When considering strength parameters, the selection of a valve for a particular application is generally based on the ‘Class Rating’, i.e. the valve thickness is suitable for a given temperature and pressure for a given material. A Liquefied Natural Gas (LNG) station operator identified three Class 600, bolted-bonnet gate valves, operating at cryogenic temperatures, as having pressure relief set-points approximately 7 barg below the 99 barg operating pressure of the process lines on which they were located. This lower set-point impeded the productivity of the lines and also presented a potential hazard from the vented gas. Therefore, to avoid venting, it was requested by the asset owner to determine whether the relief set-points on the gate valves could be safely increased to that of the process lines, without affecting the integrity of the valves. This paper presents how the stresses in the valve bodies were determined by creating a three-dimensional solid Finite Element (FE) model of the valves and adjacent pipework using PATRAN [5] with subsequent linear elastic analyses being undertaken using the general purpose FE code ABAQUS [6] for all loading scenarios. A detailed description of the subsequent fitness-for-purpose assessment to the requirements of PD5500 [10] for operating at the increased pressure is also presented considering the following failure modes; plastic collapse, incremental plastic collapse and fatigue. The results of the fitness-for-purpose assessment of the valves demonstrate that the valves will not fail by general plastic collapse, local plastic collapse or incremental plastic collapse at the increased pressure and that they are acceptable for the proposed fatigue duty. Based on the results of the work presented, and a separate functionality check by the asset owner, the set-points on the gate valves were subsequently increased to the desired level.

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