Understanding the Ever Changing Properties of LP GAS
Propane, also known as Liquefied Petroleum Gas, or LP, is a different fuel source than most typical fuels. Diesel fuel and gasoline are liquid fuels while Natural Gas is a gas vapor fuel. LP is a blend of both and has a few unique peculiarities to it. First, LP can be used in either liquid or gas vapor form. Anything in your RV uses gas vapors to run and most portable devices, such as heaters utilize gas vapor. However, liquid withdrawal is used in most engines, such as forklifts because it can be better controlled in their carburetion systems. Yet, both liquid and gas can both be obtained from the same propane tank. To understand how this works let’s take a better look at what propane is.
Propane is technically a liquid. That’s why it’s called LP. However, like any liquid, it has a boiling point. If you fill a bucket with water it just sits there. But, once you put a fire under the water and raise it’s temperature to 212° F, it begins to boil and water vapor escapes (steam). Eventually the water is all evaporated and the bucket gets empty. However, if you place water into a radiator and install a pressure cap, the boiling point of water is raised as the pressure increases. This is also the principle behind geysers, which build up huge amounts of pressure to hold back super-heated water until it just can’t no more, and then it blows.
Propane too, has a boiling point. It’s -44° F. So, if you had a bucket of propane and it was 50 or 60 below outside, technically it would not evaporate. But the minute it goes above that temperature, it wil boil and the gas will evaporate and you’ll eventually wind up with an empty bucket. However, when we pump the propane into an enclosed cylinder, there won’t be any place for it to evaporate. But, we do want to get some of the evaporated gas to run our propane appliances and there is a way to do this. By filling a propane cylinder to no more than 80%, there will be an open area above the surface of the liquid propane where the evaporating gasses can accumulate. Just a little bit of liquid propane will expand into a very large amount of propane gas and that’s the stuff we want to get.
A propane tank that is filled to 80% will have an output port with a shutoff valve that accesses the very top of the tank, where the gas is. If you have a portable tank and were to turn it upside down or lay it on it’s side you would be pumping raw liquid propane through and your grill would probably have flames shooting out of it 6′ high so you always need to use portable tanks in their upright position. On a motorhome, the tanks are mounted horizontally but the pickup tube still goes to the top of the tank. If a propane tank were to be filled beyond 80% you would have a serious fire hazard as the liquid propane was run through the system. For that reason, it’s important to watch the bleeder valve when filling a motorhome’s propane tank. Once the liquid starts to come out it’s time to stop refueling.
Propane has less BTU per gallon than gasoline or diesel fuel so it will require more gallons to do the same amount of work. But propane has a number of benefits over liquid fuels that work well in certain applications. For one thing, propane is stored in a sealed vessel. You don’t have to worry about dirty fuel, water in the fuel, or any kind of algae growth. At the initial tank fill, the LP tank is purged with methanol to remove any water vapors that may be in prior to filling. Because it is a light gas it also is very clean burning. Could you imagine how tasty your food would be if your cook top or outdoor grill ran off diesel fuel? When running engines, including generator sets, propane will be a more costly proposition due to it’s lack of energy related to other fuels. However, it is typically used in indoor applications such as forklifts were the fumes of a gasoline or diesel engine are not acceptable. Propane does have a disadvantage in that it’s low boiling point doesn’t allow it to run very well in cold weather. If you try to draw too much propane out of too small of a cylinder, it won’t be able to keep up in the gas production area. Your gas pressure will drop and your appliances will not burn very well.
Properties of LP Gas
|Pounds per gallon||4.24|
|Specific gravity of gas||1.53|
|Specific gravity of liquid||0.51|
|Cu. ft. gas per gallon liquid||36.38|
|Cu. ft. gas per pound||8.66|
|BTU per gallon||91,502|
|BTU per pound||21,548|
|Boiling point in degrees F at 14.7 psia||-44°|
|Vapor pressure at 0° F||31|
|Vapor pressure at 70° F||127|
|Vapor pressure at 120° F||196|
|Vapor pressure at 105° F||210|
|Specific Gravity of Vapor (Air = 1)||1.50|
|Ignition Temperature in Air, 0° F||920-1120|
Average LP Gas Capacities
|Nominal Size||Actual Capacity
|Lbs. of gas||BTU’s|
|10# cylinder (2.5 gallon)||2.5 gal.||11||237,028|
|20# cylinder (5 gallon)||4.8 gal.||20||430,960|
|30# cylinder (7 gallon)||7.2 gal.||30||646,440|
|40 # cylinder (10 gallon)||9.2 gal.||40||861,920|
11″ Water Column = 6 1/4 ozs. per sq. in. pressure
To estimate how long your LP gas supply will last, simply total the BTU demand of all your gas appliances and the BTU capacity of your containers at 80% full. Divide container BTU capacity by total appliance demand.
*Source NFPA Pamphlet #58-1995
Following is some more info on LP gas. Of particular interest to winter (cold weather) campers are the charts below regarding the available BTU’s from a container of LP gas in an hour. Total the possible BTU’s from your gas appliances and compare to the gas available. It may explain the mysterious winter time heating problems. This is also why your LP fueled generators don’t work very well in cold weather.
LP Gas and Cold Weather Usage
|65 Lb. Undermounted LP Tank
BTU Available at:
|30 Lb. Cylinder
BTU Available at:
|20 Lb. Cylinder
BTU Available at:
Maintaining L.P. Gas Systems
LP gas systems should be checked yearly (at the very least) for leaks and the correct operating pressure. Checking the pressure requires a manometer which most owners don’t keep in their tool boxes and so should be done by a dealer or service center. LP gas systems are designed to operate at 11″ WC (water column) or about 6 1/4 oz. per sq. in., at this low pressure it doesn’t take much of a change up or down to affect how your appliances operate.
Manometers are also used in leak checking. A popular method of leak checking is to use a manometer to monitor for a pressure loss.
The other choice is to turn the gas on and check all fittings with a leak testing solution or soapy water. Do not use anything containing ammonia. One drawback to this method is that you can not check for gas leaking through an appliance valve.
You may want to check the manufacture date of your LP cylinders, even the ones you have at home on your BBQ grill. If they go out of date while you are traveling you may not be able to have them filled. The date is stamped on to the collar of ALL LP cylinders in the form MM – YY. When a cylinder reaches its 12th birthday it must be inspected or recertified. If you find more than one date the most current should have a letter after it, most often an E, this date is good for 5 years.
Effective October 1, 1998, all cylinders with capacities from 4 #’s to 40 #’s must have an OPD (Overfill Protection Device) valve installed or it can not be refilled.
Note: This does not apply to your onboard stationary L.P. Tank of your Tiffin Motorhome. Only portable tanks need the OPD valve.
Submitted by Mark Quasius and Mike Sundberg – 3/15/06