Incidents involving propane tanks, common energy sources for homes and businesses, can pose significant hazards. Understanding the nature of a propane tank “explosion” is important, as these events are often complex and involve both physical phenomena and combustion.
Propane as a Fuel Source
Propane, with the chemical formula C3H8, is a three-carbon alkane that is a gas at standard temperature and pressure. It is a liquefied petroleum gas (LPG) derived from natural gas processing and crude oil refining. For efficient transportation and storage, propane is compressed into a liquid state and held under pressure in tanks. This allows for a substantial amount of fuel to be stored in a relatively small volume.
Propane is highly flammable, and an odorant such as ethyl mercaptan, which smells like rotten eggs, is intentionally added to it because propane is naturally odorless. This additive makes leaks detectable, serving as a safety measure. Propane vapor is denser than air, meaning that if a leak occurs, the gas tends to sink and accumulate in low-lying areas like basements or trenches, increasing the risk of ignition. The presence of an ignition source in such an accumulation can lead to a fire or explosion.
Understanding a BLEVE
A Boiling Liquid Expanding Vapor Explosion, commonly known as a BLEVE, occurs when a vessel containing a liquid, typically stored under pressure above its atmospheric boiling point, ruptures. This phenomenon is often associated with external heat exposure, such as from a fire, which impinges on the tank. The heat causes the liquid inside to warm and the pressure within the container to rise. Simultaneously, the external heat can weaken the tank’s structural integrity, especially in areas not covered by the liquid, which are less cooled.
When the internal pressure exceeds the weakened tank’s capacity, the vessel catastrophically fails. This sudden rupture leads to an instantaneous depressurization, causing a rapid transition of the superheated liquid into a vapor. This rapid expansion of vapor is the physical explosion characteristic of a BLEVE. If the substance is flammable, like propane, the rapidly expanding vapor cloud will typically ignite, forming a large fireball.
Classifying Propane Fires
Fire classification systems, such as those outlined by the National Fire Protection Association (NFPA) in NFPA 10, categorize fires based on the type of combustible material involved. Propane, being a flammable gas and stored as a liquid under pressure, is classified as a Class B fire. Class B fires involve flammable liquids or gases, including substances like gasoline, oils, and various gases. Using water on Class B fires can spread the fuel, so specialized extinguishing agents like foam or carbon dioxide are typically more appropriate.
While a propane fire falls under Class B, a propane tank “explosion” is a more complex event. The “explosion” is a BLEVE, a physical phenomenon of rapid vapor expansion after pressure vessel failure. This physical event leads to a large Class B fire if the propane ignites. The initial blast is the sudden release of stored energy and rapid phase transition, with the subsequent fire being the burning of the released flammable material. Thus, the resulting fire is Class B, but the preceding “explosion” is the tank’s mechanical failure and explosive vaporization.