Minimal criteria for Rapid Phase Transition explosion of cryogenic gases

When a criogenic gas as Liquified Natural Gas (LNG) is spilled on water or ground at a very fast rate, the phenomenon of Rapid Phase Transition (RPT) may occur: Large amount of energy are released during phase transition and explosion can occur. The related consequences should be added to the consequences of fireball when liquid is flammable. However, blast wave can be produced by simple compression of air due to volume increase along phase transition, following the classical piston model. These concepts are often analysed, erroneously, superimposing RPT phenomenon to the classical Boiling Liquid Expanding Vapour Explosion (BLEVE). Indeed, in this case, superheat temperature, shell failure mechanisms and fragment formation are the main points to be analysed whereas open release of LNG only performs physical explosion without fragments and superheat temperature cannot be reached as the very low temperature of natural gas when evaporating. Understanding the mechanism of rapid phase transition explosion by varying the LNG composition has been recently proposed by Conrado and Vesovic, on Chemical Engineering Science. However, production of blast waves needs a threshold value for pressure peak to be hazardous to people, building, equipment. Preliminary analysis aimed at identifying the minimal thermodynamic condition (i.e. temperature, amount) for the explosion to be dangerous.