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Mai'i-IMS ySS PREVENTS fW<ru>* fil< c) 'V^*' Cj ^fcJ^CTlVE CHEMICF^ X^522ams ^#e*r } t' i ( ; r Technical Memo tiWe^- C/ v M C"k' WHAT IS A BLEVE'i G-' i-LYl A1r-L ' ' f *' hj <\o ^ r SLP-TM-04-95 August 10,1995 nan E. Scheffl r Definition of a BLEVE - The definition of a Boiling Liquid Expanding Vapor Explosion (BLEVE) according NFPA 49 is: "A type of rapid phase transition in which a liquid contained above its atmospheric boiling point is rapidly depressurized, causing a nearly instantaneous transition from a liquid to a vapor with a corresponding energy release."1 A BLEVE is also accompanied by a large fireball if a flammable liquid is involved, and there is an external fire impinging on the vapor space of a pressure vessel which is a common BLEVE scenario. However, it is not necessary for the liquid to be flammable to have a BLEVE occur.1 An additional definition of a BLEVE is a major failure of a container at a moment in time when the contained liquid is at a temperature well above its atmospheric pressure boiling point.2 This definition does not qualify the cause of the container failure or the chemical or physical properties of the contained liquid. Most of the time, BLEVEs occur with pressure vessels containing liquefied gases. BLEVE explosions were first recognized in 1957, and major BLEVEs have occurred about every 10 years.3 Causes of BLEVEs - A BLEVE of a pressure vessel is likely to occur when the vessel is not strong enough to contain the pressure within the vessel. Vessels are designed to hold the pressure under normal conditions. However, if the mechanical strength of the steel is reduced, a BLEVE can occur. The two most likely failures are due to heating or mechanical conditions.2'4'7 The most common, well-known type of BLEVE occurs when a pressure vessel, partially filled with liquid, is exposed to fire. The fire exposure which occurs above the liquid filled area (1/2 to 3/4 full of liquid, typically)5 starts to weaken the steel as the temperature rises to 500-600 C (932-1112 F).6'8 The temperature of the liquid starts to increase, and the pressure rises according to the vapor pressure. When the PSV set point is reached, the relief valve opens and starts to vent vapor or vapor and liquid. The thermally induced stresses, heat weakened walls and high internal pressure combine to cause a sudden, violent rupture of the tank. The tank shatters with flying fragments and large tank parts traveling 100 to 800 m (330 to 2,600 ft).3 Most of the remaining liquid vaporizes extremely rapidly and the rest atomizes to small droplets. For flammable materials, a fireball is created by the burning vapor and liquid. A mechanically induced BLEVE can occur to a physically damaged tank. In some cases, such as with a truck tanker which has been in an accident, an immediate catastrophic failure of the tank may take place. However, in other cases, the damage to the tank may appear minor, but the stresses imposed on the damaged areas may cause failure at a later, completely unexpected time.2 A BLEVE in this situation could occur without the presence of a fire or a fireball. A hydraulically filled tank can cause a BLEVE, but usually a PSV can relieve this pressure if function ing properly. Rapid depressurization of a vessel can cause a BLEVE in the case of a superheated liquid, which could nucleate the liquid, turning it rapidly into vapor with a large increase in pressure. Thus, rapid depressurization of a tank in a fire situation is not advisable.2 ; QWESTlOiAre normaCretiefvafves designed to protect against a (BL'EV'E in the case offire? Page 1 Dow Restricted 08/10/95 (j) Primed on Recreled and Rrcvclahk* Paper DO A1R6195 CONFTDFNTTAl SLP-TM-04-95 AC\ljyWER: Odormalreliefvalves are not designed to protect against a `BL'EV'L. It is not practical to have than do so. Reliefvalves willBuy a smallamount of time Before a 'B&LltE occurs. What can happen - BLEVEs are extremely dangerous types of explosions and can cause great damage and injuries/fatalities. There are four main causes of damage:6 1. Fragments from the failed vessel can be sent up to 100-800 m (330-2,600 ft). 2. The pressure of the escaping gas can injure people who have been missed by the fragments. Buildings and structures can be damaged. 3. The burning of the escaping gas or liquid and the heat can injure people or set fire to buildings. 4. The escaping gas or liquid can mix with the air and explode producing a second pressure wave that does additional damage. The magnitude and tuning - The size of a BLEVE depends basically on how much liquid vaporizes when the vessel fails and the weight of the container pieces. Fireballs can be about 30-90 m (100-300 ft) in diameter. People have died from bums as far as 76 m (250 ft) from the larger containers and can suffer bums up to 305 m (1,000 ft) away. As an example of damage, a 15.2 m (50 ft) diameter propane sphere holding about 2,390 cu m (630,000 gal) could cause damage as far away as 4,145 m (13,600 ft) with radiation and fragmentation damage up to about 914 m (3,000 ft).5 The timing for BLEVEs to occur is a very wide range. The shortest time ever recorded is 8 min. A typical time for an non-fireproofed vessel after start of a fire is 15 min with over half of the recorded inci dents happening in this time or less. In one unusual case, a BLEVE occurred after 20-1/2 hr in an LP gas railroad tank car. Flame impingement in a localized area can speed up a BLEVE.5,7 Prevention of BLEVEs - There are many actions to be taken to prevent BLEVEs as follows: 1. Design should be made for good drainage of flammables away from vessels. This includes good sloping to direct liquid away from the tanks with remote impoundment as the best design. 2. To prevent flame impingement from weakening vessels, fire resistant insulation can be used. Water sprays, water monitors and fire hoses can be helpful to cool the vessel steel. 3. Careful venting of vapors can be done, but this may be difficult to control. Relief valves should be able to prevent pressure rise due to hydraulic filling and will give some time before tank failure occurs. However, relief valves cannot be made large enough to prevent BLEVEs. 4, There should be efforts made to avoid mechanical damage. This is difficult in the case of a tank car or truck.2,6,7 Actions to take in the case of a possible BLEVE - The actions to take if there is a possibility of a BLEVE occurring are as follows: 1. Evacuate all unnecessary personnel. 2. Apply copious amounts of water to the vapor space exposed steel and especially any concentrated fire impingement. 3. Isolate the source of fuel. Activate all Emergency Block Valves (EBVs). 4. If it is obvious that there is a good chance that there will be a BLEVE, all personnel must leave the scene and fire fighting must be done remotely.4 References: NFPA 49, "Hazardous Chemicals Data," 1994. 5. Fire Protection Handbook, 17th ed., pp 3-63 to Maninsen, W. E., "Bleve's; Causes, Effects, and 3-69, 1991. Prevention," API Committee Meeting, Sept. 6. Fire Protection Manual, Vol. 2, pp 291-297, 1981. 13-15, 1983. 7. Prugh, R. W,, "Quantitative Evaluation of 3. Lemoff, T. C., "BLEVE: Back to Basics," AIChE "BLEVE" Hazards," AIChE 22th Loss Prevention 24th Loss Prevention Symposium, Paper 93e, Aug. Symposium, Paper 74e, March 1988. 1990. 4. CHEG, 3.5.7, pp 3-174 to 3-181, 1993. 8. "Boiling Liquid Expanding Vapor Explosion (BLEVE)," SLP-TB-88-1, 1988. For address change, new subscriber or extra copies contact Sharon Hartnagle, Corporate Loss Prevention, 2020 Dow Center, Ml, (E-mail U076635) Page 2 Dow Restricted 08/10/95 DO A 1.361 96 OONF TDFNTT Al