUID COLLECTION RING LINER SUPPORT

LINER SUPPORT 3- 20  FRP liners are generally supported with the major part of the liner in tension. Supports are provided at two locations

— one near the top and the second near the breeching level

— with an expansion joint located above the breeching level. Expansio n joints in wet stack liners are a problem because they tend to collect liquid, resulting in complete liquid re

- entrainment at the expansion

- joint cavities. Therefore, expansion joints should be avoided or provided with liquid collectors. If required, expansion joints should be located 1 to 1.5 liner diameters above the roof of the breeching duct or bottom

- entry elbow to ensure that the lower liner is unobstructed for liquid collection and that liquid film flows to the liner

- drainage points. Because of stru ctural considerations to accommodate breeching openings and design loads, brick liners can be plumb or tapered. Depending upon the breeching

- opening elevation and other factors, brick liners can be supported from the stack foundation, from a concrete pedestal, or from a corbel support in the concrete column. Chimneys with breeching openings that are less than 50 ft (15 m) above the foundation would probably use a brick liner that is supported directly off the foundation. Breeching openings that are more tha n 50 ft (15 m) above the foundation should consider utilizing a concrete pedestal to support the lining. For tall brick liners with thick walls at the base, it is generally more economical to use a concrete pedestal for high breeching entries. Brick liners in high

- risk seismic locations sometimes use a corbel

- supported liner to reduce the seismic loads. The liner is constructed in short segments, which are supported from concrete ledges in the chimney, called corbels. This method of construction is not reco mmended for wet stack applications. Liquid and flue gas leaking through the brick liner can attack the concrete support system. Additionally, it is difficult to pressurize the annular

- space areas at each of the corbel sections. 3.9.1 Liner Materials Severa l types of liner materials and coating systems are available and have been used successfully in WFGD environments. The advantages and disadvantages of each system must be considered prior to the selection of the liner material or coating. The following sec tions describe the most common materials that are currently considered appropriate for wet stack applications. These materials have previously been utilized with some degree of success. Other materials and suppliers may be available and acceptable, and the y should be evaluated on a case - by

- case basis. Operating conditions, design conditions, and economics all play important roles in this decision. The advantages and disadvantages of several different liner and coating

- system options, along with estimated in stallation costs, are presented in Table 3

- 2. 3- 21  Table 3-2 Liner Material of Construction

Liner Material or Coating Advantages Disadvantages

Fiberglass

Reinforced Plastic Good corrosion resistance Easy to add liquid collection devices Maximum 300 o F (149 o C) (approx.) gas - temperature exposure Quality control during fabrication Compressive strength limitations usually requires two support levels and expansion joint.

Borosilicate

- Glass Block Good corrosion resistance Good insulator (ductwork and liner should not be insulated) Ability to retrofit to existing steel liner systems Good surface for liquid flow Cannot tolerate abrasion or physical and mechanical abuse Care in installing

Acid- Resistant Brick Good corrosion resistance Cost

- effective Liquid adheres to the porous surface. Surface discontinuities re

- entrain liquid. Not recommended in high seismic areas Maintenance of liner accessories Annulus pressurization
Protective Coating on Carbon Steel Fair corrosion resistance Ability to retrofit to existing steel liner systems Surface preparation prior to placement Frequency of repair and maintenance Limited acceptable selections

Alloy C276 Excellent corrosion resistance Welding quality control High material costs Welding seams Iron Contamination Acid cleaning Condensation 3.9.1.1 Brick Acid

- resistant brick is a solid kiln

- fired brick made of clay, shale, or mixtures of clay and shale, which conforms to the requirements of ASTM C980, Standard Specification for Industrial Chimney Lining Brick . ASTM C980 defines three types of brick, which are categorized by compressive strength, water absorption, and acid solubility. Of the three types, Type III has the most stringent requirements, allowing the least amount of water absorption and acid solubility, whereas Type I 3- 22  allows the highest levels. Because of the stringent requirements, Type III brick is the most difficult and expensive to manufacture. Because of availability limitations for Type III brick , most brick liners for FGD applications are constructed using Type II bricks. The physical properties and chemical requirements for Type I, Type II, and Type III brick, as defined by ASTM C980, Standard Specification for Industrial Chimney Lining Brick , a re presented in Table 3 -3. Table 3-3 Physical Properties and Chemical Requirements Of Acid

- Resistant Brick1 Minimum Compressive Strength, Gross Area (psi) Maximum Water Absorption by 2 - h Boiling (%)

Designation Average of 10 Individual Average of 10 Individual Maximum Average Weight Loss

by H 2 SO 4 Boil Test (%) Type I 8,500 8,000 6.0 7.0 20 Type II 10,000 9,000 4.0 5.0 12 Type III 12,000 10,000 1.0 1.5 8

1 Per ASTM C980, Standard Specification for Industrial Chimney Lining Brick 1 psi = 6.895 kPa As previously stated, chimney brick can be manufactured from clay, shale, or a mixture of clay and shale. Bricks manufactured from clay are called fireclay bricks and are typically buff in color. Bricks manufactured from shale are called red

shale bricks and are red in color. Red shale usually has a higher iron oxide content than fireclays. The iron oxide acts as a flux, which permits a lower firing temperature for shale than for fireclay. Fireclay and red

- shale brick provide comparable physical and chemical characteristics when fired at their respective temperatures. If fireclay and red

- shale brick are fired at the same temperature, red

- shale brick will typically be denser and have lower absorption characteristics than fireclay brick. When properly manufactured, both will meet the requirements of ASTM C980. Brick liners use potassium silicate mortar that conforms to the requirements of ASTM C466, Standard Specification for Chemically Setting Silicate and Silica Chemical

- Resistant Mortars . These inorganic mortars are resistant to most of the strong acids present in the scrubbed flue gas from coal

- fired power plants (except for significant concentrations of acid fluorid es or hydrofluoric acid). Potassium silicate mortars are more resistant to sulfation than sodium silicate mortars or other types of silicate

- based cements. Brick liners have been widely used by the power

- plant industry. These liners generally require the l owest initial capital

- cost expenditure in comparison to 3- 23  other types of liners. However, maintenance for a brick liner can generally be higher than that for an alloy or FRP liner. Typical maintenance for a brick liner would include repair of brick cracks, r epair or replacement of bands, maintenance of the pressurization fans, and repair of expansion

- joint seals at the rain hood and breeching. Brick liners should be designed and constructed in accordance with the recommendations of ASTM C1298, Standard Guide for the Design and Construction of Brick Liners for Industrial Chimneys

. 3.9.1.1.1 Mortar Joints Careful attention must be paid to the quality of mortar joints in a brick

- lined wet stack. All joints must be struck smooth with the brick surface in order to eliminate (to the extent possible) horizontal discontinuities on the liner surface. Mortar protub erances and depressions will act as sites for liquid collection and re - entrainment back into the gas flow. 3.9.1.1.2 Annular Space and Pressurization Sufficient space should be provided between the inside of the concrete column and the outside of the liners to allow for inspection and maintenance over the full height of the stack. A minimum annular space of 2 ft 6 in (0.76 m) is recommended. If platforms, ladders, and an elevator are located on the interior of the stack, sufficient annular space needs to b e provided for these items. The size of the annular space should account for differential movements that may occur between the concrete column and the liner as a result of wind, seismic, or thermal expansion. Damage to platforms or the liner may occur if s ufficient space is not provided. Access and adequate clearance for clean

- out doors and continuous

emissions

- monitoring equipment also needs to be considered. Ports should be accessible and provide sufficient clearance between the column and liner to insert EPA test probes. To minimize annular

- space requirements, doors can be provided in the column. Monorails can be used to assist in installation of the probes. In order to ensure personnel safety, the annular space should be well

ventilated, especially when an interior access system is used. Annulus pressurization of the brick

- liner stacks increases the rate of condensation on the liner surfaces. Leakage of pressurizing air through the liner in older brick stacks can significantly increase the condensation. Because of the porous characteristics of brick and mortar, brick liners operating under WFGD conditions should use an annulus

- pressurization system. Annulus pressurization is not needed for other liner materials. An annulus

- pressurization system consists of fans that provide ambient air under positive pressure into the annular space of the stack, ensuring that permeation of the flue gas and liquids through the liner and liner cracks is minimized. The pressure created by these fans should exceed the maximum anticipated positive pressure inside the liner by a minimum of 1 in Wg (249 Pa). Flue gas and liquids that permeate the liner 3- 24  create a highly corrosive environment and can cause damage to the liner bands and concrete column. Airtight seals are required at all openings in the stack and liner to minimize air leakage from the pressurization system. Air

- locking chambers should be provided at door locations to provide safe passage into and out of the chimney. Pressurizing fans typically have a 30,000 –70,000- cfm (14 –33-m 3 /s) flow rate and a 3 –4- in Wg (747 –1245- Pa) rise for a 500 –650- MW unit. An adjustable damper can be provided at the top of the chimney for use when reduced

- pressurization pressure and flow rate are acceptable. Excessive annulus pressurization could lead to air leakage through cracks in the brick into the inside of the liner and cause increased liquid condensation in the flue gas. 3.9.1.1.3 Target Walls and Linings Some brick liners operating under WFGD applications have experienced differential liner growth, resulting in a permanent deflection of the liner. Although the cause of this permanent deflection has not been determined, wet saturated conditions and nonunifo rm temperature appear to be contributing factors. EPRI has previously funded laboratory and field studies directed at preventing this problem. The results of these studies are presented in [21]. Precautions such as installing a target wall in the bottom of the liner should be considered. Mixing of gases with a nonuniform temperature profile should also take place prior to entering the stack. A target wall or target lining provides protection to the brick liner from the wet flue gas. Target walls or target l inings can be constructed partially or fully around the inside circumference of the brick liner, and they should extend from the bottom of the liner to 1 or 2 liner diameters above the breeching entry. A target wall is an acid

- resistant brick wall, constru cted independently of the brick liner. An air space should be provided between the target wall and the brick liner. A target lining is constructed directly against the inside surface of the brick liner. The lining can be constructed using an elastomeric co ating or a borosilicate

- glass block with an adhesive membrane, which provides a good moisture - and chemical

- resistant barrier. Another method that has been considered to reduce the direct impingement of wet gases exiting the breeching onto the interior su rface of the liner is providing a bottom entry into the brick liner. This can be accomplished by providing an alloy breeching

- thimble section that extends below the bottom of the brick liner. Target walls may be needed for good liquid collection and reduce d erosion of the brick liners in the stack

- entrance region. 3- 25  3.9.1.1.4 Liner Bands Circumferential liner bands should be provided around the liner to minimize vertical cracking and to add stability to the liner if cracking occurs. The minimum recommended ba nd size is 3/8 x 3 in (10 x 75 mm) plate spaced at 5

- ft centers. Materials of construction will depend upon the operating conditions and effectiveness of the annulus

- pressurization system. For applications in which the pressurization system is working effe ctively and the exterior of the liner remains dry, bands constructed of carbon steel with a protective

- coating system are adequate. For applications in which liquid permeates the brick and is in contact with the bands, a nickel

- alloy material should be use d for this corrosive environment. Use of carbon

- steel bands in a wet environment may require future maintenance, and possibly future replacement. Several utilities replaced their carbon

- steel liner bands because of corrosion. 3.9.1.2 FRP/GRP The most commo n material currently used for wet stack liners is fiberglass

reinforced plastic (FRP). This material is also referred to as glass

- reinforced plastic (GRP). FRP is a composite material made of glass reinforcement and a thermosetting resin. FRP is a lightweight laminated product that provides good service in cool, wet, dilute acid environments. Because of size and shipping restraints, these liners are usually filament

- wound in sections on

- site. The FRP liner’s inner surface is a corrosion

- resistant barrier th at consists of a glass mat and resin. Vinyl

- ester resins are recommended instead of the polyester resins that have been used in many past projects. Vinyl

- ester resins provide higher temperature and corrosion resistance and have a lower coefficient of therm al expansion. The structural wall of the FRP liner is composed of continuous glass strands that are helically wound around a mandrel. FRP is suitable for the low temperatures that are characteristic of wet

- scrubbed conditions and is not recommended for byp ass conditions or temperature excursions. Exposure to bypass conditions will result in loss of strength, shortening the life of an FRP liner. An inlet

- water spray
- quench system is recommended to reduce the effects of bypass temperature excursions. FRP line rs downstream of FGD systems that do not have bypass capability do not need a quench system. FRP liners should be designed, fabricated, and erected in accordance with the recommendations of ASTM D5364, Guide for Design, Fabrication, and Erection of Fibergl ass Reinforced Plastic Chimney Liners With Coal

Fired Units [7]. 3.9.1.2.1 FRP Liner Joints FRP liners are typically fabricated on

- site in 30 –40- ft (9.1 –12.2- m) sections, or “cans,” which are lifted into place within the windscreen and joined together. The cans themselves are butted together with additional glass and resin, applied to both the interior and exterior of the joint. Most of the joint material is recommended to be placed on the exterior of the liner. The amount of joint 3- 26  material on the inside of the liner should be minimized to reduce the potential for creating horizontal discontinuities on the liner surface, from which droplets may be re

- entrained back into the gas flow. The surface of the joint material must be tapered back to the base

- liner ma terial with a minimum 6:1 taper (10:1 preferred). 3.9.1.3 Alloy Stainless steel covers a wide variety of corrosion

- resistant materials. Operating parameters and environmental factors greatly affect alloy performance, especially pH, temperature, and chlori de and oxygen levels. The corrosion resistance of stainless steel is improved by increasing its molybdenum, nitrogen, chromium, and nickel contents. Molybdenum and, to a lesser extent, nitrogen improve resistance to pitting. Chromium aids in the developmen t of protective, passive surface films and increases corrosion resistance in oxidizing environments. Nickel assists in the renewal of damaged passive films and stabilizes the austenitic microstructure with its improved fabricability and weldability. On ave rage, chloride levels in the new FGD systems will likely be much higher than earlier FGD systems because of the requirement for zero

- discharge water systems. Designs specifying chlorides above 15,000 ppm have been fairly common, and some designs have antic ipated sustained operation above 30,000 ppm. Closed

- loop operation causes chloride levels to increase, thereby increasing the possibility of pitting and/or crevice corrosion of stainless steel. Stainless steels that have been used successfully in scrubbers may not perform well in FGD outlet ducts or stack liners. These locations are wetted by essentially unbuffered mist and/or condensate. Absorption of residual SO 2 can cause the pH to drop to below 2. Because of the potential for high chlorides and low pH i n the absorber outlet duct and stack, stainless steels are not recommended. For these areas, a nickel

- alloy material such as Alloy C276 or Alloy C22 is recommended. High

- nickel alloys are an effective corrosion

- resistant material in wet FGD applications. N ickel alloys containing high percentages of chromium and molybdenum provide good resistance to pitting and crevice corrosion in a high

chloride, low

- pH environment. Although several different nickel alloys may be adequate for FGD applications, Alloy C276 ( ASTM B575, UNS N10276) and Alloy C22 (ASTM B575, UNS N06022) have been shown to provide the best overall corrosion

- resistance characteristics. Alloy C276 contains 16% molybdenum and 15.5% chromium. Alloy C22 contains approximately 13% molybdenum and 22% ch romium. These alloys have performed well in numerous WFGD environments and offer approximately the same level of corrosion resistance. The two products have successfully withstood a wide range of acid concentrations and extremely high chloride levels, in c onditions ranging from 130°F (54.4°C) saturated flue gas to full system bypass. 3- 27  3.9.1.3.1 Solid Alloy Alloy C276 and Alloy C22 is produced in bar stock, plate, and sheet. Liners constructed using this material could be fabricated from solid

- alloy plate, fr om carbon

- steel plate with an alloy

- sheet lining, from roll

- clad plate, or from Resista

- Clad plate. Although liners fabricated from all of these applications are highly corrosion resistant, liners fabricated from solid Alloy C276 plate offer the most corro sion resistance. Because welding between the alloy material and carbon steel is a potential source of dilution and contamination, the use of solid

- alloy plate eliminates this concern. However, the use of solid

- alloy plate also requires the highest initial capital cost. Stiffeners on the exterior of the liner can be fabricated from carbon steel because this material is not in contact with the flue gas. Any internal stiffening, such as bracing in the ductwork, should be fabricated from alloy material. 3.9.1.3 .2 Wallpaper Lining Alloy

- sheet lining involves the welding of thin alloy sheets, usually 1/16

- in (1.6

- mm) thick, to a carbon

- steel liner. This approach has commonly been referred to in the industry as a "wallpaper lining." The sheet lining is installed wi th lapped joints. The joints exposed to flue gas must be fully seal

- welded to ensure protection of the carbon

- steel liner surface. The sheets are attached to the liner with plug welds and intermittent fillet welds. The plug welds should be covered with cap plates to eliminate any concerns about dilution. From a design standpoint, only the carbon

- steel portion of the liner should be considered structurally effective in supporting the liner. This lining process can be installed on a new or existing carbon

- ste el liner. If the lining is to be installed on an existing liner, the liner

- support steel should be evaluated to accommodate the additional load of the sheet lining. Alloy C276 sheet lining has been utilized in the stacks at a number of plants. It has been reported that after several years of operation, the material is performing well. However, buckles in the lining sheets have developed throughout the liner as a result of thermal stress. 3.9.1.3.3 Roll

- Clad Plate Alloy

- clad plate consists of alloy sheet, us ually 1/16

- in (1.6

- mm) thick, that is roll

- bonded to a thicker carbon

- steel backing plate. The two metals are mill

rolled under heat and pressure until they are integrally bonded over the entire interface. Clad plates are readily joined by welding. However , on the clad side, it is important to minimize dilution to maintain adequate corrosion resistance of the weld metal. 3.9.1.3.4 Resista

- Clad Another type of lining system that utilizes highly corrosion

- resistant alloy as a lining material is a process called Resista

- Clad. Resista

- Clad plate is a metal

- 3- 28  fabrication technique that uses a patented process in which a relatively expensive, thin sheet of corrosion

- resistant metal cladding is attached to an inexpensive substrate. The Resista

- Clad plate process is capable of cladding carbon steel to both weld

- compatible metals, such as nickel

- based alloys and stainless steels, and nonweld compatible meta ls, such as titanium. This technique allows the cost

effective use of both relatively high

- cost alloy metals, for corrosion resistance, and relatively low

- cost carbon steel, for structural support. In this technique, 1/16

- in (1.6

- mm) alloy sheet is bonded to the carbon

- steel backing by the use of resistance welding. During liner fabrication, the carbon

- steel plates are welded together, and then a batten strip of alloy is laid over the joints and seal

- welded to the alloy lining. 3.9.1.3.5 Weld Seams The hor izontal weld beads on alloy liners can cause a localized liquid re

entrainment in the stack above a velocity of approximately 55 fps (16.8 m/s). The height of the weld bead should be the minimum possible and not exceed the standard of 1/8 in (3 mm) for goo d weld quality Vertical weld seams in a wet stack, including the choke, are also typically limited to 3/16 in (5 mm). Limiting the size of the horizontal weld seams is more critical than that of the vertical weld seams with regard to minimizing liquid re -e ntrainment. If the shape of the weld bead is gradually rising and falling, liquid re

- entrainment is reduced, and higher liner

- gas velocities can be used. To achieve liner maximum velocities for smooth metal, horizontal weld beads must be ground flat and sm ooth. 3.9.1.4 Borosilicate Block Borosilicate foamed

- glass block is an inorganic manufactured product that is chemically resistant to most acids, solvents, and weak bases. Borosilicate

- glass block is supplied by Hadek, Inc., under the trade name Pennguard TM and has been successfully used in numerous wet stacks all over the world. Its closed

- cell composition is impervious to liquids and gases and provides a low thermal conductivity. This light

- weight product is manufactured in block form and is installed dir ectly against the surface that it is to protect. An adhesive membrane is used behind and between the blocks and serves as a moisture - and chemical

resistant barrier. These characteristics make it ideal for installation on liner materials that are not gener ally suitable for corrosive environments such as carbon steel or concrete. Also, because the mastic substrate provides a moisture

- resistant barrier, its installation in a brick stack will eliminate liquid seepage and bleeding through to the exterior of the liner and the need for annulus pressurization. Borosilicate block can operate at temperatures up to 960°F (516°C); however, the adhesive membrane is limited to 200°F (93°C). Because the glass block is an excellent insulator, the ductwork and liner should not be insulated if glass block is used. If insulated, the temperature of the mastic may exceed the maximum recommended temperature of 200°F (93°C) if the duct/stack system is operated dry. 3- 29  Because of its high insulative properties and low thermal mass, borosilicate block will minimize the quantity of thermal condensation within the liner during unit startup. The block also provides some level of protection to the base

- liner material in the event of a stack fire. In such a situation, the lining system wil l most likely need to be replaced, but the structural portion of the liner may be undamaged. Borosilicate

- glass block is well suited for lining ductwork. However, because the block can be easily damaged, areas that are susceptible to physical or mechanica l abuse should be protected with an abrasion

- resistant coating. Internal bracing is difficult to line with block. Membrane coatings or alloy materials are recommended for internal braces. 3.9.1.4.1 Mastic Joints Borosilicate block is attached to the substr ate material using an adhesive/membrane material. It is important that the liner substrate be fully covered with the membrane material to ensure that a continuous chemical - and moisture

- resistant barrier is formed between the inside and outside of the flue . A nominal adhesive/membrane thickness of 1/8 in (3.2 mm) is recommended behind and between each block. A 1/16

- in (1.6

- mm) thick layer should be toweled onto the liner, as well as to the sides and back of each brick, before installation. This double

- butte ring technique ensures a full bond of each block to the liner and to each other. Excess adhesive material squeezed out during placement of the block should be struck clean. Every effort should be made to minimize the quantity of adhesive material smeared o n the exposed surface of the block, because these locations can lead to liquid re

- entrainment back into the gas flow during wet operation. 3.9.1.4.2 Lining Inside of Concrete Shell Borosilicate

- glass block can be attached directly to the inside surface of a concrete shell. This technique has been used to reduce the cost of stacks and also as a method for reusing older stacks whose liners are not favorable for wet operation. The most common application is in existing stacks with tapered brick liners previously operating with flue gas reheat. After conversion to wet operation, the upper portions of the liner would be operating at gas velocities in excess of the recommended velocity for wet operation for brick

- liner material. In such a situation, the upper por tion of the brick liner can be removed down to a point at which the gas velocities are acceptable within the brick portion of the liner, and the inside of the existing larger

- diameter reinforced

- concrete shell can be used as the upper portion of the liner. In such a situation, the inside of the upper shell would be lined with borosilicate block to protect the reinforced concrete. This arrangement is presented in Figure 3

- 6, where it can be seen that a ring collector will be required at the interface between the lower and upper liner sections of the flue to collect condensate generated within the upper portion of the liner. This arrangement allows the entire liner to operate at acceptable liner velocities. 3- 30  Figure 3-6 Borosilicate Block

– Lined Upper Wind Screen There are a number of disadvantages to this approach, which must be addressed when considering such a retrofit. First is the loss of the annular space between the old liner and the stack shell. All utilities, electrical conduits, ladders, drains, and test ports located in this area will need to be moved to the exterior of the stack shell. The second disadvantage is potential damage to the concrete column if leakage through the block and membrane develops. The location of the leakage and the extent of the damage to the concrete column would be difficult to detect and EXPANSION JOINT REINFORCED CONCRETE SHELL

FOUNDATION BREECHING DUCT LIQUID COLLECTION GUTTER BETWEEN LINER SECTIONS BOROSILICATE BLOCK ATTACHED TO INSIDE OF SHELL

LINER BANDS (TYP.)

LIQUID COLLECTION RING

LIQUID COLLECTION GUTTERS

DRAIN PIPES

PRESSURIZATION FAN CONCRETE PEDESTAL EXISTING BRICK LINER

BRICK LINER REMOVED TO THIS ELEVATION LIQUID COLLECTION DRAIN FLEXIBLE CONNECTION 3- 31  evaluate in such a system. The site

- specific situation and risks involved should be evaluated when considering this syst em for a utility wet stack. 3.9.1.5 Liner

- Protective Coatings Many existing units utilizing reheat and bypass liners have been constructed using bare carbon

- steel plate. This liner material is not acceptable for wet applications. Carbon steel will corrode rapidly under wet acidic conditions. Steel liners with an existing gunite lining are also not considered acceptable for wet operation. Gunite is porous and tends to develop cracks through which liquid can penetrate and attack the substrate material. Conversion to wet operation requires removal of the gunite and relining with a corrosion

- resistant material. If liquid collection can be effectively collected without modifying the existing liner geometry, applying corrosion protection to the carbon

- steel liner should be considered. Because the available options involve extensive field construction, an extended unit outage is required to perform this work. The expense of performing these modifications and the lost revenue associated with the unit outage needs to be evaluated against the construction costs for a new stack that would require a minimal amount of unit outage. The following modification options can provide corrosion protection to the carbon

- steel liner:

 Alloy wallpaper lining

 Borosilicate

- glass block l ining.

 Protective coating Surface preparation is required before applying a lining or coating. The liner's interior surface must be blast

- cleaned. The degree of cleanliness and the surface profile for the blast

- cleaning depends upon the lining or coating b eing installed. A coating system on an existing liner typically requires weld preparation, surface preparation, testing for the pH of the surface, and if necessary, neutralizing acids. Coating also requires careful control of humidity and temperature, whic h limits installation. If alloy wallpapering is used, field measurements should be taken to verify existing dimensions before cutting and fitting the lining sheets. Existing welds must be ground flush for alloy linings and smoothly transitioned for coating s. Existing carbon

- steel liners can be lined with borosilicate

- glass block to provide corrosion protection. Installation of the block is similar to installing a new liner, except that additional surface preparation may be required for cleaning the interior surface of the liner. A wide variety of coatings are available, several of which are applicable to WFGD operations. One such coating is the flake

- glass

- reinforced, vinyl

- ester system. This system is a glass

- filled, thermosetting, resin

- based, corrosion

resi stant lining system. The vinyl

- ester resins are more durable and corrosion

- 3- 32  resistant than the polyester resins that have been used in older liner

- coating applications. This coating system should be applied in three layers. The first layer is a primer coat, which is used to prevent abrasive blasted steel from developing rust bloom. The lining is then trowel

- applied in two 30

– 40 mil layers. Toweling and subsequent rolling allows the glass

- flake filler in each layer to be properly oriented to the substrate and to achieve maximum resistance to water

- vapor permeation. In order to ensure adequate coverage, the two layers of resin can be applied using different color pigments. Historical experience indicates that this coating system may develop permeability, and li quid may eventually seep through and reach the carbon

- steel substrate. Coating systems other than the flakeline resin systems have been utilized, but with only a limited degree of success. The performance of any coating is dependent upon surface preparatio n and application. Strict quality

- control measures are required. The coatings need to be applied under good weather conditions or under a controlled environment. The coating should not be applied at temperatures less than 50°F (10°C) or greater than 120°F (49°C). In order to get optimum performance out of a coating, application and curing conditions are critical. The steel

- substrate surface requires significant surface preparation prior to application of the coating. All sharp edges and imperfections must b e ground smooth. Degreasing the surface and then sand

- blasting to a white metal blast (SSPC SP

- 5) is required. Blasting should take place only over an area that can be primed within the same day. Surface preparation and installation should carefully follow the manufacturer's written instructions. Coatings have been used successfully in some wet stack applications and have failed in others. There is some risk involved in using a coating system, and the potential for failure should be realized. Maintenance to repair chips and blisters should be anticipated. Total lining replacement is typically required after a life of about 8

– 10 years. However, coatings can represent substantial savings, and they have been included as an alternative here for cost

- comparative purposes. 3.9.1.6 Liner Insulation A wet stack with a flue gas temperature of 130°F (54°C) typically will not require insulation on the exterior surface of an alloy or FRP liner, because the liner is already operating in a wet condition. Insulation is inst alled on coated steel liners to reduce the temperature gradient across the coating, which may help in extending the life of the coating. Borosilicate

- lined stacks are not insulated to ensure that the temperature of the mastic/adhesive material between the block and liner wall does not exceed the manufacturer’s recommendations. Insulation should be considered if the condensation calculations predict a high liquid

- film thickness within the liner, which could lead to an increased potential for SLD. A thick li quid film could occur because of a high condensation rate or could be the result of a tall stack. Insulation should also be considered as a means of minimizing the potential for SLD if its minimization is a high priority; for example, such might be the cas e if the stack were located in a densely populated area. 3- 33  At 130°F (54°C), the addition of 2 in (50 mm) of mineral

- wool insulation reduces the rate of liner

- wall condensation by a factor of approximately four. The addition of more insulation will reduce th e condensation rate further, but with a diminishing rate of return. Uninsulated liners should have a protective coating on any carbon

- steel plate or stiffeners on the exterior of the liner that are exposed to the ambient environment. 3.9.2 Recommended Li ner - Gas Velocities The gas velocity within the liner is a significant factor in minimizing liquid re

entrainment into the gas stream. Liquid droplets, either carried over from the mist eliminator or condensed on cold walls, are deposited on the ductwork an d stack

- liner surfaces. As the droplets accumulate, they are pulled downward by gravity, whereas the gas drags the liquid in the same direction as the flow of the gas. When the force from the gas reaches or exceeds the forces of gravity and surface tension, the liquid is sheared from the ductwork or liner walls. Liquid then re

- enters or is re

- entrained back into the gas stream and is carried out of the stack. When this occurs, the gas velocity is referred to as the critical re

entrainment velocity. Wet stac k designers should select the liner diameter based on liquid

- collection considerations. Selecting the liner diameter so that the gas velocity is less than the critical re

- entrainment velocity (with a desirable margin for fabrication inconsistencies and pot ential future plant up

- rates) increases the opportunity for liquid to be collected within the stack rather than being emitted with the gas stream as SLD. The critical re

- entrainment velocity varies depending upon the liner's material, surface roughness, an d quality of construction. Surfaces with a high level of discontinuities and roughness (such as those of a brick liner) will be more likely to re

- entrain liquid, in comparison to smoother surfaces (such as alloy or FRP liners). Therefore, the liner diamete r will depend upon the liner material selected. For example, brick liners will require a larger diameter than an alloy or FRP liner for the same gas

- volume

- flow rate. The recommended liner

- velocity range for sizing the liner diameter when using various lin er materials was previously presented in Table 2

- 1 and is represented in Table 3

- 4 below. 3- 34  Table 3-4 Recommended Stack Velocity Range for Stack

- liner

– Diameter Sizing

* Laboratory testing of coating material is recommended to finalize liner velocity for favorable wet operation. 3.9.3 Stack - Entrance Geometry As discussed previously, there are two major stack

- entrance geometries in use today: the side

- entry breech and the bottom

- entry elbow. Each approach has i ts own unique advantages and disadvantages, and each has specific design requirements for both structural application and favorable wet operation. The stack entrance is the last good place between the mist eliminator and the stack outlet to collect and drain any liquid that enters the stack. The primary techniques are intended to promote droplet impingement onto the surfaces by 1) 

### directing droplet trajectory to the far wall or a center baffle and 2) creating a single or double gas

- swirl pattern that will help deposit droplets on the liner or baffle wall. Once the liquid is on a solid surface in the lower stack, it must be

 Allowed to drain naturally where upward gas velocities are low

 Collected and drained in a gutter or shielded area

 Prevented from re - entering the breeching area by side and top gutters on each entrance opening

 Drained to a protected sump in the bottom of the stack liner and out of the system Although single entrances are easiest to work with to obtain good liquid collection and drain age, a double

- entrance stack with a special center baffle for liquid collection and drainage can also be made to work satisfactorily. Liner Velocity Liner Velocity (ft/s) (m/s) Acid Brick Radial tolerance of construction is 1/8” (3mm) Alloy Weld bead height <= 1/8” (3mm) Fiberglass Reinforced Plastic Minimum 6:1 taper on interior joints Borosilicate Block Radial tolerance of construction is 1/8” (3mm) Coatings 

### 55* 16.8* 60 18.3 LINER MATERIAL


### 45 13.7 55 16.8 55 16.8 3- 35  Variations can be made in all of these parameters and still achieve a satisfactorily low level of SLD. However, more exten sive liquid

- collector and drain systems will be needed, and further steps may be needed to reduce condensation in the stack if velocities are in excess of stack

- liner design values. 3.9.3.1 Side

- Entry Breech Side

- entry breeches can be used on liners of any material. Because the flow gasses entering the liner must make a sharp 90° turn to vertical after passing through the breech opening, strong secondary

- flow vortices are generated, which are required for effective liquid collection. These secondary flows are stronger in a liner with a side

- entry breech than in one with a bottom

- entry elbow. Additionally, the liquid

- collection system in the lower liner of a unit with a side

entry breech is less complex

— and generally performs better

— than that of a unit with a bottom

- entry elbow. For this reason, from the perspective of effective liquid collection, a side

- entry breech is preferred to a bottom

- entry elbow for wet operation. For tall breeching openings, the vertical edges of the opening should be designed to pos sess adequate stability as a column element. Multiple and single openings under axial compression should be reinforced or proportioned to fully restore the structural capacity of the entire cross section of the liner. Brick liners utilize brick pilasters, and alloy liners utilize vertical jamb stiffeners, to provide vertical

- edge stability. For brick liners, the width of the breeching opening may dictate the diameter of the liner. The ASTM C1298 brick

- liner design guidelines recommend that the opening width not exceed one - half the internal diameter of the liner at the opening elevation. 3.9.3.2 Breech D/W, H/W Ratios Side

- entry breeching

- duct dimensions are typically rectangular, with the long dimension in the vertical direction to minimize the structural effects on the column and liner. It is beneficial to increase the height

- to
- width ratio of the breeching by transitioning the width narrower and the height taller as the breeching approaches the liner and by keeping the floor level. These precautionary step s minimize the amount of liner cross

- sectional area that is interrupted at the breeching openings. Tall, thin breech openings also result in the enhanced generation of strong secondary

- flow vortices in the lower liner. These vortices are used to push the collected liquid films to their respective drain points. Experience has shown that the breech height (H) to width (W) ratio (H/W) of the breeching

- duct opening should be in the 2

– 3.5 range for favorable wet operation. Another dimensionless ratio affecting a breech’s ability to generate strong secondary

- flow patterns is the liner

- diameter (D) to breech

- width (W) ratio (D/W). This value should be between 1.5 and 2.5 for favorable wet operation. 3- 36  3.9.3.3 Liner

- Floor Geometry Stack

- liner floors must be designed to promote the drainage of liquid that will collect in the bottom of a wet stack. Steel and alloy liners can utilize conical hoppers or floors sloped toward the breech side of the liner. FRP liners traditionally utilize a 90 ° mitered elbow, and liquid is collected in a drain at the base of the elbow. Brick liners use a built

- up sloped brick floor with a drainage system. Floors or conical hoppers should be located a sufficient distance below the breeching sill to prevent the liquid that has been collected on the stack bottom from being re

- entrained. EPRI Report CS -2520, Entrainment in Wet Stacks, recommends that the hopper for liners with side

- entry breeches be located a minimum of 1/2 liner diameter below the sill of the breeching

— both to prevent flue gas from sweeping liquid from the floor up the liner wall and to reduce the potential for generation of flow vortices on the liner floor that could re

- entrain liquid back into the gas flow [1]. This distance can be reduced to about 4 ft for cost reduction, but the draining liquid may have to be protected from the gas flow by a floor grating or by covers over the drain locations. Flow modeling can define whether protective grating or baffles are needed and what their geometry should be. F or alloy or FRP liners, th e conical hopper, sloped

- floor, or elbow material should be the same material as the liner. Sloped floors in brick liners are supported from an elevated support structure or by the concrete foundation. Brick

- liner floors consist of multiple layers of mater ials that provide corrosion and thermal protection to their support structures. These floors should be constructed of the following layers of materials: lead pan over asphaltic

impregnated felt, acid

- resistant mortar fill sloped to the drains, and a minimu m of two layers of acid

- resistant brick and mortar. The lead pan should be returned up the side of the brick liner to provide protection to the wall. 3.9.3.4 Sloped Liner Floor Most recent alloy and FRP liners with side

- entry breeches have incorporated a s loped liner floor. This floor can is commonly incorporated into the liner wall, or it can be bottom

- supported within the liner (Figure 3

- 7). Flow

- model studies performed on numerous installations have shown the advantage of a sloped liner floor, both for a reduction in system

- pressure losses as well and for the generation of robust secondary

- flow vortices in the lower liner for enhanced liquid collection. With a sloped liner floor, the secondary flows were found to be stronger and more robust over a wider r ange of plant operations than those generated in liners without a sloped liner floor. The design of the sloped liner floor also results in the formation of a small, quiet sump area below the breech entry for the collection and drainage of liquid from the liner. The final design of the sloped liner floor should be optimized as part of the liquid

- collection physical

- flow

- model study. 3- 37  Figure 3-7 Sloped Liner

– Floor Arrangements A good starting point for materi al

- estimating purposes is to assume that the top point of the sloped floor impacts the rear wall of the liner at a point 2/3 of the way up the height of the breech opposite the opening, and that its bottom point terminates 1 ft (0.3 m) below the floor of t he breech at a point approximately 1/4 of the way across the diameter of the liner. Allowing the bottom edge of the sloped floor to extend below the floor of the breech opening ensures that the bottom edge of the incoming gas stream will not be peeled off and directed into the sump area, thus ensuring that the sump remains aerodynamically quiet for effective drainage. The sloped liner floor can be fabricated as part of the liner wall, or it can be fabricated separately and supported off of the flat liner floor. In the latter case, it is recommended that a 6

– 8 in (150

– 100 mm) gap be provided between the sloped floor and the liner wall, both to allow for differential expansion and to allow liquid flowing down the liner wall to pass into the quiet area behind the sloped portion of the floor. Incorporating the sloped liner floor into the liner wall will reduce the quantity of liner material required. This consideration can be important if the liner is being fabricated from an expensive material such as an allo y. 3.9.3.5 Bottom

- Entry Elbow Bottom

- entry elbows are fabricated from cylindrical sections, typically of the same material and diameter as the liner, which have been cut into mitered sections and joined together to form a 90° elbow. The number of miter cu ts defines the elbow. For example, a three

- miter

- cut elbow has three miter cuts; a four

- miter

- cut elbow has four miter cuts (Figure 3

- 8). Three

- miter

- cut elbows are recommended for favorable wet operation. DRAIN
DRAIN QUIET SUMP AREA SLOPED LINER WALL SUPPOERTED OFF OF LINER FLOOR

SLOPED LINER FLOOR INTEGRATED

INTO LINER WALL 6- 8” GAP TO WALL STACK LINER STACK LINERBREECHBREECH ~1/3 BREECH HEIGHT

~ 1/4 LINER DIA. 3- 38  Figure 3-8 Miter

- Cut Bottom

- Entry Elbow Arrangements It is well understood that, for FRP liners, bottom

- entry elbows are a less expensive approach. Although side

- entry breeches are preferred for wet operation, bottom

- entry elbows with a properly designed liquid

- collection system can be just as effective. Their main disadvantage is that, because of the curvature in the elbow, the liquid

- collection system will require more complex shapes and the existence of strong gas

- recirculation zones exactly in the place where a key liquid

- collection ring must be located. If the ring is not properly designed, these recirculation zones can strip liquid out of the collector back into the gas flow. Bottom

- entry elbows are commonly used on FRP liners because the elbow segments can be fabricated from additional liner cans fa bricated on site. Care must be taken when fabricating elbows to ensure that the sections remain round. As with liner joints, every effort should be made to minimize the joint material on the inside of the elbow and to taper this material back to the base -l iner/elbow material with a minimum 1:6 taper (10:1 preferred) . Discontinuities in the diameters of adjacent sections can create sites for significant levels of liquid re

entrainment back into the gas flow. 3.9.3.6 Liners with Multiple Entries Stacks can be designed to enclose a single or multiple number of liners. Each liner can also be designed to accommodate a single or multiple number of operating units. Economics, plant layout, ductwork layout, and the need to perform maintenance during unit outages s hould be considered when evaluating these options. The capital cost for a stack with multiple liners is typically less than the combined costs of several stacks with individual liners. However, this savings is partially offset by the increased costs associ ated with a larger foundation, longer ductwork, and (possibly) larger pressure

- rise ID fans. Building a single large

- diameter liner to serve multiple units is not recommended, because if maintenance needs to be done on one unit, all the units 

## 3 MITER CUT ELBOW 4 MITER CUT ELBOW 3- 39  feeding into the liner will need to be taken out of service. Multiple units operating within a common liner will also have additional design considerations that will need to be addressed, such as gas

- temperature differentials, variable

flow rates, chaotic flow patterns within the liner, and continuous emissions

monitoring

- system certification. In order to monitor readings from individual units, the continuous

- emissions

- monitoring system for a liner serving multiple units would need to locate its equipment in the upstream ductwork rather than in the stack. Another concern would be that, with one unit off

- line, low

- load operations would result in a low exit velocity, causing potential downwash problems. For good liquid collection, a one

- breeching

- duct

- per - liner design conf iguration is recommended. However, if multiple entries into a single liner exist (or are required) consideration should be given to 1) placing one breech opening directly above the other or 2) using a division wall within the lower liner as a means of sepa rating the lower liner into two separate aerodynamic zones that can be optimized for liquid collection. 3.9.3.7 Division Wall A division wall is a wall within the lower liner designed to separate the lower liner into two (or more) separate aerodynamic zon es. The objective is to decouple the operation of the individual units feeding into the liner from each other so that the flow patterns and liquid collection in each zone can be optimized independently from the others. Division walls typically extend ve rtically to a point at least one liner diameter above the roof of the highest breech opening. The sides of the wall are typically attached to the liner wall, although some designs provide a gap between the division wall and the liner wall to allow for pres sure equalization between the two zones. The division wall must be able to withstand the full force of the gas jet entering the liner through the breech openings. As such, it needs to be designed to withstand the full pressure of the jet, even if the oppos ite equalizing breech is out of service. A significant quantity of liquid will be deposited on the division wall opposite the breech opening, and provisions must be made to collect the resulting liquid film and direct it to the liner wall and floor for drainage from the system. Liquid collectors will also be required along the top edge of the division wall to prevent the re

- entrainment of liquid back into the gas flow. The wall can be placed perpendicular to the incoming gas flow, or at an angle to it, to promote the motion of any collected liquid films from the division wall to the liner wall for drainage. The design, placement, and orientation of a division wall and its associated liquid

- collection system must be optimized in a physical

- flow model of the subject unit. 3- 40  3.9.3.8 Stack

- Entry Pressure Losses The total and static pressure of flue gas inside the stack is a function of the following:

 Ambient temperature and barometric pressure

 Total pressure losses in the stack entrance

 Total pressure frictional losses through the liner

 Total pressure losses across expansion joints, corbel joints, and other stack

liner breeching openings, expansions, or contractions

 Total pressure loss associated with the liner

- inlet geometry

 Pressure variation with height (stack effect)

 Static

- pressure changes across area expansions or conical contractions and chokes Total pressure loss from the stack breeching duct to the stack discharge plume is the value needed for determining the ID

- fan requirements for the stack. The maximum differential static pressure between the flue gas and the stack annulus at the same elevation is the pressure needed to design the fans for the annulus

pressurization system with brick stack liners. For a wet stack

– conversion project, flue gas pressure dat a should be collected and, after correction to saturated nonreheat conditions, compared to model study and calculated values. Details of the pressure

- loss coefficients for various liner

- entrance geometries can be found in [4]. 3.9.4 Liner - Diameter Changes Some stacks incorporate changes in the diameter over the height of the liner. These can be gradual, such as in the case of a tapered brick liner. Or they can be abrupt, such as in the case of a brick liner with multiple corbels, or in a liner in which the top sections have been removed so that the upper section of the shell can be used as the flue. A number of brick stacks have recently been commissioned in which the lower breech

- entry sections were fabricated from alloy to allow for effective liquid collection, which then discharged into a larger

diameter brick liner. The transition between these two sections incorporated a ring collector to prevent any liquid flowing down the brick portion of the liner from entering into the lower alloy portion of the line r. Gradual changes in the liner diameter are usually characterized by a reducing diameter in the direction of the gas flow. This characteristic results in an increase in the liner

- gas velocity with height. Attention needs to be given to ensure that the local liner

- gas velocity does not exceed the recommended velocity for the liner material in use. Abrupt changes in the liner diameters are typically associated with a sudden increase in the diameter of the flue. The transition from one section to another is 3- 41  an ideal location for liquid re

- entrainment back into the gas flow. In these situations, the transition should incorporate a liquid

- collection gutter to drain away the liquid from the upper section before it flows down into the higher

velocity lower sectio n. One notable exception to this rule of thumb is the stack

outlet choke, which incorporates a rapid decrease in the liner diameter in the direction of the gas flow. Stack chokes are unique in purpose and are discussed in detail in Section 3.9.6.2. 3.9.5 I nternal Structures Ideally, stacks should be smooth and clear over their entire height. However, reaching this ideal is not possible, because of the need to incorporate the many structures necessary for the proper operation of both the liner and the plant. Access into the liner in the form of ports for such purposes as testing, continuous emissions

- monitoring, and expansion joints must be incorporated into the liner. Every effort must be made to minimize these structures, and their placement should take into account their potential impact on the fluid

- dynamic and liquid

collection performance of the liner. 3.9.5.1 Liner

- Expansion Joints Alloy and FRP liners sometimes incorporate expansion joints within the liner if required by the liner

- suspension method. These joints are often located at a point less than one liner diameter above the roof of a side

- entry breech or are placed at the outle t of a bottom

- entry elbow. These locations are not favorable for wet operation because they lie in the region where most of the liner liquid

- collection occurs. Liner

- expansion joints are typically longer than their ductwork counterparts
— ranging from 2

– 5 ft (0.6

– 1.5 m) in length and 6

– 12 in (150 –300 mm) in radial depth. The liquid condensing along the stack liner flows down on the liner surface when gas velocities are below the liquid flow

- reversal velocity. This liquid will flow downward until it encounters a liner

- expansion joint. If an expansion joint is encountered, this surface discontinuity will cause all of the downward

- flowing liquid to re

- entrain back into the gas flow by the gas recirculation in the expansion

- joint cavity. Most of these re

- entrained droplets will be discharged from the stack, and many will be large enough to reach the ground before evaporating. For this reason, the liner should ideally be designed so that liner

expansion joints are not needed. If an expansion joint is required, it sh ould be located at least 1 to 2 liner diameters above the roof of the breech duct or the inlet roof of the bottom

- entry elbow. This arrangement will ensure that the surface of the lower liner, where a significant amount of the liner liquid

deposition occur s, is free from discontinuities that would lead to liquid re

entrainment. The expansion joint should be protected from the downward

flowing liquid through the installation of a liner

- ring collector above the joint, or the joint should be equipped with liqu id collectors especially designed for liner

expansion joints. Laboratory flow modeling should be used to design and develop liner expansion

- joint collectors that are effective at all operating loads for a given unit. 3- 42  3.9.5.2 Observation Ports, Test Ports, Man

- ways Observation ports, test ports, and continuous

- emissions

- monitoring (CEM) ports are required on all stacks. These ports are typically 6

– 10 in (150

– 250 mm) in diameter and are flush with the inside surface of the liner. Because of their limited num ber, no special attention with respect to their impact on the liner’s liquid

- film flow is usually given. Some plants have incorporated inverted “V”

- shaped gutters over the ports to direct the downward

- flowing liquid film around the openings on the liner wa ll. A few plants have incorporated large 2 x 2

- ft (0.6 x 0.6

- m) access doors in the liner, typically above the breech opening. The doors to these man

- ways must be flush with the inside of the liner wall. Both because of their size and of their potential for disturbing a significant quantity of the liner’s downward liquid

- film flow, these man

- ways should incorporate liquid

- diversion gutters to direct the downward

- flowing liquid film around the opening on the liner wall. 3.9.5.3 Trusses, Stiffeners, Gusset Pl ates To the extent possible, structures extending into the liner

- gas flow should be discouraged; liquid collected on these structures will be re

- entrained back into the gas flow, with little opportunity to be recollected before discharging from the stack as SLD. The liners should be designed so that all structural supports are on the outside of the liner. Some side

- entry breech openings incorporate internal horizontal bracing to stabilize the tall side - walls of the opening. This bracing is common on FRP l iners with side

- entry breeches, and it usually consists of three to four pipes, 10

– 12 in (250

– 300 mm) in diameter, spaced evenly up the height of the breech opening. Liquid will collect on these supports and will flow around to their downwind side, from wh ich droplets will be re

- entrained back into the gas flow. Liquid flowing on the breech side

- walls can also be drawn into the low

- pressure region on the downwind side of these supports, further increasing the quantity of liquid that will be re

- entrained fro m this location. To prevent liquid from being drawn into the region of flow separation on the downwind side of these horizontal supports, it is recommended that liquid

- flow

– prevention baffles, in the form of 18 in (450 mm) in diameter disks, should be plac ed on the supports
— located 18 in (450 mm) from, and parallel to, the breech side

- walls. Additional details of these disks are presented in Section 3.11.5. Unless absolutely necessary, turning vanes, vortex breakers, and other flow

control devices in the en trance region of the liner should be avoided, because these devices will act as locations for liquid collection and re

- entrainment back into the gas flow. Droplets generated from this location will have little opportunity to be recollected before exiting the top of the stack as SLD. 3- 43  3.9.6 Stack - Exit Design For most modern installations, the stack exit is simply the end of the liner. If the liner velocity is within the recommended range for the liner material used, no special attention needs to be given to this area. If the liner velocity is near or above the liquid flow

- reversal gas velocity, the outlet of the liner should incorporate a method for collecting the upward

- moving liquid film. Details of stack

- outlet liquid collectors are presented in Section 3.1 1.10. 3.9. 6.1 Liner Extensions Stack

- liner extensions are a common method of minimizing the potential for plume downwash. Extension heights are determined as a result of a computational flow

- model study. Such a study should be performed to investigate a stack’s potential for plume downw ash, using site

- specific stack design and weather conditions. Typical liner

- extension heights range from 0.5 to 1 liner diameters above the top of the stack shell. Liner extensions are typically not insulated so that they will remain above freezing during the winter months, and ice will not form on them. This situation will only apply if the liner is operating at or below the recommended velocity for the liner material used because, during the winter, the condensation rate in the uninsulated extension will increase significantly as it is exposed to the cold environment. A liner operating at or below the recommended gas velocity should be able to easily handle the increased condensate load on the liner wall. If the liner is operating above the recommended gas velocity, the extension should be insulated to minimize the increase in condensed liquid on, and potential droplet re

- entrainment from, the liner wall. Heat

- tracing may be required to control icing on the exterior of the extension and stack roof. 3.9. 6.2 Chokes Stacks that operate with a low discharge velocity sometimes have problems with downwash or insufficient plume rise. To reduce these problems, a choke can be incorporated at the top of the liner. Chokes are designed to increase the gas

- exit veloci ty by decreasing the discharge diameter. It must be noted that the use of a choke, which constitutes a "dispersion technique" under EPA stack

- height regulations, cannot result in a relaxation of the emission limitation for the facility. Chokes will increas e draft losses, and the fan

- pressure rise should be selected accordingly. Chokes will also cause a positive pressure inside the liner and increase the leaching problems for a brick liner. Chokes can be fabricated from corrosion

- resistant alloys and FRP. If a choke is installed on a wet stack liner, a choke liquid collector will be needed at the top of the choke, because the choke discharge velocity usually exceeds the liquid

- reversal velocity for the choke surface material. The choke surface material for recent choke designs is nickel alloy or FRP, even for brick liners. 3- 44  In stacks that have chokes, some of the fine droplets entrained in the gas flow will be deposited on the choke surface, and the liquid collected on the choke will lead to SLD if the local ga s velocities in the choke are high enough to exceed the flow

- reversal velocity. The gas velocity on the choke increases from the liner velocity, up to an 80

– 120 fps (24.4

– 36.6 m/s) exit velocity, which is the usual design range for chokes. The choke exit velocity normally exceeds the flow

reversal velocity. Flow

- reversal velocity is the flue gas velocity at which the flow of the liquid on the stack walls is reversed from down - to up
- flow. The magnitude of the liquid discharge is a function of the choke geom etry, the gas - velocity variation through the choke, the droplet

- size distribution, and the spatial variation of the different

- sized droplets. Flow

- reversal velocity is in the range of 70

– 90 fps for common liner materials [1]. Therefore, liquid film will be flowing upwards over a large percentage of the choke surface for most choke geometries. The liquid must be collected at the top of the choke and drained out to reduce the SLD. 3.9. 6.3 Rain

- Hood and Roof Design The stack rain

- hood shape is designed to cove r the gap between the shell and the single liner. A hood is typically installed on a stack in which the shell to liner

diameter ratio is small. Personnel access to this area will usually be limited. A stack cap should be fabricated from a corrosion

- resista nt material such as FRP and be angled downward away from the liner to promote drainage. Stack hoods or caps are engulfed by wet stack gas all along their perimeter, and icing will occur in cold climates. The amount of icing can be both minimal or large and dangerous. Hoods are typically left over from the dry stack design and are not favorable for wet operation. A liner extension above a shell with a roof is favorable. A stack roof is used on stacks in which the shell to liner

- diameter ratio is large and t he liner extends above the roof. In such a case, the annulus is larger, and personnel access is allowed to the roof. For this reason, a stack roof is structurally stronger, and it incorporates a guard rail or parapet around its perimeter. Roofs should be provided with a slope toward drains, leading the collected liquid to the ground. Typically, roofs and caps are not insulated, so that heat from the annulus can be used to eliminate icing from this area. 3.9. 7 Cyclonic Flow and Continuous - Emissions Monitors EPA 40 CFR Part 60, Appendix A, Method 1 states that the flow condition is unacceptable if the average yaw angle is greater than 20°. If potential flow

measurement problems are significant, it is recommended that a 3

- D traverse be performed to more accura tely determine the flow characteristics of each unit. These characteristics must be discussed with the regional EPA to determine acceptability. If unacceptable, an evaluation of flow

- straightening must be done. 3- 45  Although typica lly undesirable from a wet

- operation point of view, such an evaluation could include the addition of straightening vanes near the monitor location and/or a modification of the deflection plates at the base of the stack. Any modifications would probably require a three

- dimensional flow

- model study. The CEM

- system requirements for wet stack applications are generally the same as for unscrubbed applications, with the exception of monitoring opacity. Opacity monitors will not function properly in a wet stack because water droplets will result in high opacity readings. The EPA has exempted wet stack units that are otherwise affected by the acid

- rain program from monitoring opacity. However, no such exemption has been granted for new source performance standard un its. In addition, there may be local requirements for monitoring opacity. EPA regulations specify that a quality

- control program must include calibrations and calibration checks for the CEM equipment installed. Provisions for field testing need to be made

— not only to meet regulatory requirements, but to provide preventive maintenance and to ensure proper functioning of the monitoring equipment. It may be required to cut holes in existing c himney liners and concrete shells in order to get the probes through. Access into the annulus space must also be considered. For units converting to wet stack operation, flow monitors need to be recalibrated for the revised flow conditions. CEM

- system cons iderations include provisions for field testing, installation and/or modification of platforms and access systems, and compliance to EPA testing requirements. According to EPA test methods for gas sampling (EPA 40 CFR Part 60 Appendix A), flow monitors sho uld be located a minimum of eight stack or duct diameters downstream, and two diameters upstream, from any flow disturbance. If necessary, location of two stack or duct diameters downstream, and one

- half diameter upstream, from a flow disturbance may be em ployed. The purpose is to make sure that the flue gas

– sample data obtained are precise enough that the total emissions can be accurately calculated. Minimum testing requirements are satisfied if the EPA criteria for port locations are adhered to and if no cyclonic or stratified

- flow conditions exist. Extensions to existing platforms may be required to provide access to new port locations. Existing ports may be reused wherever possible. If ultrasonic flow monitoring is used, an additional platform landing w ill be required, located approximately one liner diameter above the main testing platform. Modification to existing handrails may be required at certain locations to provide clearance for the monitoring

- equipment enclosure boxes. To provide continuous acce ss to the monitoring location and ensure personnel safety, an elevator is advisable for accessing equipment on tall stacks. Platform enclosures may be needed to provide personnel and equipment with weather 3- 46  protection during the winter months. For personnel working in enclosed conditions, adequate ventilation should be ensured. 3.10 Miscellaneous Materials of Construction Because of the corrosive environment associated with FGD operations, materials on the stack other than the liner also need to be evaluated

. 3.10.1 Exterior of Shell Gas

- exit velocities on WFGD stacks should be designed for a lower velocity than for reheat or bypass stacks. Consequently, plume rise on the gas exiting a wet stack is not as high. With the gas velocity low and heavily saturated with liquid, it is common for the flue gas to come in contact with the upper portion of the stack. This condition is called downwash. As the gas comes in contact with the concrete shell, liquid condenses on the concrete. Over time, this condensate can caus e the deterioration of unprotected concrete on the upper portion of the stack. To protect the upper portion of the concrete stack against downwash, a protective

- coating system should be applied to the top of the chimney. The actual height that should be c oated is typically determined as part of a computational plume

- downwash flow

- model study. For estimating purposes, a coated height of 50

– 100 ft (15

– 30 m) can be used. Applying two coats of an acrylic

- emulsion coating is an acceptable protection system for the concrete surface. A high

- build (4

– 6 mils dry

- film thickness) epoxy coating has also been used for this application. The painting system selected should be checked for compatibility with the concrete curing compound and with any FAA painting requirement s. 3.10.2 Exterior of Adjacent Stacks A stack that is located adjacent to and downwind of another stack may be subject to the corrosive effects of flue gas impinging on its concrete surface. If the upwind stack is significantly shorter than the downwind st ack, the potential for concrete corrosion is increased. This is a common occurrence in retrofit WFGD installations, which incorporate new shorter stacks close to the old original stack. The potential for plume impingement on a taller adjacent stack can eas ily be estimated using a computational flow

- model study and can usually be incorporated into the plume

- downwash study being performed for the new stack. If the expected duration of plume impingement on the adjacent stack is high, it can be protected by app lying two coats of an acrylic

- emulsion coating system. 3.10.3 Interior of Shell for Use with Brick Liners For a stack with a brick liner, flue gas may enter the annular space of the stack if the liner is leaking or if the annulus

- pressurization system is not performing adequately. This condition can be severe enough to result in corrosion of the 3- 47  interior surface of the concrete shell. Additional protection should be provided by applying two coats of an acrylic

- emulsion coating system (or its equivalent) to the interior concrete surfaces. 3.10.4 Liner Bands Liner bands can be a potential maintenance concern if the band material or coating system is not properly selected to resist the corrosive environment. Uncoated carbon

- steel bands have failed on several s tacks. A coating system using a first coat of high

- build epoxy, with a second coat of polyurethane, can be used for this application. However, in severe environments with excessive leakage through the liner, this coating system may not be adequate, and all oy bands will be required. 3.10.5 Rain Hoods and Roofs The rain hood at the top of the stack is exposed to flue gas, so the rain

- hood material must be able to withstand this corrosive environment. Liquid re

entrained in the flue gas is deposited on the rai n- hood surface. In addition, the continuous cycle of evaporation and deposition increases the acid content of liquids deposited on the rain

- hood surface. Accordingly, the rain

- hood material must resist higher acid concentrations than the liner. FRP is an e xcellent choice, because this material is lightweight and corrosion

- resistant. Stainless steels have been used on occasion for rain hoods, but most stainless steels cannot resist the high acid concentrations that may occur at these locations. Use of coat ed carbon

- steel rain hoods is not recommended for rain

- hood applications because of maintenance concerns for the coating system. Stacks with multiple liners use a roof instead of a rain

- hood system at the top of the stack. Roofs should be provided with a minimum slope of 1/4 in/ft to permit adequate drainage. Roof materials consistent with those referenced for rain hoods should be used. In addition, concrete roofs can be utilized. Type V sulfate

resistant cement is recommended for the concrete. A coating sy stem can be applied to the top of the concrete to provide additional protection. 3.10.6 Lightning - Protection System Because most of the items associated with the lightning

- protection system are located at the top of the stack, materials of construction for these items must provide adequate corrosion resistance. The air terminals should be fabricated from a solid rod made of Alloy C276. Conductors, attachments, and fasteners located within the top 25 ft (8 m) of the stack should be lead

- covered. 3- 48  3.10.7 Plat forms and Ladders To provide adequate protection against downwash and liquid fallout, exterior platforms and ladders located within the top 100 ft (30 m) of the stack should be fabricated from a corrosion

- resistant material. These include all platform stee l, grating, handrails, ladders, ladder fall

- prevention devices, and attachments. Typically, Type 317LMN stainless steel provides sufficient corrosion protection at these locations. Items fabricated from Type 317LMN are not readily available, and material s ubstitutions may need to be evaluated. 3.10.8 Electrical Conduit All rigid steel conduits and accessories within the top 100 ft (30 m) on the exterior of the stack should be installed with a bonded PVC jacket. Conduit located within the top 100 ft (30 m) on the exterior of the stack should be supported with Type 316 stainless

- steel unistruts, clamps, and fasteners. Because of corrosion resistance and weather

- tight requirements, all electrical boxes located within this area should be NEMA 4X. 3.11 B asic Liquid - Collector - System Design Saturated gas flow and condensation on the surfaces are normal operating conditions for ducts and wet stacks. Some mist

- eliminator liquid carryover, and the liquid condensed from the large amount of water vapor present in the gas, will also increase the liquid load in the ducting and in the stack liner. Liquid

collection devices and drains are necessary to collect and drain the liquid in the absorber

- outlet ductwork and in the stack before it exits the chimney. Liquid col lectors and drains are therefore an important design consideration in a duct/stack system used for wet operation. The basic behavior of gas and liquid flows are described in Section 2, "Wet Stack Design Fundamentals," whereas the guidelines for selecting t he duct and stack geometries suitable for wet operation are discussed in Section 3.6, “Ductwork Design,” and Section 3.7, “Stack

- liner Design,” respectively. The following sections of the Revised Wet Stack Design Guide provide an update as well as additional detail for the basic liquid

- collection designs previously presented in [1] and [2]. The successful design and operation of a wet stack liquid

- collection system starts with the mist eliminators and includes all elements of the absorber duct, breeching duct, stack entrance, stack liner, and stack outlet. Each element along the gas

- flow path is important. The liquid collectors collect all the deposited and condensed liquid from the duct and stack surfaces and direct it to drains for removal from the system. The type, size, geometry, and location of liquid collectors are selected to utilize the system’s gas - shear and gravitational forces to move and collect the liquid on the duct and liner surfaces and drain it out of the system before re

- entrainment can tak e place. 3- 49  The actual design and optimization of the liquid collectors and drains is site

specific work, and it depends on the absorber type, the duct/stack geometry, the planned operating

- system operating conditions, and the gas

- velocity levels. Because t he resulting three

- dimensional gas - and liquid

- flow patterns are unique for each unit, the liquid

- collection system must be optimized for each specific unit. A flow

- model study should be performed by a flow

- modeling company experienced in state

- of

- the

- art wet stack design to determine the optimum location and configuration of flow controls, liquid

- collection devices, and drains. Liquid collectors and drains will be required on any unit operating with wet ducts and a wet stack. Because the collected liquid is acidic, the material selection for liquid collectors and drains must be evaluated. The most common material for liquid collectors are corrosion

- resistant alloy materials and FRP. For new installations, a general system arrangement and liquid

- collection system can be designed from the start for favorable wet operation. For units converting from dry to wet operation, a number of unique considerations must be addressed. The amount of vapor and liquid carryover from the mist eliminator is the same for both new and retrofit applications. However, in many retrofit applications, the gas velocities will be 5

– 10% lower than before conversion. Nevertheless, these velocities are still usually higher than the gas velocities that would be selected for the design of a new wet stack for the same gas

- flow rate. Because of the amount of liquid condensation and deposition on the duct and stack surfaces and the relatively high gas velocities, design of liquid collectors and drains is an important part of the retrofit proce ss. Mist

- eliminator operation and performance need to be reviewed before a conversion to wet operation. Mist

- eliminator washing procedures will need to be optimized to limit the amount of liquid

- droplet carryover. In some instances, it may be necessary to modify or replace the mist eliminators or mist

- eliminator wash system. Mist eliminators need to be monitored, inspected, and exchanged if damaged or if solid scaling is excessive. The pH control needs to be good to ensure that solid precipitation in the mist eliminators and the downstream duct work is minimized. Some units undergoing conversion to wet operation may have turning vanes or other flow

- control devices in the gas path between the absorber outlet and the stack. These devices are common on units wi th multiple absorption modules and interconnecting outlet ducting. If this is the case, liquid

- collection devices will probably need to be installed on them to minimize the potential for re

- entraining deposited liquid back into the gas flow. The following sections discuss the basic liquid collectors typically found in most effective wet stack installations. These discussions are designed to provide engineers who are considering wet stack operation a basic understanding of the design, location, and installa tion details of these collectors, as well as a basic understanding of their purpose and operation. Although exhaustive, this list is by 3- 50  no means complete, because many units often require custom

- designed collectors to address local gas - and/or liquid

- film flow issues unique to the unit under consideration. With the information provided in the following sections in hand, engineers should be able to understand the operation of the overall liquid

collection system. They should also have enough information to e stimate, with some degree of accuracy, the quantity and types of materials required for a liquid

collection system, along with their fabrication and installation costs. 3.11.1 Side - Wall Gutters Good liquid collection in the ducts means reduced liquid load in the stack and a reduced potential for SLD. In horizontal rectangular

- duct sections, simple slanted side

- wall collectors can be used to direct the liquid film flowing on the duct wall to the floor. These collectors are usually L

- shaped in cross section, and their sizes range from 2 x 2 in to 6 x 6 in (0.05 x 0.05 m to 0.15 x 0.15 m), depending on duct geometry, gas velocity, and expected level of liquid and solid carryover from the absorber. The L

- shape faces into the gas flow to form a collection gutter. Side

- wall gutters typically start at the roof of the duct and are sloped 10

– 15° in the direction of the gas flow (high side into the flow) to promote drainage toward the floor. These gutters can also be placed vertically if they are being used to keep the liquid film on the wall from encountering an area where re

- entrainment is likely to occur

— such as immediately upstream of a duct

expansion joint or at the edges of the breech where they meet the stack liner. The side

- wall gutters typically terminate 6

– 8 i n (15

– 200 mm) above the floor of the duct to promote drainage and prevent pluggage. Depending on the level of mist

- eliminator carryover, duct length, and duct geometry, multiple side

- wall gutters may be required down the length of the duct. Also, short sections of wall gutters may be necessary at specific locations to direct the liquid film on the duct wall away from an area where re

- entrainment might occur as a result of flow separation. These locations would be identified as part of the recommended flow

- model study. Round ducts also need to be equipped with liquid collectors. Round ducts usually require fewer collector elements, but they are more difficult to fabricate and install because they need to follow the curved surface contour inside the duct. Th ese collectors are in the form of a ring that can go completely around the perimeter of the duct with a drain at the base. Alternatively, a 10

– 20° section at the bottom of the ring can be removed to allow the collected liquid to flow along the floor of the duct toward a drain point. Similar to the basic side

- wall gutter, a round

- duct wall

- ring collector will be sloped 10

– 15° in the direction of the gas flow (high side into the wind) to promote collection and drainage to the bottom of the duct. Because the r ing will be at an angle, and not perpendicular to the axis of the circular duct, the resulting ring will be in the form of an ellipse. Circular duct liquid

- collection rings are usually L

- shaped in cross section, and their sizes range from 2 x 2 in to 6 x 6 in (50 x 50 mm to 150 x 150 mm), depending on duct geometry, gas velocity, and expected level of carryover from the absorber. The L

- shape faces into the wind to form a collection gutter. 3- 51  To be effective, the side

- wall and ring gutters must be sealed to the duct wall along their entire length. Details of basic side

- wall and round

- duct ring collectors are presented in Figure 3 -9.

Figure 3-9 Side

- Wall and Round

- Duct Liquid

- Collection Gutters 3.11.2 Ceiling Collectors Liquid will also condense and deposit on the roof of the duct. This liquid film will flow in the direction of the local gas flow. The resulting li quid

- film thickness will increase as the film moves toward the liner until the droplets fall from the roof. Many of these droplets may re

- entrain back into the gas flow. To prevent this from happening, gutters are attached to the roof of the duct to direct the moving liquid film to the side

- wall, where it will then flow onto the wall and then down to the floor. Ceiling gutters are very similar to side

- wall gutters in size, shape, and orientation with respect to the local gas

- flow direction. To be effective, the gutter must be sealed to the duct roof along its entire length. Depending on the local gas

- flow patterns, a ceiling gutter either can be a straight gutter running at a 10

– 45° angle from one side

- wall to the opposite wall, or it can be configured in th e shape of a “V,” with the point of the “V” upwind. This arrangement directs the collected liquid film to both side

- walls simultaneously (Figure 3 -10). As detailed in Figure 3

- 10, the ends of the ceiling collectors are trimmed back at a 30° angle where the y attach to the duct side

- walls. This cutback provides a means for any gas collected in the gutter to leak out of the gutter in a controlled 10-20°

6-8" (150-200mm) 20° SECTION AA

A A

A A

GAP TO FLOOR GAP OPENING 2- 6” (50

- 150mm)

ANGLE DUCT WALL SEALED TO DUCT WALL LOCAL GAS FLOW

DIRECTION

LOCAL GAS

FLOW DIRECTION LIQUID FILM FLOW PATTERNS

RECTANGULAR DUCT CIRCULAR DUCT 3- 52  manner as the liquid is directed onto the side

- wall, minimizing the potential for the scouring of the collected liq uid back into the gas flow.

Figure 3- 10 Ceiling Liquid

- Collection Gutters In some instances, the local gas velocity may be opposite to the direction of the bulk gas flow. This phenomenon is a common occurrence along the roof of vertical transitions, particularly in the transition connecting the absorber

- outlet ducting to the stack breech. If the vertical expansion is abrupt, a region of flow separation will be formed along the roof of the transition, and liquid collected and/or condensed on the roof of the transition will flow downward back toward the absorber. In such a situation, a ceiling “V” collector would need to be installed, with the point of the “V” aiming up the slope of the roof. This “V” co llector will prevent the collected liquid film from reaching the bottom of the transition roof, where it would be completely re

- entrained back into the gas flow. By orientating the “V” uphill, the downward

- flowing liquid film will be directed to the duct s ide

- walls, where it will ultimately be directed to the duct floor for collection The size, shape, and location of ceiling collectors can only be accurately determined through a flow

- model study of the unit. 10-20°

DETAIL 1

30° 

### SECTION AA

SECTION BB

DETAIL 1 A A B B

DIRECTION OF LOCAL LIQUID FILM FLOW DIRECTION OF LOCAL LIQUID FILM FLOW

DIRECTION OF LOCAL LIQUID FILM FLOW
DIRECTION OF LOCAL LIQUID FILM FLOW 2- 6” (50

- 150mm) ANGLE DUCT ROOF

SEALED TO DUCT ROOF

DIRECTION OF LOCAL LIQUID FILM FLOW DIRECTION OF LOCAL LIQUID FILM FLOW 10

- 45 ° 3- 53  3.11.3 Floor Gutters In most modern WFGD installations, the absorber is located close to the stack, with a short duct connecting them. This ducting is typically sloped 1

– 1.5° either toward the liner or back toward the absorber to enhance liquid drainage

— mainly during unit shut down. For these arrangements, floor gutters are typically not required. For older units and/or units with long and/or complicated duct runs between the absorber and the stack liner, floor gutters may be required to control the flow of collected liquid along the duct floor

— usually toward a drain. A trench or scupper installed across the duct in the floor with a drain is also effective. Floor gutters are also used to prevent liquid from flowing into ducts that are connected to the floor of the absorber

- outlet duct, as is common for units converting from reheat operation. Floor gutters are typically angled to the gas flow to promote liquid flow to a specific location such as a drain. The gutters are usually L

- shaped in cross section, and their sizes range from 2 x 2 in to 6 x 6 in (50 x 50 mm to 150 x 150 mm), depending upon duct geometry, gas velocity, and expected level of carryover from the absorber. The L

- shape faces into the wind to form a collection gutter. To be effective, the gutter must be sealed to the duct floor along its entire length. 3.11.4 Flow- Control Liquid Collectors Many new and retrofit WFGD installations will have, or will require, turning vanes located in the wet gas path between the mist eliminator and stack outlet. Liquid condensed and de posited on these devices will be dragged to the downwind or trailing edge of the device, at which point the collected liquid film will detach, re

- entraining liquid back into the gas flow. In horizontal ducts, turning vanes and baffles are very effective f or promoting deposition of suspended droplets of all sizes and are frequently recommended if additional droplet deposition is needed. In many instances, the additional cost of these devices may be justified by the combined effect of reduced pressure loss a nd the additional liquid collection generated. On the other hand, horizontal to vertical turning vanes in the stack entrance should be avoided, because droplets re - entrained from these devices will have little opportunity to be collected before exiting fr om the top of the liner. If turning vanes or other types of flow

- control devices are present in the system (or are required), liquid collectors will be necessary to minimize the potential for liquid re

- entrainment back into the gas flow. These collectors are typically located on the trailing edge of the pressure side of the vanes. However, depending upon the orientation of the vanes, collectors may be needed on the back or low

- pressure side of the vanes as well. Like the side

- wall gutters discussed previously, trailing

- edge liquid collectors are L shaped and range in size from 2 x 2 in to 3 x 3 in (50 x 50 mm to 75 x 75 mm), depending on duct geometry, gas velocity, and expected level of carryover from 3- 54  the absorber. The L

- shape faces into the wind to form a collection gutter. The gutters are typically placed on the vane or a vane extension in a slanted orientation, angled 5

– 30° relative to the perpendicular of the gas flow. This arrangement will direct the collected liquid film to the side

- wall or the floor of the duct, where it can drain onto the wall or floor and, ultimately, to the duct floor. To be effective, the gutter must be sealed to the turning vane along its entire length. Similar to the ceiling gutters, the ends of the flow

- control collectors are trimmed back at a 30° angle where they attach to the duct side

walls. This orientation provides a means for any gas collected in the gutter to leak out of the gutter in a controlled manner as the liquid is directed onto the side

wall. This arrangement mini mizes the potential for the scouring of any collected liquid back into the gas flow. The gas - and liquid

- film flow patterns in and around turning vanes can be very complicated, and the development of effective liquid collectors is difficult. It is highly r ecommended that a flow

- model study be performed to develop and optimize these collectors. 3.11.5 Internal Duct Supports and Expansion Joints To the extent possible, all structural supports should be located on the exterior of ducts used for wet operation. Internal bracing is often installed in rectangular ducts on retrofit units that originally operated dry. Horizontal bracing is also commonly found supporting the tall side

- walls of breech openings on FRP liners with side

- entry breeches. These internal support structures are generally not favorable for wet operation because they provide additional impingement surfaces and re

- entrainment sites for liquid droplets. Details of internal truss

- support designs for wet operation were previously discussed in Section 3.6.1.3. Horizontal braces in side

- entry breeches are a unique problem because of their large diameter and because they are located very close to the stack liner. When the liquid film flowing on the duct wall encounters a horizontal brace, a portion o f the film will flow onto the support and be pulled toward the center of the duct in the recirculation zone on the back or downwind side of the truss. Liquid pulled into this zone will flow along the back side of the brace, re

- entraining back into the gas flow along its entire length. Because of their close proximity to the liner, many of these droplets will pass through without being recollected. To prevent this from happening, a circular disk approximately 10

– 12 in (250 –300 mm) in diameter can be added to the brace from 6

– 12 in (150

– 300 mm) off of the duct wall (Figure 3

- 11). These disks have been proven to be effective in keeping the liquid film on the duct wall from spreading onto the support and being pulled into the center of the duct, where it will be re - entrained back into the gas flow. 3- 55  Figure 3- 11 Horizontal Brace with Liquid -Re- entrainment

– Prevention Disk Expansion joints (although required for duct design) are not effective for wet operation, beca use they form discontinuities on the duct surfaces (wall, ceiling, floor), from which the flowing liquid film can be re

- entrained back into the gas flow. The liquid films approaching an expansion joint should be collected and directed to the duct floor by appropriately designed liquid

- collection gutters upstream of the joint. It is recommended that expansion joints in horizontal ducting be drained. Liquid will naturally collect in the bottom cavity of these joints, and they are ideal locations for eliminating collected liquid from the system. Depending upon the design of the joint, the drain connection can be made through the rubber boot or through a side structural member of the joint. A 2 –3- in (50 –75- mm) drain pipe should be sufficient. The drain piping wi ll need to pass through a seal pot or run directly back to the absorber reaction tank to a point below the lowest liquid level to prevent gas from back

- flowing up the drain pipe, potentially resulting in liquid re

- entrainment back into the gas flow. If the expansion joint is not drained, it will fill with liquid. And any liquid flowing along the duct floor will flow over the joint as it is pushed by the gas flow toward the liner. Duct misalignments, or a rubber boot protruding into the gas path, may be sufficient to generate regions of flow separation that could scour liquid from an undrained joint back into the gas flow. As described previously, in Section 3.6.1.4, units with internal duct supports will typically have a brace located on either side of the e xpansion joint. If the truss support attaches to the duct floor upstream of an expansion joint filled with liquid, the recirculation zone formed on the downwind side of the attachment gusset will scour the collected liquid out of the joint back into the ga s flow. To prevent this, a cover plate should be placed over the joint, extending approximately 2 ft (0.6 m) on either side of the attachment gusset.DUCT WALL 10

- 12” (250

- 300mm )

DIA DISK

GAS FLOW 6- 12” (150

- 300mm)

OFF WALL 3- 56  3.11.6 Stack - Entrance Collectors – Side - Entry Breech The liquid

- film flow patterns in a wet stack can be defined by two basic aerodynamic zones: the lower zone, typically extending from the liner floor to a point approximately 1.5 to 2 liner diameters above the roof of the breech inlet, and the upper zone above this point (Figure 3

- 12). The lower region of the liner is defined by complex multidimensional gas - and liquid

- film flow patterns, whereas the upper zone is defined by generally well

- behaved flow patterns. Most liner

- liquid collectors are located in the lower zone.

Figure 3- 12 Upper and Lower Stack

- liner Aerodynamic Zones The stack liner opposite the breech opening on a side

- entry liner and along the outer radius of stacks with a bottom

- entry elbow generally receives more liquid deposition than any other area downstream of the absorber. Effective liquid collection in the st ack

- entrance zone is therefore critical for both inlet arrangements. As described previously, when gas flows though a 90° turn, higher gas velocities are experienced along the outer radius of the turn, and two counter

rotating secondary vortices are generated at the outlet of the turn. Similar gas

flow patterns are generated in the entrance region of liners as the gas flow turns vertically up the stack. These gas

- flow patterns both drag the deposited liquid vertically up the liner and also push them circumferentially around the inside of the liner back toward the breech opening. 1- 2 Liner Diameters Above Roof of Breech Arrows Show Liquid Film Flow Patterns

Main Droplet Impact Zone Upper Aerodynamic Zone Lower Aerodynamic ZoneBreech 3- 57  The liquid

- collection systems in the stack

- entrance region of liners with a side

entry breech or a bottom

- entry elbow are basically similar, but they have a number of geometry

- speci fic differences. Figure 3

- 13 presents a generic liquid

- collection

– system arrangement for the lower liner of a stack with a side

- entry breech. This arrangement consists of a “T”

- shaped sloped

- ring collector, typically 6 x 6 x 6 in (150 x 150 x 150 mm), used to separate the upper and lower aerodynamic zones of the liner. The upward

- facing portion of this collector is used to collect liquid flowing downward from the upper liner, whereas the downward

- facing portion of the gutter is used to help direct the verti cally moving liquid film along the rear wall of the liner circumferentially around toward the region of lower gas velocity located along the front wall of the liner above the breech opening. The liquid film in this region generally flows downward toward the top of the breech opening. To collect this liquid, a gutter is located across, and extending slightly beyond, the top of the breech opening. This gutter is typically deep, 16

– 24 in (0.4

– 0.6 m), and incorporates a number of strategically located vertical or angled tabs to prevent scouring of liquid back into the gas flow. The floor of this gutter is sloped 1

– 2° from its center point to its outer ends to promote the movement of collected liquid to drain pipes located at each end. These 6 –8- in (150 –200- mm) d rain pipes run down either side of the breech opening, terminating 8

– 12 in (200 –300 mm) above the liner floor. Liquid collected in the upward

- facing portion of the liner

- ring collector is drained into the breech

- top gutter through a drain box running down the front wall of the liner. This drain box is sealed to the liner wall and incorporates wing extensions to form two vertical gutters running along the sides of the box. These gutters provide stability to the liquid film flowing circumferentially around t oward the front wall of the liner, directing it into the breech

- top gutter. To prevent the circumferentially moving liquid film from re

- entraining back into the gas flow along the sides of the breech opening, two large “wing” collectors are located on eit her side of the breech, running from the breech

- top gutter to the bottom of the drain pipes (Figure 3

- 14). These wing collectors are generally L shaped, but have sides of 8 in (200 mm) and 18

– 24 in (450

- 600 mm). The collectors are positioned such that the 8- in (200

- mm) leg is attached and sealed to the liner wall adjacent to the breech opening, and the 18 –24- in (450

– 600

- mm) wing is roughly parallel to the liner wall

— angled slightly so that the resulting opening or gap to the liner wall at the opposite edge of the wing is 4 –6 in (100

– 150 mm) wide. The gap to the liner wall is maintained by support rods or straps located approximately every 4 ft (1.2 m) up the height of the gutter. This gutter is designed to collect the liquid film flowing circumferentially ar ound the inside of the liner from the main droplet

- impact zone on the rear wall of the liner back around toward the breech opening. In many wet stack installations, the breech

- top gutter
- drain pipes will be located inside the channel formed by this gutter

. 3- 58  3.11.7 Stack - Entrance Collectors – Bottom - Entry Elbow In units incorporating a bottom

- entry elbow, the lower

- liner liquid

- collection system is basically similar to the side

- entry breech arrangement. However, accommodations are required to account for the curvature of the ducting in the elbow (Figure 3

- 15). Similar to the side

- entry breech arrangement, the gas velocity opposite the stack inlet (in this case, along the outer radius of the elbow) will be higher than average, and two counter

- rotating secondary vortices will be formed. Liquid deposited on the outer radius of the elbow will be pushed vertically by the high

- velocity gas flow and pulled circumferentially around toward the front wall of the liner by these secondary

- flow vortices. A “T”

- shaped ring collector, shown in Figure 3

- 15 as the lower ring colle ctor, is used to separate the upper and lower regions of the liner and to help direct the circumferentially flowing liquid film in the lower section around to the front wall of the liner. When a liner

- expansion joint is used, a second vertically facing L

shaped gutter, referred to as the upper ring collector, will be necessary immediately above the joint to prevent any down

- flowing liquid from encountering the expansion joint. When the liquid reaches the region of low gas velocity above the elbow inlet, it will naturally flow downward because of the reduced vertical gas velocity in this area. Because a breech

- top gutter cannot be used, a second (or lower) ring collector is located along the uppermost angled miter cut of the elbow. This upward

- facing lower r ing, typically 6 x 6 in (150 x 150 mm), directs the collected liquid to a location on the outer radius of the elbow

— where the vertical gas velocities are not high enough to push the liquid vertically. Discharged at this point, the collected liquid will dra in downward to the floor of the inlet elbow. Similar to the side

- entry breech arrangement, the upper ring is drained into the lower ring through a drain box connecting the two rings. Because of strong gas vortices formed along the inner radius of the elbo w in the vicinity of the lower ring, numerous vertical tabs may be required in the lower ring collector to prevent these vortices from scouring liquid out of the ring back into the gas flow. If the vortices are strong enough, the ring may need to be partia lly covered in these regions. The extent of scour protection required can only be determined by a flow

- model study of the unit. 3- 59  Figure 3- 13 Typical Side

- Entry Breech Liquid

- Collection System

Figure 3- 14 Side

- Entry Breech Wing

- Collector Design SECTION AA

SECTION BB

SECTION CC 

## A

B

C A

B

C UPPER RING COLLECTOR EXPANSION

JOINT LOWER RING COLLECTOR

DRAIN BOXBREECH WING COLLECTOR BREECH TOP

GUTTER

BREECH TOP GUTTER DRAIN PIPE DRAIN BOX

WING COLLECTOR DRAIN BOX BREECH TOP GUTTER

LINER SUMP LINER DRAIN UPPER RING COLLECTOR

Horizontal Section Through Wing Collector and Breach Top Gutter Horizontal Section Through Wing Collector Below Breach Top Gutter Breech Top Gutter Drain Pipe

Wing 4- 6” (100

- 150mm) Open Gap to Wall Breech Top

Gutter Breech Wall 3- 60  Figure 3- 15 Bottom

- Entry Elbow Liquid

- Collection System with Liner

- Expansion Joint 3.11.8 Liner Collectors The gas - and liquid

- flow patterns in the stack

- entrance region of a liner are complex. In the upper portion of a liner, the gas

- flow patterns are typically well behaved, and l iquid condensing within the stack liner in this zone will flow downward if liner

- gas veloc ities are below the liquid

- reversal velocity. The lower ring collector described in the previous sections is designed to separate these two zones. The upper portion of the gutter collects the downward

- flowing liquid film, whereas the downward

- facing portio n of the gutter is used to direct the moving liquid film in the lower liner to the quiet region above the liner

- breech opening. This ring collector is typically angled 5

– 10° from horizontal to promote drainage, with the low point of the ring located on the front wall of the liner. Liquid collected in this ring is directed to the breech

- top gutter on units with side

- entry breeches, and to the lower ring collector on units with bottom

- entry elbows, through a drain box attached to the liner wall. The entrance to this drain box should incorporate a debris screen to prevent pluggage. Many stacks incorporate a liner

- expansion joint. Liquid encountering this joint will re

- entrain back into the gas flow, a significant portion of which will be discharged from the stack as SLD. Ideally, from a liquid

- collection point of view, expansion joints should not be used. If an expansion joint is necessary, every effort should be made to place the joint at a location 1.5 to 2 liner diameters above the roof of the breech or inlet duct. At this location, the joint will be situated approximately at the division point between the upper and lower aerodynamic zones of the liner, and it can be incorporated into the upper ring collector (Figure 3

- 16). This arrangement will minimize the potential for SLD caused by the expansion joint. SECTION AA SECTION BB 

### A

B A

B EXPANSION

JOINT

LOWER RING COLLECTOR DRAIN BOX

LINER DRAIN

BREECH TOP GUTTER DRAIN PIPE

SECTION CC SECTION DD 

### C D

C D UPPER RING COLLECTOR (ONLY WITH EJ IF NEEDED)

ELBOW RING COLLECTOR

ELBOW RING COLLECTOR DRAIN BOX ELBOW RING COLLECTOR ELBOW RING COLLECTOR ANTI

SCOUR TABS

ELBOW RING COLLECTOR ANTI

SCOUR TABS ELBOW RING COLLECTOR DRAIN BOX DRAIN BOX W/SLIDING CONNECTION 3- 61  Figure 3- 16 Liner

- Expansion

- Joint Placement and Incorporation Within Ring Collectors A second alternative would be to incorporate liquid collectors into t he expansion joint itself (Figure 3

- 17). Such collector concepts have been used with some success, but they require a special design and additional preventative maintenance to ensure their proper long

- term operation.

Figure 3- 17 Liner

- Expansion

- Joint Liquid

- Collector Concept 10° 

### Liner Expansion Joint Upward Facing Ring Collector

Downward Facing Ring Collector 1.5

- 2 Liner Diameters Above Roof of BreechBreech

Flue Gas Flow Downward Liquid Film Flow

Drain Expansion Joint Boot 3- 62  Only laboratory flow

- modeling can be used to design and develop liner

expansion

- joint collectors that are effective at all operating loads of a given unit. In some units, the secondary gas vortices in the lower liner may not be sufficiently strong to push the liquid collected on the rear wall of the liner circumferentially around to the front wall of the liner. This phenomenon can occur in units with either side

- entry breeches or bottom

- entry elbows and is typically associated with units in which the gas flow entering the liner has a significant vertical

- velocity component. (In other words, it is not horizontal when entering the liner.) If this situation occurs, the deposited liquid film on the rear wall of the liner will not drain properly. And as more liquid is deposited, the resulting liquid

- film thickness will increase until droplet re

- entrainment occurs off of the film’s surface. To prevent this, one or more large “V” collect ors are placed either in and/or slightly above the droplet

- impact zone on the rear wall of the liner (Figure 3

- 18). These collectors are made of “L”

- shaped material and are typically 6 x 6 in (150 x 150 mm), with the gutter opening facing downwards. These “V” collectors (sometimes referred to as “V” diverters) use the vertical motion of the liquid film to mechanically push it circumferentially away from the rear wall of the liner to a point where the secondary gas flows are strong enough to continue pushing the film around to the front wall of the liner. A gap opening and cover plate are provided at the bottom of the “V” to prevent re

- entrainment of any liquid collected on the inside of the “V” back into the gas flow. All liquid collectors in the stack liner will need to be sealed to the liner wall along their full length to prevent leakage and possible re

- entrainment of liquid back into the gas flow.

Figure 3- 18 Liner Rear

- Wall “V” Diverter 

## “V” Diverter

Main Droplet Impact Zone Liquid Film Flow Pattern Bottom Opening Cover Plate 3- 63  3.11.9 Sloped Lin er Floor Stacks with side

- entry breeches can accommodate a number of different liner

floor options, ranging from flat to conical to sloped. Of the many options available, the sloped liner floor is most favorable for wet operation. Flat and conical liner fl oors create a large open volume in the bottom of the liner, which can lead to unstable and chaotic gas - and liquid

- flow patterns as a function of boiler load. These instabilities can result in the formation of vortices that are strong enough to pull liquid off of the liner floor back into the gas flow, as well as make drainage difficult because of liquid sloshing. A sloped liner floor mitigates these issues, creating a small, quiet sump area for effective drainage, eliminating the formation of floor vortic es, and creating stable well

- defined lower liner gas - and liquid

- film flow patterns across a wide range of boiler loads. A sloped liner floor also reduces the stack

- entrance pressure losses. These benefits associated with the sloped liner floor justifies the additional cost in most installations. A typical sloped liner floor was previously presented in Figure 3

- 7. Although the optimal design is unit

- specific, a good starting point for estimating purposes would locate the high point of the floor on the back wall of the liner at a point 2/3 of the way up the height of the breech opening. The bottom point of the sloped floor would be located approximately 1/4 of the way across the liner from the breech opening, with a sump depth of 6

– 10 ft (1.8 –3 m). If the sum p depth is 4 ft (1.2 m) or less, the top of the sump opening should be covered with a grating to help prevent the flue gas from flowing into the sump and potentially scouring collected liquid back into the gas flow. The sloped floor either can be an inter nally supported structure or can be incorporated directly into the liner wall. If the sloped floor is internally supported, a 4 –10- in (100 –250- mm) gap between the sloped floor and the liner wall is recommended to allow liquid flowing down the wall to flow into the quiet area behind the sloped floor. 3.11.10 Stack - Outlet Collectors A well

- designed stack operating below the liquid

- film flow

- reversal gas velocity will have no need for stack

- outlet liquid collectors. Units with high gas velocities at the top of the liner or outlet chokes may require stack

- outlet liquid collectors if the discharge velocity of the choke exceeds the liquid

- film flow

- reversal velocity for the choke material used. The recommended choke material for alloy and brick stack liners is so lid alloy. FRP choke material should be used with FRP liners. Liquid reaching the top of the choke is discharged and falls to the ground within the near vicinity of the stack. Chokes for a wet stack need to be equipped with an effective liquid collector to prevent this liquid discharge. Some current stack and choke

- outlet liquid collectors incorporate a downward

- facing gutter at the liner outlet to collect the upward

- moving liqu id film (Figure 3

- 19). This design is only marginally effective because the pressure drop through the collection system will be higher than if it was just passing around the collector. Also, the gas flow will always follow the 3- 64  path of least resistance. Bec ause of these factors, a large quantity of liquid will collect at the point where the upward

- flowing gas separates from the liner to go around the gutter. The quantity of liquid in this area will increase until it starts re - entraining back into the gas flo w. Some gas and liquid will be collected in the downward

- facing gutter as a result of the momentum of the gas, but some bypassing of the gutter and the resulting SLD should be expected.

Figure 3- 19 Stack

- Ou tlet Liquid Collector An alternative design is presented in Figure 3

- 20, in which it can be seen that the outer top edge of the liner has been sharpened to a knife edge. This arrangement allows the liquid film to flip over the top edge of the liner outlet before it has a chance to re

- entrain back into the gas flow. Although effective, this collector design is susceptible to fouling, and such a collector needs to be cleaned on a periodic basis to ensure that deposits are not formed from which liquid could be re - entrained. A flow

- model study of any proposed stack

- outlet collector should be evaluated and optimized in a laboratory flow model before installation at a plant.



### DETAIL 1

DRAIN DRAIN



### Detail 1 Expected Detail 1 Actual Liquid Film

Build

- up Reduced Gas Velocity Within Collector

STACK LINERGAS FLOW 3- 65  Figure 3- 20 Alternate Liner

- Outlet Li quid

- Collector Detail 3.11.11 Drains Liquid collected within a wet duct/stack must ultimately be drained from the system. In general, drains can be installed in the horizontal absorber

- outlet ducts, duct

- expansion joints, stack floor, liner

- expansion

- join t collectors, and choke collectors. The preliminary wet stack design should plan an adequate drainage system by considering the drainage needs discussed in this section. The final number of drains needed and their optimum location are specified as one of t he results of the laboratory flow modeling. The absorber

- duct drains are most effective if a trench or scupper is installed across the duct in the floor with a drain. The drains, if installed in duct

expansion joints, are selected on the basis of local conditions. Sloping the floor of the absorber

- outlet ducts either back toward the absorber or toward the liner (preferred) eliminates water puddles after shutdown, when all the liquid from the walls drains to the floor. The sloping floor does not alter the li quid flow on the duct floor during operation because the gas shear dominates the liquid motion on the floor. Duct

- floor drains may be required at locations where liquid pools can build up and water re

- entrainment from the pool may take place. The best locations of the floor drains are specified by the wet stack flow model. Drains located on a flat liner floor may need to have a solid cover positioned at 8– 12 in (200

– 300 mm) above the drain opening to provide a quiet area underneath for effective liquid drainage. These covers are typically 3 x 3 ft (1 x 

## DETAIL 1 DETAIL 2

STACK LINERGAS FLOW



## DETAIL 1INSIDE LINER COLLECTION GUTTER

DRAIN 

# SHARPENED EDGE 3- 66  1m) and are supported over the drain opening by legs (not unlike a table). All drainage points should incorporate a wire cage (or equivalent) over their inlet to prevent plugging by flaking solid scale and other debris. Drains connect points of different static

- pressure levels inside the ducts and liner. To prevent the possibility of gas back

- flowing through the drain pipes

— possibly resulting in droplet re

- entrainment back into the gas flow

— they must all be individually pressure

- balanced through loop seals or drain pots with good on

- line cleaning capability. Because the liquid is acidic, the material selection for the drain line and the means of final disposal must be evaluated. FRP pipe is commonly used for wet FGD drain

- line applications. FRP pipe provides good chemical resistance to these acids, and is more economical and more readily available than stainless steel or nickel

- alloy pipes. Drain lines are sized to prevent pluggage and for ease of cleanout. T o accommodate the potential for solids plugging, drains lines should be sized to handle a larger liquid

- flow capacity than necessary. Main liner

- floor drains should be 10

– 12 in (250

– 300 mm) in diameter, because these are the final drain points in the syste m. As such, they are points of possible single

- point failures. Seal pots or siphons should be provided as needed. These devices will prevent air inflow under negative pressure, which would prohibit liquid discharge through the drain line or cause droplet r e- entrainment back into the gas flow. Different drain lines can be connected only if the static pressure at the flue gas end is the same; in other cases, pressure

- loop seals or drain pots are required. Chimney

- drain lines should use pipe crosses, tees, or “Y”s with bolted blind flanges for cleanout. To reduce pluggage, the drain lines should slope without horizontal sections. Access to cleanout connections should be provided. Outdoor FRP drain lines should have a steep

- enough slope to prevent freezing. For long, relatively flat horizontal runs of drain line, the pipe should be heat

traced and insulated. Stacks with a roof instead of a rain hood are relatively flat so as to serve as a walking surface. Because of the tendency for liquid

- fallout accumulation in this area, discharged liquid and rainwater sometimes freeze on the roof during cold weather. Ice formation at drain locations may prevent liquid from entering the drains. In order to prevent this occurrence, roof slopes should be sufficient to prevent liq uid from pooling, and multiple drains are recommended. The roof drain should be separate from the liner or choke liquid

- collector drains because of pressure differences and different disposal methods. Liquid collected from within the ductwork or stack can be routed to the waste sump or returned to the FGD

- system process. Liquid routed to the waste sump does not re

- enter the process and is eliminated. Liquid returned to the process can be routed to the absorber reaction tank. Liquid returned to the process may be of sufficient volume and pH level that it could change the operational characteristics of the FGD process. The volume and pH of the liquid returned to the process should be considered to compensate for any operational changes that may be needed. 3- 67  A s ingle stack liner that serves multiple FGD modules may produce large volumes of liquid for disposal. Drainage

- pipe diameters need to be sized accordingly. If the liquid collected from multiple FGD modules is returned to the FGD process, the volume of liqui d may be too large to be returned to a single FGD module without significant changes to its operational characteristics. 3.11.12 Post - Installation Inspections An often

- overlooked part of the liquid

- collection

- system installation process is the post

- install ation inspection. It is important to ensure that the designed flow controls, liquid collectors, and drains are installed properly. The objectives of this inspection are to ensure that the liquid

- collection system has been fabricated and installed correctly; to ensure that it is free from leaks; and to identify any deviations to the recommended design as a result of field modifications made to ease installation. Inspection of the installation is usually scheduled for a day when installation of the liquid

- col lection system is 80

– 90% complete. Such timing allows on

- the

- spot modifications to be defined, if necessary, and corrections to be made while the construction/installation crew is still mobilized on

- site. Inspections of the liquid

- collection system and dra ins should be performed during the first scheduled or unscheduled plant shutdown. If any stack

- emission incidence occurs during normal operation, the need for inspection is obvious. If SLD is not experienced, it is still important to inspect the liquid collectors to ensure satisfactory long

- term operation. Problems with solid depositions and liquid drainage can be detected and corrected during a scheduled or unscheduled outage. This inspection includes the stack

- inlet duct, stack liner, and stack outlet, if accessible. 3.12 Laboratory Flow - Modeling An experimental gas

- flow

- model study is a valuable engineering tool used to design and develop liquid

- collection systems for wet operation. The primary objectives of the wet stack flow model include 1. Minimizing the discharge of droplets from the top of the stack that are large enough to reach the ground before evaporation (SLD) 2. Investigating the potential for plume downwash and for icing on the stack liner or shell at low ambient temperatures 3. Providing velocity prof ile and swirl patterns at the CEM elevation that satisfy EPA requirements for flow uniformity Laboratory flow

- modeling provides the following benefits for wet stack design: 1. Estimation of the total liquid load on the liner surface as a result of thermal and adiabatic condensation. This estimat e is based on the liner and shell geometry, thickness, materials of construction and the flue gas

– flow rate and properties over a range of expected plant

- operating conditions and worst

- case ambient weather conditions. 3- 68  2. Design and optimization of liquid collectors in the absorber

- outlet hood, absorber

- outlet ducts, stack

- liner entrance region, and if necessary, stack

liner outlet to prevent SLD. The collectors collect liquid from duct and stack surfaces, prevent re

- entrai nment, and guide the liquid to locations where it can be drained out of the system. This development work is conducted on a scale model of the wet duct and stack system, from the outlet of each absorber’s mist eliminator to approximately four stack

- liner d iameters above the top of the breech or inlet duct. Model scales typically range between 1:8 and 1:16. Even though the absorber

- outlet

- duct region is not a part of the chimney design, the liquid

- collector designs should be developed here and provided to t he owner so that an integrated complete system of liquid collectors can be developed for the unit. Typical wet stack physical

- flow models are shown in Figure 3 -21. 3. Evaluation of the liner

- material and liner

- surface discontinuities that will be produced by the construction technique. A laboratory test determines liquid re - entrainment and drainage behavior over the expected range of unit operating conditions and confirms or selects the stack

- liner design velocity. This step can be eliminated if sufficient reliable information already exists for the selected liner material. 4. Evaluation of re

- entrainment at liner

- expansion joints in a laboratory test rig and development of a unit

- specific liquid

- collector geometry to collect and drain the liquid from the liner

- ex pansion joints. 5. Development of an optimum choke geometry (if required), including liquid collectors and drains to prevent re

- entrainment and discharge of large droplets from the top of the choke. 6. Performance of a computational 3

- D fluid

- dynamic

- model study of the upper third of the stack liner and shell to evaluate downwash of the wet plume. Downwash could lead to stack

- surface deterioration, unacceptable ground

- level concentrations of SO2, or icing problems at the top of the stack. The final choices of the liner

- extension height and the geometry of the shape of the chimney cover will be specified as a result of this work. 3- 69  Figure 3- 21 Typical Wet Stack Physical

- Flow Models Laboratory flow

- modeling has a number of limitations: 1. Physical cold

- flow models cannot accurately simulate thermodynamic processes such as thermal and adiabatic condensation. These must be evaluated using mathematical or computer

- based models. 2. The physics controlling the processes are d ifferent; droplet trajectories and deposition patterns cannot be modeled simultaneously with the liquid

- film motion in a physical

- flow model. Droplet trajectories are controlled by the gravitational and centrifugal forces acting on the droplets as they pas s through the system, whereas the motion of the liquid film on the duct and liner surfaces is controlled by gravitational and gas

- shear forces. Because these two phenomena cannot be modeled simultaneously, they must be modeled separately and the results co mbined to get a full understanding of droplet deposition and liquid

- film flow processes within the wet duct/stack system. 3. Because droplet trajectories and deposition patterns cannot be modeled simultaneously with the liquid

- film motion, physical

- flow mode ls cannot be used to accurately determine the collection efficiency of the final recommended liquid

- collection system. 3.12.1 Computer Modeling. A number of powerful computer codes are available for the evaluation of three

dimensional fluid flow and thermo dynamic processes. The applicable programs can be separated into two categories: 3

- D and 2

- D computer programs. How and where these computer codes can provide help and information for the wet stack designers is described briefly in the following sections. 3- 70  3.12.1.1 3- D Computer Programs These are a number of commercially available programs designed for general three

- dimensional thermal and fluid

- dynamic calculations. These codes can be used to evaluate the overall gas

- flow patterns and droplet trajectories, but they are incapable of predicting liquid

- film formation, liquid

- film flow, and droplet re

entrainment at a scale useful for the prediction of actual field performance. These processes must still be evaluated in a physical

- flow model of the subject unit . A complete wet stack analysis with all the necessary details

— from the absorber outlet to the top of the stack

— will not be possible using 3

- D computer code in the near future. The 3

- D computer models are very useful for calculating the steady gas

- flow pat terns in and around the ducts and stack. They are also very good at describing the trajectories of liquid droplets of all sizes and at identifying their deposition points in the wet duct/stack system when combined with the gas

- flow calculations and droplet

- evaporation rates. Three

- dimensional computer models can also be used to predict condensation rates within the liner. However, 2

- D models can do this effectively without needing to model the entire geometry of the wet stack system. Three

- dimensional compu ter models are often used to evaluate stack

- plume downwash. Details of this type of modeling are presented in Section 3.13.1. The accuracy of a 3

- D computer model is substantially a function of the data used to set up the model; the quality of the computat ional grid used; and the settings used within the code for the modeling of such factors as flue gas, material properties, turbulence, and droplet

- evaporation rates. Computer modeling should only be performed by engineers experienced in the use of the code and familiar with the modeling of the unique issues associated with the modeling of utility power

- generation systems. 3.12.1.2 2- D Computer Programs Two

- dimensional computer programs and spreadsheets are specifically written to evaluate selected flow and thermodynamic processes. Therefore, their use is important and the results are good, but they are generally limited to one detail of the complete wet stack design process. Special

- purpose 2

- D computer programs can be very useful for providing quantitative r esults for cases in which experimental models are not applicable and 3- D codes are too time

- consuming for the objectives. Their use often yields satisfactory results for 2

- D processes such as the prediction of thermal and adiabatic condensation within a we t stack liner. 3- 71  3.13 Plume Downwash and Icing Designing liners for a low

- velocity plume can decrease the amount of SLD. However, having a low

- velocity plume increases the potential for plume downwash at elevated wind velocities. During downwash episodes, saturated flue gas comes in contact with the liner extension, stack hood and stack shell. This contact can lead to deterioration of the stack construction materials as a result of exposure to acid in the flue gas. It also increases the potential for ice fo rmation on the top of the stack, which creates a potential danger to people and property in the vicinity of the stack. Details of the processes controlling the onset and extent of plume downwash were previously presented in Section 2.9.1. 3.13.1 Downwash M odeling The extent of downwash under various weather and plant

- operating conditions is typically evaluated using a computational fluid

- dynamic (CFD) model of the subject unit. A three

- dimensional model of the upper 1/3 of the stack and its surrounding envi ronment is constructed for the study. The model will then be used to study the flow patterns at the top of the stack and the likelihood of downwash for different combinations of boiler

- operating load and wind conditions. Typical results from a downwash model of a single flue liner are presented in Figure 3 -22.

Figure 3- 22 Typical Plume

- Downwash Study – CFD Model Results No Downwash onto Shell Liner Extension Stack Shell Prevailing Wind Direction 3- 72  A typical plume

- downwash study starts with an evaluation of the "as

- designed" stack (baseline geometry) at plant

- operating conditions corresponding to full and minimum load and at atmospheric conditions corresponding to the worst

- case velocities and prevailing wind directions expected during a typical full year of operation. Simulations are then performed to determine whether, and to what extent, the chosen ambient wind conditions create plume downwash. The results of the simulations performed for ambient temperatures below freezing will be used to determine the potential for ice formation at the top of the stack. If the results of initial downwash simulation are unsatisfactory (for example, downwash occurs at unacceptably low wind velocities or at a velocity for which the cumulative number of hours at or above that velocity are too high), stack

- top modifications and/or liner extensions will be evaluated as a means of reducing downwash and minimizing the likelihood of ice formation at the top of the stack. If liner extensions prove to be ineffective, the addition of a choke or other stack

t op modifications can be evaluated. For plants with multiple liners within a single stack shell, additional simulations will need to be performed as a function of the prevailing wind direction. To perform a plume

- downwash study, the following information will be required: 1. Detailed drawings of the stack shell and liner. 2. Hourly wind

- rose data (temperature, wind speed, and direction) from the plant or nearest weather station for at least one full year (3

– 5 years preferred). 3. Stack base elevation. 4. Expected plan t- operating conditions (gas temperature, flow, density) at minimum and maximum load. For stacks with multiple flues, this information will be required for each flue. 3.13.2 Windscreen Design Because the potential for plume downwash is substantially a funct ion of the windscreen to liner

- diameter ratio, the outer diameter of the stack windscreen should be kept as small as possible at the top of the stack. 3.13.3 Single versus Multiple Liners Plume

- downwash potential for a traditional single

- liner stack is in dependent of the prevailing wind direction. The potential for downwash is only a function of the stack geometry and the plume to wind

- momentum ratio. For units with multiple liners, the potential for and extent of plume downwash is substantially a functio n of the prevailing wind direction and the number and arrangement of the liners within the windscreen. 3- 73  If the prevailing wind direction is in line with the axis of a stack with two liners, the extent of downwash will be significantly less than if the wind direction is perpendicular to the axis of the liners (Figure 3

- 23). In other words, when the wind is aligned with the axis of the liners, it is more difficult to push the combined plumes over than when the wind is perpendicular to the axis and the wind is acting on both plumes simultaneously. Therefore , it is important to know the prevailing wind direction when developing the initial system design and setting the relative locations of the liners.

Figure 3- 23 Recommended Alignment of a Stack with Two Flues Stacks with three or more liners will require multiple downwash calculations with respect to the relative wind direction and individual liner

- gas velocities to establish the unit’s downwash potential. 3.13.4 Methods of Downwash Minimization If the actual or predicted cumulative duration and/or extent of downwash are not acceptable, there are a limited number of methods available to reduce them: 3.13.4.1 Liner extensions The most common method of minimizing the potential for plume downwash is to increase the distance between the top of the windscreen and the top of the liner in the form of liner extensions. These extensions minimize the potential for the plume to be pulled into the low

- pressure region formed on the downwind side of the windscreen. Typical liner

- extension heights range from 0.5 to 1.5 liner diameters above the top of the stack shell. Within these heights, the extensions should be self

- supporting. 

### Prevailing Wind Direction

Favorable Liner Orientation Unfavorable Liner Orientation Stack Shell Liners 3- 74  3.13.4.2 Chokes Because downwash is controlled by the plume to wind

- momentum ratio, increasing the liner

- exhaust velocity will have an immediate impact on reducing downwash. Although this goal can be accomplished by decreasing the liner diameter, doing so would most probably result in the liner operati ng at a velocity above the recommended value for wet operation, leading to a significant increase in SLD. Another method of increasing the liner

- outlet gas velocity is the addition of a liner

- outlet choke. This option is typically considered as a last resort; it is an expensive option with respect to both installation and operation because of the increased fan pressure required to overcome the increased pressure drop for the life of the plant. 3.13.4.3 Other Stack

- Top Modifications A limited number of n ontraditional stack

- top design modifications have been successfully used to minimize the potential for plume downwash. Chamfering the top of the windscreen (on 2 sides) can reduce both the onset and extent of downwash, particularly if the chamfering is al igned perpendicular to the worst
- case prevailing wind direction. For smaller

- diameter stacks without a windscreen, a disk placed around the stack near the outlet has been shown to reduce the onset of plume downwash. This disk can either be solid or in the form of a walkway made of grating. The diameter of the disk and its position relative to the stack outlet must be optimized by means of a plume

- downwash study. Potential approaches are highly site

- specific and they should be evaluated as part of the plume

- downwash CFD evaluation. 3.13.4.4 Protective Shell Coatings If the expected cumulative duration of plume downwash events is acceptable, or the extent of plume downwash cannot be reduced, one option is simply to live with it and ensure that the top of the stack is adequately protected with an effective shell

- coating system. 3.13.5 Stack - Top Icing Whether ice forms on the top of the stack depends on the temperature of the surface, the temperature of the mixture of saturated flue gas and cold ambient wind, and on whether water vapor will condense out of the mixture. Ice formation is most likely at plants where below

- freezing temperatures are common and where they last for extended periods of time. 3- 75  Icing usually does not cause serious ice buildups that can fa ll to the ground. However, when the icing conditions are occurring, the platform near the top of the stack, the railings, and possibly the roof, may be slippery. The potential for icing can be reduced by employing the following steps: 1. Select a stack

- liner discharge velocity that minimizes plume downwash over the expected operating range of the unit at the existing local wind conditions and that is consistent with other design objectives. 2. During cold ambient temperature conditions with high winds, run the unit at near full load. Employing this operating procedure under such conditions may be natural, because more power is consumed in below

- freezing weather. 3. Use heated annulus air or electrical heating elements to heat the stack hood, roof, or other areas where ice forms on the top of the stack. 4. Insulation is not required on sloped rain hoods constructed of a corrosion

resistant material. 4-1  

# Section 4: G uide to Developing a Specification for Wet Stacks The designer of a wet stack system must follow a logical process to develop a bid specification for a new or retrofit wet stack. This process involves evaluating specific design issues that are important to wet stack design. The engineering process that le ads to a bid specification and the completion of a wet stack design is discussed in this section. 4.1 Specification Overview The design of a new or retrofit wet stack may be broken down into two distinct phases of work:

 Phase I – Feasibility Study

 Phase II – Design Process 4.1.1 Phase I – Feasibility Study Essentially all modern wet flue gas desulfurization (WFGD) systems will operate with a wet stack. Therefore, the two major questions to be answered during the Phase 1 f easibility study are : 1) C an the exi sting stack be r eused in a retrofit application? 2) I f not, what will be the requirements for the new stack installation? The Phase I feasibility study includes the following steps:

 Address regulatory issues.

 Define existing plant considerations.

 Define o perating parameters.

 Determine if the existing stack can be converted to wet operation.

 Determine the required stack height.

 Determine liner alternatives.

 Establish liner

- material options.

 Determine the impact of liner modifications on existing constructio n. 4-2   Establish liner

- inlet options.

 Determine liner

- insulation requirements.

 Perform operating conditions versus liner

- design compatibility analysis.

 Select design velocity limit to prevent liquid re

- entrainment for each candidate liner material.

 Perform a p reliminary economic analysis for liner material and inlet

geometry options for wet stack operation.

 Select liner material(s) for wet stack operation. The feasibility study should address all considerations necessary to en sure that a wet stack is allowable and that the proposed design will work. The study should include permit considerations and plume

- dispersion modeling as well as preliminary design choi ces such as the absorber outlet

- duct arrangement, choice of inlet geometry, velocity limits for liner materials, and preliminary economics. 4.1.1.1 Identification of Issues (Phase I) A number of important issues must be addressed during the Phase I feasibility study for the design of a new or retrofit wet stack. These include the following:

 Choic e of liner materials

 Gas velocity in the liner

 Allowable gas velocity in the li ner for favorable wet operation

 Stack height

 Foundation requireme nts as affected by liner choice

 Absorber outlet

- duct geometry

 Liner

- inlet geometry

 Liquid

- collection devices and drainage

 Miscella neous materials of construction

 Operating conditions

 Model -s tudy testing to minimize system

- pressure losses and optimize liquid

collection and drainage

 Continuous

- emissions

- monitor ing (CEM) system considerations

 Outage time

 Cost A detailed discussi on of each issue that is important for new or retrofit wet stacks is included in this section of the design guide. 4-3  4.1.1.2 Evaluation of Issues (Phase I) During the wet stack Phase I feasibility study , several critical issues must be evaluated. For exampl e, w hat material will be used for construction of the stack liner ? The liner diameter is established on the basis of the liner material. Each of the specific design issues identified in this section needs to be evaluated f rom a technical and an economic st andpoint to enable the wet stack designer to decide how to proceed. 4.1.1.3 Economics (Phase I) Economic evaluation of a new or retrofit wet stack should include the following factors:

 Capital costs

 Operating costs

 Maintenance costs

 Outage costs Capital c osts for a chimney include the cost of the concrete shell, liner, foundation, and of miscellaneous items such as access platforms, doors, elevator, rain hood, pressurization system, drain system, protective coatings, and electrical system . Chimney

- maintena nce costs will vary depending up on choice o f liner material. Coated carbon

- steel liners need to be recoated periodically, whereas Alloy C276 clad liners and FRP liners require little or no maintenance. Maintenance on brick liners can include repair of crac ks, replacement of liner bands, and repl acement of expansion joints . All chimneys require periodic inspection. Outage time does not apply to a new wet stack, because the unit has not yet been placed into operation. For a retrofit wet stack, outage time dep ends up on how long it wi ll take to tie the new absorber

- inlet duct to the existing ductwork and , if applicable, reline or replace the liner within the existing stack shell. Outage costs can be minimized by scheduling the ductwork tie

- in to take place duri ng a normal maintenance outage. Outage costs per day will vary depending up on time of year (energy demand), loss of revenue, and cost of replacement power. The number of years considered in the economic analysis should encompass the design life of the FGD system. Initial capital costs, periodic or annual maintenance costs, and annual energy costs need to be converted to present value to compare the options on an equal basis. 4.1.2 Phase II – Design Process Table 4 -1 describes Phase II, which is the process for the design of a wet stack. This table provides a brief description of each step of the design process, a designation of each step of the design process, a designation of responsibility, and 4-4  a reference to the s ection of th is guide that provides the relevant background information

. The design

- process phase essentially takes the feasibility study and turns it into a bid specification. During this phase , it is necessary to establish the design crite ria, define the stack and inlet

- duct geometry, perform the flow

- model study to determine liquid

- collection devices and drains, and prepare the bid specifications and drawings. Table 4-1 Phase II Wet Stack Design Process Step Description Responsibility Reference 1. Preliminary d esign Perform component

- by

component design

. Engineer Section 2: "Specific Design Issues ” 2. Preliminary d esign r eview Adjust design for favorable wet operation. Engineer and modeling company Section 2: "Evaluation for Wet Operation ” 3. Fluid -d ynamic d esign Perform flow

- model study

. Modeling company Section 2 : "Evaluation for Wet Operation ” 4. Preparation of b id s pecification Prepare bid specifications for chimney contract

. Engineer Section 4 : "Guide to Develop Specifications for Wet Stacks ” 5. Final d esign Perform detailed design of chimney for construction

. Chimney contractor

----

Note: The utility must be an active participant in all major decisions in all steps of the design process. Several items co nsidered during the feasibility

- study phase should be revisited or finalized during the design

- process phase. Th ese include such items as the subsurface investigation and foundatio n requirements, stack and inlet

- duct geometry, and liquid

- co llection devices

. Detailed design s of the chimney foundation, concrete column, liner, and access platforms are normally provided by the chimney contractor. However, the foundation is often designed by the owner’s or general contractor’s engineer . The elect rical power - feed design is usually performed by the owner’s or general contractor’s engineer

— based on information from the chimney electrical engineer

— because the electrical raceway system must be designed ahead of time in order to be embedded in the chimn ey foundation. The chimney

- related scope of work is typically included in a separate contract. However, if schedule and site constraints warrant it, the chimney may be included in the FGD system contract. The preliminary duct and stack design i s completed using the component

- by

component guidance outlined in Section 3 , "Specific Wet Stack Design Issues." Next, the complete wet duct and stack geometry is evaluated for wet operation as 4-5  an integrated system, in which the individual components operate together as a unit. The first objective of this evaluation is to review the preliminary duct and stack design as a complete system for wet operation. How well will all the flow passages work together as a system , from the absorber outlet to the top of the stack? Ho w suitable are they to be outfitted with liquid collector s to achieve minimum SLD ? If improvements are possible by means of modification s to the duct and stack geometry, are the gains in reduced SLD worth the cost of the geometry changes? The second object ive is to design an effective liquid

- collection system that is optimized for the integrated geometry and for the specified range of operating conditions and absorber combinations. The combined wet duct and stack systems use a variety of design methods. The best methods that have been developed are described in the following subsections. The design methods and tools used to achieve these objectives are:

 Preliminary d esign r eview b ased on e xperienc e

 Laboratory flow

- modeling – 3-D

 Computer flow -m odeling – 3-D

 Computer p rogram calculations – 2-D These design methods are summarized in the first four subsections below. The subsection on "Comparison of Experimental and Computer Modeling" is an overview of the basic gas/ liquid

- flow processes that need to be evaluate d in a wet system using the design tools , and it summarizes the usefulness of the experimental and computer tools to carry out the needed evaluation. The last two subsections are a discussion of the system tests and inspections conducted after the unit is in operation to help i dentify how well the final duct/ stack geometry and the installed liquid

- collection syste m are working. If there are wet

- operation problems, the tests and inspections can identify the causes and practical solutions. 4.1.2.1 Preliminar y Design Review Based on Experience The first two steps in Phas e II are to perform a component

- by

- component design and then adjust the design for liquid

- collection requirements. The results of these first two steps should be evaluated by a preliminary desi gn review. The preliminary duct and stack

- liner design geometry for new construction must be reviewed to assess how well all individual components will work together as an integrated system. The second goal of the design modifications implemented at this p hase of wet stack design is to make the integrated geometry more suitable for liquid collection . This review should be based on actual field experience with liquid collector s designed for different power plants. This modified wet duct and 4-6  stack design is t he starting geometry for the design of the liquid

- collection devices. It also optimizes gas - flow passages by experimental flow

- modeling. It is important to get the engineer and modeling company together to review the preliminary design early in the design process , because simple changes made at this point in the process can have a big impact on the liquid

- collection performance of the entire system. 4.1.2.2 Condensation Calculations A larger portion of the liquid flow on the liner surfaces is caused by con densation th a n by deposition of the droplet carryover from the mist eliminator s. The condensation on the liner can not be measured in a laboratory

- scale model of the stack. Therefore, analytical calculations are required to define the amount of condensed

- li quid flow rate in the stack. The basic description of the adiabatic bu lk condensation and the thermal

- wall condensation processes is given in Section 2.3 , "Sources of Liquid in a Duct/ Wet Stack Sy s tem." This section discusses the methods used to predict condensation rates in the duct and the stack and also presents calculated condensation data for typical sample stacks

. Liquid condensation occurs continuously on all wet duct and stack

- liner surfaces because the flow gas is saturated and the li ner's inside surface temperat ures are lower than the gas dew

- point temperature. But the rate of condensation per unit surface area can vary significantly , depending on geometry, materials, temperatures, and wind speed. The duct - and stack

- condensation rate cannot be measured in experimental flow models. Three

- dimensional computer modeling can be adapte d for condensation calculations; however , such model s can be complex and are unnecessary for this essentially two

- dimensional process. A 2

- D condensation anal ysis is satisfactory for calculating the condensation rate that accounts for stack geom etry, mass flow, heat loss, gas

- psychrometric conditions , and the ambient atmospheric

- pressure variation along the height of the stack. Assuming axial symmetry for the stack, all the variables can be described or calculated as a function of vertical height with computer programs developed for this purpose as design tools. Numerous studies performed using these programs have drawn the following conclusions:

 The bulk condensation in the liner is a function of stack height.

 For brick liners, the pressurizing

- air leakage through the liner increases the bulk condensation by additional cooling.

 The wall condensation is directly proportional to the temperature difference between the gas and the ambient air. 4-7   The liner material does not significantly affect the wall

- condensation rate because the thermal resistance of the liner is a small fraction of the total thermal resistance between the flue gas and the ambient air.

 The wall con densation may be reduced by a factor of about 4 to 5 by insulating the outside of the alloy or FRP liners with a 2

- inch (0.05

- m) fiberglass layer. Increasing the in sulation thickness from 2 in (0.05 m) to 4 in (0.1 m) will result in a small reduction in co ndensation r ate. Therefore, more than 2 in (0.05 m) of i nsulation is seldom required. Insulation does not eliminate the liner condensation but can reduce it significantly.

 A wind

- speed increase from 20 to 40 mph (9 to 18 m/s) increases the condensation rat e by only a few percent age points because the convective

heat transfer levels off above 20 mph.

 Having more than two liners in a shell increases the total wall

- condensation rate by about 5% . This increase is mainly the result of a higher condensation rate on the liner extension above the shell. The condensation in the ductwork primarily consists of thermal

- wall condensation; adiabatic condensation (due to pressure reduction with elevation change) is negligible. Therefore, the rate of condensation here is no t as important as in the stack, because most of the condensed liquid and the deposited droplets can be collected and drained out before they enter the stack. The duct condensation rate is calculated either by a 2- D computer program or by hand

calculation for the various ductwork surfaces. The results of the condensation calculation are used to design the liquid collectors and to plan for the liquid

- disposal system. The estimated ranges of the liquid

- flow rates in the wet duct and stack of the typical 550 -M W unit are listed in Table 4 -2 . These numerical values help the stack designer to estimate the amount of liquid flow to be controlled and accounted for in a wet duct/stack system. 4-8  Table 4-2 Estimated Ranges of Flows in the Wet Duct/Stack of a Typical 550

- MW Plant Source of Liquid at the Stack Estimated Flows

• Mist

- eliminator carryover

- The vapor mass

- flow rate in the saturated gas stream is rather large. The total vapor content at saturation temperatures of 115

– 130 o F (46

– 54 o C) is equivalent to a liquid

- flow rate of

- The fine liquid mist of diameters <10 μ m with high

- efficiency mist eliminators is 1100

– 1800 gpm (250

– 410 m 3 /h)

0.3

– 1.5 gpm (0.1

– 0.3 m 3 /h)

• Duct

- wall condensation 1– 5 gpm (0.2

– 1.1 m 3 /h)

• Stack

- wall condensation 1– 6 gpm (0.2

– 1.4 m 3 /h)

• Bulk (adiabatic) condensation in the stack 6– 8.4 gpm (1.4

– 1.9 m 3 /h)

• Deposition of liquid droplets 1– 12 gpm (0.2

– 2.7 m 3 /h)

• Droplets re

- entrained from the walls (diameters 20

– 2000 μ m) 0– 10 gpm (0

– 2.3 m 3 /h)

• Liquid in drains 2– 12 gpm (0.4

– 2.7 m 3 /h

• SLD (a function of the duct/stack design and the effectiveness of the liquid collectors installed) 0– 15 gpm (0

– 3.4 m 3 /h)
4.1.2.3 Laboratory Flow

- Modeling An experimental gas - flow

- model study is a valuable engineering tool used to evaluate wet operation of the absorber

- outlet duct and stack system from each FGD module mist eliminator to the top of the stack. The primary objectives of flow

- modeling include:

 Preventing the discharge from the top of the stack of droplets that are large enough to reach the ground before e vaporation

 Investigating the potential for plume downwash and for icing on the stack liner or shell at low ambient temperatures

 Providing velocity

- profile and swirl patterns at the CEM elevation that satisfy EPA requirements

 Laboratory flow

- modeling provides the following benefits for wet stack design:

 Design and development of liquid collector s ( experimentally ) where needed in the absorber

- outlet du cts and stack

- liner entrance region to prevent large droplets from reaching the stack liner . The collectors collect liquid from duct 4-9  and stack surfaces, prevent re - entrainment , and guide the liquid to locations where it can be drained out of the system. Th is development work is conducted on a scale model of the wet duct and stack system , from the outlet of each absorber to approximately two stack

- liner diameters above the top of the breeching duct. Even though the absorber

- outlet

- duct region is not a part o f the chimney design, the duct liquid

- collector designs should be developed here and provided to the owner so that an integrated complete system of liquid collector s can be developed for the unit.

 Evaluation of the liner

- material and liner

- surface disconti nuities that will be produced by the construction technique. A laboratory test determines liquid re - entrainment and drainage behavior over the range of operating conditions represented by the range of boiler operation and confirms or selects the stack

- line r design velocity. This step can be eliminated if sufficient reliable information already exists for the selected liner material.

 Evalua tion of re

- entrainment at liner

- expansion joints in a laboratory test rig and development of a liquid

- collector design t o collect and drain the liquid from the liner

- expansion joints.

 Development of the optimum choke geometry, liquid collector s, and drains to prevent re
- entrainment and discharge of large droplets from the top of the choke.

 Wind

- tunnel tests that evaluate do wnwash of the wet plume, which could lead to stack surface deterioration, unacceptable ground

- level concentrations of SO2 , or icing problems at the top of the stack . The final geometry of chimney

- cover shape and the extension of the liner will be specified as a result of this work.

 Modeling of the absorber

- outlet ductwork, stack

- liner breeching duct, and the stack liner up to two liner diameters above the CEM elevation. This modeling measures the gas

- velocity profile and swirl angles for comparison to EPA r equirements at the CEM station. 4.1.2.3.1 Laboratory Flow

- Modeling for Wet Stack Conversion The geometry of the ductwork and the stack is inherited from the existing system and cannot be altered in most conversion cases. For this reason, gas - flow pas sages are less suitable for gas/liquid flow and for installation of liquid collector s than optimized flow passages of new or retrofit units. Experimental flow

- modeling is the best tool to evaluate existing duct/ stack geometry and to design liquid

- collection dev ices needed for a successful conversion. Because of the geometric limitations, the modeling work is more critical and more diff icult. Laboratory flow

- modeling provides the following results , specifically pertaining to the conversion of an existing system to wet operation:

 Evaluation of existing duct and stack gas - flow passages for gas/liquid flow or wet operation. Existing surface discontinuities, duct bends, and stack

- liner 4- 10  surface and geometry are evaluated , with the air and water flow simulating the fie ld conditions.

 Design of duct and stack modifications to make the geometry more suitable for wet operation. These modifications are limited to internal changes only. Internal changes are inserts, target walls, turning vanes, and baffles that make the flow more favorable for liquid collection and promote the deposition of liquid droplets entrained in the gas flow

.
 Flow- modeling to decide if reheat

- duct openings can stay open to the isolation damper or whether they must be blanked off at the duct interfaces.

 Evaluation of wet operation with existing internal trusses. Decide if they are acceptable or if they need to be equipped with liquid

- collection devices or replaced with a more suitable truss design.

 Design and development of liquid collector s. These collec tors are e xperimentally us ed where needed in the absorber

- outlet ducts and stack

liner entrance region to prevent large droplets from reaching the stack liner , to collect liquid from duct and stack surfaces, to prevent re - entrainment , and to guide liquid t o locations where it can be drained out of the system.

 Definition of the best location for drains, selection of possible drain locations within the geometric limitations, and experimental evaluation of the final drain system.

 Evalua tion of re

- entrainment a t liner

- expansion joints in a laboratory test rig and development of a liquid

- collector design to coll ect and drain liquid from liner

- expansion joints.

 Develo pment of the optimum choke

- liquid collector s and drains to prevent re - entrainment and discharge of large dro plets from the top of the choke ( if there is a choke in the existing liner ).

 Measurement of pressure losses in t he model, scaling them to field

- operating conditions , and comparing them t o the available pressure rise of the existing ID fan. Reduc t ion of pressure losses if needed.

 Wind

- tunnel tests to evaluate downwash of the wet plume that could lead to stack

- surface deterioration, unacceptable ground

- level concentrations of SO2 , or icing problems at the top of the stack. Th e final geometry of the chimney

- cover shape and the extension of the liner will be specified as a result of this work.

 Use of a model of the absorber

- outlet ductwork, stack

- liner breeching duct, and stack liner up to two liner diameters above the CEM elevation

. Measurement of the gas - velocity profile and swirl angles for compar ison to EPA requirements at the CEM station. 4.1.2.4 Computer Modeling The use of computer programs for computational fluid mechanics has been increasing during the last 20 years. How and wh en computer programs can provide help and information for wet stack designers is briefly described here. 4- 11  The applicable programs can be separated into two categories: 3

- D and 2

- D computer programs. I n the following subsections , t hese categories of computer modeling are compared to laboratory experimental flow

- modeling from the wet stack design point of view only. 4.1.2.4.1 3- D Computer Programs These are commercially available programs designed for general three

dimensional fluid

- mechanics calculations. Thes e codes have the potential t o handle all aspects of the gas/liquid

- flow in wet stacks. B ut at the present stage of development they cannot describe some of the most important wet stack flow processes, such as re

- entrainment and liquid flow on surfaces with gas flow

. Therefore, a complete wet stack analysis , with all the necessary details from the absorber to the top of the stack , will not be possible by using 3- D computer code in the near future. The 3

- D computer models are useful to calculate steady gas - fl ow patterns in the duct and stack. They are good for major flow patterns, b ut they become complex and slow

- running programs when small details are included that require larger computers ( for example, the flow in the expansion

- joint cavity, the flow around trusses, and the flow near liquid collector s). The 3

- D computer models are very good at describing the trajectories of liquid droplets of all sizes and their impact on deposition points in the duct system when combined with the gas - flow calculations. These codes can be used to ca lculate deposition on the choke

- cone surface with assumed points of droplet origin. Some of the codes come with trajectory subroutines that yield very detailed quantitative results. The condensation on the liner of a wet stack can b e d escribed by a 3

- D computer code. B ut because the process is basically two

- dimensional, 2

- D analytical codes are effective and are easier to use. Several available 3

- D computer codes are written for dispersion

- modeling only. The degree of complexity and accuracy of these codes var ies a great deal as they account for the terrain and atmospheric conditions over large distances from the stack. Computer ized modeling, verified by field

- dispersion measurements , is the best tool available at this time to predic t the plume

- dispersion process. Plume downwash at the top of the stack could be predicted by an elaborate 3

- D computer model with a high degree of accuracy. 4.1.2.5 Full

- Scale Field Tests and Inspections After the wet stack is placed into operation, the e ffectiveness of mist eliminator s and downstream liquid

- collection devices should be tested to verify that design objectives have been met. EPRI Report CS -2520, Entrainment in Wet Stacks , evaluates various methods for making carryover measurements and presents recommendations for obtaining accurate results [1]

. 4- 12  Effective operation of the liquid

- collection devices and drainage systems should also be verified during an off

- line inspection of the liner system. Gas and liquid flow on the liner and around the liquid collector s can be determined by observing the pattern of residue on the internal surfaces. If heavy solids buildup or deposits are found and are causing SLD , necessary improvements can be defined on the basis of this inspection. 4.1.2.6 Flow Measur ements at CEM Level Flow measurements can be taken to confirm the operational characteristics of the FGD system and the liquid

- collection devices. Several of these measurements are required by the EPA and are performed as part of the CEM and testing. Infor mation that can be provided from the CEM system measurements includes:

 Stack gas - flow rat es at different operating loads

 Gas

- velocity levels and yaw angles of the cyclonic flow Additional in

- place testing at the CEM level can measure the effectiveness of t he liquid

- collection system. These tests can be performed as needed to establish the liquid

- flow in the stack. The following liquid

- flow measurements are not mandatory tests performed as part of the CEM system; however, they would be beneficial in furnishi ng some or all of the following information:

 Me asurements of entrained droplet

- size distribution using video droplet analyzer ( VDA ) or similar method s

 Qualitative amount of liquid near the wall by impingement methods

 Liquid

- flow direction on wall (up, down , or slanted) by observations at the p orts covered by a Plexiglas cover

 Qualitative droplet sizes, using a suitable impingement probe expose d to the flow for a few seconds Where ver there is a large amount of entrained liquid, the droplets may interfere wit h the CEM system velocity and angle measurements. This situation shows that liquid collection and drainage must be improved , even though the SLD is acceptable. 4.1.2.7 Outage Time Outage time is the amount of time necessary for an existing unit to be out of service because of repairs, maintenance, or modifications . The outage may be caused by an emergency (unscheduled) , or it may be routine (scheduled). Outage time applies only to an existing, operating unit. Therefore, the designer does not need to consid er outage time for a wet stack on a new unit. For a retrofit FGD project, outage time is the t ime it takes to tie the new FGD

- system ductwork into the existing ductwork. Outage time for retrofit wet stacks should be minimized for economic reasons. Dependin g on unit size, energy demand, loss of revenue, and cost of replacement 4- 13  power, outage time can be very expensive. To minimize the outage time required for a retrofit wet stack project, the ductwork connections or tie

- ins should be made during a scheduled m aintenance outage. 4.2 Wet Stack Bid - Preparation Process Once the decision has been made to utilize wet stack operation, it is necessary to prepare a bid specification for the wet stack. For a new or retrofit wet stack, the chimney can either be covered in a separate contract or be part of the FGD

system contract. This section of the guide addresses key issues that should be included in a wet stack bid specification. These guidelines should not be used without appl i cation of good engineering judgm ent and consideration of site specifics. Preparing a bid specification for a wet stack is a multi

- step process. The design criteria for a wet stack are determined on the basis of plant considerations, applicable codes , and system

- performance requirements. General arrangement drawings that define duct and chimney geometry should also be included. A flow

- model study should be performed in order to optimize the absorber outlet

duct arrangement and to design the necessary liquid

- collec tion devices and drains. At this point, the bid specifications can be prepared. Exhibit drawings showing the wet stack arrangement and typical design details are included with the specifications to define the scope of work and obtain competitive bids. 4. 2. 1 Establish Design Criteria Minimum standards for design materials and construction are established in the bid specifications. The chimney contractor is normally responsible for the design of the wet stack to meet the specified operating conditions and any applicable codes, such as the latest edition of the American Concrete Institute (ACI 307) , Code Requirements for Reinforced Concrete Chimneys , or ASTM D5364, Standard Guide for Design, Fabrication, and Erection of Fiberglass Reinforced (FRP) Chimney Liner s with Coal Fired Units

. New stacks are typically designed in accordance with the latest wind and seismic s ections in the applicable steel

- stack or concrete

- chimney codes. Dynamic wind loads should be evaluated in relationship to the chimney's critical win d velocity and natural frequency. Wind effects from adjacent structures and nearby chimneys should be considered. Closely spaced chimneys can cause an amplification of the vortex

- shedding wind loads for both the new and existing chimneys. Spacing between a djacent chimneys can be increased in order to decrease the amplificati on factor for the vortex

- shedding wind loads. In addition, a subsurface investigation is usually performed to determine the most appropriate foundation type. For example, the subsurface investigation may require that a deep, pile

- supported foundation (versus a shallow, soil

- supported foundation ) be im plemented

. 4- 14  4. 2 .2 Define Absorber Outlet - Duct Geometry General arrangement drawi ngs that indicate chimney inlet

- duct size, elevation , and con figuration are typically included with the bid specifications. The duct arrangement is influenced by the number of absorbers, the size of absorber, and the operating conditions of the unit
. 4. 2 .3 Define Chimney/Liner Geometry To ensure favorable wet opera ti on, the chimney shell and liner

- inlet geometry and the liner diameter should be defined as early as possible in the initial study phase of a wet stack project. Some flexibility is required to accommodate the different scrubber arrangements and sizes available. Refer to Sections 2 and 3 of this design guide for more information on chimney

- geometry requirements. Special consideration should be paid to choosing a liner diameter that results in liner

- gas velocities favorable for the wet operation of the liner material under consideration

. 4. 2 .4 Perform Model Study Laboratory flow

- modeling should be conducted before the final design phase to design and optimize liquid

- collection devices and drains for the absorber outlet duct and stack. The objectives of a flow

- model study are as follows:

 Experimental evaluation of the absorber outlet duct and stack liner in order to assure proper geometry and favorable wet operation

 Design of flow controls to minimize system

- pressure losses

 Design of liquid

- collection devices

 Dete rmination of drainage locations

 Performance of a plume

- downwash computational fluid dynamics ( CFD) study to determine the required liner

- extension height above the top of the stack shell and/or to determine how much of the top of the stack shell should be painted with an acid

- resistant coating Laboratory flow

- modeling is a valuable tool for the design of a successful wet stack system and should be co nducted by an experienced model

- testing firm. 4. 2 .5 Determine Liquid - Collection Devices Liquid

- collection devices are critical to the successful operation of a w et stack system. After the flow

- model study is performed, liquid

- collection devices can be located and incorporated into the bid drawings. If the model study is to be performed after the chimney contract is awarded, then bid quantities need to be provided with the specifications to cover any potential liquid

- collection devices. Basic system designs from previous studies of similar units can be provided for estimating purposes. 4- 15  4. 2 .6 Prepare Bid Specifi cations The bid specifications can be generated once the feasibility study, preliminary layouts , and material selections have been completed. The bid specifications should be as detailed and complete as possible to ensure competitive bids and a successful wet stack system. Major items included in the bid specifications are discussed below in the subsection, "What to Specify in a Bid Document ." 4. 2 .7 Prepare Bid Drawings Drawings are prepared and included with the bid specifications in order to clearly defin e the scope of work. A t a minimum, the bid documents include a site plan, chimney general arrangement, typical details, and electrical drawings. 4. 2 .7.1 Site Plan A site plan included with the bid specifications conveys the arrangement of the power plant in relation to the area for the proposed construction. These drawings should include lay

- down areas for material storage and prefa brication/assembly activities. Th ese drawings will allow the contractor to plan his construction operations. It also indicates restricted or congested areas on the plant site with which the contractor will have to contend

. 4. 2 .7.2 Chimney General Arrangement A general arrangement of the chimney included in the bid documents gives the bidders overall guidelines about the scope of work. Major features of the wet stack are typically included on the chimney general

- arrangement drawing. The chimney general arrangement is only meant to serve as a guideline and is not intended to provide final design information. The chimney inlet

- duct elevation may need to be adjusted , depending upon the absorber height. 4. 2 .7.3 Typical Details Typical details that the owner or specifier wishes to incorporate into the final design are included in the bid specifications. Typical details may include platforms and handrails, rain hoods, expansion joints, duct supports, test ports and probe

- support jibs, insulation and lagging, lining systems , and liquid

collection devices such as gutters. 4. 2 .7.4 Electrical El ectrical drawings include items such as an electrical

- grounding g rid, obstruction lighting, test

- platfor m power and lighting, personnel

- elevator power wiri ng, conduit, annulus

- pressuriz ation wiring (if applicable), and electrical

equipment layout. 4- 16  4. 3 What to Specify in a Bid Document This section discusses items included in a bid specification for a wet stack. 4. 3 .1 General Requirements In the general

- requirements section of the specifi cations, the scope of work must be clearly defined. Construction schedules, submittal schedules, temporary construction power , and chimney design and construction requirements should also be specified. The general requirements must be specific either to a new chimney or to an existing chimney that is to be modified. 4. 3 .1.1 Scope of Work The scope of work for the wet stack contract must be clearly defined in the bid specifications. A complete description of the entire project may also be included in the sp ecifications. A new or retrofit wet stack, which involves constructing a new stack, is usually designed, furnished, and constructed by a chimney contractor who must comply with a performance specification that is usually prepared by an architectural/engineering firm. A conversion to a wet stack, which involves converting an existing stack to wet operation, is usually designed by an architectural/engineering fi rm that produces a construction

- contract specification. All work to be performed under a certain co ntract is clearly defined to avoid any change orders or schedule delays later. The specification should also clearly state that the contractor is responsible for furnishing and erecting all materials (if applicable), and for providing the supervision, labo r, tools , and transportation required for construction of the work specified. Work by others is included in multiple

- contract projects, to assist the contractor with on

- site coordination and scheduling issues. A general work sequence may be helpful in avoi ding unnecessary construction delays. A list of contract drawings, reference drawings , and reporting requirements is also helpful to the contractor performing the work. 4. 3 .1.2 Site Requirements Consideration is given to site requireme nts such as construc tion parking, access roads, lay

- down areas , and any plant restrictions that might impact construction efforts. 4. 3 .1.2.1 Construction Parking Construction

- parking areas may be identified to provide sufficient parking for all constructi on personnel. Constr uction

- parking areas may need to be maintained throughout the construction process and restored to the original condition after construction (if so desired). 4- 17  4. 3 .1.2.2 Access Roads The bid specifications may include a section to provide and maintain vehicular access, both to the site and within the site during construction. New access roads may need to be constructed to perform the work. Restrictions should be imposed on existing access roads which cannot support construction activities. 4. 3 .1.2.3 Lay

- Down Areas Construction lay

- down areas may need to be considered in the bid specifications. The following items should be addressed:

 Adequate area for construction

- material storage

 Prov isions for substantial, weather

- tight enclosures (if necessary for storage of materials)

 Control of temperature and humidity according to manufacturers' recommendations

 Platforms, blocks , or skids to protect mat erials from soiling or staining

 Provisions for fabrication areas and/ or enclosures if substantial on

- site prefabrication/assembly activities are planned , such as the o n- site winding of FRP liner cans 4. 3 .1.2.4 Safety Barrier A provision addressing the need for a safety barrier around the base of the chimney , which would protect personnel working below from falling objects , is typically included with the specification. Chimney c ontractors usually require a 50-ft- wide safety zone around the base of the chimney during construction of the concrete shell , exterior platforms, and ladders. A debris net on the outside of the chimney shell is typically required to catch falling objects during construction. 4. 3 .1.2.5 Coinciding Construction Activities This section alerts the contractor of any concurrent construction ac tivities in the chimney area. It allows the chimney contractor to make necessary provisions regarding coordination of construction activities between different contractors and/or the owner. For example, other constructi on activities within the safety

barri er zone cannot take place while the concrete chimney shell is being constructed. 4. 3 .1.2.6 Plant Restrictions Restrictions on construction imposed by the owner or by plant operations are no ted in this section. Restricted

- access or lay

- dow n areas may be ad dressed. Plant

- outage schedules may also be included in this section. 4- 18  4. 3 .1.3 Project Meetings and Schedules This section of the bid specifications includes requirements for the following:

 Project meetings

 Preconstruction conference

 Finalizing schedules

 P rogress meetings and methods of sub mittal

 Coordination conferences

 Safety meetings

 Schedules and reports

 Initial coordination submittals

 Work

- progress schedule

 Work

- progress reports 4. 3 .1.4 Submittals Submittal requirements specify what items must be reviewed by the engineer before fabrication or construction. Submittals include both compliance submittals and miscellaneous submittals. Compliance submittals include shop fabrication drawings, product data , and samples that are submitted by the contractor, subcontractor, manufacturer, or supplier. Miscellaneous submittals include technical reports, administrative submittals, certificates , and guarantees. 4. 3 .1.5 Temporary Utilities and Facilities Temporary utilities and facilities are specified to address water distribution, drainage, dewatering equipment, enclosure of work , heat, ventilation, electrical

power distribution, lighting, hoisting facilities, stairs, ladders , and access roads. In general, the sp ecifications usually require the contractor to furnish, install , and maintain temporary utilities required for construction, safety , and security. 4. 3 .1.6 Chimney Design and Construction Requirements This section of the bid specifications includes minimum standards for design, materials, and construction. The contractor is normally responsible for the detailed design of the wet stack, whether it be new or a retrofit construction. Specific design parameters for the piling, foundation, chimney shell, and lin er and/or lining material should be included in this section so the contractor can perform the final design. The wet stack geometry can be included in this section of the specifications unless indicated on the drawings. The operating conditions for the unit, such as maximum gas - flow rate, maximum temperature , and gas exit velocity at maximum gas flow , should be specified in this section. Specific site conditions , such as the location of nearby structures and chimneys, are considered for vortex

- shedding wind loads on the new o r existing chimneys. 4- 19  4. 3 .2 Site Work A bid specification for a new or retrofit wet stack that involves constructing a new stack includes excavation, backfill, and foundation piling ( if necessary )

. Modification of an existing stack (if co nverted to wet operation) may require the designer to analyze the existing stack and foundation, including piling, for new wind and seismic loads ( depending on applicable code requirements ) . The existing stack and foundation also need to be analyzed for ad ditional dead load, if the existing liner is revised or replaced. 4. 3 .2.1 Excav ation and Backfilling A new stack will require excavation and backfill for the chimney foundation. Dewatering requirements for the excavation during construction should be addressed. Proper fill material and backfilling procedures are also typically addressed. 4. 3 .2.2 Piling or Drilled Shafts This section addresses furnishing all the labor, equipment, and materials necessary to install foundation piling or drilled s hafts fo r the chimney structure ( if applicable ). 4. 3 .3 Concrete The concrete section of the bid specifications for a new wet stack includes such items as formwork, reinforcement, concrete materials , and quality control. The contractor submits the concrete

- mix design, which should preferably be based on field experience. However, whe n sufficient or suitable strength

- test data is not available, concrete shall be proportioned on the basis of a laboratory

- trial mix design. 4. 3 .3.1 Materials Materials of construction relating to the concrete work are usually specified. Concrete materials typically comply with applicable building codes, ACI standards, and ASTM requirements that are referenced in the concrete section of the specifications. The following items should b e specified:

 Type of Portland cem ent (and fly ash if applicable)

 Fine aggregate

 Coarse aggregate

 Mixing water

 Admixtures (such as water

- reducing type, air

- entrai ning type, retarding type, high -r ange water
- reducing type)

 Mix proportions (based on field expe rien ce or laboratory

- trial batches)

 Compressive strength 4- 20   Slump

 Air Content 4. 3 .3.2 Foundation Foundation requirements for a new wet stack are included in the concrete section of the bid specifications. In some cases involving a new stack, the chimney cont ractor is responsible for designing the chimney foundation, including the number and layout of piling ( if required ). However, the foundation is often designed and supplied by the general contractor . Pile type and capacity is generally determined by the arc hitect/engineer. 4. 3 .3.3 Chimney Column, Construction Tolerances The chimney column or concrete chimney shell may require special cement or concrete additives in the mix design to facilitate rapid construction. Coatings are usually used to withstand the p otentially corrosive environment at the top of the column. Construction tolerances for vertical alignment of center point, diameter , and wall thickness for a new chimney column conform to ACI 307. 4. 3 .3.4 Brick

- Liner Support Pedestal The liner support ped estals usually are specified to meet the same concrete requirements as the chimney column. 4. 3 .3.5 Roof Slab Specifications for a concrete roof slab should consider eithe r a corrosion

- resistant sealant, a coating applied on the concrete surface , or a sulf ate
- resistant cement

— such as ASTM C150 Type V cement. 4. 3 .4 Liner Liner materials of construction, strength requirements, construction requirements, and ambient considerations are specified in this section. 4. 3. 4.1 Materials Liner materials of construction for a new chimney liner are specifie d in the bid documents. The m aterial selected should conform to the respective ASTM standards , unless specifically noted otherwise. Refer to Sections 2 and 3 of this guide for liner

- material discussions. 4. 3. 4.2 Strength Requirements Strength requirements for various liner materials are dictated by the structural design of the liner. For example, the yield strength of a steel or FRP liner and the compressive strength of a brick liner should be specified. 4- 21  4. 3 .4. 3 Construction Requirements Construction requirements for the selected liner materials are typically included in the bid specifications. These requirements are primarily based on good construction practice, recognized and accepted throughout the industry. Unique construction requirements are also included in the bid specifications. 4. 3 .4. 4 Construction Tolerances Construction tolerances depend on the liner materials of construction selected. Construction tolerances are either specified or referenced to a specific code or standard. 4. 3 .4. 5 Ambient Conditions Ambient conditions may adversely affect the quality of liner construction , depending on the liner material selected. Certain materials , such as coatings, adhesive membranes, or mortars, may need to be stored and installed in a controlled environment to provide satisfactory performance. 4. 3 .5 Ductwork and Expansion Joints The chimney inlet

- duct and expansion

- joint section includes provisions for the contractor to design, fabricate, furnish, deliver , and erect these it ems. The ductwork and expansion

- joint section specif ies the materials of construction, design criteria , and fabrication and erection requirements. 4. 3 .6 Miscellaneous Metals Structural and miscellaneous metals are included in the metals section of the bid specifications. Miscellaneous metals include such items as platf orms and ladders, liner or roof

- support framing, test ports, and rain hood ( if applicable ). 4. 3 .7 Platforms and Ladders The materials of construction for wet stack platform s and ladders are specified. Carbon steel is typically specified for the entire ladder, except for the upper 50

– 100 f t, which should be fabricated from a more corrosion

- resistant material. The ladder should also include a Saf -T- Climb device for personnel s afety. Wet stack platforms may be fabricated from carbon steel , with the exception of the upper portion of the stack, in which case the material should be similar to the upper portion of the ladder. The extent and type of corrosion

- resistant materials spec ified will depend on the environment to which the materials will be exposed and the extent of downwash anticipated. 4- 22  4. 3 .7.1 Liner or Roof

- Support Framing Material selection should be considered wh en specifying the liner or roof

- support framing. The environment to which the materials will be exposed will dictate the material selection. 4. 3 .7.2 Test Ports Test

- port materials are typically specified and conform to applicable ASTM standards , depending on the materials selected. The materials selected will depend up on such factors as the stack

- liner material, their location on the stack, corrosive environment, intended purpose of the test port, and material costs. 4. 3 .7.3 Insulation and Lagging It is unnecessary to maintain the tempe ra ture of the saturated, absorber

- outlet flue gas above a certain minimum temperature. However, insulation may reduce the amount of condensation in a metal or FRP liner. If insulation is required, the insulation and lagging materials and design are specifi ed in the bid documents. 4. 3 .8 Access Doors Access doors are usually specified for the chimney liner at flo or level or in the chimney

- inlet ductwork for cleanout and maintenance. Access doors may be required in the chimney shel l for annular platform and t est
- port access. Access doors ma y also be specified for a brick

- liner support pedestal ( if applicable ). Addit ional liner

- access doors can be provided adjacent to liquid collector s to visually inspect the condition of the collectors and determine if the dra in pipes need to be cleaned. These access doors should be at the following locations:

 Liner

- entrance area a bove breeching duct

 Near expansion

- joint drains

 At the choke The size and location of the access doors may need to be evalu ated and determined by the flow

- modeling company. Liquid collector s and/or liquid

- film diverters may be required around access doors to prevent droplet re

- entrainment from these areas. 4. 3 .9 Protective Coatings Protective

- coating systems for each particular component of the wet stack are included in the bid specifications, if needed. Interior and exterior coating systems may also be considered for the stack shell and liner. An exterior protective coating may also be considered for the chimney inlet duct, if it is fabricated using wal lpapered construction or a clad

- plate material. 4- 23  4. 3 .9.1 S hell Exterior/Interior Protective coatings may be considered for the upper portion of the concrete shell of a wet stack to protect it from the corrosive effects of sa turated flue gas during a plume

- downwash event. The extent of coverage can be determined from a plume

- downwash study. Additional consideration is given to coating the exterior of adjacent existing stacks, which may be subject to the corrosive effects of saturated flue gas from the wet stack. 4. 3 .9.2 Liner Exterior/Interior If a liner is selected with a carbon

- steel exterior such as a n alloy

- clad or wallpapered liner , or one utilizing a borosilicate

- glass or elastomeric interior lining system , then an exterior coating system may b e specified. If a coated carbon

- steel liner is to be used, then the interior coating must also be specified. Refer to Section 2 of this guide for additional information on recommended lining systems. 4. 3 .9.3 Structural and Miscellaneous Steel The coating system selected for structural and miscellaneo us steel is evaluated on the basis of the environment to which the steel is exposed , access requirements for maintenance, the cost of the coating system, and performance history. 4. 3 .10 S pecial Construction (Tuned Mass - Damping System) Special construction may be necessary when converting an existing stack to wet operation. For example, structural modifications may be necessary when converting a steel stack to wet operation. If an existing liner is replaced with a new liner , or an existin g liner is relined with a glass

- block or coating

- lining system, the change in the overall mass of the stack could affect the natural frequency of the structure. A dynamic analysis is typically performed for vortex

shedding wind loads. If resonance at critical wind speeds is a problem, then structural modifications

— such as helical stakes, stiffeners , or the add ition of a tuned mass dampener

— may be necessary to ensure the structural integrity of the existing steel stack. 4. 3 .11 Personnel Elevator The bid specifications for a new wet stack may require furnishing and installing an electric rack

- and

- pin ion

– drive personnel elevator. Design requirements, such as conformance to applicable sections of ANSI, access to established platform elevations, minimum lifting capacity , and minimum

- design wind loads (for exterior elevators) are typically included. I f fabricated from carbon steel , t he ex posed metal work of the elevator and supporting structure should be galvanized or protected with a coating system. For the upper 50

– 100 f t of the chim ney, the mast and rack is usually specified to be Type 316 stainless

- steel material. The electrical power supply and wiring also needs be specified. 4- 24  4. 3 .12 Mechanical The following mechanical sections are typically included in the wet stack bid specificati ons. 4. 3 .12.1 Drain Piping The specifications provide for all pipes, fittings, hangers, supports , and accessories required to compl ete the chimney corrosive drain

- piping systems associated with the liquid

- collection system. Specific piping material should be specified for each application. FRP or alloy piping is normally specified for drain

pipe material. Minimum piping

- installation standards are usually also addressed. Stack

- liner liquid

- collector drains should incorporate some easily accessible method for monitoring or observing the drainage flow. This capability will be useful for determining the performance (or nonperformance) of the liquid

collection system. 4. 3.12.2 Pressurization

- System Fan s/Dampers This section of the bid specifications applies to a concrete chimney with an acid

re sistant brick liner. An annulus

- pressurization system is required to keep flue gases contained within the liner. The pressurization -s ystem section should include furnishing and installing the required number of fans, including drive motors, controls , and accessories. 4. 3.12.3 Louvers The bid specification s include a section requiring the contractor to furnish and install louvers to provide ventilation between the chimney liner and shell. 4. 3 .13 Electrical The electrical section of the bid specifications include s provisions to furnish and install all electric equi pment, wiring (including plant -i nterface wiring), grounding , and lighting necessary for a wet stack syst em. 4. 3 .13.1 R ounding Grounding for all new electrical equipment is included in the bid specification s . The grounding requirements for a new wet stack are similar to the requirements for a dry stack. Any new electrica l equipment used in a wet stack

– conver sion project needs to be properly grounded. 4. 3 .13.2 Obstruction Lighting The bid specifications include furnishing and installing obstruction lighting, including temporary obstruction lighting during construction. Obstruction lighting must comply with the governing standards of the U.S. Department of 4- 25  Transportation, Federal Aviation Administration (FAA) Advisory Circulars and the National Electrical Manufacturers Association (NEMA). 4. 3.13.3 Lightning

- Protection System (Master Label) The bid specificat ion s include provisions to furnish and insta ll a complete chimney lightning

- protection system, i ncluding a below

- grade, chimney

- ground system. The C ontractor may need to provide a Master Label C plate issued by Underwriters' Laboratories, Inc., and attach the plate to the base of the chimney. 4. 3 .13.4 Plant Interface Wiring, Power Panel, and Disconnect Switch The electrical section of the bid specifications address plant

- interface wiring, the power panel , and the disconnect/lockout switch. The contents of this section are us ually similar to standard power

- plant chimney electrical requirements. 4. 3.13.5 Pressurization

- System Fan Wiring and Controls The pressurization

- system fan wiring and controls section of the specifications usually follow the same genera l requirements as those for a dry chimney. 4. 4 What to Ask For in a Bid Document This section describes the C ontractor's responsibilities for information that is typically submitted , and inspection and testing that is usually performed , for the design and construction of a wet stack or f or wet stack modifications. 4. 4 .1 Warranties/Guarantees The Contractor is typically required to furnish a performance and payment bond. A performance bond is a guarantee that all work will be carried out in accordance with the Contract Documents. A payment bond guarantees that the Contractor will pay all lawful claim s for payment to subcontractors and material suppliers and for labor supplied in performing the work under the Contract. Normally, a one

- year warranty is specified for material and labor (one year from initial operation). If an unproven technology is being proposed by the Contractor, an extended warranty would be needed. 4. 4 .2 Design Calculations The Contractor may be requested to submit calculations that confirm the adequacy of the design. The calculations confirm that the design was performed in accordance with the codes, standards, and specification requirements specified. Design calculations are sealed by a professional engineer registered in the state where the stack is being constructed. 4- 26  4. 4.2.1 Chimney Foundation Design calculations for the chimney foundation may also be required for submittal. The following is a list of items to be included with this submittal:

 Foundation layout and size

 Foundation loads

 Soil

- bearin g pressure load or pile loading

 Facto r of safety against overturning

 Radia l and tangential design moments

 Radial and tangential reinforcing

- steel requirements

 Shearing

- stress calculations 4. 4 .2.2 Concrete Shell The concrete

- she ll calculations specify all design loads and operating conditions used. Calculations are provided for representative elevations throughout the height of the chimney. The following information is typically provided with these calculations:

 Wi nd analysis and resulting loads

 Dynamic seism ic analysis and resulting loads

 Column geometry, including diameters and wall thicknesses

 Column deflectio ns

 Thermal stresses

 Reinforcing

- steel requirements 4. 4 .2.3 Liner Calculations are also typically submitted for the line r design. The design loads, operating conditions, and material properties are specified. Load combinations applicable to the liner material are addressed per the governing code requirements. Information that is typically submitted with the liner design is as follows:

 Loads and stresses for wind, seismic, th ermal, and dead

- load conditions

 Liner geometry, including diameters and wall thicknesses

 Liner deflections

 Factor of safety against overturning (brick liners)

 Factor of safety against buckling (steel or a lloy liners)

 Joint/weld specifications 4- 27  4. 4 .2.4 Pressurization

- Fan Size The Contractor is usually required to size the pressurization fans if a brick liner is to be constructed or converted to wet operation. The information that the Contractor is required to furnish is as follows:

 Fa n manufacturer and model number

 Fan static pressure , including accessory los ses at the design point

 Fan

- volum e flow rate at the design point

 Fan

- motor manufacturer and type

 Fan

- motor horsepower rating

 Fan

- performance curves, including capacity versus static pressure, e fficiency, and shaft horsepower

 Design basis for determin ing fan size and leakage values

 Drawings of the fan housing and base frame 4. 4 .3 Materials Testing To confirm compliance with specification requirements, the Contractor is required to submit materials

- testing results and certifications for materials of construction. 4. 4 .3.1 Acid

- Resistant Brick Certified laboratory

- test reports may be submitted for each lot of brick manufactured. Brick should not be shipped until it is confirme d that the certified laboratory

- test reports comply with ASTM C980 and the specified requirements. Random

- sample testing of the brick delivered to the job site may also be performed in the field, as needed. 4. 4 .3.2 Brick

- Mortar Certifi cation The brick

- mortar manufacturer typically submits certification that the mortar c onforms to ASTM C466. Mortar

- data submittals usually include the following information:

 Mix proportions and mix methods

 Appl ication temperature information

 Curing and cur ing protection required 4. 4 .3.3 Fiberglass

- Reinforced Plastic (FRP) Material components of the FRP liner , such as resin and glass , are typically certified for compliance with the appropriate ASTM standards and specification requirements. All materials, prior to and during fabrication, as well as the completed sections of the liner , may be subject to laboratory tests. Resin

- batch 4- 28  numbers and shelf life are recorded for future reference. For ea ch batch of material, resin gel

- time tests are performed in accor dance with ASTM D2471, and resin

- viscosity tests are performed in accordance with ASTM D2393. 4. 4 .3.4 Alloy

- Material Certifications Material

- certification reports are usually furnished for all alloy materials and for the alloy

- welding materials. Certification reports should verify that the chemical composition of the material is in conformance with the ASTM or specification requirements. 4. 4 .3.5 Concrete

- Material Certifications/Mix Design

. Mill certificates may be submitted for the cement being u tilized to confirm its conformance with ASTM C150 and to ensure that the correct type is being furnished. Certifications are also required for the coarse and fine aggregate (ASTM C33) and for any admixtures (ASTM C494) that will be utilized. Concrete

- mix d es ign proportions and compressive

- strength -t est results submittals are used to substantiate the proposed mix design. 4. 4 .4 Construction Procedures The Contractor is responsible for his construction means, methods, techniques, sequences , or procedures. The Engineer or Owner usually do not dictate and approve the Contractor's construction practice. Submittal of construction procedures are only required to confirm that the work can be performed within schedule and meet the intent of the Contract Documents. 4. 4 .4.1 Foundation Construction Chimney foundations are large

- mass concrete placements that require substantial planning and coordination. Before placement, the Contractor, Engineer , and Owner review and coordinate the following issues:

 Concrete

- placement s c heme (terracing or full lifts)

 Method of concrete placement (p ump trucks, buckets, or chutes)

 Concrete

- delivery schedule

 On

- sit e concrete
- testing requirements

 Method of concrete curing

 Adequacy of formwork

 Adequate pe rsonnel and equipment available 4. 4 .4.2 Hot - or Cold

- Weather Concrete Procedures Precautions need to be taken to ensure that hot - or cold

- weather conditions do not adversely affect the quality of the concrete. ACI 305R provides the guideli nes and recommendations for hot

- weather concreting. Alt hough several options are 4- 29  available, the most commonly used hot

- weather concreting construction techniques are:

 Using ice or cold water in the concrete mix

 Applying cold

- water spray to aggregates

 Placement of concrete during the coo lest time of the day or evening ACI 306R provides the guidelin es and recommendations for cold

- weather concreting. Because of the difficult y and expense of providing cold

- weather protection, construction of the chimney shell during the winter is not recom mended. In some instances, cold

- weather situations cannot be avoided , and precautions need to be taken to ensure that the concrete is not damaged. Insulating blankets are commonly used to protect the concrete while it cures , and they are of ten used during occasional cold

- weather periods. An adequate supply of blankets or other means of protection should be readily available on

- site in cold weather. The Contractor may be requested to submit their hot - and cold

- weather concrete procedures for approval and be prepared to implement these procedures on short notice, if needed. 4. 4 .4.3 Liner Construction Liner

- construction procedures are important for scheduling and coordination. Liner materials of construction can take up a considerable amount of lay

- down and/or prefabrication area. Access to and installation of these materia ls need to be coordinated with other construction activities in the area. Scheduling and lay

down areas associated with liner materials may be submitted and incorporated within the overall project schedule. 4. 4 .4.4 Welding Procedures Submittal and implementation of welding procedures is essential in maintaining quality control of welding for a wet stack application. Because the quality of the weld affects the corrosion resistance o f an alloy liner, a high level of quality control needs to be enforced during welding. Coated carbon

- steel liners also require high

- quality welds to ensure proper performance of the coating system. For weld

- fabrication requirements for lining applications, refer to ASTM D4618, to NACE Standard RP0178 -89, Standard Recommended Practice Fabrication Details, Surface Finish Requirements, and Proper Design Considerations for Tanks and Vessels to be Lined for Immersion Service, and to NACE Standard RP0292-92, Stan dard Recommended Practice Installation of Thin Metallic Wallpaper Lining in Air Pollution Control and Other Process Equipment. Horizontal weld seams in a wet stack a re typically limited to 1/8

- in (0.003

- m) reinforcement or projection on the inside of the l iner. Horizontal welds on the inside of a choke are usually ground flush to prevent liqui d re

- entrainment. Vertical weld

- seam reinforcement in a wet stack, including the choke, is al so typically limited to 1/8 in (0.003 m). Limiting the size of the horizontal weld 4- 30  seams is more critical than the vertical weld seams with regard to minimizing liquid re

- entrainment. For all welding processes, th e Contractor prepares and submits for approval the welding

- procedure specification (WPS) and the procedure

- qualificat ion test results (PQR). For carbon

- steel welding, the welding

- procedure specification includes the non mandatory information included in Appendix E of AWS D1.1 , in addition to the mandatory information listed in Appendix IV, Table IV

- 1 of AWS D1.1. For allo y welding, the welding procedures typically address all essential and nonessential variables of Section IX of the ASME Boiler and Pressure Vessel Code. Only approved welding procedures are typically used. 4. 4 .4.5 Coating Procedures Proper preparation and application of a protective coating is important to ensure its corrosion resistance and longevity. The following submittals may be required to ensure that the coating is properly installed:

 Surface

- preparation procedures

 Application procedures, including t hickness tolerances

 Curing procedures

 Test procedures and test results

 Coating formulation with number and thickness of coats required

 Weld and seam repair procedures

 Manufacturer's standard data and recommendations for surface preparation, application, cu ring, and testing

 Quality

- control reports a nd certifications

 Temperature, humidity, and ventilation conditions to be maintaine d during application and curing 4. 4 .5 On - Site Testing and Inspection Ongoing on

- site testing and inspection are required throughout the project's construction phase. Testing confirms that the materials of construction meet contract requirements. Inspections verify that the Contractor is performing the work in accordance with specification requirements. 4. 4 .5.1 Pil ing Piling is often used to support the chimney foundation. Several types of piling systems, such as steel H

- piles or auger

- cast concrete piles, are commonly used. Tests and inspection application s for piling are as follows: 4- 31  4. 4 .5.1.1 Load Test The Contractor is typically required to install test piling to confirm design conditions and to verify that the installed piling will provide an adequate factor of safety versus the design loads calculated. The piling is tested for compression, uplift, and lat eral loads, as applicable. 4. 4 .5.1.2 Grout Cube Tests A qualified testing laboratory may be retained by the Owner to perform grout test ing for pressure

- grouted, auger

- cast piling systems. Testing is performed in accordance with CRD

- C 620 and ASTM C109. Th e results of the cube tests are used to substantiate evidence of adequate strength and uniform consistency of the grout. 4. 4 .5.1.3 Visual Inspection Piling may be inspected to ensure correct installation and location. Auger

- cast pile reinforcing steel sho uld be inspected before placement. Steel H

- piles should be inspected to confirm that the flanges are oriented properly with respect to the strong and weak axi s direction. Recommended piling

- installation tolerances are as follows:

 Piles should not exceed a variation from th e vertical of more than 1/4 in (0.006 m) per foot of pile length.

 The center of the pile head should not vary from plan location at the cutoff by more than 3 inches. 4. 4 .5.2 Foundation Inspection of the chimney foundation is required befo re and during concrete placement. 4. 4 .5.2.1 Embedded Items Placement and positioning of all embedded items should be verified. Examples of items that are typically embedded in a chimney foundation are:

 Anchor bolts

 Electrical conduits

 Grounding

 Drains 4. 4 .5.2.2 Reinforcing

- Bar Placement Reinforcing steel embedded in or projecting out of the foundation may be inspected to confirm correct number, size , and sp acing. Splice lengths and dowel

- projection lengths may also be verified. If mechanical splice connec tors are 4- 32  used, the connectors are capped off to provide protection from the wet concrete. Mud or debris is removed from the rebar before placement of the concrete. 4. 4 .5.2.3 Concrete Placement Inspection during concrete placement is performed to ensure th at the concrete is being properly placed. Inspection also confirms that the concrete is vibrated after its placement. Concrete should be placed in lift heights that permit the concrete to be vibrated into the lower section. The Contractor should select a p lacement sequence so that a minimal amount of concrete surface area is exposed at a given time. This reduces the opportunity for the concrete to develop its initial set and maintains a working surface that is easy to vibrate. 4. 4 .5.2.4 Concrete Testing Concrete testing is usually performed by a qualified lab oratory
- testing company. Before placement, the concrete is tested for temperature, slump, and air content. Concrete cylinders are taken to perform compression

- strength tests. 4. 4 .5.3 Concrete Column Inspection of the concrete column is performed during construction to confirm that the chimney is being constructed in accordance with the design drawings. 4. 4 .5.3.1 Dimensional Tolerances Dimensional tolerances for the concrete column are established by ACI 307. Vertical alignment of the column’s center point is taken by plumbing down to a reference point on the chimney foundation. The column's diameter is taped , and measurements are taken for out

- of

- roundness tolerances. 4. 4 .5.3.2 Location of Openings The orientation, elevation, and size of openings in the column should be confirmed. The breeching opening should be accurately located in order to avoid misalignment pr oblems with the absorber outlet

- duct tie

- in. Because the breeching is typically installe d before the outlet ductwork, a survey crew should position the breeching from an established benchmark that can also be used by the absorber contractor. 4. 4 .5.3.3 Reinforcing

- Bar Placement Vertical reinforcing bars should be counted and bar s izes checked for each concrete -l ift height to confirm that the bar quantities match the design requirements. Horizontal bars are checked for size and location. Splice lengths and concrete cover should also be checked. 4- 33  4. 4 .5.3.4 Wall Thickness Wall thicknesses are typ ically measured to verify conformance with the design requirements. 4. 4 .5.3.5 Concrete Testing Concrete testing is typically performed for each lift section (jump form) or for each 8

- hour shift (slip form) by a qualified laboratory

- testing company. Testin g is normally performed in the bottom of the chimney at the point of discharge from the concrete truck. The t emperature, slump, and air content of the concrete are measured and recorded. Concrete cylinders are taken to verify compressive strength. 4. 4 .5.4 Acid

- Resistant Brick and Mortar Materials of construction and construction tolerances are typically monitored for liners constructed of acid

- resistant brick and mortar. 4. 4 .5.4.1 Mortar

- Cube Strength Tests During construction of the brick liner, mortar c ubes are taken daily for compressive

- strength tests. This testing is usually performed on

- site by a qualified testing laboratory. It is recommended that two sets of mortar

- cube samples should be tested: one set that is cured in the bottom of the stack to r epresent actual in situ conditions during construction, and another set that is cured under laboratory conditions in accordance with ASTM guidelines. Because the mortar

- curing process is heat

- sensitive, compression

- strength test results from mortar cured in the bottom of the stack may indicate whether adequate heat is provided during construction in cold weather. 4. 4 .5.4.2 Visual Inspection of the Liner Inspection during construction of the brick liner may be used to confirm that the following recommended construction practices are maintained:

 Projections on the inside face of the li ner should not exceed 1/8 in (0.003 m).

 The liner's vertical axis should neither be off the theoretical axis by more than 0.1 percent of its height nor by 1 in (0.03 m), whichever is greater. Also, the center of the liner should not vary by more than 1 in (0.03 m) in 10 f t (3 m).

 Neither the diameter nor the radius of the liner should differ by more than 2% from those specified.

 During the liner's construction and curing perio d, a minimum temperature of 50° F (10°C) should be maintained in the shell's interior. 4- 34  4. 4 .5.5 Metal Liners On

- site inspection of metal liners is primarily related to welding

- quality control. Welding inspection for alloy materials is critical to ensure the co rrosion resistance of the weld in a wet environment. 4. 4 .5.5.1 Welder Qualifications For carbon

- steel welding, all welders must typically be qualified by passing tests prescribed in the AWS D1.1 Standard Qualification Procedure . Welders should have been t ested within the past 12 months , and their qualifications may be considered in effect unless the welder is not engaged in a given process of welding for more than six months. For alloy welding, all welders are typically being qualified in accordance with S ection IX of the ASME Boiler and Pressure Vessel Code. In addition, a t a minimum , the tests should meet the radiographic acceptance criteria of QW -191 of Section IX. 4. 4 .5.5.2 Welding Inspection Verification inspection and testing of welds for carbon

- stee l liners may be performed in accordance with AWS D1.1 and ASCE's Design and Construction of Steel Chimney Liners. Inspection of material and workmanship may verify conformance with the AISC ’s Manual of Steel Construction Specifications and Codes and the AI SC ’s Quality Criteria and Inspection Standards

. For alloy welding, all welds should be visually inspected by an AWS Certified Welding Inspector (CWI) , qualified per AWS Standard QC1. All alloy welds that will be exposed to flue gas are tested using liquid

- penetrant examination according to Article 6 of ASME Section V. All liq uid

- penetrant testing is performed by an ASNT Level II inspector. Certification verifying the qualifications of the welding inspector should be submitted before welding operations start
. 4. 4 .5.5.3 Weld

- Quality Workmanship Standards The Contractor usually prepares and submits workmanship samples for approval prior to the start of work. The workmanship samples are visually inspected and liquid penetrant tested prior to submittal. The appr oved workmanship samples are visibly displayed at the shop and job site for the benefit of the welders and inspectors, as examples of acceptable weld surfaces. 4. 4 .5.6 FRP Liners Inspection and testing of a n FRP liner is typically performed during the liner's fabrication and installation. 4- 35  4. 4 .5.6.1 Physical Testing Physical testing may be performed and submitted for representative wall samples. The following tests may be performed as appropriate:

 Resin/ glass ratio for interior and exterior layers per ASTM D2584

 Barcol hardness for surface cure per ASTM D2583

 Tens ile modulus per ASTM D638

 Flexural modulus per ASTM D790

 Co mpressive modulus per ASTM D695

 Coefficient of thermal expansion per ASTM E228

 Coefficient of thermal con ductivity per ASTM C117 or C518

 Specific gravity per ASTM D792

 Chemical re sistance per ASTM C581 or D4398

 Heat

- defle ction temperature per ASTM D648

 Flame retardancy per ASTM E84 4. 4 .5.6.2 Visual Inspection Wall samples may be provided by th e Contractor for use as representative samples of visual standards for the liner. The liner appearance is typically free from defects , including delaminations, cracks, bubbles, pinholes, and other defects affecting strength or serviceability. For curing co nsiderations, the air temperature in the work area is maintained at a minimum of 50 ° F (10°C). All resins and FRP components are stored in accordance with the manufacturer's recommendations. The resin temperature at its point of use should typically be betw een 60°F (15°C) and 75°F (24°C). 4. 4 .5.7 Borosilicate

- Glass Block All glass

- block lining installation s may be subject to inspection. Quality

- control procedures and requirements for installing a borosilicate

- glass -b lock lining system may be performed in ac cordance with the manufacturer’s s pecifications. Specific items that are typically inspected or tested are presented below: 4. 4 .5.7.1 Surface Preparation For installation on steel liners, any existing fly ash buildup or coating should be removed , and any abrupt contours or edges on welds should be rounded off smoothly by grinding. The interior steel

- su bstrate surface should be blast

- cleaned to white metal in accordance with SSPC

- SP5. For installation on brick liners, the condition of the inside surface of the liner should be inspected. Fly ash buildup may require that the surface be cl eaned by 4- 36  hydro blast or sandblast. Mortar joints should be inspected. Protruding mortar should be chipped off. 4. 4 .5.7.2 Visual for Oxidation After sandblasting the steel substrate, an inspection should be performed to confirm that rust bloom on the steel has not developed. A primer should be applied on the sandblasted steel substrate to protect the steel from the formation of flash rust during the period of time between comple tion of sandblasting and application of the block

- lining system. 4. 4 .5.7.3 Surface

- Contamination Tests The inside surface of the liner should be tested to confirm the removal of any existing acids. Any existing acid on the steel should be neutralized with an ammonia

- water r inse followed by a light brush

- blast cleaning in accordance with SSPCSP7. The pH of the blast

- cleaned surface should be 6 to 7. The Contractor should take pH readings in all four quadrants at 20

- ft (6

- m) intervals throughout the height of the stack. The steel

- substrate surface should also be inspected for chloride contamination. If the chloride concentration is great er than 10 μg/cm 2 , the surface is contaminated , and it should be recleaned. 4. 4 .5.7.4 Workmanship St andards The installation procedure and workmanship standards of the lining system should be verified by installing the system on a 4 x 4 -f t (1.2 x 1.2

- m) transparent Plexiglas sheet. The test panel shoul d be visually inspected, mallet

- tested, and tested fo r adhesion. Testing procedures and acceptance criteria should be in accordance with the manufacturer's requirements. 4. 4 .5.7.5 Temperature and Humidity Inspection should be performed to ensure that the lining system is being stored and installed under the proper temperature and humidity conditions. During the installation of the lining, the Contractor should measure and record air temperature, temperature of the surface being lined, moisture dew point, and relative humidity. Inspection guidelines for these issues are:

 Relative humidity in the working area where the lining system is being installed should not be greater than 90 %.

 The substrate temperature in which the block lining is installed must be at least 5°F (3°C) above the moisture dew point temperatu re.

 Temperature of the substrate, ambient air temperature in the work area, and curing temperature should be between 50°F (10°C) and 90° F (32°C). 4- 37  4. 4 .5.7.6 Visual Visual inspection of the lining system should include the following items:

 All joints must be full and of a 1/8

- in (0.003

- m) minimum in thickness.

 No air voids sh ould exist between the adhesive

- coated block and the adhesive

- coated substrate.

 The adhesive membrane is trowel

- applied. 4. 4 .5.8 Protective Coatings All protective

- coating installation work is typically inspe cted. The services of a quality

- control coating inspector certified by NACE are usually required. Items that are typically inspected or tested are presented in this s ection. 4. 4 .5.8.1 Surface Preparation The interior surface of the liner should be inspected to ensure that surface preparation is being adequately performed. The surface of the metal to be lined should be blasted to obtain a white metal surface as defined in SSPC

- SP5. The sand or grit used for blasting should be clean an d dry and produce a minimum surface profile of 2.0 mils when measured with a Keane

- Tator comparator, Model No. 372. 4. 4 .5.8.2 Visual for Oxidation After blasting, inspection should confirm that surface rust has not developed. All surfaces to be coated , as well as coated surfaces requiring additional thickness , should be primed. All sand, dust , or grit should be removed by brushing, air blasting, or vacuuming before priming. The blasted surface must be primed within the same work shift (8

– 12 h ) and before any visible surface rusting develops. In the event of surface rusting, the area must be reblasted. 4. 4 .5.8.3 Surface

- Contamination Tests Surface

- contamination tests measuring pH, chloride concentrations, and ferrous sulfate concentrations sho uld be performed before applying primer. Tests should be performed using a "kTA Surface Contamination Kit" from kTA

- Tator ( or approved equal ) . If the pH is less than 5.0, the chloride concentration is greater than 10 μg/cm 2 , or the ferrous sulfate concentration is greater than 20 μg/cm 2 , the surface is contaminated , and it should be recleaned. High

- pressure water or sodium bicarbonate solution should be used to remove the contamination. 4. 4 .5.8.4 Thickness Random wet

- film thickness readings should be taken during the coating. A minimum of four reading s should be taken per 100 sq f t (9 m 2 ). After th e coatings are fully cured, dry

- film thickness measurements s hould be taken, based 4- 38  on a 6 -f t (1.8

- m) square grid pattern. Each measurement should consist of the a

verage of three random readings taken near the center. 4. 4 .5.8.5 Temperature and Humidity Inspection should be performed to ensure that the coating system is being stored and installed under the proper temperature and humidity conditions. During the insta llation of the coating, the Contractor should measure and record air temperature, substrate temperature, moisture dew point, and relative humidity. Inspection guidelines for these issues are:

 Relative humidity in the work area should not be greater than 90 %.

 Surface temperature must be at least 5°F (3°C) above the dew point of the air in the work area.

 Temperature in the work area should be 50 –120° F (10

– 49°C), or it should be within the manufacturer's recommendations. 4. 4 .5.8.6 Spark Test The cured coating should be tested for discontinuities in accordance with NACE RP0188-90, Standard Recommended Practice for Discontinuity (Holiday) Testing of Protective Coatings. Pinholes should be repaired. 4. 4 .5.9 Elevator

- Load Test The Contractor usua lly conducts a load test of the personnel elevator , witnessed by the Owner and Engineer. The Contractor should furnish the Engineer an d Owner a copy of the load

- test procedure. The Contractor usually certifies in writing that the elevator meets all the app licable local, state, and federal codes for safety and performance. 4. 4 .5.10 Pressurization Fans Pressurization fans for brick liners will require on

- site testing , as well as start
- up upon delivery to the job site. The Contractor is typically required to perform the following procedures during the installation of the fans:

 Level the fan to the foundation using shim stocks; grout and anchor.

 Align the rotating assembly.

 Check for proper rotation of motor.

 Verify balance of fan.

 Perform functional test of pr essurization system. 4. 4 .5.11 Electrical Completion of the electrical and grounding system is typically one of the last items to be completed on a new chimney installation. Before demobilization of 4- 39  the electrical subcontractor, all electrical and groundin g components associated with the chimney should be checked. 4. 4 .5.11.1 Obstruction Lights All obstruction lights should be inspected to verify that they are operational. If strobe lights are used, the photocell should be checked to verify the correct dayt ime and evening intensity levels. The alarm contacts should also be checked to verify that the lights are not malfunctioning. 4. 4 .5.11.2 Grounding The grounding system for the chimney should be checked for continuity. The ground resistance should be meas u red and recorded at the ground

- cable risers. All physical connections between the ground cable and the air terminals should be checked. 4. 4 .5.11.3 Electrical

- System Checkout The electrical system for the chimney should be inspected. Examples of items that should be inspected are as follows:

 Confirm correct voltage level at the power panel.

 Reset transformer taps, if required.

 Test each ground

- fault

– interrupter receptacle to ensure proper operation

 Check that all ambient lights are operational.

 Verify proper position o f disconnect switches and power

- panel breakers. 4. 4 .5.11.4 Lead - and PVC

- Coated Components Grounding and conduit components at the top of the chimney require additional corrosion protection. A visual inspection should be performed to verify th at all grounding cables and fittings in this area are lead

- covered. All conduits located at the top of the chimney should also be inspected to verify that 

### polyvinyl chloride (PVC)

- coated material was used. 5-1  

# Section 5: Wet Stack Experience The original Wet Stacks Design Guide was issued in 1996. Since that time, this guide has been used as the de facto standard for wet stack design and operation; hundreds of wet stacks around the world have been designed using its recommendations. Most of these units operate well, with little if any SLD experienced under normal operating conditions. This section of t he Revised Wet Stack Design Guide reviews some of the experience gained in the industry since the issuance of the original guide. To the extent possible, experience from locations around the world has been incorporated. Also included is a discussion of the steps that can be taken in

house if a unit experiences an SLD issue

— both regarding finding the source of the issue and determining the information that should be collected before outside help is engaged. 5.1 Experience with Wet Stacks 5.1.1 General Operat ional Experience A survey of units utilizing wet stacks was performed, and for the most part, it was found that the majority of them were running well. On occasion, some units would experience SLD, but typically these events could be traced back to upsets in the WFGD absorber, maintenance issues such as fouled mist eliminators and/or plugged drains, or correlated with local weather conditions (for example, high humidity/low wind). Units experiencing SLD from the start were biased toward those that had been retrofitted with WFGD systems but kept the original stacks. For these units, the stack

- liner velocities were not favorable for wet operation from the start, and some level of SLD was not unexpected. Retrofit installations for units with multiple absorber t owers are also more likely to experience SLD issues , because these un its often have complex absorber

- outlet

- ducting arrangements and gas

routing schemes requiring complex liquid

- collection systems. To minimize costs, it is not unheard of for utilities to limit the number of liquid collectors actually installed to those nearest the stack. This practice is not recommended. A limited number of units, although designed within the recommendations of the original Wet Stack Design Guide , experienced SLD from the s tart. It was found that most of these units had been designed for the higher end of the recommended liner

- gas - velocity range and had some issues with the correct 5-2  installation of the liquid

- collection system or the overall quality of the liner construction. In many instances, simple corrections to the errors in the liquid

collection system installation were sufficient to correct the problem. For alloy liners, construction

- quality issues leading to an increased potential for SLD were primarily related to the height of horizontal weld seams; for FRP liners, the problems were related to the height and taper on the internal joints between liner segments or misalignment/roundness between adjacent segments. A more recent cause for the onset of SLD issues is relate d to increases in the stack

- liner velocity brought about by increases in the unit heat rate and, to a lesser extent, by changes in the fuel source/type/ mix used at the plant. Similar to the other issues described above, these latter issues occur at plants whose liners were originally designed for the upper end of the recommended liner

- gas velocities for favorable wet operation. A limited number of units had also underestimated the total volumetric gas

- flow rate used for the stack

- liner design basis. This e rror resulted in liner diameters that were too small, with the resulting liner

- gas velocities above the recommended design range for favorable operation. As can be seen, there is a common thread connected to the maximum recommended liner flue gas veloci ty. Units initially designed to operate at the higher end of the recommended liner

- gas velocity are more prone to SLD issues than those designed for the mid

- point or lower end of the range. For this reason, the recommended velocities for favorable wet oper ation have been slightly lowered from those provided in the original Wet Stack Design Guide . It should also be noted that the new recommendations, previously detailed in Table 2

- 1, have been provided as a single design value, not a range. By lowering the r ecommended velocities by a few feet per second, the wet stack system will be significantly more robust and capable of handling the inevitable increases in gas velocity that will turn an otherwise well

- performing stack into one experiencing some level of SL D. Although they are only now being codified in this document, these slightly lower liner

- gas - velocity recommendations have effectively been in use over the past few years. The result has been that significantly fewer units have been experiencing SLD issue s related to the system design and normal unit

- operating conditions. Issues at new plants are generally related to liquid

- collection

– system installation errors or drain pluggage caused by absorber carryover and/or the buildup of debris. An increasing numb er of plants are utilizing borosilicate

- block lining systems. In this system, the blocks are adhered directly onto a substrate

— typically, but not limited to, a carbon

- steel flue or the inside of the reinforced

- concrete windscreen, using a flexible membrane material. This material is also used between the blocks, not unlike the mortar between bricks. Units utilizing this system appear to be operating well, but they experience many of the same generic issues as alloy, FRP, and acid

- brick liners. It is very important that the blocks are installed so that they form a smooth surface, without any block edges sticking out 5-3  or recessed away from the main surface. These edges could act as sites for liquid re - entrainment back into the gas flow. This is also true for th e membrane between the bricks, which must be struck flush with the liner surface. No issues related to block misalignment have been reported, indicating the existence of good quality control during the installation process. Very few issues have been report ed when borosilicate blocks have been used in new plants designed from the start to use the system. Some retrofit applications have had SLD events related to specific unit

- geometry issues, liquid

- collection

- system drainage issues, and of course, higher tha n recommended liner

- gas velocities. Based on discussions with plant operators, most wet stack liquid

- collection systems appear to be operating well. A common theme linking plants without problems is that they perform liquid

- collection

- system maintenance o n a regular basis

— including inspections of the absorber mist eliminators, liquid

- collection gutters and baffles, and ring collectors and drains for buildup, pluggage, and debris. They also clean and flush the system drains. Photographs should be taken befo re any cleaning has been performed to document the location and extent of any buildup within the system. A documented history of the liquid

- collection

system performance will be very useful should an SLD issue occur. 5.1. 2 International Experience with Wet Stacks The United States was an early adopter of wet stack operation and, for the most part, wet stack operation has only recently been accepted for use in other areas of the world. These areas include the EU, countries hoping to join the EU, and countri es in which the plants are being funded by international agencies. As experienced in the United States, the conversion of older, previously dry, stacks to wet operation has resulted in a number of units experiencing SLD, primarily because of unfavorably high liner

- gas velocities. Because of this experience, most new wet
- scrubber installations also include a new properly designed wet stack. These units are using the latest absorber and wet stack technologies, and if operated according to prescribed procedures, they appear to be operating well. SLD is not a regulated emission in the United States. As part of the Wet Stack Design Guide revision process, efforts were made to determine if SLD was regulate d in other parts of the world. To the best of our knowledge, it is not. However, as in the United States, it is considered a nuisance, and many countries/regions recommend the installation of a liquid

- collection system to minimize the potential for SLD. 5. 1 .1.1 European Standard EN 13 084 The European Standard for free

- standing chimney design, EN 13084 Parts 1

- 8, makes no mention of the need for liquid collection. 5. 1 .1. 2 VGB PowerTech VGB PowerTech Instruction Sheet VGB

- M 643 Ue, “Chimneys for Operation without Flue Gas Reheating aft er FGD , ” discusses the formation of a liquid film 5-4  on the liner wall and the need for an effective liquid

- collection system , although no specific design details or guidance are provided. As in the EPRI/CICIND Revised Wet Stack Design Guide , recommendations for the maximum flue gas velocities inside the liner as a function of the liner material are presented. These velocities are simi lar to those recommended in this g uide for FRP, a lloys , and c oated s urface s; they are higher for a cid

- resistant brick and sligh tly lower for borosilicate block. The need to minimize horizontal discontinuities , as well as to control plume downwash , are als o discussed in this document. 5.1.1.3 China In 2006 and 2007, more than 100 GWe power

- generation capacity was installed in China. In 2008 through 2010, an additional 130 GWe of capacity was installed. Many of these plants incorporated FGD systems, and the dominant technology is wet scrubbers. M ost of these units have wet stacks; however the re is no evidence th at the se plants have incorporate d liquid

- collection systems. Discussions with the China Electric Council have reve a led that at present there are no regul ations in China controlling SLD from their plants. It is known that SLD is an issue at many plants in China , and at present there is significant interest

— both at the governmental and the plant level

— in the introduction of liquid

- collection technology to these plants. 5.1.1. 4 India No standards or guidelines related to wet st ack design and/or operation in India could be found. 5. 2 Before Unit Startup Before a wet stack is brought on line, a number of reviews and inspections should be performed to ensure that the liquid

- collection and drainage system operates as expected, with minimal potential for SLD when the unit comes on

- line. 5. 2 .1 Review of Fabrication/Installation Drawings The first inspection that should be performed is a review of the liquid

- collection system installation drawings. The liquid

- collection system design p rovided by the laboratory/flow

- model vendor is typically limited to providing the size, shape , and location of the required collection gutters, ring collectors, flow diverters, flow controls, and drains. These recommendations are then provided to the stack vendor or architecture engineering ( AE ) firm , who develops t he actual fabrication and field

- installation drawings. These drawing s should be reviewed by the liquid

- collection system designer to ensure that their recommendations have been properly interpret ed and that any modifications required for ease of fabrication or installation do not adversely impact the efficacy of the liquid

collection system. 5-5  5. 2.2 Field - Installation Inspection It is important to ensure that the liquid

- collection syste m has been in stalled properly. A second inspection should be performed in the field after the liquid

collection system has been installed , but before the access scaffolding has been removed from the liner . This way the entire extent of the liquid

- collection system can be closely inspected, and on - the

- spot modifications and corrections can be made while the construction crew and scaffolding is still available

. One of the major sources of SLD from an otherwise well

- designed liquid

collection system consists of leaks from the components ( which should be sealed to the duct and liner walls). During the pre

- start up inspection , a leak check of the various ring collectors, gutters , and drainage channels should be performed. This check can be accomplished by introduci ng water at the high point of the liquid

- collection system (typically the liner

- ring collector) and following the water’s path through the entire drainage system. If leaks are identified, they need to be sealed before the unit goes on

- line. During the pre

- startup inspection, the gutters, ring collectors, and drains should be inspected and cleared of any debris that might have collected in them during the construction process. Debris can range from snack wrappers to plastic sheeting to tools. 5. 3 After Unit Startup Some SLD may be observed during the first few weeks of operation, particularly from new stacks. Such SLD is a normal occurrence and should be expected. After fabrication, a new liner’s interior surface is often coated with a material that is nonwet ting. In alloy liners , this coating may be an o ily film or scale from the mill

- rolling process ; in FRP liners , it may be a sepa rating medium or gloss coating. Because t hese surfaces are initially nonwetting, the liquid film condensing and depositing on the liner wall will form beads that are easily re

entrained back into the gas flow . Over time, the weak acidity of the condensed liquid will etch or season the liner surface , and it will become wetting

— allowing the formation of thin liquid films that bond to th e surface via surface tension. This seasoning process may take a number of weeks to occur , and during this time some SLD may be experienced. If any stack

- emission incidence occurs during normal operation, the n eed for inspection is obvious. If SLD is no t experienced, it is still important to inspect the liquid collectors to ensure satisfactory long

- term operation. A third inspection should be performed at the first scheduled or unscheduled outage after approximately six months of continuous operation. It is important that this inspection be performed before the unit is cleaned in any way , because the solids

- deposition patterns, buildup, and water stains on the walls, roof, and duct floor are all indicators of the system’s liquid

- collection performance. Special attention should be paid to the system drains. Different problems

— such as areas of solid s buildup and liquid drainage or re - entrainment problems

— can be 5-6  detected . M odifications to the collection system can be designed for installation during a scheduled or unscheduled outage. This post

- start up inspection should include the absorber outlet, outlet duct, stack liner , and choke/exhaust plane, if accessible. The liquid

- collection system should be inspected during each outage, and all drains and liquid

- seal pots or loop seals should be cleaned. It is recommended that a photographic record be kept of the inspections; this record will be invaluable should an SLD issue arise. 5. 3.1 Liquid - Drainage Monitoring The liquid

- flow rate at each drain lo cation should be monitored for the first six mont hs of the operation after start up. The se records should show the quantity of liquid collected from each of the drains, the unit load during the measurement, an d the ambient temperature at the time the measurements were taken

. This information will be very valuable should an SLD issue be encountered. If this information is monitored on a continuous basis, trends can be detected and corrective measures taken before an SLD issue is experienced. 5. 4 What to do if SLD Occurs If SLD remains a persistent problem , the liquid

- collection system designer ( or someone skilled in the art ) should be contacted to help assess the problem. To make their initial analysis effective, the following information should be collected p rior to contacting them:

 How long has the SLD problem been occurring?

 What is the velocity of the flue gas in the stack when SLD occurs?

 Was the onset of the SLD issue related to a change in operation or to procedures at the plant , such as fuel type or ab sorber operation?

 Have there been any recent problems with the absorber chemistry?

 Have the mist eliminators been inspected since the SLD issue started , and if so , was any plugging or fouling observed?

 Can the occurrence of SLD be correlated to the mist -e liminator wash cycle?

 Has the mist

- eliminator wash system been inspected for damage?

 Can the occurrence of SLD be correlated to the local weather conditions?

 Can the occurrence of SLD be correlated to the time of day?

 How far from the stack do the droplets fall?

 What is the estimated droplet size?

 Have any droplet

- size measurements been made in the stack , and if so , what are the results?

 When was the last time the liquid

- collection system was inspected? Were there any issues such as plugging or fouling? 5-7   Has the liquid

- collection s ystem drainage

- flow rate changed? This information will provide a basis for evaluation of the SLD issue. Hopefully, it will provide the investigators with sufficient information to begin their assessment of the cause of the issue an d its elimination. Specific actions can also be taken. The first should be to measure the liquid

- flow rate exiting the stack through the liquid

- collection system drains. If these flow rates are significantly lower than the baseline or calculated values, it is likely that the drain system is plugged. An inspection of the liquid

- collection system and drains should be made at the earliest opportunity. The entire drainage path should be inspected, but particular attention should be paid to areas where the colle cted liquid is constricted. Such areas include drain ports; locations where liquid enters a drainage pathway, such as found on internally drained ring collectors; and (on units with side

- entry breaches) the drains located on either end of the breach

- top gu tter. Past experience has shown that debris tends to collect in these areas. At some units experiencing unexpected SLD, these areas have been found to be plugged with plastic sheeting; pieces of the plastic film used as a separating media during FRP liner fabrication that was not fully removed prior to startup; and sometimes even dead birds. These locations are the first places that should be inspected if SLD is encountered. During this inspection, the mist eliminators and the mist

- eliminator wash system should also be inspected for pluggage and damage. Plugged mist eliminators can result in higher than design velocity values through the unit, which could lead to liquid breakthrough and increased levels of droplets re

- entrained in the flue gas flow. Some droplets could also be re

- entrained directly from the edges of the plugged areas of the mist eliminator. Some of these droplets will not be collected in the absorber

- outlet ducting or liner and will pass through the unit, exiting from the top of the stack. 5. 4.1 Preliminary Stack Droplet Testing The location of the droplets within the liner can often be related to the source of their generation. Approximate droplet

- size measure ments and their spatial distribution within a few feet of the liner wall can be estimated using a simple droplet

- impact probe made from a 10–12- ft long 2 x 4 board painted flat black, the smoother the surface the better (Figure 5 -1)

. The probe should be o riented with the test side facing horizontally, then rapidly inserted through a test port between 2 and 8 ft (0.6 and 2.5 m)

. The probe is then quickly rotated 9 0° to place the test face directly into the upward

- rising gas flow, and it is left in this orie ntation for 3 –4 seconds. The probe is then rotated back to its original orientation and rapidly removed from the liner (Figure 5

- 2). 5-8  Figure 5-1 Droplet Probe

Figure 5-2 Droplet

- Probe Orientation Droplet spots will be observed on the surface of the probe (Figure 5 -3)

. The probe exposure time may need to be increased or decreased, depending on the number of droplets captured. The objective is to get enough droplets so that the droplet spots do not overlap and that enough spots are obtained to have some statistical significance. Tests should be repeated multiple times. The diameter of the spots should be measured and their distribution on the probe noted. Large droplets occurring in the area adjacent to the liner surface indicate that the issue could be related to droplets being re

- entrained from the liner surface. Large droplets located f a rther from the liner wall are indicative o f droplet carry over ~2 ” ~24 ”

4 ” Hard Plastic Tube ” 12 ”Stack Linier Impact Probe ~2 ” ~24 ”

4 ” Hard Plastic Tube 12 ”

Impact Probe

Insertion Orientation Test Orientation Withdrawal Orientation

Test Surface



## Gas Flow Insertion Orientation Test Orientation Withdrawal Orientation

Test Surface



## Gas Flow 5-9  from the mist eliminators or of droplet re

- entrainment from s omewhere in the absorber

- outlet ducting. The probe testing should be performed at multiple locations around the perimeter of the liner to determine if the droplets are evenly distributed across the liner or are biased toward one side. A biased distribution can provide further evidence as to the actual source of the droplets. If accessible, this technique can also be performed at the stack outlet. After each insertion, the probe should be wiped d own and allowed to dry before being reinserted into the liner.

Figure 5-3 Typical Droplet

- Probe Test Results The use of this probe technique is not intended to replace more sophisticated evaluation methods . However it is a quick, cost

- effective way to get a better picture of what is going on within the liner during the initial stages of evaluating the sources leading to SLD. An additional test that can provide valuable insight into a stack experiencing undesirable levels of SLD is to c ollect droplet samples downwind of the stack at ground level. This test can be performed by placing metal collection pans, such as a cookie sheet or a square piece of plywood covered in aluminum foil, on the ground in the area where the droplets are hitting the ground. This approach can be used to e stablish the number and size of droplets collected as a function of distance to stack and unit load. 6-1  

# Section 6: R eferences

1. Entrainment in Wet Stacks , EPRI, Palo Alto, CA: 1982. CS -2520. 2. Wet Stacks Design Guide . EPRI, Palo Alto, CA: 1996. TR -107099. 3. FGD Mist Eliminator System Design and Specification Guide. EPRI, Palo Alto, CA: 1993. GS -6984. 4. Guidelines for the Fluid Dynamic Design of Power Plant Ducts. EPRI, Palo Alto, CA: 1998. TR -109380. 5. Task Committee on Steel Chimney Liners, Fossil Power Committee, Power Division, American Society of Civil Engineers, Design and Construction of Steel Chimney Liners . New York, NY, 1975. 6. D. K. Anderson, P. E., The Planning and Design of Effective Wet Duct/Stack Systems for Coal Fired Utility Power Plants . CICIND Report, Vol. 24, No.2, July 2008, pp.37 –42. 7. American Society of Testing and Materials, Standard Guide for Design, Fabrication, and Erection of Fiberglass Reinforced (FRP) Plastic Chimney Liners with Coal Fired Units. (ASTM D 5364

- 08), West Conshohocken, PA, November 2008. 8. American Concrete Institute, Code Requirements for Reinforced Concrete Chimneys . (ACI 307

– 08), Farmington Hills, MI, November 2008. 9. VGB PowerTech Service GmbH, Chimneys for Operation without Flue Gas Reheating After FGD . Instruction Sheet VGB

- M 643

- Ue. Essen, Germany, February 2007. 10. Bernhardt Hertlein, ed., American Society of Civil Engineers, Chimney and Stack Inspection Guidel ines . Reston, VA, 2003. 11. Guidelines for FGD Materials Selection and Corrosion Protection . EPRI, Palo Alto, CA: TR

- 100680, Vols. 1 and 2. 12. Acid Deposition on Ductwork. EPRI, Palo Alto, CA: 1983. CS -3240. 6-2  13. American Society for Testing and Materials, Standard Specification for Design and Fabrication of Flue Gas Desulfurization System Components for Protective Lining Application . (ASTM D4618

- 92), Philadelphia, PA, 2010. 14. T. S. Clark. “Chimneys Subjected to Acid Gases,” Industrial and Engineering Ch emistry . Vol.15, No.3, March 1923, pp. 227 –230. 15. R. Mongia, A. Reza, et al. “Effect of Exhaust Stack Geometry on the Amount of Liquid Condensate During Plant Start

- Up,” Proceedings of the 95th Annual Conference of Air and Waste Management Association. B altimore, MD (2002). 16. H. S. Rosenberg. “Wet Stacks: Friend or Foe?”, Power Engineering. November 1998, pp.76 –81. 17. American Society for Testing and Materials, Standard Specification for Inspection of Linings in Operating Flue Gas Desulfurization Syste ms . (ASTM D4619-96), Philadelphia, PA, 2004. 18. American Society for Testing and Materials, Standard Guide for Design and Construction of Brick Liners for Industrial Chimneys . (ASTM C1298 -95), Philadelphia, PA, 2007. 19. N. J. Gardner and I. Owen, “The Be havior of Liquid Films and Drops in Relation to Liquid/Gas Separators,” Proc. Institute Mechanical Engineers

. Vol. 211, Part E, 1997, pp. 53 –59. 20. D. S. Miller, Internal Flow Systems . 2nd Edition, Gulf Publishing, Houston, TX 1990. 21. Leaning Brick Stack Liners . EPRI, Palo Alto, CA: 1989. GS -6520. A-1  

# Appendix A: Glossary
A.1 Definitions absolute humidity - the weight (or mass) of water vapor in a gas water

- vapor mixture per unit volume of space occupied. absorber - general term for those gas/liquid contacting devices designed primarily for the removal of SO x pollutants, i.e., scrubber. absorption - the process by which gas molecules are transferred to a liquid phase during scrubbing. air cubic feet per minute (acfm ) - a gas - flow rate expressed with respect to operating conditions (temperature and pressure). ambient - pertaining to the conditions (pressure, air quality, temperature, etc.) of the surrounding environment of a plant or scrubbing system. annual outage - a scheduled period of time (generally four to six weeks) set aside by the utility once per year to shut down the boiler and/or FGD system for inspection and maintenance. annulus - the space between a chimney liner and a chimney shell. base load - a generat ing station that is normally operated to take all or part of the normal load of a system and that, consequently, operates at a constant output. breeching - the section of ductwork in an FGD system between the absorber

outlet duct and the stack. British the rmal unit (Btu) - the amount of heat required to raise the temperature of one pound of water 1°F, averaged from 32 –212°F. bypass gas - flue gas that bypasses a scrubber for the purpose of raising wet flue gas temperatures above the saturation temperature. bypass reheat - a system that increases the temperature of the saturated flue gas leaving an FGD system above dew point by ducting a slipstream of particle

cleaned flue gas from the ESP exit duct past the FGD system to the absorber

outlet duct or directly to the stack. A-2  capacity factor - the ratio of the average load on a boiler for the period of time considered to the capacity rating of the boiler (actual kWh produced/theoretical kWh produced x 100). carryover - entrained solids, slurry droplets, and/or liq uid droplets that leave with the flue gas stream exiting a particular stage of a scrubber or absorber. chimney - a vertical structure at a power plant that encloses one or more flues which exhaust combustion gases. A chimney is typically constructed out of reinforced concrete. chloride - a compound of chlorine with another element or radical. choke - the constricted upper section of a steel stack or chimney liner. cladding - a thin sheet of corrosion

- resistant alloy (usually nickel alloy or titanium) that is either resistance

- welded or roll

- bonded to carbon

- steel plate. closed water loop - the water loop of an FGD system is closed when the fresh makeup water added exactly equals the evaporative water loss leaving via the stack and the water chemically or ph ysically bonded to the sludge product. column - a reinforced

- concrete chimney shell. Usually encloses one or more chimney liners. The purpose of the column is to protect the liner from weather and to act as a wind shield. computational fluid dynamics (CFD) - the use of finite element

– analysis methods to simulate heat transfer, temperature profiles, and fluid and particle movement in boilers and air

- pollution - control equipment. corrosion - the deterioration of a metallic material by electrochemical attack. damper - a plate or set of plates or louvers in a duct used to stop or regulate gas flow. dew point - the temperature at which vapor contained in saturated flue gas begins to condense. efficiency - ratio of the amount of a pollutant removed to the total am ount introduced to the normal operation. entrainment - the suspension of solids, liquid droplets, or mist in a gas stream. erosion - the action or process of wearing away of a material by physical means (friction). electrostatic precipitator (ESP) - an air

- pollution device used to remove particles from an exhaust stream by initially charging them with electrodes and then collecting them on oppositely charged plates. A-3  expansion joint - a small section of ductwork or piping that is designed to passively expand or contract as required by the flexing of more rigid duct runs, piping, or pieces of equipment as such components are exposed to varying external and internal temperatures. flue gas desulfurization (FGD) system - an SO 2 removal system that uses a wet or dry process downstream of a boiler to reduce sulfur dioxide emissions. fly ash - fine solid particles of noncombustible ash carried out of the boiler by the exiting flue gas. forced outage - the FGD system is taken out or forced out of service to make neces sary repairs or modifications regardless of boiler availability, such that the system is unavailable for service. heat exchanger - device used to transfer sensible and/or latent heat from one stream of material to another to raise or lower the temperature of one of the materials. induced draft (ID) - a fan used to move an enclosed stream of gas by creating a negative relative pressure in the stream to effectively draw the gas through the system. indirect hot air - a flue gas reheat system in which reheat is achieved by heating ambient air with an external heat exchanger using steam at temperatures of 350

– 450°. in
- line reheater - a heat exchanger installed in the wet flue gas duct downstream of the mist eliminator, usually consisting of hot water or steam coi ls used to boost the wet flue gas temperature above dew point. liner - the flue located inside of a chimney or stack that exhausts combustion gases. lining - a metal, organic, or inorganic type of material applied to a shell of an FGD system component that is intended to protect the shell from abrasion, heat, and/or corrosion. liquid

- collection devices - devices such as gutters, troughs, and drains used in absorber

- outlet ductwork or stack liners within an FGD system to collect liquid carried over from a wet scrubber. load factor - the ratio of the average load in kilowatts supplied during a designated period to the peak or maximum load in kilowatts occurring in that period. mist - dispersion of liquid particles in a gas stream, carryover from a gas

- liquid c ontact operation. A-4  mist eliminator - a piece or section of pollution hardware used to remove a dispersion of liquid particles from a gas stream. megawatt (MW) - unit used to describe the gross or net power generation of a particular facility. One watt equal s one joule per second. One megawatt equals 10 6 watts. micrometer - unit of measure equivalent to 0.0000394 in or 0.001 mm. new (as opposed to retrofit, for FGD systems) - the FGD unit and boiler were designed at the same time, or space for the addition o f an FGD unit was reserved when the boiler was constructed. NO x - a symbol meaning oxides of nitrogen (e.g., NO and NO 2 ). New Source Performance Standards (NSPS) - environmental regulations that apply to a new installation, referring primarily to the Federal NSPS that applies to installations beginning construction on or after August 17, 1971. opacity - the degree to which emissions reduce the transmission of light and obscure the view of an object in the background. open water loop - the water loop of an FGD system is open when the fresh makeup water added exceeds the evaporative water loss leaving via the stack and the water chemically or physically bonded to the sludge product. outage - that period of time when the boiler and/or FGD system is shut do wn for inspection and maintenance. Outages may be either forced or scheduled. particulate matter - finely divided solid particles entrained in the gas stream (fly ash, coal fines, dried reaction byproducts, etc.). peak load - a boiler that is normally oper ated to provide power during maximum load periods. pH - the hydrogen ion concentration of a water or slurry to denote acidity or alkalinity. plume (stack plume) - the visible emission from a flue (stack). plume downwash - the phenomenon that occurs when the flue gas exits a stack and the vapor plume drops below the top of the stack before evaporating or dispersing into the atmosphere. Usually occurs on stacks that operate at a relatively low exit velocity. parts per million (ppm) - unit of concentration t hat in wastewater applications is equal to milligrams per liter and in air

- pollution applications is equal to moles of pollutant to million moles diluent. A-5  pressure drop - the difference in force per unit area between two points in a fluid stream as a resul t of resistive losses in the stream. rain hood - the component at the top of a stack that covers the annular space. reheat - the process of increasing the flue gas temperature downstream of a wet scrubber. Reheat can be supplied by in

- line indirect hot air , direct combustion, or by partial bypass of unscrubbed flue gas. reheater - device used to raise the temperature of the scrubbed gas stream to prevent condensation and corrosion of downstream equipment, avoid visible plume, and/or enhance plume rise and d ispersion. relative humidity (also relative saturation) - the ratio of the weight (or mass) of water vapor present in a unit volume of gas to the maximum possible weight (or mass) of water vapor in unit volume of the same gas at the same temperature and pr essure. The term "saturation" refers to any gas

- vapor combination, whereas "humidity" specifically refers to an air

- water system. removal efficiency:

- particulate matter - the actual percentage of particulate matter removed by the emission

- control system (mechanical collectors, ESP, or fabric filter and FGD) from the untreated flue gas.

- SO 2 - the actual percentage of SO 2 removed from the flue gas by the FGD system.

- total unit design - the designed percentage of mass of SO 2 or particulate matter entering the stack to the mass of the material in the flue gas exiting the boiler, regardless of the removal efficiency of an individual component or the percentage of the exiting flue gas actually being scrubbed. retrofit - the FGD unit will be/was added to an e xisting boiler not specifically designed to accommodate an FGD system.

s aturated - the situation in which a gas or liquid is filled to capacity with a certain substance. No additional amount of the same substance can be added under the given conditions.

sa

turation temperature - the temperature to which flue gas drops when it is saturated by scrubbing in a wet FGD system. scale - deposits of slurry solids (calcium sulfite or calcium sulfate) that adhere to the surfaces of FGD equipment, particularly absorber/scrubber internals and mist

- eliminator surfaces. scheduled outage - a planned period of time periodically set aside for inspection and maintenance of the boiler and/or FGD system. A-6  scrubber - a device that promotes the removal of pollutant particles and/or gases from exhaust streams of combustion or industrial processes by the injection of an aqueous solution or slurry into the gas stream, i.e., absorber. sludge - the material containing high concentrations of precipitated reaction byproducts and solid matt er collected and/or formed by the FGD process (composed primarily of calcium

- based reaction byproducts, excess scrubbing reagent, fly ash, and scrubber liquor). slurry - a watery mixture of insoluble matter (usually lime or limestone). SO x - a symbol meaning oxides of sulfur (e.g., S O 2 and S O 3 ). stack - a vertical structure at a power plant that encloses one or more flues which exhaust combustion gases. stack flue - the inner duct or liner in a stack through which the flue gas is conveyed. stack

- exit v elocity - the exiting velocity of the flue gas out the top of the stack. stack

- liner velocity - area average gas velocity inside the liner. stack liquid discharge (SLD) - liquid that is discharged from a stack and falls to the ground prior to evaporating. standard conditions - a set of physical constants for the comparison of different gas volume flow rates (68°F, 29.92 in Hg, barometric pressure ). standard cubic feet per minute (scfm) - units of gas

- flow rate at standard conditions. steel stack - a vertical structure at a power plant that exhausts combustion gases. The primary supporting shell is made of steel. superficial gas velocity - the area average flue gas velocity through a mist eliminator or other component of an FGD system. temperature, dry bulb ( DB) - the temperature of a gas or mixture of gases indicated by a thermometer after correction for radiation. temperature, wet bulb (WB) - a measure of the moisture content of air (gas) indicated by a wet bulb psychrometer. total controlled capacity (TCC) - the gross rating (MW) of a unit brought into compliance with FGD, regardless of the percent of flue gas treated at the facility. turning vanes (i.e., vanes) - devices used in ductwork or chimney liners to control gas - flow direction. Usually fabricated fr om flat or curved plates. A-7  unit rating:

- gross - maximum continuous generating capacity in MW

- net - gross unit rating less the energy required to operate ancillary station e quipment, inclusive of emission

- control systems. video droplet analyzer (VDA) - uses on - the

- fly video image analysis to detect and measure the diameters of all in

- focus droplets that are entirely within the view of the camera in each video frame wallpaper - thin sheets of corrosion

- resistant alloy material welded to new or existing carbon

- steel plate. water loop - all aqueous mass flows from inlet (e.g., seal water, quench water, scrubber liquor) to outlet of an FGD system (e.g., evaporation via stack, pond evaporation, waste disposal). wet stack - a chimney, stack, or flue that exhausts sa turated, completely scrubbed flue gas. Wet stacks are located downstream from a wet FGD system. Wet stack operation does not utilize any flue gas reheat system or partial bypass. Wet stacks are equipped with corrosion

- resistant liners for handling the wet, acidic flue gas exiting the FGD system. zero discharge - a pollution regulation requiring that no effluent waste stream be discharged back into the environment, with the exception of evaporation via ponds and stacks (e.g., pond runoff or direct piping of spent slurry or waste into nearby waterways or tributaries would be prohibited). A-8  A. 2 Units and Conversion Factors To Obtain Multiply By Atmospheres Feet of water @ 4°C 0.0295 Atmospheres Inches of mercury @ 0°C 0.03314 Atmospheres Pounds per square inch 0.068 Cubic meters Cubic feet 0.02831685 Cubic meters Gallons 0.00378541 Cubic meters per second Cubic feet per minute 0.0004719 Inches of mercury @ 0°C Pounds per square inch 2.036 Kilograms per square meter Pounds per square foot 4.882 Kilowatts Btu per minute 0.01757 Millimeter Inches 25.4 Kilograms Pounds 0.4565924 Kilogram per cubic meter Pound per cubic foot 16.01846 Liters Gallons 3.785 Meters Feet 0.3048 Meters per second Feet per second 0.3048 Pascal Po unds per square inch 6,894.757 Pascal Pounds per square foot 47.88 Square meters Square feet 0.09290304 Temperature Conversions:

o F = 1.8 x o C + 32

o C = ( o F - 32)/1.8 A-9  Liquid Load Conversion: Liquid Load (grains / acf) = Q L (gal / min x 57,700) Q G (acfm) (gal/min) = (grains/acf) x 17.24 x (acfm/10 6 ) (grains/acf) = (gpm/ft 2 ) x (970.0/(ft/s)) (mg/m 3 ) = 2,288 x (grains/acf)Electric Power Research Institute 3420 Hillview Avenue, Palo Alto, California 94304-1338 • PO Box 10412, Palo Alto, California 94303-0813 USA 800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com The Electric Power Research Institute Inc., (EPRI, www.epri.com) conducts research and development relating to the generation, delivery and use of electricity for the benefit of the public. An independent, nonprofit organization, EPRI brings together its scientists and engineers as well as experts from academia and industry to help address challenges in electricity, including reliability, efficiency, health, safety and the environment. EPRI also provides technology, policy and economic analyses to drive long-range research and development planning, and supports research in emerging technologies. EPRI’s members represent approximately 90 percent of the electricity generated and delivered in the United States, and international participation extends to more than 30 countries. EPRI’s principal offices and laboratories are located in Palo Alto, Calif.; Charlotte, N.C.; Knoxville, Tenn.; and Lenox, Mass. Together...Shaping the Future of Electricity

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Programs: Integrated Environmental Controls

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