I got this document when I attended a National Fire Academy HAZMAT course in 1993. I came across it when looking for some old HAZMAT material for a new course I am developing and translated and cleaned up some of the language. It does an EXCELLENT job explaining the phenomenon we know as BLEVE (Boiling Liquid Expanding Vapor Explosion)
Crescent City, Illinois, propane explosion occurred in June of 1972 when a freight train with numerous propane cars derailed in the town. The resulting explosion leveled approximately half the city. The water tower in the foreground was over 100 feet tall.
In the past, many firefighting personnel have died and been injured as a result of transportation accidents that led to BLEVES of liquefied flammable gases. Fire Departments are normally the first public agencies to be notified when such accidents occur and accept the responsibility for taking the actions necessary to minimize injury and loss of life and property.
The incidents described in this presentation involve liquefied flammable gases and railroad car containers. BLEVES have occurred in trucks and fixed tanks containing flammable liquids or gases, and the explosions or fire behavior has been similar.
Several key points need to be discussed, which should help you understand the mechanism of these incidents and their possible effects. A good understanding is necessary to make intelligent decisions to minimize the hazard to life and property.
BLEVES can occur in containers of liquids or liquefied gases when a portion of a container is subjected to localized heating. A mechanical BLEVE of containers of liquefied gases can also occur without fire temperature (HOT BLEVE), if an object strikes the metal or fails because of mechanical damage (COLD BLEVE).
BLEVES are usually extraordinarily violent and produce severe damaging effects. Remember, a BLEVE is a Boiling Liquid Expanding Vapor Explosion. This potential is always present when mobile or stationary closed containers, tanks, and drums are damaged or exposed to fire. The primary explosion is a physical process involving the fracturing of the container, the liquid-to-vapor transformation and expansion, the propulsion of pieces, and a shock wave. The chemical properties of the material in the container, for example, flammable or non-flammable, are not factors in the explosion. However, they are of obvious importance in the effects of the blast.
Fire-caused BLEVES often occur after short periods of fire exposure. The most critical time can be expanded during fire company response and initial layouts. Most fire officers will have little reliable information about how long a tank has been exposed to fire prior to the arrival of fire companies. When you arrive at a flammable liquid or a liquefied flammable gas incident, you must realize that a violent explosion might occur at any moment if hazardous conditions exist. There is no significant safe period. If the container shell is impinged by fire or has otherwise weakened, the risk of BLEVES will continue until all contents under pressure have been burned or removed. Therefore, the most crucial situation control-decisions must be made almost instantly.
Severe heating by direct flame impingement on the tank(s) above the liquid level is the most critical hazard. Exposure heating, other than direct flame contact, will not lead to a BLEVE unless the contents are firmly unstable or reactive. A combination of both forms of heating can occur. If the flame impingement is on the container shell below the liquid level, the liquid absorbs much of the heat. And even though this will increase interior pressure as the heated liquid vaporizes, the tank metal is unlikely to fail. The excess pressure should be relieved by the safety valve. So, when you first size up, watch for flame impingement on the tank metal. Try to see if it is above or below the liquid level, remembering that all containers have a vapor space.
Mechanical damage may also have occurred to weaken the tank. Over-pressure relief devices may be jammed or otherwise rendered ineffective. Look for this kind of damage. Remember, much depends on the amount of pressure inside the tank.
Leaking gas and vapors can ignite from heat sources that are not obvious. Gas or vapors which are heavier than air, and most are, will flow to the lowest level. You will need special gas detectors to identify the pattern of some gas concentrations. Leakage of gas and vapor, visible and invisible, with or without odor, can form an explosive mixture with air at unpredictable distances, depending on the terrain, the nature of the product involved, and the weather conditions. Although the percentage of gas and air mixture required for an explosion varies with the kind of gas, this is of little significance in outdoor situations, such as our discussion.
While a BLEVE can occur with water and does in boilers and water heaters, a BLEVE in which hazardous materials are involved is a much more severe problem. If a tank car or truck contains a liquefied flammable gas, such as propane, burning the material released by the explosion introduces a damaging effect that can then be the dominant effect of the entire BLEVE incident. If the material is highly toxic, for example, chlorine or anhydrous ammonia, the released material can have the dominating effect of the BLEVE.
LP gas is transported in liquid form in vehicles, railroad tank cars, and ships. Under normal atmospheric pressure, it is a gas, but it is liquefied for economy of movement and storage, reducing its volume. For example, propane, when changed from a gas to a liquid, has its volume reduced to 1/270th of its gaseous volume. To keep propane as a liquid, pressure must be maintained, requiring tanks of greater strength than those used for gasoline.
Above the liquid level, a vapor space is always left to allow for fluid expansion due to heating from air temperature and sunlight. This area of the tank shell is susceptible to flame impingement damage. You cannot determine the liquid level merely by observation. At the top of all tanks containing flammable liquids and gases are devices designed to limit internal pressure. These are spring-loaded relief valves on tank cars in the domes, which also house fittings used for product transfer operations.
On tank trucks, similar relief devices are also located on top, but loading and unloading are most often accomplished from below. State and federal agencies regulate the standards for tank design, marking, and placarding, as well as for the transportation of hazardous materials. Liquefied gases can be released and vaporized immediately in a collision or derailment. These vapors can spread over great distances. Because LP gas vapors are heavier than air, they tend to remain close to the ground. The vapors from flammable liquids are also heavier than air. A spark or other ignition source can ignite these vapors, causing a flashback to the leaking liquid.
The liquid inside the tank is then heated, boils, and expands, increasing the temperature within the tank. The relief valve may operate to limit the excess pressure. The tank can withstand this pressure only as long as the tank metal retains its design strength. If the relief valve continues to release pressurized vapor, the liquid level drops, exposing an increasing area of metal to overheating.
Propane in a container at any temperature above its normal boiling point at -44° F, contains heat stored within itself sufficient to vaporize a large portion of the liquid if the pressure is reduced to atmospheric. At 70° F, enough heat exists in the propane liquid to vaporize almost instantly about 1/3 of the liquid propane if the pressure is reduced to atmospheric pressure.
Vaporization would produce about 270 gallons of propane vapor from each liquid gallon. Can you visualize the resulting vapor cloud from 30,000 gallons?
Heat absorbed by the metal in contact with the liquid is transferred to the liquid. This is like the action of water boiling in an aluminum saucepan. That portion of the metal in contact with the liquid is unlikely to be in any danger of failure from overheating.
However, heat absorbed by the container metal in contact with vapor is retained by the metal, which will begin to approach its melting temperature or other point of failure. This is like what happens when water boils or vaporizes in an aluminum saucepan.
Flames contacting the tank above the liquid level create high temperatures within the metal to weaken it. When this happens, despite the operation of the relief device, the pressure within the tank can cause the metal to thin and eventually tear.
In most fire-caused BLEVES involving propane containers, for example, the failure originates in the metal of the vapor space. It is characterized by the metal stretching and thinning out and beginning to tear longitudinally.
The longitudinal tear continues and then starts to become circumferential.
The tank rips apart.
The pressure drops suddenly.
Large quantities of boiling liquid vaporize, expand, and ignite immediately.
Tank pieces become flying missiles.
Usually, a cylindrical tank piece closed at one end will rocket toward its longitudinal axis, but it may be deflected and change direction.
Sections of large tanks can rocket as far as 3500 feet from the original point with devastating velocity, trailing flame and even some unburned liquid. However, it is more common for them to rocket 1000 to 2000 feet.
Different tanks may be dispersed in alignment and rocket in different directions in a wreckage. On take-off or because of how a crack propagates, the tank may pivot and move in a direction from its original alignment.
Firefighters and bystanders have been killed or dismembered by missile effects. A ground flash occurs as the mass of burning, expanding vapor is partially confined and channeled by the ground. Personnel in this area are liable to receive massive burns. The ground flash can cover an area hundreds of feet in diameter.
The blast wave or over-pressure can cause glass to shatter and structures to weaken and collapse. Although probably not the greatest threat to firefighters, the effects are serious enough to cause injuries. The blast wave is strong enough to break windows several miles away.
The mass of burning expanding vapor, including that portion in the ground flash, forms a fireball as it rises. It produces radiant heat sufficient to ignite most combustibles and cause severe flash bums up to 1000 feet beyond the fireball. The fireball rises on a thermal column, radiating heat in all directions.
When faced with a situation in which a BLEVE might occur, the fire officer in command must consider several decisions, any of which could lead to success or failure in controlling the situation. At such a time, the officer’s most important need is information.
- What exactly is the situation?
- What products are involved?
- Is it flammable liquid or flammable liquefied gas in the tank?
- Is there tank damage?
- Is the relief valve damaged or blocked?
- Are people in danger?
- What is the greatest source of water?
- How much time do we have to take action?
Decision-making is easy if the officer knows the answer to these and similar questions. But chances are that such information is not readily available.
Present and proposed marking and identification systems for containers of flammable liquids and gases make it extremely difficult for fire officers to determine what products are involved so that their hazard properties can be identified.
The hazardous materials regulations of the D.O.T. require placards to be placed on trucks, trailers, and railroad cars that carry dangerous materials. Vehicles must be placarded on the front, rear, and both sides with the hazard name in letters at least four inches tall. Railroad equipment must be placarded on both sides and ends. Placarding is not required for air shipments. But placards can become detached, destroyed, damaged, or obscured by debris, smoke, or flame.
Train crews or truck drivers may be injured or have left the scene of the accident. Waybills or manifests listing cargo content may be burned or missing. Locating survivors may be difficult. They are not required to make their identities known to fire personnel.
Different products may have their own particular hazards. A product may be toxic or explode violently for reasons other than a BLEVE. It may also be flammable, corrosive, or have mixed dangerous potential. Entering the danger zone to attempt identification of the product will entail considerable risk.
The fire officer in command must make a decision on whether to attempt to identify the products if the information is not immediately available. Only a minimum number of personnel should be exposed to risks if such attempts are deemed necessary. Every precaution, such as full protective clothing and protective hose lines, must be used to provide the highest degree of protection for those entering the danger area.
Extreme care must be used to identify the product or products’ spelling. Many chemicals have similar names but very different properties, such as
- hydrochloric acid and hydrofluoric acid
- ammonium nitrate and ammonium nitrite
A mistake in identification could be disastrous. To verify identification, write the chemical name of each product, the trade name, manufacturer, carrier, label color, and any other symbols. Transmit this immediately to fire alarm headquarters for checking.
If a decision is made to attack the fire, large quantities of water must be applied at the point of flame contact on the tank to diminish the heat of exposure. Tank vapor spaces are the most critical and high-priority areas for cooling. However, know the capabilities and limitations of hose streams.
Five hundred gallons per minute at each point of flame contact is the minimum water application required for effectiveness when flames impinge on such tanks. The water source must be capable of sustaining this flow indefinitely. If a film of water exists on the tank shell where it is exposed to flame impingement, the shell cannot be heated to more than 212°F (100°C), a safe temperature.
If a decision is made to approach the fire, only personnel essential to the operations should be put at risk. A fire officer should lead the approach. Personnel must be given clear instructions on the tactics to be employed, and the entire operation must be highly coordinated. The officer should use verbal and hand signals, which have previously been rehearsed and agreed upon by all participants.
Any attempt to extinguish the fire by shutting valves or plugging holes would be tried only after effective cooling streams strike the tank shell at points of flame contact. This is a dangerous operation and should be carried out only when necessary. It is the kind of action that requires realistic practice and training.
Protective clothing currently available provides only minimal protection against flash or heat radiation and no protection against container fragments. It has, however, saved a number of firefighters from the thermal effects of a BLEVE. Wear complete protective equipment, including SCBA.
The use of unmanned monitors reduces risk to personnel, but the period during set up is often critical in terms of BLEVE potential. The fire officer in command must carefully estimate the risks before endangering personnel in this task.
Personnel safety must be considered at all times. The following points should be kept in mind.
- Approach the fire from the sides of the container. Consider that the tank ends are most dangerous within 60° of the longitudinal axis.
- Provide wide-angle water spray protection for personnel during initial setup. Be sure to consider the distance range of hose streams.
- Use all available barriers to protect against flying missiles and thermal radiation.
- Use unmanned equipment where possible.
- Maintain observation of areas of flame contact and ensure that water application rates are adequate.
Getting people out of the zone of immediate risk may require considerable time and personnel, especially in built-up areas. It might even be impossible under some circumstances.
The NFPA strongly recommends that the possibility of BLEVE be considered at all phases of a flammable liquid and a liquefied flammable gas emergency, including the termination phase when the product is being transferred or damaged containers are being moved. Leakage during transfer can result in a fire/explosion. Containers, especially of liquefied gases, can be damaged severely enough so that a slight rise in pressure or movement can result in container failure.
The possibility of BLEVE would dictate that a minimum number of personnel should be exposed to container rupture and that hose lines should be ready for use. Additionally, the possibility of a BLEVE during any phase should be strongly considered when determining whether evacuation property adjacent to the accident should be reoccupied before the problem is terminated.
Under the most favorable conditions, this incident presents danger and threats to the firefighter. The threats also affect the nearby public, and this aspect may well have an essential bearing on final decisions on personnel commitment. If no persons or property are in the danger area, the decision should be relatively simple, and no fire attack should be made.
If only property is at risk, then be very careful about endangering fire personnel in control efforts.
When occupied premises are in the danger area, the decision will be more difficult. The fire officer will have to decide whether to start an attack that might control the fire in time to concentrate on removing exposed persons.
Unless the attack is effective, the officer in command will achieve very little, but contingencies influence the choices in each incident. The safety of the public and firefighters will be the principal consideration.
WHEN TO ANTICIPATE BLEVE
- Activation of PRV
- Sounds from PRV increase
- The pitch from PRV becomes higher and louder
- The space between flame and PRV increases
- Water turns to steam, hitting tank
Source: The National Fire Academy (Document seem to be out of publication)