Propane must be stored under pressure to remain liquid at typical ambient temperatures. If liquid propane is released and allowed to rapidly decompress to standard atmospheric pressure, its temperature immediately drops to its boiling point of approximately -44 degrees Fahrenheit (-42 degrees Celsius).
Understanding Propane’s Phase Change
Propane is stored as a liquid inside a pressurized tank to contain a usable volume of fuel. The pressure forces the molecules closer together, maintaining the liquid state even when ambient temperatures exceed the boiling point. This creates an equilibrium where liquid propane and propane vapor coexist within the tank.
The temperature of the liquid propane inside the tank is usually close to the temperature of the surrounding air or ground. The pressure exerted by the propane vapor above the liquid is directly proportional to the liquid’s temperature. As the temperature increases, the vapor pressure rises, which is why a tank sitting in the sun will have higher internal pressure than one in the shade.
Safety Hazards of Extreme Cold
Direct contact with liquid propane or its rapidly expanding cold vapor poses physical hazards. When liquid propane is spilled or vented, it flash-evaporates, drawing heat from whatever it touches, including human skin. This rapid heat loss causes a cryogenic burn, a form of frostbite, almost instantly upon contact.
Tissue damage from a cryogenic burn is similar to a thermal burn, sometimes causing the skin to appear waxy and yellow. This extreme cold can also cause materials like carbon steel, rubber, and some plastics to become brittle. Embrittlement increases the risk of equipment failure or cracking if components are exposed to liquid propane during a leak or transfer.
How Propane’s Cold Temperature Affects Usage
For propane to be used as a fuel, it must convert from its stored liquid state into a gas, which is burned by appliances. This conversion process, called vaporization or boiling, requires energy known as the latent heat of vaporization. This heat is absorbed from the liquid propane, the storage tank walls, and the surrounding environment.
As propane is drawn from the tank, the continuous vaporization process cools the liquid and the tank’s exterior. In cold weather, the environment may not supply enough heat to maintain a vaporization rate high enough to meet appliance demand. This leads to a drop in tank pressure, affecting the performance of equipment, which is often mistakenly described as the propane “freezing.”