Hydrogen blistering in rich amine flash drum of VGO HDS unit Case study illustrates how inlet flow dynamics and material selection contributed to the failure of an unlined amine flash drum
Avinash Sankpal, Arun Kumar, Aman Kumar Sahoo, Prashant Nandanwar, and Abdul Quiyoom Kochi Refinery, BPCL
H ydrogen blistering and hydrogen-induced cracking (HIC) are critical degradation mechanisms that threaten the safe operation of refinery equipment handling wet H₂S-rich amine. These phenomena can lead to unexpected failures, costly shutdowns, and compromised process safety. In particular, carbon-steel vessels without internal cladding operating in sour gas systems are susceptible to these forms of damage. The vacuum gas oil hydrodesulphurisation (VGO HDS) unit is a mild hydrocracker that operates at a cold separator pressure of 105 kg/ cm²g. The unit can handle VGO feed sulphur up to 3 wt%. This article presents a case study of hydrogen blistering in the rich amine flash drum (V-38) of a VGO HDS unit. The drum collects H₂S-rich methyldiethanolamine (MDEA) from upstream equipment and transfers it to the amine recovery unit (ARU). During a turnaround in 2024, more than 300 hydrogen blisters were discovered on its upper shell, prompting an urgent assessment of structural integrity and the root causes of degradation. The objectives of this study are to: • Investigate the root cause of hydrogen blister formation and cracking. • Understand how mechanical design flaws, turbulent flow, and wet H₂S exposure accelerated HIC. • Evaluate the vessel’s fitness for service using API 579 standards. • Analyse computational fluid dynamics (CFD) simulations to correlate flow behaviour with damage locations. • Propose effective short- and long-term mitigation strategies to prevent recurrence.
Stripper section
Hydrogen
Recycle gas knock out drum
Rich amine ash drum (V-38)
Recycle gas scrubber rich amine
Cold ash drum amine absorber rich amine
Stripper section o gas amine absorber rich amine
Sulphur recovery unit
Figure 1 V-38 process flow chart
Background and literature review Hydrogen blistering and HIC result from the diffusion of atomic hydrogen into carbon steel under wet H₂S conditions. The hydrogen diffuses and accumulates at internal voids or inclusions, forming molecular hydrogen that causes localised internal pressure. This leads to blistering and potentially interconnecting cracks. Refinery amine systems, particularly those using carbon steel for equipment exposed to sour gas, are prone to such damage. The NACE MR0175 standard prescribes material limitations in H₂S environments, yet failures still occur due to improper design, turbulent flow near nozzles, or lack of protective cladding. CFD is increasingly used in modern diagnostics to simulate flow patterns and predict corrosion-prone regions. Similar failures in amine flash drums and absorbers have been
Refining India
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