Depressurization of AGR unit, a critical and complex study for Projects

During the design of LNG plants, gas treatment, NGL recovery units and Offshore facilities, depressurization study is part of fundamental calculations for the material selection of equipment and flare network design ensuring meeting all the safety design criteria. Until the 5th release of API 521, an empirical approach was clearly defined and commonly used for all types of projects and units. Only a few exceptions on the duration of depressurization were applied on specific cases such as LPG systems or units with large inventory such as slug-catcher finger types.

However, in 2014 with the publication of the 6th version of API 521, the codes and standards knew a major update dealing with a new analytical approach based on Stefan-Boltzmann law and considering different types of fires and a stress-based calculations and performance criterion. The new methodology has been developed based on fire test data from laboratory API recommendations to comply with these requirements.

This paper is built on experiences and feedback attained through recent LNG projects in different world regions and for different facilities. It aims at highlighting the issues of the new types of fire and evaluating their impact on depressurization calculations. The use case of an Acid gas removal unit blowdown with T.EN in-house software LNGDYN® will illustrate the challenges of complex systems (multiple equipment, internals, various operating conditions...) with huge inventory for the flare design.