Purging Propane Containers. When, why, and where this procedure should be performed.
By Gary Bunzer
When one or more of a motorhome’s gas-burning appliances exhibit strange, random, or difficult-to-diagnose problems, it may signal the need to purge the propane container. Brand-new propane containers also must be purged properly before being filled for the first time. So what is purging? Does it apply to all RV propane containers? How do you know if your container needs such a service? And who can do it?
Before answering those questions, I’ll define the term “propane container.” Most motorhomes are equipped with horizontal propane containers bolted to the frame of the coach. Typical motorhome propane containers are regulated by the American Society of Mechanical Engineers (ASME), as documented in its Boiler and Pressure Vessel Code. A horizontal ASME container is called a tank. The capacity of an ASME tank usually is expressed in gallons.
Some smaller Type C coaches and older motorhomes may be outfitted with upright propane containers installed in a side compartment; it’s the same type of container usually found on towable RVs or in home gas barbecue grills. Such containers (which can be vertical or horizontal) are called cylinders, and are governed by the U.S. Department of Transportation (DOT) within the agency’s Hazardous Materials Regulations. Cylinders destined for Canadian provinces are regulated by Transport Canada (TC). The capacity of a DOT/TC cylinder typically is measured in liquid pounds.
So, generally speaking, an ASME tank is bolted to the coach, while a DOT cylinder is portable and replaceable. Both types of containers require purging at some point.
ASME tanks do not require a requalification examination, but DOT cylinders do. In the past, DOT cylinders required a first-time requalification inspection 12 years after the date of manufacture; the date is stamped onto the cylinder’s collar. But on January 23, 2017, a new rule took effect. It reduces the initial requalification period from 12 years to 10 years from the date of manufacture. The rule also reduces the period from 12 years to 10 years when the more complex volumetric expansion testing procedure is performed. Also, the requalification period increases from seven years to 10 years for DOT cylinders following a proof pressure test. A visual inspection of DOT cylinders still must be done every five years after the initial requalification testing.
New propane containers are hydrostatically tested for leaks before shipment. A container that is destined for an RV, whether it’s a tank or a cylinder, usually is filled with compressed air (or sometimes another inert gas) when it is shipped from the supplier. Air contains moisture and other contaminants. Purging removes the air, moisture, and contaminants; if the purge process is not performed before the container is filled with propane the first time, combustion problems or delivery line pressure irregularities may develop later.
Unfortunately, most new motorhome buyers don’t know whether their propane container was properly purged prior to that first filling, since a new motorhome usually is delivered to the owner with a full (or nearly full) container of fuel. I suggest asking that question during the walk-through or before officially taking delivery. Ask to see the predelivery inspection (PDI) form for validation. In fact, every new motorhome buyer should receive a copy of the signed PDI checkoff sheet from the selling dealer. Without it, it’s much more difficult to adjudicate a future problem tied to a contaminated container. It’s in the best interest of both the dealer and the buyer to have a proper purging verified in writing before the retail delivery of an RV.
Purging applies to older propane containers as well. In fact, any time a container has been opened to the atmosphere, for any reason, a new purging must take place before refilling with fresh fuel. Say, for instance, the internal float assembly fails, or the shut-off valve leaks and that component needs to be replaced; the tank must be purged after the replacement, since air has been allowed to enter. And that air contains moisture and perhaps other contaminants.
The Moisture Problem
Even a little water lying at the bottom of a propane container can be a problem. Any moisture present from the container to the inlet of the pressure regulator can cause the regulator to freeze up. There are two types of regulator freeze-up, both with negative outcomes.
One type of regulator freeze-up occurs as the fuel in the container becomes vaporized on its pressurized path to the regulator inlet. The pressure regulator has individual sections where the fluctuating container pressure is reduced in two steps to the delivery line pressure required by the appliances. The propane vapor must pass through small orifices in these two stages in order for the pressure to be reduced sequentially and smoothly. Propane’s “boiling point” is minus 44 degrees Fahrenheit. Below that, it exists in liquid form; above that, it vaporizes. So, any remnant of water will freeze in those small internal orifices and block the flow of propane vapor through the regulator.
The second type of regulator freeze-up happens externally during the winter months when mud or water or slush splashes up from the roadway, freezes, and blocks the second-stage atmospheric vent opening on the regulator. If the diaphragm inside the regulator cannot breathe, it cannot regulate, and it simply locks up. That’s why it’s important for the vent portion to be positioned downward and the regulator covered at all times during operation. Watch FMCA’s Motorhome House Calls video (https://goo.gl/jyC57I) for a detailed visual explanation.
The result of either type of regulator freeze-up is no propane flow, and the appliances will shut down just as though the container were out of fuel. Remember, propane is stored and transported as a liquid inside the container, but it is consumed as a vapor at the appliances. Also, moisture inside the container can reduce the Btu/gallon coefficient, meaning the appliances would require more propane gas to create the same amount of heat at their respective burners.
Other problems associated with moisture in the propane container include the following:
- The possibility of rust forming. With continued exposure to moisture, rust eventually can create larger pockets that may result in a breach in the container wall.
- Reduced effectiveness of the odorant in the propane. Propane inherently is odorless and colorless, so ethyl mercaptan is added to it at some point between distillation and delivery to the supplier. The infamous smell of ethyl mercaptan warns of a propane leak. But moisture in the container causes the odor of mercaptan to fade to the point that it might not be noticeable.
Another problem also can arise from failure to properly purge a new container. Attempting to fill a container with propane when it is filled with shipping air carries the risk of creating above-normal internal pressure. In some cases, this could cause the pressure relief valve to open, expelling fuel. On a DOT cylinder, the pressure relief valve is integral to the service valve. On an ASME tank, it is a separate fitting installed into the tank.
When To Purge
A container that has been in use for a while may need to be purged if more than one appliance has operational issues. For instance, the furnace doesn’t ignite right away; the water heater burner sputters; the cooktop burner flame lifts off the burner; or a burner flame is abnormally large or small, or more orange than blue in color. Any operational issue with an appliance is a cause for concern and should be investigated. It simply may be an out-of-adjustment regulator or an appliance in need of cleaning, but it could also be caused by moisture inside the container.
Who To Call
Certified and master certified RV service technicians are trained to properly purge a propane container. When seeking an aftermarket procedure, I recommend verifying with the service department that a qualified individual will perform the purging. As an alternative, most propane retailers also can do the procedure.
As mentioned, new tanks and cylinders typically are filled with air when shipped, but containers already in service will contain propane. In the case of the latter, before purging, it will be necessary to remove or burn off the container’s remaining propane. To eliminate a costly waste of fuel, try to schedule a purging service when a container is nearly empty. In some cases, however, the fuel can be captured and reused.
How To Purge
Purging is performed with propane vapor only, never with liquid propane, even though propane is stored as a liquid in the container. The purge process begins in a safe, controlled area, away from heat and spark-producing equipment. Though only fully qualified RV service techs or propane retailer personnel should perform these procedures, it’s important that motorhome owners are familiar with them. They are as follows:
- The container pressure should be at 0 psi. If a new container is filled with air, it is simply released to the atmosphere through the service valve or the fixed maximum liquid level gauge valve (also called the 20 percent valve).
- If moisture is prevalent in the tank, it is drained through the service valve (DOT/TC cylinders only). For ASME tanks, a drying agent can be introduced into the tank if necessary.
- The container is pressurized to 15 psi of propane vapor only.
- The service valve or the 20 percent fixed maximum liquid level valve is opened, and the propane vapor at 15 psi is released. Note: Some specialty purge equipment is outfitted with a dedicated vapor return hose that nullifies releasing the vapor to the atmosphere.
- The container is again pressurized to 15 psi, and the pressure is relieved or released.
- This process is repeated for a total of five purgings.
- On the fifth pressurization, the 15 psi of propane vapor is retained in the container. All fittings, valves, and container appurtenances are then leak tested.
The container then can be filled safely with liquid propane to its normal 80 percent capacity. The remaining 20 percent is reserved for vapor withdrawal and expansion. Liquid propane boils and vaporizes at the top of the container before entering the pressure regulator. Overfilling with liquid propane is hazardous and leads to regulator and appliance malfunctions. Thankfully, all new propane containers have safeguards in place to help stop the flow of incoming liquid propane during the filling process.
What Owners Can Do
Since safety is paramount, I recommend an annual test of the pressure regulator, as well as the entire propane gas distribution system, by a certified RV service technician. Check the condition of the container. In addition, the coach owner should test the propane leak detector periodically.
If the propane’s odorant is ever detected, immediately turn off the container’s service valve and schedule the coach for tests. Keep in mind that throughout the purging process and subsequent refilling of the propane container, the odor of the mercaptan may linger a little longer than normal, since the procedure allows propane to vent directly to the atmosphere.
If you’re in the market for a new motorhome, be sure to ask the dealer about the propane purging process on that new tank. And owners of existing coaches should have the propane system tested annually and keep a close eye on how well the appliances are operating. Remember, RVing is more than a hobby; it’s a lifestyle!