Propane – OSI Mobile Technologies

Propane is the fuel that allows a recreational vehicle to have many of the conveniences and comforts of home without being constantly “plugged in” to an electrical source. Almost all RVs have propane tanks which can run an absorption refrigerator for a month on a full tank, heat water, run a gas furnace, and supply fuel for a gas range. Propane has proven to be a safe and effective fuel for millions of RVs, however there are misunderstandings and confusion among many RV owners about how propane systems operate. This article will focus on the most common issues that face the RV operator.

We will first start with terms and definitions in relation to RV propane systems. “Propane” is the most commonly used term to refer to LPG (Liquid Petroleum Gas) fuel. Actually the propane stored in an RV tank or a BBQ grill cylinder is about 90% pure propane with the remainder composed of butane followed by smaller amounts of ethane and propylene. Propane exists as a liquid under high pressure in the storage tank, and it is boiled off (vaporized) before it leaves the tank to supply gas vapor to an appliance. Propane is a gas at normal atmospheric pressure above -44° F. To store propane as a liquid at 60° F it must be under 92 pounds per square inch pressure which requires a robust tank. This pressure increases to 197 psi at 110° F.

The most important component of the propane distribution system is the regulator. In RVs, the regulator assembly is actually a 2-stage regulator where the first stage takes the high pressure gas and reduces it down to an intermediate pressure (around 10 psi). The second stage of the regulator reduces the pressure down to a low level usually measured in inches of water. The final outlet pressure standard is only 11 inches of water (this is the pressure to push a column of water vertically by 11 inches) which is less than 1 psi. Regulator pressure should be periodically checked since this pressure (11 inches of water) is critical for proper operation of gas appliances. There are no repairs to propane regulators, if a regulator is out of specifications or suspected of malfunctioning it should be replaced immediately. Dual stage RV propane regulators are not expensive and can be found at almost any RV supply store.

Point #1: The regulator does not change liquid propane into a gas. The liquid propane actually boils in the tank to create vapor at the top of the tank. The pressurized vapor is fed into the regulator. All tanks have a specified operating orientation where vapor on the top of the tank is withdrawn. Feeding liquid propane, such as laying a tank on its side, into a regulator can actually damage it and may produce a dangerous over pressure situation.

Point #2: A propane tank should never be filled to more than 80% to allow for expansion and to have a vapor space on top of the tank where gas can be withdrawn to the regulator.

The next topic of tank designations probably produces more confusion with RV owners than anything else. There are 2 styles of propane containers: ASME and DOT. These refer to the standards organizations that regulate these containers. “ASME” stands for American Society of Mechanical Engineers and “DOT” stands for the Department of Transportation. ASME tanks are intended for permanent stationary mounting, usually found in a motorhome. DOT cylinders are removable and transportable and are usually found on travel trailers. The proper nomenclature for ASME containers are tanks and DOT are cylinders although most people use these terms interchangeably. To add to the confusion, ASME tanks are rated in gallons of propane and DOT cylinders are rated in pounds of propane. The conversion between these is 1 gallon is equal to 4.2 pounds of propane. It is not uncommon to hear RV operators talk about having 40 gallons of propane on board who actually have two 20 pound BBQ grill style tanks. The proper conversion in this instance is 40 pounds of propane or 9.5 gallons. Another legal requirement is that all DOT cylinders must be re-certified after 12 years, and then every 5 years after that. ASME tanks are made from thicker steel and do not require re-certification.

Point #3: ASME tanks are permanently mounted to the frame on motorhomes. The capacity is rated in gallons. There is a separate fill valve from the vapor withdrawal valve. The fill valve is a check valve, propane cannot be withdrawn from this port. There is a 80% POL service valve which is opened during filling which indicates an 80% liquid level is reached when vapor appears. RV tanks have an internal shutoff device to avoid pumping in more than 80% capacity (similar to a DOT OPD device) but can stick open (I have observed this!) resulting in overfilling unless the technician is also properly monitoring the POL overfill service valve. ASME tanks do not require periodic certification but common sense should be used for inspecting excessively rusty tanks and/or tanks that have been involved in an accident.

Point #4: DOT cylinders are not permanently mounted and are removed for refilling. Their capacity is rated in pounds of propane. The vapor service valve and fill valve are the same. Cylinders with 4 to 40 pounds of capacity are required to have an OPD (overfill protection device) to prevent overfilling. Re-certification of cylinders is required after 12 years of the initial manufacturer date and every 5 years thereafter.

There have been instances reported on RV forums of overfilling and propane service centers refusing to fill older RV tanks. A properly trained technician will fill an ASME tank with the 80% overfill valve open and will terminate the filling as soon as liquid begins to spray out of the valve. If the internal overfill cutoff device is working properly it will also stop the flow of liquid propane into the tank at the 80% level automatically. The small dial gauge included on most tanks should not be used to determine the proper fill level, as these gauges are simply not accurate enough to determine the proper fill level. There are also instances where propane service stations refuse to fill an ASME tank because of its manufacturing date being older that 12 years. There is no legal basis to refuse to fill an ASME tank that is older than 12 years except if the technician determines it is in too poor condition for propane storage. The 12 year certification requirement comes from the confusion between DOT cylinders and ASME tanks. If a “trained certified” operator cannot tell the difference between DOT cylinders and ASME tanks, it will be prudent to take your propane business elsewhere.

Another safety feature found on Acme screw-on connectors (right hand twist to tighten / hand tighten only) used with DOT cylinders is the excess flow valve. The purpose of this valve is to limit the flow of high pressure propane in the event of a severe leak or line break. It is a simple valve with a small spring loaded metal ball. When there is excessive gas flow the pressure differential will cause the valve to shut down the flow (but not entirely) until the pressure builds back up on the outlet side. However this valve sometimes causes issues with RVs. If the cylinder valve is suddenly turned on with no other pressure in the distribution system this valve can activate. If the stove or some other appliance is turned on and then the main cylinder valve is opened the pressure drop can be enough to activate the excess flow valve. Also if there is a line break in the propane system this will greatly reduce the flow (as it is designed to do). If your complaint is the stove really doesn’t heat up very well with a good flame or the furnace barely is heating, then check to see if the excess flow valve activated. First verify there are no leaks in the system and all appliances are off. Turn off the main cylinder valve for several seconds (in a quiet environment a click can be heard indicating the valve has reset). Then slowly turn on the main valve and check for proper propane flow on a stove burner.

Point #5: Propane tanks and cylinders do not spontaneously “blow up”. All DOT cylinders and ASME tanks are fitted with pressure relief valves. Industry standards, modern manufacturing quality controls and certifications / re-certifications assure container integrity. Unless the container is penetrated by a high powered rifle bullet or a very severe impact it is almost impossible for a cylinder or tank to explode. DOT tanks have a pressure relief valve set at 375 psi, ASME tanks at 312 psi. The most common causes of a pressure relief valve opening are severe overfilling where a temperature change causes the internal pressure to increase above limits, or very high temperature as in the case of a vehicle/RV fire. In the case of overfilling, RV owners report that the pressure relief valve opening can sound like a gunshot but in this case it is not an explosion- the valve is doing what it is designed to do. There are videos of RV/vehicle fires on the web that have on-board propane tanks. When the tank/cylinder gets hot and the internal pressure builds enough for the pressure relief to activate it usually shows up dramatically as a flamethrower, but not an explosion. All tanks and cylinders have pressure relief valves, even 1 pound camping lantern cylinders.

There is one last comment which involves purchasing propane. DOT cylinder exchange services at big retailers have gained popularity. Warehouse clubs also promote cylinder refill services at seemingly greatly discounted prices. The key word here is seemingly since the consumer is not always getting what they think they are paying for. Almost all of these discount store and easy exchange services only fill 20 pound BBQ cylinders with 15 pound of propane. They promote this is a “safer” fill level for the consumer where in reality a 20 pound cylinder is actually rated for 20 pound of propane at 80% of its volume filled. Always read the fine print to see that these businesses are actually only filling cylinders to 15 pounds of propane. Traditional hardware stores, farm supply stores, and propane dealers will sell propane by the gallon. For example, a fully empty 20 pound cylinder will take 4.7 gallons to fill. The price of propane from an filler that sells by the gallon is roughly the same as for a gallon of gasoline. Note that many of the 15 pound cylinder exchange sellers actually end up costing 2 or 3 times the normal retail rate for a gallon of propane.