An Engineering Critical Assessment of a Service-Damaged Pressure Vessel

Petrochemical and energy utility industries require a methodology which will allow them and the governing regulatory authorities to make technical and financially sound decisions for the repair or replacement of pressure vessels which suffer damage during service. ECA (Engineering Critical Assessment) procedures, which are based on the principles of fracture mechanics, can be used to assess the risk of operation of a damaged vessel by predicting the amount of damage which can lead to premature failure, and thus determine the need for repair or replacement of the vessel. The total cost to repair or replace a damaged vessel can be high not only in terms of economics of repair but also that of loss of production and safety of workers and the public. Considerable savings can thus be realized if unscheduled shutdowns can be avoided. Valid ECA procedures would allow both the owners and insurance underwriters of pressure vessels to assess the integrity of the vessel, thereby avoiding costly and unnecessary repairs at the same time reducing the possibility of failure during service to an acceptable low level.

In Canada, a co-operative program between Metal Technology Laboratories of CANMET, a branch of Natural Resources Canada and a number of representative organizations from the petroleum, gas transmission and electrical utility industries, as listed in Table 1, was initiated to investigate the validity of ECA of a service damaged vessel. The vessel, a compressor knock-out drum, shown schematically in Fig 1, was selected as a typical example of pressure vessels being currently used in sour service in petrochemical plants. As input to the ECA, non-destructive inspection was performed in order to measure the length and depth of flaws in the pressure vessel, using magnetic particle inspection (MPI) and manual ultrasonic inspection. Two automated ultrasonic techniques were used in selected locations to provide additional data. Table 2 presents the depths of the major flaws. Based on the results of the initial MPI and manual ultrasonic inspection an area of the shell free of flaws was removed to provide test specimens for material property evaluation. A replacement patch was subsequently welded into the vessel. A hydrostatic burst test, monitored with acoustic emission, was carried out to confirm the assessment.