(175c) Mechanical Integrity Considerations in LNG Depressurization

Mechanical Integrity Considerations in LNG Depressurization

A Sensitivity Analysis

Jordi Dunjó, Hana Gilliam, and Daniel Nguyen^‡

ioMosaic Corporation

93 Stiles Road, Salem, New Hampshire 03079

                                                ^‡ Presenter (nguyen@iomosaic.com)

Abstract

In a typical LNG installation, a rapid depressurization can cause cryogenic temperatures in both the upstream and downstream of the connecting process equipment and piping. This phenomena is sometimes referred to as auto-refrigeration, compromising the equipment’s mechanical integrity and posing a risk for 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 compositions, 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, and thermal insulations. The primary objective is to identify and illustrate the key factors that influence the mechanical integrity of a typical LNG installation.

Key words: LNG, Depressurization Systems, Mechanical Integrity, Embrittlement, Vessel Wall Dynamics, Vessel Wall Segmentation Approach