Here is a quick video of how a PRV works on propane tanks when propane inside of a tank expands and the internal pressure rises. This is video from last nights structure fire. Pressure Relief Valve (PRV) The most critical safety device on a 420-pound propane tank is the pressure relief valve. Its main function is to prevent excessive internal pressure from causing the tank to rupture or explode. During a fire, heat causes the propane inside the tank to expand and the internal pressure to rise. W