Minimum Pressurization Temperature and Pressure-Temperature Envelope of a 1-1/4Cr-1/2Mo Steel Heavy Wall Vessel Available to Purchase

An aged heavy wall pressure vessel for high temperature-high pressure service was to be reused in a new service. The vessel was built in the late sixties with 1-1/4Cr-1/2Mo alloy steel. The integrity of “old vintage” Cr-Mo steel vessels for high pressure service depends heavily on whether the material of construction has sufficient toughness and if they have remained crack free. Cracks or discontinuities may trigger brittle fracture. Because brittle fracture is more likely to occur when the vessel is cold, the planned hydrotest for future conditions as well as the normal start-up and shutdown procedures needed to be established to reduce the risk of brittle fracture by unstable crack propagation. A fracture mechanics and current engineering practice assessment was performed to determine the recommended hydrotest temperature and Minimum Pressurization Temperature (MPT) to avoid brittle fracture. A safe pressure-temperature envelope for future start-up and shutdown procedures was also developed. The evaluation was based on removal of actual test samples using LR Capstone’s proprietary device, the Scoop Sampler^SM. Results showed that methods based on chemistry, while conservative, can be overly restrictive.