There is a significant variation in the blowdown behavior of the pressure relief valves (PRV) commonly supplied on certain commercial propane tanks. Determining how different pressure relief valve characteristics affect the survivability of a propane tank in a fire and the associated hazards, was the goal of these tests.

To determine the effect of PRV blowdown, six 1890 L (500 gallon) propane tanks were exposed to an array of torch fires and thermally ruptured. A computer controlled valve was used to simulate the desired characteristics of a commercial PRV. In all cases the simulated PRV was set to open at 1.9 MPag, and blowdown was varied from 5% to 45%.

The impact of the blowdown was determined by observing the nature of the failure, the time to failure, and the energy stored in the liquid and vapor phases at failure. Blast, radiation, and projectile hazards were also observed and recorded in order to determine the impact blowdown has on these pressure vessel failure hazards.

An important finding of this testing is that the blowdown setting does impact the tank's survivability. It was consistently observed that large blowdown resulted in delayed failure. As a result, the PRV was open for a longer time, with a resulting reduction in hazards because of the lower fill at failure. This is believed to happen because of the lower average stress state in the tank with increased blowdown.