Propane is a hydrocarbon fuel that exists as a gas under normal conditions, but it is stored and transported as a liquid. This change in state is achieved through compression, making it possible to contain large amounts of fuel in portable tanks and storage vessels. The ability to switch between liquid storage and gaseous use makes propane an efficient and versatile energy source.
The Dual Nature of Propane Storage
Inside any standard propane tank, the substance exists simultaneously as a liquid and a vapor. The bulk of the fuel is stored as liquid propane, which settles at the bottom. Above this liquid, propane vapor forms, which is the gas used by the appliance.
This combination is maintained in equilibrium under pressure. The vapor is the fuel source drawn out to power grills or heating systems. Tank gauges measure the level of the liquid propane, not the pressure of the vapor.
Because propane is a liquid that expands significantly with temperature changes, tanks are intentionally never filled to 100% capacity. The industry standard maximum fill level is 80%, leaving 20% of space for the liquid to expand safely without risking over-pressurization. A gauge reading therefore indicates the volume of stored liquid fuel available.
Why Propane Must Be Compressed
Propane is compressed into a liquid primarily for logistical efficiency and convenience. At standard atmospheric pressure and temperature, propane occupies a large amount of space as a gas. To make the fuel portable and storable in practical quantities, it must be condensed into a liquid state.
This phase change achieves significant volume reduction; one gallon of liquid propane expands into approximately 270 gallons of usable propane gas. This compaction ratio makes it economically feasible to transport propane via trucks and store it in tanks for residential use. The required pressure to maintain this liquid state is relatively modest, typically needing about 177 pounds per square inch (psi) at 100 degrees Fahrenheit.
The pressure inside the tank, known as vapor pressure, is directly linked to the ambient temperature outside the container. As the temperature rises, the liquid propane absorbs heat and vaporizes more rapidly, which increases the pressure inside the vessel. This relationship between temperature, pressure, and volume is the fundamental physics governing propane storage. The compression is a necessity that allows for the safe and practical containment of a large energy reserve.
The Journey From Liquid to Usable Fuel
When an appliance is turned on, the stored liquid propane transforms back into a usable gas through vaporization. The liquid absorbs heat from the surrounding air and tank walls, causing it to boil, even at temperatures as low as -44 degrees Fahrenheit. This boiling produces the propane vapor that fills the space at the top of the tank.
Once the vapor is created, it is drawn out of the tank and must pass through a pressure regulator before reaching the appliance. Propane is stored at high internal pressures, often ranging from 100 to 200 psi, which is too high for safe use by household equipment. The regulator reduces this high storage pressure to a low, consistent pressure, typically measured in ounces or inches of water column, suitable for the appliance burners.
A sudden release of liquid propane, such as from a leak, carries a safety risk due to rapid evaporation that causes chilling. Direct contact with liquid propane can cause cold burns or frostbite because the liquid quickly draws heat away from any surface it touches. This is why the system uses the stable, regulated vapor rather than the volatile, high-pressure liquid.