Majority of modern design codes and regulations for pressurised equipment mandate that pressurised equipment are equipped with depressurising facilities so that in the event of an over-pressurising scenario or during emergency shut-downs the equipment can be safely depressurised.

In the process industry, depressurisation calculations are usually done in accordance with the requirements of API 521 standard. For equipment with a wall thickness greater than 25mm, this standard recommends that depressurising facilities are designed to reduce the pressure of a vapour containing system exposed to external pool fire from the initial internal pressure to the final safe pressure within 15 minutes. There are cases where the depressurising time is even further shortened from 15 minutes by design engineers, e.g. for LPG applications or jet fire scenario.

Once depressurisation facilities are sized for the fire-case, depressurising calculations are carried out in order to determine the minimum metal temperatures at coincident pressures reached in the equipment in a non-fire depressurising scenario (called cold-case). This will enable design engineers to analyse equipment for potential brittle fracture of equipment during cold-case depressurisation.

Whilst the above mentioned methodology is usually adequate for majority of applications, there may be occasions that achieving API 521 recommended fire-case depressurisation time would require a large depressurising valve. This can potentially cause:

• Significantly fast depressurising (and subsequent auto-refrigeration) in the cold-case leading to very low metal temperatures and the need for costly materials, particularly in cold climate environments;

• Damage to equipment internals due to high depressurising rate;

• Overloading the existing flare network in a brown field project.

Increasing the depressurising time can alleviate the operational and/or economic issues arising from rapid depressurisations. However, slower-than-usual depressurisation increases the risk of rupture during fire-case, as equipment will be subject to heat for a longer period of time whilst still pressurised.

This paper describes a methodology and identifies the necessary steps for assessment of pressurised equipment for slow depressurisations. The method is based on the provisions in the latest editions of API 521, API 579-1/ASME FFS-1 and Finite Element Analysis (FEA). A sample high pressure vessel is analysed in this paper for both cold and fire depressurisation.

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