Mechanical Integrity Considerations in LNG Depressurization

In a typical LNG installation, a rapid depressurization can cause cryogenic temperatures in both upstream and downstream connected process equipment and piping. This phenomenon, sometimes referred to as auto-refrigeration, can compromise the equipment’s mechanical integrity and pose a risk of material embrittlement. As vessel metal walls are exposed to temperatures below the minimum design metal temperature (MDMT), permanent damage is possible. The potential for brittle failure is even more pronounced for a non-fire scenario. The level of severity depends on the initial pressure, initial temperature, content inventory, depressurizing rate, fluid composition, surrounding conditions, and heat transfer mechanisms.

Emergency depressurizing valves must therefore be sized to ensure a reasonable compromise between the impact of pressure and temperature. This paper examines the effects of different liquid levels, depressurizing valve sizes, vessel wall thicknesses, thermal insulations, and fluid compositions. The primary objective is to identify and illustrate the key factors that influence the mechanical integrity of a typical LNG installation, particularly at the mid to lower end of methane fluid compositions, and their impacts on carbon steel.

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