With a better understanding of the components and operation of your motorhome’s propane system, you can enjoy the benefits of this compressed fuel with confidence.
By Gary Bunzer
Propane Safety System — PDF includes diagrams of an ASME horizontal motorhome tank and a pressure regulator.
You own a motorhome, so you are probably mindful of the benefits of traveling with a container of propane that supplies fuel for many of the vehicle’s appliances, even though you may have some trepidation about it as well. Although flammable, propane is a relatively safe commodity. With a little preventive maintenance and some common sense, it can be utilized without fear.
As used in the motorhome realm, propane is a true gas that conforms to the principles of Boyle’s Law. This law states that the pressure of confined propane (and other true gases) is related to its temperature when the volume is constant, and if the volume varies, as in a motorhome application, it directly affects the pressure and the temperature. Do a quick Google search for “Boyle’s Law” and you can while away a whole afternoon or more. When propane vapor expands as the pressure is reduced, it expands in all directions, and quickly. This is why a small external propane leak should not necessitate a full-blown panic. Even though it is heavier than air, propane gas disperses quickly in real-world applications. Its expansion rate is approximately 270 to 1, which means 1 gallon of liquid propane in the tank will expand to 270 gallons of propane vapor when it boils. In addition, it has a rather narrow flammability zone. To burn, the propane-to-air mixture must be between 2.5 percent and 9.6 percent. Yes, it demands respect, but certainly not fear.
Propane gas is odorless and colorless in its natural state, so in order to alert users to a leak, an odorant called ethyl mercaptan is added during the distilling process. This sulfur compound gives the gas a rotten egg or skunk oil odor. A few pounds of mercaptan typically are added to every 10,000 gallons of propane. This makes propane distinguishable in concentrations one-fifth of the lower limit of combustibility, as required by NFPA 58, the Liquefied Petroleum Gas Code. RVers should be able to readily detect the odorant in propane.
The bottom line is propane can be compressed to a liquid, can be stored easily, and is relatively safe, making it the perfect fuel for use in a motorhome.
Let’s look at four major components of the typical motorhome propane system: the container, the pressure regulator, the distribution piping, and the appliances.
Most motorhomes are equipped with an American Society of Mechanical Engineers (ASME) propane tank. Some smaller Type C motorhomes or older coaches may be equipped with twin Department of Transportation/Transport Canada (DOT/TC) cylinders, common predominantly to towable RVs. ASME tanks usually are bolted to the frame of the motorhome, oftentimes exposed to the elements. DOT/TC cylinders usually are housed in a dedicated storage compartment, fully isolated from the living portions of the motorhome.
Unlike DOT/TC cylinders, ASME tanks require no recertification inspections. Since most FMCA member coaches are equipped with a typical ASME tank, we’ll focus only on that type of container here. (Feel free to contact the author — email@example.com — if you have specific questions relating to DOT/TC cylinders.)
All ASME tanks incorporate specific external components and fittings, including the main service valve, also called the vapor withdrawal valve; a separate fill valve; a fixed maximum liquid level gauge (20 percent outage valve); a safety relief device; a visible sight gauge (usually equipped with a remote sensor); mounting brackets; and, oftentimes, protective plates.
In addition, other devices are located inside the tank, such as the backflow check valve, the overfilling prevention device (OPD), an excess flow check valve, a liquid level float assembly, and internal tubes for vapor withdrawal and the maximum liquid level gauge.
Note: OPDs have been required on ASME tanks only since 1984, so older motorhomes may not be equipped with an OPD.
All motorhome owners should be able to locate the external tank components readily, as periodic inspections are important for the overall safety of the propane system and the motorhome. Everyone who travels in the motorhome should know where the main service valve is located. It’s the first thing you’ll turn off if the odor of ethyl mercaptan is detected inside the motorhome. Turning this valve fully clockwise will shut off the propane flow to the entire system.
On ASME tanks built from 1993 onward, the excess flow check valve is integral to the service valve. In the event of a collision that sheared the connection at an open service valve, this check valve would restrict the gas flow through the valve when it exceeded a preset limit of 200 cubic feet per hour. On tanks built prior to 1993, the excess flow check valve was incorporated into the left-hand-threaded POL fitting, the fitting attached directly to the service valve outlet. Excess flow check valves are required by NFPA 1192, the Standard for Recreation Vehicles.
The safety relief valve used prior to 1993 was also integral to the service valve itself. Since then, it is a separate component attached to the tank. Safety relief valves inside the service valve will open when the tank pressure exceeds 375 psi, while the independent safety valves will open at 312.5 psi. On newer motorhomes, the only consumer requirement for the safety relief valve is to be sure the plastic dust cover remains intact.
The backflow check valve, sometimes called a double backflow check valve, is a mechanical device used in conjunction with the fill valve that permits the flow of vapor or liquid in only one direction. Backflow check valves are primarily engaged when filling ASME tanks and to prevent propane from escaping into the atmosphere after the tank is filled. As a safety measure, both the fill valve and the excess flow check in the service valve will prevent gas from escaping if either is accidentally sheared off during a collision.
The OPD is a float-operated, automatic shutoff of sorts, also integral to the separate fill inlet fitting on the tank. It limits the filling of the tank to its maximum of 80 percent. Overfilled propane containers can cause a plethora of problems that manifest themselves at the regulator and the individual appliances. Severely overfilled tanks can contaminate the entire piping system and render appliances useless.
The fixed maximum liquid level gauge, which is often inaccurately called a 10 percent valve, is visible on the outside of the container. It’s not really a gauge at all, but rather a threaded valve that indicates when the tank reaches the 80 percent level during filling. Propane retailers will open this valve while propane is being pumped into the tank and will stop the filling process as soon as liquid propane emits from the opening, hence its name. Consider it a redundant check along with the OPD to minimize the possibility of overfilling the container.
As RV owners, we can verify the tank was filled properly by opening this valve immediately after having the tank filled, when parked on a level surface. If liquid appears (as a white mist or fog), it’s overfilled. If only vapor escapes, it was filled to the correct level, a maximum of 80 percent.
Although we store propane as a liquid, the appliances inside the motorhome burn propane vapor. Some ASME tanks are also equipped with a purely liquid withdrawal connection to supply propane-fired engines and/or RV generators that may require liquid propane instead of vapor.
The heart of the propane system is the main pressure regulator, located near the connection at the service valve outlet. I consider it one of the hardest-working devices on a motorhome. That’s why I recommend that motorhome owners carry a spare regulator while traveling. If the regulator is rendered faulty, the result will be no comfort heating, no hot water, no cooking, and, potentially, no refrigeration.
Today’s pressure regulator is a two-stage device that reduces the varying tank pressures, which can exceed 250 psi, to the steady pressure required by the appliances. Reducing this pressure in two steps helps to eliminate regulator freeze-up and allows for a smooth delivery of fuel. The first stage reduces container pressure to approximately 10 psi to 15 psi. The second stage reduces it further to 11 inches of water column, about four-tenths of 1 psi. That’s not a lot of pressure. Then again, not much is needed, but it must remain steady, which is the never-ending job of the pressure regulator.
From this point on, think of pressure in terms of water column inches instead of psi. A water column inch is a much smaller unit of measurement used to quantify the rather slight propane operating line pressure. The analogy would be like measuring distances in either miles or inches. Each would be accurate, but the inch measurement would be more exact. For years the four propane appliances found on typical motorhomes have been designed to operate at a delivery pressure between 10 and 14 water column inches. For optimum appliance operation, therefore, the operating line pressure is set by professional RV service technicians at 11 water column inches, which falls about in the middle of this designated spectrum.
Many of today’s more efficient appliances require less pressure and less fuel to operate at their optimum. These appliances may include their own internal regulator to further reduce the 11 water column inches of pressure to what is required by the appliance. These so-called “third-stage” regulators are nonadjustable. The common denominator among the four propane-burning appliances is the main regulator located at or near the propane container.
An important thing to understand about the regulator is that it must be able to “breathe” as the internal diaphragm expands and contracts inside the regulator body. That is, as fuel enters, the diaphragm expands slightly into an upper chamber above the diaphragm. These upper chambers are exposed to the outside atmospheric pressure through a vent in each stage. If the chamber were not vented, allowing air to be expelled as the diaphragm flexes, it would not be able to move. Here’s the key: the vent openings for each stage must be kept clean and free from moisture, dirt, mud, snow, or anything that may plug or block them.
Typically, the first-stage vent opening is a fairly tiny hole in the regulator housing. It doesn’t take much to block it. Likewise, the vent portion of the second stage, though more prominent, is also critical for proper operation and must be kept clean. In fact, it is a mandate that the second-stage vent always be positioned downward (within 45 degrees of vertical). This position minimizes the possibility of dirt or mud collecting on the vent screen.
If you buy a replacement regulator, be sure the vent configuration is correct for your application. A recent trip to a local RV parts and accessory store sadly revealed only one style of regulator in stock that had the vent portion positioned in only one direction. Even more disappointing, the RV store manager was not even aware of the different configurations available, or why.
Some unwary owners may find they have a vertically designed regulator (for DOT cylinder applications) installed in a horizontal position, whereby moisture may enter the regulator body, shortening its life. Check the vent opening on your regulator and ensure it is positioned downward. For regulators mounted vertically, the vent (dime-size part) must be on the long axis of the regulator. For regulators mounted horizontally, the vent must be perpendicular to the long axis of the regulator. The key here is that the vent be pointed downward within 45 degrees of vertical. Unfortunately, this may be impossible when a vertical regulator is installed horizontally, which happens more often than one would think.
Finally, and most importantly, always keep the regulator protected with a plastic cover. If your regulator cover has become lost or damaged, pick up another during your next visit to a service center. This is the single most effective method of protecting the regulator.
Downstream of the regulator, propane fuel is delivered to the appliances through a variety of pipes, hoses, and tubing. You’ll likely find an assortment that includes black iron pipe, copper tubing, and flexible rubber hoses. All connections must remain leak-free, and since motorhomes twist and rack while going down the road, it’s important to have the propane system tested for leaks periodically. Fittings and connections can and do work loose over time.
Black iron pipe manifolds may be secured along one of the frame rails of the motorhome with branch lines routed to each appliance via copper tubing. In the case of appliances mounted in a slideout, flexible hoses may be used to tap into the main manifold.
All joints and connections in black iron pipe will employ SAE male and female pipe threads requiring a propane-approved sealant of some type. Sealant, by the way, should be applied only to male threads. Not all sealants are approved for propane use. As an example, many of us have seen white Teflon tape used as a sealant for plumbing fittings at home or on a motorhome. Typically, the common white Teflon tape is not approved for LP gases. Yellow Teflon tape is approved for combustible gases. The intent here is to remind everyone to carefully read the product label to verify that the specific sealant can be used on propane connections.
Flare connections for copper tubing require no sealant on the brass-to-brass fittings. Flexible hoses may be outfitted with flared or pipe connections at either end. It is imperative that a propane leak test be performed whenever any connection in the piping system is disconnected or “opened.” Motorhome owners are encouraged to bubble test each fitting after reconnecting any propane line, tube, or hose. A bottle of inexpensive children’s blowing bubbles is a cheap alternative to the more expensive combustible gas leak detection fluid. Avoid using liquid detergent that contains ammonia or chlorine derivatives, which can damage brass fittings.
Periodically check the connection of the piping and tubing at the various attachment points along their routes. With the proliferation of slideouts today, it’s not uncommon to find an appliance that ceases to work with the slideout in one position or the other. An improperly secured flexible propane hose or one that kinks when the room is in either the extended or the travel position is typically the cause.
Even though many coach manufacturers are designing all-electric motorhomes, by far the majority of motorhomes are equipped with three or four propane-burning appliances. A cooktop/oven, refrigerator, furnace, and water heater allow users to be totally independent and self-sufficient, ready to travel anywhere.
Although few problems develop within the cooktop or range, annual preventive maintenance on the other three appliances is paramount to long-term, trouble-free RVing. A definitive set of procedures should be performed on each appliance at least once per camping season in order to optimize the propane system. Cleaning and servicing procedures will include partial or complete removal, disassembly, and cleaning of crucial components found within. Appliance orifices, burners, vents, ducting, etc., should be cleaned, tested, and verified annually. The last thing you would want is to have the water heater go out in the middle of a dry-camping excursion, for example.
Certified or master certified technicians are trained in the exact procedures for each appliance. When it comes to your motorhome’s propane system, trust only professional service technicians. I have written detailed consumer-oriented procedures for the major appliances for previous issues of FMC magazine, so a quick search through the online archives at www.fmcmagazine.com may help you in that endeavor if you choose to perform some of these preventive procedures yourself.
Rate Of Vaporization
One confusing aspect concerning appliances that do not ignite or perform properly is a phenomenon called “container refrigeration.” It is possible to exceed the rate of vaporization, which is similar to the symptom of running out of fuel, when in fact the tank still contains fuel. This is because vaporization cools the propane to the point where there is little or no container pressure. Propane tank refrigeration becomes more of a concern as the ambient temperature drops. In other words, in order for all appliances to function in very cold winter climates, a full propane container may be required to maintain sufficient container pressure. (For additional tips on winter RVing, see the “Winter Travel” article on page 50 of the November 2009 issue of FMC magazine.)
As a rule of thumb, when using a motorhome in very cold climates, keep the propane level in the container above 50 percent at all times. The liquid propane, as you’ll learn if you dig into Boyle’s Law, must absorb heat from the atmosphere to boil and produce vapor to operate the appliances. The heat transfer occurs primarily through the container walls as wetted by the liquid inside. The lower the fuel level, the less wetted surface there will be inside the container, which reduces the vaporization rate.
Propane System Maintenance And Safety Practices
Okay, so what can RV owners do to remain proactive when it comes to propane utilization and safety? Actually, there’s quite a lot. See whether you concur.
1. Remember, safety is always the highest priority. Propane on skin is so cold it can cause serious burns. Always wear personal safety gear such as rubber gloves and eye protection when working on or near the propane system.
2. Never allow the ASME tank to be overfilled. Granted, it’s not always possible to watch or be nearby while the tank is being filled, but it’s always possible to check the fixed maximum liquid level gauge after every filling and before you open the service valve. Always alert the retailer immediately if the container appears overfilled.
3. Never use a wrench to forcibly tighten the service valve or the fixed maximum liquid level gauge. If either leaks during normal hand manipulation, it’s time to have that component replaced.
The service valve incorporates O-rings around the stems and working parts. If a leak develops around the stem of the service valve, it often can be corrected by plugging the outlet with a brass or plastic POL plug (that left-hand fitting) and opening and closing the service valve two or three times. The movement of the O-ring back and forth on the seating surface wipes the surface clean of any impurities.
4. Open the service valve fully and listen immediately at the pressure regulator for a certain sound. There should be a distinctive “phfht” sound, indicating the regulator has attained its lockup pressure when the service valve is first opened with the appliances turned off. If a consistent “hiss” or humming sound is heard, this means a large leak may be present and the system should be leak-checked before being put back into service.
In certain applications, the service valve may actually seep a small amount of propane vapor from around the stem during its travel from fully closed to fully open. When the service valve is closed, a lower seat closes to prevent gas from leaking out of the system. When the valve is opened all the way, the valve stem presses against an upper seat to stop any gas leakage. This sealing action at the “open” position is sometimes called “back-seating” and applies to most gas valves. In this manner, the bonnet and stem are both protected from leaks in the fully open and fully closed positions. It is recommended to open the valve fully into the back-seated position whenever the service valve is turned on and the appliances are being used. If an O-ring leak cannot be corrected by working the valve back and forth, it will be necessary to replace the service valve.
5. Bubble test any fitting or connection if the system has been opened for any reason. When tightening propane connections, always use two wrenches: one to tighten the fitting and one to hold fast its mating component.
6. Check the tank mounting hardware periodically. If a mounting bolt needs to be replaced, always use the same size and grade of bolt.
7. Inspect the tank annually. Look for dents, gouges, scrapes, rust, or any other physical damage. An oily residue anywhere around a tank fitting or on the tank itself may indicate a leak or a pinhole in the container. Contact a certified service technician or a propane retailer if any doubt exists.
8. Inspect and clean the pressure regulator at least once per camping season and immediately after driving through mud, slush, or snow. Verify the vents are clean and free of debris. Always keep the regulator covered.
9. Annually, have a certified technician perform a timed pressure drop test on the entire propane system and have the regulator pressure adjusted to 11.0 water column inches. A regulator lockup test should also be performed every year. Propane regulators can wear out over time, so always carry a spare. Replacement regulators are preset at the factory to within the 10 to 14 water column inches requirement. Simply have it verified when you get back to civilization.
Setting the pressure (adjusting the regulator output pressure), measuring the lockup pressure, and performing the timed pressure drop test should be attempted only by a certified RV technician. There are specific steps involved and special equipment needed to perform these three tasks properly.
10. Have the refrigerator, furnace, and water heater cleaned and serviced each year or every other year at the very least. If you read the fine print on any appliance manufacturer’s literature, it will state this mandate somewhere in the verbiage. As conscientious coach owners, it is to our benefit to optimize every major system on the motorhome even though such preventive maintenance usually is not covered by any warranty or extended warranty plan.
11. Test the operation of the propane leak detector inside the motorhome. Simply open the valve of a small butane cigarette lighter near the sensor on the leak detector panel. Do not ignite the lighter; just open the valve to release a small amount of butane. The leak detector should sound within a few seconds.
By adhering to these simple maxims, and having professionals perform the other required services, you will be doing all that you can to maintain a safe, well-performing propane system. Just remember, RVing is more than a hobby; it’s a lifestyle!