Boiling Liquid Expansion Vapor Explosion (BLEVE)

BLEVE or Boiling Liquid Expanding Vapor Explosion can be defined as a major failure of a container at a moment in time when the contained liquid is at a well temperature above its atmospheric pressure boiling point. A type of explosion that can occur when a vessel containing a pressurized liquid is ruptured. A BLEVE is what happens, for instance, when a closed container of flammable liquid is exposed to strong heating.

Characteristic

If you have:

§  Pressurized and isolated vessel

§  Flammable liquid

§  Heating sources nearby

You have the chance to experience the BLEVE within 14 minutes in average.

* H.R. Wesson & J.R. Lott, “Effectiveness of fire resistant coatings applied to structural steels exposed to direct flames contact, radiant heat fluxes, and mechanical and cryogenic thermal shock”, AGA Transmission Conference, St. Lois, 1977.

Significant Bleve Accident

Several significant BLEVE accidents are as follows:

1. Feyzin, France in 1966

2. Kingman, Arizona in 1973

3. San Juan Ixhuatepec, Mexico in 1984

4. Others:

§  Waverly, Tennessee in 1978

§  San Carlos, Spain in 1978

§  Quebec, Canada in 1993

§  Burnside, Illinois in 1997

Bleve Type

There are 3 common types of BLEVE:

1. Thermally Induced BLEVE

2. Mechanically Induced BLEVE

3. Pressure Induced BLEVE

Thermally Induced

- The scenario generally is as follows:

1. A partially filled pressure vessel is subjected to high heat flux from a fire.

2. The liquid temperature starts to increase, causing pressure increase also within vessel.

    When the relief valve pressure setting is reached, starting to vent vapor.

3. Temperature on tank shell that is not in contact with the liquid increase dramatically.

4. The heat weakens the tank shell and Thermally Induced Stress are created near the vapor/liquid interface.

5. The heat, stress, and high internal pressure combine to cause a sudden violent tank rupture.

6. Tank fragments are propelled away to some distance at great force.

7. Most of the remaining liquid vaporizes rapidly and the rest is mechanically atomized to small drops. A fireball is created by the burning vapor and liquid.

Mechanically Induced

§  When a pressure vessel that is partially filled with liquid is mechanically   damaged.

§  This type of BLEVE is very rare for storage tanks, but is not uncommon in transportation accidents.

§  One good example is the tank car incident at Kali Krasak Bridge near Magelang, Indonesia in 1992.

When the tank car had an accident, rolled and the imposed stresses damaged the tank and failed catastrophically. Then the great explosion destroyed the bridge

Pressure Induced

§  When a pressure vessel is allowed to become completely filled with liquid. The temperature rises and there is no pressure relief valve or the pressure relief capacity is insufficient from exceeding the strength of the tank.

§  This type of BLEVE have occurred in several accident involving small, portable LPG cylinders for domesticuse.

§  However this BLEVE is rarely happened on a vessel with pressure relief valves.

BLEVE poses 4 main types of hazards:

1. Overpressure

2. Flying Shrapnel Object

3. Rocketing Tank Parts

4. Fireball

Over Pressure

§  There are 2 sources of overpressure:

1. Expansion of the vapor that was present in the tank

2. Flashing rapid change from liquid to vapor

§  A BLEVE can cause neighboring vessels, if within few meters, to shift from their supports, resulting in failure of connected piping, thus causing accident to propagate.

§  Overpressure wave can also cause serious injuries, especially those who do not wear protective clothing

Flying Shrapnel Object

§  Hazards posed by pieces of metal tank that are scattered when a tank ruptures are difficult to quantify.

§  The hazards will depend on energy that is transmitted to:

§  Pieces of the fragments

§  Sizes of the fragments

§  Weight of the fragments

§  Actual data gathered on the distances show that larger fragments of 125 m3 (33,000 gal) tank can reach as long as 460 m (1,500 ft). Smaller fragments could be thrown to several thousand feet.

§  NFPA course, Handling Hazardous Materials Transportation Emergencies, recommends an evacuation distance of 760 m (2,500 ft).

Rocketing Tank Parts

§  The “end tubes” of several bullet tank could travel in greater distance. For instance, a large segment of a 36 m3 (9,500 gal) tank could reach approximately 1,200 m (3,900 ft

Fireball

§  Fireball created by combustion of the mixture of vapor and liquid that is explosively dispersed by the sudden rupture of the tank.

§  Sudden expansion of compressed vapor and large quantity vapor from liquid flashing create a large ball of liquid droplets and vapor.

§  The heat of burning dispersed liquid and vapor causes a powerful thermal updraft which interacts with the burning fuel/air mixture to create shaped “ball of fire”.

How to Prevent Bleve

§  Proper design is applied

§  Prevent the fire

§  Prevent the heat of the fire

§  Avoid overpressure

Proper Design

§  Pressure vessel and its associated piping system

§  Sufficient relief valve capacity

§  Material strength

§  Sitting of major hazards installation

§  Fire protection system design

Prevent the Fire

§  Head shields and shelf couplers on railroad tank cars. This type of modification would be beneficial to prevent spill and fire following the derailments.

§  Proper diking and drainage would be very useful forpreventing liquid pools in case of accidental release.

Prevent Heat Radiation

§  Insulation. Ordinary type of insulation and jacketed with a steel skin demonstrate the effectiveness of insulation as a preventive measure. Other type of insulation is epoxy in tumescent coatings.

§  Advantage of the coatings:

§  Low moisture permeability

§  High adhesion to the substrate

§  Low differential expansion at steel interface

§  Bonded fixings not necessary

§  High mechanical strength

§  Low corrosivity

§  Good surface finish

§  Water. With sufficient water sprayed into the tank, it is possible to keep the tank and its contents below 100°C (212°F). Type of application are such as a fixed water spray nozzles, deluge systems and monitor nozzles.

Avoid Over Pressure

§  Keep the tank shell full with liquid throughout entire time of fire exposure

§  Admit water into the tank

§  De-pressurization system

Bleve Mitigation

§  Since BLEVE has uncertain and very short time to occur, NFPA recommended to evacuate the area and allow the impinging fire to burn itself out.

§  Other alternative could be to remove the vessel from the impinging flame (for the mobile tank).

§  And also to keep vessels cool and evacuate the area.

Could LNG tank bleve?

§  LNG tanks are not designed for pressure, and even if subjected to external fire, cannot be over pressurize to a level that would cause a BLEVE event. LNG tanks won't BLEVE because.

§  tanks are doubled walled (outer concrete wall up to 2 ft thick)

§  outer shell will prevent direct flame impingement on the inner tank

§  insulation between the outer and inner wall will greatly slow heat transfer to the LNG.

§  The tanks are also spaced sufficiently that the radiant heat from a fire on one tank would not cause a cascading failure of the other tanks. The outer concrete tank will be designed to withstand heat from an adjacent tank fire. Fire water systems will be stationed around the tanks and operating areas and would be used to keep surrounding equipment and facilities cool in case of a fire.

Conclusion

§  Have occurred with LPG tanks

§  Have never occurred with LNG fixed tanks.

§  Occurs when tank is heated by direct flame contact

§  The fire increases the temperature of the product (liquid) inside the tank.

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