Methodology to Determine Relief Load for Trans-Critical Pool Fire

Adequate pressure relief design is of high priority in a variety of industries in order to maintain compliance with governmental regulations including OSHA 1910.119 and Pressure Equipment Directive 97/23/EC.  Accurate relief sizing reduces the potential risk of damages to personnel, equipment, and production schedule. One of the most common overpressure scenarios for relief sizing is external pool fire. Among the most challenging types of fire analysis is a trans-critical pool fire, where the process fluid is subcritical at the onset of relief then progresses to become fully supercritical during the fire.  Often, process simulation is required to get accurate fluid properties at different times during the fire, then a relief load is estimated using the ‘alternative method’ outlined in the 6^th edition of API Std 521, Section 4.4.13.2.4.4, which examines the change in specific volume over time. This paper will examine, through process simulation, the effect of using the API alternative, wetted, and unwetted methods to determine the required relief load for trans-critical fires.