Propane tanks are pressurized vessels designed to safely store liquefied petroleum gas, a fuel commonly used for grilling, heating, and other applications. Understanding the relationship between external temperature and the internal pressure of these tanks is fundamental to safe handling and storage. Heat directly affects the liquid propane inside, causing pressure to increase and potentially challenging the tank’s structural limits. Knowledge of safe temperature ranges is crucial for preventing equipment damage or accidents.
The Relationship Between Heat and Tank Pressure
Propane is stored as a liquid inside the tank, but it must convert to a gas, or vaporize, to be used by an appliance. This conversion process, known as boiling, occurs constantly, and the resulting vapor creates the internal pressure. The pressure inside the tank is directly determined by the temperature of the liquid propane, not the amount of propane present, provided both liquid and vapor are inside.
As the ambient temperature rises, the liquid propane absorbs heat and vaporizes more rapidly, causing the internal pressure to increase. For example, a standard tank at 70°F registers an internal pressure of about 145 pounds per square inch (psi). If that same tank is heated to 110°F, the pressure can elevate to around 220 psi, demonstrating the non-linear relationship between temperature and pressure. This rapid pressure increase underscores why heat exposure is the primary operational hazard for propane tanks.
How the Pressure Relief Valve Protects the Tank
Propane tanks are equipped with a pressure relief valve (PRV) to manage high internal pressures. The PRV is designed to automatically open and vent a small amount of propane gas when the pressure reaches a specific safety threshold. This mechanism prevents the tank from exceeding its maximum safe working pressure due to heat buildup.
For common portable tanks, the PRV is set to activate at a pressure around 250 psi. When this pressure point is reached, the valve will “weep,” releasing a controlled amount of vapor that can be heard as a hissing sound. Once the internal pressure drops back to a safe level, the spring-loaded valve automatically re-seals. This venting action prevents the tank from reaching the point of structural failure from overpressure.
The Temperature That Causes Tank Failure
While the pressure relief valve handles internal pressure increases from ambient heat, it cannot protect the tank from structural failure caused by external fire. The ultimate danger point is not a specific internal pressure, but the temperature at which the tank material loses its strength. Steel, the most common material for propane tanks, begins to weaken substantially when exposed to fire conditions.
The temperature required to compromise the structural integrity of the steel tank is approximately 1,200°F. When a tank is engulfed in fire, the metal shell is heated beyond its yield strength, meaning it can no longer contain the internal pressure, even if the PRV is venting. This structural failure leads to a catastrophic rupture, often referred to as a Boiling Liquid Expanding Vapor Explosion (BLEVE), where the superheated liquid propane instantly flashes into vapor upon the tank’s breach.
Guidelines for Safe Propane Tank Storage
Preventing the tank from reaching dangerously high temperatures is the most effective safety measure a user can take. Propane tanks should always be stored outdoors in a well-ventilated area to prevent gas accumulation in case of a leak. Store the tank at least 10 feet away from any door or window that leads into a building.
Tanks should never be stored in direct sunlight, as solar radiation can quickly raise the tank’s surface temperature above the recommended maximum of 120°F. Placing the tank in a shaded location helps keep the temperature within the safe range of -40°F to 120°F. Tanks should be kept away from any source of ignition or heat, such as grills, furnaces, or vehicles, and must always be stored in an upright position to ensure the pressure relief valve functions correctly.