Propane, commonly known as Liquefied Petroleum Gas (LPG), is a versatile fuel source used for everything from heating homes to fueling barbecue grills. It is stored as a compressed liquid. This state is necessary for efficient storage and transportation, as propane is naturally a gas at standard atmospheric pressure. For instance, one gallon of liquid propane expands into approximately 270 gallons of usable gas.
The Physics of Liquid Storage
Keeping propane liquid inside the tank relies on the relationship between pressure and temperature. Propane’s natural boiling point is extremely low, around -44°F (-42°C), so the tank applies significant pressure to force it into a liquid at ambient temperatures. Inside the tank, liquid propane and vapor coexist in equilibrium. This vapor pressure is dynamic and changes with the external temperature; on a 70°F day, the tank pressure is typically around 145 pounds per square inch (psi), increasing to about 172 psi on a 100°F day. Maintaining this high pressure keeps the propane molecules tightly packed and condensed as a liquid, which the robust steel tank is designed to safely contain.
How Liquid Propane Becomes Usable Fuel
Liquid propane must transition back into a gas before it can be used by an appliance. When the tank valve is opened, a regulator reduces the pressure, allowing the liquid to boil and convert into vapor. The appliance then draws this vapor from the top of the tank for combustion. This process is continuous: as vapor is drawn out, the liquid boils to replace the lost gas and maintain pressure. This phase change, known as vaporization, requires energy drawn from the liquid propane and the tank walls.
In cold weather or during periods of high demand, the rapid loss of heat during vaporization can cause the exterior of the tank to feel cold or even frost up.
Why Tanks Are Only Filled to 80%
Propane tanks are only filled to 80% capacity as a mandated safety measure. This 20% empty space, known as headspace, is necessary to accommodate the thermal expansion of the liquid propane. Liquid propane expands dramatically when heated, at a rate nearly 17 times greater than water over the same temperature increase. If a tank were filled completely, a rise in ambient temperature would cause the liquid to expand and quickly fill the remaining volume, creating immense hydrostatic pressure. Since liquids are largely incompressible, this pressure risks tank failure and rupture. Therefore, the 80% limit ensures a sufficient vapor space remains to safely absorb and manage the pressure increase, even on the hottest days.