Propane (liquefied petroleum gas) is safely stored in steel tanks, but heat is the primary safety consideration because it directly affects internal pressure. The steel casing has a high melting point, so failure under normal circumstances is not due to the metal physically breaking down from ambient heat. Instead, the safety of a propane tank is defined by engineered limits designed to manage the internal pressure created by temperature increases. Understanding these limits and the tank’s built-in safety features is important for safe handling and storage.
How Heat Increases Propane Tank Pressure
Propane is stored as a liquid under pressure, but it must convert to a vapor before use by an appliance. As the surrounding temperature rises, the liquid propane inside the tank begins to vaporize rapidly. This vaporization process significantly increases the internal pressure within the sealed container. For example, a standard 20-pound tank might have an internal pressure of about 145 pounds per square inch (psi) at 70 degrees Fahrenheit, which climbs to approximately 172 psi if the temperature reaches 100 degrees Fahrenheit.
To accommodate this thermal expansion, propane tanks adhere to an 80% fill rule, meaning they are never filled completely with liquid. This regulation ensures a 20% vapor space remains at the top of the tank to act as a buffer for the liquid propane to expand as the temperature increases. Without this vapor space, even a moderate temperature increase could rapidly push the internal pressure beyond the tank’s designed limits.
The Role of the Pressure Relief Valve
The tank’s primary defense against excessive heat and pressure buildup is the Pressure Relief Valve (PRV). This spring-loaded mechanism is designed to open automatically when the internal pressure reaches a predetermined safety threshold. The PRV’s setting is mandated by federal safety standards, such as those set by the Department of Transportation (DOT) or the American Society of Mechanical Engineers (ASME).
Standard DOT tanks often have their relief valves set to activate at approximately 250 psi, while other tanks may be set higher, commonly around 375 psi. When this threshold is met, the PRV opens and safely vents excess propane vapor into the atmosphere until the internal pressure drops. This mechanism defines the tank’s operational limit for withstanding heat, as it is designed to vent rather than rupture.
Structural Failure and the BLEVE Hazard
Catastrophic tank failure is not caused by high ambient air temperatures but by exposure to an intense, sustained heat source, such as a localized fire. This scenario can lead to a Boiling Liquid Expanding Vapor Explosion (BLEVE), which represents the tank’s absolute physical failure limit. A BLEVE occurs when fire impinges directly on the tank shell above the liquid level, in the area containing only propane vapor.
The steel in the vapor space is not cooled by the liquid propane, causing it to overheat rapidly and lose structural strength. Although the steel’s melting point is high, its tensile strength decreases significantly when exposed to fire’s extreme temperatures. The internal pressure, simultaneously increasing due to the heat, then overwhelms the weakened metal, causing the tank to tear open suddenly. This mechanical failure is followed by the superheated liquid propane flashing into a massive volume of vapor, creating a powerful explosion and subsequent fireball.
Guidelines for Safe Storage Temperatures
To prevent the internal pressure from activating the PRV, simple storage practices must be followed. Propane tanks should never be stored in enclosed spaces where temperatures can spike dramatically, such as hot garages, sheds, or vehicles left in the sun. The recommended safe ambient temperature range for storage is below 120 degrees Fahrenheit (49 degrees Celsius).
Tanks should be stored outdoors in a shaded, well-ventilated area, avoiding direct sunlight exposure which quickly raises the tank’s surface temperature. Following these guidelines ensures the tank remains within its normal operating temperature range. This precaution keeps the internal pressure well below the level required to trigger the pressure relief valve.