Wet hydrogen sulphide (H2S) service environments create a specific and severe cracking risk in carbon and low-alloy steel pressure equipment. The combination of H2S, water, and susceptible steel produces atomic hydrogen that diffuses into the steel lattice — initiating sulphide stress cracking (SSC), stepwise cracking (SWC), and stress-oriented HIC (SOHIC).
Damage Mechanisms and PAUT
Detection
Sulphide Stress Cracking (SSC)
SSC initiates at the steel surface in
regions of high tensile stress — typically at welds, HAZ, or stress
concentration locations — and propagates perpendicular to the applied stress.
SSC cracks are tight, planar, and often surface-breaking or near-surface. PAUT
shear wave inspection from the pipe or vessel surface, with appropriate focal
laws for the near-surface zone, is the primary detection technique. ACFM
provides complementary surface-breaking crack detection capability.
Stepwise Cracking (SWC) and SOHIC
SWC and SOHIC involve hydrogen blisters and
planar cracks that connect through the wall in a stepwise pattern perpendicular
to the principal stress. The critical detection challenge is the vertical
(through-wall) connectivity of the stepwise crack path — which may not be
detectable with the standard horizontal beam angles used for lamination
scanning. PAUT with steep-angle focal laws and TOFD supplement are required to
characterise SOHIC-type cracking through the wall.
Calibration to Wet H2S Crack Morphology
Wet H2S PAUT inspection procedures are calibrated using reference blocks
containing electrolytically induced hydrogen damage and machined notches that
simulate the crack orientations and aspect ratios of SSC, SWC, and SOHIC. This
ensures that the detection sensitivity established in calibration is relevant
to the actual damage population that the inspection is designed to find.
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Refinery sour service pressure vessel inspection —
reactors, separators, absorbers
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Amine unit vessel and piping wet H2S inspection
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Sour gas processing pressure equipment inspection
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Hydroprocessing
reactor and heat exchanger inspection in H2S-containing service
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Pipeline and piping in sour service — internal H2S
cracking assessment
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Fitness-for-service assessment input for sour service
equipment with identified cracking
Reports include defect location referenced to vessel geometry,
through-wall position, crack depth and extent, damage mechanism classification
(SSC / SWC / SOHIC), and fitness-for-service disposition against API 579, ASME
FFS-1, or NACE MR0103 damage assessment criteria. Reporting is structured to
support the asset integrity management and regulatory inspection requirements
of the sour service industries.