Fitness for Service Assessment of Carbon Steel Vessel With Localized Deformation During PWHT

Post weld heat treatment (PWHT) is applied to welded pressure vessels and piping to relieve residual stress built up during welding. During the PWHT, pressure vessel made of carbon steel is heated up to a minimum temperature of 595 °C for a holding period as required based on the thickness (Refer ASME Section VIII, Division 1, UCS 56) [1]. However, for equipment susceptible to carbonate stress corrosion cracking, PWHT is required irrespective of thickness at a temperature range of 649 °C to 663 °C as per WRC 452 [2]. A process column built in 1983 and operating in carbonate stress corrosion cracking environment was observed to have widespread corrosion under insulation at reboiler supports. Material of construction for the column was SM41B (Japanese Industry Standards Carbon Steel). Four Insert plates of various sizes were welded at same circumferential band during turnaround. The insert plate repair sizes were relatively large. Local PWHT was required to be performed with the equipment in vertical condition. Due to risk of uneven temperature distribution and resultant local thermal stresses, spot PWHT or Bulls-eye method of PWHT was not considered. Full circumferential local PWHT was considered for this equipment. During the actual PWHT process, localized overheating occurred, and some areas of deformation were observed in the equipment. A multi discipline review was performed to understand the root cause of the localized overheating and subsequent deformation. This paper describes the methodology and results of the fitness-for-service (FFS) assessments that were performed based on API 579-1/ ASME FFS-1, 2016 [3] to assess the integrity of the column. Based on the assessment performed, the equipment was found to be fit for service and continued safe operations.