Physics

What is a Bleve



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BLEVE is an acronym for Boiling Liquid Expanding Vapor Explosion. Even if the term is unfamiliar to you, you might be familiar with the dramatic results of a BLEVE from news reports of chemical plant fires or train derailments. This is a brief introduction to BLEVEs with some additional notes for those interested.

BLEVEs occur when a vessel containing liquid above its normal (atmospheric pressure) boiling point fails catastrophically1,2. With the sudden drop in pressure, the liquid undergoes rapid transition to the vapor phase (i.e. boils) resulting in a pressure wave and flying debris propelled by the rapidly expanding vapor cloud3. If the vapor cloud ignites, you can get a spectacular fireball.

The vessels types that might be involved include tank trucks, rail cars, 55 to 85-gal. drums, high pressure cylinders, intermodal containers and fixed facility storage tanks.

Many people think you can have a BLEVE only with flammable liquids. This is not true. Perhaps the most common example would be your basement hot water heater. In a fire, these can and do BLEVE. Since the resultant cloud of water vapor is not flammable, you won't get a fireball, but you do get the pressure wave and shrapnel from the explosion.

BLEVE incidents are closely associated with fires. The fire both provides a heat source to raise the temperature and vapor pressure of the contained liquid, and may weaken the container metal causing vessel failure. However, you can have a BLEVE without a fire. Some materials like LPG (liquefied petroleum gas) have normal boiling points well below zero (about -42 C for LPG) and are stored and transported under considerable pressure. These could BLEVE in the absence of fire if some other factor led to sudden vessel failure4. These pressure vessels are very robust and designed to withstand considerable abuse (e.g., train derailments), so such a non-fire event would be very rare.

In a major incident, if there is a potential BLEVE hazard, on-scene emergency responders will want you to evacuate the area. A standard evacuation radius is one half-mile away from the incident5. They are not being overly conservative. To view an actual incident, go to http://www.youtube.com/watch?v=Xf3WKTwHpIU. In this case, you will see not only the huge explosion and fireball, but a large portion of railcar that was blown over 3600 feet away - significantly more than a half mile. You might want to pad your evacuation distance accordingly.

Before you high-tail it, however, if you are witness to the incident, particularly if it involves fire, share your information with the response team or another responsible authority. Depending upon the level of thermal protection, railcars and other vessels (and their support structures) are designed to last for a considerable time in a fire6. Knowing exactly when the fire started will assist the responders with their hazard assessment.

Notes:

A liquid above its boiling point is not actually boiling while contained in a vessel unless a pressure relief device has been activated. Instead, the pressure in the vessel builds up as the vapor pressure of the liquid increases with temperature. If the vessel fails, the sudden drop in pressure allows the liquid to flash (rapidly boil) making a large amount of vapor. It is the rapid expansion of this vapor that causes most of the damage. Think of the pressure cooker in your kitchen. You are able to heat the liquid in the vessel above its normal boiling temperature (thus speeding up the cooking process) because the vessel allows pressure to build rather than allowing the liquid to boil off. You control the cooking temperature by setting the relief device to regulate the pressure build-up. You would not use a badly damaged pressure cooker (would you?) because the weakened metal might no longer withstand the pressures it was originally designed to handle. Consider then the potential damage to pressure containers in a train derailment or chemical facility fire. If vessel integrity is compromised, there is increased risk of a BLEVE. Expansion ratios tell you the change in volume to expect as a given material transitions from the liquid to vapor (gas) phase. The range is broad, typically 500-1000, with some outliers. E.g., the expansion ratio for anhydrous ammonia is 855, for chlorine 458, and for propane 270. So, one gallon of liquid anhydrous ammonia, upon release from its container, will want to expand to 855 gallons (114 cubic feet) of gas. An anhydrous ammonia railcar has a 33,500 gallon capacity - that's a lot of potential gas. Pressure vessels are normally protected from simple overpressure failure by relief valves or rupture disks. Still, vessel failure might result from a number of factors alone or in combination that can cause the vessel to fail below its design pressure and/or render the relief device ineffective (e.g., consider an upside down railcar or tank truck) allowing excessive pressure buildup. Vessel failures can result from mechanical damage, corrosion, shrapnel from nearby explosions, and sustained flame impingement (Note 6). Other factors, notably the potential release of toxic materials, could make the evacuation zone much larger and non-symmetrical as dictated by wind conditions. Generally, you would want to evacuate cross-wind. Toxic plumes can extend several miles downwind. If impinging a wetted surface, fire poses no immediate threat to vessel integrity even without thermal insulation because the liquid provides a heat sink and the metal, through conduction, will not heat above the liquid temperature. That's why it's OK for you to set your tea kettle on the stove burner. The metal gets no hotter than the boiling point of the liquid inside. If you allow the tea kettle to boil dry, however, then the metal begins to heat up and you might ruin your kettle. Similarly, if flame impinges the vapor space of a container, the metal will heat up and weaken. Thermally protected vessels may withstand hours in a pool fire (large pool fires can reach 1200 degrees C), but torch fires can burn much hotter (consider the propane torch you use for cutting) and result in much shorter time to metal failure. Torch fires are less likely to occur, but are possible. In a train derailment, you might have a railcar sitting in a pool fire and venting through its relief valve. If the vented material is flammable, it may catch fire. If the resultant torch flame impinges onto another railcar, that car is in greater danger of failure.

References: The Environmental Resource Center's "Emergency Response Guidebook" www.ercweb.com; the "Hazardous Materials Field Guide," Bevelacqua & Stilp, Delmar Publishers www.firesci.com; and the NIOSH (National Institute for Occupational Safety and Health) "Pocket Guide to Chemical Hazards"

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  • InfoBoxCallToAction ActionArrowhttp://www.youtube.com/watch?v=Xf3WKTwHpIU
  • InfoBoxCallToAction ActionArrowhttp://www.ercweb.com/
  • InfoBoxCallToAction ActionArrowhttp://www.firesci.com/