Reference A 1 1 1 2 51b75b NBS

PUBLICATIONS

NATL INST OF STANDARDS & TECH R.I.C. MRCIP ftfi.QnAQ All 102516756

IN DOI M UU OUHv? McHenry, Harry l/Examlnatlon of a pressu QC100 U56 NO. 86-3049 1986 V19 C.1 NBS-P EXAMINATION OF A PRESSURE

VESSEL THAT RUPTURED AT THE

CHICAGO REFINERY OF THE

UNION OIL COMPANY ON

JULY 23, 1984

Harry I. McHenry

T. Robert Shives

David T. Read

J. David McColskey

Charles H. Brady

Patrick T. Purtscher

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

March 1986

Sponsored by

Occupational Safety and Health Administration

U.S. Department of Labor

Washington, DC 20210

QC

100

. U56

86-3049

1986

NBSIR 86-3049

MBS

RESEARCH

INFORMATION CENTER

EXAMINATION OF A PRESSURE

VESSEL THAT RUPTURED AT THE

CHICAGO REFINERY OF THE

UNION OIL COMPANY ON

JULY 23, 1984

Harry I. McHenry

T. Robert Shives

David T. Read

J. David McColskey

Charles H. Brady

Patrick T. Purtscher

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Bureau of Standards

Boulder, Colorado 80303

March 1986

Sponsored by

Occupational Safety and Health Administration

U.S. Department of Labor

Washington, DC 20210

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

This failure investigation was conducted by an interdisciplinary team of

NBS personnel. Responsibilities for the major areas of investigation were

as follows:

H. I

.

McHenry Principal Investigator

T. R. Sh ives Gaithersburg Coordinator

U. Bertocc

i

Corrosion

C. H

.

Brady Metallography

J. D. Me Co Is key Testing

R. Mar inenko Microchemical Analysis

J. A. Norris Chemical Analysis

P . T . Purtscher Fractography

D. T. Read Fracture Mechanics

R . E. Schramm Inspect ion

T. A. Siewert We Id ing

Many others made important contributions to this investigation,

including E. Escalante, R. J. Fields, D. E. Harne, M. J. Kaufman,

R. D. Kriz, N. Sanchez, B. A. Stevenson, A. C. Van Orden, D. P. Vigliotti

and R. 0. Voth. The field investigation conducted in the week following the

incident was performed by B. W. Christ and R. Shull.

The authors acknowledge the critical reviews provided by E. Passaglia,

E. N. Pugh, and M. B. Kasen.

The investigation was conducted under the direction of K. J. Yotz, the

Project Officer for OSHA, and R. P. Reed, the Project Officer for NBS.

NOTE: An initial investigation was performed by a private company of

engineering consultants. This company is referred to by name in the text of

this report in order to state clearly the relationship between the initial

investigation and the subsequent one conducted by NBS. This does not imply

any endorsement or criticism by NBS.

lil

CONTENTS

EOHEWORD i

LIST OF FIGURES v LIST OF TABLES ix

LIST OF ACRONYMS x

EXECUTIVE SUMMARY 1 Materials Evaluation 2

Metallurgical Studies 2

Fracture Mechanics Analysis 3

Causes of the Failure 4

INTRODUCTION 5

PRESSURE VESSEL 12D701 7

Description of the Vessel 7 History of the Vessel 9

Failure of the Vessel 10

Summary Comments 11

PRESSURE VESSEL SEGMENTS 13

Magnetic Particle Inspection 19

Ultrasonic Inspection 21 Dimensional Checks 22

Summary Comments 26

MATERIALS EVALUATION 27

Chemical Analysis 27

Tensile Properties 27

Charpy Impact Toughness 29

Fracture Toughness 33

Rockwell Hardness 36

Metallography and Microhardness 36

Summary Comments 49

v
CORROSION AND HYDROGEN CRACKING TESTS 51

Hydrogen Pressure Cracking 52

Hydrogen-Induced Cold Cracking 55

Stress Corrosion Cracking 59

Galvanic Corrosion 61 Fracture Mechanics Test for Stress Corrosion Cracking 65

Chemical Analysis of Corrosion Products 67

Summary Comments 72

FRACTURE SURFACE OBSERVATIONS 73

Information Received from the Fire Marshal's Investigation 73

Visual Observations 74

Metallography 80

Fractography 84

Microchemical Analysis of the Black Deposits 89

Summary Comments 92

FRACTURE MECHANICS ANALYSIS 93

Fracture Toughness Tests in an MEA/^S Solution 94

Fracture Toughness Tests on Hydrogen-Charged Specimens 95

Summary Comments 101

CONCLUSIONS 103

REFERENCES 105

APPENDIX A - FIELD INVESTIGATION 107

APPENDIX B - MAGNETIC PARTICLE INSPECTION 139

APPENDIX C - ULTRASONIC INSPECTION FOR DELAMINATION 159

APPENDIX D - ROCKWELL HARDNESS 173

APPENDIX E - y-GROOVE CRACKING TEST 185

APPENDIX F - REPORT OF ANALYSIS BY THE ELECTRON PROBE MICROANALYZER

OF FRA.CTURE SURFACE SPECIMENS 195

vi
LIST OF FIGURES

Figure 1. Schematic of pressure vessel 12D701 8

Figure 2. Locations of the pieces of pressure vessel 12D701

investigated by NBS: Rings 1 , 2, and 3 and

Plates 1 OA and 1 OB 14

Figure 3. Rings 1, 2, and 3 showing locations of all cutouts 15

Figure 4. Specimen locations in plates cut from Ring 1 18

Figure 5. Specimen locations in plates cut from Ring 3 20

Figure 6. Summary of magnetic particle inspection results 23

Figure 7 . Wall thickness measurements near the fracture surface 25

Figure 8. Charpy specimens and their notch locations 31

Figure 9. Single-edge-notch-bend specimens for fracture

toughness testing 34

Figure 10. Notches in fracture toughness specimens 35

Figure 11 . Macrostructure of Weld 1/2 at CC 1.5 m 39

Figure 12. Microstructure near crack B in HAZ 1 shown in Figure 11 39

Figure 13- Macrostructure of Weld 2/3 at CC 1 .8 m and microstructure of HAZ 3 40

Figure 14. Microhardness of HAZ 3 in Weld 2/3 at CC 1 .8 m

at the location shown in Figure 13b 42

Figure 15. Macrostructure and microstructure of vertical

weld A in Course 2 43

Figure 16. Microhardness of vertical Weld A and its HAZ in Course 2

at location B shown in Figure 15a. Etchant: 2 % nital 44

Figure 17. Microstructure of Weld 1/2 at CC 1 .0 m and base metal

cracks on the inner surface of Course 2 45

Figure 18. Microhardness of the HAZ of the Monel-to-steel weld

at the location shown in Figure 17a 4?

Figure 19. Hydrogen pressure cracking in Course 1 at CC 3.9 m 43

Figure 20. Specimen for measuring susceptibility to

hydrogen pressure cracking c

.

vii
Figure 21 . Blistering on surface of specimens after

hydrogen pressure cracking tests 54

Figure 22. Metallography of cracking in specimens after hydrogen

pressure cracking tests 57

Figure 23- Specimen for y-groove weldability test 58

Figure 24. Cross section of y-groove test specimen of

Course 1 steel welded to Course 2 steel showing

cracking in HAZ 1 60

Figure 25. Test apparatus for galvanic corrosion studies 63

Figure 26. Specimen for fracture mechanics test of

stress corrosion cracking susceptibility 66

Figure 27. Fracture surfaces of specimens after completion

of fracture mechanics tests of stress corrosion

cracking susceptibility. Arrows point to

fatigue precracks 68

Figure 28. Sketch of fracture surface showing pre-existing

cracks (shaded areas), fracture path, and direction

of fracture propagation 76

Figure 29. Section of fracture removed for metallographic

studies with arrows showing cracks on inside

surface of HAZ 2 78

Figure 30. Piece cut from Ring 1 where the fracture passed

through Course 2 79

Figure 31- Metallographic sections through the fracture

surface at the location of the initiation crack 81

Figure 32. Microstructure of Weld 2/3 at CC 7.6 m, metallography of cracks in HAZ 2 and HAZ 3»

and microhardness of HAZ 3 82

Figure 33- Macrostructure of Weld 2/3 at CC 5.9 m and

microhardness values of HAZ 2 and HAZ 3 83

Figure 34. Macrostructures at two locations where a black deposit was not found on the fracture surface 85

Figure 35. FMI [1] photographs of the macrostructure

of Weld 1/2 and fractograph at CC 5.2 m. The letters show corresponding areas 87

Figure 36. FMI [1] photographs of the macrostructure of

Weld 1/2 and fractograph at CC 7-3 m. The letters

show corresponding areas 88

vlii
Figure 37. Fractographs of the crack in HAZ 2 (shown in Figure 32) of

Weld 2/3 at CC 7.6 m 90

Figure 38. SEM fractographs of hydrogen pressure cracks in Course 1 91

Figure 39. CTOD vs. crack extension, Aa, curves for the Course 1 side of Weld 1/2 after hydrogen charging. Tested

at 38°C (100°F) 100

Figure Cl. Ferritic steel plate with machined slots used for

procedural check 162

Figure C2. Oscilloscope photographs of measurements made

on reference plate in Figure Cl 163

Figure C3. Samples of typical oscilloscope traces 166

Figure C^4 . Scan of Specimen 1 (Ring 1, Course 1) 168

Figure C5. Scan of Specimen 2 (Ring 3, Courses 2 and 3) 169

Figure C6. Scan of Specimen 3 (Ring 3, Courses 2 and 3) 170

Figure C7 . Results of the ultrasonic inspection for

delaminations near the fracture surfaces in Rings 1 and 2 ....171

Figure D1 . Hardness traverses of the weld between

Course 1 and Course 2 175

Figure D2. Hardness traverses of the weld between

Course 2 and Course 3 176

Figure D3. Hardness traverses of the weld between

Course 3 and Course 4 177

Figure D4 . Hardness traverses of the weld between

Course 4 and Course 5 178

Figure D5. Hardness traverses across vertical

weld A in Course 2 179

Figure D6 . Hardness traverses across vertical

weld B in Course 2 1 8

0

Figure D7 . Hardness traverses across vertical weld C in Course 2 1 8

1

Figure D8. Hardness traverses across the vertical weld in Course 1 182

Figure D9 . Hardness traverses across the vertical weld in Course 3 1 8

3

Figure DIO. Hardness traverses across the vertical weld in Course 5 134

IX
Figure El. Plate and groove dimensions for the

y-groove cracking test 188

Figure E2. Schematic of the weld progression in the center section 188

Figure E3. Typical cross section of the weld at the center

(test) section of Specimen 3 193

Figure E4 . Typical crack along the HAZ in the Plate 1 side

of the groove, extending 20% of the leg length

from the weld root 194

Figure FI. Examples of the EDS spectra 199

Figure F2. WDS maps of specimen 25~3/8C2 201

Figure F3. WDS maps of specimen 25~3/8C2 202

Figure F4. WDS maps of specimen 25~3/8C20 203

Figure F5. Energy dispersive spectra of the pitted area of

specimen 25~3/8C20 204

x
LIST OF TABLES

Table I. Chemical compositions (weight percent) of the

steel plates used for Courses 1, 2, and 3 and

of Welds 1/2, 2/3 and the vertical weld in

Course 2 28

II. Tensile properties of Courses 1, 2, and 3 and Weld 1/2 30

Table III. Charpy V-notch impact toughness of Courses 1 and 2, Weld 1/2, and HAZ 2 of Weld 2/3 32

Table IV. Fracture toughness test results 37

Table V. Results of hydrogen-pressure cracking tests 56

Table VI. Slow strain rate test results 62

Table VII. Results of the galvanic corrosion tests 64

Table VIII. Results of the fracture mechanics test for

stress corrosion cracking 69

Table IX. Chemical composition (weight precent) of scale

found behind the Monel liner 71

Table X. Fracture toughness test results for specimens

tested at 38°C (100°F) in an MEA/P^S solution 96

Table XI. Fracture toughness test results for hydrogencharged specimens tested in air 99

Table AI. Design information for pressure vessel 1 2D701 120

Table All. Plate thicknesses according to code and

construction drawing 125

Table BI. Magnetic particle inspection of Ring 1, Course 1 141

Table BII. Magnetic particle inspection of Ring 2, Course 2 147

Table Bill. Magnetic particle of Ring 3, Course 2/3 153

Table ci. Graphical symbols used for ultrasonic measurements 165

Table El. Results for the y-groove cracking tests 192

Table FI. EDS qualitative analysis of rough specimens from

fracture surface 198

xi
LIST OF ACRONYMS

ASME American Society of Mechanical Engineers

ASTM American Society for Testing and Materials

AWS American Welding Society

BS British Standard

CC c ircumferent ial coordinate

CTOD crack tip opening displacement

EDS energy dispersive spectrometer

FMI Fire Marshal's investigation

HAZ heat-affected zone

HK hardness, Knoop

HRA hardness, Rockwell A HRB hardness, Rockwell B MEA monoethanol amine

MPI magnetic particle inspection

NACE National Association of Corrosion Engineers

NBS National Bureau of Standards

OSHA Occupational Safety and Health Administration

SEM scanning electron microscope

SENB s ingle -edge -notch bend

SRM standard reference material

TL specimen orientation such that loading is normal to the rolling

direction and crack propagation is parallel to the rolling

d irect ion

WDS wavelength dispersive spectrometer

xii
EXECUTIVE SUMMARY

Ir. July 23, '984, a pressure vessel ruptured at the Chicago Refinery of

one Cm i : r. Oil Company and caused an explosion and fire that killed seventeen

people and resulted in extensive property damage . The Occupational Safety

ini Health Administration (OSHA) requested the National Bureau of Standards

MBS to conduct an investigation into the cause of the failure. This report

summarizes the results of these studies and identifies the probable causes of

the failure.

The ruptured pressure vessel was an amine absorber tower used to strip

hydrogen sulfide from a process stream of propane and butane. It was a '8.8 m ( 6 1 . 5 ft.) high, 2.6 m (8.5 ft.) diameter tank made of 25 rm (1.0 in.)

thick plates of type ASTM A51 6 Grade 70 steel. It was constructed in

accordance with the ASME Boiler and Pressure Vessel Code by welding six

cylindrical sections together between two ellipsoidal ends. The cylindrical

sections, called courses, were designate< Courses f 7'’“ ^ ~ tn U o— ^ J - - w— bottom to top. The vessel was built In 1968 and put Into service in 97 C

;

thereafter it was shutdown and inspected at two year intervals.

In '974, Course 2 was replaced because extensive hydrogen blister

cracking was discovered in the original plate. It was replaced on the site

with a new plate of the sane grade of steel, which was welded in place by th«

shielded metal arc process. At the time of this reoair, hydrogen blister

cracking was also detected in the adjacent plate, Course '

. During the next

scheduled shutdown in 1976, a Monel liner was installed on the bottom cf the

vessel, including most of Course 1 , to correct the problem. Ho further

modifications were made in the subsequent biennial inspections.

About fifteen minutes prior to the explosion, a unit operator was notified of a leak in the wall of the pressure vessel. The operator, while

attempting to close the hydrocarbon inlet valve, noticed that the leak rad

increased in size. He ran for cover and was knocked over by a vapor blast

just before the explosion. The pressure vessel ruptured into two main

pieces. At the time of the rupture, the upper '4 m -5 ft. section cf tee

vessel was propelled nearly 1 km '0.6 mi. away. The lower portion cf tr vessel remained in its original position at the refinery. The fr = *

.

• was primarily in Course 1, adjacent to the field weld joining Course 1 t

Course 2 (the replaced section).

1
After the incident, the Fire Marshal of the State of Illinois impounded

the vessel pieces. Three ring sections and two plates were cut from the

vessel for a failure investigation conducted by Packer Engineering

Associates, for the Fire Marshal. The ring sections, each 0.9 m (3 ft.)

high, included sections containing the mating fracture surfaces, i.e., the

upper portion of Course 1 and the lower portion of Course 2, and a section

centered on the repair weld joining Courses 2 and 3- Several small pieces

were removed from these sections in the Fire Marshal's investigation and were

not available to NBS . The main part (over 99$ by weight) of the ring

sections and test plates were provided to NBS. The present investigation

consisted of an evaluation of the vessel materials, metallurgical studies,

and a fracture mechanics analysis.

Materials Evaluation

Tests conducted to evaluate the materials of construction revealed that

the steel plates and the welds met the specified requirements for chemical

composition and tensile properties. Corrosion tests showed that the steel

plates were susceptible to hydrogen pressure cracking in an aqueous solution

(pH = 3) saturated with hydrogen sulfide (H 2 S), but not to stress corrosion

cracking in an amine solution containing H

2 S. The galvanic corrosion

tendencies between the steel and the Monel liner were measured and found to

be insignificant. Fracture property measurements indicated that the welds

were tougher than the plate.

Metallurgical Studies

Various metallurgical techniques were used to examine the fracture

surface, the path of the fracture, and secondary cracks. When NBS received

the vessel segments, the fracture surfaces were covered with rustlike

deposits and evidence regarding the origin of the fracture was not clearly

discernible. The fracture path followed the HAZ on the Course 1 side of the

weld for most of the circumference. The fracture surface did not have either

the chevron patterns character istic of brittle fracture or the shear lips

characteristic of ductile fracture. Under low-power magnification, the

texture of the fracture surface appeared fibrous or "woody."

2
After the fracture surface was cleaned with high-pressure steam, a black

deposit, previously observed and photographed in the Fire Marshal's investigation, could be seen. The black deposit extended from the inner surface to

beyond midthickness over nearly thirty percent of the circumference.

Chemical analysis of the black deposit revealed it to be rich in sulfur.

Since the deposits almost certainly formed prior to fracture, they delineated

cracks that existed in the vessel at the time of rupture.

Metallography, fractography and microhardness measurements revealed that

these pre-existing cracks initiated adjacent to the repair welds in areas of

hard microstructure that are susceptible to hydrogen stress cracking. The

cracks initiated along the inside surface of the vessel at the fusion line

and propagated along a straight path, through the hardened region in the HAZ

toward the base metal. Upon reaching the base metal, crack propagation

continued in a zigzag path. Fractographic examination of typical crack

surfaces revealed that the zigzag cracking was due to hydrogen pressure

cracking. Cracks of this type formed on both sides of the repair welds

joining replacement Course 2 to Courses 1 and 3» In the Course 1 HAZ, one such crack, about 800 mm (31 in.) long, had propagated nearly through the

wall thickness prior to the final rupture. When the crack depth exceeded

90 to 95 % of the wall thickness, the remaining ligament of material tore,

causing a leak that spread the approximate length of the pre-existing crack

and initiated the final fracture. Growth of the crack was facilitated by its

uniform depth over most of its length.

Fracture Mechanics Analysis

A fracture mechanics analysis was conducted to determine if the 800 mm

(31 in.) long crack was sufficient to cause rupture of the vessel at the

operating stress level of 35 MPa (5.1 ksi). The fracture criterion used was

the crack tip opening displacement (CT0D), a measure of the driving force for

fracture. The fracture toughness by this criterion, i.e., the critical CTOD

at which fracture occurs, was measured to be a minimum of 0.17 mm (0.0066 in)

for specimens notched in the HAZs of Courses 2 and 3. The CTOD of the 800 mm

(31 in.) long crack was calculated to be 0.064 mm (0.0025 in.). Thus, by the

CTOD criterion, the crack should have been stable, and sudden rupture should

not have occurred. However, a key feature of the vessel's condition was not

accounted for in this analysis: the steel plates of Courses 1 and 2 had been

3
charged with hydrogen as indicated by the presence of blisters, delaminations, and hydrogen-induced cracking on these plates. Therefore, additional

fracture toughness tests were conducted on steel samples notched in HAZs of

Courses 1, 2, and 3 and charged with hydrogen. The measured CTOD values

ranged from 0.064 to 0.096 mm (0.0025 to 0.0038 in.). The agreement between

these measured values and the calculated CTOD indicates that fracture of the

vessel was possible at a stress level of 35 MPa (5.1 ksi) for an 800 mm

(31 in.) long crack because the toughness of the steel was reduced by

hydrogen embrittlement.

Causes of the Failure

The incident at the Chicago Refinery was caused by the rupture of a pressure vessel. The vessel fractured along a path that was weakened by

extensive cracking adjacent to a repair weld joining a replacement section to

the vessel. These pre-existing cracks initiated in areas of hard microstructure known to be susceptible to hydrogen stress cracking. This hard

microstructure formed during the repair welding of the replacement section,

and at the surface, it was not tempered by subsequent weld passes or thermal

treatment. The cracks grew through the vessel wall due to hydrogen pressure

cracking. When the depth of the largest of these pre-existing cracks

exceeded 90 to 9 5% of the wall thickness, leakage occurred because the thin

ligament of steel remaining in the cracked section ruptured. Because the

depth of the pre-existing crack was relatively uniform, the thin ligament

continued to tear and caused the through crack to grow to a length of about

800 -mm (31 in.). This crack triggered the final fracture at the operating

stress level of 35 MPa (5.1 ksi) because the toughness of the vessel steel

was reduced by hydrogen embrittlement.

4
INTRODUCTION

On July 23, 198*4, there was an explosion followed by a fire at the

Chicago Refinery of the Union Oil Company of California. Seventeen refinery

employees were killed, and property damage was extensive. Eyewitness reports

and refinery records indicate that the substance that exploded was a mixture

of propane and butane that leaked from a ruptured pressure vessel, designated

12D701. This vessel, a 18.8 m (61.5 ft.) high steel cylinder, was an amine

absorber tower used to strip hydrogen sulfide gas (H 2 S) from the propane and

butane process stream. After the fire, the upper 1*4 m ( *46 ft.) section of

the pressure vessel was found approximately 1 km (0.6 mi.) from its original

site. The lower section of the vessel remained attached to its base.

Soon after the incident, the Occupational Safety and Health Administration (OSHA) requested the National Bureau of Standards (NBS) to conduct an investigation into the cause of the pressure vessel failure. A field

investigation was conducted during the week of July 30, 198*4; it is

summarized in Appendix A. At this time, access to the vessel was restricted,

and the investigation was limited to visual observations of the ruptured

vessel and the failure site.

From the time immediately following the failure until August 8, 1985, the

failed pressure vessel was in the custody of the Illinois State Fire Marshal.

During this period, a failure investigation was conducted by Packer

Engineering Associates for the Fire Marshal; hereafter it is referred to as the

Fire Marshal's Investigation (FMI) [1], On the basis of a metallurgical

examination of the vessel, the FMI concluded that the failure was caused by

hydrogen induced cracking in the hardened area adjacent to a repair weld.

This weld was done in the field during the replacement of a segment of the

vessel.

The present investigation began in August 1985 with a brief review of

the information available from OSHA and the FMI regarding pressure vessel

1 2D 70 1 and its failure. On the basis of this review, five major tasks were planned to reveal the probable causes of the failure:

5
1. Examination of the vessel pieces

2. Materials evaluation

3. Corrosion and hydrogen cracking studies

4. Fracture surface observations

5. Fracture mechanics analysis

This report begins with general information about pressure vessel 1 2D 701

and its failure and continues with sections covering the results of the major

tasks and the probable causes of the pressure vessel rupture.

6
PRESSURE VESSEL 12D701

The ruptured pressure vessel was an amine absorber tower designated

12D701. The vessel was part of the unsaturated gas plant (Unit No. 12)

located at the south end of Union Oil's Chicago Refinery in Lemont, Illinois.

In this section information obtained from OSHA regarding the vessel and its

failure is summarized.

Description of the Vessel

Pressure vessel 1 2D 70 1 was a 1 8 . 8 m (61.5 ft.) high, 2.6 m (8.5 ft.)

diameter, cylindrical tank shown schematically in Figure 1. Its six

cylindrical sections are designated Courses 1 through 6. The welds joining

the courses are identified with the same numbers; e.g., the weld joining

Course 1 to Course 2 is Weld 1/2. It was constructed of 25 mm thick steel

plates to the requirements of the ASME Boiler and Pressure Vessel Code,

Section VIII [2]. Its design and operating conditions were:

Design Operating

Pressure 1.6 MPa (230 psi) 1.4 MPa (200 psi)

Temperature 60°C (140°F) 38°C (100°F)

The materials for the shell and heads were:

Steel Plate ASTM A516 Grade 70

Welding Materials ASME SA233 for shielded metal arc welding (SMAW)

ASME SA558 No. EL12 for submerged arc welding (SAW)

ASTM A558 No. F70-xxxxx for the SAW flux.

Preheating and postweld stress relief were not specifically required.

The function of the vessel was to remove H

2 S from the propane/butane

process stream by a counterflow process. A 20$ solution of monoethanol

amine (MEA) in water was used to absorb the H

2 S from the mixture of proparn

and butane. Under the operating conditions of 1.4 MPa (200 psi) and 38°<

(100°F), the mixture was a liquid. The hydrocarbon liquid contaminated wi*

H

2 S entered near the bottom of the vessel and, owing to its lower density,

7
Figure 1. Schematic of pressure vessel 12D701.

8
flowed upward to the outlet at the top of the vessel. The lean MEA entered

the top of the vessel and, as it flowed downward, absorbed the H

2 S from the

Hydrocarbon liquid. Contact between the liquids was increased by a series of

nine perforated trays spaced at 1.5 m (5.0 ft.) intervals. The trays had

open sections that alternated left and right and directed the counterflowing

liquids. The hydrocarbon liquid stripped of H

2 S exited the vessel at the top

and continued to the next processing step. The MEA, rich in H

2 S, exited at

the bottom of the vessel, went through a regenerator that removed the H

2 S,

and then recirculated through the vessel.

History of the Vessel

Pressure vessel 12D701 was built in 1968 and put into service in 1970.

Thereafter it was shut down and inspected for corrosion damage at two-year

intervals.

During the 1972 inspection, hydrogen blisters and de laminations were detected in Course 2, and as a result, Course 2 was replaced during the 1974

shutdown. The new course was made of the same type of steel plate: 25 mm

(1.0 in.) thick, ASTM A516 Grade 70, in the as-rolled condition. The new

course was installed in three segments, each of which was 2.7 m (9.0 ft.)

high (the plate width) and 1/3 of the circumference in length. The segments

were welded in place by the shielded metal arc process. Details of the

repair procedures were not made available to NBS.

During the 1974 inspection, numerous small hydrogen blisters were found

in Course 1. In 1976, more blisters were found and delaminations were detected by ultrasonic inspection. As a remedial action, a Monel liner was

installed during this shutdown. The liner, 2.5 mm (0.10 in.) thick, covered

the bottom head and Course 1 up to the bottom of the first tray [about 250 nr (10 in.) below Weld 1/2], In the region where the tray was open (the dowr.-

comer), the Monel liner extended up to Weld 1/2. The Monel liner was welded to the vessel by the gas metal-arc process with plug welds at the

center width of the Monel strips and seam welds at the edges. Installation

procedures were not made available to NBS.

No modifications or repairs were made to the pressure vessel taring •

1978, 1980, and 1982 shutdowns. In 1984, leaks were found in the Mor. . and they were repaired by welding.

9
Failure of the Vessel

At 5:38 p.m. on Monday, July 23, 198-4 there were ten short blasts from

a whistle at the Chicago Refinery, indicating something was wrong. An

eyewitness report by a unit operator described the events as follows.

A refinery worker observed a leak in pressure vessel 1 2D701 and notified the

operator who was stationed near the vessel. The operator checked the vessel

and saw a vapor plume about 6 m ( 20 ft.) in length coming out of the vessel

near the propane-butane inlet valve. He also walked around the vessel to see

if there were any other leaks, and none was observed. Then he called the

fire department, and presumably, this is when the whistle sounded. The

operator wanted to close the inlet valve, but access to the valve from a ladder mounted on the side of the vessel was blocked by the vapor plume.

Therefore, he put on a breathing apparatus, obtained a ladder, erected it near the inlet valve, climbed the ladder and partially closed the valve.

Then he noticed that the leak had increased. He climbed down the ladder just

as the fire department was arriving. He estimated that the crack was about

0.6 m (2 ft.) long at this time. As he started to run for cover, a vapor

blast knocked him over, and as he was crawling behind a concrete barrier, he saw a yellow flash. Presumably, the vapor blast occurred when the vessel

ruptured, and the yellow flash occurred soon after when the vessel contents

exploded. The explosion occurred at 5:53 p.m., 15 min. after the whistle

sounded.

The explosion was followed by a fire that caused another vessel to

explode at 6:20 p.m., adding more fuel to the fire. The fired continued out

of control until after midnight and was not completely extinguished until

2 p.m. on July 24. Seventeen refinery workers were killed by the explosion

and fire.

10
Summary Comments

The information received at the outset of this investigation

indicated that:

1 . The vessel had a history of hydrogen-induced blisters and

de laminat ions in Courses 1 and 2.

2. The source of hydrogen was the H

2 S in the process stream.

3. The H

2 S tended to collect in the lower portion of the vessel

because of the counterflow process.

4. The fracture was preceded by one leak that grew in size for more

than 15 min.

5. At the time of rupture, the crack was approximately 0.6 m (2 ft.)

in length; it was located between the ladder and the inlet valve at

the approximate level of the valve.

6. The failure of pressure vessel 12D701 caused the explosion and

fire.

11

PRESSURE VESSEL SEGMENTS

The pressure vessel ruptured into two main pieces. At the time of the

rupture, the upper 14 m (46 ft.) section of the pressure vessel was propelled

about 1 km (0.6 mi.) from its original location. Apparently, vaporization of

the liquefied gases in the vessel served as the propellant when the pressure

was released by the rupture. The crash landing flattened the vessel along

most of its length. It evidently landed on its top, which was crushed,

and thus the fracture surface did not strike the earth on impact.

The lower portion of the pressure vessel remained erect and cylindrical

in its original position in the refinery. It was exposed to the fire, but

experienced only limited fire damage. For example, the white paint on the

outside surface remained intact, but it did become powdery.

The fracture path was primarily in Course 1 , adjacent to the field weld

joining Course 1 to Course 2 (the replaced section), as shown in Figure 2.

The failure investigation was conducted on segments removed from the upper

and lower pieces of the vessel as part of the FMI . These consisted of

Rings 1, 2, and 3 (three cylindrical sections) and Plates 1 0A and 1 0B

(two square plates) cut from the vessel at the locations shown in Figure 2.

Samples were removed from each segment and retained as part of the FMI. The

remainder of the main segments were shipped to NBS (Boulder, Colorado) for

this investigation. In addition, some of the pieces removed in the FMI were

sent to NBS and included in this investigation. These are shown as speckled

areas in Figure 3- The locations of the samples not received by NBS are

shown as blank areas in Figure 3.

The circumferential coordinates (CC) given in Figure 3 are used

throughout the report. The zero position is the centerline of the

hydrocarbon inlet piping. When the vessel is viewed from above, the

circumferential coordinates are numbered clockwise with zero at 12 o'clock.

A particular position along the circumference is identified by CC # m, where # m is the coordinate in meters. The reference piece for marking the

coordinates was Ring 1, which remained nearly circular despite the failure.

Position marks were scribed on the outside surface at 305 mm (1 ft.)

13
Figure 2. Locations of the pieces of pressure vessel 1 2D 70 1 investigated by

NBS: Rings 1, 2, and 3 and Plates 1 0A and 10B.

14
Course 3

Ring 3

Course 2

Ring 2

Course 2

Ring 1

Course 1

Figure 3. Rings 1, 2, and 3 showing locations of all cutouts.

Cutout rocolvod by NB8

Cutout not received by NBS

pgj] NB8 cutout

15

intervals. The position marks were transferred to Ring 2, which was severely

deformed. This was done by matching physical features on the mating fracture

surfaces and then assuming that the two rings matched between these reference

points. Finally, the coordinates were transferred from Ring 2 to Ring 3.

Here, the only reference points were vertical welds A, B, and C. The

position marks between these welds were placed at the same intervals as on

Ring 2.

Test plates and specimens for metallurgical examination were taken from

the vessel pieces for testing and evaluation by NBS; their locations are also

shown in Figure 3. The test plates, relatively large pieces, were removed

from Rings 1 and 3 with an oxyacetylene torch. The torch-cut edges were removed by cutting a 75 mm (3 in.) strip from the plate edges with a saw. These plates were the main source of material for test specimens. The

specimens for metallurgical examination were removed with a hand-held jigsaw

and magnetically mounted portable drill press. Use of these tools prevented

local overheating of the samples.

Ring 1 is a 0.9 m (3 ft.) high cylindrical section of predominately

Course 1 steel that is partially lined with Monel. Ring 1 is the uppermost

portion of the part of vessel 12D701 that remained attached to the base, and

thus it includes the lower fracture surface. It was exposed to the fire at

the time of the incident. A test plate, approximately 1.8 m (6 ft.) x 0.5 m

(1.5 ft.) in size, was torch-cut from Ring 1 with the Monel liner in place.

The liner was then removed by grinding off the seam welds that attached the

liner to the vessel. The specimens used to evaluate the properties of

Course 1 material were located in this plate, as shown in Figure A. Between

CC 1.62 m and CC 2.29 m, the fracture deviated from the heat affected zone of

Course 1 (HAZ 1 ) and ran through the Course 2 plate. This section of steel,

also shown in Figure 4, was removed during the FMI and sent to NBS in a separate shipment.

Ring 2 is a 0.9 m (3 ft.) tall cylindrical section of Course 2 material.

It includes the upper fracture surface and most of the repair weld that

joined Courses 1 and 2 (Weld 1/2). Ring 2 is the lowest portion of the part

of vessel that was propelled from the original site and flattened upon

impact; therefore, its fracture surface was not exposed to fire. Numerous

specimens were removed from the fracture surface of Ring 2 and used for

metallurgical studies.

17
3.91 m 4.70 m

Specimen Type Identification

Tensile N4, 04

Fracture Toughness (compact) 66

,

531

Charpy Impact E4, F4, . - 14

Chemical Analysis M4

Slow Strain Rate Tensile K4

,

L4

Hardness A4

Hydrogen Pressure Cracking B4, C4

Galvanic Corrosion J4

Metallography P4

Figure 4. Specimen locations in plates cut from Ring 1.

18
Ring 3 was torch-cut from pressure vessel 12D701 about 0.45 m (1.5 ft.) on either side of the Weld 2/3. Like Ring 2, it was crushed upon impact and

was not exposed to the fire. A 2 m (6.6 ft.) long section was torch cut from

Ring 3 and subsequently saw cut into four test plates. Material from these

test plates was used to evaluate Courses 2 and 3 and Weld 2/3. The specimen

locations are shown in Figure 5. Weld 2/3 is a repair weld that joined the

replacement plate, Course 2, to an original plate, Course 3, and thus it is

similar to the weld that failed.

Two 0.6 m x 0.6 m (2 ft. x 2 ft.) plates were removed from the vessel

above the replacement course. Plate 1 0A includes Courses 4 and 5, Weld 4/5,

and the vertical weld in Course 5. Plate 1 0B includes Courses 3 and 4,

Weld 3/4, and the vertical weld in Course 4.

Magnetic Particle Inspection

The inner surfaces of each pressure vessel ring and Plates 1 0A and 1 0B were subjected to magnetic particle inspection (MPI) in the vicinity of each

weld. A minimum of 100 mm (4 in.) above and below each weld was examined in

all regions where accessibility permitted. The MPI was done in a darkened

room using the wet fluorescent method.

The areas of the vessel subject to MPI were cleaned in several steps:

1 . brushed with a fine-wire brass brush and then a plastic fiber brush

2. washed with 1,1,1 tr icholorethane and allowed to dry

3. sprayed with a chlorinated solvent cleaner

After the Solvent had evaporated from the flaws, magnetic particles

in an oil suspension were sprayed on the areas to be inspected. An ac magnetic yoke with a probe separation adjustable to 150 mm (6 in.) was

used to magnetize the region and cause the particles to collect at

discontinuities. A 1 00 W black light was used to illuminate the fluorescent

particles, which have a yellow-green color. The locations of all isolated

indications of discont inuites and regions with numerous indications were

recorded.

19
0.9 1m 1.41m 1.42 m 1.08

2.06 m 2.46 m 2.73 m

Scrap

Course 2 A2 B2 C2 D2 E2 F2 G2 H2

Scrap

f-*-Charpy Specimens

12 J2

K2 • - X2

Course 3 Scrap

Scrap

Ring 3

Specimen Type Identification

Tensile B, C, U, V

Fracture Toughness A5 , B5 , C5 , A6 , B6...E6, A2 , B2...H2

B, C, U, V

Charpy Impact K2 , L2. . . X2 , F, G. . .L Chemical Analysis F3 , A, M, W

Slow Strain Rate Tensile A3 , B3, C3 , D3

Hardness E3

Hydrogen Pressure Cracking 12 , J2, X, Y

Galvanic Corrosion N

Metallography S Stress Corrosion Cracking P, Q, R, T

Figure 5. Specimen locations in plates cut from Ring 3-

.90 m

.74 m

?n
The results of the MPI are summarized in Figure 6 and Appendix B. The

indications observed were of the linear type associated with cracks open to

the surface. There were numerous small indications in the base metal and in

the HAZ of Welds 1/2 and 2/3. The three vertical welds (A, B, and C) joining

the replacement segments of Course 2 were inspected, and only two small

indications were detected. Several small ( < 1 0 mm long, 0.4 in.) indications

were observed in the vertical weld in Course 1 , and two isolated indications

were found in the vertical weld in Course 3. No indications were observed in

Plates 1 0A and 1 0B . The results listed in Appendix B generally agree with

the inspection results of the FMI [1].

The test plates removed from Rings 1 and 3 were inspected by MPI over

the entire inner surface and on each of the saw-cut edges. Two isolated

25 mm ( 1 in.) indications were detected on the surface of the Ring 1 test

plate. The edges of the Ring 1 test plate had numerous de laminations that

appeared to be hydrogen pressure cracks similar to blisters. Most of the

cracking detected on the edges was within 6 mm ( 0.23 in.) of the inside

surface.

On the Ring 3 test plate, no indications were found in the weld or HAZ of Weld 2/3. Small cracks were found near three separate arc strikes.

The saw-cut edges of the Ring 3 test plates were inspected and no indications

of delamination were detected.

Ultrasonic Inspection

The ultrasonic inspection task is summarized in this section. Details

regarding interpretation of the signals and the complete results are presented in Appendix C. The test plates taken from Rings 1 and 3 were ultrasonically tested for

evidence of delamination in order to avoid taking specimens from damaged

material. A pulse echo technique was used. Measurements were made at

25.4 mm (1 in.) intervals on the outside surface of the vessel using 12.7 mm

(0.5 in.) diameter piezoelectric transducers with a resonant frequency of

10 MHz. Extensive regions of delamination were found in the Ring 1 test

plate, particularly between the edge cut at CC 3-91 m and the vertical weld

at CC 4.32 m. No regions of de lamination were detected in the test plates

cut from Ring 3 .

21
After detection of such extensive delamination in the Ring 1 test plate,

it was decided to survey Rings 1 and 2 by the same technique (except that a

5 MHz transducer was used) to assess the extent of delamination in the

vicinity of the fracture. In Ring 1, measurements were taken 25 mm (1 in.)

and 76 mm (3 in.) below the fracture surface at 51 mm ( 2 in.) intervals

around the circumference and 0.33 m (1 ft.) below the fracture surface at

0.3 m (1 ft.) intervals. In Ring 2, measurements were taken 51 mm (2 in.)

and 102 mm (4 in.) above the fracture surface at 0.3 m (1 ft.) intervals.

Fewer measurements were taken because the Ring 3 test-plate results

(Appendix C) indicated a lower susceptibility to de laminat ions in Course 2.

The results indicate that Course 1 had several areas of delamination and

the ultrasonic signals frequently had moderate noise levels between the

outer-surface echoes. This noise is an indication of discontinuities such as inclusions in the steel plate. In contrast, no clear indication of delamination was found in Course 2, and typically the ultrasonic signals were free of

noise, indicating that the Course 2 plate was cleaner than the Course 1 plate. No clear indication of delamination was found in Course 3, but the

ultrasonic signals frequently had moderate noise levels between the

outer-surface echoes. This suggests that the cleaniness levels of Courses 1 and 3 were similar.

Dimensional Checks

The thickness of Courses 1 and 2 in the vicinity of Weld 1/2 was spotchecked with a micrometer. The results, summarized in Figure 7, indicate

that the thickness was typically equal to or greater than the nominal wall

thickness of 25.4 mm (1.0 in.). The minimum thickness measured was 23.4 mm

(0.92 in.), which was well within the corrosion allowance for the pressure

vessel.

22
Course 3

Ring 3 Court# 4

Ring 3

Court# 2

Ring 1

Court# 1

Inspection Boundary

Figure 6. Summary of magnetic particle inspection results.

o <25 mm 25-61 mm

"I" >51 rrw#

23

Ox

8.0

\

0

o 25.4 - 26.0 mm

• 24.6 - 25.4 mm

m 23.4 - 24.6 mm

—2.0 POSITION, meters

Figure 7. Wall thickness measurements near the fracture surface.

25
Summary Comments

The pressure vessel fractured into two main pieces along a path adjacent

to the field weld joining Course 1 to Course 2 (replaced section). The

failure investigation was conducted on three ring sections and two plates cut

from these pieces.

The vessel segments included both fracture surfaces, the field weld

joining Courses 2 and 3, and samples of the original construction welds joining

Courses 3> and 5. Inspection of these pieces revealed:

1. Indications of several hundred cracks in the vicinity of

the field welds joining Course 2 to the vessel by MPI

2. Few indications of cracks in the vicinity of the original

fabrication welds or near the vertical field welds within Course 2

3. Numerous indications of delamination in Course 1 by ultrasonic

inspection

M. No clear indications of delamination in either Course 2 or Course 3 by ultrasonic inspection

5. No significant reduction in the thickness of the vessel due to

corrosion in the vicinity of the fracture

26
MATERIALS EVALUATION

A series of tests was conducted on the steel plates and welds of

pressure vessel 1 2D 70 1 . The purpose of these tests was to characterize

the materials of construction.

Chemical Analysis

The chemical compositions of the steel plates used for Courses 1, 2,

and 3 of Welds 1/2, 2/3, and of vertical Weld A in Course 2 were determined

by atomic emission spectroscopy. The procedures were in accordance with

ASTM E 415 [3]. The vacuum spectrometer was calibrated for the runs using

NBS Standard Reference Materials 1 261 -65, 1 1 6 1 , 1 1 62, 1 1 64, 1 222, 1 227, 1 1 34,

1136, C11 73, 1 1 39a, 33d, 36 b, 55e, lOg, 1218 and Reference Materials 50b, and

BAS 405.

The results for the steel plates are presented in Table I along with

the chemical requirements specified for ASTM A5 1 6 Grade 70. All three

plates met the specified requirements. The compositions of Courses 1 and 3 were essentially the same, suggesting that both plates came from the same

heat. The composition of Course 2 differs, as expected, since the Course 2

steel was from a different heat. The principal difference between the heats

is that the Course 1 and 3 steel had significantly higher sulfur (0.028$ vs. 0.017$) and aluminum (0.08$ vs. 0.05$) contents. Sulfur and aluminum are inclusion formers, and thus the cleanliness of Course 2 steel was likely to

be superior to that of the steel for Courses 1 and 3.

The results for the welds are also presented in Table I along with

chemical requirements specified for AWS E7018 welds. All the welds met the

requirements specified for AWS E7018. The carbon, manganese, and silicon

contents are typical for these welds. The samples taken from Weld 2/3 and

the vertical weld were analyzed twice. The small variations in the measured

values were within the range expected for welds.

Tensile Properties

The tensile properties of the steel plates used for Courses 1, 2, and 3 and of Weld 1/2 were measured using the procedures of ASTM E 8 [4]. Two

tests on each material were conducted at room temperature, and 0.2$ offs-’

27
Table

I.

Chemical

compositions

(weight

percent)

of

the

steel

plates

used

for

Courses

1,

2,

and 3

and

of

Welds

1/2,

2/3

and

the

vertical

weld

in

Course

2.

CO •o

nJo

•rH

4->

a) >

oo

00

X X X X X

Cfl m cO 03 03 E E E E E

O LO o o co

VO O- oo CM o • • • • • T— O o o o

[0- X

CO CM OO O i>- LO vO LO Ov X— X O oo CM OO r

o O CM O O o O oo O O o o o O O

o o O o O O o o o O <0.

LO X E- CM o 0- LO LO OO X T a

v

CM oo X o O CM o O o o oo oo o o o o o o o

O o o <- o o o o o o o o

V

o co r— 0- oo CO oo LO

LO o CM oo X

CM x

o o x o

o o o o o o o o o o o

o o o r- o o o o o o o o

LO ov o oo LO ov -=r CM LO vO o CM CM CM T— CM X o o X o

o o o r- o o o o o o o

o o o — o o o o o o o o

\y

o LO CTv LO OO LO LO to- LO vO X x— OO LO CM OO O O O X

O

o o o o O O O O o o O

o o o r- o O O O O o o o

V

co

<d

cfl

I—

I

a.

CDa>

4-> oo

X X X LO on

VO CO cx) 03 CM oo

X O E E E • • LO t- , o 1 1 1 1 1 1 1 <c LO 1 1 1 1 1 1 1 1 1 CD co on o on 1 1 1 1 1 1 1 s: a CM o o OO <

E—1 CT5 co e O o o o o

< o

OO CO on co CM oo on

LO o CM o CO X oo o o CO CM CO

CM o o x— CM o o o o o o o

on O o o O o o o o o o o

CD 10- LO o CM CM on a>

co CM X X— CM on O X o O o o LO E CM o o O CM O o o O o o o

Z5 CM o O o o T— O O o o O o o o

o

0- on o CM oo oo oo

OO o CM T CO X— O o o CM CO

CM o O CM o o O o o o o

O o O r- O o o O o o o o

4-0 c

0)e

CD

i

1 w

JZl ‘rH •(—I £_ •rH O ^ <

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28

bal.

bal.

bal.

bal.

bal.

bal.

bal.

bal.

bal.
y i

•

' Id strength, ultimate tensile strength, percent elongation, and percent

reduction of area were measured. The plate properties were measured with

50 mm (2 in.) gage length, 12.7 mm (0.5 in.) diameter specimens taken from

the midthickness level of the plate and oriented in the transverse direction.

The weld properties were measured with 25 mm ( 1 in.) gage length, 6.4 mm

(0.25 in.) diameter specimens taken along the length of the weld such that the

entire test section was in the weld.

The results are presented in Table II along with the tensile requirements specified for ASTM A516 Grade 70 and AWS E7018. The steel plates and

weld metal exceed their respective tensile requirements. The tensile properties of the plates are essentially the same, and the yield strength of

the weld metal (477 MPa, 69.2 ksi) is significantly greater than that of the

plate (300 MPa, 43-5 ksi).

Charpy Impact Toughness

The Charpy impact toughness was measured as a function of temperature on specimens taken from Courses 1 and 2, Weld 2/3, and HAZ 2 of Weld 2/3- The

tests were conducted in a 165 J (122 ft*lbf) capacity impact machine in

accordance with the procedures of ASTM E 23 [5] . The specimen conf igurat ion

and the notch locations are shown in Figure 8. The notches were oriented

perpend icular to the plate surface such that the fracture of the plate speci- mens proceeded in the rolling direction (TL orientation), and the fracture of

the weld metal and HAZ specimens proceeded along the length of the weld.

Test temperatures of 38, 0, -40, and -51 °C (100, 32, -40, and -60°F) were selected to cover the range of the ductile-to-brittle transition and to

permit estimation of the 20 J (15 ft*lbf) Charpy V-notch transition

temperature.

The results are presented in Table III. The weld metal and HAZ toughness

exceeded that of the base metal. The estimated 20 J (15 ft»lbf) transition

temperatures were 0°C (32°F) for the base metal (Courses 1 and 2), -51 °C

(-60°F) for the weld metal (Weld 2/3), and -40°C (-40°F) for the HAZ of

Course 2.

29
Table

II.

Tensile

properties

of

Courses

co

0)

s, <soC fes. o

co O' cr> 05 lo <—

43-43- =t 43- lo lo

00 tC— M3 I

4-> o3

T3

CD CCCO

C

•r-H S

4-> ajbOCO

05 CO 05 00 cm

CM CM CM CM OOm CM

43- on

CM CM

Cd

CM

T31—I

CD

T3S

on

-oc

c0

CM 0

x: ^ CM <

M3 43- LO D— 05

4-5 -rH . • • . • • 1 OO t— C— C- ^r O

C— 0- t- t>- D—

CD

w

i4-> 00

CD

4-5

O

03 CM S M3

•pH

4-5 CO 05 lo onm ln CM 1—1 0- on on on on T t CO

3) S LO in lo lo LO LPi dCT5 on

LO f10 LO

.c

4-5

05 f43- on

43- 3T

CM 3T

43- 43- 3T 43"

CO CM o on

43- 43- coon

t- 00

00 crs M3 MO

CD

c.

4-5 00

x>

CD

•rH cO C35 LO M3 r— co CM on <—

>H Ou

O

O

O

O CO 05 M3 to- CO on OO on on CvJ CM CM 43- 43-

CO

CD

4-5 03 1—1

Cu

CD

CD

4-5 00

CD

CO

S-, 3OO

on on CM CM M3

«- o

LO C— <

CD S TO

E- 03 00 L < O

CM

IS

30

AWS

E7018

413,

min.

(60,

min.)

496,

min.

(72,

min.)

22,

min.
Weld Metal

Base

Metal

///////

////'////>%#: //////////$%

////////// ////////// M: //////////

/V n i < k i ' ' ////////

'//////// //////// '////////

'////////

'////////

'////////

'//////// '//////// '////////

/ / / / //// mm.

z/z/lml' ////&£$

////W:\ ////Mi // //®W. ///mm mm

W%&:/ //

/

mm/ / / / "W*- / / / /

mm / / / / /

mm/ / / / / __

Course 2

mm? ///////

"Wk ///////

mz-.Y ////// "m //////

wm/ / / / /

mm / /// mm/////

m. / / / /

/:/ Z / Z W,/ / /

//////

\

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/////. //////)

//////, //////

A

//////,. //////>

///////[

Course 3

Weld Metal

Charpy

T/4

///77/// ywfam

>•>•)<>•>• >'>' >> >>><>• ////////////7mm ///////////// 7-m

./ ./ .< / /u / / / / / / m

/ / / - / / / . m z z z z .’ z z ///// z / ')?}/.

*mmz / / mm/// /

.... mm///// /////

//////

Mmy//////

\/ ////////////// /mm

\ ////////////////m>: )/////////////////m ////////////// //&£:

\// ///////// /////mx

'///////////// //mm -///////////// / / mm*/ r///////////// / mmm

* /////////////.////' X////// //'// ////J*.-- Course

mm///////////, m*/////////// -/////// m/////////

/

wy////////

A

mm ///////

A

Wy/y //////7

mmz////// a

'/zz/z //////

WzZ /////

A

V* iW' .tf. - —' — . J Course 3

HAZ 2

Charpy

y / / ////// ////////

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.’ f > } z ) } ) r ///////////////

z A7 7 7 7 7 7 7 7 7 7 '7 7 7 7 '7 7 z /////////////////// /////////////////// ///////////////////

yzzzzzzzzzzzzzzzzzz '////////////////// '//////////////////

f ////////////////// '////////////////// '//////////////////

////////, / / / / ' /

,

zzzzzzzzzzzzzzz /////////////// /////////////

z

^'7

'7 77 7 7 7 7 7 7 7 7 7 7 7

/ / '/z

/z

' / A

' /

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zzzzzzzzzzzzzzz. ///////////////////

//////////////////z //////////////////Z ZZZ/ZZZ/ZZZZZZ/ZZ/z //////////////////z //////////////////z ////z/zz/zz/z/zz

Base Metal

Charpy

Courses 1 & 2

Figure 8. Charpy specimens and their notch locat ions.

31
Table

III.

Charpy

V-notch

impact

toughness

of

Courses 1

and

2,

Weld

1/2,

and

HAZ 2

of

Weld

2/3.

IXoO

OO VO VO

CM CM on o V V V s v_/ oOO CO LO VO C\J on co co vo

Ct-t JOP

Cm

lD

bOX

CDC

IXPO

03CXE

JCOP0

X'1 >

>>CXX

03 JXo Xo

CMonooO

IX, 0

X"1 ooO

X"I Xo0

VO1 0oLO1

LO

LO CO IP VO

<- X- CO CM

CM CM CM CM I I O O

lo on

LO LO

CM X"

CO X" <

VO

X- CO VO

O VO

CM <-

CO X- VO LO

X" X" O <—

<- LO

X- LO I I

LO LO OO VO

'-M CO

LO LO

CTV X"

CDC

CD

O

X CS1 3

-P X3

03 CO CD X CD P

CD P ,

CM

r—1

C\J

O & cO 03 \ CD E ,—1

CD CD P <

<P

CD

X CO CO (D <M

HX

X S T3 <

r—1 3 3 1—1

X) CD

O o •o <D P

CO CD

O o 1—1

03

CD P CD CD H OO 3s X

32

Weld

2/3;

HAZ 2

8.1

(6.0)

23

(17)

52

(38)

57

(42)

6.1

(4.5)

34

(25)

52

(38)

84

(62)
Fracture Toughness

Fracture toughness was measured by the crack tip opening displacement

(CTOD) method as a function of temperature on specimens taken from Ring 3.

The tests were conducted in accordance with BS 5762 [6], "Methods for Crack

Opening Displacement Testing" using full-thickness (to the extent practical),

single-edge-notch-bend (SENB) specimens. The specimens, shown in Figure 9, were 50 mm (2 in.) wide and were tested in three-point bending over a 200 mm

(8 in.) span. The specimens were in the TL orientation, i.e., the crack was perpend icular to the plate surface and parallel to the weld.

Before the notches were machined, the specimens were etched to reveal

the HAZ . The notch location was scribed on the specimen as shown in

Figure 10a. The notches were typically located in the HAZ, as shown in

Figure 10b on the Course 2 side of Weld 2/3. This side had a straighter HAZ

than the Course 3 side because Course 2 was the lower side of Weld 2/3. In

the horizontal welding used to replace Course 2, the lower course served as a shelf to build up the weld and the upper course had a double bevel preparation. Two specimens were notched on the Course 3 side of the weld to spot

check the toughness values of the Course 3 HAZ.

Before fatigue precracking, the specimens were given a precompression

treatment to ensure a straight crack front [7]. A compressive strain of 1$ was applied to the crack tip region in the through-th ickness direction.

The specimen was squeezed between two 19 mm (0.74 in.) diameter disks

centered in the notch plane 9 mm (0.35 in.) ahead of the notch tip. A

specimen with the indentations due to precompression is shown in Figure 10c.

After precompression, the specimens were precracked in fatigue at room

temperature. The typical length of fatigue extension was 2 mm (0.08 in.)

resulting in a total crack length of 25 mm (0.98 in.).

The specimens were tested in a servohydraulic testing machine with a 100 kN (22,000 lbf.) capacity. Tests were conducted at 38 , 0, and -40°C

(100, 32, and -40 °F). These temperatures were selected to represent the range

of temperatures possible at the crack tip at the time of the incident. The

service temperature was 38°C (100°F). The temperature of -40°C (-40°F) could

have occurred due to the cooling effect of evaporating propane, which was leaking through the crack just prior to the failure. The temperature of 0°C

(32°F) was simply an intermediate temperature.

33
Figure 9. Single edge notch bend specimens for fracture toughness testing.

34
c. Specimen after testing showing region

of precompress ion.

d. Fracture surface of specimen tested

at 38°C (100°F)

Figure 10. No tches in fracture toughness specimens.

35
The results of the fracture toughness tests are summarized in Table IV.

The scatter in the results was due in part to variations in the notch

locations with respect to the weld. The weld metal toughness was higher than

the base metal or HAZ toughness. Thus, if the crack front was partially

located in the weld, higher values were measured. The specimens were notched

in groups so that the test results for one group of tests could be reviewed

and before notching the next group. Owing to the toughening effect of the

weld, the notches in later tests were moved farther away from the weld metal.

This tended to reduce the scatter in the later tests conducted at 38 °C

(100°F), i.e., specimens identified as A2, C2 , and C5 in Table IV.

Rockwell Hardness

Rockwell hardness traverses were taken on cross sections of Welds 1/2,

2/3, 3/4, and 4/5, and the vertical welds in Courses 1, 2 (Welds A, B, and C),

3, and 5. The traverses were nominally parallel to the plate surface.

Each traverse sampled the hardness of the weld, the base metal, and the HAZ

on each side of the weld. The Rockwell A scale (HRA) was used for all

specimens, and the Rockwell B scale (HRB) was also used for Weld A in Course 2 and for Weld 2/3.

The results of the Rockwell hardness traverses are presented in

Appendix D. In general, the hardnesses of the base metal, HAZ, and weld metal were uniform, indicating an appropriate matching of the tensile strengths of

the base metal and the weld. The peak hardness in the HAZ of all but one of

the specimens was within 2 HRA points of the peak hardness in the weld.

The one exception was the HRB traverse on Weld 2/3, where the peak hardness

in the HAZ was 106 HRB compared with a peak of 98 HRB in the weld. The

possibility of high hardness values in the repair welds done to replace

Course 2 was studied further by measuring microhardness.

Metallography and Microhardness

The metallography to evaluate the steel and welds of pressure vessel

12D701 discussed in this section includes: 1) sections through Welds 1/2

and 2/3 and through a vertical weld in Course 2 showing the microstructures

of the weld, HAZ, and base metal; 2) a section through Weld 1/2 showing the

microstructure near the Monel-to-steel weld and surface-breaking cracks in

the base metal; and 3) longitudinal and transverse sections through a

36
Table

IV.

Fracture

toughness

test

results.

^-N

C-- /-S LO to C~—

T CO

ON LO MD o LO =T C— co to vO CO X O t X o O o O o o O O o O o o o o O o c o O o o o o o o o O o

•H

v—^ v—' v'—' v—' QOHO co CO to vO c~- cO CO CM CM CO LO g co on *— CM CO CO CM <— *- o o o O o o o O O o o

CO CO co to o o LO LO CO on VO

g =T

O CO vO

T

—

T LO CM -=r CO > E • • CO o O o O O o o X CO CO oo to CM CM CM LO

cC • • E2 o LO CO CO LO O CO LO LO CO

CL. -X CO OO CO CO CO CO CO CO on

LO

r , CO CM o\ o CO CO

T -=T

C vO vO vO LO CO -=T cO LO LO vO LO

ro E CM CM CM CM CM CM CM CM CM CM CM

CO CO to CO LO CO to OO CO C— OO e CO o O O O CO o O CO O O 3 E LO LPl LO LO LO LO =r LO LO

LO -=r O'. co

O on CM to

OQ § o co o ON CO CO o CO

O o CO

C\J CM CM CM CM o o O O CM CM O o O O

O CD ^ o o O O CO CO O

S- Dl,T

T

T

T 1 i i « 1 3 o V—

v_/ Nw/ V_/

O

w

03SCDaE CO CO OO OO o O o o O CO o

CD o on on co on -=r CO H o i i i i

CO CO \ N c\j CM

•o •o c 1—1 1 1 CD CD CD E 3 3 •rH CM CM CM LO CM CM CM LO CM CM in O Cm Dd X O O OQ U-t Q X O Cm <C < CD o o a

C/3 C\J CO M N

<C < X X

37
delaminated section of Course 1 . Microhardness measurements were used to

evaluate HAZ hardness and to assist in identifying microstructures.

Weld 1/2 : The section through Weld 1/2, shown in Figure 11, was taken from

Ring 1 at CC 1 .5 m in the region where the crack propagated through Course 2

away from the weld. The daub of black that can be seen on the Course 1 side

of the inner surface is a bead of Monel from the seam weld that joined the

liner to the vessel. The crack on the Course 2 side of the inner surface

(A in Figure 11) was stretched open, presumably at the time of final

fracture. Apparently, the crack existed prior to the failure because the

material on the weld side of the crack was not stretched, as revealed by the

discontinuity in the HAZ.

The crack on the Course 1 side of the outer surface (B in Figure 1 1

)

initiated at the fusion line of the weld that joined Course 1 to the original

Course 2. The large bead size of the weld adjacent to crack B indicates that

this is a submerged arc weld made during initial fabrication of the vessel

that was not completely removed for the replacement of Course 2. The crack,

shown at higher magnification in Figure 12, had a light etching corrosion

product on the surface in contrast to the black deposits found in cracks on

the inside surface.

Weld 2/3 : A specimen was removed from the part of Ring 3 ( CC 1.8 m) where

vertical Weld A in Course 2 intersected Weld 2/3. The macrostructure of

Weld 2/3, shown in Figure 13a, reveals the expected shape for a weld done in

the horizontal position, i.e., a square lower edge (Course 2) and a V-shaped

upper edge (Course 3). The sequence was to weld from the outside first and

then complete the weld from the inside.

In the photograph of the HAZ 3 microstructure near the inside surface,

Figure 13b, Weld 2/3 is in the upper right, and the base metal of Course 3 is

in the lower left. Within the HAZ, there is a coarse-grain region nearest

the weld, then a fine-grain region, and finally, a partially transformed zone next to the base metal. The microstructure of the coarse-grain region, shown

at higher magnification in Figure 13c, has the acicular appearance of mar- tensite or martensite-bain ite. The presence of martensite was also indicated

by microhardness measurements taken near the inner surface in both HAZ 2 and

HAZ 3- In both cases, the peak hardness values were in excess of 400 on the

38
Figure 12. Microstructure near crack B in HAZ 1 shovm in Figure 11.

Etchant: TL nital

Figure 11. Macrostructure of Weld i/2 at CC 1.5 m. Etchant: 27<> nital

6 0jum

39
Course

Course

Figure 1 3

|

a. Macrostructure of Weld 2/3 at CC 1.8 m

i . d.

Course 2

o. d.

3

b. Macrostructure of HAZ 3

c. Microstructure of HAZ 3

250 /xm

i . d

.

25 /xm

Macrostructure of Weld 2/3 at CC 1.8 m and microstruc ture of HAZ 3.

Etchant: 27, nital

40
Knoop scale (HK) . The results for HAZ 3 are shown in Figure 14. Figure 14a

shows the same location as Figure 1 4 b after making the hardness indentation.

The values given in Figure 1 4b are above 400 HK in the eoarse-grain region

tnd over 300 HK in the fine-grain region. The higher magnification photomicrograph, Figure 14c, reveals martensite in the eoarse-grain region of the

HAZ. The martensite that formed adjacent to the plate surfaces was not

•

• mpered by subsequent weld passes. Microstructures containing hard

martensite are known to be susceptible to hydrogen stress cracking.

Vertical Weld A, Course 2 : The metallography of vertical Weld A in Course 2

is shown in Figure 15. This is a field weld made at the time Course 2 was replaced. The weld was done in the vertical position, and as shown in

Figure 15a, the joint preparation was symmetrical, and fewer weld passes were

used to fill the joint than those used for the horizontal repair welds. The

microstructure of the HAZ at the inside surface, shown in Figure 15b, does

not show the eoarse-grain region that cqn be seen in the horizontal repair

weld in Figure 13b. This is because the microstructure in the eoarse-grain

region is ferrite and pearlite, as shown in Figure 15c, i.e. , the same phases

that exist in the other parts of the weldment. A microstructure of ferrite

and pearlite is softer and more resistant to cracking than the martensite

seen in the horizontal repair welds. Microhardness values in the HAZ ranged

from 206 to 240 HK, as shown in Figure 16.

Monel-to-Steel Lap Weld : The specimen shown in Figure 17 was removed from

Ring 2 (CC 1.0 m) where the fracture path left a flap of Course 1 material

containing the Monel-to-steel weld seam. This section also contained

sur face-break ing cracks (labeled A, B, and C in Figure 17a) in the base metal

of Course 2, which had previously been detected by MPI . As shown in

Figures 17b and 17c, these cracks have a stepwise appearance typical of

hydrogen pressure cracking. Crack C, shown at higher magnification in

Figure 17c, is connected to a delamination such that one appears to be an extension of the other. The specimen also shows a region of submerged arc welding near the outer surface of Course 1. Presumably, this was the weld

that joined Course 1 and the original Course 2 and was not completely removed

41
HAZ We Id

\2 6 5 334

414

330

365

\

491

\

433

\

49 2

\

330

260

39 3

\\

481

437

\

b. Knoop hardness values

(tracing of Fig. 14a)

c

.

Figure 1 A .

Microstructure at location of maximum hardness in Figure 1 4a

Mi crohardness of HAZ 3 in Weld 2/3 at CC 1.8 m at the location

shown in Figure 13b. Etchant: TL nital

42
0.25 mm

b. Microstructure of HAZ at A in Figure 15a

25/xm

c. Microstructure of HAZ

Figure 15. Macrostructure and microstructure of vertical weld A in Four.

Etchant: 27o nital

43
We Id

240

206

/

HAZ

216

/

6 0iim

a - °°P hardness indentations 5 . Knoop hardness values

(tracing of Fig. 16a)

Microstructure of location of maximum hardness in Figure 16a

Figure 16. Microhardness of vertical Weld A and its HAZ in Course 2 at

location B shown in Figure 15a. Etchant: 2 % nital

44
b. Crack at location A in Figure 17a

c. Cracks at location C in Figure 17a

Figure 17. Microstr uc ture of Weld 1/2

inner surface of Course 2.

at CC 1 .0 m and base metal cracks on the

Etchant: 27„ nital

45
at the time of the repair. Finally, there is a delamination in Course 1 labeled D that terminates at the fusion line, as if it were unable to penetrate into the weld.

The microstructure on the steel side of the Monel-to-steel joint is

shown in Figure 18. Monel that laps over onto Course 1 has apparently been

eaten away by crevice corrosion, as shown in Figures 18a and 1 8 b . This

suggests that the Monel was locally anodic to the steel. This is contrary to

what is expected on the basis of the position of these metals in the electromotive series. The microhardness indentations, Figures 1 8b and 18c, indicate

hardness values of 479 and 497 HK adjacent to the Monel. The microstructure

at higher magnification, Figure 1 8 d , shows that the high hardness values are due to the presence of martensite.

Delamination in Course 1 : A sample was cut from Course 1 in an area where

ultrasonic and magnetic particle inspection indicated delamination of the

plate. Longitudinal and transverse sections through the sample are shown in

Figures 19a and 19b, respectively. The transverse section is the view that

would be observed in a section through a circumferential weld. At a higher

magnification (Figure 1 9 d ) , its appearance is similar to that observed on several metallographic samples and to a sketch (shown here as Figure 19c)

of what is called "hydrogen-induced stepwise cracking" in NACE Standard

TM-02-84 [7].

46
Weld 1/2

Course 1

Shown

Figure

in

1 8b

Mi crostr uc ture of the Monel to steel weld

b. Knoop hardness indentations

2 am

HAZ

c. Knoop hardness values

(tracing of Fig. 1 8 b

)

Mone 1

Base

Metal

HK 500

Figure 18 . Microhardness of the HAZ of the Monel-to-steel weld at thlocation shown in Figure 17a. Etchant: TU nital

47
c. Sketch of cracks referred to as hydrogen- induced stepwise cracking

in NACE Standard TM-02-84 [7]

d. Microstruc ture near cracks in Figure 19b

1 mm

Figure 19. Hydrogen pressure cracking in Course

Etchant: 27o nital

at CC 3.9 m.

48
Summary Comments

Tests to evaluate the steel plates and welds used for initial construe

t ion and for replacement of Course 2 indicated that:

1. The steel plates and welds met the specified requirements for

chemical composition and tensile properties.

2. The toughness of the repair welds was significantly higher than

that of the steel plates.

3. The tensile strength and hardness of the welds and plates were

comparable.

4. The properties of the steel used to replace Course 2 were similar

to those of the original steel.

5. Field welding done to replace Course 2 resulted in hard regions

in the HAZs near the plate surfaces. The hard regions contained

martensite, a microstructure known to be susceptible to hydrogen

stress cracking.

6. Installation of the Monel liner resulted in a hard microstructure

in the vessel steel where the Monel was welded to the steel.

7. Hard microstructures were not found in the HAZs of the original

construction welds or in the vertical field welds within Course 2

49

CORROSION AND HYDROGEN CRACKING TESTS

Pressure vessel 1 2D701 had a history of damage by corrosion and hydrogen

cracking. Replacement of Course 2 and installation of the Monel liner were necessitated by the detection of blisters and de laminat ions in Courses 1 and 2. It is likely that the extensive cracking observed by MPI was caused

by corrosion or hydrogen- induced cracking. A limited testing program was undertaken to help clarify the mechanisms of the cracking observed in the

vessel.

There are many manifestations of stress corrosion cracking and hydrogeninduced cracking, and various terms are used to describe these phenomena.

For the purpose of this report, stress corrosion cracking refers to cracking

due to the combined action of corrosion and tensile stress. In this case

the corrosive environment was an aqueous solution of MEA (20% by volume)

containing H

2 S. Hydrogen-induced cracking is used as a general term to

describe cracking by several mechanisms including two of interest in this

investigation: hydrogen pressure cracking and hydrogen stress cracking.

In the pressure vessel, the source of hydrogen was H

2 S, which decomposed at

the steel surface. The hydrogen then permeated into the steel where it was free to diffuse as atomic hydrogen.

Hydrogen pressure cracking occurs owing to the formation of molecular

hydrogen at internal defects such as inclusions. As hydrogen continues to

enter the steel and diffuse to the defect, high pressures develop, interfacial separation occurs, and eventually the steel enclosing the hydrogen

deforms plastically and ruptures. Blisters and de laminations are formed by

this mechanism. This type of cracking is observed in steels, including lowstrength steels, exposed to corrosive environments containing H

2 S.

Hydrogen stress cracking occurs by a mechanism that remains unknown.

However, it has been extensively studied and many of the characteristics are well-known. Hydrogen stress cracking occurs in high-hardness steels under

sustained load at tensile stresses below the yield strength. Hydrogen

diffuses to regions of stress concentration, and in susceptible materials,

brittle cracking occurs. This type of cracking was observed in the studies

discussed in the Fracture Surface Observations section. Another form of

hydrogen stress cracking characteristically occurs in the welding of steels

and is referred to as hydrogen-induced cold cracking.

51
Hydrogen Pressure Cracking

The susceptibility of the vessel steels to hydrogen pressure cracking

was evaluated using the procedures of NACE Standard TM-02-84 [8]. NACE

refers to this type of cracking as stepwise cracking induced by hydrogen

absorption from aqueous sulfide corrosion. Two base metal specimens each

were tested from Course 1 and Course 2. Weld 2/3 was tested using two

specimens oriented transversely to the weld, and thus contained steel from

Courses 2 and 3- The orientation of the specimen with respect to the curvature and grain orientation of the plate is shown in Figure 20. This is the same orientation with respect to curvature as that specified in the NACE

standard. However, the sections examined metallograph ically were parallel to

the direction of rolling, whereas the NACE standard specifies sections

transverse to the rolling direction. The NACE specification was developed

for line pipe where the rolling direction is parailel to the pipe axis,

whereas in pressure vessels, the rolling direction is perpendicular to the

cylindrical axis.

The tests were conducted by placing* the specimens in substitute

seawater prepared according to ASTM D 1141 [9] and then saturating the sea water with H

2 S. After saturation, H2S was bubbled through the solution

continuously for the duration of the tests. All specimens were immersed

simultaneously for a duration of 96 h. The specimens were then removed from

the solution, cleaned, and examined.

All specimens developed blisters of the type shown in Figure 21.

In the welded specimens, blisters formed on the base metal, but not on

the weld. Note that the specimens shown in Figures 21 e and 21 f were not

metallograph ically prepared and etched, and the welds cannot be seen. The

specimens were sectioned and examined for hydrogen pressure cracking at the

locations shown in Figure 20, and the susceptibility to cracking was characterized in terms of the ratios defined by NACE:

Crack Sensitivity Ratio, CSR = I(. a—x-.b)_ x W x T

1 0056, ( 1 )

Crack Length Ratio, CLR = x 1 0056, w

Ijl T

(2)

Crack Thickness Ratio, CTR = x 1 0056, (3)
tt

Faces to be examined

Figure 20. Specimen for measuring susceptibility to hydrogen pressure

crack ing.

53
10 mm

iiiiiiiiiiimmiiiimiiiim iiniiiimiiiiiiiiiiiiiiiiiiii

c. Specimen X from Course 2 d. Specimen Y from Course 2

e. Specimen 12 from Weld 2/3; f - Specimen J2 from Weld 2/3;

blisters are in Course 3 blisters are in Course 2

Figure 21. Blistering on surface of specimens after hydrogen pressure

cracking tests.

54
where a = crack length,

b = crack thickness,

W = section width,

and T = specimen thickness.

The results, summarized in Table V, indicate that the A5 1 6 Grade 70

steel from both Courses 1 and 2 is susceptible to hydrogen pressure cracking

in the NACE solution [8].

The results also show that Course 1 steel is more susceptible to hydrogen pressure cracking than the Course 2 steel. This is consistent with the

lower levels of inclusion formers, sulfur and aluminum, in the Course 2

steel. No cracking was observed in the welds, but cracks did form in the

HAZ. Metallographic sections of typical cracks are shown in Figure 22. The

linkage of several hydrogen pressure cracks into a surface -break ing crack is

shown in Figure 22a. The size and orientation of the hydrogen pressure

cracks is not simply related to the microstructure, as shown in Figure 22b.

The cracks that developed in HAZ 3 of Weld 2/3 are shown in Figure 22c.

Hydrogen-Induced Cold Cracking

Magnetic particle inspection revealed numerous cracks in the HAZ of

Welds 1/2 and 2/3. These cracks may have formed at the time Course 2 was replaced or during operation of the vessel. A possible cause of cracking at

the time of repair welding is hydrogen-induced cold cracking. This type of

cracking tends to occur in the HAZ of steel welds when the hardness is high

and hydrogen is present.

A series of y-groove cracking tests were conducted to evaluate the

susceptibility of the steel plates from Courses 1 and 2 to cold cracking

due to welding. The tests were carried out in accordance with Japanese

Industrial Standard JIS Z 3158 [8] using the specimen shown in Figure 23.

The procedures and results are discussed in Appendix E. In the first

y-groove tests, one specimen of Course 1 material and one specimen of

Course 2 material were welded by the shielded metal arc method at room

temperature using dried electrodes of type AWS E 7018. No cracking was observed in either specimen.

55
Table V. Results of hydrogen-pressure cracking tests.

Specimen Sect ion Crack Sensitivity Crack Length Crack Th ickness

Location Ratio, % Ratio, % Ratio, %

Course 1

B4 12

3

1.7

0.3

0.9

80.3

24.2

27.6

3.8

2.7

3.4

Avg. 1 .0 44.1 3.3

C4 1 0.1 23.8 1 -3

2 5.2 67.2 7.8

3 2.1 59.5 3.5

Avg. 2.5 50.2 4.2

Course 2

X 1 0.6 45.1 2.8

2 0.1 14.5 1 .4

3 0.2 24.2 2.6

Avg. 0.3 27.9 2.3

Y 1 0.1 8.5 1 .

1

2 0.0 8.1 0.3

3 0.2 25.5 2.1

Avg. 0.1 14.0 1 .2

Weld 2/3

12 1 (Course 2 side) 0.0 0.0 0.0

2 Weld 2/3 0.1 6 .

6

2.9*

3 (Course 3 side) 0.2 10.4 3.1

Avg. 0.1 5.6 2.0

J2 1 ( Course 3 s ide) 0.1 6.9 3.0

2 Weld 2/3 0.0 0.0 0.0

3 ( Course 2 s ide) 0.0 0.0 0.0

Avg. 0.0 2.3 1 .0

* Crack in HAZ of Course 2 side

56
a Specimen CA from Course 1 (as polished)

60 |itm

E

b. Cracks in Course 2 of Specimen 12 of Weld 2/3

c. Cracks in HAZ 3 of Specimen 12 of Weld 2/3

60 jum

Figure 22. Metallography of cracking in specimens after hydrogen ]

• cracking tests.

57
Figure 23. Specimen for y-groove weldability test.

58
Next, a specimen was prepared that simulated two conditions that may

have existed at the time of the repair: 1) the Course 1 steel may have been

hydrogen charged at the time of repair welding because of the prior service,

and 2) welding may have been done on a cool day (-1°C, 34°F) without preheat.

The test was conducted on a duplex specimen consisting of Course 1 steel on

one side and Course 2 steel on the other. The Course 1 side of the specimen

was charged with hydrogen by soaking it in the NACE solution (NACE Standard

TM-01 -77 ) [10] for 80 h. The NACE solution is an acidic (pH = 3) saltwater

saturated with H

2 S. The duplex specimen was cooled to 1 °C ( 3 ^4 °F ) and welded

using the standard procedures [8], It cracked at the root of the weld on Course 1 ( hydrogen-charged) side. A cross section of the specimen is

presented in Figure 24a. The root cracking, shown in Figure 24b, occurred in

the coarse-grain part of the HAZ. The microhardness in the vicinity of the

crack was typically 350 on the Vickers scale indicating the presence of

martensite.

The results indicate that Course 1 was susceptible to cold cracking if

charged with hydrogen and welded without preheat. Thus, cold cracking could

have occurred at the time Course 2 was replaced. However, the numerous

cracks observed by MPI in Course 2 cannot be attributed to cold cracking on

the basis of these tests. Also, if cold cracking was prevalent in Course 1

,

it may have occurred on either the inside or the outside surface of the

vessel. The HAZ of Weld 1 on the outside surface of Ring 1 was checked for

cracking by MPI in the limited regions where the fracture did not occur along

HAZ 1 ; no cracks were found.

Stress Corrosion Cracking

The susceptibility of the vessel steels to stress corrosion cracking in

the MEA/H

2 S environment was evaluated by slow strain rate tensile testing.

Tests were conducted at room temperature on 4 mm (0.16 in.) diameter, 25 mm

(1 in.) gage length specimens in a screw-driven tensile machine at a constant

displacement rate of 10 ^ cm/s (0.4 x 10 ^ in./s). Base metal specimens were taken from Courses 1 and 2 and a transverse weld specimen was taken across

Weld 2/3. One control specimen of each material type was tested in air and

59
Course 1 Course 2

b. Root cracking in y-groove test in HAZ 1

Figure 24. Cross section of y-groove test specimen of Course 1 steel

welded to Course 2 steel showing cracking in HAZ 1.

60
Hi'.* specimen was tested in a 20$ aqufeous solution of MEA containing approx imately 0.83 molar H0 S. The results are summarized in Table VI. The

strength and ductility values are essentially the same in air and in the

MEA/HpS environment. Thus, the materials did not exhibit any susceptibility

to stress corrosion cracking under the conditions evaluated.

Galvanic Corrosion

The galvanic couple between the Monel liner and the steel vessel was studied by measuring the current flow between steel (from Course 1) and

Monel (from the liner) in the MEA/H

2 S solution. The experiments indicated

2

that the current density was typically quite low, on the order of 1 yA/cm

(6.5 yA/in. 2

). The tests were conducted using the apparatus shown in

Figure 25.

The steel surfaces not facing theppMonel were masked with lacquer. The

2 areas of the electrodes were 12.2 cm (1.9 in ) for the steel and 62.2 cm

2

(9.6 in. ) for the Monel. In the first test, steel electrodes were placed on either side of the Monel electrode, one at a spacing, d, of 1 mm (0.04 in.) and

the other at a spacing of 10 mm (0.4 in.). Current measurements were initially

made at room temperature; then the bath temperature was raised to 52°C

(126°F) and a second set of data was recorded. The bath was held at 41 °C

(106°F) overnight and a final reading was taken after 24 h. The experimental

results, presented in Table VII, indicate a particularly high current flow

after 84 min. The results also indicated a change of polarity. The steel

was initially anodic with respect to the Monel, but changed to cathodic

within 20 min. for an electrode separation of 1 mm (0.04 in.) and within

84 min. for an electrode separation of 10 mm (0.04 in.).

Two additional experiments were conducted under simplified conditions to

confirm the observations made in the first experiment. The simplifications

were 1) to use a single electrode couple and 2) to conduct the test at constant temperature. Experiment 2 was conducted at room temperature with an electrode separation of 10 mm (0.4 in.). Experiment 3 was conducted at

42°C (108°F) with an electrode separation of 8 mm (0.3 in.). The results of

these tests are summarized in Table VII. Both tests revealed a reversal of

polarity, i.e., the steel was initially anodic but became cathodic with

61
C

O -

•pH cO

4-> CD CO CO io CO 0O LO O S- C\] C\J 00 00 00 OO 3 <X X3 CD C

CC -H

c

o

*1—

1

p>

03

faO C O PT OO r- 0O CO O • • • • • •

i—

1

O O^i T— OO OO o

x on c\j OO OO x x

t— PT OO cO co x •pH • » • • * • co 00 00 co cr\ CTC LO x t- b- co CO C— [— — — ^ —p ^-p

co

Tensile rength,

MPa

b- CTs co o o o

-P pi CTc CTi t— CO LO 00

i—

i

00 X- X x x LO LO

ate

test

resu

4-> C

CDS

c 00 co 00

s_ o 00 00 00 p X X X

c •r—

1

s N X •i—

1

> p < S- < S- <

03 C •pH X •pH X •rH X

x <C S < S < S

I

.

Slow

st

n

of

ngth 00 00 oo oo > O CD iH X CD CD CD CD X X

00 00

CD -p CO CO CO CO

1 1 CCJ CD S- P L SU X3 "O

JD O 50 3 3 3 3 rH r—

1

03 O 03 O O O O CD CD H X O O O O O 2 2

62
N2

Figure 25. Test apparatus for galvanic corrosion studies.

63
Table VII. Results of the galvanic corrosion tests

Summary of Experiment 1

Time Temperature

(°C) ( °F

)

d - 1 mm (0.04 in.) d = 10 mm (0.4 in.)

Current

pA Status of Steel vs. Monel CurrentpA Status of Steel vs. Monel

0 21 (70) 15 Anod ic 100 Anod ic

14 min. 21 (70) 0 — 10 Anodic

20 min. 21 (70) 20 Cathodic 6 Anod ic 48 min. 21 (70) 20 Cathodic 3 Anodic

Heat Turned On

56 min. 26 (79) 20 Cathodic 5 Anodic

72 min. 44 (111 ) 45 Cathod ic 8 Anod ic 77 min. 49 (120) 70 Cathodic 0 — 84 min. 52 (126) 5000 Ca thod ic 1600 Ca thod ic 96 min. 52 (126) 110 Cathodic 30 Cathodic

2 h 15 min. 47 (117) 25 Ca thodic 19 Cathod ic Overnight 41 (106)

24 h 41 (106) 2 Cathod ic 50 Cathodic

o t opp 0 d

Summary of Experiment 2

Time Temperature

°C (°F)

d -

Current

pA

1 0 mm (0.4 in.)

Status of Steel vs. Monel

0 22 (72) 100 Anodic

15 min. 22 (72) 10 Anodic

60 min. 22 (72) 5 Anod ic

2 h 10 min. 22 (72) 0 — 2 h 20 min. 22 (72) 0.4 Cathod ic

2 h 35 min. 22 (72) 0 — 4 h 22 (72) 1

1

Anod ic? 6 h 20 min 22 (72) 0 — 8 h 22 (72) 1 Cathod ic 12 h 22 (72) 12 Cathod ic 24 h 22 (72) 12 Cathod ic 48 h 22 (72) 20 Cathodic

Stopped

Summary of Experiment 3

d = 8 mm (0.3 in.)

Temperature Current Status of

°C (°F) pA Steel vs. Monel

0 35 (95) 1000 Anod ic 20 min. 40 (104) 40 Anod ic 35 min. — 10 Anod ic 55 min. 43 (109) 0 — 2 h 42 (108) 10 Cathodic

4 h 42 (108) 8 Cathodic

16 h — 5 Cathod ic 24 h 42 (108) 5 Cathodic

64
respect to the Monel. After the initial current decayed, the current

density dropped to approximately 1 pA/cm within 35 min. in both tests. The

polarization reversal and the low current densities were attributed to the

passivating effect of a black deposit that was presumed to be a compound of

iron and sulfur.

Fracture Mechanics Test for Stress Corrosion Cracking

The susceptibility of the vessel steel to stress corrosion cracking

* along the HAZ was evaluated using the bolt -loaded specimen shown in

Figure 26. The test environment was the MEA/H

2 S solution at room temperature. Exposure time was 1008 h. Four specimens from Weld 2/3 were tested,

two with the crack in HAZ 2 and two with the crack in HAZ 3-

The tests were conducted in accordance with the procedures used by Novak

and Rolfe [11]. The specimens were loaded to a stress intensity level of

approximately 22 MPa/m (20 ksi/in.), the maximum stress intensity that met

the ASTM E 399 criteria for plane strain [12]. Load was applied by torquing

the bolt to a predetermined displacement level of 0.14 mm (0.0055 in.).

After loading, the specimens were immersed in the MEA/H

2 S solution for the

duration of the test. The specimens were checked during the course of the

test, but no crack extension was observed. Careful examination after 1008 h

again revealed no crack extension at the surface.

Two of the specimens were unloaded after immersion for 1008 h, one from

HAZ 2 and the other from HAZ 3. The specimens were unloaded by turning the

* The results of the stress corrosion cracking tests presented in this

section are not considered reliable by NBS because the fatigue precracking

loads were approximately 40$ higher than the sustained loads used during

the stress corrosion test. The influence of higher fatigue loads is not

known, but good practice would have been to use lower fatigue loads than

the sustained loads. The procedures and results are presented herein as a record of the work done. The mistake was not discovered until completion

of the tests, and the influence of the mistake on the results is not

known.

65
6.30

~*r 8 1 .3 mm

Figure 26. Specimen for fracture mechanics test of stress corrosion

cracking susceptibility.

66
bo Li until it was loose. The displacement change as the crack was unloaded

was measured with a clip gage. After unloading, the specimen was loaded in a

.. -raw-driven tensile machine to its initial displacement, measured by the name clip gage. The load to achieve the displacement was measured and

recorded. The specimen was then unloaded and cooled in liquid nitrogen to

77 K (-321 °F). The cooled specimen was loaded to failure immediately after

taking it from the liquid nitrogen. The crack length at fracture (and at the

end of the stress corrosion test) was measured on the exposed fracture

surfaces.

The fracture surfaces of the broken specimens were examined using a X30

optical microscope to see if any sustained-load cracking had occurred during

the test. No evidence of cracking beyond the initial fatigue crack front was observed in either specimen. Using the initial crack size and the load

required to open the specimen to its final displacement level, the stress

intensity, K, was calculated to be 20.9 MPa/m (19.0 ksi/in.) and 20.8 MPa/m

(18.9 ksi/in.) for the specimens of HAZ 2 and HAZ 3> respectively. The

fracture surfaces of the two specimens are shown in Figure 27. The other two

specimens were returned to the solution for the indefinite future to evaluate

the effect of longer exposure times. The test results are summarized in

Table VIII.

Chemical Analysis of Corrosion Products

The test plate removed from Ring 1 was taken from a location covered

by the Monel liner. When the liner was removed, a heavy black deposit was observed on the steel plate. The deposit had two distinct textures. In one

region, it was sticky and came off in large flakes; this is referred to as Scale 1. In another region, it was dry and more powdery; this is referred to

as Scale 2. Samples of Scale 1 and Scale 2 were removed and were analyzed by

emission spectroscopy for elements such as iron, chromium, nickel, and

manganese, and by the gas fusion method for carbon and sulfur.

The emission spectroscopy procedure was based on ASTM Committee E 02

Suggested Method, SM1 1 -22 "Semiquantitative Spectrograph ic Analysis of

Miscellaneous Materials by the Graphite Dilution Technique" [13]. The

samples were dried at 54 °C (130°F) for 4 h f ground in a mu llite mortar and

67
C2Q C3Q C2T C3T

Figure 27. Fracture surfaces of specimens after completion of fracture

mechanics tests of stress corrosion cracking susceptibility.

Arrows point to fatigue precracks.

68
Table

VIII.

Results

of

the

fracture

mechanics

test

for

stress

corrosion

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69

stress

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Novak

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[

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.
dried further overnight at 54 °C. Duplicate 1 mg specimens were taken from

both scale samples, mixed with graphite, and volatilized in a dc arc. The

spectra were photographed and visually compared with commercial reference

materials treated in the same manner. The gas fusion analyses for sulfur and carbon were performed in a com- mercial instrument calibrated with NBS Standard Reference Materials (SRMs).

For sulfur, SRM 129c was used for calibration and SRMs 133b and 368 were run

as controls. For carbon, SRM 12h was used for calibration and SRMs 1 1 h , 129c

and 133b were run as controls. Four-gram samples were taken of each scale

and dried at 115°C (239°F) for 48 h. Test portions of these samples weighing

10 to 25 mg were put into ceramic crucibles, covered with 1 g of high purity

iron and 1 g of tungsten accelerator. Twelve test portions of scale were

analyzed.

The results of the chemical analyses, summarized in Table IX, indicate

that the main constituents of the black deposits are iron, sulfur, and

carbon. The high sulfur content, approximately 15 % by weight, is significant

because the probable source of the sulfur was sulfide corrosion of the steel

vessel by the aqueous H

2 S environment. This same corrosion process generates

hydrogen at the steel surface, and thus, it is a probable source of the

hydrogen that damaged the steel. The probable sources of the other elements

detected in the black deposits were iron, manganese, and silicon from the

steel; copper and nickel from the Monel; sulfur from the corrosion product;

and carbon from the hydrocarbons in the process stream. The differences in

the results for Scales 1 and 2 are not considered to be significant.

70
Table IX, Chemical composition

Monel liner.

(weight precent) of scale found behind the

Method Element Scale 1 Scale 2

Emiss ion Spectroscopy A1 0.05 0.05

Ca < 0.05 T < 0.05 T

Cr 0.05 0.05

Cu 2 2

Fe >20 >20

Mg 0.01 0.01

Mn 1 1 Na 0.1 0.1

Ni 2 2

Si 0.2 0.2

Gas Fusion S 16 ± 4

C\J +l

C 14 ± 2 9 ± 1

71
Summary Comments

Tests to evaluate the susceptibility of the vessel materials to corrosion and hydrogen cracking indicated that:

1. The steels used for Courses 1 , 2, and 3 were all susceptible to

hydrogen pressure cracking in a hydrogen-charging environment.

2. The vessel steels and the repair welds were not susceptible to

stress corrosion cracking in an MEA/H

2 S environment.

3. The galvanic corrosion tendencies between the steel and the Monel

liner were insignificant.

4. Hydrogen-induced cold cracking could have occurred in the HAZ of

Course 1 at the time Course 2 was replaced; however, this mechanism

of cracking does not account for the extensive cracking observed in

the vicinity of the repair welds.

72
FRACTURE SURFACE OBSERVATIONS

The most important pieces of evidence in many failure investigations are

the fracture surfaces. The examination of fracture surfaces can often reveal

where the fracture began, the nature and size of the initial defect that

triggered the fracture, the path and direction of propagation, and the relative fracture resistance of the material. This was indeed the case for the

present investigation.

Information Received from the Fire Marshal’s Investigation

The investigation by NBS significantly benefited from the FMI on the same vessel pieces [1]. As the first step of the investigation, three NBS

metallurgists visited Packer Engineering and carefully reviewed their

findings with emphasis on the fracture surface observations. This was an obvious first step for practical reasons, but it was also important for a fundamental technical reason. The FMI began more than one year before NBS

received the vessel pieces. During this period, the fracture surfaces

deteriorated due to corrosion, and some portions of the surface were removed

for the FMI studies.

Fortunately, a photographic record in color of the fracture surfaces in

both the original condition and after cleaning was available to NBS from the

FMI. The photographs were taken at 150 mm (6 in.) intervals around the

c ircumference before any specimens were removed from the fracture surface.

Areas of black deposit were visible on the photographs of the Ring 2

fracture surface. These deposits were analyzed in the FMI and found to be

rich in sulfur [1], The source of the sulfur was assumed to be the H

2 S in

the pressure vessel. Thus, FMI concluded that the deposits delineated cracks

that existed in the vessel prior to the catastrophic fracture. The black

deposits indicated that pressure vessel 1 2D 701 was cracked to depths

exceeding half its wall thickness over nearly 40% of its circumference.

Photographs of the Ring 1 fracture surface did not reveal the black

deposits, presumably because the deposit was oxidized during the fire. Two

sets of color photographs made from the original negatives of the cleaned

fracture surfaces (Rings 1 and 2) were obtained, assembled into montages of

both fracture surfaces, and used by NBS in the present investigation.

73
The metallographic and scanning electron microscopy (SEM) studies of the

FM I were also reviewed by NBS. Copies of the photographs taken to document

the fractographic studies were obtained by NBS. These were of particular

interest because the condition of the fracture surfaces may have deteriorated

between the time of these studies and the start of the NBS investigation.

Metallurgists at NBS carefully reviewed the FMI . The observations and

interpretations made by NBS personnel on the photographs from the FMI are

part of this study.

Visual Observations

Upon receipt of the pressure vessel pieces, the fracture surfaces were visually examined in the original condition with the unaided eye and with

a low-power (up to 35X) optical microscope. The surfaces were covered with

an adherent, dark, rustlike layer that obscured many of the features.

However, several notable features were apparent. For about 80% of the

c ircumference, the fracture occurred in Course 1 adjacent to Weld 1/2, i.e.,

in HAZ 1. The remainder of the fracture path was on the other side of

Weld 1/2, in HAZ 2, except for a 0.6 m (2 ft.) portion where the crack

deviated away from Weld 1/2 and into Course 2. In the region where the

fracture path was confined to the HAZ, the fracture surface had a fibrous

(sometimes called woody) appearance at low magnifications, and there was no evidence of the chevron markings characteristic of brittle fracture or of the

shear lips character istic of ductile fracture. Additional observations were

made after the fracture surfaces were cleaned with high pressure steam and by

scrubbing with brushes and detergent. Examination of the fracture surface of

Ring 2 revealed the black deposits on the fracture surface were readily

visible, but the contrast was not as sharp as on the FMI photographs.

The fracture surface of Ring 2 was examined with an optical microscope

in the regions corresponding to those where the black deposits were found on

the mating fracture surface. The deposits had a reddish color. When a sharp

probe was used to dig into the surface deposits, the red color became even

more apparent. Presumably, the black deposits reacted with oxygen and became

a red oxide of iron during the fire.

74
The montage of the Ring 2 fracture surface was the principal means of

locating and sizing the regions on the fracture surface that were covered

with a black deposit. On the basis of the findings of the FMI, we assumed

that the black deposits delineated cracks that existed in the vessel prior to

the catastrophic rupture. These are referred to as pre-existing cracks. The

location and length of the pre-existing cracks are summarized in Figure 28.

The crack depths are not indicated in this figure because of uncertainties in

the depth measurement.

One of the pre-existing cracks was located between CC 7.4 m and

CC 8.2 m. The depth of this crack exceeded 80$ of the wall thickness over most of its length and exceeded 90% of the depth for about half its length.

The location of this crack coincided with the location of the leak, as reported by the eyewitness, i.e., between the ladder ( CC 6.7 m to CC 7.2 m)

and the inlet valve ( CC 8.3 m) at the approximate level of the valve (the

level of Weld 1/2). The location reported by the eyewitness is considered

reliable because the vapor plume from the leak interfered with his efforts to

climb the ladder and close the valve at the time of the leak. Thus, we assumed that the catastrophic rupture initiated at this crack.

In addition to locating the regions of pre-existing cracks, Figure 28

summarizes the observations regarding the paths and directions of fracture

propagation. These observations were made directly on the fracture surfaces

of Rings 1 and 2. The crack that initiated the fracture, referred to as the

initiation, was 800 mm (31 in.) in length and was in HAZ 1 between CC 7.4 m and CC 8.2 m. The fracture propagation from the CC 7.4 m end occurred along

HAZ 1. It temporarily jumped into HAZ 2 between CC 4.3 m and CC 3.9 m and

again between CC 3.1 m and CC 2.5 m. At the locations where the fracture

jumped out of HAZ 1 ( CC 4.3 m and CC 3*1 m) , the fracture tunneled a short

distance under Weld 1/2 along HAZ 1. This suggests that the crack was propagating in the counterclockwise direction (see Figure 28) and continued

in this direction for a short distance beyond the place where it jumped to

HAZ 2. A similar indication of counterclockwise propagation was observed at

CC 2.75 m, where the crack veered upward through Weld 1/2 into Course 2. The

path of the fracture within Course 2 was clearly delineated by chevron

markings that pointed to the origin. It propagated through Course 2 in the

75
POSITION, meters

Figure 28. Sketch of fracture surface showing pre-existing cracks (shaded

areas), fracture path, and direction of fracture propagation.

76
• >unterclockwise direction until the CC 1.5 m position, where it connected to

• h-- crack coming from the other direction. A snail piece of the vessel was

:n 1 ir.ing at this location. It is assumed that the piece broke free when the

two cracks, which were traveling in opposite directions, connected. The

fracture propagation from the CC 8.2 m end may have been temporarily pinned

by the re in forcement for the hydrocarbon inlet piping. After passing around

th>' rein forcement, the crack propagated from CC 0.1 m to CC 1 . 5 m through

HAZ 1 .During the final stages of fracture, the vessel started to tip over and

plastically strained the remaining material that was located in the vicinity

of CC 1.5 m. This can be seen from the opened cracks on the inside surface

of the piece cut at CC 1 m (Figure 29) and on the inside surface of a piece

cut at CC 1.5 m where the fracture passed through Course 2 above Weld 1/2

(Figure 30). Metallography of the plastically stretched cracks taken from

these pieces is shown in Figures 11 and 17.

The path of propagation was unusual in two respects: 1) Fracture

occurred perpend icular to the axial stresses rather than perpendicular to

the hoop stresses, which are twice the magnitude of the axial stresses.

2) The fracture path was confined to a path near the HAZ for over 90$ of the

c ircumference. Normally, once a fracture begins to run, it has sufficient

strain energy to follow a more arbitrary path.

The fracture initiated perpendicular to the axial stresses because the

initiation was a very large crack (800 mm, 31 in.) that was oriented perpendicular to the axial stress. In the MPI , all crack indications longer than

25 mm ( 1 in.) were along the HAZ of the circumferential welds (Welds 1/2

and 2/3). In comparison, the vertical welds and their HAZs (in Rings 1, 2,

and 3) had relatively few crack indications.

The crack was confined to the HAZ of the circumferential welds because

it was a low energy fracture. There was essentially no shear lip formation

at the crack border until the final stage of cracking; shear lips developed

when the crack veered up into Course 2. The plate orientation was such that

the minimum toughness path was in the circumferential direction. The

Charpy toughness results indicated that the weld metal toughness ( 1 60 J,

1 2A ft*lbf) was considerably higher than the HAZ toughness (70 J, 5^ ft*lbf).

Thus, deviation from the HAZ was energetically unfavorable.

77
MET.

Fracture Surface

Course 1 HAZ

Weld 1/2

Course 2

Figure 29. Section of fracture removed for metallograph ic studies with arrows showing cracks on inside surface of HAZ 2.

78
Fracture Surface

79

Figure

30.

Piece

cut

from

Ring

1

where

the

fracture

passed

through

Course
Metallography

Metallography was used to investigate the nature of crack initiation,

subcritical growth through the thickness, and propagation around the circumference.

The metallographic studies of the FMI were extremely informative. The

sections of particular interest were those through the fracture surface in

regions of black deposit. Etched cross sections of the mating fracture surfaces were taken at each location (Plates 44, 51, 58, and 62 in the FMI report

[1].) In reviewing these cross sections as part of the present study, we noted that in each case, the crack intersected the inside surface in the HAZ

,

close to the fusion line. The crack made a straight trace through the HAZ,

typically 1 to 2 mm (0.04 to 0.08 in.), and then zigzagged through the base

metal to about the midthickness level. It then entered the HAZ and propagated through the HAZ or the weld metal to the outside surface.

The area on the fracture surface identified as the initiation in

Figure 28 was examined metallograph ically and the observations were compared

with those made on the FMI specimens. Two etched sections taken through the

initiation at the CC 7.4 m and CC 7.6 m locations are shown in Figure 31.

Note that in both cases the crack passed through the HAZ at the inside

surface, propagated through the base metal near the HAZ for most of the

thickness, and passed through the weld at the outside surface. In the

specimen at CC 7.4 m (Figure 31b), the crack intersected the surface in the

HAZ near the fusion line, the location expected on the basis of the FMI

metallography and consideration of microhardness. However, the specimen at

CC 7.6 m (Figure 31a) was slightly different. The crack intersected the

inside surface in the HAZ about 1 mm (0.04 in.) away from the fusion line.

The Knoop microhardness measured adjacent to the crack ranged from 209 to 251

(HK 500), a hardness level deemed not susceptible to crack initiation. Thus,

crack propagation along (or to) the surface may have occurred.

Several long cracks were detected in the HAZ of Weld 2/3 above the

initiation site in Weld 1/2. Metallographic sections were taken through the

cracks at CC 7.6 m (Figure 32) and at CC 5.9 m (Figure 33). These cracks

were examined metallographically and then broken open to expose the fracture

surface for fractograph ic examination. In both cases, a black deposit was

80
i. d.

Course

Course 2

Figure 31 .

Course 1

o. d.

a. Section at CC 7.6 m

i . d

.

Course 1

o . d

.

b. Section at CC 7.4 m

Metallographic sections through the fracture surface at the

location of the initiation crack.

81
Crack in HAZ 3 and microhardness

indentations at location 3

in Figure 32a

200 ^TM

1 mm

b. Crack in HAZ 2 at location A

in Figure 32a

d. Knoop hardness values (HK 300)

(tracing of Fig. 32c)

Figure 32. Microstruc ture of Weld 2/3 at CC 7.6 m, metallography of cracks

in HAZ 2 and HAZ 3, and microhardness of HAZ 3.

82
i . d.

b. Knoop microhardness values of HAZ 3 at location A in rig. do a 200/im

Knoop hardness indentation of HAZ 2 at locations B and C in Figure 33a 200/xm

CO o ^ cm <r 04 m m

324

' /

385 |

1

339 ;

id l l

• oo in 04

' sO sO o m , m | m

|| 330

' 306

328

/ j

d. Knoop hardness values (HK300)

at locations B and C (tracing of Fig. 33c)

Figure 33- Macrostructure of Weld 2/3 at CC 5.9 m and microhardness values

of HAZ 2 and HAZ 3-

83
found on the crack surfaces that was similar to the deposit found on the

Ring 2 fracture surface. Thus, black deposits did form on cracks open to the

inside of the vessel. This supports the idea that the black deposits on the

fracture surface indicated cracks that existed at the time of the fracture.

The crack in HAZ 2 of Weld 2/3 at CC 7.6 m (Figure 32b) had the same features observed in sections taken through regions of black deposit on the

fracture surface. The crack intersects the surface at the fusion line, runs

in a straight line through the HAZ, and then zigzags through the base metal

to about the midthickness level, where it stops in the HAZ.

The crack in HAZ 2 of Weld 2/3 at CC 5.9 m, propagated through the HAZ

for nearly 10 mm (0.4 in.), yet it had the same key features observed in the

other specimens. The crack intersected the surface near the fusion line

and had straight edges while located in the hardened part of the HAZ.

Microhardness indentations along this straight edge are shown in Figure 33c.

When it propagated through the HAZ that had been tempered by a subsequent

pass, it changed to the zigzag mode and continued in this mode into the base

metal.

Several sections were taken through the fracture surface to determine

the path of the fracture in locations that were not marked by a black

deposit, i.e. , whether the crack was located in the HAZ or in the base metal

near the HAZ. In general, the crack path included some HAZ near the surface,

but most of the crack front passed through the base metal. This can been

seen from the sections taken from Weld 1/2 at CC 1.0 m shown in Figure 15 and

from Weld 1/2 at CC 1.3 m and Course 1 at CC 5.7 m shown in Figure 34. Thus,

the path of the catastrophic fracture is more correctly described as being

near the HAZ rather than through the HAZ.

Fractography

The fracture surfaces were examined with a scanning electron microscope

(SEM) in an effort to identify the micromodes of fracture of the initial

defect and the propagating crack. In addition, cracks detected near Weld 2/3 were broken open and examined to clarify the mechanism of crack formation.

It was not possible for NBS to conduct meaningful SEM studies of the

vessel's fracture surface because we could not clean the fracture surfaces

without damaging the microscopic features. However, the low magnification (XI 0)

84
Course 2

Macrostructure of Weld 1/2 at CC 1.3 m

b. Enlargement of Figure 3 4a at

inner surface

1mm

1mm

d. Enlargement of Figure 3^c

at inner surface

Figure 3^. Macrostruc tures at two locations where a black deposit was not

found on the fracture surface.

85
fractographs from the FMI were quite informative and two representative

samples, PE 103 (CC 5.2 m) and PE 114 (CC 7.3 m) , are discussed below.

The metallographic section and corresponding fractograph of specimen

PE 103 are shown in Figure 35. This specimen was taken at CC 5.2 m in a region of black deposit. The metallographic section has the characteristic

appearance discussed previously: a straight-sided crack intersecting the

surface in the HAZ near the fusion line and beginning a zigzag path through

the base metal about 1.5 mm (0.06 in.) below the surface. The fractograph

clearly shows the transition from a straight-sided crack in the HAZ to

fibrous (zigzag) texture at a depth of 1.5 mm (0.06 in.). In this case the

final fracture passed through the weld, and a marked transition from fibrous

to uniform fracture texture can be seen on the fractograph. Examination of

the fractographs from the FMI at higher magnification in the region of

uniform texture indicates that the fracture mode through the weld was

due tile -dimple fracture.

The metallographic section and corresponding fractograph of specimen

PE 114 are shown in Figure 36. This specimen was taken at CC 7-3 m in a region free of the black deposit. The metallographic section shows that the

fracture passed through the HAZ and base metal. The fracture intersects the

inside surface in the HAZ near the fusion line and the fibrous texture is

somewhat finer near the surface. Yet, the distinct change in texture noted

in specimen PE 103 was not apparent.

The cracks near Weld 2/3 were thought to be similar to the ones existing

in Weld 1/2 except that they had not been exposed to fire and corrosion by

the vessel fracture. Thus, we expected that there would be less corrosion

damage on these crack surfaces and that improved fractograph ic observations

could be made.

The crack in Weld 2/3 at CC 7.6 m was located directly above the initiation site and is thought to be equivalent to the pre-existing cracks near

Weld 1/2. This crack, shown in Figure 32b, was broken open at room temperature to expose the fracture surface for SEM studies. There was a heavy,

loose black corrosion product near the inside vessel surface, but the end of

the crack had a very thin black deposit. This region was examined by SEM, and

the fractographic features of the crack were compared with the features of

86
i . d.

o. d.

b. SEM fractograph of the fracture sur at the location shown in Figure 35a

Figure 35. FMI [ 1 ] photographs of the macrostructure of Weld 1/2 and

fractograph at CC 5.2 m. The letters show corresponding areas

87
Course 2 Course 1

i . d.

a. Macrostr uc ture of Weld 1/2 at CC 7.3 m

o. d.

b. SEM fractograph of the fracture surface

at the location shown in Figure 36a

Figure 36. FMI [1] photographs of the macrostruc ture of Weld 1/2 and

fractograph at CC 7.3 m. The letters show corr espond ing areas.

88
• freshly exposed fracture surface created by breaking the crack open.

At low magnifications, the features of this crack and the freshly exposed

franture are remarkably similar. In both cases, there are regions of dimples

roughly aligned with the plate surfaces. At higher magnification, as shown

in Figure 37, one can see dimpled regions separated by smooth areas often

containing splits associated with inclusion stringers. On the freshly

rxposed fracture surface, the regions separating the rows of dimples have a rougher texture and there are fewer splits.

The fracture features on the crack surface shown in Figure 37 are characterist ic of hydrogen pressure cracking. The smooth regions develop

where the metal splits due to internal pressure developed by hydrogen that

collected at the inc lus ion-matr ix interface. These features are compared

with fractographic observations made on a delamination induced in a Course 1 specimen by charging the specimen with hydrogen using the NACE solution

(acidified seawater, pH = 3). The specimen was pulled apart to expose the

surfaces of the de laminat ion. Fractograph ic examination of the delaminated

surface revealed smooth regions separated by cleavage facets, as shown in

Figure 38. The similarity in the smooth regions of the delamination

(Figure 38) and the crack (Figure 37) supports the contention that the

subcritical crack growth observed in the pressure vessel was due to hydrogen

pressure cracking.

Microchemical Analysis of the Black Deposits

Several samples of the fracture surface were removed from regions marked

by the black deposit and used to identify the principal chemical constituents

of the deposit. The samples were ground and polished perpendicular to the

fracture surface and then analyzed by energy dispersive X-ray spectrometry

and also by wavelength dispersive X-ray spectrometry. The results indicate

that the black deposit is rich in sulfur. A report of the procedures and

results is given in Appendix F.

89
a. Fractograph of pre-existing crack

that was originally covered with

a black deposit

b. Fractograph of a freshly exposed

surface (broken open in the

laboratory)

Figure 37. Fractographs of the crack in HAZ 2 (shown in Figure 32) of

Weld 2/3 at CC 7.6 m.

90
L. 150 pm

t » 30 ym

Figure 38. SEM fractographs of hydrogen pressure cracks in Course 1.

91
Summary Comments

Metallurgical examination of the fracture surfaces and cracks found in

Weld 2/3 revealed:

1 . Black deposits on the fracture surface delineated cracks that

existed in the vessel prior to rupture. The cracks were open to

the inside surface of the vessel and extended to depths beyond

midthickness for almost 4

0

% of the circumference.

2. A black deposit delineated an 800 mm long pre-existing crack in the

vicinity of the leak that preceded rupture.

3. The fracture path was confined to the HAZ in Course 1 adjacent to

the repair Weld 1/2 for about 8 0% of the c ircumference.

4. The pre-existing cracks delineated by black deposits and similar

cracks found in the HAZ of Weld 2/3 initiated at the inside surface

of the vessel adjacent to the field welds in regions of hard

microstructure.

5. The pre-existing cracks initiated and followed a straight path

through the hard part of the HAZ to a depth of 1 to 2 mm. Crack

initiation of this type is attributed to hydrogen stress cracking.

6. At a depth of 1 to 2 mm, the cracks reached a softer microstructure

and began to zigzag through the wall thickness. Crack propagation

of this type is attributed to hydrogen pressure cracking.

92
FRACTURE MECHANICS ANALYSIS

A fracture mechanics analysis was conducted to determine the role of

stress, flaw size, and material properties in the vessel failure. The analysis was based on the crack tip opening displacement (CTOD) criterion, i.e.,

fracture occurs when the CTOD of a crack in the vessel exceeds the fracture

toughness of the steel, which can also be expressed in terms of CTOD. The

CTOD of a crack in the vessel is calculated using Irwin’s equation [14]:

CTOD =

where K =

4

IT

o/ira Y ( a//Rt) ,

(4)

and

oa

R

t

applied stress,

half length of a through-thickness crack,

radius of pressure vessel,

thickness of the vessel steel at the location of the fracture,

0.2/t offset yield strength of the steel,

modulus of elasticity.

The parameters R and t are dimensional values known for the vessel, and

0 y S

and E are material properties known for the vessel steel. The function

Y(a//Rt") is the geometrical term used to calculate the stress intensity for a

c ircumferential crack in a cylindrical shell [15]. The axial stress is

readily calculated for the case of a pressure vessel operating at a known

pressure

o = || = 35 MPa (5.1 ksi) , (5)

where p

= operating pressure = 1.38 MPa (200 psi)

,

R = radius = 1 .3 m (4.3 ft.),

and t = thickness = 25.4 mm (1.0 in.).

The crack length, can be estimated from the length of the black deposit

that marked the initiation. The black deposit existed from CC 7.40 m to

CC 8.20 m, and it delineated a part-through crack that was 800 mm (31 in.)

long. The leak occurred because the depth of the crack reduced the remaining

ligament of material to the extent that it could no longer support the

93
operating pressure of the vessel. Assuming failure at the tensile strength,

o ts , of the steel, leakage occurred when the crack depth, h, reached the

value

From examination of the color photographs of the fracture surface, we estimated that this crack depth (0.93t) existed between CC 7.57 m and CC 7.82 m. When the ligament in this region broke and caused the leak, the length of the

through-thickness crack was approximately 250 mm (10 in.). This crack then

propagated through the thin ligament of steel remaining in the area of the

initial part-through crack, i.e., the crack attained the length of the black

deposit. Thus, the value of crack length used in the fracture mechanics

analysis was 800 mm (31 in.).

Substituting 2a = 800 mm (31 in.) and a = 35 MPa (5.1 ksi) into

equation (4) results in a CT0D value of 0.064 mm (0.0025 in.). This means

that fracture is predicted to occur if the fracture toughness is less than

0.064 mm (0.0025 in.). However, the fracture toughness results indicated a minimum toughness level of 0.17 mm (0.0067 in.). Thus, the fracture

mechanics analysis predicts that an 800 mm (31 in.) long crack along the weld

in the pressure vessel would be stable, i.e., it would leak but not fracture.

But, of course, the pressure vessel failed. Uncertainties in the

analysis assumptions for stress and flaw size cannot account for the 2.6-fold

difference between the calculated driving force for fracture (applied

CT0D = 0.064 mm, 0.0025 in.) and the measured fracture toughness (critical

CT0D = 0.17 mm, 0.0067 in.).

Fracture Toughness Tests in an MEA/H

2 S Solution

One of the main differences between the fracture toughness tests and the

pressure vessel failure was the different environment. The tests were con- ducted in air at 38°C (100°F), but the vessel contained a mixture of hydrocarbons, MEA, and HgS. The environment in the vessel may have reduced the

fracture toughness of the steel. To assess this possibility, two fracture

toughness tests were conducted in an MEA/^S solution.

h = t 1 ( 6 )

94
The tests were conducted on specimens from Weld 2/3 with the notch in

HAZ 2. The solution was a 20% solution of MEA (commercial grade) in water

with a 0.5 molar concentration of H

2 S. A precracked SENB specimen (Figure 9) was installed in a test machine equipped with a box for holding the MEA/H

2 S

solution. The solution was poured into the box until the specimen was completely immersed. The solution was heated by water flowing through a copper coil in the bottom of the box. The temperature was controlled at

38° ± 1 °C (100°F) for 10 min. prior to immersing the specimen. Testing was started approximately one hour after immersion of the specimen.

The fracture toughness testing procedure was the same as that used for

the tests in air, except that the loading rate was lower. The test sequence

was to load the specimen over a period of about 15 min. at a constant

displacement rate of 6 x 10

-

^ mm/s (2.4 x 10

-

^ in./s) from zero to the stress

intensity (40 MPa/m, 36 ksi/in. ) calculated for an 800 mm (31 in.) long

crack. This load value was held for about one hour, and then the loading was resumed at the initial displacement rate and continued past maximum load.

The fracture toughness results in the MEA/H

2 S solution are summarized in

Table X. The CT0D values of 0.19 mm (0.0075 in.) and 0.24 mm (0.0094 in.)

and the corresponding load-displacement records were essentially the same as those for minimum-toughness specimens tested in air at 38 °C (100°F). There

was no apparent crack extension during the hold time; i.e., the loaddisplacement records did not show any incremental displacement at the

constant load. Thus, in these tests, the MEA/H

2 S environment had no significant effect on fracture toughness.

Fracture Toughness Tests on Hydrogen-Charged Specimens

Another significant difference between the conditions of the fracture

toughness tests and those of the pressure vessel failure was the condition of

the material. In the case of the pressure vessel, the steel was charged with

hydrogen during the several months of service following the spring 1984

inspection. Hydrogen was known to enter the pressure vessel steel because of

the hydrogen-pressure cracks observed in this investigation. In contrast,

the steel used for the fracture toughness specimens probably had a low

hydrogen content, because the hydrogen was able to diffuse out of the steel.

95
co

•I

—I p3I—I o

co oo

oo X PO

053Oop

CD3

•o

CO0PO2

CDa,

03 pCO

[ij oo QOHO

Ol O

CMO

O

COonp

03 TD

CD P

CO

CD P O

(P

=r

coc

CDs

•I—I o

CDQ.

CO

X

03E2

Oh J*

CO

LOcn o

lo 3op

co p

r-H 3

CO

CD3P

COCD P

IP

00

OOO

LP

CTL LP

00

COO

LP

CO

CO

CDCP

bO3OP

CQ

LPO

OO

O

00

CD33PO33P

OO\CM

TOi—I 3 X

CD PO3H 33E

•l

—I O3

Q,

00 PO

00N<p

LO<

LOw

96
Thus, a series of fracture toughness tests was conducted on specimens that

had been charged with hydrogen to determine whether hydrogen lowered the

fracture toughness of the steel.

The specimens were machined and the notches were compressed to ensure straight fatigue cracks, and then the specimens were precracked by fatigue.

The specimens were immersed in the NACE solution (salt water and acetic acid

at pH = 3) at room temperature for periods of 96 to 120 h. During the final

hours of exposure, the temperature of the NACE solution was raised to 38 °C

(100°F). Each specimen was removed from the solution, installed in the test

machine within 3 min. and loaded immediately at a constant displacement

rate, 6 x 10~^ mm/s (2.4 x 10

-

^ in./s) for SENB specimens and 4 x 10

-J4 mm/s

(1.6 x 10

-5 in./s) for compact specimens. During loading, typically one hour

to failure, the specimen was enclosed in a box in which that the temperature

was controlled to 38 ± 1 °C (100°F).

Two types of tests were conducted on separate groups. The first group

consisted of three SENB specimens taken from Ring 3 and notched near HAZ 2

(1 specimen) or HAZ 3 (2 specimens). These were tested in accordance with

the procedures of BS 5762 [6]. The CTOD values measured were 0.096 mm

(0.0038 in.) in HAZ 2 and 0.070 mm (0.0028 in.) and 0.064 mm (0.0025 in.)

in HAZ 3. The HAZ 3 values are similar to the calculated driving force of

0.064 mm (0.0025 in.), and thus provide a reasonable explanation for the

fracture. Thus, it was decided to conduct additional tests on specimens

notched in HAZ 1 of Weld 1/2, the location where the vessel failed.

The HAZ 1 specimens were taken from Ring 1 between CC 1.62 m and

CC 2.29 m. This piece was cut from the portion of Ring 1 where the fracture

propagated through Course 2, away from Weld 1/2. The fracture toughness

specimens were compact specimens of the IT configuration (width = 50.8 mm, 2.00 in. and height = 61 mm, 2.40 in). The CTOD at maximum load was calculated using the analysis method given in the Draft ASTM Test Method for Crack

Tip Opening Displacement (CTOD) Testing:

6 = K

2

(1 - v

2

) r (W - a ) V

A P o P 2a E

ys

r ( W - a ) + a

p 00 (7)

97
where r

p

= 0.4 = rotation factor,

W = specimen width,

a

Q

= initial crack length,

V

p = plastic part of load line displacement,

v = Poisson's ratio,

0 y S

= 0.25S offset yield strength,

and K = stress intensity factor

K = YP/B/W, (8)

where P = load,

B = specimen thickness,

and Y = stress intensity factor coefficient;

Y = (2 + a/W) [0.886 + 4.64 a/W - 13-32(a/W) 2 (9)

+ I4.72(a/W) 3 - 5.6 (a/W)^]/( 1 -a/W) 3/2

,

where a = crack length.

The CTOD values measured in HAZ 1 at 38°C (100°F) were 0.071 (0.0028 in.) and

0.081 mm (0.0032 in.). These values were similar to those measured in HAZ 3 with SENB specimens and only slightly higher than the calculated driving

force of 0.064 mm (0.0025 in.). The results are summarized in Table XI.

In addition to measuring CTOD in the compact specimens, crack extension

was measured as a function of load using the unloading compliance method of

ASTM E 813 [16]. The results are presented in the form of CTOD versus crack

extension (Aa) curves in Figure 39. From these curves, the CTOD at the

initiation of tearing was found to be 0.032 mm ( 0.0013 in.) by the procedures

of the Draft ASTM Test Method for CTOD Testing (November 1985). This

extremely low value of CTOD indicates low tearing resistance and means

that cracks that are shorter than the critical size can grow at the stress

levels that occur in the pressure vessel.

98
Table

XI.

Fracture

toughness

test

results

for

hydrogen-charged

specimens

tested

in

air.

to- oo LO CO CM on CM CM cm on ^

S o O O o o . o O O o o c • • . * . H (±> o o o o '—

' ' w

QOHO

v

D o ,

CTn 0- M0 f— OO

g

O o o o o g • * . . • o o o o o

OO T- <— ^=r CM > T— CM CM

g . • # . . > e o o o o o X o CM LO MO r- m • • • • • S2 M0 M0 Oo CM

a. o* CM CM CM <- CM

M0 a> oo 0- <— * • • • . g lo LO LO oo *—

03 E CM CM CM cm on

CO CO CO o CO . • • . • g o o o X) o S E lo LO LO x- lo o i— on • • • • . g o o on cm

CD E CM CM CM CM H E- o o < <

CD CD CD CD

X

X

a 2 X 2 2 S

DJ DO DO o o H 00 00 00 o o

on OO

X

N \ \ CM CM <-

X3 TO TO i—1 i—1 1—1

CD CD CD 3 3: 3 C C Cm X X

CDO

O M0

O M0 lO

O M0 T- E —

a CQ O mo on

•rH J_> CM on <— LO O 03

CD

O

N

N

cx o < < <

00

X

X

X

X

99
CTOD,

mm

Aa, mm

a. Specimen 66

Aa, mm

b. Specimen 531

Figure 39. CTOD vs. crack extension, Aa, curves for the Course 1 side of

Weld 1/2 after hydrogen charging. Tested at 38°C (100°F).

Fracture analysis was conducted using the CTOD value at maximum

load.

100
Summary Comments

A fracture mechanics analysis based on the CTOD criterion was conducted

by using best estimates of the stress, crack size, and material properties

for the vessel at the time of rupture. The axial stress was computed to be

35 MPa (5.1 ksi) for the normal operating pressure and the nominal vessel

dimensions. The crack length was measured to be 800 mm, the length of the

black deposit on the fracture surface in the vicinity of the leak. A

through-thickness crack was assumed because ligament tearing, resulting in

the leak, occurred prior to final rupture. The fracture toughness values

used were measured in the CTOD tests. The results of the fracture mechanics

assessment were:

1. The CTOD value at rupture was calculated to be 0.064 mm, but the

minimum critical CTOD measured in the first series of fracture

toughness tests was 0.17 mm (0.0067 in.). Thus, this analysis

indicates that the vessel should not have ruptured.

2. The critical CTOD measured after hydrogen charging ranged from

0.064 to 0.096 mm (0.0025 to 0.0038 in.). The drop in toughness

was attributed to hydrogen embrittlement.

3. Using the toughness values for hydrogen-embrittled steel, the

calculated CTOD for the vessel is similar to the measured CTOD for

the material. Thus, fracture would be expected to occur under the

conditions estimated to exist in pressure vessel 1 2D 701.

101

CONCLUSIONS

1 . The pressure vessel fractured along a circumferential path adjacent to a repair weld made on site when a section of the vessel was replaced.

The steel plates and welds used for initial construction and for

replacement of the vessel section met the specified requirements for

chemical composition and mechanical properties.

3. The repair welding procedure used for the circumferential welds caused a hard, crack-susceptible microstructure to form in the heat-affected zone. Near the vessel surface, this microstructure was not tempered by subsequent weld passes. Similar microstructures were not found near the

vertical repair welds or the original construction welds because lower

cooling rates are associated with their respective welding procedures.

4. Cracks developed during operation of the vessel at the inside surface in

the hard, crack -susceptible microstruc ture adjacent to the circumferential

repair welds. Their probable cause was hydrogen stress cracking.

5. These surface cracks propagated in a zigzag path through the vessel

wall. Their probable cause was hydrogen pressure cracking.

6 . When the deepest of the pre-existing cracks extended through more than

ninety percent of the wall thickness, the remaining ligament of steel

tore, causing a leak. The ligament continued to tear until the length

of the through crack was approximately 800 mm ( 31 in.).

7. Catastrophic rupture occurred at an axial stress level estimated to be

35 MPa (5.1 ksi)

,

which is less than ten percent of the strength of the

steel. The fracture occurred at such a low stress level because the

vessel was weakened by:

• The 800 mm long pre-existing crack that initiated the fracture and

other pre-existing cracks around the circumference along the path of

the fracture.

• Hydrogen embrittlement of the steel that reduced the fracture

resistance of the steel by more than half.

103
8 . The source of the hydrogen

embrittlement of the steel

stream.

that caused the pre-existing cracks and

was the hydrogen sulfide in the process

the

104
References

1. Failure Analysis and Code Compliance Review of the Monoethano lamine

Absorber Vessel 1 2D 70 1 at the Union Oil Refinery, Lemont, Illinois,

Packer Engineering Associates, Inc., Naperville, Illinois, submitted to

the Illinois Office of the State Fire Marshal, Springfield, Illinois 1 985.

2. Boiler and Pressure Vessel Code, Section VIII, Pressure Vessels,

American Society of Mechanical Engineers (ASME), New York,

New York, 1968.

3. Method for Optical Emission Vacuum Spectrometric Analysis of Carbon and

Low-Alloy Steel, ASTM E 415, American Society for Testing and Materials,

Philadelphia, Pennsylvania, 1985.

4. Methods of Tension Testing Metallic Materials, ASTM E 8, American

Society for Testing and Materials, Philadelphia, Pennsylvania, 1985.

5. Methods for Notched Bar Impact Testing of Metallic Materials,

ASTM E 23, American Society for Testing and Materials, Philadelphia,

Pennsylvania, 1985.

6. Methods for Crack Opening Displacement (COD) Testing, BS 5762, British

Standards Institution, London, 1979.

7. Test Method for Evaluation of Pipeline Steels for Resistance to Stepwise

Cracking, NACE Standard TM-02-84, National Association of Corrosion

Engineers, Houston, Texas, 1984.

8. Method for y-Groove Cracking Test, Japanese Industrial Standard,

JIS Z 3158, Japan Welding Engineering Society, Tokyo.

9.

Specification for Substitute Ocean Water, ASTM D 1141, American Society

for Testing and Materials, Philadelphia, Pennsylvania, 1980.

10. Test Method for Testing of Metals for Resistance to Sulfide Stress

Cracking at Ambient Temperatures, NACE Standard TM-01-77, National

Association of Corrosion Engineers, Houston, Texas 1977.

11. Novak, S. R. and Rolfe, S. T., Modified WOL Specimen for Kj scc Environmental Testing, Journal Of Materials, 4(3), 1969, pp. 701-728.

12. Standard Test Method for Plane-Strain Fracture Toughness of Metallic

Materials, ASTM E 399, American Society for Testing and Materials,

Philadelphia, Pennsylvania, 1983.

13- Semiquantitat ive Spectrograph ic Analysis of Miscellaneous Materials by

the Graphite Dilution Technique, ASTM E 02 SM1 1 -22 , in Methods for Emission

Spectrochemical Analysis, American Society for Testing and Materials,

Philadelphia, Pennsylvania, 1982.

105
14. McClintock, F. A. and Irwin, G. R., "Plasticity Aspects of Fracture

Mechanics, in ASTM STP 381, American Society for Testing and Materials,

Philadelphia, Pennsylvania, 1965, pp. 84-113.

15. Rooke, D. P. and Cartwright, D. J., Compendium of Stress Intensity

Factors, Her Majesty's Stationery Office, London, 1974, pp. 327~329.

16. Test Method for Jj

c

, A Measure of Fracture Toughness, ASTM E 813,

American Society for Testing and Materials, Philadelphia,

Pennsylvania, 1985.

106


APPENDIX A - FIELD INVESTIGATION

December 1984

Prepared by: Bruce W. Christ

Fracture and Deformation Division

Institute for Materials Science

and Engineering

National Bureau of Standards

Boulder, Colorado 80303

Robert Shull

Metallurgy Division

Institute for Materials Science

and Engineeering

National Bureau of Standards

Gaithersburg, Maryland 20899

107

ABSTRACT

At the July 30, 1984 invitation of the Regional Administrator of the

Occupational Safety and Health Administration in Chicago, Illinois, the

National Bureau of Standards conducted a field investigation of a July 23,

1984 incident at the Union Oil Company Refinery located at Lemont, Illinois.

Drs. Bruce Christ and Robert Shull visited the incident site with OSHA

Compliance Officers between July 30 and August 3, 1984.

A 1 6-year old, 1 6 .

mm (55~ft) tall amine absorber tower, constructed in

1968 and designated pressure vessel 12D701, had ruptured into two pieces and

caused fires which led to extensive damage at the south end of the refinery.

Custody of the ruptured pressure vessel was in the hands of State Fire

Marshal's Office.

The NBS field investigation focused on visual examination and photography of both the site and the two major pieces of the ruptured pressure vessel and on participation in witness interviews conducted by OSHA Compliance

Officers from the Calumet City, Illinois Area Office.

An eye witness reported that the rupture of pressure vessel 12D701 originated at a leak. Longitudinal tensile stress from internal pressure caused

the through-wall defect at the leak to grow circumferentially and eventually

to separate the vessel into two major pieces. The circumferential fracture

path followed a zone of low fracture resistance in the vicinity of the girth

weld joining Shell Course 1 and replacement Shell Course 2. The through-wall

defect at the leak developed sometime between May 1974, when the replacement

shell course was welded into place, and July 23, 1984 when the leak was

reported.

Pressure vessel 12D701 sustained substantial corrosion cracking (blister

cracking) on the inside walls in the vicinity of the inlet piping near the

bottom of the vessel. The delamination via corrosion was a continuing process

during the life of the vessel; it was so severe as to require replacement of

the original Shell Course 2 in 1 974 after six years of service, protection of

Shell Course 1 and the bottom head with a Monel interior liner in 1976, and

weld repair of the corroded liner in April 1984.

Numerous refinery records and much technical data and analysis on test

specimens taken from the pressure vessel are needed to facilitate an impartial assessment of the causes of the catastrophic fracture of pressure vessel

1 2D701

109

APPENDIX A - FIELD INVESTIGATION

CONTENTS

1.0 General Information

1.1 Reference

1 .2 Background

1.2.1 Sources of Information about the Incident

1.2.2 Description of Incident

1.2.3 Recovering Components of Pressure Vessel 1 2D 701

1.2.4 Function of Pressure Vessel 1 2D701 in the

Refinery Process

1.2.5 Custody of the Components of Pressure Vessel

1 2D701

1.2.6 Preliminary NBS Reports to OSHA/U.S. Department

of Labor

2.0 Description of Pressure Vessel 1 2D 7 0

2.1 Sources of Information

2.2 Construction

2.3 Shells and Heads

2.3-

1 Materials, Dimensions, and Fabrication

2.3-

2 Calculation of Minimum Wall Thickness

2.3-

3 Corrosion Allowance on Thickness

2.

3-

3-1 General Requirements for Corrosion

Allowance

2.

3-

3-

Corrosion Allowances for Pressure

Vessel 12D701

2.4 Valves

3.0 Inspection and Maintenance History

3.1 Documents Available During the Field Investigation

3.2 Replacement of Shell Course 2

3-3 Inlet Piping Penetration of Girth Welds

3-4 Installation of Internal Metal Liner

4.0 Description and Assessment of Fracture from Visual

Observations

4.1 Initiation of Fracture

4.2 Characteristics of the Fracture

4.2.1 Orientation and Location of Fracture Path

4.2.2 Appearance of Fracture Surfaces to Unaided Eye

4.3 Videotapes of Fracture Surfaces

4.4 Implications of the Fracture Characteristics

111
5.0 Discussion

5.1 Probable Cause of the Refinery Incident

5.2 Through-Wall Defect in Pressure Vessel 1 2D 701

5.3 Circumferential Separation in the Vicinity of a Girth Weld

5.4 Corrosion of Pressure Vessel 1 2D 701

5.4.1 Corrosion Caused by Hydrogen Sulfide

5.4.2 Galvanic Corrosion

5.5 Information Needed for Assessment of Fracture

6.0 Conclusions

7.0 References

112
1.0

GENERAL INFORMATION

1.1 REFERENCE

U.S. Department of Labor, Occupational Safety and Health Administration,

Regional Office, Chicago, Illinois. This field investigation was conducted

at the request of Mr. Frank L. Strasheim, Regional Administrator, in memoranda dated July 30, 1984, to Dr. R. P. Reed, Chief of the NBS Fracture and

Deformation Division, Boulder, Colorado, and to Dr. L. Testardi, Chief of the

NBS Metallurgy Division, Gaithersburg, Maryland. The request memoranda followed telephone calls from Mr. Kenneth Yotz, Assistant Regional Administrator

for Technical Support in the OSHA Chicago Regional Office, to Dr. Robert

Shull of the NBS Metallurgy Division on July 26, 1984 and to Dr. Bruce Christ

of the NBS Fracture and Deformation Division on July 27 and 29, 1984. OSHA

Cost Coding Number 4400-E-2-F-501 -22000-21 1 2-000 supported this work.

1.2 BACKGROUND

1.2.1 Sources of Information about the Incident

Information about the incident described below was obtained principally

from: staff of the OSHA Regional Office in Chicago; staff of the OSHA Area

Office in Calumet City, Illinois; employees of the Union Oil Company petroleum refinery at Lemont, Illinois; and the Herald News of Joliet, Illinois,

the July 29, 1984 edition.

OSHA Regional Office staff members who provided information include:

Mr. Frank L. Strasheim, Mr. William Murphy (temporary assignment),

Mr. Kenneth Yotz, and Mr. James Sumpter. Some discussions were also held

with Mr. Richard Fiore, an attorney representing the OSHA Regional Office.

OSHA Area Office staff members who provided information include

Mr. William Funcheon (Office Director) and the following Compliance Officers:

Mr. James Whyte, Mr. Joseph Howicz, Mr. Michael Ypsilanti, Mr. Gregg

Mitchell, and Mr. Young Paul Li. Copies of refinery documents pertaining to

construction, inspection maintenance, and repair were received from

Mr. Funcheon via mail on August 7, 1984 at NBS in Boulder, Colorado.

113
Refinery employees who provided information include Mr. Ronald Runge

(Chief of Plant Operations), Mr. Daniel Sobieski (Chief Laison Staff. Person),

Mr. Denton, Mr. Bassett, Mr. H. Kolff van Oosterwijk, Mr. E. Schaschl, and

Mr. Emil Bereczky.

The July 29, 1984 Herald News of Joliet, Illinois carried a timetable of

events purporting to list occurrences during the first 24 hours of the incident at the refinery.

In addition, some information was obtained from an alleged eyewitness,

who claimed to be on a hill about one mile east of the refinery in the beginning moments of the incident.

This information was provided to Drs. Bruce W. Christ and Robert Shull,

who visited the site of the incident during the week of July 30, 1984. Both

are technical experts in the fields of materials science and engineering.

Dr. Christ arrived at the site around 4:30 p.m. on Monday July 30, 1984.

Dr. Shull arrived at the site around 8:30 a.m. on July 31, 1984. Drs. Christ

and Shull were accompanied and guided by OSHA staff members from the Area Office in Calumet City, Illinois at all times while at the site. All information from refinery employees was transmitted through, or in the presence of,

OSHA staff members.

At Dr. Christ's request, OSHA arranged to have two professional photographers at the site during the week of the field investigation. The photographers obtained several videotapes of important aspects of the incident and

provided the tapes to the OSHA Area Office in Calumet City.

Drs. Christ and Shull assisted the OSHA Compliance Officers via technical interpretation of observations at the incident site and technical assistance during interviews by Compliance Officers taking witness statements.

Drs. Christ and Shull left the site at 3:15 pm on Friday, August 3, 1984.

Drs. Christ and Shull provided technical assistance to various OSHA staff

members via telephone conversations initiated by OSHA in the weeks following

their visit to the site.

1.2.2 Description of Incident

A refinery worker observed a leak in an upright, 2.6 m (8.5 ft.) diameter, cylindrical pressure vessel which was about 19.4 m (63.5 ft.) high (from

114
th*" base ring to the top). The vessel, designated 1 2D 701, was an amine abrber tower located in the unsaturated gas plant at the south end of the

refinery. This pressure vessel nominally operated at 1.4 MPa (200 psig) and

38°C (100°F), and nominally contained about 45 k& (12,000 gal.) of liquid

propane (C^), liquid butane (C^), and aqueous amine solution (MEA). The purpose of this amine absorber tower in the refinery process was to remove hydrogen sulfide from the hydrocarbon process stream.

The refinery worker, who observed the leak, notified a plant supervisor

close to the pressure vessel at about 5:38 p.m. on Monday, July 23, 1984. The

supervisor immediately came to the scene and observed the leak in pressure

vessel 1 2D 7 01. The supervisor attempted to mitigate the leak by climbing a 4.6 m (15 ft.) ladder attached to pressure vessel 12D701 and manipulating a valve. Forceful jetting of vapor leaking from the pressure vessel near the

valve forced the supervisor to abandon the effort to manipulate the valve.

While retreating from the leaking pressure vessel, the supervisor observed

that the leak was growing larger. The supervisor took cover under some refinery equipment about 21 m (70 ft.) from the pressure vessel.

Shortly thereafter, a major explosion occurred, which caused ground

tremors to be felt about a mile east of the refinery. The time of this

explosion was estimated to be 5:53 p.m. Apparently, this explosion occurred

when the pressure vessel completely separated circumferentially into two

major pieces. The upper portion of the pressure vessel, approximately 12 m

(40 ft.) long was launched into the air. The lower portion of the pressure

vessel remained standing almost upright on its base ring. A white cloud

formed at ground level near the ruptured pressure vessel; then a fire

started. Black smoke began to fill the air. This smoke obscured the flight

path of the launched portion of the ruptured pressure vessel; a second major

explosion occurred at about 6:20 p.m. Firefighters were arriving at the scene

by this time. Fires were brought under control by midnight. The explosions

and fires created several acres of charred wreckage and debris.

It was the consensus of refinery employees that the rupture of pressure

vessel 12D701 was the initial event that led to the fires and extensive

damage at the south end of the refinery.

115
1.2.3 Recovering Components of Pressure Vessel 1 2D 701

Upper Portion . The upper 12 m (40 ft.) of pressure vessel 1 2D 701 was found

about 915 m (3000 ft.) from its original location in the refinery. It had

been propelled in a southeasterly direction to a field outside the refinery.

By chance, it landed on an electric power line transmission tower. The

landing impact collapsed the tower and made a crater about 3.7 m (12 ft.) deep

and perhaps 6m (20 ft.) in diameter.

The weight of this section of pressure vessel 1 2D 7 0 1 was estimated to be

roughly 29 Mg [(40 ft./55 ft.) (78,807 lb.) = 57,529 lb. = 28.8 T] . At the

time of separation, pressure vessel 1 2D 7 0 1 was thought to be practically full

of liquid; a rough estimate indicates that the amount of liquid in the upper

portion was approximately 33 k£ [(40 ft./55 ft.) (12,000 gal.) = 8760 gal.]

The upper portion, with the associated piping, was loaded onto a flatbed trailer and trucked to a parking lot inside the refinery, where it was examined visually. A safety relief valve remained attached to the top head.

The circumferential fracture path closely followed a girth weld which

joined two shell courses of pressure vessel 12D701.

The upper portion was flattened along most of its 12 m (40 ft.) length.

The cause of the flattening was not evident during the week of July 30 - August 3, 1984. The upper portion showed some crushing of the top head,

suggesting that it had landed top down. Soil adhered to various locations on

the upper portion, including the outside surface near the head and the inside

wall near the fracture surface. The inside of the upper portion at the open

end smelled intensely of hydrocarbon vapors during the week of the field

inspection

Lower Portion . The lower 4.6 m (15 ft.) of pressure vessel 1 2D 701 remained

standing in the refinery nearly upright on its base ring. On July 30, 1984,

the inlet pipe and distributor were still connected to the lower portion, and

the lower portion was about 2/3 full of an unknown liquid with a strong odor.

A liner installed inside the lower portion of pressure vessel 1 2D 7 0 1 was visible above the liquid. The visible portion appeared to be intact. The

top edge of the liner was fastened to the inside wall around the entire

circumference. The top edge of the liner appeared to be welded in place

116
il n,; a girth weld which joined the lower two shell courses of pressure

vessel 12D701. The fracture path nearly coincided with the top edge of the

liner

The lower portion of pressure vessel 1 2D 701 was removed from its base on

July 31, 1 98^4 . It was loaded onto a flatbed trailer, trucked to the parking

lot inside the refinery where the trailer carrying the upper portion of

pressure vessel 12D701 was standing, and parked alongside this trailer.

Construction workers dismantling the lower portion indicated that the

J-bolts holding the pressure vessel to its base had been stretched about

6 mm (0.25 in.). These bolts were not examined further during the week of

the field investigation.

1.2.4. Function of Pressure Vessel 1 2D 7 0 1 in the Refinery Process

Pressure vessel 12D701 was an amine absorber tower. Its function in the

refinery process was to remove hydrogen sulfide from a steam of liquid

propane and liquid butane. Normal operating conditions of the pressure

vessel were: full of liquid; about 38°C (100°F); about 1.4 MPa (200 psig)

When the pressure vessel was in operation, the process stream containing

propane, butane, and hydrogen sulfide entered the vessel at the bottom, rose,

and exited at the top. Aqueous amine solution (MEA) entered the vessel at

the top, decended, and by contacting the upward-flowing hydrocarbon stream,

removed hydrogen sulfide. The aqueous amine solution enriched with hydrogen

sulfide collected in the bottom 3 to 4.6 m (10 to 15 ft.) of the pressure

vessel and was periodically drawn off.

Information about the operating pressure and temperature of pressure

vessel 12D701 just prior to the rupture was not available during the week of

the field investigaton, nor was information about the chemical composition of

the process stream over a period of time prior to the rupture. Knowledge of

the operating pressure and temperature and of the chemical composition of the

process stream would be useful in assessing the fracture.

1.2.5 Custody of Components of Pressure Vessel 1 2D 70

The State Fire Marshal’s office had custody of the components of pressure

vessel 1 2D 701 during the week of the field investigaton. Accordingly, the

NBS investigative role involved activities that did not disturb the ruptured

117
vessel, e.g., visual and photographic observations and some measurements with

a hand-held tape. The NBS investigators were not involved in any metal

cutting operations.

1 .2.6 Preliminary NBS Reports to OSHA/U.S. Department of Labor

Two preliminary reports describing the NBS field investigation of the

incident were sent from NBS to OSHA, dated August 10, 1984 and August 29,

1984. A1

118
2.0 DESCRIPTION OF PRESSURE VESSEL 12D701

>.1 SOURCES OF INFORMATION

The information reported in this section was taken principally from

•ngineoring drawings^ ’

A

^ provided to OSHA Compliance Officers by oil

refinery personnel in July 1984. Stress-pressure-wall thickness calculations

wore based on Section VIII - Division 1 of the ASME Boiler and Pressure

Vessel Code.^ Some information in this section came from direct observations made on recovered components of pressure vessel 12D701.

2.2 CONSTRUCTION

Pressure vessel 1 2D 70 1 was constructed in 1968 in accord with the ASME

Boiler and Pressure Vessel Code - Specification UW. The as-constructed vessel was a cylindrical shell standing vertically. The vessel itself was about

1 6 . 8 m (55 ft.) high and about 2.6 m (8.5 ft.) in diameter. The vessel was supported on a pedestal about 2.6 m (8.5 ft.) high. Nominal wall thickness

was 25 mm (1 in.). The vessel was constructed of six shell courses and an ellipsoidal top and bottom head.

2.3 SHELLS AND HEADS

2.3.1 Materials, Dimensions, and Fabrication

ASTM A5 1 6 Grade 70 was specified for the six shell courses. Plate

dimensions were: 269 cm x 843 cm x 2.5 cm (106 in. x 332 in. x 1 in.). The

cylindrical shells were fabricated by shaping the plate to a cylindrical

configuration and joining the edges with longitudinal welds. Table AI

summarizes some design and construction data.

The top head was ellipsoidal in shape. ASTM A5 1 6 Grade 70 steel plate

was the specified material. The top head had an outside diameter of 2.6 m (104 in.) and a minimum thickness of 21 mm ( 0 . 8 1 4 in.).

The bottom head was ellipsoidal in shape. ASTM A5 1 6 Grade 70 was the

specified material. The bottom head had an outside diameter of 2.6 m (104 in.) and a minimum thickness of 22 mm (0.865 in.). Presumably the extra

119
Table A1 . Design information for pressure vessel 12D701^3

Specif ication ASME UW

Type of construction Welded

Year of construction 1968

Identifying symbol (marking) 12D701

Design pressure (new) 1 .6 MPa (230 psi)

Hydrostatic test pressure 2. A MPa (3^5 psi)

Operating temperature 60°C ( 1 90 °F

)

Specified shell plate thickness 25. 9 mm ( 1 in.

)

Specified top head plate thickness 21 mm (0.819 in.)

Specified bottom head plate thickness 22 mm (0.865 in.

)

Corrosion allowance (shell) 3 mm ( 1 /8-in .

)

Corrosion allowance (top and bottom heads) 3 mm ( 1 /8-in.

)

Specified shell plate thickness (new) 25.9 mm (1 -in .

)

Specified structural steel (heads and shells) ASTM A51 6 Grade 70

Tensile strength 980 to 621 MPa

(70,000 to 90,000

Yield strength 260 MPa, min.

(38 ksi, min.)

Carbon concentration of steel 0.27 wt.% (max.

)

Maximum allowable stress at operating temperature* 120 MPa (17,500 ps

Weight, empty 36 Mg (78,807 lb.)

Weight, full of water 126 Mg (276,682 lb.

Water, capacity 95 ki (12,000 gal.

*from Table UCS-23 of ASME Boiler and Pressure Vessel Code^

120
thickness of the bottom head, as compared to that. of the top head, was intended to provide an allowance for corrosion due to the corrosive fluid that

concentrated in the bottom of the pressure vessel during normal operation.

The shell courses and heads were joined by girth welds. Documentation

about the welding procedures was not available for review during the field

in vestigat ion

2.3.2 Calculation of Minimum Wall Thickness

Sections and UG-32^ b in the General Requirements part of Section

VIII of the ASME Boiler and Pressure Vessel Code specify procedures for

calculating the minimum wall thickness of components of vessels sustaining

internal pressure. Calculations of minimum wall thickness for the shell and

heads of pressure vessel 1 2D 701 appear below.

Minimum Thickness of Shell Plates

t PR

SE - 0.6P

( A 1 )

where t

P

S

R

E

minimum thickness of shell plates in inches, exclusive of

corrosion allowance

design pressure = 1 .6 MPa (230 psi)

maximum allowable circumferential tensile stress at 60°C (140°F

121 MPa ( 17,500 psi) for ASTM A51 6 Grade 70 steel (from Table

USC-23/Section VIII - ASME B & PV Code) AAc

inside radius of shell (before corrosion allowance is added in)

8.4 m (332 in.)/(2Tr) = 1.3 m (52.8 in.)

joint efficiency = 1 for fully radiographed joints

Substituting the above values into Equation 1 gives

t = 18 mm = {(230 psi x 52.8 in. )/[( 17 ,500 psi x 1) - 0.6(230 psi)] = 0.70 in.

}

Therefore, the minimum thickness of shell plates, exclusive of the corrosion

allowance, was 18 mm (0.70 in.).

121
Minimum Thickness of Ellipsoidal Heads

t = PD

2 SE - 0.2 P (A2)

where t = minimum thickness of head in inches after forming, exclusive of

corrosion allowance

P = design pressure = 1.6 MPa (230 psi)

D = inside diameter of the headskirt or inside length of an ellipsoidal

head

D = 2.6 m (102.3 in.) for bottom head.

D = 2.6 m (102. 14 in.) for top head.

S = maximum allowable circumferential tensile stress at 60°C (140°F)

= 120 MPa (17,500 psi) for ASTM A51 6 Grade 70 steel (from Table USC - 23/Section VIII - ASME B & PV Code A4c

).

E = lowest efficiency of any joint in the head = 1 for fully radiographed joints.

Substituting the above values into Equation A2 gives

t = 17 mm [(230 psi x 102.4 in.)/[(2 x 17,500 psi x 1 ) - (2 x 230 psi)]

= 0.675 in.]

Therefore, the minimum thickness of heads, exclusive of corrosion allowance, was 17 mm (0.675 in.)

2.3.3. Corrosion Allowance on Thickness

2.3.3. 1. General Requirements for Corrosion Allowances

Section UG-25 in the General Requirements part of Section VIII of the

ASME Boiler and Pressure Vessel Code A4cl discusses corrosion allowances.

Paragraph (b) states, "Vessels or parts of vessels subject to thinning by

corrosion, errosion or mechanical abrasion shall have provisions made for the

desired life of the vessel by a suitable increase in thickness of the

material over that determine by the design formulas, or by using some other

suitable method of protection." Paragraph (2) states, "The user or his

122
-mated agent shall specify corrosion allowances other than those required

by the rules of this Division." Essentially, the code leaves it up to the user or designated agent to decide on the amount of corrosion allowance.

2. 3. 3. 2 Corrosion Allowance for Pressure Vessel 12D701

An engineering drawing for pressure vessel 1 2D 701 indicates that the

specified corrosion allowance was 3 mm (1/8 in.) for shell plates and for

heads.''- Adding this corrosion allowance to the minimum thickness to support

internal pressure (calculated in Section 2.3.2) gives the specified minimum

plate thickness at initial construction. Results appear in Table All.

Comparing these results with the plate thicknessess appearing in the

engineering drawing^ f0 r shells and heads indicates that the actual plate

thickness intended for use in initial construction exceeded the specified

minimum plate thicknesses.

2.4 VALVES

Documentable information about the valves on pressure vessel 12D701 was

not abundant during the field investigation. Presumably the inlet and outlet

piping had valves to regulate flow of the process streams, but the valves

were not identified during this week. A valve identified by refinery

employees as a safety relief valve was attached to the upper portion of

pressure vessel 12D701 as it lay on the flatbed trailer in the refinery

parking lot during the field investigation, but it was not examined at that

time. The valve that the plant supervisor attempted to manipulate while a leak was occurring (see Section 1.2.2.) was not positively identified during

the field investigation. It would be useful to identify and locate all the

valves which regulated internal pressure in pressure vessel 1 2D701 and

ascertain the reliability of their performance.

123
3.0

INSPECTION AND MAINTENANCE HISTORY

3.1 DOCUMENTS AVAILABLE DURING THE FIELD INVESTIGATION

The documentation about the inspection and maintenance history of pressure vessel 1 2D 7 0 1 dealt principally with observations of inside wall corrosion and wall thickness measurements. Some information about the replacement

of Shell Course 2 in 1 97 ^ and installation of the metal internal liner in

1976 was also available. A summary of Equipment Inspection Reports^ cited

highlights of biennial inspections between April 1972 and April 1984.

The inspection records that led to repairs by welding were not available. These repairs were thought to be located in the area where the vessel

ruptured and separated. Information about the repairs themselves, thought to

have been made a few months prior to the date of the incident, was not

available.

3.2 REPLACEMENT OF SHELL COURSE 2

The April 1972 and April 1974 Equipment Inspection Reports^ described

severe inside wall corrosion near the bottom of pressure vessel 12D701 in the

form of blister cracking. Metallurgical examination in May 1972 of a throughwall sample trepanned from the wall of the vessel near the bottom evaluated

the corrosion. ° The size of the sample and the location from which it was taken were not specified. Wall thickness measurements made on original shell

Course 2 in April 1972 with ultrasonic techniques indictated thicknesses

ranging from 12 mm (0.46 in.) to somewhat more than 25 mm (1 in.). Inspection records showed that one region had thicknesses ranging from 10 to 16 mm

(0.39 to 0.63 in.). These values are well below the minimum thickness for

shell plates, 18 mm (0.700 in.) listed in column a of Table All.

A June 26, 1972 memorandum report^ summarized the visual observations

of corrosion, the metallurgical evaluation of the through-wall sample trepanned from a severely corroded region, and results of wall thickness mea- surements with ultrasonic techniques. It reiterated a recommendation in the

124
Table

All.

Plate

thicknesses

according

to

code

and

construction

drawing

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CD cl 03 c CD E o 1—1 x: CD O CD Q. rH 43 43 43 E CD Cl 03 43 03 O x: O 1—1 O •—

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125
May 1972 Equipment Inspection Report, namely, that the original Shell Course

2 be replaced. The recommended replacement took place between March 28 and

May 10, 1979, according to a memorandum dated September 5, 1979. The April

1979 Equipment Inspection Report^ suggested that one contractor started the

replacement and a second one finished it.

Since the fracture path ran through heat-affected zone of Shell Course 1

it would be of interest to know if the heat-affected zone of Shell Course 1 from the time of original construction had been removed before replacement

Shell Course 2 was welded into place.

The Equipment Inspection Reports^ between 1976 and early 1989 did not

indicate significant deter iorat ion of replacement Shell Course 2. Five

gouges on the southeast side of the shell approximately 0.9 m (16 in.) above

the weld seam were reported in April 1978.^5 However, these gouges are not

mentioned in subsequent Equipment Inspection Reports. In April 1989, it was reported that all ultrasonic thickness readings taken on the shell of

pressure vessel 1 2D 70 1 were above the nominal thickness.

3.3 INLET PIPING PENETRATION OF GIRTH WELDS

The inlet piping for the hydrocarbon process stream penetrated the

girth weld joining Shell Course 1 and replacement Shell Course 2. The inlet

piping was welded in place. Documentation about the welding procedures used

for this penetration were not available for review during the field

investigation.

A noteworthy feature of the inlet piping was the distributor at the end,

inside the pressure vessel. The distributor was a perforated pipe designed

so as to spread the incoming hydrogen-sulf ide-rich, liquid hydrocarbon stream

throughout the pressure vessel cross section. The position of the distributor inside the pressure vessel was at about the height of the girth weld

joining Shell Course 1 and replacement Shell Course 2.

3.9 INSTALLATION OF INTERNAL METAL LINER

The lower portion of pressure

liner, which had been installed in

vessel 12D701 contained an internal metal

May 1976. The May 1976 Equipment

126
Inspection Report stated that, "The entire surface of shell plate ring #1 had

many blisters ranging in size from 1/4" to 1" diameter." This report also

indicated that the liner was installed "to arrest the hydrogen attack."

Monel* was the liner material. The specified thickness of the liner was not

reported. The May 1976 Equipment Inspection Report establishes that the liner was welded to the inside bottom wall of pressure vessel 1 2D 70 1 and that the

top edge of the liner was welded approximately along the girth weld joining

Shell Course 1 and replacement Shell Course 2, as observed on July 30 and 31 , 1984.

A 5 Equipment Inspection Reports J indicated that the Monel liner appeared

to be in good condition in 1978 and 1980. The liner was not mentioned in a 1982 Equipment Inspection Report. However, a discussion of the liner in a 1984 report^ indicated that "...eight locations showed signs of seepage of

liquids from welds. Lining was heated to drive off trapped liquid and these areas were weld-repaired."

The condition of the liner as it was observed on July 30 and 31, 1984 is

described in Section 1.2.3, "Lower Portion," of this report.

*Monel is a nickel-copper alloy containing about 66.5% nickel, 30% copper,

\% manganese, \% iron, and about 0.2 to 0.5% silicon and carbon, balance

residuals. Three types of Monel (400, R-405, K-500) are listed on page 68 of

the 1984 Materials Processing Data Book published by the American Society for

Metals, Metals Park, Ohio. It would be of interest to learn how widespread

is the practice of installing protection liners of Monel in the petrochemical

industry

127
4.0

DESCRIPTION AND ASSESSMENT OF FRACTURE FROM VISUAL OBSERVATIONS

4.1 INITIATION OF FRACTURE

According to an eyewitness account, the fracture originated at a leak

that forcefully jetted hydrocarbons from pressure vessel 12D701 (see Section

1.2.2). The observation of a single leak indicated the existence of a single

through-wall defect and suggests a single-fracture origin. The area of the

leak apparently had undergone repair work by welding some months prior to

detection of the leak. Documentation about such repair work was not

available during the field inspection.

The same eyewitness account indicated that the initial leak grew larger

while it was being observed. This observation suggests propagation of a through-wall defect, indicating that the defect was unstable under the

operating conditions of the pressure vessel. (Information about these

operating conditions was not available during the field inspection.) In

fracture mechanics terminology, the observation of defect growth indicates

that the through-wall defect causing the leak exceeded the critical defect

size for the local conditions of fracture toughness and applied stress.

4.2 CHARACTERISTICS OF THE FRACTURE

4.2.1. Orientation and Location of Fracture Path

a. The fracture path was circumferential.

b. The fracture path followed closely the girth weld and associated

heat-affected zone between Shell Course 1 and replacement Shell

Course 2. Much of the fracture path was in the heat-affected zone rather than in the weld metal.

c. At only one location did the fracture path run through parent

metal. This occurred in the northwest quadrant of the vessel's

c ircumference at a distance of about 1 .5 m (61 in.) from the inlet

pipe penetration. The total length of the fracture path in parent

metal was about 76 cm (30 in.).

128
d. The fracture path nearly coincided with the top circumferential

edge of the internal metal liner, which was welded to the inside

wall of pressure vessel 1 2D 701 approximately along the girth weld

between Shell Course 1 and replacement Shell Course 2.

e. The fracture path was in the immediate vicinity of the inlet pipe

and distributor, which introduced and distributed a hydrogensulfide-rich hydrocarbon stream into the vessel (see Section 3-3).

Moreover, the fracture path was in the vicinity of the highest

level of the hydrogen-sulf ide-r ich aqueous amine solution : which

collected in the bottom of the vessel (see Section 1.2.4).

4.2.2 Appearance of Fracture Surfaces to Unaided Eye.

a. Fracture surfaces on the upper and lower portions of pressure

vessel 1 2D 70 1 were severely corroded. Soil adhered to several

locations on the fracture surface on the upper portion of the

vessel.

b. The fracture generally had the appearance of rather low ductility,

as evidenced by the rather flat fracture surfaces around the entire

c ircumference.

c. The fracture separated some welded joints associated with the inlet

piping penetration (see Section 3*3).

d. A possible fracture origin site* was noted in the southwest quadrant

of the vessel's c ircumference, approximately 1.3 m (53 in.) from the

inlet piping penetration. The noteworthy feature on the fracture

surface at this location was a through-wall flat spot about 76 to

1 30 mm ( 3 to 5 in.) long.

*This possible fracture origin was in the vicinity of what appeared to be a series of ladder lugs welded to the outside wall of pressure vessel 12D701.

See Section 1.2.2 for a description of the leak by an eyewitness.

129
4.3 VIDEOTAPES OF FRACTURE SURFACES

Videotapes were made of both fracture surfaces while the upper and lower

portions of pressure vessel 1 2D701 were lying on their sides on the flatbed

trailer in the refinery parking lot on August 1 and 2, 1984 (see Section

1.2.3). The tapes were provided to the OSHA Area Office in Calumet City,

Illino is.

4.4 IMPLICATIONS OF THE FRACTURE CHARACTERISTICS

Pressure vessel 12D701 ruptured into two major pieces, with the separation occurring along a circumferential fracture path that roughly followed

the girth weld joining Shell Course 1 and replacement Shell Course 2. A

circumferential fracture path in an internally pressurized cylindrical shell

is an unexpected occurrence. It demonstrates that rupture occurred in re- sponse to longitudinal stress, not circumferential (hoop) stress, which is

twice as large for a given internal pressure.

The maximum design longitudinal stress, approximately 60 MPa (8,750 psi)

for the ASTM A516 Grade 70 steel specified for pressure vessel 12D701,* is

well below the specified ultimate tensile strength of ASTM A516 Grade 70

steel, namely 480 to 620 MPa (70,000 to 90,000 psi). A ^ The difference of approximately 410 to 550 MPa (60,000 to 80,000 psi) between the maximum design

longitudinal stress and the specified ultimate tensile stress ensures that

c ircumferent ial fracture will not occur in normal circumstances. The circumferential path of the fracture indicates that intended design conditions

did not prevail at the time of rupture. According to Equation A1 in Section

2.3.2, the deviation from design conditions might have been a decrease in

wall thickness below the minimum allowable wall thickness. Perhaps an increase in pressure (P in Equation A1 ) above normal operating pressure

occurred in combination with a decrease in wall thickness.

*This stress is one-half of the maximum design hoop stress of 2.5 MPa

(17,500 psi) cited in Table USC-23/Section VIII-ASME Boiler and Pressure Vessel

Code

130
1'nore is a third possibility, namely, the existence of a circumferential

no if unusually low fracture resistance (low toughness) in the structural

mat-rial in the region that became the fracture path. Localized deteriorati n of the fracture resistance of structural material, i.e., a loss of

toughness, could have occurred from the heating effects of welding and/or

action of corrosive fluids. Indeed, much of the fracture surface was in the

weld heat -affected zone* and simultaneously at a location where highly

corrosive fluids were concentrated inside the pressure vessel. Excessive

corrosion was known to occur at this location in the early life of the

pressure vessel, as evidenced by such remedial measures as replacement of

Shell Course 2 six years after initial construction (see Section 3.2) and by

installation of the internal metal liner in May 1976 (see Section 3.9).

It can be speculated that a circumferential zone of low fracture

resistance might also have developed from galvanic corrosion involving the

Monel internal liner, the pressure vessel steel, and the fluids inside the

pressure vessel. Extensive analysis is required to evaluate this speculation

about galvanic corrosion.

It can also be speculated that a circumferential zone of low fracture

resistance might have developed from local refrigeration associated with

vaporizing and expanding hydrocarbons jetting from a leak. Local cooling of

the steel could result from supplying the heat of vaporization and/or

from the Joule-Thompson cooling associated with expanding gas. Such local

cooling might have decreased the temperature in the vicinity of the growing

leak to below the ducti le-to-brittle transition temperature of the steel,

*There may be three heat-affected zones to consider. The first would have

been created when Shell Course 1 and Replacement Shell Course 2 were welded

together in 197-4. The second would have been created when the internal metal

liner was welded to the inside wall in the vicinity of the first heataffected zone in 1976. The heat-affected zone associated with the liner

might overlap the first heat -affected zone. A third heat-affected zone might

exist on Shell Course 1 from the time when it and the original Shell Course 2 were joined during original construction, or from the time when these two

Shell Courses were separated in 1979.

131
thereby creating a zone of low fracture resistance at the circumferentially

growing crack front.* Extensive analysis is required to evaluate this

speculation about localized cooling.

In summary, the possible causes of a cir cumferential zone of low

fracture resistance include:

a. Low toughness in the heat -affected zone of the shell steel caused by

heating associated with the girth welding at the time of initial

construction when replacement Shell Course 2 was installed and/or

when the internal metal liner was installed

b. Deterioration of the heat -affected zone by corrosion, e.g., blister

cracking, on the inside wall due to exposure to hydrogen sulfide

c. Low toughness of steel at the leak due to localized cooling from

vaporization of liquid hydrocarbons and/or Joule-Thompson cooling

associated with jetting hydrocarbons causing local temperature at

the leak to fall below the ductile-brittle transition temperature of

the steel

Further study of the fracture beyond the visual observations cited in

Sections 4.1 thorugh 4.3 is necessary to reach a satisfactory explanation of

the cause of fracture. In view of the severe distortion of the fracture

surface on the upper portion of pressure vessel 12D701 after the fracture it seems reasonable to focus initial investigative efforts on the undistorted

fracture surface on the lower portion. This latter fracture surface is

probably more representative of conditions at the time of rupture, despite

heating from the refinery fires on July 23, 1984.

^During the field investigation, refinery employees mentioned that it is common refinery practice to spray steam on a leak.

132
5.0

DISCUSSION

5.1 PROBABLE CAUSE OF THE REFINERY INCIDENT

It appears that the rupture of pressure vessel 1 2D 701 was the initiating

'.•vent that led to the fires and extensive damage at the south end of the

Union Oil Company petroleum refinery in Lemont, Illinois on July 23, 1984.

The principal evidence available during the field investigation in support of

this view was the eyewitness report (see Section 1.2.2).

5.2 THROUGH-WALL DEFECT IN PRESSURE VESSEL 1 2D701

The observation of a leak (see Section 1.2.2) implies the presence of a through-wall defect. It is not known how long it took for the defect to

develop and penetrate the wall. However, it can be said that the time

interval for development of the through-wall defect was between the spring of

1974, when replacement Shell Course 2 was installed and July 23, 1984 when

complete wall penetration led to a visible leak. Equipment Inspection

AS Reports J indicate more deterioration by corrosion of Shell Course 1 than

replacement Shell Course 2 during this time interval.

The possibility exists that the through-wall defect developed over a short period of time and was due to causes other then corrosion. However,

plausible explanations for development of a through-wall defect in a short

time period were not reported during the field investigation.

5.3 CIRCUMFERENTIAL SEPARATION IN THE VICINITY OF A GIRTH WELD

Pressure vessel 12D701 ruptured into two major pieces via a separation

occurring along a circumferential fracture path that closely followed most of

the girth weld joining Shell Course 1 and replacement Shell Course 2 (see

Sections 1.2.3 and 3.2.1. for a more complete description). A circumferential fracture path in an internally pressurized cylindrical shell is an unexpected occurrence. It indicates that the rupture occurred in response to

longitudinal tensile stress, not c ircumferent ial tensile stress, which is

twice as large as the longitudinal tensile stress for a given internal

133
pressure. The circumferential fracture path suggests the existence of a highly localized, circumferentially oriented zone of unexpectedly low

resistance to fracture under longitudinal stress. Since the fracture path

closely followed most of the girth weld joining Shell Course 1 and replacement Shell Course 2, the region of the girth weld* is a candidate zone for

mechanical, metallurgical, and fractographical examination to see if it might

be a zone of low fracture resistance.

5.4 CORROSION OF PRESSURE VESSEL 12D701

5.4.1 Corrosion Caused by Hydrogen Sulfide

The function of pressure vessel 12D701 in the refinery process was to remove hydrogen sulfide from a liquid hydrocarbon stream (see Section 1.2.4).

Hydrogen Sulfide in known to be corrosive to steel under the operating conditions of the pressure vessel; consequently, this pressure vessel was designed

with a corrosion allowance amounting to a 3 mm (1/8 in.) wall thickness above

that required to sustain internal pressure. ^3

Equipment Inspection Reports^ showed that pressure vessel 1 2D701 sus- tained substantial corrosion cracking (blister cracking) on the inside walls

in the vicinity of the inlet piping near the bottom of the vessel. The piping introduced a hydrogen-sulf ide-r ich hydrocarbon stream into the vessel.

The deterioration via corrosion was a continuous process during the life of

the vessel, which was so severe as to require: 1) replacement of the original Shell Course 2 in 1976 after six years of service, 2) installation of a Monel internal liner in 1976 to protect Shell Course 1 and the bottom head,

and 3) weld repair of the corroded Monel liner in April 1984.

The rupture of pressure vessel 12D701 on July 23, 1984 due to longitudinal stress from internal pressure suggests that all corrosion processes

causing deterioration were not being adequately monitored and counteracted.

The focus of refinery inspection was usually on blister cracking. Other

corrosion processes, e.g., galvanic corrosion, should also be considered.

*The region of the girth weld includes the weld metal, the heat-affected zone

of the base plate, and the unaffected base plate near the weld.

134
5.4.2 Galvanic Corrosion

It may be speculated that galvanic corrosion might have occurred between

the Monel Liner and the shell plate steel. The necessary electrical contact

between the liner and shell was ensured by welding the liner to the shell. It

is plausible that the process fluids could have acted as an electrolyte. It

is also plausible that Monel is more noble that the shell plate steel,

setting the stage for anodic solution of iron in the steel.

This speculation about possible galvanic corrosion needs to be evaluated

by experimental measurements directly on the Monel from the liner shell

steel, which was in contact with the Monel and typical process fluids.

5.5 INFORMATION NEEDED FOR ASSESSMENT OF FRACTURE

Some aspects of the girth weld joining Shell Course 1 and replacement

Shell Course 2 are described in Sections 2.3.1, 3.3, and 3.4. Only very

limited information about this region of pressure vessel 1 2D 701 was available

during the field investigation. Much more information is needed to assess

the possible causes of low fracture resistance listed in Section 4.4 or to

assess other possible causes of fracture.

Records needed include all available construction and materials

specifications, all maintenance and repair specifications, all welding

procedures, and all inspection reports dating from the time the pressure

vessel was constructed. Of particular interest would be information in these

records pertaining to Shell Course 1 and replacement Shell Course 2.

Records describing operating conditions prior to and at the time of

rupture are also needed. Of particular interest would be information on pressure, temperature, and chemical composition of process fluids.

Finally, records describing valve specifications and valve inspection

reports are needed.

In addition to the foregoing records, a substantial amount of technical

data needs to be generated on the structural material specimens taken from

the 1) heat -affected zone, 2) weld metal, 3) base plate and 4) Monel internal

liner of the ruptured pressure vessel, specifically:

135
a. Unambiguous identification of the origin of fracture

b. Results of nondestructive inspection of cracks in the vicinity of

the fracture surface

c. Measurements of wall thickness in the vicinity of the fracture

surface (see Table All for minimum required thickness according to

Table USC-23 Section VIII of the ASME Boiler and Pressure Vessel Code)

d. Chemical composition of metal samples taken near the fracture

e. Metallographic and fractographic analysis of specimens taken from

the fracture surface and nearby regions

f. Mechanical properties of test specimens taken from the ruptured

pressure vessel. Properties of interest include tensile properties,

hardness, and especially, toughness

g. Theoretical fracture mechanics analysis of part-through and throughwall circumferential cracks

h. Results of testing model structures, if needed

i. Theoretical fluid mechanics analysis to assess refr igeration effects

on toughness of a through-wall leak

A j. Corrosion test results assessing the effects of process fluids on structural material

k. Corrosion test results assessing the extent of galvanic corrosion

effects between the Monel internal liner and the pressure vessel

steel in the presence of process fluids

Expert technical analysis will be required of the records and the units

of these tests.

136
6.0 CONCLUSIONS

b.1 The rupture of pressure vessel 12D701, a 16 year old, 16 m (55 ft.)

high amine absorber tower constructed in 1968, was the initiating event that

led to the fires and extensive damage at the south end of the Union Oil

Petroleum Refinery in Lemont, Illinois on July 23, 1984.

6.2 The rupture of pressure vessel 1 2D 7 0 1 originated at a crack that leaked.

Longitudinal tensile stress from internal pressure caused the through-wall

defect at the leak to grow circumferentially and to separate the vessel into

two major pieces. The circumferential fracture followed a path of low

fracture resistance in the vicinity of the girth weld joining Shell Course 1 and replacement Shell Course 2.

6.3 The through-wall defect at the leak developed sometime between

May 1974, when replacement Shell Course 2 was welded into place, and July 23,

1984 when the leak was reported.

6.4 Pressure vessel 1 2D 701 sustained substantial corrosion cracking (blister

cracking) on the inside walls in the vicinity of the inlet piping near the

bottom of the vessel. The deterioration via corrosion was a continuing

process during the life of the vessel, which was so severe as to require

replacement of the original Shell Course 2 in 1974 after six years of

service, protection of Shell Course 1 and the bottom head with a Monel

interior liner in 1976, and weld repair of the corroded liner in April 1984.

6.5 By 1972, wall thickness of the original Course 2 of pressure vessel

12D701 had been deteriorated by corrosion to values as much as 30? less than

the minimum wall thickness required by the ASME Boiler and Pressure Vessel

Code

6.6 Numerous refinery records and much technical data and analysis, as described in Section 5.4 of this report, are needed to facilitate an important assessment of the causes of the catastrophic fracture of pressure

vessel 1 2D701

137
7.0 REFERENCES

A1 . Preliminary NBS Reports to OSHA/U.S. Department of Labor.

a) "Preliminary Report of NBS Field Investigation of Incident at Union

Oil Company’s Refinery at Lockport, Illinois on July 23, 1984." To

Mr. William Funcheon, Calumet City Area Office, Calumet City, Illinois

60409. August 10, 1984.

b) "Incident at Union Oil Company's Chicago Refinery, Lockport,

Illinois on July 23, 1984." To Mr. Frank Strasheim, Chicago Regional

Office, Chicago, Illinois 60604. August 29, 1984.

National Bureau of Standards, Boulder, Colorado.

A2. Engineering drawing for Union Oil Company at Lemont, Illinois - Chicago

Refinery - Unsaturated Gas Unit 12 - Plot Plan. C. F. Braun and Co.,

Alhambra, 'California. November 22, 1967.

A3- Engineering drawing for "(1) Unsaturated C^-Ci, Amine Absorber" for Union

Oil Company, Lemont, Illinois and C .F .Braun and Company, Alhambra,

California. Buffalo Tank Division, Bethlehem Steel Corporation.

August 12, 1968.

A4. 1968 ASME Boiler and Pressure Vessel Code. Section VIII - Division 1

a) Paragraph UG-27/Thi ckness of shells under internal pressure.

b) Paragraph UG-32/formed heads, pressure on concave side.

c) Table USC-23/Maximum Allowable stress values in tension for carbon

and low-alloy steel.

d) Paragraph UG-25/corros ion

ASME, New York. 1968.

A5. Equipment Inspection Reports: April 1972; April 1974; May 1976; April

1978 T/A; 1980 T/A; 1982 T/A; April 1984 T/A. Provided by refinery em- ployees in July 1984 to Compliance Officers from OSHA Area Office in

Calumet City, Illinois.

A6. "Blisters in the C^-C^ Amine Absorber." Letter report from

Mr. E. Schaschl, Engineering Associate, Union Research Center, Brea,

California to Mr. Hans Kolff van Oosterwijk, Chicago Refinery, Lemont,

Illinois. May 3, 1972.

A7. "C^-C^ Amine Absorber and Fuel Gas Contactor". Memorandum report from

R. Y. Salisbury, Inspection Division, Union Oil Company of California to

J. E. Ganzman, Operations Division, Union Oil Company of California.

June 26, 1972.

A8. "Inspection Report, March 1974 T/A, UNSAT Gas Plant No. 212." Memorandum

report from L. W. Virtel, Inspection Supervisor, Union Oil Company of

California to Mr. H. Kolff van Oosterwijk, Chicago Refinery, Lemont,

Illinois. September 5, 1974.

A9 . ASTM Designation A516: Standard Specification for Pressure Vessel

Plates, Carbon Steel, for Moderate-and Lower-Temperature Service . American Society for Testing and Materials, 1916 Race Street,

Philadelphia, Pennsylvania.

138
APPENDIX B - MAGNETIC PARTICLE INSPECTION

December 1985

Prepared by: J. David Me Co Is key

David T. Read

Thomas A. Siewert

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Bureau of Standards

Boulder, Colorado 80303

139

Magnetic particle inspection of Ring 1, Course 1.

Location

Comments Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications

0-0.15 BM 1 6 -30 Edge of feed-through

Npipe, 6 mm to 102 mm

from FS

0.25-0.41 BM 1 3-6 -36 13 mm to 89 mm from FS

0.42 BM 1 6 1 3 mm from FS

0.45 BM 1 6 1 9 mm from FS

0.43-0.55 BM 1

IIA -25 3 mm to 76 mm FS

0.55-0.65 BM 1 (cutout) 3-19 4 Splitting on edge of

cutout, 19 mm from FS

0.65-0.80 BM 1 ^6 -25 13 mm to 76 mm from FS

0.77 BM 1 25 1 19 mm below FS

0.85 BM 1 13 2 19 mm below FS

0.85 BM 1 3 3 6 mm below FS

0.88 BM 1 3 3 At vertical fillet

weld, 1 3 mm from FS

0.91 -1 .07 BM 1 3-6 8 Three are 3 mm long;

five are 6 mm long,

13 mm to 64 mm from FS

(Monel liner starts

here)

.

1 .09 BM 1 13 1 38 mm from FS

FS

BM

HAZ

i . d

fracture surface

base metal

heat affected zone

inside diameter (surface)

141
Table BI, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm

Ind ications Comments

1 .42 BM 1 10 1 Apparent blister, 13 mm from FS

1 .46 BM 1 13 1 114 mm from FS

1 .59-2.04 BM 1 (cutout) 6-25 Multiple splitting

indications on edges of

cutouts.

2.04-2. 18 BM 1 9 15 25 mm to 152 mm FS

2.06 BM 2 6 3 3 mm from FS surface.

2.21 -2.34 BM 1 9 5 25 mm to 1 27 mm from FS

2.36-2.44 BM 1 9 3 13 mm to 76 mm from FS

2.68 BM 1 6 1 25 mm from FS

2.84-3.06 HAZ 1 Indications at fusion

line of Monel attachment weld, and intermittent indications

along fusion line of

HAZ 1 .

2.91 HAZ 1 13 1

2.95-3.09 HAZ 1 140 1

3.18 BM 1 13 1 1 3 mm from FS

3.24 BM 1/

fillet weld

^1 9 4 Vertical weld for tray

rib has 4 indications

transverse to fillet

weld. Located at

32 mm, 127 mm, 184 mm, and 229 mm below girth

weld (Monel liner stops

here)

.

3.49-3.57 BM 1 6 1 2 13 mm to 51 mm below FS

142
Table BI , continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

3.57-3.66 BM 1 ( cutout) 22-51 Multiple splitting,

about 3 mm from i.d.

along sawcut edges

3.67 BM 1 3-6 Multiple indications,

19 mm to 76 mm from FS

3.71 BM 1 13 1 50 mm from FS

3.78 BM 1 6 1 50 mm from FS

3.84 BM 1 19 1 25 mm from FS

3.84-4.09 HAZ 1 250 1 Along HAZ 1

4.05 BM 1 6 1 50 mm below FS

4.11 BM 1 6 2 25 mm and 38 mm below

FS

4.11-4.15 HAZ 1 38 1

4.17-4.18 HAZ 1 19 1

4.22 BM 1 9 1 6 mm below FS

OJ BM 1 3-6 Multiple indications,

6 mm to 50 mm below FS

4.24 HAZ 1 25 1

4.27 HAZ 1 13 1 Extends to cutout

4.27 BM 1 3-13 4 50 mm to 76 mm below FS

4.28-4.37 BM 1 ( cutout) 6-25 1

1

Multiple splitting

within 3 mm of i.d.

along saw-cut edge of

cutout

.

4.31 Vertical weld 3-9 6 3 mm to 50 mm below

cutout surface

143
Table BI, continued

Location

Circumference Base Metal,

m Weld, or HAZ

and Course

Number

Length

mm Indications Comments

4.37-4.42 BM 1 £6 Multiple indications,

13 mm to 127 mm below

FS

4.41 BM 1 6 1 6 mm below FS

4.45 HAZ 1 6 1

4.51 BM 1 32 1 9 mm below FS

4.59 BM 1 3 3 6 mm below FS

4.60 BM 1 6 1 25 mm below FS

4.62 BM 1 6 1 13 mm below FS

4.63 BM 1 6 1 9 mm below FS

4.63 BM 1 19 1 25 mm below FS

4.63 BM 1 6 1 32 mm below FS

4.67 BM 1 9 1 9 mm below FS

4.80 BM 1 6 1 19 mm below FS

4.85 BM 1 5 1 9 mm below FS

CO OO BM 1 6 1 25 mm below FS

4.90 BM 1 6 1 38 mm below FS

4.95 BM 1 5 3 3 mm below from FS

4.97 BM 1 6 1 25 mm below from FS

4.98 BM 1 6 1 13 mm below from FS

4.98-5.00 BM 1 3 5 wi thin 13 mm of FS

5.02 Vertical weld/

bracket

51 1 within 51 mm of FS

(vertical indication)

5.02 Vertical weld/

bracket

9 1 9 mm from FS

144
Table BI, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

5.02 Vertical weld/

bracket

9 1 29 mm from FS

5.03 BM 1 16 1 6 mm to 22 mm below FS

5.41 BM 1 5 1 25 mm below FS

5.41 BM 1 5 1 9 mm below FS

5.49 BM 1 6 1 3 mm below FS

5.69 BM 1 70 1 44 mm below FS

5.69 BM 1 9 1 50 mm below FS

5.76 BM 1 6 1 3 mm below FS

5.91 BM 1 6 1 1 02 mm below FS

6.30 BM 1 9 1 3mm below FS

6.72 HAZ 1 6 1

6.91 BM 1 6 1 3 mm below FS

6.91 BM 1 16 1 9 mm below FS

6.91 BM 1 13 1 29 mm below FS

7.20 BM 1 6 5 6 mm below FS

7.22 BM 1 16 1 3 mm below FS

7.22 BM 1 13 1 13 mm be low FS

7.22 BM 1 9 1 16 mm below FS

7.22 BM 1 16 1 19 mm below FS

7.22 BM 1 6 1 29 mm below FS

145
Table BI, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

7.22 BM 1 27 1 35 mm below FS

7.22 BM 1 3-1 3 Multiple indications,

38 mm to 89 mm below

FS

7.23-8.3 No indications

146
r »b 1 91 1. Magnetic particle inspection of Ring 2, Course 2

Location

Circumference Base Metal,

m Weld, or HAZ

and Course

Number

Length

mm Indications Comments

0.03 BM 2 (cutout) 57 1 127 mm above FS (on saw cut)

0.03 BM 2 (cutout) 5 1 13 mm above FS

0.03 BM 2 (cutout) 5 1 83 mm above FS (on saw cut)

0.03 BM 2 6 1 32 mm above FS

0.03 BM 2 6 1 25 mm above FS

0.03 BM 2 13 1 89 mm above FS

0.03 BM 2 6 1 108 mm above FS

0.03 BM 2 6 1 121 mm above FS

0.04 BM 2 44 1 70 mm from FS, at fu~

sion line of flange

weld

0.04 BM 2 1-6 Multiple indications

from FS to 152 mm above

FS

0.16 HAZ 2 22 1

0.16 BM 2 64 1 19 mm above FS

0.38 BM 2 50 1 25 mm above FS

0.50 BM 2 3 1 35 mm above FS

FS - fracture surface

BM - base metal

HAZ - heat affected zone

i.d. - inside diameter (surface)

147
Table BII, continued

Location

Circumference Base Metal, Length Indications Comments

m Weld, or HAZ mm and Course

Number

0.55-0.64 BM 2 (cutout) 3~9 3

0.67 HAZ 2 3 1

0.70 BM 2 9 1

0.64-0.87 Weld 1/2 3

0.95 BM 2 25 1

0.99 HAZ 2 9 1

o on HAZ 2 165 1

1 .03 BM 2 22 1

1 .03 BM 2 9 1

1 .03 BM 2 6 1

1 .04 BM 2 9 1

1 .04 BM 2 9 1

1 .04-1 .27 HAZ 2 1 78

1 .24 BM 2 9 1

1 .30 BM 2 76 1

1 .31 HAZ 2 32 1

1.38 HAZ 2 13 1

1 .42 BM 2 13 1

Splitting indications

on saw-cut surfaces

32 mm from FS

38 mm above FS

Multiple transverse

cracks in weld, 6 mm to

32 mm above FS

35 mm above FS

25 mm above FS

25 mm above FS

38 mm above FS

35 mm above FS

32 mm above FS

32 mm above FS

51 mm above FS

Apparent continuous

indication 32 mm above

FS

57 mm above FS

51 mm above FS

32 mm above FS

51 mm from FS

57 mm above FS

148
Table BII, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

1 .45 BM 2 6 1 44 mm from FS

1 .46 BM 2 3-6 Multiple indications,

25 mm above FS

1.59 BM 2 6 1 89 mm from FS

1 .63 BM 2 6 1 86 mm above FS

1 .69-1 .76 BM 2 (cutout) 6 1 Bottom of cutout, open

to inner surface of

vessel

1 .78 BM 2 6 1 76 mm from FS

1 .79 BM 2 6 1 76 mm from FS

1 .79 BM 2 6 1 102 mm above FS

1 .85 BM 2 5 1 6 mm from FS

2.16 HAZ 2 6 2 1 3 mm from FS

2.23 HAZ 2 3 1 19 mm from FS

2.23 BM 2 13 1 64 mm above FS, possible blister

2.26 BM 2 9 1 38 mm above FS, possible blister

2.28 BM 2 9 1 44 mm above FS

2.29 BM 2 9 1 44 mm above FS

2.29 BM 2 6 1 41 mm above FS

2.34 HAZ 2 6 1 25 mm above FS

149
Table BII, continued

Location

Circumference Base Metal,

m Weld, or HAZ

and Course

Number

Length

mm Indications Comments

2.35 HAZ 2 6 1 25 mm above FS

2.35 BM 2 6 1 38 mm above FS

2.41 BM 2 19 Multiple indications,

38 mm above FS

2.39-2.44 Weld 1/2 50 1 Monel/weld interface,

6 mm above FS

2.48 BM 2 6 1 44 mm above FS

2.60 Weld 1/2 89 1 Monel/weld interface,

25 mm below FS

2.64 HAZ 2 6 1 32 mm above FS

2.65 HAZ 2 6 1 32 mm above FS

2.65 BM 2 5 1 44 mm above FS

2.67 HAZ 2 1 Large, open crack at

HAZ 2

2.67 BM 2 13 1 44 mm above FS

2.68 BM 2 6 1 57 mm above FS

2.73 BM 2 6 1 22 mm above FS

2.86 BM 2 6 1 6 mm from FS

2.92 BM 2 13 1 29 mm above FS

3.07 BM 2 16 1 19 mm from FS

3.07 BM 2 6 1 32 mm above FS

3.15 BM 2 16 1 32 mm above FS

150
fable B 1

1

, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

3.23-3.59 Weld 1/2 <13 Multiple transverse

indications at HAZ 2;

multiple indications

parallel to weld, some about 13 mm long

3.59-3.69 BM 2 (cutout) 83, 25 2 On saw-cut surfaces,

83 mm indication at

3-59 m, 6 mm above FS,

and 25 mm indication at

3.59 m, 16 mm above FS

(splitting indications)

3.71-3.9^ HAZ 2 Indications at 3.71 m, 3.85 m, and 3-89 m

4.27 HAZ 2 9 2

4.32 HAZ 2 25 1

4.45-4.53 BM 2 (cutout) 13 1 19 mm above FS

(saw-cut edge)

4.67 HAZ 2 8 1 29 mm from FS

4.74 HAZ 2 13 1 29 mm from FS

4.95 HAZ 2 9 1 29 mm from FS

5.27 Weld 1/2 13 1 6 mm from FS

5.32 HAZ 2 8 1 32 mm from FS

5.56 Weld 1/2, BM 2 19 1 Transverse to weld,

19 mm above FS

5.75 HAZ 2 16 1 29 mm from FS

5.84-6.25 Weld 1/2 3-6 Multiple transverse

cracks in weld

5.89 HAZ 2 19 1 32 mm above FS

151
Table BII, continued

Location

Cir cumf erence

m Base Metal,Weld, or HAZ

and Course

Nu mber

Length

mm

Ind ications Comments

5.92 HAZ 2 19 1 32 mm above FS

5.99 HAZ 2 25 1 32 mm above FS

6.18 HAZ 2 19 1 30 mm above FS

6.22-6.27 HAZ 2 Multiple small indications, 30 mm above FS

6.38-6.63 Weld 1/2 6 Multiple transverse

indications in weld

6.93-6.99 HAZ 2 Multiple indications

7.16-7.66 BM 2 (cutout) No indications

7.66-8.3 No indications

Vertical weld A has

Vertical weld B has

Vertical weld C has

no indications.

one indication (vertical) 330 mm one 3 mm indication, 22 mm aboveabove cutout,

cutout.

6 mm long.

152
Bill. Magnetic particle inspection of Ring 3, Course 2/3

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

0.01 BM 3 3 3 3 mm from weld

0.28 BM 3 13 1 6 mm from weld

0.32 HAZ 2 6 1

0.43 HAZ 2 19 1

0.47 BM 3 3 5 25 mm from weld

0.60 HAZ 3 6 1

0.55 HAZ 3 13 1

0.62 BM 3 6 2 19 mm from weld

0.62-0.63 HAZ 2 1 14 1

0.65 HAZ 2 13 1

0.84 NBS CUTOUT FOR SPECIMENS BEGINS HERE

1.14 BM 2 Arc strike, 6 mm from

weld

2.41 BM 3 Arc strike, 13 mm from

weld

C"- C\J BM 3 Arc strike, 13 mm from

weld

2.90 NBS CUTOUT FOR SPECIMENS ENDS HERE

2.94 BM 3 9 1 6 mm from weld

2.96 BM 3 9 1 13 mm from weld

2.98 BM 3 6 3 6 mm from weld

2.98 BM 2 3 1 3 mm from weld

153
Table Bill, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

3.00 BM 3 3 2 19 mm from weld

3.02 BM 2 6 2 19 mm from weld

3.12 BM 3 6 1 "X" shaped indication,

13 mm from weld

3.15 BM 3 6 2 6 mm from weld

3.15-3.28 AREA INACCESSIBLE

3.31 BM 2 3 1 6 mm from weld

3.31 BM 3 6 2 13 mm from weld

3*33-4.03 TORCHED CUTOUT

4.03 BM 3 13 2 At torch-cut edge of

cutout

4.09 HAZ 3 6 1

4.09 HAZ 2 3 1

4.14 BM 2 9 1 6 mm from weld

4.19 HAZ 2 19 1

4.24 BM 3 6 2 9 mm from weld

4.27-4.37 BM 3 6 1

1

13 mm from weld

4.32 BM 2 6 1 6 mm from weld

4.43 BM 3 6 2 1

3

mm from weld

4.46 BM 2 3 2 6 mm from weld

4.46 BM 3 6 2 6 mm from weld

CO ^r BM 2 6 1 9 mm from weld

154
Table Bill, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm Indications Comments

4 . 53 BM 3 6 1 3 mm from weld

4.58 HAZ 3 3 1

4.59 HAZ 3 3 1

4 . 61 -4 . 66 BM 3 51 1 Weld pass above fillet

weld

4.70-4.78 BM 3 6 -10 13 mm from weld

4.80 BM 3 6 1 3 mm from weld

4.83 BM 3 9 1 19 mm from weld

4.83 HAZ 2 3 1

4.86 BM 3 6 1 6 mm from weld

4.85-4.90 HAZ 2 51 1

4.89 HAZ 3 3 1

4.91 HAZ 2 13 1

4.97 HAZ 2 25 1

4.97 HAZ 3 13 1

4.92 BM 3 3 2 3 mm from weld

4.97-5.21 TORCHED CUTOUT

5.21 -5.32 HAZ 3 1 1 4 1

5.22 HAZ 2 6 1

5.24-5.84 HAZ 2 603 1

5.26 BM 3 25 1 Along fusion line of

fillet weld

155
Table Bill, continued

Location

Circumference

m Base Metal,

Weld, or HAZ

and Course

Number

Length

mm

Ind ications Comments

5.70-5.75 TORCHED CUTOUT

5.82 BM 2 13 2 25 mm from weld

5 .8*1-6. 05 TORCHED CUTOUT

6.07-6.12 Weld 6 4

6.21 BM 3 13 1 1

3

mm from weld

6.43 BM 3 9 1 13 mm from weld

6.45 BM 3 9 1 1

3

mm from weld

VO CO BM 3 6 1 6 mm from weld

6.55 BM 3 3 2 9 mm from weld

6.87 BM 2 13 1 6 mm from weld

7.32-7.67 HAZ 2 356 1

7.34 HAZ 3 25 1 Curvy

7.37-7.44 HAZ 3 70 1

7.44-7.54 HAZ 2 102 1

7.56-7 .68 HAZ 2 1 27 1

7.66 HAZ 3 3 1

7.67-8.03 BM 3 356 1 Along fusion line of

fillet weld

7.73 HAZ 3 13 1

7.87 HAZ 3 13 1

7.90 HAZ 2 3 1

156
Table Bill, continued

Location

Circumference Base Metal, Length Indications Comments

m Weld, or HAZ

and Course

Number

mm

8.02 BM 3 3 2 13 mm from weld

8.07 HAZ 3 3 1

7.96 BM 3 Transverse indication

across fillet weld

8.24 HAZ 3 3 1

8.3 HAZ 3 6 1

8.3 HAZ 2 6 1

Vertical weld at 4.27 m in Course 3:

6-mm indication on vertical weld fusion line,

6-mm indication on vertical weld fusion line,

133 mm from weld

210 mm from weld

Vertical welds A, B, and C in Course 2 had no indications.

157

APPENDIX C - ULTRASONIC INSPECTION FOR DELAMINATION

December 1985

Prepared by: Raymond E. Schramm

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Bureau of Standards

Boulder, Colorado 80303

159

ULTRASONIC INSPECTION FOR DELAMINATION

The purpose of the ultrasonic inspection was to detect possible

de lami nat ions parallel to the top and bottom surfaces. The approach was to

direct ultrasonic energy vertically into the steel plate. Echoes from the

two outer surfaces occur at well defined times, and signals other than these

indicate the presence of reflectors (flaws) at intermediate positions. This

method is also frequently used for thickness measurements.

Commercially available equipment was used in a standard configuration.

It consisted of a pulser to excite the ultrasonic transducer, a receiver

operated in the pulse-echo mode to detect the signals, and an oscilloscope

to display the output. The 12.7 mm (1.0 in.) diameter transducers were piezoelectric devices producing longitudinal waves at a resonant frequency of

5 or 1 0 MHz. The acoustic couplant necessary to transfer energy between the

transducer and the steel plate was a mixture of petroleum jelly and mineral

oil.

As a check on equipment and procedure, a reference plate about the same thickness as the tank was prepared from ferritic steel. It contained two

machined slots that served as intermediate reflectors (Fig. Cl). Measurements at three locations are shown in Figure C2 . Each echo seen on the

oscilloscope trace indicates a round trip through the thickness. Many more echoes occur but the time scale was set to see the detail on just the first

few. A nominal handbook value of the sound velocity (5.9 km/s, 3-7 mi./s) was used to measure depth.

All measurements were made on the outside or convex surface of the tank.

In most instances this meant going through a layer of paint. The only surface preparation was wire brushing by hand to remove any loose flakes.

The paint did not present any obvious problems. The measurement procedure

was to apply couplant, move the transducer and apply pressure to ensure good

acoustic contact, observe the oscilloscope trace, and record the observation

on a graph.

161
A B C

Figure Cl . Ferritic steel plate with machined slots used for

check (side view, dimensions in mm). A, B, and C

transducer positions for oscilloscope photographs

procedural

ind icate

in Figure C2

.

162
b

HMHE3I

c

Figure 02. Oscilloscope photographs of measurements made on reference plate

in Figure Cl

.

a) Transducer position A. Echoes at multiples of about 8.6 ys indicate a round trip distance of 50.8 mm (2.0 in.) or depth of 25.4 mm (1.0 in.).

b) Transducer position B. Echoes at multiples of about 4.4 ys indicate a round trip distance of 26.0 mm (1.0 in.) or depth of 13.0 mm (0.51 in.).

c) Transducer position C. Echoes at multiples of about 6.4 and 8.6 ys

indicate round trip distances of 37.8 mm (1.5 in.) and 50.8 mm (2.0 in.) or depths of 18.9 (0.74 in.) and 25.4 mm (1.0 in.). The transducer

straddles the edge of the slot here.

163
The information from the oscilloscope trace of the ultrasonic signal

fell into two basic categories:

1 . The presence of echoes from the outside surfaces

2. Other signals occurring at an earlier time, indicating the presence

of reflectors in the interior of the plate

These observations were divided into five signal levels. Degrees 1 and 2 represent what would be expected of material with through-depth integrity.

Degree 3 is the result of conditions likely to precede de laminat ion, whereas

degrees 4 and 5 are the consequences of laminar separation. The five graphical symbols used and their meanings are given in Table Cl. The photographs

in Figure C3 are examples of signal appearance. Figure C3a shows a very

clean baseline with strong signals indicating first and second echoes from

the inside surface (noted by markers). In Figure C3b the baseline displays

some small noise, but the only strong signals are again from the inside surface. In contrast, Figure C3c indicates strong reflections from intermediate

depths through the wall thickness; since the inside-surface reflections are still present, the delaminations causing the intermediate echoes are probably

tightly closed. Figure C3d shows only signals from intermediate depths and none from the inside wall; here the de laminations appear to have opened up so that ultrasonic waves did pass through them.

Initial inspections (10 MHz transducer) were made of three plates cut

from the tank sections:

Specimen 1 - Ring 1, Course 1, 3-91-4.70 m, 0.3 m (12 in.) high

(contains a vertical weld)

Specimen 2 - Ring 3, Courses 2 and 3, 2.06-2.73 m, 0.25 m (10 in.) above

and below the weld joining the two courses Specimen 3 ~ Ring 3, Courses 2 and 3, 1.42-1.88 m, 0.28 m (11 in.) above

and 0.23 m (9 in.) below the weld joining the two courses.

Measurements over the entire area of these specimens were done on a 2.54 cm

(1.0 in.) grid. Owing to surface roughness, there were some locations where

it was not possible to obtain good acoustic coupling (especially near the

welds), and no observations were recorded.

164
Tib If Cl. Graphical symbols used for ultrasonic measurements

Degree Symbol Explanation

1

2

3

4

5

o

(?

(8)

Clean baseline between outer-surface echoes - indicates homogeneous plate.

Slight noise on baseline between strong

outer-surface echoes - indicates small amount of

discontinuity through plate thickness.

Moderate noise on baseline between strong

outer-sur face echoes - indicates increasing

amount of discontinuities.

Well-defined intermediate return(s) with

moderate-to-strong outer-surface echoes - indicates likely delamination that is still

tight enough to transmit sound.

Strong intermediate return(s) with no outer-surface echoes - indicates complete

discontinuity due to delaminat ion.

NOTE: See Figure C3 for examples of oscilloscope traces.

165
a O b 9 Kiniwi

ilHSa NrapvoHira

n'itiMili II IMW

c Q d •

Figure C3* Samples of typical oscilloscope traces. Markers near top of each

photo indicate position of first two echoes from outer surfaces.

Refer to Table Cl. x-axis: 2 V/division; y-axis: 2 ys/division.

a) Degree 1. Specimen 2 [2.11 m on circumference, 0.05 m (2 in.) below weld].

b) Degree 2. Specimen 2 [2.11 m on circumference, 0.20 m (7.9 in.) above

we Id]

.

c) Degree 4. Specimen 1 [4.24 m on circumference, 0.20 m (7.9 in.) above

bottom of plate]

.

d) Degree 5. Specimen 1 [4.29 m on circumference, 0.20 m (7.9 in.) above

bottom of p late]

.

166
The results in Figures C4 through C6 indicate that major problems with

de lamination were found only in Specimen 1 (Course 1); large areas on both

sides of the vertical weld produced intermediate returns of the ultrasonic

signal, indicating material separation through the depth (degrees 4 and 5) as well as the moderate baseline noise from discontinuities that can precede

actual separation (degree 3). Specimen 2 appeared quite sound (degrees 1 and 2) as did Specimen 3 where only one small area gave degree 3 signals.

The arrows in Figures C4 and C5 indicate the locations correspond ing to the

oscilloscope traces in Figure C3.

A second inspection (with the 5 MHz transducer) was a survey around the

entire tank c ircumference in both Rings 1 and 2. In Ring 1, measurements

were made at 2.5 and 7.6 cm ( 1 and 3 in.) below the fracture surface at

5.1 cm (2 in.) intervals around the circumference; additional measurements

were taken at 33.0 cm (13 in.) below the fracture at 30.5 cm (12 in.) intervals. In Ring 2, measurements were made at 5.1 and 10.2 cm (2 and 4 in.)

above the fracture surface at 30.5 cm ( 1 2 in.) intervals. The survey results

are in Figure C7 . The area immediately above the fracture in Ring 2 showed

only a few spots of returning degree 3 signals (moderate baseline noise);

from the degree 1 and 2 ultrasonic echo returns; from the degree 1 and 2

ultrasonic echo returns, nearly all areas of the material appeared to be

sound. However, below the fracture in Ring 1, there was a considerable

mixture of areas, some showing considerable delamination (degrees 4 and 5)

while others were normal. Specimen 1 (Fig. C4 )

,

also from Ring 1, showed

much damage.

167
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168

CIRCUMFERENTIAL

LOCATION,

meters
0

. 20

-

169

CIRCUMFERENTIAL

LOCATION,

meters
o

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

o 9 9 o O o o o o o o o o o

o 9 9 o O o o o o o o o o o

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170

CIRCUMFERENTIAL

LOCATION,

meters

Figure

C6.

Scan

of

Specimen

3

(Ring

3,

Courses

2

and

3)-
171

CIRCUMFERENTIAL

LOCATION,

meters

APPENDIX D - ROCKWELL HARDNESS

December 1985

Prepared by: Patrick T. Purtscher

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Bureau of Standards

Boulder, Colorado 80303

173

Ident if icat ion

Number

Hardness,

HRA

Base Metal 1 50.5

Cl 2 51 .5

3 51 .5

4 51 .0

HAZ 5 52

Cl 6 54.5

Weld Metal 7 54

8 52.5

9 52.5

HAZ 10 54

C2 1

1

51 .5

Base Metal 1 2 50.5

C2 13 50

14 50.5

15 50

Identification numbers refer to the indentations, from left

to right, in the photographs.

175
Traverse 1 Traverse 2

Ident if icat ion Hardness, Identif ication Hardness

Number HRA Number HRB

Base Metal 1 51 .5 1 88

C2 2 53-5 2 90.5

3 54

HAZ 4 55 3 104.5

C2 5 57.5

Weld Metal 6 56.5 4 98

7 55.5 5 94.5

8 56 6 95.5

9 55.5 7 96

10 55.5 8 95

1

1

55.5 9 95.5

1 2 57 10 94.5

HAZ 13 57.5 1

1

106

C3 1 4 55.5 12 95.5

Base Metal 15 54 13 94

C3 16 54

17 53.5

176
Traverse 1 Traverse 2

Identification Hardness, Identif icat ion Hardness,

Number HRA Number HRA

Base Metal 1 50.5 1 52

C3 2 51.5 2 52.5

HAZ 3 51.5 3 52.5

C3 4 51 .5 4 53

5 52

Weld Metal 6 52.5 5 53

7 53-5 6 53.5

8 53-5 7 54

9 53-5 8 54

10 53-5 9 54.5

1

1

52.5

HAZ 12 54.5 10 53-5

C4 13 52 1

1

52.5

14 52

Base Metal 15 51.5 12 52.5

C4 16 52 13 52.5

14 86 HRB

Traverse 1

Course 3

Traverse 2

Course 4

Figure D3. Hardness traverses of the weld between Course 3 and Course 4.

177
Traverse 1 Traverse 2

Ident if icat ion Hardness, Ident if icat ion Hardness,

Number HRA Number HRA

Base Metal 1 52.5 1 52.5

C5 2 53.5 2 53.5

3 53.5 3 54.5

HAZ 4 54.5 4 55.5

C5 5 55 5 55.5

6 57.5

Weld Metal 6 51 7 57.5

7 51.5 8 58

8 53-5 9 58

9 50.5 10 57.5

10 48

HAZ 1

1

57 1

1

57

C4 1 2 54.5 12 57.5

13 56.5

Base Metal 13 54

C4 1 4 54.5 1 4 54.5

15 53.5 15 53.5

i . d.

Figure D4 . Hardness traverses of the weld between Course 4 and Course 5.

178
Traverse 1 Traverse 2

Ident if icat ion Hardness, Identif ication Hardness

Number HRA Number HRB

Base Metal 1 50.5 1 85.5

2 50.5 2 85.5

3 51 3 86.5

4 50.5 4 87.5

HAZ 5 52 5 86.5

6 54 6 91.5

Weld Metal 7 53 7 88.5

8 52 8 88.5

9 51 .5 9 86.5

10 53 10 87.5

1

1

90

HAZ 1

1

54.5 12 90

12 53-5 13 86.5

13 51

Base Metal 1 4 51 1 4 85.5

15 50.5 15 85.5

Traverse 1

Traverse 2

i . d.

Figure D5. Hardness traverses across vertical weld A in Course 2.

179
Traverse 1 Traverse 2

Identif ication Hardness, Identif ication Hardness,

Number HRA Number HRA

Base Metal 1 51.5 1 52

2 52 2 52

3 51.5 3 52.5

4 51.5

5 50.5

HAZ 4 50.5 6 51

5 50.5 7 52

6 53 8 53

7 53

Weld Metal 8 51.5 9 51

9 51.5 10 52

10 51 .5 1

1

51 .5

HAZ 1

1

52 12 53

1 2 52 13 52.5

13 51 1 4 50.5

Base Metal 1 4 51.5 15 51

15 51 .5 16 51 17 51

i . d.

Figure D6 . Hardness traverses across vertical weld B in Course 2.

180
Traverse 1 Traverse 2

Identification

Number

Hardness,

HRA

Identif ication

Number

Hardness,

HRA

Base Metal 1 49.5 1 86.5

2 49.5 2 86.5

3 49.5

HAZ 4 51.5 3 86.5

5 53.5 4 89.5

Weld Metal 6 50.5 5 88

7 50.5 6 90

8 50.5 7 89.5

9 51.5 8 90

9 91.5

HAZ 10 53 10 89

1

1

53 1

1

86

Base Metal 1 2 50.5 12 87

13 50.5 13 87

1 4 51

Traverse 1

Traverse 2

R

* «Aii »Aii A . i -0>-A. J5 J5. . *

i . d.

Figure D7 . Hardness traverses across vertical weld C in Course 2.

181
1

1

-

Traverse 1 Traverse 2 Traverse 3

Identification

Number

Hardness,

HRA

Identif ication

Number

Hardness,

HRA

Identif ication

Number

Hardness

HRA

Base Metal 1 50.5 1 48 1 52

2 51.5 2 50.5 2 52

3 50.5

4 50

HAZ 3 52 5 50.5 3 52

4 53.5 6 53 4 54

5 54.5 7 53 5 54

6 54

Weld Metal 6 54 8 52 7 53.5

7 54 9 52 8 53-5

8 54 10 51 9 53.5

9 53-5 11 51 10 53

10 54

1

1

53

12 52

HAZ 13 53 12 51 1

1

53

14 53 13 50 12 53.5

15 52 13 52.5

Base Metal 16 51 1 4 49 1 4 52.5

15 49.5

Traverse 1

Traverse 2

Traverse 3

Figure D8 . Hardness traverses across the vertical weld in Course 1.

182
Traverse 1 Traverse 2

Ident if icat ion

Number

Hardness,

HRA

Ident if icat ion

Number

Hardness,

HRA

Base Metal 1 53-5 1 52

2 53-5 2 52.5

3 52.5

4 52.5

HAZ 3 53 5 51 .5

4 53.5 6 52.5

5 53 7 53

6 53

Weld Metal 7 52 8 52.5

8 52 9 53.5

9 52 10 54.5

10 51.5 1

1

54

12 54

13 54

14 53.5

HAZ 1

1

51 15 54.5

12 51 16 53

13 50 17 52

Base Metal 14 50.5 18 51

15 51 19 52

16 50.5 20 51.5

i . d.

Figure D9 . Hardness traverses across the vertical weld in Course 3.

183
Traverse 1 Traverse 2

Ident if icat ion

Number

Hardness,

HRA

Ident if icat ion

Number

Hardness

,

HRA

Base Metal 1 50 1 51 .5 2 52.5 2 52

3 53 3 51 .5

4 52 4 51.5

HAZ 5 52 5 52

6 52

7 53.5

8 54

Weld Metal 9 53.5 6 52.5

10 54 7 52

1

1

53.5 8 52.5

12 53-5 9 52

13 54 10 52

1 4 53.5

HAZ 15 54 1

1

53

16 53.5 12 53

17 53

Base Metal 18 52.5 13 52

19 53 1 4 52.5

Traverse 1

Traverse 2

i . d.

Figure DIO. Hardness traverses across the vertical weld in Course 5.

184
APPENDIX E - y-GROOVE CRACKING TEST

December 1985

Prepared by: Thomas A. Siewert

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Bureau of Standards

Boulder, Colorado 80303

185

y-GROOVE CRACKING TEST

INTRODUCTION

This test evaluates the cracking tendency of a base material-electrode

combination to cold (also known as delayed or hydrogen) cracking. In

this study, all tests were performed with rebaked electrodes, which reduced

the hydrogen contribution of the electrodes to a low and constant level.

Thus the test evaluated only the cracking sensitivity of the heat -affected

zone (HAZ) in the base material. The test compared three material combinations and conditions that might have been present during the replacement of

Course 2 of the vessel.

TEST DESCRIPTION

The test consists of a weld between two pieces of base material, each

200 mm by 75 mm (7.9 in. by 3-0 in.) by the plate thickness. As the name

implies, the test has a groove in the shape of a lower-case y (Fig. El).

The groove shape is produced by forming a butt joint along the 200 mm

(7.9 in.) side of the plates, one having a single bevel and the other a double bevel, and both having a 30 degree bevel angle.

The important feature of this test is the severe, but constant, lateral

restraint imposed by the weld on the surrounding HAZ. This restraint is

established and controlled by (1) fixing the root opening at 2 ± 0.2 mm

(0.08 in.) when measured normal to the bevel surfaces and (2) filling the

outer 60 mm (2.4 in.) of the groove at each end of the plate with stringer

beads before the center (test) section is welded. A 4 mm (0.16 in.) diameter

type AWS E7018 electrode was rebaked at 250 °C (482°F) for at least 4 h before

being used for both the restraint and the test welds in each specimen. The

welding was performed by the shielded metal-arc process at 170 ± 10 A and

26 ± 3 V, with a travel speed of 10 to 15 cm/min (3-9 to 5.9 in./min. ).

187
Figure El. Plate and groove dimensions (mm) for the y-groove cracking test.

Figure E2. Schematic of the weld progression in the center section.

188
The center 80 mm (3 in.) length of the groove was evaluated for cracking.

After the restraint welds had been completed, the specimen was allowed to

stabilize at the desired temperature before the final weld was begun. The

weld began on the plate surface to eliminate arc starting effects, traveled

down into and along the groove, and ascended again to the plate surface to

eliminate arc crater effects (Fig. E2). The time the electrode was in the

groove and the length of the groove were measured to determine whether the

desired travel speed was achieved.

TESTS OF THREE SPECIMENS

The first specimen was composed of two plates from Course 1 , with no subsequent treatments other than slight straightening and machining of the

bevels. The plates were separated with 2 mm (0.08 in.) shims and clamped to

align the restraint welds. Even with continuous welding, the plate

temperatures did not exceed 150°C (302°F), as measured with temperatureindicating crayons. The specimen was allowed to cool to 20°C (68°F) before

the center 80 mm (3 in.) section was welded. A travel speed of 16 cm/min.

(6.3 in. /min.) was measured for this center section. After the completion of

welding, the specimen was kept at 20°C (68°F) during a one-week crack

incubation period.

The second specimen was composed of two plates from Course 2, with no subsequent treatments other than slight straightening and machining of the

bevels. The clamping and welding details were the same as for Specimen 1.

A travel speed of 15 cm/min (5.9 in. /min.) was measured for the center

section. As for Specimen 1, Specimen 2 was kept at 20°C (68°F) during a one-week crack incubation period.

The third specimen was composed of one plate from Course 1 and one plate

from Course 2. Both plates were straightened slightly and the groove bevels

were machined. The plate from Course 1 was charged with hydrogen by

immersion for 80 h in the NACE (Standard TM-01-77) solution; the plate from

Course 2 was not hydrogen charged. Since the hydrogen began to diffuse from

the plate immediately after removal from the charging bath, the specimen was prepared for the test weld as rapidly as possible. Within 10 min. after

189
removing the Course 1 plate from the solution, it was welded to the Course 2

plate at the specimen ends (the restraint welds). The welding conditions

were essentially the same as those used for Specimens 1 and 2. Immediately

after the restraint welds were completed, the specimen was placed in a refrigerator and cooled to 1 °C (3^°F). This temperature reduced the loss of

hydrogen from the plate before the test weld and simulated the lower

temperatures that may have existed in the vessel during fabrication. For the

first three days after completion of the test weld, Specimen 3 was kept at

1 °C (3^°F). For the final four days of the crack incubation period, the

specimen was kept at 20°C (68°F) to simulate the increased diffusion that

would occur as the vessel was heated.

CRACKING EVALUATION

Since hydrogen cracking is a phenomenon based on diffusion, the

examination for cracking must be delayed until the cracking has time to occur. This procedure allowed a seven-day incubation period for the hydrogen

to have its effect on the weld region. At the end of that time, the weld

surface was examined by a dye-penetrant technique.

After this nondestructive examination, the center (test) weld was sectioned transverse to the weld at five evenly spaced locations. These

sections were also examined for cracking in the HAZ . The cracking was expressed as a percentage of the HAZ length normal to the welding direction,

CL

n (%) = ^ ,

where CL

n is the percentage of cracking along the HAZ root and toe of the

bead on a section normal to the weld direction, i is the length of individual

cracks along the HAZ, and I is the total length of the interface between the

root and toe of the weld.

We also monitored the crack growth during the seven-day crack incubation

period. The weld surface was inspected with a magnetic particle technique on

the third, fifth, and seventh day of the test.

190
RESULTS

The results of the examination of the three specimens are summarized in

Table El. No cracking was found in Tests 1 and 2 during either the non- destructive examination of the weld bead surface or the subsequent examination of the weld cross section on the seventh day after welding.

No cracking was found in Test 3 during the nondestructive examination of

the weld bead surface. However, cracking was found in the examination of the

weld cross section. A typical section through the weld, Figure E3, reveals a rounded bead with good penetration into the sides. The crack is visible in a higher magnification view, Figure E4, extending from the root along the

Plate 1 (hydrogen-charged) side of the groove. The crack was always in the

HAZ on the Plate 1 side of the groove and was consistently near 20% of the

leg length (distance from the toe to the root of the weld).

REFERENCE

El. Yurioki, N.; Suzuki, H.; Ohshita, S . ; and Saito, S. Determination of

Necessary Preheating Temperature in Steel Welding. Welding Journal 62

(6), 1983, pp. 147 s-153s.

191
Table El. Results for the y-groove cracking tests.

Test

Number

Base

Material

Mater ial

Condition

Preheat

Temperature Surface

Cracks

Cross Section

1 Course 1 welded to

Course 1

As Received

As Received

20 °C (68°F) None None

2 Course 2

welded to

Course 2

As Received

As Received

20 °C (68°F) None None

3 Course 1 welded to

Course 2

H

2 Charged

As Received

1 °C ( 3^ °F) None

. * 20 %

Cracking confined to HAZ on the Course 1 side of the specimen.

192
4 mm

Figure E3. Typical cross section of the weld at the center (test) section of

Specimen 3-

«

*

193
0.4 mm

Figure E4. Typical crack along the HAZ in the Plate 1 side of the groove,

extending 2 0% of the leg length from the weld root.

194
APPENDIX F - REPORT OF ANALYSIS BY THE ELECTRON PROBE MICROANALYZER

OF FRACTURE SURFACE SPECIMENS

October 1985

Prepared by: Ryna Marinenko

Gas and Particulate Science Division

Center for Analytical Chemistry

National Bureau of Standards

Washington, D.C. 20234

Approved by: Dale E. Newbury

Harry L. Rook

Gas and Particulate Science Division

Center for Analytical Chemistry

National Bureau of Standards

Washington, D.C. 20234

NOTE: The specimens submitted by Robert Shives for analysis were taken from

the following locations:

Ring 1 at CC 2.0 m ( G-6-5-1

)

Ring 1 at CC 5.5 m (G-1 8-1

)

Ring 1 at CC 7.5 m (24-C1) - not analyzed

Ring 1 at CC 7.7 m (25-3/8C1 ) Ring 2 at CC 1 .0 m (G-3-5-2)

Ring 2 at CC 7.2 m (25-3/8C2)

Ring 2 at CC 7.2 m ( 25-3/ 8C20)

195

REPORT OF ANALYSIS BY THE ELECTRON PROBE MICROANALYZER

OF FRACTURE SURFACE SPECIMENS

DESCRIPTION OF SPECIMEN

Seven metal pieces, each with one corroded surface were submitted for

electron microprobe analysis. Each piece is about 1 cm x 5 mm x 5 mm. The first four specimens are labeled G— 1 8—1 (red, inside), G-3- 5~2 (brown),

G-6-5-1 (black), and G-6-5-1 (dark brown). Three others, which were submitted later, are labeled 25-3/8C1 (RD25C1), 25 - 3/8C2 ( DK25C2 ) , and 24-C1

.

Also, there is one sample mount containing two cross sections, one labeled

25-3/8C2 and the other labeled "out. "

REQUEST

Qualitative analysis of the corrosion layer on the metal fracture

surface.

RESULTS

The rough specimens were mounted for in situ analysis with the electron

microprobe using the energy dispersive detector. Several areas were analyzed

on each specimen with a point beam. A short description of the appearance of

the region analyzed on each specimen and the elemental results of the analysis are listed in Table FI. For each analysis, the major elements are listed

on the first line, lower-concentration elements, estimated to be less than

10 percent, are on the second line, and those elements present in very small

quantities, probably less than one percent, are on the third line. The major

elements on the first line are listed in order of the largest peak first.

Iron, which originates from the steel matrix, is the major constitutent in

most of the spectra. Sulphur and/or silicon occur in large amounts in many

areas. Several other elements, including sodium, magnesium, aluminum,

phosphorus, chlorine, potassium, calcium, titanium, manganese, chromium, and

zinc, are generally present in much smaller amounts. A few examples of the

EDS spectra observed are in Figure FI. Spectrum A was taken from the white

197
Table FI. EDS qualitative analysis of rough specimens from fracture surface (Union Oil)

Specimen Spectrum Region Analyzed

G-18-8-1

R1 (Red) 1 REDFe

Si, S , Ca ,Mn Zn , Mg , A1 , P , K

WHITE CRYSTALLINE

S+Fe

Si , Ca ,Mn

A1 , Zn , P ,Mn ,Mg

BLACK

Fe+S

Si,P,Ca

Al ,K,Mg ,Mn

2 Fe

S i , P , S , Ca , Mn

A1

SSi ,Fe ,K,A1 ,P Ca ,Mn , Zn ,Mg

Fe

S , S i , Mn

Al ,P,C1

G-3-5-2

(Brown) 1 YELLOW-REDFe

Si , P ,

S

A1 , K , Ca , Mn , Zn , Cl

WHITE CRYSTALLINE

S , Si , Fe Al,K,Ca

Ti, Cl , P ,Mg , Zn

DARK

FeSi,P,S,Ca,Mn

Al ,K, Zn ,Mg , Cr

2 Fe

Si+Al ,P ,

S

Zn,K, Ca,Mn,Ti

FeSi+P , S , Cl , Ca , A1

K , Ca , Zn , Mn ,Mg , Ti

Fe

S i+Al , K ,

S

Ca , T i , Mn , Zn , P , Mg

3 FeSi+Al, P,S

Zn,K,Ca,Mn,T

Si+Fe

A1,S

K, Cl , P , Ca ,Mn , Zn ,Mg

4 Ca+Fe , Si+Mg

P , S , K , Zn , T i , Mn , A1

G-6-5-1

(Black) 1 DARK RED

Fe+S

Si,P,Mn,Ca

Zn , A1 ,K

WHITE CRYSTALLINE

Si+Fe, S+Mg,Ca

Ti

K , Mn , A1

BLACK

Fe+S

S i , Ca , K , Mn , P , Al

Mg , Zn

1 SFe , Ti , Si

Ca

Fe

Si, S P , Ca ,Mn , Zn

3 REDDISH WHITE

S+Fe

Zn, Si

A1 ,P,Ca,K,Mn

G-6-5-1

(Dark brown) 1 DARK RED

Fe+Al

Si , S Mn , Zn , Ca , Mg , P , Ti

WHITE CRYSTALLINE

Al+Fe

S , Si

Ca ,Ti

BLACK

Fe

Mn

2 Fe Fe+S

S , A1 , Mn Al , Si , Cl , Ca

25-3/8C1

(RD25C1) 1 REDFeS+Si,Mn

WHITE CRYSTALLINE

P+Ca , Fe

Mg,Mn,Al,Si,S,Cl

DARK

25-3/8C2

(DK25C2) 1 WHITE CRYSTALLINE

Si+Fe

S , Ti

Ca, Al , P , Cl ,Mn , Zn

DARK

Fe+S, Si

Mn

K, Ca

2 S+Fe

Si

K,Ca,Mn,P,Zn

198
CUSOt: S.KtKBJ

f. S

1

R

m f ?

• • *

1

B fl _J>1 «*

1

8 •

• A 8

irS-65536 IF5-3276B ivi.fi 11/B8S 4* DK25C2 15/1.4 11/985 — A

-

S.

A. White crystalline material in RD25C1 B. Dark material in DK25C2.

C. White crystalline material in DK25C2 D. Reduced scale of C.

Figure FI. Examples of the EDS spectra (all are Sepctrum 1 in Table FI).

199
crystalline material in RD25C1 (spectrum 1 in Table FI) B was taken from the

dark material in DK25C2 (spectrum 1 in Table FI) and C was taken from the

white crystalline material in DK25C2 (spectrum 1 in Table FI). Spectrum D is

a reduced scale of C. Based upon the qualitative analyses of the fracture

surfaces, two samples were selected for analysis in cross section.

The sample mount with the two cross sections was carbon coated.

Digital X-ray maps were taken of three different pitted areas on the metal

surface of these specimens. These maps are prepared by positioning the

electron beam point-by -point on the specimen in an array pattern with a preselected distance between points. At each point X-ray counts for the

elements of interest are integrated for a selected time period. The maps in

this report were taken at a magnification of 300X, which covers an area slightly larger than 300 x 300 ym. A 128 x 128 point array was used.

Therefore, there were approximately 2.6 ym between the centers of adjacent

points. The three maps were taken with wavelength spectrometers (WDS) on the

elements iron, sulfur, and silicon. The WDS maps were ratioed to identical

maps taken of the standard elements. To do this, each point of the unknown

map is ratioed to the corresponding point of a map taken of a standard

specimen containing the element of interest. This was done to correct

spectrometer defocussing, which occurs in low-magnification WDS maps.

These maps can be displayed on an image analysis system in black and

white or in color. In each of Figures F2-FA are shown the individual black

and white maps for each element and the composite color maps for each of the

three pitted areas. The areas in Figures F2 and F3 were from the large

specimen denoted 25_ 3/8C2, while the area in Figure FA was from the small

specimen labeled "out" and denoted 25~3/8C20 in our notation. The color maps

were prepared by overlaying the iron, sulfur, and silicon ratio maps. Iron

is red, sulfur is green, and silicon is blue. Silicon was not included in

Figure F2 because there was so little observed in the silicon map of this

area. There is also little silicon in the area of Figure F3. In both of

these maps, sulfur and iron are the major components of the pitted region.

In Figure FA there is a lot of silicon in the pitted region and sulfur is

located primarily along the interface between the iron metal and the siliconrich pit. Since other elements were observed in the EDS spectra of this

region, an EDS map was prepared of this pit in order to include more elements

200
(FEW2U MT

A . Sulfur B. Iron

v_. . Composite color map

Figure F2. WDS maps of specimen 25-3/8C2.

201
KFEWJCi .MT MUCH

MMIl

THE CMT

te*f

Figure F3. WDS maps of specimen 25-3/8C2.

202

8*il!

ni

e.Ill.i

KKsISb
Figure F4. WDS maps of specimen 25-3/8C20.

203

ip.

I

JJJ

I

fu

hki
Figure F5. Energy dispersive spectra ot the pitted area of specimen 25-3/8C20.

204
in a single-mapped region. (Only three elements can be observed at the same

time with WDS since there are only three wavelength spectrometers on our instrument. ) These EDS maps are displayed in black-and-white in Figure F5.

Some elements such as aluminum, potassium, and phosphorus are present with

silicon in the pitted region. Other elements found frequently in other EDS

spectra of the fracture surface, calcium and magnesium, are not present to

any noticeable extent in this region.

ARCHIVAL STORAGE

Laboratory notes on the analysis of these specimens are in the laboratory book labeled Service Analysis on pages 57- 75. EDS spectra of the rough,

corroded surface of the specimens are on FLEXTRAN floppy disk 1570A. The WDS

digital maps of the cross sections and standards are on RT-11 floppy disks

106, 108, 109, and 110, and the EDS maps are on disks 111 and 112. The

spectra for the EDS maps are on Flextran floppy disk 1572. These maps are stored on the VAX 780 computer in directory SYS$IMAGE: [MARINENKO. UN0IL] . The

names of the maps on the computer can be traced back to the original maps

with the MAPS LOG (small green book). The maps used in this report are numbered 1 28-1 37.

205
NBS-1 14A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication D ate

SHEET (See instructions) NBSIR 86-3049 March 1986

4. TITLE AND SUBTITLE

Examination of a Pressure Vessel That Ruptured at the Chicago Refinery of the

Union Oil Company on July 23, 1984

5. author(S) Harry I. McHenry; T. Robert Shives ; David T. Charles H. Brady; Patrick T. Purtscher

6. PERFORMING ORGANIZATION (If joint or other than NBS. see in struction s)

Read; J. David McColskey;

7. Contract/Grant No.

NATIONAL BOREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Occupational Safety and Health Administration

U.S. Department of Labor

Washington, DC 20210

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here) On July 23, 1984, a pressure vessel ruptured at the Chicago refinery of the

Union Oil Company causing an explosion and fire that killed seventeen people and

resulted in extensive property damage. The Occupational Safety and Health Administration (OSHA) requested NBS to conduct an investigation into the cause of the

failure. The pressure vessel fractured along a path that was weakened by extensive

cracking adjacent to a repair weld joining a replacement section to the vessel.

These pre-existing cracks initiated in areas of a hard microstructure known to be

susceptible to hydrogen stress cracking. This microstructure formed during the

repair welding of the replacement section, and at the surface, it was not tempered

by subsequent weld passes or thermal treatment. The cracks grew through the vessel

wall due to hydrogen pressure cracking. When the depth of the largest of these

pre-existing cracks exceeded 90 to 95% of the wall thickness, leakage occurred

because the thin ligament of steel remaining in the cracked section ruptured.

Because of the uniform depth of the pre-existing crack, the thin ligament continued

to tear, causing the through crack to grow to a length of about 800 mm. This crack

triggered final fracture at the operating stress level of 35 MPa because the tough- ness of the vessel steel was reduced by hydrogen embrittlement.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

corrosion; failure analysis, fracture mechanics; hydrogen embrittlement; hydrogen

induced cracking; hydrogen sulfide; magnetic particle inspection; mechanical properf- -j o g • nrpgQiirp vppiFipl ; steel ; ultrasonic inspection

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

(T3 Uni i mi ted

| | For Official Distribution. Do Not Release to NTIS

J Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

|TK] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

218

15. Price

USCOMM-DC 6043-P 80

U. C GOVERNMENT PRINTING OFFICE: 1986-671-758/45,200 REGION NO, 8