Why Bleed the Air Out of a Propane Tank
About Propane Tanks
Propane tanks are reinforced storage vessels designed to hold liquid propane at extremely high pressure. They can be small enough to carry around or large enough to hold thousands of gallons. Propane is highly flammable, and the most common uses for propane tanks are to provide fuel for barbecue grills, home and RV heating systems and industrial equipment.
Relief Valves and Propane Tank Pressure
Because propane tanks store liquid propane at such high pressure, they are fitted with safety relief valves. When the pressure inside a propane tank rises to a level that may be unsafe, air will automatically bleed out through this valve. In most cases, pressure rises inside propane tanks when they're left outdoors on hot days, which heats up the air trapped inside the tanks. The propane, which is heavier than air, sinks to the bottom of the tank, while the air fills the top of the tank near where the safety relief valve is installed. Without a safety valve, the pressure of the hot air trapped inside a tank can reach dangerous levels under high-temperature conditions. If these pressure levels build to a breaking point, the tank could rupture, causing a violent explosion and producing a great risk of fire.
How Propane Relief Valves Work
The stem of a propane tank safety-relief valve is wrapped in a heavy metal spring. The spring is calibrated so that its pressure will keep the relief valve closed whenever pressure conditions inside the tank are normal. If and when the pressure rises to a point where it equals the tension pressure on the spring, it allows the valve to open slightly, which automatically bleeds air out of the tank and relieves the pressure. An audible hissing noise may be heard coming from a tank that is releasing pressure in this way. The valve will only open completely if the pressure inside the tank surges significantly. When the valve opens all the way, it often makes a loud popping sound, and the pressurized air is released very quickly. Although this is by design as a safety precaution, the somewhat violent action of a safety valve that springs open suddenly can damage the valve. Propane tanks with relief valves that have fully opened should be inspected immediately for damage and possible service by a trained technician.
