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FAILURE OF AUSTENITIC STAINLESS STEEL PIPING IN HYDROCARBON GAS
PROCESSING PLANT - A CASE STUDY
Saleh Ali Al Sulaiman
Sr. Insp. & Corr. Engineer
G. Santhosh Kumar
Inspection Engineer
Ripan Kapoor
Inspection Engineer
Inspection & Corrosion Team
Al-Tameer Building
Kuwait Oil Company
The paper discusses about the failure of Stainless Steel piping in hydrocarbon gas processing
plant at KOC. The detection methodology is discussed together with inspection, testing and
assessment program that was introduced to evaluate the piping. Deployment of advanced
inspection technique, Time of Flight Diffraction (ToFD), is also discussed.
In the subject case study, the Stainless Steel piping failure mode is typically that of Stress
Corrosion Cracking (SCC) due to chlorides. SCC is as such classified as a catastrophic form of
corrosion and unexpected failures of stainless steel piping occur with disastrous consequences.
Microstructures and other testing techniques for failure analysis are also discussed along with
photomicrographs of failed specimens observed under Scanning Electron Microscope (SEM).
The case study shows the importance of monitoring the vulnerable piping by NDT techniques to
ascertain the health of stainless steel piping in hydrocarbon gas service which may get affected
due to presence of corrodants as the gas stream composition changes over a period of time.
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INTRODUCTION
Kuwait Oil Company handles the oil resources of the country and produces approximately 2.0
million barrels per day of crude oil. Natural gas produced along with the oil is separated in the
separators installed in the Gathering Centers. The gas from the separators and the compressed
tank vapor is sent to the gas booster stations for further compression where they are scrubbed in
the inlet gas scrubbers to remove the liquid carry over. The compressed gas is dehydrated and
exported to the consumers.
GAS ANALYSIS
Over a period of time the oil wells have turned from sweet to sour. This leads to major internal
corrosion problems in the plant pressure vessels and piping. Moreover the water content in the
gas and oil also has a high amount of chlorides which affects the stainless steel piping
installations.
FAILURE AT GAS PLANT
On March 9, 2005 minor oozing of gas & oil occurred from the bottom portion of weld joint inlet
piping (42”dia, stainless steel 321) of a low pressure gas suction scrubber. The suspected
welding seam was cleaned and visually inspected for the presence of any surface cracks. Soap
Solution and Dye penetrant testing was also carried out. However presence of any surface
indication was not revealed. Ultrasonic scanning of the suspected location was carried out, with
special calibration devised to detect the presence of indications in the wall thickness of the
piping. During ultrasonic scanning several linear indications were observed having
characteristics similar to that of Stress Corrosion Cracking (SCC). Thereafter radiographic
examination was carried out which revealed the presence of several cracks similar to SCC. The
radiographic examination also revealed large amount of deposits inside the piping which can be
the probable source of corrosion mechanism.
Due to the above observation, it was recommended to immediately stop the operation of
concerned gas stream piping and replace with new piping. Moreover, all the other piping in the
plant and similar piping in another gas booster station was also extensively inspected to detect
for similar cracking present at any other location.
INSPECTION METHODOLOGY ADOPTED
Stress Corrosion Cracking (SCC) is typically characterized by branched & tight transgranular
cracks. It is also known that the SCC cracks do not follow a linear path. These features
associated with stress corrosion cracks make their detection all the more difficult by applying
conventional ultrasonic methods. A specialized calibration was devised to enable the detection of
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such tight cracks in preliminary stages. All the piping of plant was subjected to extensive
ultrasonic scanning by using this calibration, due to which a few more areas were identified
having similar cracks.
These new locations identified in ultrasonic scanning having stress corrosion cracking were also
subjected to radiographic examination which however did not reveal any indication.
Hence to confirm the presence of indications detected in ultrasonic scanning, it was decided to
carry out Time of Flight Diffraction (ToFD). A new calibration notch was devised and used for
the calibration of this technique. The ToFD scans confirmed the presence of indications at these
locations. A major finding of the ToFD scan was it revealed that one of the cracks was breaking
the outside surface also.
These piping loops were also immediately recommended for replacement in view of stress
corrosion cracking giving sudden catastrophic failures. Some locations in various piping where
indications of preliminary nature were observed were also marked for monitoring as per the
assessment priority decided based on indication severity.
INVESTIGATION
The affected piping (42” dia.) was dismantled and a spool piece was cut from the same where
major indications were observed. The internal surface of the piping was subjected to dye
penetrant testing which revealed several cracks having typical pattern of stress corrosion
cracking.
It could be observed that the corrosion damage had initiated at a number of discreet locations in
the 6 o’clock position by crevice corrosion mechanism which progressively developed into
several cracks. The cracks, for the majority of their length appear to penetrate through
approximately 80% of the wall thickness (the major crack had penetrated throughout the
thickness of piping leading to a leak). The cracking was not confined to any particular direction
since the crack orientation was in both longitudinal and transverse direction. The crack had
traveled across the weld joint in either directions. Large amount of branching was also visible.
The major influencing factors were observed to be; position in pipe where corrosion had
initiated, material of construction being stainless steel, chloride content in the gas stream, carry
over of large amount of sludge and liquid particles, condensation and accumulation of deposits at
the bottom most position.
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NON DESTRUCTIVE TESTING
It is pertinent to note that the cracks were not visible on the outside surface of the piping and the
plant was on stream at the time of inspection. In view these failures of stainless steel piping, it
was decided to conduct the non destructive inspection of the balance plant piping (on stream) to
ascertain their health. The ultrasonic flaw detection was carried out on a special notch block of
proprietary design, which could ascertain sensitivity good enough to detect fine size cracks
typical of stress corrosion cracking. The scanning was carried out primarily near the weld joints
in both circumferential and longitudinal directions. During scanning of around 432 joints of
various sizes, minor indications were observed in a few joints and some parent metal area also.
These indications were further analyzed by using different combinations of probes. As a
confirmatory measure, radiographic examination was carried out on the suspected portions. In
one joint a large crack was revealed, however several other cracks observed in ultrasonic
scanning were not visible. Other locations suspected to have cracking did not reveal any crack in
radiographic examination. It may also be noted that a large amount of deposit was also visible in
some of the joints during radiographic examination.
This piping loop was shutdown and replaced. The dismantled pipe was cut and a spool piece was
prepared. The deposits sticking on the inside surface of the pipe were carefully removed and sent
for chemical analysis.
The inside surface was cleaned and a dye penetrant test was carried out. The dye penetrant test
revealed several branched cracks typical of SCC.
METALLOGRAPHY
Subsequently a portion of the failed area in the cut piping spool was identified for microstructure
analysis. The macrostructure clearly reveals large branched cracks. Locations were marked for
the microstructure examination. One sample was prepared for microstructure examination of
inside surface of the pipe and other for the cross section of pipe. The samples were cut by
machining. Electrolytic polishing was carried out by 10% oxalic acid. The microstructure clearly
revealed presence of transgranular cracks with several branches. The microstructure of the cross-
section revealed that the major crack had propagated through 80% of the wall thickness.
It is observed that the cracks are initiating from the parent metal crossing the weld metal and
propagating on the other side into the parent metal. Large pits are also visible with several cracks
originating from the same. It can also be revealed that the severity of cracking is more in HAZ
than in weld metal or parent metal. Microphotographs also reveal the cracks to have a
transgranular pattern. It can also be seen that the weld is highly sensitized (probably during
welding) and an intergranular attack has also occurred with several carbide precipitates seen at
the grain boundaries. However intergranular corrosion is not a primary source of failure.
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Another sample from unaffected region was also examined. Where the cracking was not present,
the area did not have any presence of corrosion and the microstructure was typical that of SS 321
material.
FRACTOGRAPHY
Fractography of the samples was carried out using SEM-EDX. The fracture surfaces were having
corrosion products which were analyzed by EDX. EDX analysis showed the corrosion products
to be rich in oxygen and chlorine. Examination of the fracture surface revealed a rough surface
CHEMICAL ANALYSIS AND HARDNESS SURVEY
Vickers hardness test using 10kg load was carried out on the samples at the locations of weld,
HAZ and parent metal & all the readings were below the maximum limit of NACE
MR0175/ISO15156, which is 22HRC (equivalent to HV 248).
An optical emission spectrometer was used to carry out the chemical analysis of the weld and
parent metal area of the pipe sample. The chemical analysis confirmed the material composition
as SS 321 and weld to be of SS 347.
FAILURE ANALYSIS
From the various examinations, tests and analysis carried out it is evident that the primary failure
mechanism was chloride stress corrosion cracking. Crack branching was evident secondary to
the main fracture. Branch like step wise cracking was evident in dye penetant testing &
metallography, which is typical of stress corrosion cracking phenomena. The chloride
environment may have been due to either high chloride content of the gas streams on a
continuous basis or condensation of liquids on the lowest surface on the weld joint due to the
typical weld joint geometry, and concentration of chlorides over a period of time. Though the gas
plants are not expected to have high chloride contents, the recent gas analysis revealed the
chloride content in the gas streams in the region of around 500ppm, which is very high. The high
amount of stresses required for the mechanism were due to the residual stresses in the weld joint.
Moreover the operating temperatures must have remained above 60deg C which promoted the
failure phenomena.
In addition to the above factors it is also pointed out that the joint design of the weld seam was
one of the main contributors for this phenomena. After cutting the piping spool piece it was
observed that chamfering was done on both the plate edges to prepare the weld joint. The
chamfering on each side was around 10mm. This gave rise to a large groove wherein
accumulation of deposits and condensation was possible. Due to the accumulation of deposits
over a period of time, crevice corrosion and pitting must have started beneath them. This is
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evident from the photograph which shows corrosion of the metal surface on the bottom portion
of the weld joint and rest of the area remaining intact.
CONCLUSIONS
The most probable cause of failure was due to the combination of several factors like poor weld
joint configuration allowing deposits to get accumulated, H2S environment, carry over of liquid
and solid particles in the gas stream, condensation of liquids, high concentration of chlorides in
gas stream, probable ingress of oxygen during outages, residual stresses in the weld joint,
operating temperatures exceeding 60deg C etc.
Hence it is concluded that for the operations of piping for gas service having high chloride
contents, alternate material than SS 321 should be used. If SS 321 is used then the parameters
such as gas composition & temperature should be strictly monitored and maintained so as to
avoid any catastrophic failure on account of chloride stress corrosion cracking of austenitic
stainless steels.
Precise & timely detection of the cracking phenomena in the plant piping by applying special
inspection techniques resulted in averting a major piping failure and thereby saving precious
human life, plant, machinery and production.
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Figure 1: Photograph of piping which was detected with SCC.
Crack Location
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Figure 2: Dye Penetrant testing on the internal surface of the cracked piping.
Figure 3: Photograph of radiographic examination of the weld joint showing crack.
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BEND 1 BEND 2 BEND 3
BEND 4 BEND 5 BEND 6
BEND -7 BEND -8 BEND -9
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Figure 4: ToFD scans of Header piping showing cracks in the mid wall.
BEND -10 BEND -11 BEND -12
BEND -13 BEND -14 BEND -15
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Figure 5: Photomacrograph of the cracked piping.
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Figure 6: Photomicrographs showing SCC cracks.
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Figure 7: Photomicrograph of the cross section of weld joint showing crack
propagating from parent metal through the weld joint.
Figure 8: Photomicrographs showing crack propagation near the ID of the pipe
surface.
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Figure 9: Fractography
Scanning electron micrograph showing a detail
of fracture face prior to cleaning illustrating the
presence of corrosion products
Scanning electron micrograph showing detail
of fracture face after chemical cleaning. Note
the evidence for crack branching & a generally
transgranular fracture path
Scanning electron micrograph showing a detail
of fracture face after cleaning. Note the
evidence for secondary branched cracking
Scanning electron micrograph showing detail
of section taken from weld. Detail of crack tip.
Points indicate positions of EDX spectra.
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Figure 10: Fractography & EDX Spectra
Scanning electron micrograph showing detail of
section taken from weld. Detail of crack
branching and corrosion product within the crack.
Box indicates the position of detail shown in
figure below
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Figure 11: Chemical composition of the piping sample.
Figure 12: Hardness measurement of failed location.
Identification Scale Load (Kg) Range
Weld HV 10 191-224
HAZ HV 10 176-210
Parent Metal HV 10 198-209
Composition % Element
Base Metal Weld SA312 TP 321
C 0.032 0.034 0.08
Si 0.6 0.94 0.75
Mn 0.78 1.87 2
Cr 19.3 19.5 17.0-20.0
Mo 0.16 0.078
Ni 9.82 9.82 9.0-13.0
Al 0.022 <0.0002
Co 0.2 0.2
Cu 0.43 0.021
Nb <0.004 0.75
Ti 0.71 0.087 5 x C (min)
V 0.065 0.073
W 0.19 0.097
Pb 0.009 <0.002
Fe 68 66.3
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Figure 13: Chemical analysis of water samples in Gas Streams.
RESULTS TEST UNIT
C-101 C-103 C-203
PH 5.2 6.0 6.2
Conductivity µ mhos 2,167 317 147
Total Dissolved Solids ppm 1517 222 103
Suspended Solids ppm 6,212 422 281
P Alkalinity CaCO3 ppm Nil Nil Nil
Total Alkalinity CaCO3 ppm 9 30 35
Caustic Alkalinity ppm Nil Nil Nil
Carbonate CO3 ppm Nil Nil Nil
Hydrogen Sulphide H2S ppm Nil Nil Nil
Sodium Na ppm 248 1.0 3.0
Sulphate SO4 ppm 320 42 18
Chloride Cl ppm 514 33 16
