“Maybe tone it down with the explosions.” — Michael Bay
Most of what we know about explosions is wrong.
Most of what we “know” about explosions is what we’ve learned from the movies, which is the only thing called an explosion that we ever see. But what we see in the movies are not explosions, but fireballs, because fireballs last long enough to be captured on film. Explosions, on the other hand, are over so quickly that they make terrible cinema.
Which explains why news accounts frequently describe events as “explosions” that are not. The reporters don’t know any better. Most photographs of explosions are not of the explosions themselves, but of the debris launched by the explosion, or of a fireball that has been mistaken for an explosion.
What Is An Explosion?
An explosion is “a sudden expansion in volume in which energy is transmitted outward as a shock wave.”
The increased use of closed-circuit video cameras makes it much easier to study potentially explosive events at process facilities. When the event is captured on video, there is an opportunity to look at the event frame-by-frame, using nearby objects to scale the event. To determine that an event was an explosion, however, doesn’t require a frame-by-frame examination. It simply requires identifying that there is a point at which the camera shakes, the point at which the shock wave passes around the camera. Fireballs do not create shock waves.
In process facilities, there are primarily four types of explosions that concern us. The first is the detonation of an explosive. These are typically compounds that contain an oxidizer as part of the molecular composition, and it is the sudden and catastrophic decomposition of these compounds into gasses that creates the explosion. Ammonium nitrate and ethylene oxide come to mind as examples, and the personnel handling these compounds should be aware of the explosive hazards.
The other three types of explosion hazards are much more general. They are pressure vessel explosions (PVEs), boiling liquid-expanding vapor explosions (BLEVEs), and vapor cloud explosions (VCEs).
Pressure Vessel Explosions
A PVE occurs when a pressurized vessel fails catastrophically. This failure may be the result of deterioration of the vessel. However, we typically think of the cause as extremely high pressure that takes the vessel above its yield strength. This is not the Maximum Allowable Working Pressure (MAWP), but a factor of 3½ to 6 times higher, depending on when and where the vessel was fabricated.
The pressure can come from any of several sources, including compressed air, pressurized nitrogen, steam, or high-pressure process fluids. The source of high pressure may be the decomposition or combustion of process materials, or the increase of pressure that comes with heating process materials, but neither chemical reaction nor external heating are a requirement of PVEs.
All that is required is that pressure in the vessel be high enough to result in brittle fracture, at which point all the stored energy is released suddenly as expanding gas and a shock wave.
Boiling Liquid-Expanding Vapor Explosions
BLEVEs are perhaps the most misunderstood of the process explosions. This is exacerbated by the EPA’s mischaracterization of BLEVEs in the Risk Management Planning rule, 40 CFR 68. A BLEVE is a specific form of a PVE, where the contents of the exploding vessel are a liquid well above its normal boiling point.
When a liquid is heated, its vapor pressure increases. At the liquid’s normal boiling point, its vapor pressure equals atmospheric pressure, so there is nothing to hold the vapor back when it is exposed to the atmosphere. In a pressure vessel, however, increasing the temperature increases the vapor pressure, but the vessel pressure keeps the material liquid well above its normal boiling point.
Water at a pressure of 60 psig, for instance, will remain liquid up to 307°F. If a vessel containing water is heated to 307°F, the pressure will increase to 60 psig. If that is the pressure at which the vessel fails, it will release that liquid at 95°F above its normal boiling point of 212°F, meaning that it will want to vaporize instantly. The volume of the liquid water increases by a factor of almost 1600 when it vaporizes—an explosion.
The necessary property of a BLEVE is that a material is kept a liquid in a pressure vessel well above its normal boiling point. Water in a boiler can bleve (yes, we have turned that acronym into a verb!), as can any liquified gas: cryogenic oxygen, cryogenic nitrogen, liquified natural gas (LNG), liquified petroleum gas (LPG), propane, butane, etc.
There is no combustion or decomposition in a BLEVE. However, if the liquified gas is a flammable liquid, the BLEVE can lead to a VCE.
Vapor Cloud Explosions
While a BLEVE is an explosion resulting from the sudden vaporization of a liquid, a VCE is an explosion resulting from the ignition of a flammable vapor cloud. Neither water nor nitrogen can be in a VCE. LNG, LPG, propane, butane, and any other flammable material that can form a flammable mixture with air have the potential to be involved in a VCE.
This is why OSHA and the USEPA insist that VCEs be considered the worst-case release for flammable materials in their process safety management regulations, even though VCEs are rare.
For a flammable material to be involved in a VCE, it must satisfy the five requirements of the explosion pentagon: fuel, oxidizer, ignition source, dispersion, and confinement. Fuel is easy enough—the flammable material is the fuel. If the material is released into the atmosphere, then air serves as the oxidizer. Ignition can come from many sources. It is the lack of dispersion and confinement that keeps most releases of flammable materials from exploding in VCEs.
Flammable materials don’t explode as liquids. They must be dispersed as vapors or as finely divided aerosols. Otherwise, they will only burn. Burning is not trivial, but it is not an explosion with a shock wave. Even when dispersed as vapors and aerosols, flammable materials will not result in a VCE if they are not contained. Congested process equipment and piping is sufficient to create the confinement necessary for a VCE, but a vapor cloud in the open will ignite as a vapor cloud fire—a fireball—without the accompanying shock wave of an explosion.
Words Have Consequences
Reporters call everything explosions because that makes for punchier copy and because they don’t know better. For those that do know better, the distinction between PCEs, BLEVEs, VCEs, and fireballs is a quibble. “Just semantics.” But semantics is about what words mean. What we mean by these words impacts what we do about them. And confusion about what we mean impacts what others do about them. So, make sure you understand the difference between PCEs, BLEVEs, VCEs, and fireballs, and use the terms correctly. It will help you to make the world a safer place.