Engineering Critical Assessment of a Service-Damaged Pressure Vessel Under Sour Service Conditions

The use of engineering critical assessment (ECA) of “fitness for service”, for the evaluation of flaws and local wall thinning in pressure vessels is receiving a lot of attention in the petrochemical and utility industries. A rigorous methodology based on fracture mechanics and accurate flaw sizing can be used to assess the risk of operation of a damaged vessel by predicting the amount of damage that can lead to premature failure.

In Canada, a cooperative program between Materials Technology Laboratory, a division of Natural Resources Canada and several representative organizations from the petroleum, gas transmission and electrical utility industries has been conducted. Its aim was to promote the acceptance of an ECA methodology for service damaged pressure vessels by the various codes and regulatory bodies.

Phase 1, featured the hydrostatic burst test of a damaged vessel under sweet conditions (1-2) and has been reported in a previous issue of this journal (3). The results of the hydrostatic burst test carried out in Phase 1 showed that the test vessel failed at about 5 times the design pressure in spite of the presence of extensive imperfections. The fracture mechanics analyses, both simple and complex, gave a conservative prediction of burst pressure.

The second phase has featured the hydrostatic burst of a damaged pressure vessel under wet sour gas conditions. The vessel, a sour glycol separator, shown schematically in Figure 1, was fabricated in 1973 according to ASME codes. It was 5.72 m (18'-9") in length and 1.52 m (5'-0") inside diameter, with a measured wall thickness of 33.5 mm (1.32"). The vessel was post weld heat treated after fabrication in accordance with the ASME code.