Open Fire Heat Absorption

We are trying to evaluate some low pressure rated equipment relief requirements, design pressure ~2.5 psig. The vessels and equipment are indoors in a well drained, sprinkler protected environment with a good fire fighting brigade. Due to the low pressure rating of the systems we are leaning toward API-2000 methodology. However when calculating heat absorption rates we are getting nearly double the vapor generation rates of API-521 using the 21,000 BTU/hr/SF heat flux equation.
Has anyone applied the allowable environmental F factor of 0.5 (good drainage/impounding)when using API-2000 methodology for similar scenario's?
Debating using API-521 methodology vs API-2000 with F=0.5 for acceptable heat absorption.
Any thoughts appreciated.