Boiling Liquid Expanding Vapor Explosion - Mechanism

Mechanism

There are three characteristics of liquids which are relevant to the discussion of a BLEVE:

1. If a liquid in a sealed container is boiled, the pressure inside the container increases. As the liquid changes to a gas it expands - this expansion in a vented container would cause the gas and liquid to take up more space. In a sealed container the gas and liquid are not able to take up more space and so the pressure rises. Pressurized vessels containing liquids can reach an equilibrium where the liquid stops boiling and the pressure stops rising. This occurs when no more heat is being added to the system (either because it has reached ambient temperature or has had a heat source removed).
2. The boiling temperature of a liquid is dependent on pressure - high pressures will yield high boiling temperatures, and low pressures will yield low boiling temperatures. A common simple experiment is to place a cup of water in a vacuum chamber, and then reduce the pressure in the chamber until the water boils. By reducing the pressure the water will boil even at room temperature. This works both ways - if you increase the pressure beyond normal atmospheric pressures you could suppress the boiling of hot water far beyond normal temperatures.
3. When a liquid boils it turns into a gas. The resulting gas takes up far more space than the liquid did.

A BLEVE typically starts with a container of liquid which is held above its normal, atmospheric-pressure boiling temperature. Many substances normally stored as liquids, such as CO₂, oxygen, and other similar industrial gases have boiling temperatures, at atmospheric pressure, far below room temperature. In the case of water, a BLEVE could occur if a pressurized chamber of water is heated far beyond the standard 100°C (212°F). That container, because the boiling water pressurizes it, is capable of holding liquid water at very high temperatures.

If the pressurized vessel, containing liquid at high temperature (which may be room temperature, depending on the substance) ruptures, the pressure which prevents the liquid from boiling is lost. If the rupture is catastrophic, where the vessel is immediately incapable of holding any pressure at all, then there suddenly exists a large mass of liquid which is at very high temperature and very low pressure. This causes the entire volume of liquid to instantaneously boil, which in turn causes an extremely rapid expansion. That expansion is so rapid that it can be classified as an explosion; fully capable of inflicting severe damage on its surroundings.

Imagine, for example, a tank of pressurized liquid water held at 204.4°C (400°F). This vessel would normally be pressurized to 250PSI above atmospheric ("gauge" pressure). Were the tank containing the water to split open, there would momentarily exist a volume of liquid water which is

• at atmospheric pressure, and
• Four hundred degrees.

At atmospheric pressure the boiling point of water is 100°C (212°F) - liquid water at atmospheric pressure cannot exist at temperatures higher than 100°C (212°F). It is obvious, then, that 204.4°C (400°F) liquid water at atmospheric pressure must immediately flash to gas causing an explosion.

It is important to note that a BLEVE need not be a chemical explosion - there does not need to be a fire - however if a flammable substance is subject to a BLEVE it may also be subject to intense heating, either from an external source of heat which may have cause the vessel to rupture in the first place or from an internal source of localized heating such as skin friction. This heating can cause the substance to ignite, adding a secondary explosion caused by the primary BLEVE. While any BLEVE can be extremely devastating, a flammable substance such as propane can add significantly to the danger.

While the term BLEVE is most often used to describe the results of a container of flammable liquid rupturing due to fire, a BLEVE can occur even with a non-flammable substance such as water, liquid nitrogen liquid helium or other refrigerants or cryogens, and therefore is not usually considered a type of chemical explosion.