11^^^^ United States Department of Commerce

National Institute of Standards and Tectinology

NIST Special Publication 780

Guidelines for

Pressure Vessel

Safety Assessment

Sumio Yukawa
NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20899

DATE DUE

Demco. Inc. 38-293
NIST Special Publication 780

Guidelines for

Pressure Vessel

Safety Assessment

Sumio Yukawa

Materials Reliability Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80303

Sponsored by

Occupational Safety and Health Administration

U.S. Department of Labor

Washington, DC 20210

Issued April 1990

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office

and Technology Washington: 1990

Special Publication 780

Natl. Inst. Stand. Technol.

Spec. Publ. 780

75 pages (Apr. 1990)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
CONTENTS

Page

ABSTRACT vii

1. INTRODUCTION 1

2. SCOPE AND GENERAL INFORMATION 1

2 . 1 Scope 1

2.2 General Considerations 3

3. PRESSURE VESSEL DESIGN 4

3.1 ASME Code 4

3.1.1 Section VIII of ASME Code 5

3.1.2 Scope of Section VIII 5

3.1.3 Summary of Design Rules and Margins 6

3.1.4 Implementation of ASME Code 9

3.2 API Standard 620 10

3.2.1 Scope of API 620 12

3.2.2 Design Rules 12

3.2.3 Implementation of API 620 12

3.3. Remarks on Design Codes 14

4. DETERIORATION AND FAILURE MODES 14

4.1 Preexisting Causes 14

4.1.1 Design and Construction Related Deficiencies. . 14

4.1.2 Brittle Fracture 15

4.2 Inservice Deterioration and Damage 16

4.2.1 General Material Loss 16

4.2.2 Localized Attack and Cracking 17

4.2.3 Material Property Degradation 22

5. INSPECTION METHODS AND IMPLEMENTATION 22

5.1 Role of Organization Involved 23

5.1.1 ASME Code 23

5.1.2 API Standards 24

5.1.3 National Board 24

5.1.4 ASNT Recommended Practice 25

5.1.5 ASTM Specifications 25

5.1.6 NACE Recommended Practices 25

iii
Page

5 . 2 Examination Methods 27

5.2.1 Visual Examination (VT) 27

5.2.2 Liquid Penetrant Test (PT) 27

5.2.3 Magnetic Particle Test (MT) 29

5.2.4 Radiography (RT) 32

5.2.5 Ultrasonic Testing (UT) 32

5.3 Detection Probabilities and Flaw Sizing 35

6. RECENT CRACKING EXPERIENCE IN PRESSURE VESSELS 36

6.1 Deaerator Service 36

6.2 Amine Service 39

6.3 Wet Hydrogen Sulfide Service 40

6.4 Ammonia Storage Service 42

6.5 Pulp Digester Service 42

6.6 Summary of Service Cracking Experience 44

7. PERIODIC INSPECTION REQUIREMENTS AND RECOMMENDATIONS 45

7.1 National Board Inspection Code and API 510 45

7.2 Recommendations for Specific Applications 45

7.3 Institute of Petroleum Code 47

8. DAMAGE AND CRACK SEVERITY EVALUATION AND REPAIR 47 ;

8 . 1 Damage Evaluation 47 ] 8.2 Repair by Welding 47

9. INFORMATION FOR SAFETY ASSESSMENT 50

10. ACKNOWLEDGMENT 50

11. REFERENCES 54

APPENDIX A - ASME CODE SECTION VIII, DIVISION 1 REPORT FORM .... 57-58

APPENDIX B - REPORT CONTENT FOR API STANDARD 620 LOW PRESSURE . . . STORAGE TANK 59

APPENDIX C - NACE AMINE CRACKING SURVEY QUESTIONNAIRE 60-62

APPENDIX D - INSPECTION PRIORITIES FOR WET H2S CRACKING SURVEY ... 63

APPENDIX E - NATIONAL BOARD'S INSPECTION REPORT FORM 64

APPENDIX F - API 510 REPORT FORM FOR INSPECTION RESULTS 65

APPENDIX G - INSTITUTE OF PETROLEUM CODE FREQUENCY OF PERIODIC . . . INSPECTION 66

APPENDIX H - NATIONAL BOARD REPORT FORM FOR WELD REPAIR OR ALTERATION 67

J
LIST OF FIGURES

Page

Figure 1. Illustration of some major parts of a pressure

vessel 2

Figure 2. Examples of acceptable nozzle-to-shell welds in

Section VIII, Div. 1 of the ASME Code 8

Figure 3. Marking of ASME Section VIII pressure vessels 11

Figure 4. Marking of low pressure storage tanks constructed

in accordance with API Standard 620 13

Figure 5. Illustration of non-branching and branching stress

corrosion cracks 20

Figure 6. Concept of threshold stress or stress intensity

factor (Kjs^,^,) in stress corrosion cracking 21

Figure 7. Appearance of hydrogen induced blisters in carbon steel. 23

Figure 8. Principles and application of magnetic particle testing. 30

Figure 9. Illustration of magnetic particle indications due to

various causes 31

Figure 10. Longitudinal and shear waves utilized in ultrasonic

examination 33

Figure 11. Principles of pulse-echo ultrasonic technique 33

Figure 12. Basic features of angle beam ultrasonic examination

of a butt weld 34

Figure 13. Simplified flow diagram for feedwater deaerator/storage

system 37

Figure 14. Simplified process flow diagram of amine plant 39

Figure 15. U.S. and European Guidelines for ammonia storage vessels 43

Figure 16. National Board Stamp or Nameplate for a weld repaired

vessel 49

V
LIST OF TABLES

Page

Table I SUMMARY OF ASNT RECOMMENDED PRACTICE SNT-TC-lA

"QUALIFICATION AND CERTIFICATION PROGRAM FOR NDE

PERSONNEL" 26

Table II SUMMARY OF NDE METHODS 28

Table III SUMMARY OF INSPECTION GUIDELINES AND RECOMMENDATIONS ... 46

Table IV SUMMARY OF NBIC AND API REPAIR WELDING REQUIREMENTS ... 48

Table V INFORMATION AND DATA USEFUL FOR THE SAFETY ASSESSMENT

OF STEEL VESSELS AND LOW PRESSURE STORAGE TANKS 51

vi
ABSTRACT

This document presents a technical overview and information on metallic

pressure containment vessels and tanks. The intent of the document is to

provide OSHA (Occupational Safety and Health Administration) personnel and

other persons with information to assist in the evaluation of the safety of

operating pressure vessels and low pressure storage tanks.

The scope is limited to general industrial application vessels and tanks

constructed of carbon or low alloy steels and used at temperatures between -75

and 315 °C (-100 and 600 °F) . Information on design codes, materials,

fabrication processes, inspection and testing applicable to these vessels and

tanks are presented. The majority of these vessels and tanks are made to the

rules and requirements of ASME Code Section VIII or API Standard 620.

The causes of deterioration and damage in operation are described and

methods and capabilities of detecting serious damage and cracking are

discussed. Service experience in several applications where 30 to 50%

incidence of cracking has been found is described. Guidelines and

recommendations formulated by various groups to inspect for the damages being

found and to mitigate the causes and effects of the problems are presented.

A summary of the needed or useful information for the various factors and

items involved in the safety of these vessels and tanks is included to assist

in deciding whether further technical evaluation of safety concerns is

required

.

Key Words: API Standards; ASME Code; design; failure; guidelines; inservice

examination; nondestructive testing; pressure vessels; reliability; safety;

service experience; steel.

vii
1
GUIDELINES FOR PRESSURE VESSEL SAFETY ASSESSMENT

Sumio Yukawa

1 . INTRODUCTION

This document presents a technical overview and information on pressure

vessels and low pressure storage tanks. This overview and information are

intended to help identify potentially hazardous conditions and to assist in the

evaluation of safety for continued operation. The vessels and tanks of concern

are relatively large metallic containers used to contain liquids and gases at

various temperatures and pressures.

This document has been prepared primarily for use by the Occupational

Safety and Health Administration (OSHA) of the Department of Labor. The

purpose of the document is to provide OSHA personnel and other interested

persons with background and current technical information regarding the

operational reliability and safety of pressure vessels and tanks. This will

aid in deciding whether additional engineering evaluation to assess continued

safe operation is warranted.

Although pressure vessels designed and constructed to one of the

recognized design codes have had an excellent safety record, some recent events

indicate a basis for concern about continuing reliability and safety,

especially when coupled with the current trend of extending service usage.

Recent inspection programs for vessels in several types of applications have

revealed cracking and damage in a considerable number of the vessels inspected.

These results are discussed in detail later in this document.

2. SCOPE AND GENERAL INFORMATION

2.1 Scope

Pressure vessels are produced and used in a wide variety of geometrical

shapes, capacities, and sizes for use in a large number of applications.

Examples range from relatively small and simple air compressor tanks to very

large and extremely complex nuclear reactor pressure vessels. The scope of

this document might be termed the "mid- segment" of this total application

range. Figure 1 illustrates a schematic pressure vessel with some of the main

features and terminology.

More specifically, the type and applications of pressure vessels addressed

in this document are characterized by the following features

:

Stationary and unfired.

Used for pressure containment of gases and liquids,

• Constructed of carbon steel or low alloy steel, and

• Operated at temperatures between about -75 and 315 °C (-100

and 600 °F)

.

1
Nozzle (typical)

Bolted Joint

1

J

Typical Weld Seams

Figure 1. Illustration of some major parts of a pressure vessel.

This definition includes pressure vessels and low pressure storage tanks widely

used in process, pulp and paper, petroleum refining, and petrochemical

industries and for water treatment systems of boilers and steam generation

equipment. (In this document, the term "pressure vessel" generally will be

meant to include low pressure storage tanks.)

This scope categorization excludes vessels and tanks used in many other

applications and also excludes other parts of a pressure containment system

such as piping and valves. Some of the major applications and items not

covered in this document because of this scope limitation are:

2
• Vessels used as fired boilers,

• Vessels used in high temperature processes (above 315 °C, 600 °F)

or at very low and cryogenic temperatures

,

• Vessels and containers used in transportable systems,

• Storage tanks that operate at nominally atmospheric pressure,

• Piping and pipelines,

• Safety and pressure relief valves, and

• Special purpose vessels, such as those for human occupancy.

2.2 General Considerations

Safety and hazard evaluations of pressure vessels need to consider the

consequences of a leakage or a rupture failure of a vessel. Hammer [1] in one

chapter of his book discusses "Pressure Hazards" and describes two consequences

of a complete rupture. One is the blast effect due to sudden expansion of the

pressurized fluid. The second consequence is damage and injury caused by

fragments if fragmentation type rupture occurs. For a leakage failure, the

hazard consequences can include the whole range from no effect to very serious.

If the leakage occurs into a closed space, suffocation or poisoning can occur

depending on the nature of the contained fluid. Physical consequences include

fire and explosion for a flammable fluid.

It is of interest to put some perspective on the potential human hazards

arising from pressure vessel operation. The National Board of Boiler and

Pressure Vessel Inspectors collects and publishes an annual incident report [2]

for pressure vessels (and also a separate report for boilers) within its

jurisdictional scope. The number of injuries and deaths attributable to

pressure vessel failures over the past few years were as follows:

Year Inj uries Deaths

1984 437 73

1985 269 78

1986 99 44

1987 44 5

These figures cover all types of pressure vessels, not just the category

covered in this document, and include tens of thousands of vessels in

operation. There are some limitations on the figures listed above in that

reporting of the incidents is voluntary and generally for vessels registered

with the National Board. Some less serious incidents or those not involving

injuries or fatalities may not be reported. Also, some incidents may not

involve the pressure vessel per se but an associated part such as the piping or

a relief valve

.

In spite of the limitations, the figures indicate a very good overall

record. However, recent experience indicates an apparent trend of increasing

3

I
deterioration and problems with pressure vessel reliability in some specific

types of service. These concerns have derived in part from some serious

failures such as the one in 1984 at a petroleum refinery; this failure resulted

in an explosion, a fire, and 17 fatalities [3]. Surveys of vessels in several

specific applications indicate deterioration and cracking problems greater than

expectations; these survey results are described in detail later in section 6.

3. PRESSURE VESSEL DESIGN

Most of the pressure or storage vessels within the scope of this document

and currently in service in the United States have been designed and

constructed in accordance with one of the following two design codes:

• Section VIII of the ASME (American Society of Mechanical Engineers)

Boiler and Pressure Vessel Code, commonly referred to as the

ASME Code [4] , or

• API (American Petroleum Institute) Standard 620 [5].

In addition, some vessels designed and constructed between 1934 to 1956 may

have used the rules in the "API-ASME Code for Unfired Pressure Vessels for

Petroleum Liquids and Gases." This code was discontinued in 1956.

A summary description of the scope and major features of the ASME Code,

Section VIII, and API 620 are presented in the following; the descriptions are

limited, and the design codes should be consulted for all detailed information.

There are codes and standards for many of the other applications, components, and parts listed earlier that are not within the scope of this document.

These include other Sections of the ASME Code, API Standards, ANSI (American

National Standards Institute) Piping Codes, and governmental agency rules.

3.1 ASME Code

The first edition of the ASME Code was the 1914 edition developed and

published in response to an appeal to the ASME from manufacturers and users of

steam boilers "...to formulate standard specifications for the construction of

steam boilers and other pressure vessels and for their care in service." Over

the intervening years , this Code has grown in scope and coverage so that the

1986 edition contains 11 Sections and occupies several feet of shelf space.

Chuse's book [6] provides an informative description of the history of the ASME

Code and the role of various groups involved in its implementation. In

addition, it discusses the technical considerations for various applications.

A shorter general description of the main features of the Code is available in

Yokell's paper [7]. Both of these references also discuss the legal and

jurisdictional aspects of the ASME Code.

Of the 11 Sections in the ASME Code, three are concerned with heating and

power boilers and two are concerned with pressure containment components for

nuclear power plants. Rules for pressure vessels for general applications are

contained in Section VIII which is the Section of primary relevance for vessels

in the scope of this document. In addition, three other Sections of the Code

4
have associated relevance since they contain additional rules and requirements

which are invoked in Section VIII by reference. These three are:

• Section II, Material Specifications,

• Section V, Nondestructive Examination, and

• Section IX, Welding and Brazing Qualifications.

Reference to these Sections are made at appropriate points in this document.

3.1.1 Section VIII of ASME Code

This Section contains the rules for the design, fabrication, inspection,

and testing of pressure vessels for general application and covers the

following features and items:

• List of acceptable materials,

• Allowable design stresses for the listed materials,

• Design rules and acceptable design details,

• Acceptable forming, welding, and other fabrication methods,

• Bolting materials and design,

• Inspection and testing requirements, and

• Requirements for pressure relief devices.

Section VIII consists of two Divisions, 1 and 2. Vessels for moderate

pressures and temperatures and therefore thinner walls (up to about 50 to 75

mm, 2 to 3 in) are usually made to Division 1 requirements while Division 2 is

used for higher pressures and temperatures or more severe duty vessels. The

alternative rules of Division 2 require more design analysis but permit higher

design stresses. The higher design cost is often offset by a decrease in the

amount of material used.

3.1.2 Scope of Section VIII

The rules of Section VIII, Division 1 do not apply for certain

applications and circumstances; of these, several of the more pertinent are:

• Fired process tubular heaters,

• Pressure containers which are integral parts of rotating or

reciprocating machinery or which serve as hydraulic or pneumatic

cylinders,

• Piping systems and piping components.

5
• Small hot water supply storage tanks, and

• Vessels of any size having an internal or external

operating pressure less than 0.1 MPa (15 psi)

.

Division 2 of Section VIII has essentially the same limitations on the scope of

application.

3.1.3 Summary of Design Rules and Margins

The following discussion concentrates on the design basis and rules of

Division 1 since it is the more general purpose and widely used part of Section

VIII of the ASME Code.

The Code lists a large number of acceptable materials covered by

specifications with either SA- or SB- prefix for base materials and SFA- prefix

for weld filler materials. The chemical composition, manufacturing methods,

and minimum properties specifications for each material are given in Section II

of the Code. The ferrous metal alloys (carbon, low alloy, high alloy stainless,

and heat resisting steels) are in the SA- group and the nonferrous metal alloys

(aluminum, copper, nickel, and titanium alloys) are in the SB- group. In most

cases, the SA- and SB- specifications are identical to or nearly identical to

the numerically corresponding ASTM (American Society of Testing and Materials)

A- or B- specifications, and the SFA- specifications are identical to the AWS

(American Welding Society) A- specifications.

Section VIII has approved for use most but not all of the materials listed

in Section II. In ASME Code terminology, the term "low alloy steel" includes

steels containing up to 9% chromium (Cr) and 1% molybdenum (Mo). However, the

temperature range addressed in this document puts a practical maximum of around

3% total alloy content (for example, 2.25 Cr-1 Mo) as the highest alloy content

alloy steel likely to be considered. A typical ASME Code specification is

SA516-Grade 70 which defines a C-Mn plate steel often used for pressure vessel

construction (and is identical to ASTM A516-Grade 70 but with ASME Code

verification)

.

The overall design approach of the ASME Code is to provide an adequate and

safe margin against a bursting failure of the pressure vessel at the design

pressure. Experimental studies have shown that the bursting failure pressure

of vessels is strongly related to the tensile strength of the vessel material.

This is valid as long as the strength properties are only temperature dependent

but not time dependent, that is, below the temperature where the material

strength properties are affected by creep. For the temperature range and

materials of concern in this document, time dependent creep strength is not a design consideration.

For Section VIII, Division 1 materials at temperatures above -29 °C

(-20 °F) and below the creep range, the maximum allowable design stress is

established as follows:

6
• For -29 to 38 °C (-20 to 100 °F) , the lesser of one-fourth of the

specified minimum tensile strength or two -thirds of the

specified minimum yield strength at room temperature.

• Above 38 °C (100 °F) , the lesser of one-fourth of the tensile

strength or two- thirds of the yield strength at the elevated

temperature

.

For most of the carbon and low alloy steels used in Division 1, the allowable

stress is governed by the tensile strength criterion. The yield strength

criterion is included to prevent excessive distortion of the vessels made from

materials that can have a very low yield strength relative to the tensile

strength. Based on these criteria, Section VIII, Division 1 pressure vessels

can ideally be expected to have a margin of four or greater between the

allowable design pressure and the expected bursting failure pressure. This is

based on experimental results that the failure strength of a simple pressurized

cylinder is approximately equal to the tensile strength of the material.

This margin can be decreased or diminished by several factors:

• Welds and other types of joints,

• Nozzles and other penetrations through the vessel wall which act

as stress raisers,

• Brackets, supports, and other geometrical details which may be

attached by welding and become a stress raiser, and

• Cracks and other material damage which may be initially present or

develop with use.

The Code minimizes the effects of the first three factors by providing rules

for acceptable designs and by specific limitations. Welds, especially in

conjunction with nozzles and openings, are locations of special concern and the

Code prescribes acceptable designs; figure 2 illustrates a few of many

acceptable designs.

The inspection requirements for materials and the inspection and postweld

heat treatment requirements for welds minimize the effects of the fourth factor

in the as - fabricated condition. This is further enhanced by the hydrostatic

test (or an alternative pneumatic test) performed after completion of

manufacture where successful performance indicates an absence of a serious

defect or crack-like discontinuity. Additionally, the increased notch

toughness requirements very recently added to Section VIII, Division 1 in the

1987 Addenda to the Code will provide further protection against the effects of

cracks and discontinuities. The main features and the rationale for the new

toughness rules are discussed by Selz [8]. Very briefly, the new rules consist

of exemption curves as a function of thickness for various groups of steels and

Charpy impact test requirements for steels not included in the exemption

curves

.

7
8
The fabrication rules in Section VIII include requirements for identifying

each major material stock, and rules and tolerances for the cutting and

forming. For welded construction, preheat and postweld heat treatment

requirements are specified. In addition, a written welding procedure

specification (WPS) and qualification of the procedure and the welders who will

use the procedure are required. These specification and qualification

requirements are prescribed in Section VIII, but the details of their

preparation and execution are referred to and provided in another Section of

the Code . The intent of these requirements is to ensure that the margin

against failure is not diminished below an acceptable value.

The inspection rules of Section VIII include performance requirements and

acceptance standards for nondestructive examination (NDE) of materials and

fabrication welds. Similar to the welding format, the NDE requirements are

prescribed in Section VIII, but the details of the techniques are contained in

another Section.

The other important part of the inspection rules concerns the hydrostatic

or, alternatively, the pneumatic pressure test. The standard hydrostatic test

requirement of Section VIII, Division 1 is pressurization to 1.5 times the

maximum allowable working pressure (MAWP) , which is usually the same as the

design pressure. The rules provide an alternative pneumatic pressure test

procedure when a hydrostatic test is not possible or practical. The purpose of

the overpressure test is to ensure the overall structural integrity and leak

tightness of the pressure vessel. The factor of 1.5 implies that the operating

pressure will not be greater than 2/3 of a test pressure that the pressure

vessel has satisfactorily survived in the final fabricated condition.

For pressure relief and safety valves, Section VIII specifies the

performance requirements but does not include detailed requirements for design

and testing.

Section VIII is a design and construction code. As such, it does not

explicitly have provisions regarding maintenance of the safety margin in

service. It does require that the design include a corrosion allowance

(increased thickness) to account for material wastage from general corrosion.

However, provisions for periodic inspections or evaluations of any other form

of deterioration are not included in Section VIII rules.

3.1.4 Implementation of ASME Code

By itself, the ASME Code has no legal standing. However, the Code has

been adopted wholly or in part by most States and many cities and other

jurisdictions in the United States, and by all the Provinces of Canada. The

jurisdictional implementation is accomplished through legislative action by a governing body requiring that pressure vessels for use within its jurisdiction

must comply with the ASME Code rules.

The enforcement of the legal requirement is the responsibility of

designated officials in the jurisdiction. Since the vessels are often

manufactured in a jurisdiction other than where it will be installed,

reciprocity is desirable. For this and other reasons, the chief inspectors of

9
applicable states and large cities in the U.S. and Canadian provinces formed

the National Board of Boiler and Pressure Vessel Inspectors, often referred to

as the "National Board." This is an independent, non-profit organization that

promotes the adoption and use of uniform set of rules and requirements in all

of the jurisdictions and reciprocity between jurisdictions. The reciprocity is

now common so that manufacture in one location and installation in another is

usually possible.

The ASME has certain procedural requirements to ensure that a manufacturer

is capable of making vessels to the applicable Code rules and to verify that

the material, design, fabrication, and examination requirements are fulfilled.

These actions in the case of Section VIII include:

• Certification permitting the Manufacturer to build ASME vessels;

this certification is issued after a review verifying the

Manufacturer's capability.

• Third party inspection and verification that all requirements have

been fulfilled for each vessel.

• Marking of each vessel with the official ASME stamp and the

preparation of a Data Report for the vessel.

The Official ASME stamps and the information required to be in the permanent

stampings on the vessel for Section VIII, Divisions 1 and 2 vessels are shown

in figure 3. A Data Report form for a Division 1 vessel is attached in

Appendix A to this document showing the information required.

Several additional details about the marking and Data Report can be noted.

If the third party inspection is done by an inspector who holds a National

Board Commission, the vessel can also be registered with the National Board.

In the case of a vessel to be owned and used by the vessel manufacturer, the

third party inspection can be done by an inspector in the manufacturer's

employ. For a class of smaller vessels, the "UM" stamp may be used (not

included in fig. 3). These vessels have fewer inspection requirements, and the

Data Report (Appendix A) is not required; instead, a Certificate of Compliance

form is used.

3.2 API Standard 620

One of the limitations of Section VIII, Division 1 of the ASME Code is

that it does not apply to vessels with an internal pressure less than O.lMPa

(15 psig) . American Petroleum Institute's (API) Standard 620, "Recommended

Rules for Design and Construction of Large, Welded, Low-Pressure Storage Tanks"

[5] provides rules for lower pressure vessels not covered by the ASME Code. For

tanks that operate at nominally atmospheric pressure, another API Standard (API

650, "Welded Steel Tanks for Oil Storage") applies.

There are many similarities between API 620 and Section VIII, Division 1 of the ASME Code; the following describes the major differences.

10
Certified by

u

Name of Manufacturer

T r ^osi at ^ OF

(Max. allowable working pressure)

W (if arc or °F at ,osi gas welded

)

(Min. design metal temperature)

RT (if radiographed)

HT (if postweld (Manufacturer's serial number)

heat treated)

(Year built)

Division 1 Vessels

Certified by

©

(Name of manufacturer)

psi at °F

(Design pressure)

OF

(Min. permissible temperature)

HT (if postweld

heat treated) (Manufacturer's serial number)

(Year built)

Division 2 Vessels

Figure 3. Marking of ASME Code Section VIII pressure vessels. (Additional

information is required for low temperature service, for type of

construction, for extent of radiographic examination, and for

special service vessels.)

11
3.2.1 Scope of API 620

The major aspects of the scope and limitations of API 620 are as follows:

• Intended for large, field-assembled tanks for containment of

gases and liquids primarily associated with the petroleum

industry

.

• Internal pressures no greater than 0.1 MPa (15 psig)

.

• Metal temperatures between -37 and 93 °C (-35 and 200 °F)

;

Appendices provide rules for lower temperature applications.

• Tank materials limited to carbon steels.

3.2.2 Design Rules

Some of the differences between API 620 and Section VIII, Division 1 of

the ASME Code include:

• List of acceptable carbon steels categorized by minimum design

metal temperature

.

• Allowable design stress based on the lower of 30% of the

specification minimum tensile strength or 60% of the minimum

specification yield strength.

• Hydrostatic or combination hydrostatic -pneumatic test

at 1.25 times the nominal pressure rating.

• Exceptions to postweld heat treatment requirements when such

treatments are impractical due to physical size.

Overall, these differences are a slight relaxation of the Section VIII,

Division 1 rules in consideration of the lower operating pressures.

Like Section VIII, API 620 has no explicit rules regarding inspection and

evaluation in operation. However, API has another standard (API 510) for

inservice inspection and rerating of tanks; this standard is discussed later.

3.2.3 Implementation of API 620

Upon approval of an application from the manufacturer, the API authorizes

the official API Standard 620 symbol to be stamped on vessels made by

authorized manufacturers. This symbol and the additional information required

to be included in the stamping is indicated in figure 4.

In addition, the manufacturer is required to prepare a report summarizing

all data on the tank and a conformance and certification form. The information

to be included is shown in Appendix B.

12
API 620 Symbol

Information required in the marking:

1. Official API Standard 620 symbol

2. Manufacturer's name

3. Manufacturer's certificate of authorization number

4. Manufacturer's serial number

5. Nominal capacity

6. Design pressure for gas or vapor space at the top of the tank

7. Maximum permissible specific gravity of liquid contents to be

stored

8. Maximum elevation to which tank may be filled for liquid of

maximum specific gravity and design pressure at top of the tank

9. Maximum elevation to which tank may be filled with water for test

or purging purposes

10. Year of completion

11. SR for stress relieved vessel

12. XR for radiographed vessel

Figure 4. Marking of low pressure storage tanks constructed in accordance

with API Standard 620.

13
3.3 Remarks on Design Codes

It is useful to recall the philosophy underlying most design codes such as

the ASME Code when evaluating the adequacy of a code for particular situations.

The ASME Code and other codes are consensus documents that are intended to

provide minimum requirements for adequate safety for the operational conditions

considered and included in the design. Since they are minimum requirements,

the owner is expected to specify, and the designer and the manufacturer should

include additional requirements when it is anticipated that the equipment will

experience severe and/or not fully known service conditions. This caveat is

especially important in general purpose design codes such as Section VIII,

Division 1 of the ASME Code.

A more difficult and subtle problem regarding the application of design

codes occurs when service conditions change in time after some period of

operation. Temperatures may increase or decrease more frequently, pressures

and flow velocities may become more variable and cyclic, the composition of the

process fluids may be slightly different, down- time care may become less

carefully controlled, and greater demands may be put on old equipment. The

owner of the pressure vessel may not be fully aware of the technical effects of

these changes which were not addressed in the original design.

It is important to recall that the two design codes discussed above are

design and construction codes. They do not contain rules and procedures for

the inservice inspection, examination, and evaluation of the equipment. There

is a growing awareness of the needs in this area and several organizations have

been initiating or expanding their role in developing recommended practices,

guidelines and evaluation criteria for this purpose. These activities are

described later in this document in section 7.

4. DETERIORATION AND FAILURE MODES

A relatively large margin for reliability and safety is included in the

design of pressure vessels and tanks. However, lack of understanding of all

service conditions in design, poor quality control during manufacture, and

changes in service conditions can erode this margin. A number of articles and

books are available which discuss these factors. Among these, Thieisch's book

[9] provides much general and specific information about deterioration

mechanisms and failure behavior for pressure vessels and piping.

In general, conditions diminishing the safety margin can arise from

inadequacies during design and manufacture, or from operational conditions,

that is, preexisting before service or service - induced . These are described in

greater detail in the following, but with the major emphasis on service - induced

causes since these are the most pertinent for this document.

4.1 Preexisting Causes

4.1.1 Design and Construction Related Deficiencies

Although design and construction deficiencies may not cause immediate

reliability and safety problems, they can sometimes be the underlying reason

14
for later inservice problems. These preexisting situations include:

• Inadequate design considerations for the preservice,

operational and down- time conditions.

Poor design details such as lack of flexibility, severe

geometrical stress risers and sharp changes in

thickness

.

• Improper materials either by wrong design selection or

mistakes in identification; this includes both base

materials and welds or other joint materials.

• Undetected defects in the base material and in the

fabrication joints (welds).

• Incorrect heat treatments and cleaning procedures.

In most instances, a defic

conditions does not lead t

cause a failure during the

iency or error in one or

o an immediate failure,

hydrostatic test.

more of these preexisting

Usually, only gross errors

4.1.2 Brittle Fracture

The possibility of a sudden and unexpected failure due to brittle fracture

is an important consideration in safety and hazard assessment. This kind of

failure can occur either due to preexisting conditions or to a combination of

preexisting and service - induced conditions. Brittle fracture requires a combination of three factors:

• Existence of a crack or crack- like defect,

• A crack located in a high stress region, and

• A material with low notch toughness.

The initiating defect may exist because of its location in an uninspected

region or a detection failure in the inspection. High stresses can be caused

by geometrical stress raisers or by iocked-in (residual) fabrication stresses,

usually from welding. Welds that have not been thermally stress relieved are a prime source of residual stresses. Notch toughness is a measure of the

material's sensitivity to brittle fracture. The value of notch toughness

depends on temperature for carbon and low alloy steels with the material having

a low value, or brittleness , at lower temperatures and transitioning to much

higher toughness at higher temperatures. A typical carbon steel may have this

"transition" in behavior over a 55 °C (100 °F) temperature range. For some

grades of carbon steels, room temperature lies within the range of this

transition. For other kinds and grades of steels, the transition may be at

very low temperatures. This transition behavior does not involve any change in

the physical characteristics of the material; it is a change in the response to

mechanical factors.

15
These features explain why brittle fracture failures tend to occur when an

adverse combination of the following conditions exists:

• Operation at low temperatures

,

• Welds in the as -welded (not stress relieved) condition ,

• Incomplete or inadequate inspection, and

• Low notch toughness steel.

These characteristics of brittle fracture also explain why it can sometimes

occur in service after a successful preservice hydrostatic test. Service

conditions may include temperatures much lower than the hydrostatic test

temperature, and crack- like defects may be produced or enlarged in operation.

The latter effect is an important reason for including the possibility of

brittle fracture in the evaluation of service - induced cracking damage. It may

be noted that the new notch toughness rules adopted in Section VIII, Division 1 of the ASME Code [8] will provide additional margin against brittle failure for

vessels manufactured in the future.

4.2 Inservice Deterioration and Damage

Deterioration and damage to vessels and tanks as a result of operational

service and attendant shutdown and down- time conditions produce three general

classes of problems:

• Wastage and general loss of material,

• Localized attack and cracking, and

• Alteration of material properties

.

There are a number of material, temperature, and environment related

attack and deterioration mechanisms in each of these classes but the scope of

this document eliminates some from consideration. For example, the material

and temperature limits mean that material wastage by severe oxidation and

embrittlement by high temperature exposure do not need to be considered.

Similarly, certain kinds of localized corrosion peculiar to high alloy

stainless steels are not pertinent. With these limitations, the following

provides further information about specific mechanisms in each category listed

above

.

4.2.1 General Material Loss

The two most common forms of general material loss that can occur in

carbon and low alloy steel parts are corrosion and erosion. The ASME Code

requires that the designer account for corrosion loss. However, in some cases,

the corrosiveness of the fluid may not be fully communicated to the designer.

Within the range of carbon and low alloy steel grades, chemical composition

does not have a major influence in most cases of general corrosion and

16
therefore, material selection is not a primary factor. Severe cases of general

corrosion require stainless steels or other corrosion resistant materials.

Erosion tends to occur in the piping system and valves more than in

vessels and tanks because the wear is accentuated by high fluid velocity.

Particulate matter content and two-phase flow also can increase the erosion

rate. Turns, junctions, and area changes where the fluid flow has to change

direction or velocity are regions most susceptible to erosion. Erosion by

aqueous fluids often involves the loss of an adherent oxide scale which in turn

appears to be related to the chromium content differences even within the low

alloy grades. Thus, material selection of either the base material or weld

materials can have a role in some instances of erosion.

The main safety consequence of deterioration by general material loss is

the reduction in thickness and load carrying area which eventually can result

in an overstress failure. Because of the relatively large safety margin

included in pressure vessel design codes, considerable general material loss

can be tolerated under nominal working pressure conditions, and field

experience confirms this expectation.

4.2.2 Localized Attack and Cracking

Unlike general material loss, localized attack and cracking can have a severe consequence much greater than in proportion to the amount of material

degraded. This form of damage can be divided into several categories depending

on the underlying cause:

• Stress related,

• Environment (chemical) related, or

• Combination stress and environment related.

The most common purely stress related localized damage is fatigue

cracking. The cyclic stress responsible for fatigue can arise from purely

mechanical sources such as pressure cycling or from stresses produced by

thermal differentials in temperature cycling. Temperature cycling can be

caused by system characteristics such as intermittent or periodic flow,

frequent start- stop operation and problems with associated components such as a leaking valve. Changes in production schedules or rerouting of flow paths

external to the vessel or tank may result in a greater intensity of cyclic

stressing causing a condition that was previously benign to become critical.

Fatigue cracking resulting from cyclic stressing can involve either the

enlargement of a preexisting discontinuity or the initiation and growth of

crack where none existed before. The location in the first case will be

determined completely by the location of the existing discontinuity and the

rate of growth will depend on the intensity of stresses at the location. In the

second case, the cracking often initiates and grows in regions of high stress

such as at geometrical transitions and at or near welds.

17
Occasionally, a system related condition like "water hammer" can be a source of cyclic or varying pressure and stresses. Obviously, improper or poor

control of flow, pressures and temperatures are a source of abnormal and

varying stresses.

The second category of localized attack listed above, namely, that due to

chemical attack by the environment alone without the necessity for stress,

occurs in one of several ways:

• Pitting corrosion resulting in numerous surface cavities.

Selective galvanic corrosion in the region between two

electrochemically different metals.

Selective corrosion attack along a metallurgically altered region,

commonly the weld heat affected zone (HAZ) , or

• Corrosion attack in crevices resulting from the concentration of

the aggressive chemical specie(s)

.

It is impossible to list the many combinations of chemical species,

concentrations, metallurgical conditions, temperatures, and geometries where

problems due to localized chemical attack have been observed. Specialized

reference articles and handbooks are available for detailed discussion of the

problem and precautions. However, some commentaries on the safety consequences

will be helpful.

Pitting corrosion attack generally does not pose a safety hazard for

pressure vessels because the rate of attack is relatively small compared to the

usual thickness of the vessel wall. Severe through wall pitting attack is a leakage problem in thinner wall parts such as heat exchanger tubing.

The other three types of selective attack listed above can lead to

significant safety problems because, in the extreme, they can produce a cracklike discontinuity. Additionally, the localized susceptible regions can be

located in areas difficult to inspect. The crevice under the weld backing

material is an example.

The third category of localized attack is stress corrosion cracking (SCC)

;

it results from the combined action of stress and environment. The occurrence

of SCC requires a combination of three conditions:

• Susceptible material or material condition,

• Chemically aggressive environment, and

• Sufficiently high stress.

SCC will not occur if the magnitude of any one of the three conditions is not

sufficient

.

18
There are several distinctive characteristics about SCC which can be

summarized by the following:

• Very little or no general corrosion in the surface region around

the cracking, and virtually no corrosion of the crack surfaces.

• Cracking on a plane transverse to the principal stress direction

in the region; this may not always coincide with the direction of

primary loading due to local perturbations.

• In cross section, the cracking may proceed as a single continuous

crack or with a branching pattern.

• Metallurgically , the cracking can be through the grains

( transgranular) or along the grain boundaries ( intergranular)

.

Sketches in figure 5 schematically illustrate some of the major features of

SCC.

Since three factors are involved, generalizations about environments that

can cause SCC are difficult even when restricted to a specific class of

material. However, experiments and service experience have identified

environments that can or have caused SCC in carbon and low alloy steels, and

these have been tabulated and described in many references, for example, Logan

[10]. The listing below from Logan and other sources gives the major damaging

environments for carbon and low alloy steels:

• Hot or boiling caustic (sodium hydroxide) solutions, the cause of

"caustic embrittlement"

,

• Hot or cold nitrate solutions,

• Wet hydrogen sulfide, the cause of "sulfide cracking",

• Anhydrous ammonia, possibly aggravated by air and carbon dioxide

contamination

,

• Amine solutions, and

• Hot, oxygenated water.

Experience and statistics for vessels in service in several of these

environments are discussed in greater detail in section 6.

The metallurgical condition of the material is an important determinant of

the severity of the SCC problem. In general, sensitivity to SCC increases with

hardness and strength. Therefore, high strength bolts and the HAZ of welds

without a postweld heat treatment (not stress relieved) are examples of

susceptible materials and conditions.

19
Non-Branching (intergranular)

Branching Crack

Figure 5. Illustration of non-branching and branching stress corrosion

cracks. (Both can be either intergranular or transgranular .

)

20
Stress is the third required ingredient for SCC and high stresses, both

applied and residual, increase the severity of the problem. There has been

much effort to determine a lower limiting threshold stress for SCC, or more

recently, the limiting fracture mechanics quantity "threshold stress intensity

factor, Kjscc" illustrated in figure 6, and these values are very useful for

design.

—Threshold Stress

0' K,scc

Time

Figure 6. Concept of threshold stress or stress intensity factor (K^g^c)

in stress corrosion cracking.

However, very little of this kind of data exists for carbon and low alloy

steels in the environments of interest; in addition, using these as a design

basis means that careful attention has to be paid to eliminating or minimizing

stress concentration details and sources of residual stresses such as severe

machining and welds in the as -welded condition.

In addition to SCC, some environments can accelerate fatigue crack growth.

For carbon and low alloy steels, hot water containing small amounts of

dissolved oxygen appears to be such a detrimental environment. This problem of

the interaction between the environment and fatigue crack growth is a relatively recent area of study and a listing of detrimental environments is

incomplete

Stress corrosion cracking and environmentally assisted fatigue crack

growth have major and severe safety and hazard consequences for two reasons.

The resulting crack- like defects have a detrimental effect on structural

integrity that far outweighs the amount of material affected. In addition, SCC

21
and fatigue cracking often occur in high stress regions. For these reasons,

sec and fatigue cracking are damage mechanisms of major concern for pressure

vessel safety assessment.

4.2.3 Material Property Degradation

A number of operating conditions can change the properties of materials.

Some of the well known among these include high temperature thermal exposure

and nuclear radiation. However, within the material and temperature scope of

this document, only one service environment is of major concern in this regard.

This is the degradation caused by ingress of hydrogen into carbon and low alloy

steels from a hydrogen producing reaction at the metal surface. Aqueous

solutions containing hydrogen sulfide is a prime example of an environment

known to cause the generation and uptake of the hydrogen into steels.

A loss of ductility in ordinary tensile tests caused by hydrogen

dissolved in steels has been known for a long time. Recent tests [3] indicate

that fracture mechanics quantities, such as fracture toughness and tearing

resistance, can also be decreased by the presence of dissolved hydrogen.

Additional studies are needed to develop a full understanding of dissolved

hydrogen effects on fracture mechanics properties and the results would be an

important consideration in evaluating the safety and hazards of vessels

operating in hydrogen producing environments.

The effects of dissolved hydrogen on ductility and toughness are

manifested without the formation of any internal physical discontinuities.

However, if the amount of hydrogen ingress becomes excessive, a damage

condition known as "blistering" can occur. It is characterized by irregularly

spaced, small-to-fairly large swellings on the surface of the steel. Crosssectioning through these swellings shows that voids have formed on a plane

parallel to steel surface. Figure 7 shows the surface appearance of blistering

and cross sections of blisters.

A small amount of blister formation would generally not have a major

detrimental effect on structural integrity and safety margin. This is partly

because the planes of responsible voids are nearly parallel to the vessel

surface and therefore not subjected to pressure stresses. However, blister

formation is an indicator that hydrogen ingress into the material has occurred,

and that other forms of localized cracking and degradation of properties may be

present

.

5. INSPECTION METHODS AND IMPLEMENTATION

A working understanding of nondestructive examination (NDE) methods and

their capabilities and limitations in the inspection of vessels and tanks is an

important element in the safety assessment of these structures. The total NDE

scope involves a number of organizations whose activities cover the formulation

of NDE requirements and acceptance standards, the development and validation of

NDE techniques, and the qualification and certification of NDE personnel.

The first part of this section provides a brief description of

organizations involved in the NDE of pressure vessels and the relationship

22
among them. This is followed by a sximmary of the major NDE methods and some

remarks about the capabilities and limitations of each method.

5.1 Role of Organizations Involved

5.1.1 ASME Code

Section VIII of the Code contains examination requirements, acceptance

standards, and personnel qualification requirements specific to the materials

Magnified Cross Section Appearance

Figure 7. Appearance of hydrogen induced blisters in a carbon steel.

23
I

1 and fabrication processes permitted in this Section of the Code. In addition, I Section VIII refers to Section V, "Nondestructive Examination" [11] of the Code ! for requirements and guidelines relating to the general aspects of NDE ! techniques and personnel qualification.

Specifically, Section VIII requires that personnel performing radiographic

examination of welds shall be qualified and certified to a written practice.

The guideline for this purpose is the ASNT (American Society for Nondestructive

Testing) recommended practice which is described later. For other NDE methods,

Section VIII requires the manufacturer to certify personnel competency but

specific use of the ASNT recommended practice as the guideline is not required.

Overall, the ASME Code uses the format that if the design Section has no

specific personnel qualification requirements, then the requirements of Section

V of the Code applies which in turn is often an ASNT recommended practice.

5.1.2 API Standards

API Standard 620, for the design and fabrication of low pressure storage

tanks, requires that the NDE methods when specified be in accordance with

Section V of the ASME Code. The acceptance standards for the specified NDE

methods are essentially identical to ASME Section VIII, Division 1 requirements. API has no specific requirements regarding the qualifications of

the personnel performing the NDE tests and evaluations.

API has another standard, API 510, for the inservice inspection of vessels

and tanks used in the petroleum and chemical industries [12]. Usually, this

inservice inspection is done under the direction of a third party inspector

whose qualifications are those required by the inspector's employer.

API 510 also permits inservice inspection to be done under the direction

of an inspector employed by an owner-user (the Owner-User Inspector) . In this

case, the inspector is required to have one of several alternative education

and experience qualifications which in brief are:

• Engineering degree plus 1 year of relevant experience, or

• A 2 -year engineering or technology certificate plus 2 years of

relevant experience, or

• High school education or equivalent plus 3 years of relevant

experience

.

API 510 has no specified certification requirements for the personnel

performing the NDE.

5.1.3 National Board

To aid in their efforts to maintain uniformity in the construction,

inspection, and repair of pressure vessels, the National Board of Boiler and

Pressure Vessel Inspectors issues a Manual entitled "National Board Inspection

Code" [13]. This Manual covers both initial and inservice inspections.

24
For inservice inspection, the National Board Inspection Code (NBIC) is

intended for application to installations other than those covered by API 510.

NBIC inservice inspections can be performed by Authorized Inspectors or by

Owner-User Inspectors. Authorized Inspectors are third-party individuals who

hold National Board Commissions and who are authorized by the applicable

jurisdictions. Owner-User Inspectors also must hold a National Board

Commission and be authorized by the jurisdiction but are employed by the owneruser of the pressure vessels. The education and experience requirements for a NBIC Owner-User Inspector are essentially identical to those described above

for an API 510 Owner-User Inspector.

Like API 510, the National Board Code does not have specific certification

requirements for the personnel performing the examinations.

5.1.4 ASNT Recommended Practice

The ASNT in their Recommended Practice No. SNT-TC-lA [14] provides initial

qualifications, training guidelines, and examination requirements for three

qualification levels of personnel performing NDE. The three levels are I, II,

and III in order of increasing qualification. Table I summarizes the main

features of SNT-TC-lA to provide more information about the three levels of

certification.

This recommended practice is used by many organizations as a guideline for

their internal competency testing and qualifying of NDE personnel, and by

design codes and inspection agencies as a requirement for personnel

certification

5.1.5 ASTM Specifications

ASTM (American Society for Testing and Materials) issues many

specifications and test methods for NDE. The ASME Code has adopted and

included ASTM specifications and m.ethods which are relevant to pressure vessel

applications in its Section V on NDE. In these cases, the ASME Section V

methods and procedures are identical to the corresponding ASTM specification.

5.1.6 NACE Recommended Practices

NACE (National Association of Corrosion Engineers) has issued or is

preparing recommended practices for the inspection of vessels in some

applications that have been experiencing problems. In some cases, the

recommended practice includes a requirement that the NDE must be done by

personnel holding a specified ASNT Level certification. Details are given

later in connection with pressure vessel cracking experience.

25
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5.2 Examination Methods

The application of NDE methods involves many considerations about

materials and fabrication, structural geometry, and accessibility for

examination. A detailed discussion of each of these methods and applications

is beyond the scope of this document but references such as those by McMasters

[15], McGonnagle [16], and Chapter IV of the API Guide [17] can be consulted

for additional information.

Of the various conventional and advanced NDE methods, five are widely used

for the examination of pressure vessels and tanks and the discussion in this

section will be limited to these five. The names and acronyms of these five

are

Visual Examination VT

Liquid Penetrant Test PT

Magnetic Particle Test MT

Gamma and X-ray Radiography . . RT

Ultrasonic Test UT

There is a significant difference in the capabilities and therefore

applicability between the first three methods as a group and the last two. VT

PT and MT can detect only those discontinuities and defects that are open to

the surface or are very near the surface. In contrast, RT and UT can detect

conditions that are located within the part. For these reasons, the first

three are often referred to as "surface" examination methods and the last two

as "volumetric" methods.

Table II summarizes the main features of these five methods; additional

commentary on each is presented in the following.

5.2.1 Visual Examination (VT)

A visual examination is easy to conduct and can cover a large area in a short time. It is very useful for assessing the general condition of the

equipment and for detecting some specific problems such as severe instances of

corrosion, erosion, and hydrogen blistering. The obvious requirements for a meaningful visual examination are a clean surface and good illumination.

5.2.2 Liquid Penetrant Test (PT)

This method depends on allowing a specially formulated liquid (penetrant)

to seep into an open discontinuity and then detecting the entrapped liquid by a developing agent. When the penetrant is removed from the surface, some of it

remains entrapped in the discontinuities. Application of a developer draws out

the entrapped penetrant and magnifies the discontinuity. Chemicals which

27
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28
fluoresce under black (ultraviolet) light can be added to the penetrant to aid

the detectability and visibility of the developed indications. The essential

feature of PT is that the discontinuity must be "open," which means a clean,

undisturbed surface.

The PT method is independent of the type and composition of the metal

alloy so it can be used for the examination of austenitic stainless steels and

nonferrous alloys where the magnetic particle test is not applicable.

5.2.3 Magnetic Particle Test (MT)

This method depends on the fact that discontinuities in or near the

surface perturb magnetic flux lines induced into a ferromagnetic material. The

magnetic field can be induced into the part by various means. For a component

such as a pressure vessel where access is generally limited to one surface at a time, the "prod" technique is widely used. The essentials of this technique

and its application for examining a weld seam are illustrated in figure 8. The

magnetic field is produced in the region around and between the prods (contact

probes) by an electric current (either AC or DC) flowing between the prods.

The ferromagnetic material requirement basically limits the applicability of MT

to carbon and low alloy steels.

The perturbations of the magnetic lines are revealed by applying fine

particles of a ferromagnetic material to the surface. The particles can be

either a dry powder or a wet suspension in a liquid. The particles can also be

treated to fluoresce under black light. These options lead to variations such

as the "wet fluorescent magnetic particle test" (WFMT)

MT has some capability for detecting subsurface defects. However, there

is no easy way to determine the limiting depth of sensitivity since it is

highly dependent on magnetizing current, material, and geometry and size of the

defect. A very crude approximation would be a depth no more than 1.5 to 3 mm

(1/16 to 1/8 in)

The sketches in figure 9 illustrate the appearance of MT indications

associated with cracks and discontinuities that might occur in and near welds.

A very important precaution in performing MT is that corners and surface

irregularities also perturb the magnetic field. Therefore, examining for

defects in corners and near or in welds must be performed with extra care.

Another precaution is that MT is most sensitive to discontinuities which are

oriented transverse to the magnetic flux lines and this characteristic needs to

be taken into account in determining the procedure for inducing the magnetic

field.

29
Prod technique for magnetic particle inspection of welds

(From: Welding Handbook, Vol. 5, 7th ed. , Am. Weld. Soc.)

Examining a welded tank by magnetic particle method

(From: Principles of Magnetic Particle Testing,

MAGNAFLUX, 1985)

Figure 8. Principles and application of magnetic particle testing.

30
WeldSurface crack in a weld.

WeldWeld

Indications

Indications

Diffused indications along weld - base

metal junction typical of subsurface

flaws in or near fusion line.

Scattered small indications in areas next

to welds probably due to service

induced conditions.

Figure 9. Illustrations of magnetic particle test indications due to

various causes

31
5.2.4 Radiography (RT)

The basic principle of radiographic examination of metallic objects is the

same as in any other form of radiography such as medical radiography. Holes,

voids, and discontinuities decrease the attenuation of the x-ray and produce

greater exposure on the film (darker areas on the negative film)

.

Because RT depends on density differences, cracks with tightly closed

surfaces are much more difficult to detect than open voids. Also, defects

located in an area of a abrupt dimensional change are difficult to detect due

to the superimposed density difference. RT is effective in showing defect

dimensions on a plane normal to the beam direction but determination of the

depth dimension and location requires specialized techniques.

Sets of reference radiographs for various materials and product forms

showing typical kinds of defects are available from ASTM. They include E 186,

E 280 and E 446 for steel castings and E 390 for steel fusion welds.

Since ionizing radiation is involved, field application of RT requires

careful implementation to prevent health hazards.

5.2.5 Ultrasonic Testing (UT)

The fundamental principles of ultrasonic testing of metallic

similar to radar and related methods of using electromagnetic and

waves for detection of foreign objects. The distinctive aspect of

inspection of metallic parts is that the waves are mechanical, so

equipment requires three basic components:

• Electronic system for generating electrical signal.

• Transducer system to convert the electrical signal into mechanical

vibrations and vice versa and to inject the vibrations into and

extract them from the material.

• Electronic system for amplifying, processing and displaying the

return signal.

For volumetric examination, two kinds of waves can be induced in metallic

materials; longitudinal waves and shear waves as illustrated in figure 10.

Ultrasonic testing can be done in several different modes but the pulse-echo

technique illustrated in figure 11 is probably the most widely used for

examination of structural equipment because of its convenience and flexibility.

In this mode, very short signal pulses are induced into the material and

waves reflected back from discontinuities are detected during the "receive"

mode. The transmitting and detection can be done with one transducer or with

two separate transducers (the tandem technique). Figure 12 shows the

essentials of UT examination of a weld and adjacent region by the angle beam,

single transducer technique.

materials are

acoustic

UT for the

the test

32
^ Plastic Wedge

7

; i 5 ttitt

Longitudinal Shear

J

= Particle Motion | = Wave Direction

Figure 10. Longitudinal and shear waves utilized in ultrasonic examination.

Flaw Reflection

Transducer

Ji^ Flaw

Initial Pulse J

\

1 1 ^

Time — /

/

CRT Display

Back Reflection

Figure 11. Principles of pulse-echo ultrasonic technique.

33
Figure 12. Basic features of angle beam ultrasonic examination of a butt weld.

34
The most common way of displaying the detected signal Is a time based

display of the amplitude of the signals on a CRT screen as shown schematically

in figure 11. Since the wave velocity is constant, the position of the

reflected signal from a discontinuity on the time scale is a good measure of

its location within the part.

Although the amplitude of the reflected signal in UT provides some measure

of the size of the discontinuity, the effect of many other factors

(orientation, geometry, type of discontinuity, distance) are involved. To

account for some of these reasons, the amplitude is often reported in relative

values. Two normalizing indices commonly used for this purpose are:

• Amplitude of the back reflection, or

• Amplitude of the reflection from a flat bottomed hole (FBH) at the

same location as the detected indication.

Amplitudes are then reported as % Back Reflection or %FBH.

Unlike radiography, UT in its basic form does not produce a permanent

record of the examination. However, more recent versions of UT equipment

include automated operation and electronic recording of the signals.

Ultrasonic techniques can also be used for the detection and measurement

of general material loss such as by corrosion and erosion. Since wave velocity

is constant for a specific material, the transit time between the initial pulse

and the back reflection is a measure of the travel distance and the thickness.

5.3 Detection Probabilities and Flaw Sizing

The implementation of NDE results for structural integrity and safety

assessment involves a detailed consideration of two separate but interrelated

factors

• Detecting the discontinuity, and

• Identifying the nature of the discontinuity and determining its

size

Table II has notations indicating the ideal sensitivity of each NDE technique.

This information indicates the capabilities of the methods under ideal,

laboratory environment conditions with experienced test personnel. Many

conditions, some of which were noted above for each method and which will be

inherent to actual examinations, will make the real detection capability less

than the ideal sensitivity. Also, since human factors are involved,

quantification of capabilities can only be based on experimental data from

replicate and round-robin tests expressed in probabilistic terms.

Much of the available information on detection and sizing capabilities has

been developed for aircraft and nuclear power applications and is summarized in

Bush's comprehensive discussion of NDE reliability [18]. This kind of

information is very specific to the nature of the flaw, the material, and the

35
details of the test technique, and direct transference to other situations Is

not always warranted. However, data for one case of a round- robin examination

of surface fatigue cracks In a very high strength steel serves to Illustrate

the nature of the problem. In this case, MT and UT were able to detect cracks

of surface flaw lengths In the 2 to 3 mm (0.08 to 0.1 In) range with 90%

probability of detection at 95% confidence level while the probability was zero

by RT. Unfortunately, there has been no systematic studies of this kind for

cracks and flaws that might be found In pressure vessels for general

applications

.

Once detected, the size of the discontinuity and If possible Its exact

type needs to be determined. These determinations are much easier for surface

discontinuities compared to embedded ones. Later discussion In section 6 will

Indicate that surface cracking seems to be the predominant problem In vessels

of Interest In this document. In this case, the flaw sizing problem becomes

one of determining the depth dimension.

The overall reliability of NDE Is obviously an Important factor In a safety and hazard assessment. Failing to detect or undersizing existing

discontinuities reduces the safety margin while oversizing errors can result in

unnecessary and expensive outages. High reliability results from a combination

of factors:

• Validated procedures, equipment and test personnel,

• Utilization of diverse methods and techniques, and

• Application of redundancy by repetitive and independent tests.

Finally, it is useful to note that safety assessment depends on evaluating

the "largest flaw that may be missed, not the smallest one that can be found."

6. RECENT CRACKING EXPERIENCE IN PRESSURE VESSELS

The Introduction noted that surveys and service experience are indicating

damage problems occurring in pressure vessels in several application areas.

These problems are discussed in greater detail in this section. The

applications covered are vessels and tanks in deaerator, amine, wet hydrogen

sulfide, ammonia storage and pulp digesting service.

6.1 Deaerator Service

Deaeratlon refers to the removal of non-condenslble gases, primarily

oxygen, from the water used in a steam generation system. Figure 13

schematically Illustrates the function of the deaerator vessel in the flow

stream. Deaerators are widely used in many industrial applications including

power generation, pulp and paper, chemical, and petroleum refining and in many

public facilities such as hospitals and schools where steam generation is

required. In actual practice, the deaerator vessel can be separate from the

storage vessel, as illustrated in figure 13, or combined with a storage vessel

into one unit.

36
Non-condensibles

Condensate

Make-up water

Low pressure steam

Deaerator

Storage

to Boiler

Figure 13. Simplified flow diagram for feedwater deaerator/storage system.

Typical operational conditions for deaerator vessels range up to about 2.1

MPa (300 psi) and up to about 150 (300 °F) . Nearly ail of the vessels are

designed to ASME Code, Section VIII, Division 1 rules resulting in vessel wall

thicknesses up to but generally less than 25 mm (1 in). The vessel material is

almost universally one of the carbon steel grades.

Following some serious deaerator vessel failures in 1982 and 1983, a NACE

(National Association of Corrosion Engineers) Task Group undertook a survey of

industry experience in this application. A summary of the survey results have

been reported by Robinson [19] and show that cracking had been detected in over

30% of the 84 vessels in the survey. Case histories of some cracking incidents

have been described by Franco and Buchheim [20] and survey results in specific

industries have been provided by Winters [21] and by Vormelker [22]. The last

two references report cracking incidences of 42% and 50%. An update of the

NACE Task Group effort is given in a recent paper by Kelly et al. [23].

37
Analysis of the survey data and other investigations has determined the

following features about the cracking:

• Water hammer is the only design or operational factor that

correlates with cracking.

• Cracking is generally limited to weld regions of vessels that had

not been postweld heat treated.

• Corrosion fatigue appears to be the predominant mechanism of crack

formation and growth as indicated by the studies of Herro [24],

Copeland, et al. [25] and others.

The weld and welding practice parameters that are involved in the sensitivity

of and localization to weld regions have been discussed by Gooch [26].

The failures and the survey results have prompted several groups to

prepare inspection, operation and repair recommendations. The groups are TAPPI

(Technical Association of Pulp and Paper Industry) , the National Board of

Boiler and Pressure Vessel Inspectors, and NACE. The main features of the

TAPPI recommendations have been published by Beckwith et al. [27] and a summary

of the NACE recommendations has also been published [23,28]. The National

Board guidelines are scheduled to be in an Appendix to the next edition of the

Inspection Code and NACE's proposed recommended practice is planned to be

published in 1988 or 1989. For inspection, all recommendations suggest:

• Special attention to the internal surface of all welds and heataffected zones (HAZ)

.

• Use of the wet fluorescent magnetic particle (WFMT) method for

inspection

.

The TAPPI and the NACE recommendations also contain additional items:

• Inspection by personnel certified to ASNT's SNT-TC-lA minimum

Level I and interpretation of the results by minimum Level II.

• Reinspection within 1 year for repaired vessels, 1-2 years for

vessels with discontinuities but unrepaired, and 3-5 years for

vessels found free of discontinuities.

In addition, both TAPPI and NACE give general and specific recommendations for

operating practice to minimize damage and for repair procedures.

Whenever crack indications are found in the inspections, the structural

integrity and safety of the vessel for continued operation has to be evaluated.

Copeland et al. [25] has reported the results of a fracture mechanics analysis

for one group of deaerator vessels. They concluded that cracks transversely

oriented to the weld direction may be acceptable for continued service without

repair provided the pressure stresses were fairly low (less than 52 MPa, 7.5

ksi) which was the case for many vessels in this group. Repairs were

recommended if the stresses were higher or if the cracks were parallel to the

38
weld direction for all stresses. These conclusions were for a particular

group of vessels with specific material, material properties, and design

parameters and would not necessarily apply to other cases. However, it does

demonstrate the use of fracture mechanics analysis to evaluate whether removal

of all crack indications are necessary or not.

6.2 Amine Service

The amine process is used to remove hydrogen sulfide (H2S) from petroleum

gases such as propane and butane. It is also used for carbon dioxide (CO2)

removal in some processes. Amine is a generic term and includes

monoethanolamine (MEA) , diethanolamine (DEA) and others in the amine group.

Figure 14 shows a simplified flow diagram of an amine treatment plant. These

units are used in petroleum refinery, gas treatment and chemical plants.

SWEET

GAS ^

OUT

CONDENSER

ABSORBER/

CONTACTOR

ACID

GAS

IN

LEAN

AMINE

RICH

AMINE

RICH/LEAN

HEAT

EXCHANGER

ACCUMULATOR

REGENERATOR

STEAM

REBOILER

Figure 14. Simplified process flow diagram of amine plant [29]
The operating temperatures of the amine process are generally in the 38

to 93 °C (100 to 200 °F) range and therefore the plant equipment is usually

constructed from one of the carbon steel grades. The wall thickness of the

pressure vessels in amine plants is typically about 25 mm (1 in).

Although the possibility of cracking of carbon steels in an amine

environment has been knovm for some years, real concern about safety

implications was highlighted by the 1984 failure of the amine process pressure

vessel mentioned earlier. While the complete investigation of this incident

showed that hydrogen induced cracking and not amine cracking was the primary

cause [3], the incident prompted further actions on amine process equipment.

One of the actions was a survey of cracking experience in amine service units.

The survey results have been reported by Richert et al. [29]. The form used by

NACE in the survey is included as Appendix C to this document.

Overall, the survey found about 40% cracking incidence in a total of 294

plants. Cracking had occurred in the absorber/contactor, the regenerator and

the heat exchanger vessels, and in the piping and other auxiliary equipment.

Several of the significant findings of the survey were:

• All cracks were in or near welds.

• Cracking occurred predominantly in unstress relieved (not

PWHT) welds.

• Cracking occurred in processes using several kinds of amines

but was most prevalent in MEA units.

• WFMT and UT were the predominant methods of detecting the cracks;

internal examination by WFMT is the preferred method.

Information from laboratory studies of this problem by Lyle [30] and

Schutt [31] indicate that pure amine does not cause cracking of carbon steels

but amine with carbon dioxide in the gas phase causes severe cracking. The

presence or absence of chlorides, cyanides, or hydrogen sulfide may also be

factors but their full role in the cracking mechanism are not completely known

at present.

Currently, API is preparing a Recommended Practice for vessels in amine

service. It is expected to contain recommendations on the type and frequency

of examination for cracking as well providing information on design, operating

experience, and cracking mechanism. Preparation of the Recommended Practice is

expected to be completed in 1988 or 1989.

6.3 Wet Hydrogen Sulfide Service

Wet hydrogen sulfide refers to any fluid containing water and hydrogen

sulfide (H2S) . Hydrogen is generated when steel is exposed to this mixture and

the hydrogen can enter into the steel. As discussed earlier, the resulting

dissolved hydrogen can cause cracking, blistering, and embrittlement . A recent

article by Warren [32] provides a concise and informative discussion of the

general and specific effects of hydrogen on steels.

40
The harmful effects of hydrogen generating environments on steel have been

known and recognized for a long time in the petroleiom and petrochemical

industries. In particular, sensitivity to damage by hydrogen increases with

the hardness and strength of the steel and damage and cracking are more apt to

occur in high strength steels. To minimize this problem in equipment made of

carbon steels and subject to wet H2S environments, both NACE and API have

Recommended Practices [33,34] that gives a guideline limit on the hardness of

the weld.

Recently, in line with the emphasis on improved and more thorough

inspections being used on amine service equipment, the petroleum refining

industry initiated an inspection program for vessels in wet H2S service. The

suggested priorities and schedule for the inspection program is shown in

Appendix D. Also, the WFMT method of examination was to be used. An interim

report of the results which included the results for 189 vessels has been

reported by Merrick [35]. Cracks of varying severity were detected in 31% of

the vessels. This is a considerably higher incidence than was expected and is

attributed in part to the use of WFMT, a more sensitive examination method.

The implications of the survey results are still being studied, but some

of the findings from the survey and associated investigations are:

• Significant cracks can initiate from very small hard zones

associated with weldments; these hard zones are not detected by

conventional hardness tests.

Initially small cracks can grow by a step-wise form of hydrogen

blistering to form through thickness cracks.

NACE/API limits on weld hardness may not be completely effective

in preventing cracking.

Thermal stress relief (PWHT) appears to reduce the sensitivity to

and the severity of cracking.

Wet hydrogen sulfide has also been found to cause service cracking in

liquefied petroleum gas (LPG) storage vessels. Cantwell [36] has reported on

the results of a recent inspection survey which showed a 30% incidence of

cracking for 141 inspected vessels. A considerable portion of the total found

is attributed to preexisting fabrication flaws which are being detected by more

sensitive inspection techniques such as WFMT. However, the results clearly

show that inservice cracking has also occurred.

The service cracking in the LPG vessels occurs predominantly in the weld

heat affected zone (HAZ) . The vessels are usually spherical with wall

thickness in the 20 to 75 mm (0.8 to 3 in) range. The vessel materials range

from typical grades of carbon steels up to alloy steels with tensile strengths

over 690 MPa (100 ksi)

The source of the hydrogen sulfide is believed to be carry-over

("breakthrough") from the treating process into the storage vessel. In common

with the general trend of wet hydrogen sulfide cracking, the incidence in LPG

41
storage vessels is higher for the as -welded condition and for higher strength

steels

.

Cantwell [36] provides recommendations for new and existing vessels to

minimize the risk of a major failure. Among these are:

• Use lower strength steels for new vessels.

• Schedule an early inspection for vessels more than 5 years in

service.

• Improve monitoring to minimize breakthrough of hydrogen sulfide.

• Replace unsafe vessels or downgrade to less severe service;

usually, lower pressure service.

6.4 Ammonia Storage Service

Careful inspections of vessels used for storage of ammonia (in either

vapor or liquid form) in recent years have resulted in evidence of serious

stress corrosion cracking problems. Statistics reported at a meeting on this

problem [37] indicate cracking in approximately one-half of the vessels

examined.

The vessels for this service are usually constructed as spheres from one

of the carbon steel grades, and they operate in the ambient temperature range.

The water and oxygen content in the ammonia has a strong influence on the

propensity of carbon steels to crack in this environment. Figure 15 shows the

U.S. and European guidelines for operation and inspection frequency.

Recent laboratory studies by Lunde and Nyborg [38] indicate general

consistency with these guidelines.

The reported information indicates a tendency for the cracks to be in or

near the welds in as -welded vessels. Cracks occur both transverse and paralle

to the weld direction. Thermal stress relieving seems to be a mitigating

procedure for new vessels, but its efficacy for older vessels after a period o operation is dubious partly because small, undetected cracks may be present.

6.5 Pulp Digester Service

The kraft pulping process is used in the pulp and paper industry to diges

the pulp in the papermaking process. The operation is done in a relatively

weak (few percent) water solution of sodium hydroxide and sodium sulfide

typically in the 110 to 140 °C (230 to 285 °F) temperature range. Since the

early 1950s, a continuous version of this process has been widely used. Nearl

all of the vessels are ASME Code vessels made using one of the carbon steel

grades with typical design conditions of 175 to 180 °C (350 to 360 °F) and 1 MPa (150 psig)

.

42
European practice

UJ

O E

o3

I

lOCOO

1 000

100

10

A

/ /

B / c

10 100 1000

OXYGEN IN LIQUID PHASE

(ppm w/w)

A Inspect at normal frequency

8 Inspect ot 'eost twice normal frequency

C Oo not operote in triis area

Cither reduce oxygen level or add water

to bring operating conditions into Zone AorB

UJ

9 £

O Q.

Q:

UJI

10000

1000

1 oo

10

US practice

A

B

1 10 too 1000

OXYGEN IN LIQUID PHASE

( ppm w/w)

A Inspect ot normal frequency B Operate carefully

Figure 15. U.S. and European Guidelines for ammonia storage vessels [37].

These vessels had a very good service record with only isolated reports of

cracking problems until the occurrence of a sudden rupture failure in 1980

[39]. Since then, TAPPI (Technical Association of the Pulp and Paper Industry)

has organized and coordinated a program of inspection, determination of causes,

and repair recommendations. The progress and results of this program have been

summarized by Bennett [40].

The inspection survey has revealed that about 65% of the properly

inspected vessels had some cracking. Some of the cracks were fabrication flaws

revealed by the use of more sensitive inspection techniques but most of the

cracking was service - induced . The inspection survey and analysis indicates the

following features about the cracking:

• All cracking was associated with welds.

• Wet fluorescent magnetic particle (WFMT) testing with proper

surface preparation was the most effective method of detecting

the cracking.

43
Fully stress relieved vessels were less susceptible.

• No clear correlation of cracking and non- cracking could be found

with vessel age and manufacture or with process variables and

practices

.

• Analysis and research indicate that the cracking is due to a caustic stress corrosion cracking mechanism although its

occurrence at the relatively low caustic concentrations of the

digester process was unexpected.

Currently, preventive measures such as weld cladding, spray coatings, and

anodic protection are being studied, and considerable information has been

obtained [41]. In the meantime, the recommended guideline is to perform an

annual examination.

6.6 Summary of Service Cracking Experience

The preceding discussion shows a strong influence of chemical

environmental conditions on cracking incidence. This is a factor that is not

explicitly treated in most design codes. In fact, it would be difficult to

include this factor in general design codes considering the wide variety of

operating environments for various applications. Therefore, quantitative rule

for the determination of the detrimental effects of various environments are

not given in most design codes. Instead, service experience is the best and

often the only guide to inservice safety assessment.

For vessels and tanks within the scope of this document, the service

experience indicates that the emphasis of the inspection and safety assessment

should be on:

• Vessels in deaerator, amine, wet H2S , ammonia and pulp digesting

service

,

• Welds and adjacent regions,

• Vessels that have not been thermally stress relieved (no PWHT of

fabrication welds) , and

• Repaired vessels, especially those without PWHT after repair.

The evaluation of the severity of the detected cracks can be done by

fracture mechanics methods. This requires specific information about stresses

material properties, and flaw indications. Generalized assessment guidelines

are not easy to formulate. However, fortunately, many vessels in the

susceptible applications listed above operate at relatively low stresses, and

therefore, cracks have a relatively smaller effect on structural integrity and

continued safe operation.

44
7. PERIODIC INSPECTION REQUIREMENTS AND RECOMMENDATIONS

Rules and recommendations for periodic inservice inspection and evaluation

can be very detailed and complete or relatively general and brief. Section XI

of the ASME Code, "Rules For Inservice Inspection of Nuclear Power Plant

Components," is an example of a very complete document with rules and

requirements for inspection frequency, inspections methods, acceptability

criteria, evaluation methods, and repair or replacement procedures. However,

this is a special purpose document for a specific application. Of necessity,

general application documents on inservice inspection have to be much more

general in content and usually, shorter in length.

Several general documents on inservice inspection have already been

mentioned. In addition, some recommendations developed for specific applications which have experienced serious cracking incidence have also been

discussed. For consolidation and convenient reference, these requirements and

recommendations are summarized in table III supplemented by additional remarks

below

7.1 National Board Inspection Code and API 510

These two are discussed together since the inservice inspection

requirements of the two are similar; the specific documents are API 510 [12]

and NBIC (National Board Inspection Code), [13]. Both documents are for

general application and both cover rerating, alteration, and repair in addition

to inservice inspection requirements. API 510 is intended for pressure vessels

used in the refinery and petrochemical industries and NBIC is for all other

applications

API 510 and NBIC both use general corrosion rate as a guide for

determining inspection frequency; the specific requirement is:

• The maximum period between inspections to be the lesser of onehalf of the remaining corrosion life or 10 years.

The recommended examination method is the visual method augmented by other

methods as appropriate.

Forms used by the National Board and by API to report the results of an

inservice inspection of pressure vessels are included as Appendix E and F,

respectively

7.2 Recommendations for Specific Applications

Table III contains entries for several specific applications discussed

earlier which have had significant cracking incidence in the past few years.

The entries are not complete because some of the recommendations are still in

preparation. Also, some of these are quite specific about inspection interval

and frequency and examination method while others are more general. However,

the table provides a good summary of guidelines for this important aspect of

pressure vessel safety.

45
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7.3 Institute of Petroleum Code

The last entry in table III lists information contained in a pressure

vessel inspection code [42] used in the United Kingdom for the petroleum and

chemical industries. Although this Code does not apply in the United States,

one item in it is very pertinent. This is the item concerning the recommended

frequency of inspection which is summarized in table III. Additional details

of this part of the code are included in Appendix G.

The inspection frequency requirements of this Code are more specific than

those in the API and National Board rules, and they are categorized by class o vessel and record of prior inspections. The first inservice inspection is

required within the first 2/5 years of operation, depending on the class of

vessel. Successive inspections can be at longer intervals if prior inspection

results show a damage-free condition.

8. DAMAGE AND CRACK SEVERITY EVALUATION AND REPAIR

Assessing the severity of deterioration or cracks revealed by inservice

inspections requires a thorough technical analysis. If the assessment

indicates that a repair or modification is necessary to restore structural

integrity, they need to be done with careful preparation and execution.

Consideration of specific details for each vessel and application are required

Consequently, only some general and procedural guidelines are presented in thi

document

8 . 1 Damage Evaluation

The proposed or tentative recommendations and guidelines for the cases of

significant cracking described earlier are that if the depth of cracking or

damage is less than the corrosion allowance, careful removal of the crack and

blending the cavity with the surrounding is the recommended action.

If the damage depth is greater than the corrosion allowance, detailed

engineering analysis is required to evaluate the options of allowing continued

operation with the damage for some interval of service, removing the damage

without repair, or repairing the damage. Fracture mechanics methodology for

performing this type of evaluation was noted earlier. The evaluation should

also include an analysis to determine whether further damage can be minimized

by operational modifications.

8.2 Repair by Welding

If the technical evaluation indicates that a repair is necessary to

restore structural integrity, welding is the usual method of repair. In the

United States, weld repairing of vessels and tanks within the scope of this

document will usually be done in accordance with the rules and requirements of

API 510 [12] or the NBIC [13]. The major provisions for repair welding in

these two codes are summarized in table IV.

47
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48
Procedurally, both codes require that the repair plan be reviewed and

certified by a registered or experienced engineer. Authorization to proceed

with the repair is required from an Inspector and all welding must be done by

qualified welders. In general, the repair weld should be postweld heat treated

(PWHT) , especially for vessels in cracking susceptible service. However, this

may not always be possible and alternatives are provided in these codes.

Pressure vessels repaired according the NBIC rules are required to be

marked with an "R" stamp by stamping or nameplate. Figure 16 shows the "R"

symbol and the information required in the stamping.

•— STAMPING OR NAMEPLATE OF A BOILER

OR PRESSURE VESSEL REPAIRED BY WELDING

(name of repair firm)

No.,

(National Board

Repair symbol

stamp no.)

[date of repair(s)]

Stamping or nameplate shall be applied adjacent to the original manufacturer's

stamping or nameplate. A single nameplate or stamping may be used for more than one repair to a boiler or pressure vessel provided it is carried out by the same repair

organization. The date of each repair shall be stamped on the nameplate. This date

should correspond with the date on the Report of Welded Repairs. Letters shall be

at least 5/32 in. (4 mm) high. (Ref. R-403, page 48.)

Figure 16. National Board Stamp or Nameplate for a weld repaired vessel.

49
In addition, the National Board requires the preparation and distribution of

Form R-1, Report of Welded Repair or Alteration. A copy of this form is

included as Appendix H. API does not have a formalized stamping to indicate

repairs, but API 510 does require that the records of the repair be maintained

by the owner or user of the vessel.

Overall, repair welds are usually made under less than ideal shop

fabrication conditions, and careful attention to all aspects of welding must b^ exercised to avoid a condition that may be more prone to damage and

deterioration.

9. INFORMATION FOR SAFETY ASSESSMENT

This document has discussed a large amount of information on the design

rules, inspection requirements, service experience, and damage mitigation

relevant to pressure vessels and low pressure storage tanks used in general

industrial applications. To serve as a summary and as a reminder, table V

outlines the information and data that are necessary or useful to assess the

safety and hazard implications of operating vessels and tanks.

10. ACKNOWLEDGMENT

The assistance and helpful comments of Dr. H. I. McHenry of the National

Institute of Standards and Technology, Boulder, Colorado in the preparation of

this document are gratefully acknowledged. Thoughtful comments and remarks

provided by OSHA personnel who reviewed a draft of this document are also

acknowledged and greatly appreciated.

50
TABLE V

INFORMATION AND DATA USEFUL FOR THE SAFETY ASSESSMENT OF STEEL

VESSELS AND LOW PRESSURE STORAGE TANKS

INTRODUCTION AND SCOPE

This outline summarizes information and data that will be helpful in assessing

the safety of steel pressure vessels and low pressure storage tanks that

operate at temperatures between -75 and 315 °C (-100 and 600 °F)

VESSEL IDENTIFICATION AND DOCUMENTATION

Information that Identifies the specific vessel being assessed and provides

general information about it include the following items:

Current Owner of the Vessel

Vessel Location

Original location and current location if it has been moved

Vessel Identification

Manufacturer's serial number

National Board number if registered with NB

Manufacturer Identification

Name and address of manufacturer

Authorization or identification number of the manufacturer

Date of Manufacture of the Vessel

Data Report for the Vessel

ASME U-1 or U-2, API 620 form or other applicable report

Date Vessel was Placed in Service

Interruption Dates if not in Continuous Service

DESIGN AND CONSTRUCTION INFORMATION

Information that will identify the code or standard used for the design and

construction of the vessel or tank and the speecific design values, materials,

fabrication methods, and inspection methods used include the following items:

Design Code

ASME Code Section and Division, API Standard or other design

code used

Type of Construction

Shop or field fabricated or other fabrication method

ASME VIII, Division 1 or 2 Vessels

Maximum allowable pressure and temperature

Minimum design temperature

API 620 Vessels

Design pressure at top and maximum fill

Additional requirements included such as Appendix Q (LowPressure Storage Tanks For Liquefied Hydrocarbon Gases) and

Appendix R (Low- Pressure Storage Tanks For Refrigerated

Products

51
TABLE V (con't.)

Other Design Code Vessels

Maximum design and allowable pressures

Maximum and minimum operating temperatures

Vessel Materials

ASME, ASTM or other specification names and numbers for the

major parts

Design Corrosion Allowance

Thermal stress relief (PWHT, Postweld heat treatment)

Design code requirements

Type, extent, and conditions of PWHT performed

Nondestructive Examination (NDE) of Welds

Type and extent of examination performed

Time when NDE was performed (before or after PWHT or

hydrotest)

SERVICE HISTORY

Information on the conditions of the operating history of the vessel or tank

that will be helpful in safety assessment include the following items:

Fluids Handled

Type and composition, temperatures and pressures

Type of Service

Continuous, intermittent or irregular

Significant Changes in Service Conditions

Changes in pressures, temperatures, and fluid compositions and

the dates of the changes

Vessel History

Alterations, reratings , and repairs performed

Date(s) of changes or repairs

INSERVICE INSPECTION

Information about inspections performed on the vessel or tank and the results

obtained that will assist in the safety assessment include the following items:

Inspection(s) Performed

Type, extent, and dates

Examination Methods

Preparation of surfaces and welds

Techniques used (visual, magnetic particle, penetrant test,

radiography, ultrasonic)

Qualifications of Personnel

ASNT (American Society for Nondestructive Testing) levels or

equivalent of examining and supervisory personnel

Inspection Results and Report

Report form used (NBIC NB-7, API 510 or other)

Summary of type and extent of damage or cracking

Disposition (no action, delayed action or repaired)

52

I

i
TABLE V (con't.)

SPECIFIC APPLICATIONS

Survey results indicate that a relatively high proportion of vessels in

operation in several specific applications have experienced inservice related

damage and cracking. Information on the following items can assist in

assessing the safety of vessels in these applications:

Service Application

Deaerator, amine, wet hydrogen sulfide, ammonia or pulp

digesting

Industry Bulletins and Guidelines For This Application

Owner/operator awareness of information

Type, Extent, and Results of Examinations

Procedures, guidelines and recommendations used

Amount of damage and cracking

Next examination schedule

Participation in Industry Survey for This Application

Problem Mitigation

Written plans and actions

EVALUATION OF INFORMATION

The information acquired for the above items is not adaptable to any kind of

numerical ranking for quantitative safety assessment purposes. However, the

information can reveal the owner or user's apparent attention to good practice,

careful operation, regular maintenance, and adherence to the recommendations

and guidelines developed for susceptible applications. If the assessment

indicates cracking and other serious damage problems, it is important that the

inspector obtain qualified technical advice and evaluation.

53
11. REFERENCES

[I] Hammer, W. . Occupational Safety Management and Engineering. 2nd ed.

,

Prentice-Hall, 1981, Chapter 18, "Pressure Hazards."

[2] , "Incident Report," published annually in April issue. National Board

Bulletin . National Board of Boiler and Pressure Vessel Inspectors,

Columbus, Ohio.

[3] McHenry, H.I., Read,D.T. and Shives , T.R., "Failure Analysis of an AmineAbsorber Pressure Vessel," Mat . Perform. . Vol. 26, Aug. 1987, pp 18-24.

[4] ^ "Rules for Construction of Pressure Vessels," Section

VIII, Divisions 1 and 2, ASME Boiler and Pressure Vessel Code, 1986 ed.

[5] , "Recommended Rules for the Design and Construction of Large, Welded,

Low-Pressure Storage Tanks," API 620, 7th ed. , Sept. 1982, Rev. 1, April

1985.

[6] Chuse, R. , Pressure Vessels: The ASME Code Simplified. 6th ed., McGrawHill, 1984.

[7] Yokell, S., "Understanding the Pressure Vessel Code," Chem. Eng. Vol. 93,

No. 9, May 12, 1986, pp 75-85.

[8] Selz, A., "New Toughness Rules in Section VIII, Division 1 of the ASME

Boiler and Pressure Vessel Code," Mechanical Eng. . Vol. 110, April 1988,

pp 84-87.

[9] Thielsch, H., Defects and Failures in Pressure Vessels and Piping . 2nd

ed. , Reinhold, 1975, Chaps. 16 and 17.

[10] Logan, H.L., The Stress Corrosion of Metals . Wiley, 1966.

[II] , "Nondestructive Examination," Section V, ASME Boiler and Pressure

Vessel Code, 1986 ed.

[12] , "Pressure Vessel Inspection Code," API 510, 4th ed. , July, 1985.

[13] , "National Board Inspection Code," National Board of Boiler and

Pressure Vessel Inspectors, Columbus, Ohio, issued biennially.

[14] , "Recommended Practice No. SNT-TC-lA," Amer. Soc. for Nondestructive

Testing, Aug. 1984 ed.

[15] McMaster, R.C., and McIntire.P. (eds.). Nondestructive Testing

Handbook, 2nd ed. , Vols. 1-5, Ronald Press, 1985-1987.

[16] McGonnagle , R.C., Nondestructive Testing . 2nd. ed. , Gordon and Breach,

1969.

54
, Guide for Inspection of Refinery Equipment . 2nd ed. , Chapter IV,

"Inspection Tools," API, 1976.

Bush, S.H., "Reliability of Nondestructive Examination," Vols. 1, 2 and 3,

NUREG/CR-3110, U.S. Nuclear Regulatory Commission, Oct. 1983.

Robinson, J.O., "Deaerator Cracking Survey: Basic Design, Operating

History and Water Chemistry Survey," NACE Corrosion 86, Paper No. 305.

Franco, R.J. and Buchheim, CM., "Case Histories of Deaerator Failure

Analyses," NACE Corrosion 86, Paper No. 306.

Winters, M.A. , "An Oil Company's Overview of Deaerator Cracking," Mat

.

Perform. . Vol. 25, Oct. 1986, pp 9-19.

Vormelker, P.R. , "Deaerator Inspection and Analysis," NACE Corrosion 87,

Paper No. 214.

Kelly, J. A., Guzi, C.E., and Laronge , T.M., "An Overview of NACE Task

Group T-7H-7's Report on Deaerator Cracking," NACE Corrosion 88, Paper No.

350.

Herro , H.M. , "Corrosion Fatigue and Deaerator Cracking," NACE Corrosion

87, Paper No. 213.

Copeland, J.F., Eastman, A.D., and Schmidt, C.G., "Fatigue and Stress

Corrosion Cracking Evaluations in Deaerators," NACE Corrosion 87, Paper

No. 216.

Gooch, T.G., "Review of Welding Practice for Carbon Steel Deaerator

Vessels," Mat. Perform. . Vol. 25, Dec. 1986, pp 30-38.

Beckwith, R.J., Sawtell, J.O., Stokes, H.A., and Yorgiadas , S., "System

Design, Specifications, Operations and Inspection of Deaerators," Tappi

Journ. . Vol. 70, July 1987, pp 45-47.

, "Proposed Guidelines for Inspection of Boiler Feedwater Heater

Vessels and Storage Tank Welds," Mat. Perform. . Vol. 23, Nov. 1984, pp 65-

66.

Richert, J. P., Bagdasarian, A.J., and Shargay, C.A., "Stress Corrosion

Cracking of Carbon Steel in Amine Systems," Mat . Perform

.

. Vol. 27, Jan.

1988, pp 9-18.

Lyle , F.F. Jr., "Stress Corrosion Cracking of Steels In Amine Solutions

Used In Natural Gas Treatment Plants," NACE Corrosion 88, Paper No. 158.

Schutt, H.U. , "New Aspects of Stress Corrosion Cracking

In Monoethanolamine Solutions," NACE Corrosion 88, Paper No. 159.

Warren, D. , "Hydrogen Effects on Steel," Mat. Perform. . Vol. 26, Jan.

1987, pp 38-48.

55
[33] , "Methods and Controls to Prevent In-Service Cracking of Carbon Steel

Welds in P-1 Materials in Corrosive Petroleum Refining Environments," NACE

Std. RP0472-87, 1974, Revised May 1987 (pub. in Mat. Perform. . Vol. 26,

Oct. 1987, pp 57-61)

.

[34] , "Controlling Weld Hardness of Carbon Steel Refinery Equipment to

Prevent Environmental Cracking," PUBL 942, 2nd ed. , American Petroleum

Institute, Nov. 1982.

[35] Merrick, R.D., "Refinery Experience With Cracking in Wet H2S

Environments," Mat. Perform. . Vol. 27, Jan. 1988, pp 30-36.

[36] Cantwell, J.E., "LPG Storage Vessel Cracking Experience," NACE Corrosion

88, Paper No. 157.

[37] , "Panel Discussion on Stress-Corrosion Cracking in Ammonia Storage

Spheres," Plant/Operations Progress . Vol. 2, Oct. 1983, pp 247-259.

[38] Lunde , L. and Nyborg, R. , "The Effect of Oxygen and Water on Corrosion

Cracking of Mild Steel in Liquid and Vaporous Ammonia," Plant /Operations

Progress . Vol. 6, Jan. 1987, pp 11-16.

[39] Smith, K. , "MacMillan Bloedel Digester Accident Shows Need for Frequent

Inspection," Pulp and Paper . Oct. 1981, pp 66-69.

[40] Bennett, D.C., "Cracking of Continuous Digesters: Review of History,

Corrosion Engineering Aspects and Factors Affecting Cracking," Pulp and

Paper Industry Corrosion Problems . Vol. 4, Proc . 4th Intl. Symp . on

Corrosion in the Pulp and Paper Industry, Swedish Corrosion Inst., 1983.

[41] Singbeil, D. and Garner, A., "A Test Method to Evaluate Stress Corrosion

Cracking In Pressure Vessels," Mat. Perform. . Vol. 27, Feb. 1988, pp 38-

46.

[42] , "Pressure Vessel Inspection Safety Code," Part 12. IP Model Code of

Safe Practice in the Petroleum Industry . The Institute of Petroleum,

London, 1976.

56
APPENDIX A - ASME CODE SECTION VIII, DIVISION 1 REPORT FORM

FORM U-1 MANUFACTURER'S DATA REPORT FOR PRESSURE VESSELS

As Required by the Provisions of the ASME Code Rules, Section VIII, Division 1

1. Manuf«ciur«d and cartifiad by -.

2. M«nuf«ctuf»d for

O

© (N«mt and MdTM* of purch«t«r|

3. Location of mttallation , ©

© (Nam* and aodratti

4. Tvp«_ © © ®

5. Tha chamical and phytical prop«rtiat of all parti n^aat tna raquiramantt of matartat ip*cif ication* of tha ASME Boitar and Pratsura Vmsai Coda Tha datign.

conttruction, and worknr\an«hip conform to ASME Rulai, Sactton v 1 1 1. Oivition 1 © ©

hems 6- 1 1 ind. to be completed for s/ng/e wall vessels, lackets of lacketed vessels, or shells of heat exchangers ©

7. Saanrtt © @

0 Ih ft ,n . © ©

@ @

M T Iwno ( ©

B. HMds (a) Mati (hi Marl

iSpcc No Grad«i No Gra<s«i

Location (Too Bottom Endtl Minimum Co"0«iOn

AllOMrance

CroMn

RaOius Radius

<

E iiiDiicai Rai'O

1

Conica*

At>«i Angl«

Hamisprt«ncai nadiui

Fiai 0'ameT«'

1

i Si0« lO P'Vtture

1 {Convai Of Coricavii

1

(•1 ® @

"

1

, 1

ibi

1

If ramovabia, bottt uaad (datcriba othar fastenings) .

(V) 9 Typa of Jackat .

10 Jackat Closura _

. Proof Tast

(Man Spec No G' S./e No i ®

lOMCTitM as OQM A b9< ate ^

. If bar, giva d<rnansions_

11. MAWP_ ®

Hydro . pnau., or comb tas prass..

. psi at rrwM tamp.

.

(55)

F Min, tamp (whan lass than -20 F )_ _ p»i.

. If boltad. dascnba or akarch

tt0ms 12 artd 13 to bm complatad for tube sections

1 2 Tub«aha«Tt ^2 ©

13. Tubaa

Siaiiona'v Man iSoec No G' © Floaiing Man iSpec No Gr i ©

Otam (in I iSubiaCI 10 or«fts<jre) Co<' Allowl (Ti I

Co" AllovK \

Mati (Sp«c No G' Nom Thh (in of Gaugai

Anacri <W«ided Boitcdi

Type iSiraigi^' or u

ttamt 14- 1 7 incf to bo complotadjor innor chomt0f$ of /ackoiad vatto/s or eharyr^mls of haat mMChangors

,4. sh.., ^ 6i) 6^ ©

15. Saams: ©n T ii«oi (H f will 3.

@A T i^t o, fv<

16. HMdt: (a) Mati. (SpK No Gradai. (b) Mati . (S»M No G'aOti

Location (Too •onom Endil MMMmum CoTOiton Atkmanca

Crown

Rad«ut

Knuctia

RmIiui

Eiitotical RatioConical AoaB Angle

Hamiapn«rica> RadiuiFiai Oiamattf

S<0« to ^'Mtur*

(Convai Concavai

() —

(b)

1

If rcmovabi*. boln utad (OMcrlb* othar <Mt>ningil

17. MAWP. ©

Hydro., pr>«u.. or comb, tatt praas.

.

psi at max . tamp.

iMati Spac No Gr Sm No)

F Mm tamp (whan lass than -20 F )

.

©

Thit Form (EOOIOS) may Ix obtainad from tha ASME Ordar Oapt.. 22 Law Oriva, Box 2300, Fairfiald. NJ 07007-2300

57
APPENDIX A - ASME CODE SECTION VIII, DIVISION 1 REPORT FORM (Con't.)

Form U-1 (Back)

18. No2zl«t. lnip*ction «nd Safvty V*lv« Op^ningt:

lln*«1 OmAM >Mt< nc 1 MM ® ®

11 \

: ^©-^

1 1

h' — -

1

1 1

1

1

11

1 1

'

1

1

1

1

19 Supports: Sk )rt _lY«S or nol lWh*r« and r>owl

20. Rvmnrkt Manuf«ctur«f s Partial Data Raportt proparly lOantifiad and ngnad Dv ComrrMutonad inipacTOrt hava bMn furniihsd for tha follonving

'tarns Qt tha raport ^o@@@@@®@®®®

CERTIFICATE OF SHOP COMPLIANCE

Wa cartify that tha ttatamanti mada m thpt raport ara correct ar>d rhat all dataiii ot datign. matanai. construction, artd workmanship of this va form TO tha ASME Coda for Prassura Vasa«is, Sacnoo^ ill. Division 1 U Caftificata of Authorization No —_ ajtpiras

Data Co nama S*gnad- ®

CERTIFICATE OF SHOP INSPECTION

Vassal constructed by . at

I. tha undarsignad, hoidtng a valid commission isauad by tha National Board of Boiiar and Prassura Vassal inspactors and/or ttta Stata or Provmca of and ampioyad .

. hava inspactad tha prasaura vasMi daacribad m this Manufacturar'i Data Raport on^ 19_ _. and itata that, to tha bast of my k nowiadga 9r\<3 baiiaf . tha Manufacturar hat constructad this

prassura vassal >n accordanca with ASME Coda. Saction vii i . Division 1 8v signing this cartif icata naithar tha i nspac tor r>or his amployaf makat any war

ranty. exprassad or imphad, concarning tha prassura vassal datcribad m iha Manufacturer'] Data Raport Furtharmora. naithar tha Inspector nor his am

pioyar shall ba habia m any manner for any personal miury or propany damaga or a loss o* any kind arising from or connactad with this inspection. Date . Stgnad. J9- -Commrssiont , _©_

NM < Bm*« Sui* < ® CERTIFICATE OF FIELD ASSEMBLY COMPLIAHCE

We cartify that the f >aid assembly construction of all parts of this vassal conform vviih the r«Qu if emants of Section VIM. Division 1 of tha ASME Boiler

and Pressure vassal Code

"U" Certificate of Authorization No

Data Co name , ®

. Signed . .@

ffi CERTIFICATE OF FIELD ASSEMBLY INSPECTION

the undersigned, holding a valid commission issued by the National Board ot Boiler and Pressure Vessel inspectors and/or tha State or Province of

_and employed by.

.have compared the statements <n this Manufacturer's Data Report

With the described pressure vessel and state that parts referred to as data items not included in the

certificate of shop inspection, have been inspected by me and that, to the best of my knowledge and belief, tha Manufacturer has constructed and assem

bled this pressure vessel m accordance with ASME Code. Section VMf. Division 1 The described vessel was inspected and subtacied to a hydrostatic test

of _osi. 8y signing this certtdcata neither the inspector nor his employer makes any warranty, aspressad or implied, cortcernmg the pressure

vessel described m rhis Manufacturer s Data Report Furthermore, neither the inspector nor his employer shall be liable m any manner for any personal

iniury or property damage or a loss of any kindarismg from or connected with thu inspection

Data Signed VSy r ftfT^mi««inn« — ®line' wf**OH»^r»\- %imt ''O

58
APPENDIX B - REPORT CONTENT FOR API STANDARD 620 LOW PRESSURE STORAGE TANK

RECOMMENDED SCOPE FOR MANUFACTURER'S REPORT (See Par. 5.27)

It is not the intent to set down rigid rules for the preparation of the manufacturer's report, inasmuch as the extent of the information which it contains, with

the accompanying supplementary sketches, graphs of

tests, and possibly special items wanted by the purchaser

as shown on purchase orders, cannot possibly be listed here.Although it is recommended that there be a certifitificate for each tank supplied, this is intended for simplification in keeping the records of futtire inspection in separate files for convenience. When a group of tanks

is being constructed on one order and in one general

location, some specific form of reporting other than a manufacturer's report may be preferred by both parties.

It would seem desirable that the details on each

contract be settled when the purchase order is placed,

if not in the proposal then as information given in the inquiry.

When parts of the structure are shop assemblies which are stress-relieved, as called for in Par. 3 25 and

4.18, the plans should so indicate in the customary

general notes given thereon. When more than minor repairs or changes and /or

additions are made to the structure in the field for any

reason, it is assumed that both the manufacturer and

the purchaser will want to have a record thereof at- tached to the manufacturer's report. A suggested wording for certification is:

WE CERTIFY, that the design, materials, construction, and workmanship on this low-pressure tank conform

to the requirements of API Standard 620: Recommended Rules for Design and Construction of Large, Welded, LawPressure Storage Tanks.

Date 19 Signed by

( Manufacturtr

)

I have inspected the lank described in this manufacturer's report dated , and state that to the

best of my knowledge the manufacturer has constructed this tank in accordance with the applicable sections of API

Standard 620. The tank was insftected and subjected to a test of psig.

Date 19 Inspector

5.13 DATA REQUIRED FROM MANUFACTURER

ON COMPLETED TANKS

If specified in the purchase order, the manufacturer

shall supply marked copies of plans (or a separate

sketch) showing the location of all plates, with means

of identifying each plate with the heat numbers, which

markings shall be checked by the inspector. A copy

shall be attached to the manufacturer's report

5.27 MANUFACTURER'S REPORT AND

CERTIFICATE

5J7.1

The manufacturer, upon completion of all tests and

inspections on each tank, shall prepare a report summarizing all the data on the tank, iiKluding foundations

if provided by him, and shall attach to the report all drawings and charts, as required by other paragraphs

in this section of the rules (see Par. S.13).

5^7.2

The manufacturer shaU furnish and fill out a certifi- cate for each tank, attesting that the tank has been constructed to these rules (see Appendix M). This certificate shall be signed by the manufacturer and the purchaser's inspector. This certificate, together with the

official symbol placed on the tank, shall be a guarantee

by the manufacturer that he has complied with all applicable requirements of these rules.

5.27.3

If the purchaser so requests, the manufacturer shall attach to the report copies of the records of the qualifi- cation test of welding procedures, of welders, and/or

of welding operalore (tee Par. 4.07 and 4.08).

59
APPENDIX C - NACE AMINE CRACKING SURVEY QUESTIONNAIRE

CODE NO

(assigned by NACZ Headquarters)

It Is recognized that in many locations several amine streams share a common

regenerator. Fill out the questionnaire for each absorber/contactor and

indicate the relationship with the regenerator.

I PROCESS

Type of plant: ammonia plant chemical plant

refinery field production gas plant

Startup date Unit design circulation rate

(U.S. gpm)

Source of acid gas stream (i.e. what unlt(s) does amine plant service)

Is feed stream: liquid gas

Types of Amine Used Range of Acid Gas Loading Acid Gas in Feed

(dates) Cone (X) Mole Gas/Mole Aailne (Vol t)

Type From (dates) To Lean Rich H^S

Is a reclaimer used? Yes No

(Reclaimer duty X of regenerator feed)

Reboller: amine outlet temp CP)

heat medium temp CF)

Quality of circulating amine: frequency of testing

location of sample: rich lean

typical values: Iron (ppm)

CI (ppm)

cyanide

TDS (ppm)

Other

degradation products/heat stable aalts (specify units)

Quality of reflux water: Is it totally refluxed? yes no

Are filters used on amine stream? Tes No TypeTs) of filter

used?

Additives to amine*: soda ash

caustic soda: fresh spent comb

injection point

how is caustic level controlled?

Types of Inhibitors Used Manufacturer/ Range of

Type From (dates) To Brand Name Cone (ppm) Injection Point

•Please give as complete a history as possible, including additives used in the

past. Please attach additional sheets of explanation if necessary.

This Is general information necessary to gain data for .both cracked and

non-cracked equipment.

60
APPENDIX C - NACE AMINE CRACKING SURVEY QUESTIONNAIRE (Con't.)

II EQUIPMENT (carbon steel)

Complete the following table for stress relief CSR) history (use the

following abbreviations for SR: F - at fabrication, L - Late, N - No).

If Clad

What wnj TYPE

Equipment Mat'l butt socket seal ext attacheent

Absorber /Contactor

Regenerator |

I 1 ~|

Piping* Rich ZZIZIIZIZIZZI

Lean

Storage Tanks " Letn/Rlch

Exchanger Shell

Reboiler Shell

Other Vessels

Valves

I Pumps ]

What is the maxloua operating temp (*Fl absorber/contactor

•nd pre$> (p*fg) for: [ 1 ( ) regenerator

[ 1 ( ) storage tank

( 1 ( )

*If using stainless piping, specify location and reason for use, (example: from regenerator to condenser because of corrosion).

Ill INSPECTION

Absorber/ Piping Rich/Lean

Contactor Regenerator Rich Lean Tanks Exchanger

Years of amine

service at last

inspection

Inspection Method^

2 Surface Preparation

If A£ was used, what was the method? standard trlangulation

special .

Have inspection methods been modified over life of piping/equipment?

No Yes If yes, in what way and why?

(1) Preface for external or internal inspection with small e or 1 and

use the following abbreviations: VT - Visual Testing;

UT(S) ultrasonic shear wave; UT(L) " ultrasonic longitudinal wave; MT(B) - dry magnetic part; MT(W) - wet magnetic part; WFMT - wet

fluorescent magnetic part; PT dye penetrant; AE acoustic

emission; RT " radiographic testing.

(2) Use following abbreviations: WB wire brush; PB " power brush;

SB sandblast; CC chemical cleaning.

61
APPENDIX C - NACE AMINE CRACKING SURVEY QUESTIONNAIRE (Con't.)

IV CLEAh'IN'G TECHNIQUES AT StfUTDOWN

Is a water wash used before steanout? Yes No

Has this plant been steamed out without a water wash? Yes No

If yes, how many times?

Is any chemical cleaning used? Yes No Type:

During routine shutdown procedures do you transfer hot amine to tankage?

Yes No

If yes, does amine transfer through lines or to tanks where you've

reported cracking? Yes No

VI CRACKING HISTORY*

Have cracks been detected? Yes No

Cause/method of crack detection: Leakage

Inspection: on stream turnaround

Inspection: VT UTCST" UT(L)

mt(b7" mkwT' wfmt

PT RT A£

Location of cracking (if conveolenc use back for sketch).

-type of equipment age of equipment

-if cracks are in piping, specify location

-type of weld (e.g. internal attachment, opposite external weld, shell

vertical, etc)

-was weld: shop field repair

-was crack transverse or parallel to weld?

-was crack associated with weld defects? Yes No Not known

If yes, please describe

-was crack location stress relieved? Yea No Time 4 temp if knowa

-what inspection techniques were used at fabrication at crack location?

Hardness at crack location Method

Has metallography been performed? Yes No ; If yes, were

cracks: (check as many as applicable)

branched Intergranular transgranular mixed mode

« cile filled type: oxide sulfide other (specify)

Process conditions at crackt normal process temp (*F) max process temp (*F)

pressure (p«lg)

amine - rich

- lean

For multiple occurrences, please attach additional pages of explanation.

Was the failed component exposed to higher temperature amine than the

maxiiiua operating temperatures reported above (especially tankage and

lines during shutting down procedures)? Yes No

Material of construction at crack Thickness (Inches)

Was material lined or clad? Yes No

How many cracks? How deep?

Methods of repair:

Stress relieved after repair? Yes No Method (time and temp if

known)

62
APPENDIX D - INSPECTION PRIORITIES FOR WET H2S CRACKING SURVEY

TABLE 1 — Inspection program for equipment exposed to wet HjS > 50 wppm and cyanides > 20 wppm

Equipment Intpaction

Existing non-PWHT vessels

History of cracking blistering, or HIC

Previous welded repairs/alterations

(witrt^vitnout PWHT)

No prior welded repairs 'alterations

Existing PWHT vessels

History of cracking

History ot Dlistering or HIC.

Previous welded repairs/alterations (witn/witnout PWHT)

WFMT 100°o o( shell and head welds, internal attachment welds, and weld repairs/alletaiions at next scheduled turnaround

WFMT, I00°o ol weld repairs and alterations, and spot exammauon

ot other welds at the next scheduled turnaround WFMT on a spot basis ol shell and head welds in selected vessels

at next turnaround

WFMT 100% ol shell and head welds, internal attachment welds, and weld repairs/alterations at next scheduled turnaround.

WFMT, I00°.o ol weld repairs and alterations, and spot examination

ot other welds at next scheduled turnaround WFMT I00°o ot weld repairs and alterations at next scheduled

turnaround

No prior welded repairs/alterations No special inspection required

TABLE 2 — Inspection program for equipment exposed to wet H^S > 50 wppm

Equipment Inspection

Existing non-PWHT vessels

History ol cracking, blistering, or HIC.

Previous welded repairs/alterations (with/withoul PWHT)

No prior welded repairs.'alterations

Existing PWHT vessels

History ol cracking

History ol blistering or HIC,

Previous welded repairs/alterations {wilh/without PWHT)

WFMT. lOO°/s ol shell and head welds, internal attachment welds, and weld repairs/alterations at next scheduled turnaround WFMT I00°o ot weld repairs and alterations ot selected vessels

at next scheduled turnctround.

No special inspection required.

WFMT. IOC'S of shell and head welds, internal attachment welds, and weld repairs/aiterations at next scheduled turnaround

WFMT. 100% of weld repairs and alterations, and spot examination

ol other welds at next scheduled turnaround

WFMT. 100°» of weld repairs and alterations of selected vessels

at next scheduled turnaround

No prior welded repairs/alterations No special inspection required

63
APPENDIX E - NATIONAL BOARD'S INSPECTION REPORT FORM

FORM NB-7 PRESSURE VESSELS

REPORT OF INSPECTION Sundw4 Pom lor Jwltdtctlon* Oparating Under th* ASME Cod*

10 >MHC AMD TTTU Of »tmOH TO WfHOM nCQUMSWCNT) WCHC UAJklNCD

I HEWtY CCKTffY T>«t IS * rnjt NCMSm Of MV mSnCTION

IMMATUMt or MVfCTOn lOCNT NO Cim.OVCD IT I'll I I I I ym Hnm mm X utMind Irom Tm H«Won«l loK ol »o»«r wd Pr»nui» v««««< lii«p«cwii. 10M Cwi»» A»«^ CoTt.. qm *un

64
APPENDIX F - API 510 REPORT FORM FOR INSPECTION RESULTS

APPENDIX B—EXAMPLE OF

INFORMATION FOR PRESSURE

VESSEL INSPECTION RECORD

POPMO/CTE

FORM NUMBER _

O^ER OR USER

.

VESSEL NAME

oescmmoN

NAME OF PROCESS

.

LOCATION

INTERNAL DIAMETER

TANGENT LENQTH/HEIQHT

SHELL MATERIAL SPECIFICATION

.

HIAO MATIRIAk tPtClFICAnON _ INTERNAL MATERIALS

OWNER OR USER NUMBER

.

AIRISOlCnON/NB NUMBER

MANUFACTURER

MANUFACTURER'S SERIAL NO . DATE OF MANUFACTURE

CONTRACTOR

ORAWINa NUMBERS

.

NOMINAL SHELL^THICKNESS

.

NOMINAL HEAD THICKNESS _ DESIGN TEMPERATURE

MAXIMUM ALLOWABLE WORKING

PRESSURE

DESIGN COOE

JOINT EFFICIENCY

.

TYPE HEADS

MAXIMUM HYDROTESTED PRESSURE

.

DESIGN PRESSURE

REUEF VALVE SET PRESSURE . CONTENTS

TYPE JOINT

FLANGE CLASS

COUPUNG CLASS

NUMBER OF MANWAYS

.

SPECIAL CONDITIONS

.

SKETCH OR

LOCATION

DESCRIPTION

THICKNESS MEASUREMENTS

LOCATION

NUMBER

ORIGINAL

THICKNESS

REQ. MINIMUM DATE

THICKNESS

COMMENTS Method

mspociof

Um to-" on«i shMts. as n«c«ssary O65
APPENDIX G - INSTITUTE OF PETROLEUM CODE FREQUENCY OF PERIODIC INSPECTION

Tabu a CI»M B N'on-Srttutory {UK only)

Insptction Ptriod (months)

EqtapmtM Gradt 0 Gradt I Gradt 11 Gradt III Rtvitw

Procen Prcuun VeucU and

Proces* Vteuum VeueU

24 36 7a 108 7a

Prettura Storage Vetacb 60 60 90 120 90

Heit Exchanger* J4 36 7a 108 7a

Protective Safety Device* 14 36 60

3.4 Inspection Grmding Allocation

for CUm B Equipment

3.4.1 Inapection Grade o

All equipment shall b« deemed to be in Grade o and ahall remain in this Grade

until a first thorough inspection it carried out, except a* permitted in sections 4.5, 4.6 and 4.7.

3.4.3 Inspection Grade I Equipment should be allocated to this Grade when the conditions of service arr such that:

(a) Deterioration in whole or in part is possible at a relatively rapid rate, or

(i) There is little evidence or knowledge of operational effects on

which to predict behaviour in service.

3.4.3 Itupection Grade U

Equipment should be allocated to this Grade when the conditions of service are such that:

(a) Deterioration in whole or in part hai been shown to be at a reasonable and predictable rate consistent with

the increased inspection interval given for the item under this Grade,

or

(6) Evidence or knowledge of actual, behaviour in service is sufficiently

reliable to justify the inspection

inter\-al permitted by this Grade.

The intervals recommended in Table

2 are maxima. Intervals less than those

allocated to Grade II but in excess of

those allocated to Grade I may be

stipulated if more appropriate to the

conditions.

3.4.4 Inspection Grade m

Equipment may b« allocated to this Grade, when the item has successfully concluded a period of service in Grade II and service conditions are such that:

(a) Deterioration in whole or in part has been shown to be at a low and

predictable rate consistent with the

increased inspection interval given

for the item in this Grade, or

(A) Evidence and knowledge of actual

service conditions are sufficiently accurate and reliable that an

increased inter\-al is justified. The intervals recommended in Table

2 are maxima. Intervals less than the maxima for Grade III but in excess of

those for Grade II may be stipulated

where appropriate to the conditions. Other fartors to be considered in the choice of Grading are detailed in sections

4.7.2, 5.2 and 6.2.

3.4.5 Impection Review

Equipment shall be subject to an

Inspection Review when:

(a) Registered items are allocated to Grade III inspection intervals. This

is so that a reassessment may be made of the factors which led to a Grade III allocation being made

and whether any changes have

occurred since the last thorough

inspection which may lead to a

possible shortening of the interval which may be allowed to elapse to the next thorough inspection, (see sections 4.2.3 and 5.7.), (b) Significant changes take place in the conditions of service of any

registered items in any Grading

allocation which would affect its deterioration in whole or part, and

(f) Following an abnormal Jicident which has or could have affected the safety of operation of the equipment.

66
APPENDIX H - NATIONAL BOARD REPORT FORM FOR WELD REPAIR OR ALTERATION

FORM R-1, REPORT OF WELOEO REPAIR OR Z ALTERATION

as rvqulred by th« prorltlont of m« NtlkMiat Soard ln»p«ctlon Cod*

t Wof* c>«fiomi«d by

2 0«n«r

3 Locanon of mtiailatton

4 Unit '(Mnilftc«iK>n

5 idanlifving not.

8 Oescnotton o* wort

CERTIFICATE OF COMPLIANCI

Th« undariignad ctrtittM tn«( in* slit»frt«ni9 m«)« m ims 'toofi ife corrvct ana lhal iii dMign mai«fiii consrruction «na worhmarsnio

on thps conform lo tn« Na(ion«l Bo*ra insp^ciion Cod*

C«nillc«ii ot AutnoriulKwi no.

Dit* 19 _

lymboi •ID"'**

CERTIFICATE OF INtPCCTION

Tr>« undersigned. hokJlng • v«ik] Commiuion issued by TH« Ninonai aovd of 8oii«f and Pr^ttuf Vmmi ln»o*clors M>a c*rt<fic«i« ol conv

oetency issued by ihe stsi* Of orovtnce ot •nd empwyed by

ol _ r>as ins[)ecied trte worn described m ihis dais rtoort on . _ and itaie ihai lo

ir>e best ol my Knowledge and beiiel inis •kx* t\M» oeen done m accordance with ine National Board msoecnon Code

8r Signing mig cenificaia neiirvcr ihe urtoersigned nor my amoioyer mahes any Marraniy aioresMd or imoHed concerning ih« wor%

described m inis report Funhermore neitner the understgned nor my emoioyer shall oe nabte m any manrter 'or any oersoriei mtury

orooerty demege or k>«s ol any hind aristng Irom or connected with tnis inspection, eiceo* sucn naetllty n mmy Oe oro«>0ed m Doocy ol in- sura/Ka wnicn ine undersigned s insurance company may luue upon aeid obieci arxj then only m accordance wnn ir^ lerms ol said poncy

0«te_ _ Signed _ _ComrT>iBSK>ns_

TM« (erm maf be otttahMtf from The Mettotul ftoert ol totter and ^reaaura VcMal inapartofB. lOU CruppM A«*.. CotuffAua. OH 4323» -m •

67
a
NIST-114A

(REV. 3-89)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NIST/SP-780

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBLICATION DATE

4. TITLE AND SUBTITLE

Guidelines for Pressure Vessel Safety Assessment

5. AUTHOR(S)

Sumio Yukawa

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Occupational Safety and Health Administration

U.S. Department of Labor

Washington, D.C. 20234

10. SUPPLEMENTARY NOTES

DOCUMENT DESCRIBES A COMPUTER PROGRAM; SF-185, F1PS SOFTWARE SUMMARY, IS ATTACHED.

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

This document presents a technical overview and information on metallic pressure containment

vessels and tanks. The intent of the document is to provide OSHA (Occupational Safety and

Health Administration) personnel and other persons with information to assist in the evaluatior

of the safety of operating pressure vessels and low-pressure storage tanks. The scope is

limited to general industrial application vessels and tanks constructed of carbon or low alloy

steels and used at temperatures between -75 and 315 °C (-100 and 600 °F) . Information on design

codes, materials, fabrication processes, inspection and testing applicable to these vessels

and tanks are presented. The majority of these vessels and tanks are made to the rules and

requirements of ASME Code Section VIII or API Standard 620. The causes of deterioration and

damage in operation are described and methods and capabilites of detecting serious damage and

cracking are discussed. Service experience in several applications where 30 to 50% incidence

of cracking has been found is described. Guidelines and recommendations formulated by various

groups to inspect for the damages being found and to mitigate the causes and effects of the

problems are presented, A summary of the needed or useful information for the various

factors and items involved in the safety of these vessels and tanks is included to assist in

deciding whether further technical evaluation of safety concerns is required.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

API Standards; ASME Code; design; failure; guidelines; in s e r vice examination ; nondestructive

testing; pressure vessels; reliability; safety; service experience; steel.

13. AVAILABIUTY

UNLIMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

XXX

XXX

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

75

15. PRICE

ELECTRONIC FORM





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