Comprehensive reliability analysis of blistered ‘LPG’ wash water vessel in FCC unit – Part II Engineering Critical Assessment of blistered vessel

•One wash water vessel suffered severe hydrogen blistering after 2 years of operation.•ECA of the blistered vessel was required to continue operation till new vessel is installed and commissioned.•The in-service assessment – a challenging task due to lack of reliable NDT data and service induced degraded material data.•ECA was done using in-house RLA software and vessel was allowed to run under derated condition.•Post replacement; reliability study by API579 and RLA software confirmed in-service ECA was directionally correct.

The wash water vessel of the INDMAX unit – an improvised FCC unit based on patented technology from Indian Oil Corporation Ltd. had suffered from severe hydrogen blistering damage after 2 years of operation. A decision was taken to replace the vessel but considering the lead time of supply of new vessel, it was decided to quantify the extent of damage and to carry out Engineering Critical Assessment (ECA) using an in-house RLA software, before taking the vessel in service. The vessel was declared fit for further service under derated condition. There after 6 months of operation, the vessel was replaced by a new vessel and post replacement the failed vessel was subjected to detailed failure analysis and ECA.The findings of the failure analysis carried out on the failed pressure vessel post replacement have been reported as part I of the paper. This paper describes the detailed ECA analysis carried out during service as well as post replacement of the vessel. The study indicated that the employed methodology for ECA during field study was directionally correct. Further, it was recommended, that in case of blisters, the best way is to follow Level I of the API-579, followed by Level II. In case loss in thickness occurs over large blistered area, API 510 guidelines should be followed and ratability judged. However, under extremely severe condition of blistering, a rigorous analysis based on Level III may be applied provided reliable NDT data for defect sizing is possible.