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See also the discussion on energy: rvtech/energy/ - The RV dealers association has a good rundown at http://www.rvda.org/rvtechnician/lpgas.htm -
Propane is the fuel of choice for RV heating purposes. It can be handled and stored as a liquid which is convenient for commerce and energy storage capacity yet is easily converted to a gas to facilitate mixing with air for combustion. There are a lot of rules and regulations about the handling, use, and storage of propane in order to promote its safe use.
Propane supplies have a perfume added so that leaks are readily noticed by smell.
Storage tanks for vehicles are usually rated by the weight of the propane that they will store. At a bit over four pounds a gallon, a medium sized thirty pound tank will hold about seven gallons. A forty pound large tank will hold about ten gallons. All propane tanks are designed to be fully filled when liquid level is at the 80% mark. This leaves a fifth of the tank available for expansion due to heating, if necessary.
To keep propane a liquid requires that it be kept under pressure so propane tanks have to be built to withstand this pressure. This pressure also means that feed systems do not have to depend upon gravity or pumps to get the fuel out of the tank. Tanks are supposed to be inspected every few years to make sure that they still appear able to handle the pressure. You can see a date stamp of the last inspection near the top of your tank.
If propane is taken from the tank near the top, it comes our as a gas. If it is taken out from near the bottom of the tank, it comes out as a liquid. RV's use propane as a gas. Forklifts, high capacity generators, and other large fuel flow needs use propane bottles designed to feed liquid propane.
Trailers usually have vertically mounted propane tanks on the trailer tongue. Motor homes and propane powered vehicles will likely have horizontal tanks underneath the floor. Each type of tank will have special filling apparatus and valves to assure only 80% filling and proper feeding (liquid or gas).
Overfill Protection Devices are a new requirement for portable (40 pound or less vertical) propane tanks. These valves have a tube going down into the tank with a float to stop filling when the tank is 80% full of liquid. The also have an easy to use shutoff valve on the top and quick connect threads. Older style backwards threaded POL connectors can still be used with these new valves.
Mounted near the propane tanks is a regulator to reduce the fairly high propane gas pressure being fed from the tank to a uniform low pressure just enough to back water up a pipe eleven inches high. In trailers with two tanks, these regulators may also have a switch to automatically convert from an empty tank to a full one so you can refill the empty one without interrupting the propane supply and having to re-light all the pilot lights.
Propane pipes in RV's are usually copper with flair fittings. The pipe size is usually around a half inch with a size smaller pipe used for single appliances and a size larger for main feedlines. As much of the piping as possible is routed on the outside of the RV so that any leaks won't cause a buildup of gas inside the RV. There will also normally be a shutoff valve for each appliance on the pipe before it heads inside.
Propane has to be mixed with air and ignited to burn and produce heat. Safety mechanisms in gas appliances try to make sure that propane is not released unless it is burned. The manual method is often used for RV stoves - turn the knob and touch off the burner. Pilot lights are going out of style but they used a small flame to keep a thermocouple warm. When heat is needed, a valve that needs both a thermostat and the thermocouple to agree that it is OK opens up to the burner that is lit by the pilot. The most common modern method is to electronically generate a spark to ignite the burner after it has been turned on.
All of the automatic systems use some sort of sensor to make sure that the propane is being burned. If the thermocouple is cold or fails in pilot system it will shut off the gas. The automatic systems are also designed so that any failure will shut off the gas. Automatic systems typically need energy from the battery so they can add to battery drain and won't work on a dead battery.