(5b) Mitigating Low-Temperature Metal Embrittlement during Overpressure Scenario

High pressure natural gas flashed to atmospheric pressure result in low temperatures due to Joules Thomson effect. In LNG or gas plants there are systems and piping which are exposed to low temperatures during relief scenario.

Tube rupture is a commonly considered relief scenario for shell and tube exchangers. In closed loop refrigeration systems, a high pressure natural gas typically flows through the tube side and a low pressure refrigerant flows through the shell side. During a tube rupture scenario, the natural gas mixes with refrigerant and is relieved through a relief valve located on the shell side of the exchanger to prevent catastrophic equipment failure due to overpressure. However, the exchanger and piping must also be protected against low temperature embrittlement which can potentially be caused by natural gas as it gets cold while expanding from high pressure to low pressure.

Based on normal operating conditions the initial material selection was done based on the typical standard metallurgy used in the industry for this exchanger and piping. However, a steady state analysis of the tube rupture scenario revealed that the metallurgy may not be able to handle the colder temperatures generated during this scenario. This prompted several discussions relating to the impact of this finding such as:

• Are steady state results overly conservative?
• How much of the metallurgy will get impacted?
• How much is the time to consequence for this scenario?

In this presentation we will show how advanced tools such as computational fluid dynamics (CFD) and dynamic simulation were used to analyze the scenario and provide mitigations. The presentation will illustrate the temperature profiles, the critical areas, and where and how mitigation was implemented.