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Gas Fires

A gas fire is usually preceded by an explosion. The explosion may be so minor that it does not leave evidence that an explosion occurred. Any gas that will burn in the normal concentrations of oxygen in air is a flammable gas. As with other gases or vapours flammable gas will burn only when its concentration in air is within its combustible range and the mixture is heated to its ignition temperature.

The hazards presented by a gas that is confined in its container are different from those presented when the gas escapes from its container. We shall discuss them separately, even though these hazards may be present simultaneously, in a single incident.

When a confined gas is heated, its pressure increases. If enough heat is applied, the pressure can increase sufficiently to cause a gas leak or a container failure. In addition, contact with flames can reduce the strength of the container material, possibly resulting in container failure. To prevent explosions of compressed gases, pressure relief valves are installed in tanks and cylinders.

Another cause of explosion is a very rapid buildup of pressure in a container. The pressure cannot be released through the safety valve. Tanks and cylinders are also subject to explosion when flame impinges on their surface causing the metal to lose its strength. Flame impingement above the liquid level is more dangerous than that on the container surface area that is in contact with the liquid. Heat from flames above the liquid is absorbed by the metal itself; below the liquid line most of the heat is absorbed by the liquid. This is not a safe condition because absorption of heat by the liquid also causes a dangerous, although less rapid, pressure increase. Spraying the surface of the container with water can help keep the pressure from explosive force. Cooling with water is not a guarantee that an explosion can be averted, especially when flame impingement is occurring.

Compressed or liquefied gas represents a great deal of energy kept in check¹ by its container. When the container fails, this energy is released - often very rapidly and violently. This type of failure is called Boiling Liquid Expanding Vapour explosion, or BLEVE². Most BLEVEs commonly occur when LP gas³ container is exposed to excessive heat. The heat flux must be greater than the capacity of the relief valve. As a result, pressure within the tank rises intill the metal fails. This sudden release of internal pressure of the vaporized gas has a reaction similar to that of a rocket. The container ends may be propelled for considerable distances as the pressure is released.

Ignition of the vapour is immediate. Liquefied petroleum gas may fall to the ground as a cold spray, due to the refrigeration effect of the expanding gas.

The effect of a BLEVE on tank cars, which carry up to 30,000 gallons⁴ of propane, are spectacular since the long end of the tank car resembles a rocket in size, shape and weight.

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