BLEVE is an acronym that was first coined by three Factory Mutual (FM) researchers. On April 24, 1957, a substantial container failure occurred as a result of overheating a mixture of formalin (a solution of formaldehyde gas in water) and phenol in a chemical reactor at an FM research facility. In the analysis that followed the incident, it became evident that the physical model of the container, due to overpressurization, was also applicable to any liquid that was at a temperature well above its normal boiling point at the moment of vessel failure. Liquefied gases (oxygen, nitrogen, nitrous oxide, methane, etc.) are good examples. Because of this observation and to avoid using excessive wording, the term "BLEVE" was introduced by FM researchers James B. Smith, William S. Marsh, and Wilbur L. Walls.
Walls went on to work for the NFPA as a fire protection engineer and made many presentations on BLEVEs. In his November 1978 NFPA Fire Journal article "Just What Is a BLEVE?" Walls refers to the NFPA definition: "a major container failure, into two or more pieces, at a moment in time when the contained liquid is at a temperature well above its boiling point at normal atmospheric pressure."
While this definition is a broad one, Walls points out that it is an exact one. He suggests that some confusion over the type of BLEVE has led to misapplication of the term itself. For instance, the actual mechanism of a BLEVE is a physical reaction in which the material rapidly, and instantaneously, converts from a liquid to a gas. It is merely a change of state that yields pressure. This reality is in contrast to a chemical reaction in which the material is converted to other materials, especially gases, chemically, such as with high explosives. BLEVE is actually a misnomer because it is not technically an "explosion." Make no mistake, however; the pressure involved with a liquefied gas changing state can be extreme and violent.
Walls also points out that the definition of BLEVE is independent of the cause of the container failure. For a BLEVE to occur, the container has to be under pressure, the pressure has to exceed the strength of the container, and the container has to be weakened in some way (impact, corrosion, fire). Walls goes on to discuss different types of BLEVEs such as containers failing from flame impingement. If the liquefied gas is flammable and released because of a BLEVE, the important and dangerous dimensions of fireballs and ignition of vapor clouds have to be considered. Walls warns that the impression that BLEVEs are solely restricted to flammable, liquefied gases is false. BLEVEs occur with many types of liquefied gases, flammable and nonflammable.
A. Michael Birk, a professional engineer and a professor with Queens University at Kingston, Ontario, Canada, offers this viewpoint on BLEVEs: "You get a BLEVE when a vessel holding a "pressure" liquefied gas fails catastrophically. It does not matter how the container fails. It can be by fire impingement, impact, corrosion, etc. The BLEVE is the boiling liquid expanding vapor explosion that happens when the tank opens up fully." He goes on to say, "A BLEVE is a physical explosion of compressed vapor and rapidly vaporizing liquid. Upon vessel failure the vapor space sends out a shock wave from the liquid flashing to vapor. If the material is flammable, a fireball may follow it. The rapid explosion can also cause projectile effects."
How does this apply to amateur rockets:
If you or your rocket team are using: tanks that have been fabricated by the team (rolling and welding aluminum or stainless steel sheeting), a tank not fabricated by a ASME rated welder or fabricator, a commercial tank above its rated pressure, a tank with an unknown pressure rating, or a tank that has been previously damaged; and your team is using: liquefied gases such as oxygen, nitrogen (for testing), methane, or nitrous oxide, your rocket is susceptible to a BLEVE.
A BLEVE can happen at anytime your tank(s) are pressurized and contain liquefied gas propellants. The BLEVE will generate a high-pressure blast wave with high-velocity and razor sharp fragments. If the propellant is a liquefied flammable gas, like methane, a large fireball may also be generated.
Extreme caution should be used under the above conditions. A BLEVE is extremely dangerous and could result in DAMAGE or DESTRUCTION of your rocket, your launcher, and your personal property; and result in INJURY or DEATH of you, your team members, or observers.
Precautions to be used:
- If possible, use a commercial tank at it's rated pressure.
- If you want to fabricate your own metal tank, use an ASME rated welder or fabricator.
- All welds should be inspected and have 100% penetration and free of pin holes and cracks.
- Do not use damaged tanks.
- Damage may be in the form of scrapes, scratches, dents, impact, corrosion, or fire.
- Do not use tanks with defects or poor welds (less than 100% penetration, cracks, or voids).
- Protect your tank(s) from damage during handling, shipping, and transport.
Attach Safety Devices
- Proof test tanks to 150% of maximum expected operating Pressure.
- Attach a relief valve to the top of each propellant tank that is set for 125% of the maximum expected operating pressure.
- Attach a remote-controlled vent valve to the top each propellant tank that can be activated independently from the main control system.
- If possible add a remote-controlled vent valve to the pressurant tank.
- Attach a pressure transducer to the top of each propellant tank and to the pressurant tank that can be monitored remotely.
- Utilize a main pressurization valve that can be remotely opened and closed located between the pressurant and propellant tanks.
- Incorporate the Rocket Emergency Depressurization System (REDS) attach to the remote controlled vent valves.
- ONLY pressurize your tanks when all participants are safe, which is defined as behind a substantial protective barrier, in a bunker, in a blockhouse, or at a safe distance away.
- Continuously monitor propellant tank pressures.
- DANGER. If propellant tank pressures are rising above the maximum operating pressure, close the main pressurization valve and depressurize the propellant tanks.
- DO NOT come out from behind a protective barrier or approach the rocket or static firing setup when the propellant tanks are pressurized, pressure relief valves are venting, or the tanks are on fire.
- DANGER. Close the main pressurization valve and depressurize all tanks before you approach rocket or static firing setup.
- IMPORTANT. If the automatic controls fail and cannot depressurize your rocket or static test stand, use the remote controlled vent valves or REDS to depressurize the propellant tanks.