A Study On Degree Of Conservatism Of Two Safety Assessment Methods For Piping And Vessels With Defects

Flaws (defects), such as local thinning due to corrosion, inevitably exist in piping and pressure vessels. These defects can be evaluated using the fitness for service (FFS) method provided in the GB/T 19624 standard. Recently, the GB/T 35013 standard (Fitness -for-service assessment of pressure equipments), which provides another FFS assessment method for local thinning on piping and vessels, has been published. The evaluation method for local thinning in the GB/T 35013 standard is generally similar to that in the API 579-1/ASME FFS-1 (2016), but is totally different from that in the GB/T 19624. In this paper, the differences of the local thinning assessment methods in the GB/T 35013 (or API 579-1/ASME FFS-1 (2016)) and the GB/T 19624 are summarized, and the degree of conservatism comparisons between these standards are presented. The maximum allowable operating pressure (P-r(max)) that takes into account the effect of defects for straight pipe sections and vessels under different defect sizes and safety factors in these two standards have been calculated and compared. Monte Carlo simulations have been carried out to estimate the failure probability (P-f) and reliability of the straight pipe sections and vessels with different safety factors using the GB/T 19624.