Dear All,
Having been discussing the emergency fire depressuring criteria explained in API 521, we came across that for jet fire causing potetnial vessel failure and leakage, the recommendation in API 521 section 4.6.6 and section A. 3.5.4.2 is to depressurize to 7 barg while taking into account the impact of system's design pressure (and hence thickness) when dictating the required depressurization duration (Yet min 15 minutes). Jet fire heat input would definitely be calculated by API 521 analytical method equation A.1.
The above would be comparatively conservative than the pool fire emergency depressuring criteria mentioned for 1 inch thick plates of reducing the pressure to 50% of the design pressure in 15 minutes (the time to have the plate temperature reaching the limit of 593 C).
I was discussing with some consultant regarding the jet fire requirement of depressurization, they recommended not to enforce the criteria of 7 barg in minimum 15 minutes as this would lead to large flare system capacity whilst jet fire may be remotely applicable in some cases depending on the safety emergency response plan of the plant.
However, in my opinion, design shall cover jet fire scenarios and accordingly the appropriate criteria shall apply.
I need your views on the criteria for jet fire fire depressuring and whether specific deviations would be accepted?