(66a) Using Dynamic Analysis for Accurate Assessment of Pressure Relief and Blowdown System Performance

Blowdown/flare systems typically provide the last line of defense to prevent loss of primary containment or escalation when a loss of containment or fire has occurred.  Sizing of the flare system must be sufficient to ensure timely blowdown of all systems without overpressuring the flare headers or knockout drum.   Moreover, recent work by the API-521 committee indicates that blowdown times for thinner-walled vessels may need to be shortened from the original design.

Traditional flare modelling assumes that the header and knockout drum are at steady-state conditions.  This can be overly conservative, since it does not take credit for the finite time required to “pack” the flare system.  In new designs, this can lead to oversizing.  In existing systems that need to be retrofitted with larger blowdown valves to address current practices for blowdown time, this could exaggerate overpressure issues, leading to unnecessary modifications.

A more rigorous, fully dynamic flare model can be used to produce more efficient designs, or to demonstrate the sufficiency of existing systems.  This paper details how such a dynamic analysis, using fully coupled process and flare system models, has been applied to a number of oil & gas facilities in Alaska, for which steady-state analysis indicated excessive pressure.  It shows how the transient behavior of the flare system can be accurately predicted for blowdown events, avoiding unnecessary modifications.