Propane/LPG Sidebar
ROBERT BURKE
Published: July 1996
Also See: Weyauwega Wisconsin Propane Fire
Propane and LPG are liquefied compressed gases. The only thing keeping the compressed gases a liquid, is the fact that they are under pressure. If the pressure is released from the container of a liquefied gas, such as when a Boiling Liquid Expanding Vapor Explosion (BLEVE) occurs, all of the liquid turns into a gas instantly. This occurs because the liquid in the tank is already above its boiling point. The only thing keeping it a liquid is the pressure. Propane and LPG (Liquefied Petroleum Gas) are two very common flammable liquefied compressed gases. LPG is a mixture of butane, isobutane, propane, propylene, and butylenes. Propane has a flammable range of 2.4% to 9.5% and LPG 1.9% to 9.5% in air. Propane and LPG have boiling points of -44° F and -40° F respectively. LPG has a flash point of -100° F. Both materials are above their boiling points under ambient temperature conditions in many parts of the country year round. This makes the materials very dangerous when a leak or fire occurs, especially if there is flame impingement on the container. The vapor density of propane is 1.56 and LPG is 1.5. Both propane and butane vapors are heavier than air. The vapors will tend to hug the ground and seek low places like basements and confined spaces.
On the rail propane and LPG are shipped in insulated pressure rail tank cars. The rail cars are insulated to keep flame impingement from the tank surface (Figure 1.1). Because the materials are already above their boiling points, flame impingement, radiant heat transfer, or increases in ambient temperature, can cause the materials to boil faster. Faster boiling causes an increase in pressure within the container. Even though the containers are specially designed to withstand pressure and have relief valves provided to release excess pressure, there are limits to the pressure they can tolerate. If the pressure build-up in the container exceeds the ability of the tank to hold the pressure or the relief valve to relieve the pressure, the container will fail. Flame impingement on the vapor space of the tank will cause the metal to weaken and fail causing a BLEVE.
When dealing with emergencies involving pressure containers and flammable gases, great caution should be taken. Flame impingement on the vapor space of a container is a "no win" situation. If a BLEVE is going to occur, it's just a matter of time. To try to fight a fire under those conditions is to play Russian Roulette. The NFPA Fire Protection Guide says that BLEVE times range from 8 to 30 minutes with the average time being 15 minutes. There usually is no way of knowing how long the flame impingement has been going on prior to the fire department arrival. There is no way to know exactly when the BLEVE will occur. If the only threat to life is that of the emergency responders, then there is little reason to risk the lives of the responders needlessly. If the impingement is on the liquid space, the liquid will absorb the heat for a period of time. The liquid will boil faster as it absorbs the heat. There will be an increase in pressure within the tank as the liquid boils faster. This can still be a dangerous situation if not handled properly. Conditions involving the tank must be monitored constantly for changes, including liquid level, pressure increases, and signs of tank failure.
Precautions should be taken to ensure personnel safety when fighting fires involving flammable gases. Flammable gas fires should not be extinguished until the source of the gas has been shut off. It is much safer to have the gas on fire and know where it is then to have the gas leaking and going where it wants to. Flammable gases are more dangerous than flammable liquids. Most flammable gases do not have a flash point. Many flammable gases have wide flammable ranges. Flammable gases can be heavier or lighter than air.
Related:
About the Author - Robert Burke
Robert Burke is the Fire Marshal for the University of Maryland. He is a Certified Fire Protection Specialist (CFSP),
Fire Inspector II, Fire Instructor III, Fire Investigator, and Hazardous Materials Specialist, and has served on state
and county hazardous materials response teams. He is a veteran of twenty-four years in fire and emergency services,
with experience in career and volunteer departments. He has attained the rank of lieutenant, Assistant Chief, and
served as a Deputy State Fire Marshal. He is an adjunct instructor at the National Fire Academy and the Community
College of Baltimore, Catonsville Campus. He is the author of books titled "Hazardous Materials Chemistry For
Emergency Responders" and "Counter-Terrorism for Emergency Responders". Robert can be reached on the Internet at
robert.burke@worldnet.att.net
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