The BLEVE Effect.

The Boiling Liquid Expanding Vapor Explosion (BLEVE) effect depends crucially on a phase change from liquid to vapor that might occur during a loss of containment. This process involves ordinary physics with which most people are already familiar.

If a sample of water is placed in an evacuated chamber then the water will boil even at room temperature. To be more precise, the vapor pressure describes the pressure that a vapor-filled void will have when in equilibrium with a liquid. An important subtlety is that the vapor pressure is a property associated with the liquid rather than the vapor itself. To appreciate this subtlety, consider an open container of water (at room temperature) that is placed inside a larger sealed chamber so that a void exists above the water. Now, imagine that the chamber is suddenly evacuated. Under these circumstances the room-temperature water will start to boil (ie., vaporize) and continue to do so until the pressure in the chamber matches the vapor pressure of the water. If the pressure in the chamber is maintained below the vapor pressure of the water (say, by connecting the vacuum pump directly to the chamber) then the water will continue to boil until no liquid remains. This does not require any increase in temperature.

This is what happens in a BLEVE of a high pressure vessel filled with water -- such a BLEVE is just a very fast episode of the process described in the above paragraph. The time scale of a catastrophic failure can be on the order of milliseconds. Such a rapid loss of containment generates a transient bubble-like outward propagating pressure disturbance (ie., a shock wave) inside of which the pressure drops below the water's vapor pressure. A small fraction of the water inside this bubble instantly vaporizes and very rapidly expands to approximately 1600 times its initial volume. This rapid expansion is the explosion referred to in “BLEVE” and it represents a violent conversion of internal energy into kinetic energy.

A video of several BLEVE tests conducted by NASA is available that readily shows the violence of such an event. Dr. A. M. Birk² is a Mechanical Engineering Professor at Queen's University who researches BLEVE events. After reviewing the tape, he estimates (based on thermodynamical arguments) that the flash fraction in the video represents perhaps 2% of the total volume of water with the remaining volume of water being blasted into mist rather than vaporized. Thus, Dr. Birk's estimation is that the video shows a violent explosion that qualitatively resembles a BLEVE but one with physics different than BLEVE.

The relevance of this section to the PTV is best understood by watching the video and appreciating the potential violence of a high pressure containment failure.