A Re-Examination of Propane Tank Tub Rockets Including Field Trial Results

Abstract

When a tank containing a pressure-liquefied gas fails, one mode of failure involves the tank being propelled large distances by the released two-phase material. This mode of failure is called a tub rocket and it can pose a severe hazard to the public because of its unpredictability. Field tests were recently conducted to study the effect of explosive devices on propane tanks. The tests included tanks of various sizes up to 2000 L (500 gal).In most cases, the tests resulted in punctured tanks with transient two-phase jet releases. In some cases, the jet releases were sufficient to propel the tanks over considerable distances. In a small number of tests involving 470-L tanks, the explosive device resulted in the clean removal of a tank end, and this resulted in near-ideal launches of tub rockets. In one case, the rocket was launched vertically, and in another, the rocket was launched near 45 deg elevation angle giving a tub range of 370 m. In other cases, the explosive devices resulted in punctures, and in some of these, the resulting two-phase jet propelled the tanks over considerable distance. These examples gave a good opportunity to re-examine tub rocket models for tanks containing liquefied gases. This paper describes a theoretical model involving two-phase critical flow propulsion of cylindrical tanks. Three different critical flow models are compared, including the homogeneous equilibrium model (HEM), the homogeneous frozen model (HFM), and the Henry-Fauske model (HFK). Range predictions are compared with existing data and a model previously developed. Model predictions are calibrated to the field trial results described in the foregoing and then used to predict realistic ranges for various sizes of storage and transport tanks.