repair and failure analysis of a crack in an ammonia converter

2002 AMMONIA TECHNICAL MANUAL

Repair and Failure Analysis of a Crack in an Ammonia Converter

This paper reviews the repair of a crack in a weld in the shell of an ammonia converter in Trinidad. It highlights the inspection performed, the repairs conducted and the metallurgical findings that

identified the cause of the failure.

Michael B. Rugh Kellogg Brown & Root, Inc.

Stanley Reid PCS Nitrogen Trinidad Ltd.

Introduction

CS Nitrogen Trinidad Limited operates four ammonia plants and one urea plant with an annual production of 2,000,000 tons of am-

monia and 700,000 tons of urea. The incident occurred in the 04 plant which was

started up in 1998. The converter uses the Kellogg Brown & Root�s KAAP technology and operates at 89635 Kpa (1300 psi) and 371 deg. C (700 deg. F). The converter is designed to ASME Section VIII, Division 2, and its dimensions are 23.5m long, 3.35m diameter and 100mm thick. The material of construc-tion is SA-302- Gr. B for the lower shell section and SA-387-Gr. 11, CL2 for the upper shell section and heads.

At 5:00 p.m. on January 17, 2002 during normal plant operations, a small fire was observed emanating from the insulation on the 105-D ammonia converter of the 04 Plant. The fire was seen on the converter shell at a location approximately one-third of its height. The fire was extinguished by operating personnel and there was no injury to personnel, no environmental impact and only minor property damage.

After the plant was shut down and insulation re-moved in the vicinity of the fire , attempts to locate the leak site by visual, infrared thermography, and ex-plosimeter testing proved futile. It was only after the converter was repressured to 700 psi that the location of the leak site was identified. The crack was located in a longitudinal weld (LWL-14, Fig. 1).

Converter Repairs

Repair Plan

Immediately following the discovery of the leak, PCS Nitrogen Trinidad Ltd contacted Kellogg Brown & Root (KBR) and a repair plan developed. The plan was designed to meet all the conditions of the National Board Inspection Code (NBIC). The following compa-nies were involved in various aspects of the repairs:

� KBR � Repairs, welding advisor � IESCO - Inspection services � CUST-O-FAB � Welding, fabrication � NBIC � Authorized Inspector � Local Inspection Companies

P

AMMONIA TECHNICAL MANUAL 2002

• PCS Corporate • KBR�S Head Office • Metallurgical Consultants - Laboratory Ser-

vices • G. R. Prescott - Consultant The key factors that influenced the repair strategy

included: • The removal of a forensic sample to estab-

lish the cause of failure • The converter was full of catalyst charge,

which is pyrophoric, and under nitrogen blanket.

• Vessel was in hydrogen service, appropri-ate repair methods were critical

• The repairs had to be done in accordance with NBIC

• Thorough inspection required to identify any other defects

Inspection

A thorough inspection of the converter shell circum-ferential and longitudinal welds and base metal in the SA-302 Gr B section was performed using eight inspec-tion techniques (Fig. 2).

Ultrasonic shear wave was used to characterize the crack. It measured approximately 100mm on the outer surface and 345mm on the inner surface. The crack started 270mm above the girth weld CWL-6, ran across the width of the girth weld and extended 18mm into the shell.

Ultrasonic examination for High Temperature Hy-drogen Attack using backscatter, velocity ratio and spec-trum analysis was conducted in the SA-302 Gr B section on seam welds and heat affected zones and on nozzle at-tachment welds. No detectable HTHA was found.

Ultrasonic Time Of Flight Diffraction A and D scans were performed on all girth and longitudinal welds on the bottom 3 sections of the converter. This inspection revealed the following:

• A 50mm long area with scattered inclu-

sions 75mm above the crack at LWL-14. The largest slag inclusion was less than 12mm long and the rest were point indica-tors. This was repaired.

• Numerous minute point indicators at CWL-6 located 1.96mm from MH 3. A sample was taken for metallurgical analysis.

• At CWL-8, there was a 45mm long indica-tion coming out of the cladding to a height of 9mm. This was presumed to be a previ-ous repair and will be monitored.

• At CWL-5 an indication 15mm long was found near the back wall and appeared to be a slag inclusion. Its height was 2.5mm. This will be monitored.

Sixteen replicas were taken and the results showed

good fusion and microstructure apart from random in-clusions (Fig. 3).

Over 800 hardness readings were made and the weld hardness averaged 200 BHN and base material av-eraged 180 BHN. No hardness values were found above 225 BHN.

Repairs

Out Gassing and Temperature Maintenance

Prior to the removal of the large forensic sample at LWL-14, the area was out gassed of hydrogen by heat-ing the location to 427 deg. C (800 deg. F) and holding for 12 hours. The sample removed at CWL-6 was 38mm thick and the hydrogen bake out was held for 6 hours. A preheat of 204 deg. C (400 deg. F) was main-tained during cutting and welding processes and sus-tained until the PWHT began.

Repairs at LWL-14

It was impossible to access the inner wall of the con-verter at this location due to vessel design and the con-verter having a full charge of catalyst. The crack was cut out by drilling and sawing and the size of the material re-moved was 125mm (outside width) X 71mm (inside width) X 356mm long. Because of the large void, a back-ing bar, 5mm thick and of SA-302 Gr. B material was used. A design review of the converter showed that the backing bar would not impact on the ASME primary membrane stress levels or integrity of the vessel for its in-tended use. The backing bar was seal welded and the cavity filled and capped, using the TIG process.


Repair of Slag Inclusion in LWL-14

The defect which was 50mm long and at a depth of 50mm was removed by mechanical grinding and the re-pairs made after the major repair was completed. Re-pairs were replacement in kind and no calculations were required.

Repairs to Boat Sample at CWL-6

A sample 320mm long X 75mm OD and 38mm thick was removed from the girth weld at CWL-6 for metallurgical analysis. The repairs were similar to that performed for the slag inclusion at LWL-14.

Post Weld Heat Treatment (PWHT)

Prior to the PWHT of the repairs a uniform tempera-ture of 204 deg C (400 deg. F) (minimum) was estab-lished around the circumference of the shell at CWL-6. For the PWHT the temperature was uniformly brought up to 427 deg. C (800 deg. F) with no gradient in heated zone greater than 120 deg. C (250 deg. F) per 5m inter-val. The area was then heated at 38 deg. C (100 deg. F) per hour to 630 deg. C (1200 deg. F) and held for four (4) hours, ensuring that no temperature deviation in the heated zone exceeded 38 deg. C (100 deg. F). The tem-perature was reduced to 427 deg. C (800 deg. F) at 38 deg. C (100 deg. F) per hour, allowed to cool to 315 deg. C (600 deg. F) and then the insulation removed.

Inspection of Welded Repairs

Prior to weld repairs the base material was inspected using magnetic particle inspection and ultrasonic flaw detection. These inspection methods were also utilized after the completion of the first, second and third layers of welding and thereafter at every third layer.

After PWHT, the converter was allowed to cool and UT flaw detection on the repaired areas showed no defects.

Metallurgical Analysis

The main sample (forensic) with the crack taken from LWL-14 and the boat sample from CWL-6 con-taining the point reflectors identified from the TOFD in-spection were delivered to Houston for metallurgical analysis. The analyses performed included scanning electron microscopy, energy dispersive X-Ray spectros-copy, metallographic examination, Vickers hardness

testing and chemical analysis. The samples were sec-tioned for examination of locations of interest (Fig. 4).

Metallurgical Findings

Main Sample from LWL-14

The mid-length of the sample (Section D) showed a large crack that coincided with the fusion line across most of the wall thickness. Also, there were differences in appearance of weld passes originating on the inside and outside surfaces of the converter (Fig. 5). Weld passes originating from the inside surface were much wider and thicker with larger HAZ than those from the outside surface, indicating higher heat input during welding from the inside. The size of the weld passes from the outside surface was typical of lower heat input, narrow gap welding, while the weld passes from the in-side surface were part of an obvious repair weld.

Scanning electron microscopy on a small cracked sample from the inside surface of sample D revealed the fracture to be intergranular (Fig. 6). The results of the electron dispersive X-ray spectroscopy of the sample showed a low alloy steel containing manganese and mo-lybdenum, but no corrosion products.

Vickers hardness testing using a 5Kg load on the sample at section A and D revealed high hardness val-ues (299) in the HAZ. Also, chemical analysis of the sample indicated material of SA-302 Gr B.

Boat Sample at CWL-6

The sample was sectioned and inspected using wet fluorescent magnetic particle and no indications were found. After surface grinding and polishing, no linear or spherical indications were found at low magnifica-tion. However, at 400 magnification the weld showed numerous very small spherical inclusions, the largest measured 0.013mm in diameter (Fig. 7). Both the hardness values and the chemical composition were ac-ceptable. These inclusions are not detrimental to the performance and are well within the ASME code limits for Section VIII, Division 2 vessels.

Discussion

The cracks initiated at the inside surface in the HAZ on one side of a repair weld and continued across the wall thickness of the converter, generally following the fusion line. The cracking was intergranular and was


caused by hydrogen embrittlement. There was no evi-dence of high temperature hydrogen attack (HTHA).

The inspection of the SA-302 Gr. B Section of the converter shell was thorough and even indications as small as 0.013mm were identified. Areas with signifi-cant indications were repaired and the repair quality was not compromised. The repairs met the requirements of the NBIC and received an R-stamp.

Conclusions

The leak was the result of a poorly made weld repair in the longitudinal weld LWL-14 during fabrication. A crack occurred in the repaired area. Either the pre-heat was not maintained at the required temperature or the PWHT was improperly applied, or both. The result was that a crack developed due to hydrogen embrittlement and it propagated through the 100mm thick wall, adja-cent to the weld fusion line. The high hardness values in the HAZ�s are evidence of poor workmanship in the original repair weld.

The inspections conducted were thorough and re-pairs well executed and met the requirements of NBIC.

References

1. Prescott, G.R., �Process Equipment Problems Caused By Interaction With Hydrogen � An Over-view� AIChE Paper N0. 4F, 2000.

2. Heuser, A., �Repair of a Thick-Walled Ammonia Synthesis Converter Containing a Leak� AIChE , 1991.

3. Prescott, G.R., �Hydrogen Induced Cracks In 2 1/4 Cr-1Mo Welds� , MPC Second International Con-ference On Steels With Hydrogen In Petroleum In-dustry Pressure Vessel And Pipeline Service, 1994.

4. Prescott, G.R., �History and prediction of Hydrogen Attack of C-1/2 Mo Steel�, MPC Second Interna-tional Conference on Interaction of Steels With Hy-drogen In petroleum Industry pressure Vessel And Pipeline Service, 1994.

NORTH NORTHSOUTH EASTWEST

COURSE 1

COURSE 2

COURSE 3

COURSE 4

COURSE 5

COURSE 6

COURSE 7

MH2

MH 4

CWL 1

CWL 2

CWL 3

CWL 4

CWL 5

CWL 6

CWL 7

CWL 8

DEVELOPMENT OF 04 AMMONIA CONVERTER - 105D

WELDREPAIRS

INCLUSIONS41" 34"

77"

AREAS WHERE SLAGINCLUSIONS WERE LOCATED

SA

302

-B

LWL

14

BOAT SAMPLE

AREA WHERE CRACK WASLOCATED

AR

EA

S IN

SP

EC

TE

D

264"

1.77"

144"

0.59"

Previous Weld repair

Figure 1. Development of Converter Showing Location of Indications


Figure 2. Inspection Techniques

FUSION LINE

WELD PARENT

Photomicrograph r5 Nital Etch x50 Typical Microstructure of Parent and Weld Material with Fusion Line Visible.

(Course 7 - Vertical Weld South).

Figure 3. Typical Replica along Seam Weld


Figure 4. Large Sample from LWL-14 Cut for Analysis


Figure 5. Large Crack along Fusion Line and Weld Repair


Figure 6. Scanning Electron Microscopy of Cracked Sample Section D


Figure 7. Microstructure of Boat Sample at CWL-6

repair and failure analysis of a crack in an ammonia converter

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