Brittle Fracture Assessments of Offshore PSVs

Abstract

Pressure safety relief valve needs to be designed and operated to assure metal temperatures are not lower than the Minimum Allowable Temperature (MAT) to prevent brittle fractures. Design and fitness for service codes include general procedures to prevent brittle fractures. Design procedures in the codes are very conservative whereas fitness for service codes in some cases lack details. In the absence of a detailed brittle fracture assessment procedure for valves subject to significant low temperatures as a result of either Joule-Thompson effect or auto-refrigeration, an approach involving pressure based stress ratio method of ASME/API 579, Part 3 has been adopted and implemented. The initial and very conservative approach involved a worst case combination of the upstream pressure while calculating the stress ratio and a comparison of the newly established MAT with the downstream temperature. This conservative approach resulted in the disqualification of numerous PSVs studied in this work. Valve replacement and associated lost production time leads to high costs. A sophisticated and appropriately conservative brittle fracture assessment approach involving the use of computational fluid dynamics (CFD) followed by finite element method analysis (FEA) based stress analysis was adopted based on the concepts defined in ASME/API 579 and is presented in this paper. Predictive CFD analysis establishes more realistic temperatures and pressures in relation to the actual operating conditions. The CFD predicted pressure and temperature field is used to determine the stresses in the valve body using FEA methods. The stress analysis is followed by an intermediate brittle fracture assessment based on the procedures described in API 579 Part 3 using the actual PSV body metal temperatures and stresses established using the stress analysis. A discussion on the allowable stresses and stress components to be used in this intermediate assessment is also presented. If the PSVs cannot be qualified with this intermediate brittle fracture assessment, fracture mechanics evaluations are carried out to establish the limiting flaw sizes for the valves. In addition, the flaw tolerances of the valves are also established using reference flaw approach described in API 579, Part 9.