A large external fire can cause multiple discharges to the disposal system due to vaporization of liquids within the equipment heated by the fire.
Determination of Credibility
As the source for an external fire of significant extent is generally a flammable liquid, processing units that contain only gaseous materials or only non-flammable liquids may not require consideration of the external fire contingency.
Determination of Impact on the Collection and Disposal System
Consideration should be given to the source and location of flammable substances, the provision for liquid drainage, and the effects of natural and man-made barriers. In the absence of special drainage or barrier considerations, API Recommended Practice 521 recommends assuming that a fire will cover a surface area of between 2,500 and 4,000 ft2, typically in a circular shape. All relief devices protecting equipment requiring external fire relief, and located within the assumed fire zone, are assumed to discharge simultaneously into the disposal system.