.^^'^ "' 'o.

z ""V

v>,r of ^^^ NBS TECHNICAL NOTE 966
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

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proposes new engineering practices; and develops and improves mechanisms to transfer

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research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

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The National Bureau of Standards was reorganized, effective April 9, 1978.
M.J. Orloski and R.S. Wyly

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

P

Jordan J. Baruch, Assistant Secretary for Science and Technology

J<^- NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

^^ STANDARDS

tJBttABy

AUG 2 3 1978

Performance Criteria and

o^^oc-cer

Plumbing System Design ,!t°-,-^ ' ' ion^ ir^rA ^W^P nbAblf'

icon's

C-2.

Issued August 1978
National Bureau of Standards Technical Note 966

Nat. Bur. Stand. (U.S.), Tech. Note 966, 61 pages (Aug. 1978) CODEN. NBTNAE

U.S. GOVERNMEhfT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.40 Stock No. 003-003-01963-1

(Add 25 percent additional for other than U.S. mailing).
FOREWORD

This paper gives the current status of the development of a performance

approach to design methods in plumbing. The long-term implication of

this performance approach is also discussed.

It is expected that this paper will be of particular interest to plumbing

designers and code officials.

As an introduction to the subject, the traditional approach to plumbing

system design, based on continuous flow, is reviewed. Then, recent research

is discussed in some technical detail in order to explain the significance

of time profiles of hydraulic parameters under dynamic conditions (representative of transient flow phenomena in plumbing systems).

The application of performance criteria based on this type of data can

reduce excessive design factors and provide useful criteria for evaluating

innovative systems. Illustrative examples are given for reduced-size venting

satisfying the recommended performance criteria.

I

L

111

CONTENTS

Page

FOREWORD iii

ABSTRACT 1

1.0 INTRODUCTION 1

2.0 BACKGROUND ON PERFORMANCE-ORIENTED STUDIES 5

2.1 An Approach to the Systematic Correlation of User Needs,

Performance Evaluation and Design Criteria 5

2.2 Discussion of Traditional Models for Plumbing Design and the

Current Status of Design Models for Plumbing 9

3.0 EXAMPLE OF PERFORMANCE RESEARCH IN PLUMBING AND ITS APPLICATION

TO PLUMBING DESIGN 17

3.1 Discussion of Traditional Vent Model for Tall Stacks in Short

Stack Applications and NBS Studies of Short Stack Systems 17

3.2 Development of Laboratory Performance Data for One Criterion ... 20

3.3 Discussion of Findings on Flow Parameters and Trap Performance . 27

3.4 Application of Findings for Vent Sizing Criteria for

^_ Residential Systems with Short Stacks, and Field Performance

f Validation 31

4.0 SUMMARY OF NEEDS TO IMPROVE PLUMBING MODELS THROUGH PERFORMANCEORIENTED RESEARCH ! 37

4.1 Laboratory Research Needs for Plumbing Systems 37

4.2 Field Research Needs for Improved Precision in Estimating

Peak Hydraulic Loads and Assigning Fixture Unit Ratings 39

[O CONCLUSIONS 43

[0 REFERENCES 47

Appendix A, Sizing Reduced-Size Vents, Procedures and Examples 50

Appendix B , Definitions 52

Appendix C, Units and Conversion Factors 54
LIST OF FIGURES

Figure 1. . Schematic of Performance Concept of Housing 4

Figure 2. Basic Relationship of Elements and Attributes 4

Figure 3. A General Illustration of Performance Requirements

(Qualitative) for Plumbing Systems 6

Figure 4. Illustrative Example of 2.1.2 (Drain-Waste-Vent)

"Controls" (Measures) i.e. Performance Criteria

and/or Evaluation Aids 6

Figure 5. Example of Basic RCTC Format of Individual Performance

Statements 7

Figure 6. Some Mathematical Models for Soil or Waste Stack

De s ign 10

Figure 7. Some Mathematical Models for Vent Stack Design 10

Figure 8. Principal Pairameters for Design of Horizontal Drains 12

Figure 9. Variables in System Capacity 12

Figure 10. Arbitrary Assumptions in Applying Mathematical

Models to Vent Design in Tall Stacks 16

Figure 11. Difference Between "Single-Run" and "Cumulative"

Measurements of Trap-Seal Reduction (dH). (In

each case, the peak pressure (dP) is averaged for

the four runs . ) 21

Figure 12. NBS Laboratory Component Stack Schematic 23

Figure 13. Component Stack Data With Flushometer Water Closets , 23

Figure 14. NBS Laboratory Townhouse System Schematic , 25

Figure 15. Townhouse System Data on P-traps 25

Figure 16. Sample of Data for a "Time Slice" Averaging

Period of 1.0 s Duration and the Associated Cumulative

Trap Seal Reductions (dH) from a Four-Run Test

(i.e. systematic procedure of Figure 11) 26

Figure 17. Comparison of P-Trap Data in Townhouse System

and in 5-story Simple Stack for Multiple Flushes 30

VI
FIGURES—CONTINUED

Figure 18,

, Figure 19.

Figure 20.

; Table 1.

Table 2.

! Table 3.

Table 4.

Photo 1.

Photo 2.

Photo 3.

Photo 4.

Page

Principal Coordinated Inputs Needed to Update Hunter

Curve for Drainage (a similar illustration could be

constructed for Water Supply) 40

Model for the Development of Performance Criteria 44

Schematic Indicating Groups Representing Cross-Section

of Interests in Plumbing System Specifications , 44

LIST OF TABLES

Height of Fall Effect and Air Circulation Evidence 2 7

Levels of Peak Suction Producing a 25 mm (1.0 in) ' Trap Seal Reduction in WC-traps and P-traps for Single

and Multiple Flushes 29

Summary of Principal Results of Laboratory Tests on

a Full-Scale Townhouse Drainage System with ReducedSize Vents 29

Recommended Criteria for Selecting Dry-Vent Sizes for

1-2 Story Sanitary Drainage Systems 32

LIST OF PHOTOS

NBS Laboratory Townhouse, Second-Story Fixtures !•

NBS Laboratory Townhouse, First-Story Fixtures 19

Andrews Air Force Base Field Test Houses, Aerial

View of D, E, and F Units with Reduced-Size Vents

(Photo Courtesy Andrews Air Force Base) 34

Typical Lavatory Installation with Instrumented

P-Trap and Water Supply Lines at Andrews Air

Force Base 34

VI

1

PERFORMANCE CRITERIA AND PLUMBING SYSTEM DESIGN^

By M. J. Orloski and R. S. Wyly

ABSTRACT

An overview is presented indicating how the performance approach to plumbing

system design can be used to extend traditional methods to innovative systems.

Identification of the plumbing performance needed in a built system is used

to classify current design criteria intended to furnish this level of performance. Some current design criteria may provide a higher level of performance

than is actually needed by the user. In other cases, no standard test method,

criterion, or evalution technique exists. Putting existing knowledge into

a performance format increases the utility of this knowledge and facilitates

identification of needed research to fill the gaps. Some of the mathematical

models now used for system design and pipe sizing in plumbing codes are reviewed in the context of performance-oriented research. The results of

experimental work in plumbing systems with reduced-size vents (smaller than

allowed by codes) are presented as an example of the use of the performance

approach, and illustrate a case where performance criteria permit relaxing

of vent design practice. Conceivably the re-examination by plumbing designers

of traditional design criteria against measured user needs could be beneficially

extended to other areas of plumbing design such as water distribution, storm

drainage, and plumbing fixtures. Beyond this, it has been recognized that

uniform guidelines for evaluation of innovative systems , based on research

findings, are essential for wide acceptance of performance methods, particularly

by the regulatory community.

Key words: Performance, Plumbing systems. Reduced-size venting

1. INTRODUCTION

The principal objective in the application of the performance approach to

plumbing system design and installations is to provide a basis for evaluating

the adequacy of alternative methods of achieving satisfactory functional

operation for the user. This is accomplished by introducing performance

requirements and their related test methods (criteria and evaluation) as a way of facilitating the development, evaluation, and acceptance of innovative

systems which are adequate to meet the basic needs of the user.

^ Portions of this paper have been presented to the American Society of

Plumbing Engineers and the American Society of Sanitary Engineering at their

national meetings.

Definitions used in this paper are given in Appendix B, page 52.
In other words, a quantitative statement of what is needed or wanted (a

performance criterion) seems better than specifying just one acceptable method

of sizing or constructing a given component. However, introduction of performance criteria applicable to a given product or system is difficult because

the user's fundamental needs and service expectations have to be identified

and expressed in measurable terms. The constraints of the present prescriptive

specifications as a basis for systematic improvement of plumbing system design

through research are described in the following quote:

"Neither performance attributes of interest nor the limit

states of concern are explicitly expressed in most existing

specifications. This lack of clarity has made it difficult

to improve specifications through research, as there can be no certainty that a specification provision is improved if

neither the response of concern nor the desired performance

attribute is clearly def ined. " [1

1*^

In this present paper, the plumbing system and its elements are first discussed

generally in the context of user needs and the performance approach, as

r.elated to a complete housing system, in order to describe an orderly basis

'I for relating the 'performance attribute of interest' and 'the limit states

Cjif concern'. A performance matrix framework is presented in Section 2.1 as

a format and a basic tool for systematic identification and classification

of the basic user needs (requirements), and of the significant measures

(criteria) that may be used for determination of the satisfaction of the

requirements. The concept of the existence of both the need and the opportunitji

to correlate design criteria and performance criteria, in particular through

research on plumbing, is introduced in Section 2.2 with a brief review of some of the key mathematical models used in traditional drain-waste-vent (DWV)

design and in the establishment of pipe sizing requirements in the codes.

A discussion of the shortcomings of the vent mathematical model for tall

stacks (vertical distance of water fall greater than 6 m) as applied to

Numbers in brackets refer to literature references at the end of paper.
short stacks (vertical distance of water fall less than 6 m) is presented

in 3.1. This provides an example of the performance approach in which the

practical application of the knowledge that air flow rates predicted by the

traditional theory do not occur in systems with short stacks suggests that

these pipes are larger than necessary to protect trap seals from effects

due to pneumatic fluctuations in the vent. In 3.2, the guidelines are described

for the systematic development of performance data in which trap-seal reduction

measurement (one performance criterion) is compared with the associated air

demand and pneumatic suction in the vent (the traditional design criteria).

The recommended sizing criteria resulting from the study are presented in

3.4 and this is followed by a short description of the scope of a subsequent

field study for field validation of design criteria based on the laboratory

findings

.

Major research needs as applied to the plumbing mathematical models in a broad research plan are discussed in 4.1 for the laboratory, and in 4.2

for field investigations. In the conclusions, 5.0, the long-term implications and benefits of utilizing performance methods to supplement traditional methods are presented.
LIVING UNIT

STRUCTURE AND ENVIRONMENT

Strong & Tight

BUILDING

FOUNDATION

Safe & Functionally Effective

Plumbing

9:

Electrical

SYSTEMS + SUB-SYSTEMS

Hardware

Fixtures

I "' -I

Appliances

Figure 1. Schematic of Performance Concept of Housing

LIVING UNIT

- 00

1—

LU

LU

LU

1

BUILDING

2

SYSTEMS

3

SUB-SYSTEMS

LU

ATTRIBUTES X

QC

ZD

\—

CJ

cc

I—

00

A. STRONG

1 1 1 1

B. TIGHT

1—

UJ

CD

CC

>

LU

C. SAFE

FUNCTIONALLY

°- EFFECTIVE

Figure 2. Basic Relationship of Elements and Attributes
2. BACKGROUND ON PERFORMANCE-ORIENTED STUDIES

2.1 AN APPROACH TO THE SYSTEMATIC CORRELATION OF USER NEEDS, PERFORMANCE

EVALUATION AND DESIGN CRITERIA

In the generalization of the performance approach in relation to a residential

building system, the structure and environment together furnish a living

unit with certain basic Attributes, For the user, a living unit should be

Strong and Tight (the structure), and Safe and Functionally Effective (the

interior environment). The living unit in turn is comprised of Elements:

the building, its systems, and their subsystems. See Figure 1. These are interrelated through providing essential utilities for the function of the

building to its occupants, see Figure 2, and in turn these parts can be

systematically broken down into interrelated components, for a finer and

finer grid at all levels of detail, relating back to the overall Attributes

of Strong, Tight, Safe and Functionally Effective.

Within these categories, the basic user needs, or qualitative Requirements

are enumerated. These Requirements are the primary means by which basic

user needs are indexed within the performance framework relating Elements

and Attributes. (The examples given in this paper should not be construed

as comprehensive or complete.) Figure 3 is an illustrative example of the

qualitative Requirements for a plumbing system. Figure 4 illustrates

possible criteria for the Requirements of Figure 3, and Figure 5 presents

the basic format of RCTC statements for Requirement C2. 1.2-2 "Healthful",

of Figure 4.

This acronym is defined as follows:

R Requirement

C Criterion

T Test Evaluation

C Commentary

It represents one type of a performance statement format.
^^

V^

ELEMENTS

2. SYSTEMS

LIVING UNIT \ 2.1 PLUMBING SYSTEM 2.2 2.3

2.1.1 WATER

SUPPLY

2.1.2 DRAINWASTEVENT2.1.3 HYDRAULIC

LOAD AND

FIXTURES ATTRIBUTES

UJ

CC

1—ZD

CC1—oo

A. STRONG 1 DEFLECTION

2 TORSION

1 DEFLECTION

2 TORSION

1 DEFLECTION

2 TORSION

B. TIGHT

1 SUPPORTS

2 LEAKAGE

1 SUPPORTS

2 LEAKAGE

1 SUPPORTS

2 LEAKAGE

1—UJoc

>

UJ

C. SAFE

1 HAZARD

PROTECTED

2 HEALTHFUL

1 HAZARD

PROTECTED

2 HEALTHFUL

1 HAZARD

PROTECTED

2 HEALTHFUL

FUNCTIONALLY

^EFFECTIVE

1 ADEQUATE

CAPACITY

2 COMFORTABLE

1 ADEQUATE

CAPACITY

2 COMFORTABLE

1 ADEQUATE

CAPACITY

2 COMFORTABLE

Figure 3. A General Illustration of Performance Requirements

(Qualitative) for Plumbing Systems

CO

1—UJ

UJ

2. SYSTEMS

^x^-

21 PLUMBING

SYSTEM

LIVING UNIT

2.1.2 DRAIN-WASTE

VENT

i AHRIBUTES 1 UJ

QC

IZ) hC_) ZD

CC

1—

CO

A. STRONG

1 DEFLECTION

2 TORSION

VERTICAL — STATIC

B. TIGHT

1 SUPPORTS

2 LEAKAGE

-^ SAG -- MOISTURE

1—UJo

CC

>

UJ

C. SAFE 1 HAZARD PROTECTED

2 HEALTHFUL

-——» PRESSURE -^ TRAP SEAL

RETENTION

n FUNCTIONALLY

EFFECTIVE

1 ADEQUATE CAPACITY

2 COMFORTABLE

> VOLUME

SELF-CLEANIf

HORIZONTAL etc IMPOSED etc

ALINEMENT etc CORROSION etc

TEMPERATURE etc.

CROSS FLOW etc

FLOW RATE etc.

OUIET etc See Figure 5 for Criterion C2.1.2-2.1 as an example

Figure 4. Illustrative Example of 2.1.2 (Drain-Waste-Vent)

"Controls" (Measures) i.e. Performance Criteria

and/or Evaluation Aids
2 SYSTEMS

2.1 PLUMBING SYSTEM

2.1.2 DRAIN-WASTE -VENT

C. SAFE C. 2.1.2-2 HEALTHFUL (REQUIREMENT)

CRITERION C 2.1.2 - 2.1 Trap-seal retention

With water-sealed trap systems, trap seal depth should not be less than 50 mm (2 in) nor more than 100 mm (4 in) and trap seal reductions

under loading conditions representative of normal use should not exceed

50 percent of the full-seal depth, in any trap.

TEST FOR C 2.1.2 - 2.1 Criteria for selecting test loads for sanitary DWV systems are given

as tentative guidelines* . The performance of DWV systems that depend

on water-seal traps should be evaluated according to the following:

(a) For self siphonage: Maximum reduction 50 percent trap-seal depth

in four successive test runs under identical loading conditions, with

the trap refilled before each run.

(b) For induced siphonage (from combined effects of vacuum and back

pressure) : Cumulative reduction 50 percent trap-seal depth in idle

traps, after a series of four successive test runs under identical

loading conditions without trap refill.

COMMENTARY ON C 2.1.2 2.1

In water-sealed trap systems, the important, primary criterion of performance is the ability to maintain the trap seal, not the ability

to maintain a specified limit on pneumatic pressure, although there is some relationship between the two. Because the relationships between

dynamic pneumatic pressure and trap-seal reduction rate are not adequately defined, it seems reasonable to adopt a maximum trap-seal

reduction of 50 percent of the seal depth as the criterion, since a greater limit tends to produce an unstable condition and retention

of 50 percent of the seal depth provides some reserve for evaporation,

and is adequate to resist entry of sewer gases and foul air under normal circumstances.

*The load selection guidelines presented in Chapter H of [2] were comprised of Table la of [3] and two other loading tables which

were developed from the Hunter model and certain assumptions.

Figure 5. Example of Basic RCTC Format of Individual Performance

Statements •
Identifying and ordering basic user needs, or Requirements, within a selected

performance framework of Elements and Attributes, however, is only a first

step and is primarily an indexing scheme for classifying the information as comprehensively as possible. The key issue is, how is it to be determined

whether a Requirement is met?

A performance statement must somehow include what is to be measured. This

paper focuses on some simple examples (Figure 5) of one type of performance

statement: Requirement, SiCriterion, Test, and Commentary (RCTC), to point

out the importance of standard test methods or other systematic evaluation

techniques to determine compliance wi th a Requirement. The language of

performance specifications , and several basic classification methods are discussed in depth in [4],

In an RCTC performance statement, each Requirement has associated with it one or more Criteria. A Criterion denotes some physical or chemical characteristic or property that is subject to measurement and that can be used as a meaningful measure or indication of adequacy in the satisfaction of a performance Requirement. Each Criterion in turn has its own "Evaluation" or "Test"

which gives a systematic procedure or test method by which it is determined

whether the Requirement has been met, and (Optional) "Commentary" which

explains the rationale of the Criterion if this is helpful, or may discuss

the limits of the present evaluation approach. Solutions acceptable under

the model codes should necessarily be accepted under a performance standard,

because such solutions are generally considered to have been found adequate

by the test of experience. Ideally, the performance statement (a Requirement, Criterion, Test Evaluation, and Commentary) should contain definitive

guidance, preferably in the form of a definitive method of evaluation with

acceptance limits, and should be applicable to an innovative method or material. In this paper, "Evaluation" will be used to mean "Test Evaluation"

for uniform evaluation techniques including the judgment of a group of persons qualified to make such a determination. If an exact method of evaluation

cannot be defined, general recommendations should be provided to reduce subjectivity of evaluation. The present state of knowledge for application of

8
the performance concept is not sufficient, however, for complete coverage

of all the plumbing subsystems and conceivable materials. In areas where

the state-of-the-art does not yet provide for realistic presentation of performance language, it is very important to evaluate the knowledge concerning

the existing specification-type standards as a basis for estimating compliance

of innovations with the performance 'Intent of these standards, as determined

through the combined, studied judgment of experienced persons duly charged

with this responsibility.

2.2. DISCUSSION OF TRADITIONAL MODELS FOR PLUMBING DESIGN AND THE CURRENT

STATUS OF DESIGN MODELS FOR PLUMBING

Examination of certain generally accepted plumbing design criteria shows

that design guidelines and limits of acceptability are based on data derived

from simple laboratory systems under conditions of continuous flow. Such

conditions are nontypical since it is known that dynamic processes predominate. Of special interest is the fact that the models do not relate hydraulic

and pneumatic parameters with trap-seal reduction (further discussed in

Section 3.2). Much of the accumulated body of data was obtained by methods

that could benefit from the more sophisticated measurement techniques now

available. In addition, conservative safety factors and simplifying assumptions have been applied in interpreting and utilizing the data in the development of mathematical models, which are a means of generalizing the available

data and arriving at a safe level of performance. These mathematical models

do, however, provide basic guidelines for the design and acceptance of

specific traditional systems which historically are adequate in performance.

In current physical models for plumbing stack design, it is assumed that

water is discharged into the stack and, see Figures 6 and 7, after falling

a distance L^. (terminal length), reaches a speed V^. (terminal velocity)

and clings to the wall in annular flow, for the rest of the fall [5].

Figures 6 and 7 present examples of working equations derived for specific

conditions from the more general dimensionally consistent equations given

in [5]. Allowable water flow, Q^, and the associated air flow, Q^, are
SOIL OR WASTE

STACK

(from NBS Monograph 31)

TERMINAL VELOCITY 3.0 - 6.1 m/s

(10 - 20 FPS)

Vt = Ci (Qw/Ds) Lt

TERMINAL LENGTH 3.0 - 4.6 m

(10 - 15 FT) Ds

Lt = C2 (Vt)2

WATER FLOW

Qw = C3 (rs) 5/3 ,D^) 8/3

STACK FULLNESS (1/3 to 1/4)

Figure 6.

r, = f(t,Ds) C^, C2, C3 constants dependent on

material roughness

Some Mathematical Models for Soil or Waste Stack

Design

Figure 7.

(from NBS Monograph 31) LENGTH OF VENT

Lv=C4(Dv)X^

(for a 0.25-kPg (1-in. W.G.) pressure drop)

AIR FLOW RATE V C5 (rs)^^^n-rs)(Ds)^^^

MINIMUM BRANCH VENT DIAMETER

Db = Dv/2 or Ds/2

wfiichever is less but

not less ttian 35mm (XA in)

C4, Cg constants dependent on material roughness

Some Mathematical Models for Vent Stack Design

10
computed by the assximption of a stack fullness ratio, r (area of the

water ring to the internal area of the pipe) and ratio of mean air speed

to water speed, V^/V^. Allowable vent length, L^, or diameter, D^ , (one

of these must be assumed) is computed on the basis of the peak demand air

flow, Q , the allowable pressure drop, dP, and a material-dependent roughness factor. In the design of drain-waste-vent systems, the traditional

limit of air pressure fluctuations has been + 0.25 kPa (+ 1.0 in W.G.)^ [6, 1\

(However, in terms of the fundamental need, or Requirement, of the user

for protection of health, this implies adequate trap-seal retention. )

In the design practice for nominally horizontal sanitary drains it is assumed

that the water in the drain should flow at a mean velocity not less than

0.6 m/s (2 ft/s) when one-half full, and should not in any event flow more than one-half full under design load [8] (see Figure 8). A complicating

factor in the design of horizontal lines is the 'hydraulic jump' which is

not well defined through adequate mathematical models. The Manning Formula

for uniform continuous open channel flow is not appropriate for the dynamic

system, nor is the relationship of the hydraulic jump to stack capacity well

understood for transient flow. A hydraulic jump is an energy dissipation

phenomenon in a nominally horizontal conduit (pipe or channel) by which

the depth of flow (in the downstream direction) increases suddenly, possibly

filling the cross section of the conduit. Development of insight into and

understanding of this phenomenon in plumbing systems is important in the

evaluation of some innovative systems (single stack systems, for example)

where a full building drain and back pressure at the base of the stack

can choke air circulation, and cause ejection of sewage through floor drains

or other fixtures connected near the base of the stack.

In previous studies of horizontal drains at NBS [8], a controlled surge

flow was utilized in a horizontal drain as a means of simulating a hydraulic jump and some of its effects. In general, cresting of the jump

occurred much further downstream of entrance than the 10 drain diameters

commonly assumed [9]. It was found that as the duration of the surge was

Units and conversion factors are given in Appendix C, page 54.

11
n

BUILDING

DRAIN

(NBS Monograph 86)

HYDRAULIC DISCONTINUITY

"^ ^-^^ critical IN CONTINUOUS FLOW,

HYDRAULIC JUMP MAY OCCUR

DESIGN VELOCITY: AT LEAST 0.6 M/s (2 F/s) —

DESIGN DEPTH: 1/2 Dq MAXIMUM

Downstream

of Jump

DESIGN FLOW RATE: Vi RATE PRODUCING

FULL PIPE AT JUMP

Figure 8. Principal Parameters for Design of Horizontal

Drains

LOAD FLOW PIPING

XZZJ

FIXTURES

DURATION

PEAK

ADDITIVES

TIME

STACK FULLNESS

DRAIN DEPTH

AIR CIRCULATION

PRESSURE LIMITS

SIZE

LENGTH

en

J J J ROUGHNESS

SLOPE

St,^^

Figure 9. Variables in System Capacity

12
decreased, the capacity of the drain to carry the flow without completely

filling, was increased. Also, this relative increase in drain capacity

was inversely proportional to the drain slope.

A further study is needed of the dynamics of this type of flow especially

in relation to stack load conditions. Such a study could provide a better

understanding of the dynamics of the plumbing drainage system at the base

of the stack where the horizontal drain (designed for 0.6 m/s) may not be

capable of receiving a continuing discharge from the stack (designed for

3.0 - 4.6 m/s). A more general definition of conditions determining the

drain capacity in this type of experimentation is needed and a more realistic application of the laboratory data depends on improved definition of

the representative time-profile of the surge from data under field conditions. This is discussed further in 4.2.

To focus on the system as a whole. Figure 9 depicts the basic parameters

associated with hydraulic load profile (i.e. load vs time) and hydraulic

carrying capacity. Many of these have been discussed in part in Figures 6,

7 and 8. Perhaps the analogy of the changing traffic load pattern across

a bridge would be helpful in describing the time profiles of hydraulic

parameters. The hydraulic load profile corresponds in the analogy to the

changing vehicular traffic pattern (Load in Figure 9) on a bridge, while

the hydraulic carrying capacity may be described both as analogous to achieving smooth movement on the bridge without stoppage of the probable maximum

number (design load) of vehicles that will be carried simultaneously (Flow

in Figure 9) and to maintaining the strength of the bridge (Piping in

Figure 9) for the peak loads during the loading period.

The design hydraulic load in plumbing systems is derived from the discharge rate

calculated from the estimated use of the plumbing fixtures in the system. The

frequency with which this peak design load occurs has been difficult to document in real systems under service conditions. It is^ however, widely believed

that the customary Hunter Curve [10, 11, 12] load- estimating calculations

13
result in pipe sizes larger than needed for proper functioning of the

plumbing system.

In transporting the hydraulic load, two flow parameters, stack fullness (r ) and water depth in the drain (Hj) are primary determinants of the hydraulic

carrying capacity. Design values of r and Hj (these are related to the volume rate of water flow) in turn affect the type and assumed amount of venting

(volume rate of air flow) needed. The purpose of venting the wet system

with air is to preserve the fixture trap seals from suction (self or induced

siphonage), and back pressure. To this end, pneumatic pressure excursion

in the system is limited by design to + 0.25 kPa (+1.0 in W.G.). Actually,

these pressure excursions affect the calculated air demand, Q (a pressure

drop value is included in constant Ca cf Figure 7) , and also limit allowable

connected fixture (unit) loads (which are a function of design peak water

flow rate, Q^).

The pipe size directly affects r in vertical flow and Hj in horizontal flow,

for a given flow rate. The overall configuration of the system is also important. Straight pipe of proper size, without fittings, offers the least

resistance to flow. Changes in direction, the presence of fittings, changes

in pipe length and slope will alter the carrying capacity of the system.

New, smooth pipe offers the least resistance to flow, but documentation of the

increase of effective pipe roughness over time in plumbing drains and vents

and its limitation on the fluid carrying capacity (of the pipe) would permit

better prediction of carrying capacity under service conditions. Some

engineers have recommended an adjusted vent diameter based on observed fouling in service in applying the equations of Figure 7, but this is not standard

practice.

In summary, the relationships between design parameters and performance are

not adequately accounted for in present design practice, due to the lack of

definition of some relevant phenomena and due to the fact that significant

correlations between some of the parameters are either not recognized, or are

not satisfactorily established. As an example of an incomplete correlation,

14
taking into account the reduction in cross-sectional area due to fouling

in service may tend to increase design sizes of new pipe, but a better

definition of the dynamic hydraulic carrying capacity may tend to decrease

the design sizes of new pipe.

15
I

SEAL DEPTH

REDUCTION, A H

TRAP-SEAL

DYNAMICS

BEHAVIOR

(static conditions)

+ m

CONSERVATIVE

CORRELATION

BETWEEN aH AND DYNAMICa P J V 1.

DRAIN WASTEVENT FLOW

(steady state)

MODELS

"bhh

hh^

W t7 t'

t' t7 t?

LOAD

PREDICTION

(probability theory)

r T

(/

Va =Vw

AIR AND WATER

MEAN SPEEDS EQUAL ' V

A"

<^

<<^....

z.

u^

<

^

<

WATER FLOW

IN AT ONE POINT

HIGH ON STACK

(worst case hydraulic load)

NO NETWORK

CIRCULATION

NO Q^ IN BUILDING SEWER-^

Figure 10. Arbitrary Assumptions in Applying Mathematical

Models to Vent Design in Tall Stacks

16
3. EXAMPLE OF PERFORMANCE RESEARCH IN PLUMBING AND ITS

APPLICATION TO PLUMBING DESIGN

3.1 DISCUSSION OF TRADITIONAL VENT MODEL FOR TALL STACKS IN SHORT STACK

APPLICATIONS AND NBS STUDIES OF SHORT STACK SYSTEMS

Having briefly reviewed the broad framework within which performance criteria

may be identified and ordered with regard to basic user needs and to the framework of traditional plumbing design, an illustrative example of a performanceoriented study will be presented using the NBS work on venting criteria.

In traditional systems, it is customary that a continuous air path be maintained within the system from roof to sewer. This is provided through design,

as discussed in Section 2.2, that seeks to maintain a core of air within the

falling water in the stack, and to limit the peak water depth in the building

drain and sewer to a depth about 1/2 the pipe diameter. In traditional

design, however, no definitive account is taken in calculated air demands

of the fact that in many systems, particularly with short stacks, air demand

is significantly reduced (from that predicted from the usual model [5]) by

pneumatic pressures differing slightly from atmospheric pressure, but still

within the + 0.25 kPa code-prescribed limit. Thus many systems are designed

for calculated air flow rates much greater than actually occur under particular pneumatic conditions, because the assumption is made that the pneumatic

pressure does not differ from atmospheric, and this makes Q^ a maximum value

which is utilized in the vent sizing equation to give a maximum size of vent.

Furthermore, the basic assumptions [5] in accepted venting theory drawn from

the mathematical models for calculated air demand in tall stacks that have

been described are especially conservative for systems with short stacks —

water fall distance less than 6.1 m (20 ft). These assumptions, depicted

in Figure 10, are: (1) all the air brought down the soil (or waste) stack

with falling water from discharging fixtures is exhausted in an equal amount

out the top of the vent stack, without losses; (2) there is no slippage

between the air and water during fall in the stack (i.e. mean speeds of

17
air and water assumed equal); (3) worst case hydraulic load (with all the

water introduced at the top of a tall stack in most of the tests) was utilized in experiments from which air demand data were drawn; and (4) no specific contribution to venting was assumed either from air circulation

within the vent network or relief from the building drain and sewer.

The calculation of air flow rates under the above assumptions made in

venting theory for tall stacks led to designs of oversize vent pipes for

piany residential (short stack) designs. For this reason, a planned laboratory study of hydraulic and pneumatic parameters in systems with reducedsize vents was undertaken. The National Bureau of Standards has been

carrying out laboratory work on reduced-size vents (smaller than those

presently allowed by model plumbing codes) over a number of years. The

earliest laboratory work was sponsored by the National Association of Home

Builders, and more recently by the Tri-Services Investigational Committee

on Building Materials of the Department of Defense.

In the early work, two full-scale systems and several partial DWV systems

were studied [13] . The principal measurement in the full-scale system was trap-seal retention (a performance criterion). Principal measurements in

the partial DWV systems were peak air flow rate and corresponding pneumatic

pressure in the vent (design criteria). This investigation provided an empirical basis for further work to broaden the vent-sizing criteria developed from the initial work so as to apply to individual and branch vents,

and to differentiate between vent sizes for one- and two-story configurations [14] .

In the more recent laboratory investigation, a full-scale two-story "mixed"

system (where the pneumatic suction affecting the P-trap was the limiting

performance criterion) was studied, see Photos 1 and 2, after first experimenting with a simple component stack with flushometer water closets which

was used to isolate the water closets in order study pneumatic suction

related to water-closet trap-seal performance [14] . Test loads in the

18
Photo 1.

NBS Laboratory Townhouse,

Second-Story Fixtures

Photo 2.

NBS Laboratory Townhouse,

First-Story Fixtures

19
full-scale system selected on the basis of guidelines given in [3] covered a wide range of operating conditions from normal to severe loadings including

some tests with the vent terminals closed and a few tests with the building

drain submerged. (Tests with the building drain submerged and an adequate

range of tests with additives were not included in the earlier NBS-NAHB study.

In the recent tests, the experimental approach was to obtain a correlation

under dynamic conditions between the peak vent suction (0.2 s averaging

period), the peak air flow (0.2 s averaging period), and P-trap and WC-trap

trap-seal reduction. The averaging period refers to the time period selected

for measurement for which the (data acquisition system) computer would retain

a single value of the (pressure, air flow rate or building drain depth)

parameter representing an average value of the many values scanned in that

time period. The broad need for systematic research for better definition

of time profiles of hydraulic and pneumatic parameters in relation to values

obtained i(inder conditions of continuous flow is discussed further in 4.1.

3.2 DEVELOPMENT OF LABORATORY PERFORMANCE DATA FOR ONE CRITERION

A systematic approach to measurement which is reproducible and accurate is

essential for the collection of performance data if such data are to have

a general applicability. As discussed in Section 1.1, the uniform evaluation guidelines for judging the compliance of a system to a performance

Requirement are important in order to assure an evaluation technique which

will have general applicability from system to system.

The basic procedure that was followed in the recent laboratory tests was

to discharge a pre-selected combination of fixtures four times (four runs)

and to record selected peak pressures and to read the idle traps after

each run. This gave a cumulative value of trap-seal reduction after four

runs. In some cases, the traps were refilled before the beginning of each

run and the (4) values averaged. This produced a single-run dH value.

These evaluation procedures are described in the Test section of Figure 5.

The procedures are depicted schematically in Figure 11.

20
SYSTEMATIC TEST PROCEDURES FOR

TRAP-SEAL REDUCTIONS, dH

(TEST = FOUR RUNS = 1234)

SINGLE- RUNdH

(average reading)

(TEST FOR C 2.12 -2.1 (a)

from Figure 5

CUMULATIVE dH

(reading after final run)

(TEST FOR C 2.1.2 - 2.1 (b))

from Figure 5

PEAK PRESSURE OBSERVED OR RECORDED DURING RUN (OPTIONAL)

Figure 11. Difference Between "Single-Run" and "Cumulative"

Measurements of Trap-Seal Reduction (dH). (In

each case, the peak pressure (dP) is averaged for

the four runs.

)

21
Development of these gviidelines for deciding what and how measurements are

to be made is an evolutionary process both analytical and empirical. As

mentioned in Section 2.2, the current mathematical models for plumbing design

do not specifically relate pneumatic pressure fluctuation and trap-seal

retention. Rather it is assumed that vent sizes calculated to limit pressure

drop to 0.25 kPa are required to protect trap seals. In the NBS Laboratory

studies of reduced-size venting, the limit of acceptable trap-seal reduction

was set at 25 mm (1 in) as representative of the performance intended

in service. During the tests, the pneumatic peak suction in the vent was measured also, for comparison with the values predicted for air flow

at a 0.25 kPa pressure drop [5] from theoretical computations.

In the recent NBS laboratory work, a component stack comprised of a 5.5m

(18 ft) long 78 mm (3 in) diameter PVC stack with back-to-back flushometer

water closets at the top was studied initially. Test conditions could be

carefully controlled in this simple configuration. An adjustable butterfly

valve was used to simulate the resistance of different sizes or lengths

of vent pipe. A schematic of the simple system showing the measurement

points is given in Figure 12. Tests (4 runs per test) were first carried

out with the butterfly valve open, then with 3 different degrees of restrictioi

and finally with the valve closed, each series for supply line pressures

207, 345, 483 kPa (30, 50, and 70 psi). Each water closet was flushed

4 times alone, then both were flushed 4 times together, for each valve

setting, in general.

In Figure 13, the data from the single WC flushes in the simple system is

presented. Of special interest is the much smaller rate of increase in the

trap-seal reduction (dH) values than in the peak vent suction (dP) values,

as the valve is closed. In particular, the peak vent suction, dP, is

1.00 kPa (4.0 in W.G.) for a 25 mm (1.0 in) trap-seal reduction, dH, for

The metric size equivalents given are simple conversions of the nominal

pipe sizes in the U.S. Customary Units, and therefore do not represent

either actual or nominal metric pipe sizes.

22
5.5m 18 ft)

(a)

cr^

78-mm(3-in) PVC 0©

MEASUREMENT POINTS

(A) VENT AIR FLOW, Qg

(P) VENT SUCTION, dP

® TRAP DEPTH SENSOR, dH

® LOAD

VALVES IN VENT LINE (^ BUHERFLY VALVE

GATE VALVE

Figure 12. NBS Laboratory Component Stack Schematic

CD

= 1.49 (6.0)r

1.24 (5.0)- safe level /experimental finding ^ 1.00 (4.0)-

Q-o

o

00oo

LU

a.<

0.75 (3.0)

0.50 (2.0)

0.25 (1.0) — 000 (00)

°o'

yO JX>-

.^

no '^ Ob

-.TRADrriONAL

<CC

-dHDESIGN LIMIT i-A.

.^-

• f g h

o

t—o

=3aUJ

cc - 50 (1.0) ^UJGO ^

25 (1.0) ^ 78 mm PVC

t- (3in)

SUPPLY PRESSURE: 345 kPa (50 psi)

I

_i_ J I I L.

5.5m(l8ft)

(0.0) —

Q- 9.0 80 70 6.0 5.0 40 3.0 2.0 1.0 0.0

(143) (127) (III) (95) (79) (63) (48) (32) (16) (0) DESCENDING SCALE »- VENT

CLOSED

Q^, PEAK AIR FLOW SUPPLY, i/s (GPM)

Figure 13. Component Stack Data With Flushometer Water Closets

23
single flushes. Also important, but not shown in Figure 13, is that the

corresponding peak vent suction, dP, for four runs wLthout refilling the

idle trap between runs, was 0.75 kPa (3.0 in W.G.) for a 25 mm (1.0 in)

cumulative trap-seal reduction, dH. In other words, a single WC flush which

produced (in the restricted vent) a peak suction level in the vent of 1.00 kPa

(4.0 in W.G.), or four consecutive flushes (no refilling) that each produced

a peak suction level in the vent of about 0.75 kPa (3.0 in W.G.) reduced

the idle WC trap seal in the back-to-back configuration by 25 mm (1.0 in).

After completing the study of the simple system, a full-scale two-story

townhouse system was tested in the laboratory. A schematic of the townhouse

system shows the geometry and fixture distribution of the system (Figure 14).

The reduced-size vent sizes are marked. The system had 9 fixtures; one and

a half bathrooms upstairs, and a half bath plus a kitchen sink and clotheswasher downstairs, see Photos 1 and 2. The system was tested over a variety

of conditions including one with the main vent terminals closed and the

building drain submerged. Reduced-size vents were installed 152 mm (6 in)

above the flood rim of the fixtures served.

Because of the mild suctions produced in the townhouse system under representative test loads, data relating pneumatic pressure excursion and trapseal reduction near trap-seal failure levels were not numerous. In this

system of mixed fixtures, the P-traps failed more readily than the WC-traps.

Trap failures occurred only when the vent terminals were completely closed,

in accordance with the test plan. Figure 15 gives specific information

about the dP-dH relationship for P-traps in the failure region.

For a representative test with the vent terminals closed, the time profile

of the pressure level of vent suction is presented in Figure 17, with the

corresponding cumulative trap-seal reductions of the idle fixtures. The

data (fed to an on-line plotter from a mini-computer which processed the

24
FIXTURE VENT

CONFLUENT VB^T

NOMINAL PIPE DIAMETffi CONVERSIONS MM IN*

16 '/2

21 3/4 27 1 35 u

41 V/2 53 2

*Schedule 40 pipe.

ASTM 02665

Figure 14. NBS Laboratory Townhouse System Schematic

50. (2.0)-

o

haLU

cc

I <LUa.<cc

25 (1.0)

• SINGLE RUN dH

^CUMULATIVE dH

(EACH SYMBOL IS FOUR RUNS)

50 Percent Retention Performance Criterion • ^^ /•

^-'

(0.0) 25

(1.0)

\ 0.50

(2.0)

0.75

(3.0)

dP, PEAK SUCTION IN VENT, kPa (IN W.G.

Figure 15. Townhouse System Data on P-traps

25
PRESSURE

OOkPa

(0 IN W G )

PRESSURE -TIME PROFILES

25 kPa —I

(10 IN W G

1 7VENT

/TERMINALS

lCLOSED

H

•-5

Rl KJ

1- PRESSURE TAP SYSTEM

IT

L2 W2 lT K^ TESTED

i- L5 v<1 c

1 4 ^H^^

«=o^p<-

HYDRAULIC LOAD EACH RUN

Wl + W2 + 81

(SIMULTANEOUSLY DISCHARGED;

2

SYSTEMATIC TEST PROCEDURE

CUMULATIVE

FIXTURE

TRAP-SEAL REDUCTION AFTER FOUR RUNS

LIL2L5 W5

C4S3

mm (in.) 16 (0 63)

19 (07F')

15 (0,59)

10 (0.381

16 (0.63)

7 (028)

Figure 16. Sample of Data for a "Time Slice" Averaging

Period of 1.0 s Duration and the Associated Cumulative

Trap Seal Reductions (dH) from a Four-Run Test

(i.e. systematic procedure of Figure 11)

26
data to a single average value for each 1-second "averaging period")

represent the peak suction produced in the soil stack vent for a discharge

of two water closets and a bath tub, all in the second story level. The

greatest trap-seal reduction was 19 mm (0.75 in) even though the suction

at the measured point was over 0.25 kPa (1.0 in W.G.) for a cumulative

time of 4 seconds for the four runs. These data provide additional evidence

to that of Figures 13 and 15 that idle trap seals can tolerate repetitive,

brief applications of rapidly oscillating suction beyond the traditional

design limit value of 0.25 kPa (1.0 in W.G.).

3.3 DISCUSSION OF FINDINGS ON FLOW PARAMETERS AND TRAP PERFORMANCE

Results from the experimental work indicated that both the vents and building drain were important elements of the air circulation network. Empirical

data were obtained in the townhouse tests indicating the presence of recirculation within the vent network, and of a difference in the peak air

demand from a first or second story discharge. These data are given in

Table 1, an excerpt of Table 9 in [14]. It is believed air circulation

within the vent-drain network, with the main vent terminal closed, furnished

the air flow required by the operating fixture since the levels of vent suction measured were nearly the same for both open and closed vent terminals.

Table 1. HEIGHT OF FALL EFFECT AND AIR CIRCULATION EVIDENCE

(0.2s Averaging Period)*

Water Closet Discharged

TEST C N D I T I N S VENT OPEN VENT CLOSED

i\ame Location

<)^, peak air demand at vent terminal

dP, peak vent suction

in stack of active

fixture

Qg, peak

air demand

at ve n

t

terminal

dP, peak vent suction

in stack

of active

fixture

(1/s) (gpm) kPa (in W.G. ) (1/s) (gpm) kPa (in W.G.)

W2 Second story (1.8) 28. 0.082 0.33 0.062 0.25

W5 First story (0.5) 8. 0.010 0.04 0.006 0.02

Ihis averaging period it> different from that for Figure 16. See also Section 4.1.

27
A depth monitor was utilized in the tests to measure the peak water depth

In the building drain, intended by design not to exceed one-half drain diameter at design flow rate. For a discharge of four fixtures, the most fixtures used in a test an unlikely load the measured water depth in the

nominal 4-in building drain was a maximum of 6 cm. The hydraulic load

producing this level was: Wl + W2 + W5 + S3A, i.e. the simultaneous discharge

of the three water closets and one bowl of the double-bowl kitchen sink.

As a summary of the experimental relationship of trap-seal performance and

pneumatic pressure, data are presented in Table 2, giving a comparison

of the different levels of peak suction producing a 25 mm (1.0 in) trap

seal reduction in WC traps and P-traps, for single and multiple flushes.

Although a water closet trap can tolerate a higher level of peak suction

(to produce a 25 mm trap-seal reduction) than a P-trap, the P-trap criterion

is the limiting value in a system of mixed fixtures. The finding is based

on the data from reverse trap water closets having trap-seal depth of approximately 76 mm (3 in) and 1 1/2-in P-traps having trap-seal depths of a approximately 51 mm (2 in). Another experiment, made on a ten-story system, [15],

indicated that WC-trap-seal depth may be the limiting criterion for back

pressure.

Table 3 presents a summary of the principal results, in terms of trap-seal

performance, of the tests on the full-scale system following the systematic

test procedures of Figures 5 and 11. There were no trap-seal reductions

of significance unless the main vent terminal was closed.

The recommendation for a design value for dP of 0.37 kPa (1.5 in W.G.) is [14] a conservative value based on multiple flush values for dP (0.45 kPa) for

P-traps which was obtained in this study by extrapolation. This extrapolated

value (0.45 kPa) was further confirmed by additional data which were obtained

in a preliminary NBS laboratory study of the dP-Q^-dH relationship in a

3. 5-story stack. P-trap data relating dP and dH is presented in Figure 17.

28
Table 2. Levels of Peak Suction Producing a 25 mm (1.0 in) Trap Seal

Reduction in WC-traps and P-traps for Single and Multiple Flushes

Test Procedure

Peak Suction Measurement (0.2s)

WC-trap vent

(from Component

Stack Data)

P-trap vent

(from Townhouse Data)

Single flushes

kPa (in W.G.)

1.00 (4.0)

0.75 (3.0)

kPa (in W.G.)

0.62 (2.5)

0.45 (1.8) Multiple (4) flushes

Table 3. SUMMARY OF PRINCIPAL RESULTS OF LABORATORY TESTS ON A FULL-SCALE TOWNHOUSE DRAINAGE SYSTEM WITH REDUCED-SIZE VENTS

Fixtures Discharged^

(Active Traps)

TEST CONDITIONS

Vent Terminals and Building Drain Open

Additives

(firs

runs

)

(first and third

b

Idle Traps

Failed After

Four Runs Without

Refill'"' (Cumulative

dH)

Vent Terminals Closed and/or Building Drain Submerged

Addi t ives

(all four

runs )

Vents

Closed

Drain Submerged

Idle Trap

Failed After

Four Runs Without

RefiH*"-

( Cumulative

dH)

Number of

Idle Traps

LI+Wl

.

Bl+Wl.

S3a+Wl+W2 C4+W1+W2

.

C4+W1+W2. C4+W1+W2.

B1+W1+W2. B1+W1+W2

.

B1+W1+W2.

S3a+Wl+W2+W5

S3a+Wl+W2+W3

S3a+Wl+W2+W5

DET/fwdu in S3A. None

PD in W2

PD in Wl None

DET/fwdu in S3A. None None

DET in C4 PD in Wl

BB in Bl BB in Bl

DET/fwdu in S3A. fwdu in S3A DET/fwdu in S3A.

fwdu in S3A. None

None

None

None

fwdu in S3A. None None

None None

None None

fwdu in S3A. fwdu in S3A. fwdu in S3A.

Both. Main

.

Both.

None

.

Main.

None

.

Both.

Both. Sink.

Both. None

.

Both.

Both. Both.

Both.

No. . No..

No . .

Yes.

No. .

Yes.

No. . No. . No. . No..

Yes. Yes.

No. . No. . No..

S3A - fwdu compartment of a double bowl sink

S3B - non-fwdu compartment of a double bowl sink

Wl, W2, W3 - water closets

LI, L2 , L5 - lavatories

81 - bathtub

fwdu = food-waste-disposal-unit

DET = granulated detergent

BB = bubble bath

PD = paper diaper

'^ Reduced by 25 mm (1 inch from full-seal depth)

29
P-TRAP DATA

FOUR-RUN CUMULATIVE dH

FOUR-RUN AVERAGE dP

TERMINAL

"RESTRICTED

AIR FLOWMETER

-WATER IN

2 2-2.8 l/s

( 35-45gpm)

PRESSURE TAP

E

Eo

I-o

Q

UJ

CC

_l<

Ll) in

cl<

X

5

3

2

I '(4 in) 100 mm O 4TH FLOOR P-TRAP DATA

25-

(1.0)

(00),

50 PERCENT RETENTION

PERFORMANCE

CRITERION

O

CQ

o

I I 0.25 0.50

(1.0) (2.0) dP, VENT PEAK (0.2s) SUCTION, kPa(inW.G)

^

W2

L5

LI KJ

Bl

W5

C4 ^

-VENT

TERMINALS

CLOSED

Wl +W2+BI

DISCHARGED

S3

A LAVATORY P-TRAP DATA

(Adata from Fig. 15 for Cumulative dH) Figure 17. Comparison of P-Trap Data in Townhouse System

and in 5-story Simple Stack for Multiple Flushes

The 5-story stack was intended to serve as a multistory counterpart to

the "component stack" used in the study of short stacks in which the test

conditions could be carefully controlled. The design of the experimentation

for the 5-story, simple stack provided for the systematic addition/change

of branch piping and fittings, so that the simple stack could serve as the core of a study of more complex systems. With varying water flow rate

and variations in venting and fitting geometry, the pneumatic pressure

fluctuations and trap-seal behavior could be studied under dynamic conditions. Results obtained in limited experimentation on the 5-story stack

provide encouragement that with a suitable, expanded, test program such

as that outlined in 4.1, a correlation for dynamic dP-Q -dH could be

established for a range of designs, for systems of different heights.

30
The value of 0.375 kPa for design pressure drop has previously been

recommended Independently by Lillywhite and Wise [16] In connection with

a procedure for sizing of stacks for single-stack drainage systems. Their

recommendation for the equivalency of 25 mm (1 In) of trap-seal reduction

and 0.375 kPa suction was based on practical experiments In multistory

buildings up to 25 stories In which stack suction was compared with trapseal reduction of English wash-down water closets [17],

3.4 APPLICATION OF FINDINGS FOR VENT SIZING CRITERIA FOR RESIDENTIAL

SYSTEMS WITH SHORT STACKS, AND FIELD PERFORMANCE VALIDATION

The research described Is performance oriented because data were obtained

simultaneously relating the traditional specification type design criteria

(pipe sizes based on specific air flow rates and peak suction levels) and

a basic user need for protection of health through adequate trap-seal retention -- in the context of the performance framework described in Section 2.

The findings indicated that the widely held assumption that a fluctuating

suction of 0.25 kPa (1.0 in W.G.) is roughly equivalent to a trap-seal reduction of 25 mm (1.0 in) is erroneous. The data support a design pressure

drop in the vents of 0.37 kPa (1.5 in W.G.) rather than the 0.25 kPa (1.0

in W.G.) on which the present codes are based. Utilization of these findings

in the existing design criteria could result in a 50 percent increase in

allowable dry vent pipe length, or a diameter reduction of about 8 percent,

in all systems. Perhaps the most important result was the development of new criteria for 1-2 story systems that provide for vent size reduction

of 1 to 4 commercial pipe sizes.

The RSV recommendations are presented in Table 4 and the sizing procedures

and examples are given in detail in Appendix A (both from [14]). The selection

parameters identified are: (1) vent function, (2) elevation of trap, and

(3) the fixture unit load served by the vent. In the sizing sequence, the

primary elements, or fixture vents are sized first. A fixture vent provides

the primary or sole ventilation for a trap or group of traps located at the

base of the vent. Where two or more fixture vents are connected, the joint

31
Table 4. Recommended criteria for selecting dry-vent sizes for 1-2 story sanitary drainage systems^

SIZING

SEQUENCE

First

.

Next.

Last.

SELECTION CRITERIA

Function of vent

d FIXTURE VENTS:

Individual

(serves single trap).

Common

(serves two traps).

Stack vent (Main vent at top of a

soil or waste stack).

- CONFLUENT VENTS:

For two fixture vents . .

For three fixture vents .

For four or more fixture vents.

^ARTERIAL VENT:

Vent stack, 1 or Stack vent serving as I a relief vent J

Elevation of trap '' (determines distance

of water fall)

ft Up to 8

8 tol6 ....

Up to 8 . . .

8 to 16.... Up to 8 . . .

8 to 16...

Not applicable

.

Maximum load "^ (served by vent)

Not applicable

Not applicable

Not applicable

FU

1 to 3. 4 to 6.

1 to3.

4 to 6.

1 to3.

4 to 6.

1 to 6.

1 to6...

7 to 15.. 16 to 30.

1 to6...

7 to 15.

.

16 to 30.

Not applicable

Not applicable

Not applicable

10.

10. 30.

30.

MINIMUM VENT SIZE

(nominal pipe diameter)

.%.

M.

.1.

.1.m

.1%.

.i^i

One pipe size larger than the

largest fixture vent served.

' In most cases, increase one

pipe size over largest fixture vent served.

* Area of pipe selected must

equal or exceed the value computed from:

AcONPLVKNT = y AlaROEST- IL-^SBRVBD

AM (36 ft max),

.in (120 ft max)

AVi (30 ft max). AVo (100 ft max).

32
Table 4, Footnotes

* This table may also be applied to 3-level split configurations in which the total height of water fall between the highest

fixture and the main building drain does not exceed 16 ft.

*> Elevation of trap above first lower (vented) horizontal

fixture branch, (vented) soil or waste stack offset, or building drain branch that serves the trap.

Figure A I

TRAP

DISCHARGE ELEVATION —u

^Fixture unit values for usual plumbing fixtures found in

residences [18]. For consistency, these values should be used

for sizing the dry vents. The applicable code values, even if different from those given below, may be used for sizing the wet piping.

Common

symbol

Fixture Load

P-1 Tank WC

FU

4 P-2. P-3. P-4

Lavatory

Bathtub

Shower

122 P-5

P-6 F.D

Sink w/fwdu and DWM

Clothes washer

Floor drain

3

3

3

** Fixture vent: Any single vent that provides the sole or primary ventilation for a trap or group of traps located

at the base of the vent. These sizes are valid for vent lengths up to 25 ft. For longer lengths, increase by one pipe size.

^ Confluent veni: A vent pipe that serves two or more

fixture vents.

Exceptions requiring two pipe size increas.T

(Three) fixture vents served Confluent vent

in in in in

1 1 1 1^2 1% IM I'A 2 % 1 VA 2 % 1J4 lA 2

1 1 \M 2

1 \M \A 2 IH I'A \M 3 IM Wi IK2 3

« Cross-sectional areas by which to calculate confluent vent

size using the square-root formula are as follows:

Nominal

Internal cross-sectional areas

diameter

pipe Schedule 40 Copper tube M DWV

inM

%

1lA

1/2

2

1712 0.304

0.533 0.864

1.495

2. 036 3.355

7.393

m2

0.254

0.517

0.874

m2

1.317 1.865 3.272

3 7.235

^ These sizes (a one-pipe-size reduction derived from table 14 of [5]) are on ihe basis that significant pressure relief occurs through the building-drain and building-sewer route,

significant circulation occurs in the branches of the vent network, and air demand is minimal in short stacks. If flooded sewer conditions are anticipated, the arterial vent

size, obtained as indicated, should be increased one pipe size.

pipe is sized in the next step as a confluent vent by means of square-root

formula which relates the sum of the areas of the fixture vents served to

that of the confluent vent. Last in the sizing sequence is a consideration

of the air circulation in the system as a whole. The arterial vent sizing

criteria apply to DWV systems of more than one story and are intended to

provide circulation and back-pressure relief. The criteria are meant to be

supplemented by engineering judgment and close attention to details of

installation. In a separate study based on an analysis of data in [12, 13,

14] , Brownstein has made similar recommendations in the context of guidelines for plumbing designers on reduced-size vents for short-stack residential

DWV systems [19]

.

33
Photo 3,

^, — Andrews Air Force Base

4'^ Field Test Houses,

•*^ *-! Aerial View of D, E, and F

y' ,*. '^ Units with Reduced-Size Vents

Jj (Photo Courtesy Andrews Air Force Base)

FIELD STUDY UNITS

"D" 4-BR TOWNHOUSE (a and b) "E" 3-8R TOWNHOUSE (e Q?T#f) V

"F" 4-BR DUPLEX {a and b)

V

'I A

F/a IL

r.j' T^^BL.^,

F/b

X

Photo 4.

Typical Lavatory Installation

with Instrumented

P-Trap and Water Supply Lines at Andrews Air Force Base

34
To verify the utility of the laboratory findings in service, and to study

the effect of service parameters such as frost closure of vents, natural

loadings, and sewer pressure fluctuations, NBS has been carrying out a field

study (to be reported separately). The criteria derived from the laboratory

work were used in designing the vent systems for 3 types of homes: a 3-bedroom

townhouse, a 4-bedroom townhouse, and a 4-bedroom duplex (See photo 3).

Principal field measurements taken for study of reduced-size vent performance

were the trap-seal retention (see photo 4) of P-traps and selected vent

pressures near some of the selected traps.

In built systems, the performance intended (by the pressure-level-limit

design criterion) is adequate trap-seal retention. This is more easily

measured in built systems, following systematic testing guidelines, than

pneumatic pressure excursion and peak air demand in the vent. Thus there

is widespread interest in hydraulic test procedures for innovative systems

involving measurement of trap-seal reduction. It has been recognized,

however, that standardization of these procedures requires better information on service load patterns as a basis of test load selection [20].

The laboratory studies indicated that a cumulative 25-mm trap-seal reduction

would not be likely unless the peak transient suction in the vent was on

the order of 0.45 kPa. In the RSV field study, the test procedures utilized

in the laboratory were adapated to the field in tests carried out before

occupancy. The systematic test procedures of Figures 5 and 11 were followed.

That a transient suction beyond 0.25 kPa (up to 0.45 kPa) does not cause a serious trap-seal reduction was confirmed in the field studies based on peak

readings visually observed on magnetically-coupled pressure gages connected

to selected vents. (Some tests were made with the vent terminal through

the roof closed, in accordance with the test plan.)

In the field validation of the adequacy of laboratory-based design criteria

for sizing RSV, an opportunity was provided for a pilot study involving

measurement and data acquisition methods relating to water and energy usage.

NBS provided sensing devices and an automatic data recording system which

35
recorded the real time when triggered by a sensor. Several houses were instrumented with flow switches (to register on or off) on the hot and cold

water supply lines to the fixtures, in conjunction with remote indicating

devices on the house water meter, and on the watt-hour meter connected to

the water heater. The principal objective of the pilot study was to develop

measurement and data recording methods, although the data provided some definition of the time distribution of use at a number of water outlets.

Comprehensive data on water usage, as discussed in 4.2 is needed as a major

building block of the performance approach, as it impacts on many areas of

plumbing design, plumbing standards development, and test procedures utilized

in laboratory research, especially as related to the design and evaluation

of innovative approaches.

36
4. SUMMARY OF NEEDS TO IMPROVE PLUMBING MODELS THROUGH

PERFORMANCE-ORIENTED RESEARCH

4.1 LABORATORY RESEARCH NEEDS FOR PLUMBING SYSTEMS

Traditional approaches to the evaluation of plumbing systems by current

design criteria are not sufficiently flexible for evaluating innovative

systems. In this paper we have broadly described, using the examination

of the vent model as the basis of a simple example, the utility of performance methodology to extend traditional methods. We have also discussed

the organization of existing knowledge into the performance (format) framework to maximize the utility of the available information to different

groups of users and to reveal gaps in existing knowledge that suggest

research needs.

The recent work shows that there is a significant opportunity to improve

the precision and scope of experiments and plumbing design criteria for

hydraulic (water supply and drainage) carrying capacity and for hydraulic

load-producing effects of fixtures. This is possible because of recent

state-of-the-art advances in instrumentation and data acquisition systems

capable of making meaningful measurements of a number of the rapidly

changing dynamic parameters typically occurring in a plumbing system at the same time. Statistical and mathematical techniques applied to such a data

base would permit systematic and rational reduction of some of the large

safety factors that have in the past been utilized to assure a safe level

of performance in the absence of adequate analytical information.

A broad laboratory research program should be designed to take into account

the interrelated areas of : I. Drain-Waste-Vent System Carrying Capacity,

II. Hydraulic Load Imposed by the Fixtures, and III. Metrology of Plumbing

Dynamics. Suggested principal research topics within a laborabory investigation to address hydraulic parameters in these areas are:

37
I. Drain-Waste-Vent System Carrying Capacity : A more definitive dynamic

correlation of peak air flow demand, pneumatic pressure and the

resulting trap-seal reductions from input Hydraulic Load as derived

from II, taking into account the following field conditions:

A. Changes in direction (offsets, fittings)

B. Additives (solids, suds)

C. Soil or waste stack length (distance of water fall)

D. Changes in flow resistance (effective roughness factor)

in service due to fouling and corrosion

I. Hydraulic Load Imposed by Fixtures : Better definition of the dynamic

load of individual fixtures and combinations of fixtures for generation

of a representative time-profile of hydraulic load as input to utilize

in experimentation and computation in relation to I, taking into account:

A. Time distribution of discharge (time-profile) of single

fixtures and combinations of fixtures

B. Attenuation of discharge rate as affected by geometry

of branch piping connected to fixtures

C. Volume of water consumption of different fixtures

D. Peak and average flow rates and duration of flow

I. Metrology of Plumbing Dynamics : Better correlation between peak values

measured and averaging period ("time-slice" width) used in data acquisition/

analysis for I and II, needed as a contribution to a common base for:

A. Uniform, reproducible measurements among laboratories

B. Analysis and utilization of data for development of useful mathematical

models, e.g., for trap-seal behavior and system capacity

C. Adapting the laboratory measurement approach to field data collection,

and interpretation of field peak-value measurements

38
A specific benefit of improving the present method of expressing fixture

loading effects by their hydraulic parameters in a common format for use

in plumbing system design would be better correlation of the discharge vs. time-profile with system carrying capacity in terms of the allowable number

of connected fixtures. If combined with documented data on frequency of use

in service, as discussed in the next section, this would also contribute

to improved precision of the fixture unit ratings assigned to the various

fixtures and appliances. This improved precision would be reflected in

reduction in the degree of overdesign of plumbing systems that is generally

believed to exist with the traditional design criteria [21, 22],

4.2 FIELD RESEARCH NEEDS FOR IMPROVED PRECISION IN ESTIMATING PEAK

HYDRAULIC LOADS AND ASSIGNING FIXTURE UNIT RATINGS

Although laboratory evaluation can define the hydraulic characteristics of

fixtures and appliances i.e., duration of discharge "t", and flow rate, "q",

the overriding research need is for field studies to define the time

distribution of fixture operation during periods of maximum usage. From

these data, updated values of "T", (average time between operations during

peak use periods) could be derived for various fixtures in different

occupancies. The three parameters, "t", "q", and "T" are utilized in

the probabilistic calculation of loads and fixture unit ratings [10]

for sizing water supply and DWV systems for buildings, see Figure 18.

Planning for the collection of usage data for interpretation in relation

to hydraulic design (service loads) requires that sociological, seasonal,

and geographic parameters be included in the program, and that data be

obtained continuously over a substantial period of time for meaningful,

statistically significant, interpretation of results, e.g. whether water

is "saved" under a particular set of conditions, whether specific numerical

changes in fixture-unit ratings can be recommended, etc. The feasibility

and impact of such studies depends greatly on the development of prototype

instrumentation and data collection suited to the scope and number

of raw data measurements that should be recorded and analyzed.

39
Figure 18. Principal Coordinated Inputs Needed to Update Hunter

Curve for Drainage (a similar Illustration could be

constructed for Water Supply)
The pilot study described 3.4, emphasizing field measurement and data re- cording methods, is considered an initial step in the development of a uniform methodology of defining real-time usage patterns in plumbing systems.

Such pilot studies are essential for the establishment of test methods that are viable under field conditions. Measurement procedures appropriate to the

laboratory may be difficult to adapt to field conditions where solids and

suds, as well as electrical disturbances caused by weather and a lack of control of flow conditions, add to the problem of keeping a total system operational and of remotely recording measurements that are meaningful [23, 24].

But some of these measurement system problems have to be addressed in the

research if meaningful nev/ criteria are to be developed to facilitate cost

reduction, materials conservation, and water and energy conservation.

For example, knowledge of representative fixture use patterns in service is

needed as the basis of an updated Hunter Curve and its associated Fixture

Unit ratings. Updated definition of t, T, and q would provide a basis for

improved precision of hydraulic load prediction (Q) for water distribution

and drainage systems in buildings, for individual and on-site water supply

and treatment systems, and for sewage treatment and disposal systems. - This improved precision in load estimation is needed for the reduction

of excessive safety factors in design, and for the realistic testing

of innovative systems.

The Hunter Curve was derived from the binomial probability theory in such

a fashion that it provides for any particular given system of one type

of fixture, an estimated (design) peak discharge rate. The design rate

is the greatest value of the two obtainable from the following criteria:

1. The rate which probably will occur, in the aggregate, in excess

of 1 percent of the time by the smallest possible amount.

2. The rate which probably will be exceeded in the aggregate less

than 1 percent of the time by the smallest possible amount.

41
To generalize the application of the theory, Hunter developed the "fixture

unit concept" which provides a means of synthesizing the basic curves as described above Into a single curve for estimating loads from typical systems comprising several types of fixtures, or "mixed" systems.

In recent times, engineers have metered the building water service on various

occasions In limited studies to compare observed peak demand with that predicted from the Hunter Curve. Generally they have found the actual demand

to be considerably less than that predicted from the Curve. However, there

has been no work to systematically adjust the currently used values of the

basic parameters of the Hunter model (t, T, q and the "fixture unit" ratings

of the various fixture types) that determine the predicted hydraulic load

(Q). Rather, only empirical modifications of the ordinate of the Curve have

been suggested.

A long-range comprehensive national field program Is needed. This work should

use uniform, technically adequate methods of experimental design, data acquisition and analysis, and should Involve all groups having a substantial Interest

In the subject of updating load prediction for plumbing systems (e.g. updating

of the Hunter Curve).

42
5. CONCLUSIONS

The role of basic research in developing viable performance evaluations is

depicted in Figure 19. This diagram illustrates the need for improved analytical capability that would permit both tentative design criteria and innovations based on such criteria to be evaluated directly in field studies or on the basis of reliable prediction (Figure 19, broken lines) of performance.

Presently, the step-by-step procedure (Figure 19, solid line from 1 to 5)

involves laboratory and field studies for validation.

However, research to strengthen the analytical capability, whether by computation or standard test method, would have significant impact on uniform and

realistic procedures. For example, there is at present no standard method

for determining and expressing the hydraulic characteristics of plumbing

fixtures and their appliances, i.e., how much water a particular fixture uses

or wastes, or what peak flow rate is generated, or the flush performance of

water closets. This becomes of major concern to manufacturers and code

officials in light of today's emphasis on energy and water conservation when

legislation increasingly mandates the use of "water-conserving" fixtures [25]

.

In another example, there are no standard procedures for predicting long-term

fouling effects under service conditions.

The scope of this paper has been concerned with what the performance approach

is, concentrating on DWV hydraulic carrying capacity in the discussions, and

with how performance methodology could be utilized in the evaluation of

innovative plumbing systems. Comprehensive performance evaluation of plumbing

systems on the basis of function, efficiency, and economy requires not only

adequate knowledge of hydraulic carrying capacities and the characteristic

patterns of service hydraulic loads as has been described, but also requires

better definition of parameters that affect durability and maintainability

under service conditions. In some cases, the system evaluation requires

interdisciplinary interaction such as for evaluation of (the installed system)

fire safety. Whether combustible piping material is fire-safe depends on

the plumbing and other building systems considered together, as installed.

43
5 CRITERIA IN USE

Findings disseminated through

publications to technical committees of plumbing

model code groups

4 FIELD STUDIES

v.^X--t'

May be desired to confirm the adequacy of laboratory- based design

criteria, at least to the extent that basic

analytical procedures are incomplete

<a

2 FUNDAMENTAL MEASURES

AND MODELS

Provide a means of generalizing the

available data on hydraulic capacity

and load patterns.

3 APPLIED RESEARCH

Can develop reproducible

evaluation procedures utilizing hjndamental measures and

models.

Figure 19. Model for the Development of Performance Criteria

Researcher

Building Official

'also Architect "also Manufacturer Figure 20. Schematic Indicating Groups Representing Cross-Section

of Interests in Plumbing System Specifications

44
Broad evaluation of the installed system should also be based on knowledge

of the psychological needs of the users of plumbing systems, in addition to

the technical parameters. For example, the unwanted sound generated by the

plumbing system (apart from functional capability) impacts daily on the user's

living activities if the noise is excessive. Study in the area of behavioral

sciences in relation to building technology has long been neglected and is

only now beginning to receive recognition as an important part (to the user)

of performance evaluations [26]. The assignment of responsibility for

development and implementation of interdisciplinary criteria is not specifically delineated at the present time, so these criteria tend to be neglected.

Evaluation of plumbing systems on the basis of performance would in the long

term enable innovative designs to be evaluated on their performance in terms

of user needs (and on this basis enter the marketplace). The present need

to systematically develop uniform criteria for design and evaluation of

innovation in plumbing has been recognized nationally in the ANSI A40

(committee) project, Minimum Requirements for Plumbing [27]. An A40 subcommittee has recently undertaken the drafting of a performance standard

for plumbing as a means of classifying and evaluating the 'performance

attributes of interest' and the 'limit states of concern'. This effort

could serve as a starting point for plumbing system interests to evolve

the uniform design and evaluation methods. Figure 20 is a schematic of

the groups representing the cross-section of interests which could ultimately benefit from improved plumbing system and design evaluation criteria,

and installation specifications.

Drawing again from the work of Fenves and Wright [1], the following quote

seems to describe what a broadened base of plumbing specifications (broadened

to include performance methods) is intended to accomplish:

"The quality of the built environment, including its functionality

and safety, is directly dependent on the quality of the specifications controlling the design. Ventre [28] has argued that because

of the 'diverse, dispersed, detached, and discontinuous' nature of

45
the building industry, specifications, and especially their legal

embodiments in the codes, represent essentially the only 'collective

memory' of the industry."

And finally, the performance approach, then is not a sweeping replacement

of existing codes, but something to be assimulated within this 'collective

memory' that can be used as a complementary methodology to aid code administrators in evaluating innovative designs. The performance approach also

seeks to put information in a systematic format for easier access by (possibly

adaptable to computer retrieval by) the different groups of users involved.

With systematic test methods and evaluation procedures which can be uniformly

used and enforced, performance evaluation and design methods can provide a needed supplement to traditional prescriptive methods.

46
6. REFERENCES

[I] Fenves, S. J. and Wright, R. N. , "The Representation and Use of

Design Specifications", prepared for presentation at the Symposium

of Structural and Geotechnical Mechanics, University of Illinois,

Urbana, Illinois, October 2-3, 1975.

[2] Guide Criteria for the Evaluation and Design of Operation Breakthrough Housing Systems, Report 10-200, 4 volumes. National Bureau

of Standards (September 1970).

[3] Wyly, R. S. and Sherlin, G. C. , "Performance of a Single-Stack DWV

System Utilizing Low-Angle Stack Branch Confluence and Bottom Shunt

Venting", National Bureau of Standards (U.S.), Building Science

Series 41, 24 pages (April 1972).

[4] Fenves, Steven J., Rankin, Kirk, and Tejuja, Hotchand K. , "The

Structure of Building Specifications", National Bureau of Standards,

(U.S.) Building Science Series 90, 83 pages (September 1976).

[5] Wyly, R. S. and Eaton, H. N. , "Capacities of Stacks in Sanitary

Drainage Systems in Buildings", National Bureau of Standards (U.S.),

Monograph 31, 52 pages (July 1961).

[6] Recommended Minimum Requirements for Plumbing in Dwellings and

Similar Buildings, Final Report of Subcommittee on Plumbing of the

Building Code Committee, National Bureau of Standards (U.S.),

Elimination of Waste Series (July 3, 1923).

[7] French, J. L. , Eaton, H. N. , and Wyly, R. S., "Wet Venting of

Plumbing Fixtures", National Bureau of Standards (U.S.), BMS 119,

(December 1950).

[8] Wyly, R. S., "Investigation of the Hydraulics of Horizontal Drains

In Plumbing Systems", National Bureau of Standards (U.S.), Monograph 86,

41 pages, (December 1964).

[9] Konen, Thomas P., "A Review of the Hydraulics of Gravity Drainage,

Waste and Vent Systems", Proceedings of the 1977 American Society of

Sanitary Engineering Annual Meeting, p. 65.

[10] Hunter, Roy B. , "Methods of Estimating Loads in Plumbing Systems",

National Bureau of Standards (U.S.) BMS 65 (1940).

[II] Hunter, R. B. , "Water Distributing Systems for Buildings", National

Bureau of Standards (U.S.), BMS 79, (1941).

[12] Plumbing Manual, Report of Subcommittee on Plumbing-Central Housing

Committee on Research, Design, and Construction, National Bureau

of Standards (U.S.) BMS 66 (1940).

47
[13] Wyly, R. S., Sherlin, G. C. and Beausoliel, R. W. , "Laboratory

Studies of the Hydraulic Performance of One-story and Split-level

Residential Plumbing Systems with Reduced-Size Vents, National Bureau

of Standards (U.S.), Building Science Series 49, 53 pages (March 1974).

[14] Orloski, M. J., and Wyly, R. S., "Hydraulic Performance of a FullScale Townhouse Drain-Waste-Vent System with Reduced-Size Vents,"

National Bureau of Standards (U.S.), Building Science Series 60,

43 pages (August 1975).

[15] Konen, T.P., "The Effect of Water Closet Type on Drainage System

Capacity", Plumbing Engineer, Vol. 2, No. 4, (July/August 1974).

[16] Lillywhite, M. S. T. , and Wise, A. F. E., "Towards a General Method

for the Design of Drainage Systems in Large Buildings", (British)

Building Research Establishment, CP 27/69 (August 1969).

[17] Wise, A. F. E., Payne, H. N., and Griffiths, T. J., "Sanitary

Services for Modern Housing", Proceedings of the Public Works and

Municipal Services Congress, November 1966.

[18] National Standard Plumbing Code, National Association of Plumbing Heating

Cooling Contractors, and the American Society of Plumbing Engineers,

(1978).

[19] Brownstein, Edward, "Reduced Size Venting Design," Plumbing Engineer,

Vol. 6, No. 3, (May/June 1978).

[20] Maybeck, Edward M., "Field Performance Testing of Innovative Plumbing

Systems", Plumbing Engineer, Vol. 5, No. 4., (July/August 1977).

[21] Benazzi, Robert V., et. al. , "Water Requirements and Procedures for

Estimating the Demand for Water in Buildings," Proceedings of the

CIB Commission W62 Symposium on Drainage and Water Supply for Buildings,

Washington, D.C., September 1976 (in press).

[22] Fowell, Andrew J., et. al. , "Water and Water-Related Conservation in

Buildings", Proceedings of the CIB Commission W62 Symposium on Drainage and Water Supply for Buildings, Washington, D.C., September 1976

(in press).

[23] Martig, Kenneth W. , Jr., "Open Channel Flow Monitoring or Metering,

That is the Question" , Proceedings of the Symposium on Flow in Open

Channels and Closed Conduits, February 23-25, 1977, National Bureau

of Standards (U.S.), Special Publication 484 (October 1977).

[24] Grant, D. M. , "Instrument Errors in Open Channel Flow Measurement

Systems" Proceedings of the Symposium on Flow in Open Channels and

Closed Conduits, February 23-25, 1977, National Bureau of Standards

(U.S.), Special Publication 484 (October 1977).

[25] "Big Surge in Water/Energy Code Action", Domestic Engineering,

Volume 231, No. 3, p. 62 (March 1978).

48
[26] Klra, Alexander, "The Bathroom", The Viking Press, 652 Madison Avenue,

New York, N.Y., 10022 (1976).

[27] "Performance Code Time?", Contractor Magazine, November 15, 1976.

[28] Ventre, F. T. , "Social Change of Technological Innovation: The

Regulation of Building Construction", unpublished doctoral dissertation,

Department of Urban Studies and Planning, Massachusetts Institute of

Technology, Cambridge, Massachusetts (May 1973).

49
Appendix A. Sizing Reduced- Size Vents, Procedure and Examples

A.l. General Approach

It is recommended that the piping schematic first be marked with applicable fixture unit ratings by

fixture. The sizing sequence, using table 4, is: STEP 1—Fixture vents STEP 2—Confluent vents STEP 3—Arterial vent

It is important to identify on the DWV piping

schematic the various vent types given in table 4. A fixture vent is a vent pipe that provides the sole or primary ventilation for a trap or group of traps

located at the base of the vent. A confluent vent is a vent pipe that serves two or more fixture vents. The

fixture vents need not all join the confluent vent at a

single point. That is, confluent vents should be sized on the basis of the fixture vents served, not on the

basis of the branch vents that may connect directly

to the confluent vent. For example, figure A.l

shows that Confluent Vent 2 serves Fixture Vents

1, 2, and 3 which do not all connect at the same point.

FIXTURE VENT I

L^

_CqNFLUE_NT_ 'V VENT I

IFIXTURE

I VENT 2

I

I

_cqNn.uENj VENT 2

FIXTURE VENT 3 ^ H

J^

Figure A.l Schematic showing fixture and confluent vents.

The arterial vent (applicable only to systems greater

than one story) may be recognized as the main artery

of the vent system serving other vents and as the

most direct route for the relief of potential back

pressure in the building drain.

A.2. Example of Use of Table 4 to Size RSV

Sample calculations of how the vents in the

townhouse system were sized, step-by -step, are as follows

STEP 1 —Fixture vents. The appropriate fixture- unit loads for single fixtures were obtained from the

National Standard Plumbing Code [18] , for the

fixtures in the townhouse system, as listed in foot- note c of table 4. From this, the connected fixture- unit load by stack was determined as listed in table A.l for the five stacks. From the geometry

of the system however, W5 was considered to be

vented equally through stacks 4 and 5. On this

basis, the FU loads vented by these two stacks were

recalculated as listed in table A.2. Table A.l. Connected fixture-unit load by stack

Stack

desig- nation

Fixtures Load

FU

12

34

5

Ll + Wl + Bl

L2 + W2

S3C4

L5 + W5

7

5

3

3

5

Table A.2. Estimated fixture-unit loads vented through stacks 4 and 5

Stack

desig- nation

Fixtures Load

4

5 C4 + i/^W5

FU

53

The results of the determination of the FU loads

for the five stacks are shown in figure A.2. Fixture

unit loads considered to be vented through each stack

are shown, rather than the loads connected. The

DWV vent piping is standard size to 6 in above the

fixture flood-rim level. To complete STEP 1, the early tentative criteria

[13] now incorporated in table 4) were utilized to

size the fixture dry vents for stacks 1-5. This

produced the sizes shown in table A.3.

Stack 2

Stack I

C4 (7

Figure A.2 Schematic showing Fixture Unit loads vented through each stack.

50
rviu.K A. 3 Fixlurr iliy iciit sizes for tonnliouso systpin

Slack

natioii

Selection Criteria

Fixture

vent type

Stack vent.

Stack vent.

Individual. Common . . Common . .

Elevation

of trap(s)

ft 8 to 16.

8 to 16. Up to 8 Up to 8 Up to 8

Load

FU

7

5

3

5

3

Nominal

pipe

size

m

r.4

STEP 2—Confluent vents. As an aid to the

computation of sizes for confluent vents, figure A.

3

was prepared showing, not only the sizes for the

fixture dry vents, but also the unsized elements

X. Y, and Z, (the vent header). The sizes for these

elements were determined by computation using

the square-root relationship given in table 4, and

selecting the next larger pipe size from footnote g

of table 4. The relationship is:

^"^COXFLUE.VT — '\-^LARGEST '^^-^SERVED

where

(A.l)

-4co.vF£.c'RVT = theoretical internal cross-sectional area required in the confluent vent

Alargest =^ctual internal cross-sectional area

for the largest fixture vent served

by the confluent vent

'^AsERVED =s\-\m of actual internal cross-sectional areas of all the fixture vents served

by the confluent vent

Jl^

r^

o.

^^1

To C4

(5FU)

I<

To L5

I l/4-<t>

I

I a W5 I (3 FU)

To L2 a w2

I5FU)

i^l 1/2

To LI.BI a Wl

(7 FU)

Sizing confluent vents by eq (A.l) results in a one-pipe-size increase for all cases where two fixture vents (from Js to 2 in) are served. Where three

fixture vents (any combination of three of y, to 2 in) are served, eq (A.l) produces eight exceptions

to a one-pipe-size increase. These require a twopipe-size increase over the largest fixture vent

served and are listed as footnote f of table 4.

For confluent vents serving four or more fixture vents, the confluent vent size should always be

calculated by use. of eq (A.l).

Confluent vent sizing of X, Y, and Z, the elements

of the vent header of the townhouse system, is tabulated in table A.4. This completes STEP 2.

Table A.4 Confluent vent sizes for toicnhouse system

Confluent vent

X

Y

Nominal sizes

of fixture vents served

1,1'^

1,1'i, ^4...

hVLn,i}2.

Nominal size confluent vent

m

Increase one size 1';

Increase two sizes 2

(see table 4, fj

Use eq (A.l) area and

select nearest larj^er commercial size 2

(See table 4, g)

STEP 3—Arterial vent. Utilization of these sizing

criteria to provide a pressure relief route for the

building drain may take precedence over the sizes calculated for this route in STEP 1 or STEP 2. For the NBS experimental townhouse system, the

arterial vent was chosen as Stack 1. The FU load on the system was 23 FU, and a flooded sewer was

not made a condition, thereby making the arterial vent size, IJo in. This size was already the size of Stack 1 by STEP 1; the vent ternnnal size was

2 in by STEP 2. Thus no size change was called

for to meet the V,i size determined in STEP 3. When the final vent sizes are determined as just

described, a DWV piping schematic should be marked

to complete the procedure.

A. 3 Construction Note

Familiarity with the RSV sizing procedures and

the rationale behind them is needed when they are applied to the on-site as-built wet piping configuration. This can be particularly- important because

construction constraints and other field conditions

may result in on-site changes in the planned wet

system that would require significant modifications

in the planned sizing of the reduced-size dry vents.

h IGUKK A.3 Schematic shoiring four fixture vents and three confiurnl vents (X, Y, and Z) for the townhouse system.

51
Appendix B - Definitions

1. Arterial vent - A vent in DWV systems of more than one story, intended

to provide circulation and back-pressure relief in multi-story systems.

2. Attributes - The overall characteristics the occupant desires for his

living unit (in this paper, Sound, Durable, Safe, and Functionally

Effective).

3. Averaging period - Time period selected for measurement for which the

(data acquisition system used in the townhouse investigation) computer

would retain a single value representing an average value of the many

values scanned for that (time) period,

4. Confluent vent - A vent pipe that serves two or more fixture vents.

5. Criterion - Some physical or chemical characteristic or property that

is subject to measurement and that can be used as a meaningful

quant'itative measure or indicator of adequacy in the satisfaction of a (performance) Requirement.

6. dH - The amount of decrease in trap-seal depth from full-seal depth.

7. d_P - Maximum value of vent suction obtained during a run for a selected averaging period (see also "averaging period").

8. Elements - The interrelated built parts of the living unit (in this

paper, major categories are the building, its systems (of which plumbing

is one), and their subsystems).

9. Emissions - Blowback of water or gas into occupied spaces due to

positive pressure.

10. Evaluation - A systematic procedure or standard test method for

determining if a Criterion has been met.

11. Fixture vent - Any single vent pipe that provides the sole or primary

ventilation for a trap or a group of traps located in the proximity

of the base of the vent.

12. Performance (approach) - An approach to design and evaluation that

centers on the idea that the performance of products, devices, systems,

or services can be systematically described and measured in terms of user Requirements without regard to their particular combination of

physical and chemical characteristics, their design, or the method of

their construction. In this paper, the view is taken that a structure

and its environment together furnish a living unit with certain overall

Attributes desired by the user (see also Attributes). The living unit

in turn is comprised of built Elements (see also Elements) which can be

systematically related to the Attributes by (performance) Requirements

(see also Requirements).

52
13. Qa

~ Maximum value of air demand flow rate obtained during a run for

a selected averaging period (see also, "averaging period").

14. Requirement - A qualitative statement of basic user needs.

15. Time profile - The change in magnitude of a hydraulic parameter for

a time period on the order of 10 to 300 seconds. For this study, some data smoothing was achieved by means of a data collection

system which averaged instantaneous values to a single value

for a (pre-selected) "averaging period" on the order of a fraction

of a second (the response time of a trap-seal) (See also "averaging

period").

16. Units, abbreviations (see also Appendix C) -

cm centimeter

ft foot, or feet

gpm gallons per minute

kPa kiloPascal (see Pascal)

l,i liter m meter

min minute

Pa Pascal, = Newton per square meter, by definition

psi pounds per square inch

s second

W.G. Water Gage

53
Appendix C, Units and Conversion Factors

LENGTH

in mm ft m

40 -1- 1000 100- - 30

35 - - 900 90- : =

800 80- - 25

30 - - 700 70 -

- 20

25 - - 600 60- -

20 - - 500 50 - - 15

15 - - 400 40 - -

AREA

in mm

1 In = 25.4 mm

0.0394 in = 1 mm

30

20-

10 -

- 5

1 ft = 0.305 m

3.281 ft = Im

1000 -I

- 0.5

600 -

500 -

400 -

300 -

200

1000-^0

in ^ nim

ft' 100

90

60

50 -

40 -

30 -

20 -

7.0

- 3.0

2.0

0-1-0

ft' m'

1 in' = 6.451 X 10-4 m' 1 ft' = 9.290304

X 10-2 m'

1536.098 in' = 1 m'

10.763910 ft' = 1 m'

VOLUME FLOW RATE PRESSURE

( us ) gpm S/t

100—1- 6.0 90-

70

50-

40-

30 -

20-

10-

2.0

- 1.0

ft^ /min m^/s

1000 - - .45 900 -

- .40 800 -

500 -

.30

.25

1 ( US ) gpm=6.309 1 ft ^ /min=4.719474

X 10"' liter$/$ x IQ-* mVi

in W.G. kPa*

10 2.50

1.75

1.00

90 -\

80

70 -

kPa

- 600

60 -

50

40 -

30 -|_ 200

20 --- 100

10 -~

1 in W.G.=0.2488 kPa 1 psi=6.895 kPa

4.02 in W.G.=1 kPa 0.145 psi=l kPa

- 400

300

15.85 ( US ) gpm 2118.9 ftVmin=l mV» • k = times 1000

=1 iiters/$ Pa = Newton / meter' (N/m '), by definition

54
NBS-n4A (REV. 7-731

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. iniHI.K AlION OR Ri;i^ORr NO. TN 966

2. Gov't Accession

No.

3, Recipient's Accession No.

4. TITl.h AND SUBTITLK

PERFORMANCE CRITERIA AND PLUMBING SYSTEM DESIGN

5. Publication Date August 1978

6^ Performing Organization Code

7. AUTHOR(S)

M.J. Orloski and R.S. Wyly

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit Nc 742 4190

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) 13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant bibliography or literature survey, mention it here.) An overview is presented indicating how the performance approach to plumbing system

design can be used to extend traditional methods to innovative systems. Identification

of the plumbing performance needed in a built system is used to classify current design

criteria intended to furnish this level of performance. Some current design criteria

may provide a higher level of performance than is actually needed by the user. In other cases, no standard test method, criterion, or evaluation technique exists. Putting;

existing knowledge into a performance format increases the utility of this knowledge

and facilitates identification of needed research to fill the gaps. Some of the

mathematical models now used for system design and pipe sizing in plumbing codes are reviewed in the context of performance-oriented research. The results of experimental

work in plumbing systems with reduced-size vents (smaller than allowed by codes) are presented as an example of the use of the performance approach, and illustrate a case where performance criteria permit relaxing of vent design practice. Conceivably the re- examination by plumbing designers of traditional design criteria against measured user

needs could be beneficially extended to other areas of plumbing design such as water

distribution, storm drainage, and plumbing fixtures. Beyond this, it has been recognizee

that uniform guidelines for evaluation of innovative systems, based on research

findings, are essential for wide acceptance of performance methods, particularly by the

regulatory community.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Performance; plumbing systems; reduced-size venting.

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