The Effect of Pressure Relief Valve Blowdown and Fire Conditions on the Thermo-Hydraulics Within a Pressure Vessel

If a pressure vessel containing a pressure liquefied gas (PLG) is exposed to a severe fire, there is a chance the tank will rupture and possibility even explode. It has been shown that the energy stored in the liquid phase is important in the outcome of such an accident. In order to better understand tank failure and energy storage, it is desired to understand the complex thermo-hydraulic response of the lading prior to a thermally induced rupture. In the summers of 2000 and 2001, a series of controlled fire tests were conducted on horizontal 1890 litre (500 US gallon) propane tanks. The test tanks were instrumented with pressure transducers, lading and wall thermocouples, and an instrumented flow nozzle in place of a pressure relief valve (PRV). A computer controlled PRV was used to control pressure, while high momentum, liquid propane utility torches were used to heat the tank. PRV blowdown and fire conditions were varied in this test series while all other input parameters were held constant. It was found that the lading response and energy storage varied according to the fire conditions and PRV operation. The location and quantity of the burners affected the thermal stratification within the liquid, and the swelling and frothing at the liquid/vapour interface. The blowdown of the PRV affected average tank pressure, average liquid temperature and time to destratify. PRV operation caused enhanced convective cooling on the vapour space wall. The degree of blowdown dictated the pressure drop, and thus the liquid flashing and subsequent swelling were affected. This paper will discuss these thermohydraulic responses and their role in the tank failure.