Propane is a widely used fuel, stored as a liquid in steel tanks for everything from backyard grills to home heating systems. While tanks are engineered with multiple safety features to prevent catastrophic failure, a true explosion remains possible under extremely specific and sustained heat conditions. This rare but serious event is known as a Boiling Liquid Expanding Vapor Explosion (BLEVE), which occurs when the tank structure itself is compromised.
The Role of the Pressure Relief Valve
Consumer propane tanks, such as the standard 20-pound barbecue cylinder, are equipped with a mechanical safeguard called a pressure relief valve (PRV). This device is the primary mechanism designed to prevent the container from rupturing due to excessive internal pressure. Propane’s liquid contents naturally expand significantly when subjected to heat, causing the internal pressure to rise.
The PRV is a spring-loaded plug calibrated to open automatically when internal pressure reaches a predetermined safety threshold, typically around 375 pounds per square inch (psi). When the valve opens, it rapidly vents expanding propane vapor into the atmosphere, preventing the tank’s structure from being overstressed. This controlled safety measure results in a loud hissing sound and a jet of gas, which may ignite if a fire is present.
Once the pressure inside the tank drops back to a safe level, the spring mechanism forces the valve closed, sealing the tank again. This continuous venting and resealing action manages the pressure buildup caused by external fire exposure. The PRV ensures a propane tank exposed to heat is far more likely to simply vent and burn off its contents in a controlled manner than to explode.
When Catastrophe Strikes: Boiling Liquid Expanding Vapor Explosion (BLEVE)
A true explosion, or BLEVE, requires the failure of the tank’s steel shell. This catastrophic outcome is usually the result of direct and prolonged flame impingement on the tank’s vapor space, which is the area above the liquid propane level. When the tank is exposed to fire, the liquid propane inside absorbs the heat, keeping the metal shell below the liquid level relatively cool.
The portion of the tank above the liquid level, containing only propane vapor, is not cooled by the liquid and reaches dangerously high temperatures. The intense heat causes the steel shell in this vapor space to rapidly lose its mechanical strength and tensile integrity. The metal can no longer contain the extreme internal pressure, which is simultaneously being driven up by the heat.
When the weakened metal fails, the tank ruptures instantaneously, leading to a sudden, massive drop in internal pressure. This pressure drop causes the superheated liquid propane to flash into vapor, expanding to many times its liquid volume in a fraction of a second. This rapid phase transition generates a powerful shockwave, hurling tank pieces outward as high-velocity shrapnel.
If the escaping vapor cloud finds an ignition source, the result is a huge, expanding fireball, dramatically increasing the destructive radius. The BLEVE occurs only when the tank’s safety mechanisms and structural integrity are overwhelmed by sustained, localized heat exposure.
Safe Storage and Emergency Response
Minimizing the risk of a propane-related incident begins with strict adherence to proper storage guidelines. Propane cylinders must always be kept outdoors in a well-ventilated area, as the gas is heavier than air and can pool in low, enclosed spaces. Never store a propane tank, even an empty one, inside a home, garage, shed, or enclosed vehicle, as temperature fluctuations can cause dangerous pressure changes.
Tanks should be stored upright and secured to prevent tipping, which ensures the PRV is positioned correctly to vent vapor rather than liquid. It is also important to store tanks away from any ignition sources, such as pilot lights, electrical devices, or sources of high heat like furnaces or direct sunlight.
Transporting Propane Tanks
If you must transport a tank in a vehicle, always keep it upright, secure it, and ensure the vehicle is well-ventilated, such as by keeping windows open.
Emergency Response
If a propane tank is exposed to fire, the immediate response is to prioritize distance and safety. Do not attempt to move a tank that is involved in a fire, as the structural integrity may be compromised. Evacuate the area immediately and move to a safe distance, generally at least several hundred feet away. Once safely removed from the scene, contact the fire department or emergency services immediately and inform them that a pressurized propane tank is involved in the fire.