Propane (C3H8) is a three-carbon alkane hydrocarbon, often stored and transported as liquefied petroleum gas (LPG). It is compressed into a liquid state, which significantly reduces its volume compared to its gaseous form. When a container of this fuel is exposed to heat, the stored material undergoes predictable physical and chemical transformations that have serious safety implications.
Propane’s Phase Change and Pressure Dynamics
Propane is stored as a liquid, but the gaseous vapor above the liquid is used as fuel. The pressure inside the tank is governed entirely by the temperature of the liquid propane. Even at ambient temperatures, the liquid constantly vaporizes, generating internal pressure.
As the temperature rises, the increased rate of vaporization causes the saturated vapor pressure to rise proportionally. For example, at 70°F (21°C), the internal pressure is approximately 127 pounds per square inch gauge (psig). If the temperature increases to 105°F (41°C), the pressure climbs to around 200 psig.
As long as liquid propane remains, the internal pressure is solely a function of the temperature, as the liquid phase continuously supplies vapor. The liquid also expands slightly as it warms, contributing to the total volume inside the container.
Tank designers account for this thermal expansion by intentionally leaving a small vapor space, which is why tanks are never filled to 100% capacity. This empty space prevents a hydraulic rupture under normal operating temperatures.
Thermal Decomposition Under Extreme Heat
When a propane tank is subjected to heat far beyond normal conditions, such as during a prolonged fire exposure, the propane molecules begin to break down chemically in a process called pyrolysis or thermal cracking. This reaction is distinct from vaporization and requires much higher temperatures, typically between 1,292°F and 1,652°F (700°C to 900°C).
At these temperatures, the stable three-carbon alkane molecule (C3H8) fractures into smaller, simpler, and more reactive hydrocarbon compounds. The primary products include methane (CH4), ethane (C2H6), ethene (C2H4), and molecular hydrogen (H2). This molecular breakdown is irreversible and changes the chemical composition of the fuel mixture.
The creation of these smaller molecules increases the total volume of gas within the container. Newly formed products are highly flammable and have different combustion characteristics than the original propane. This transformation also increases the internal pressure due to the generation of entirely new molecules.
The cracking process can also produce heavier, unsaturated hydrocarbons, such as propene and acetylene. This chemical cascade occurs simultaneously with the physical pressure increase. The resulting mixture of gases is more volatile and has a higher energy content.
Managing Pressure and Catastrophic Risk
The primary defense against pressure buildup from normal heating is the pressure relief valve (PRV), which is mandatory on all propane containers. This spring-loaded device is set to open automatically, typically around 375 psig, to vent excess propane vapor into the atmosphere. The PRV maintains the container’s structural integrity by releasing pressure that builds up when the tank is heated.
The PRV is designed to prevent rupture under standard over-pressure conditions, but it is not a fail-safe against an external fire. When a tank is exposed to intense flame impingement, the metal shell rapidly loses mechanical strength as its temperature rises.
Simultaneously, the relief valve may not be able to vent the rapidly expanding gas and cracked chemical products fast enough. This prevents the pressure from staying below the tank’s now-weakened burst point.
This combination of extreme internal pressure and weakened container material leads to the catastrophic failure known as a Boiling Liquid Expanding Vapor Explosion (BLEVE). A BLEVE occurs when the tank metal in the vapor space, which is not cooled by the liquid propane, is heated past its yield strength.
The metal tears violently, instantly releasing the superheated liquid propane. This liquid flashes into a massive cloud of vapor that expands up to 270 times its liquid volume. The subsequent ignition of this enormous vapor cloud creates a devastating fireball and an explosive shockwave.
This explosion is caused by the instantaneous, explosive phase change of the liquid into vapor upon the container’s mechanical failure. Intense heating transforms propane from a stable liquid fuel into a volatile, high-pressure hazard.