Hooking up a refilled LP cylinder? Check this out!


By Russ and Tiña De Maris
If you own an RV and it’s not a motorhome, chances are close to 100 percent that you have portable LP containers — properly called cylinders — that you remove to have filled. Those nifty little containers are great — you don’t have to move the RV out of camp when you run out of LP, just haul them into town for a fill-up. But the hang-up can sometimes come when you go hook those cylinders back up to your rig — sometimes the gas just won’t flow. What’s the problem?

Most states require your cylinder to be equipped with an OPD, or “overfill protection device.” You’ll know if you have one by the distinctive three-cornered shut-off valve. The theory is that cylinders should not be filled beyond 80 percent of their rated capacity. The extra 20 percent allows for safe expansion of LP that might occur, for example, if you filled your container in a cold climate and then immediately headed somewhere hot. The OPD valve would provide a safety margin for heat-caused expansion of the gas. In practice, not all OPD valves cut off the flow of LP at the 80 percent mark, or so say some LP filling station operators.

Inside of a cylinder equipped with an OPD valve is a small float that is pushed up as the level of LP rises in the cylinder. When the 80 percent level is reached, bingo! The float closes off the valve, stopping the inward flow of LP. But there can be a problem. You might hook up your freshly filled cylinder and be unable to get your gas to flow out. Some blame this on a “hung up” OPD float and recommend inverting the LP cylinder to solve the problem. DON’T! While the chance is slight, you could get liquid propane into your gas line, rather than the gaseous form of LP. Liquid propane in a line is a fire or explosion hazard waiting to happen.

If you get a “hung up” LP cylinder, there are a couple of things to do. First, close the cylinder service valve and disconnect the gas line fitting. Reconnect the fitting and SLOWLY open the gas service valve, as in “barely crack the thing, then slowly turn it wide open.” Why? We’ll come to that in a minute. If it still doesn’t help, close the service valve, disconnect the gas connection, and locate a solid surface, like a smooth concrete walkway or solid earth. Make sure there aren’t any sharp objects like pointy rocks in the way. Grasping the cylinder by the safety collar up topside, give the cylinder a controlled drop onto the hard surface a couple of times. This often releases the “hang up,” and you’ll be good to go.

Now, about that problem that you may have resolved by simply opening the valve slowly: That’s potentially an issue with your “pigtail.”

The assembly that connects your propane cylinder to the LP pressure regulator is often called a pigtail. That’s a throwback to the days when those assemblies were primarily a brass fitting connected to a coil of copper line. To allow safe expansion and contraction, and to reduce the chances of breakage due to vibration, the copper lines were coiled, resembling a pig’s tail. In the late 1970s, fire codes changed and mandated pigtails be made from rubber (later thermoplastic) tubing. Even then, the actual fitting to the cylinder valve was a left-hand thread fitting that screwed into the inside of the LP service valve, known as a POL valve. Why POL? Ah, an acronym for the company that originally developed them, Prest-O-Lite.

U.S. Patent Office Illustration

But when the new OPD valve became required, a new fitting was recommended, called ACME. No acronym here, and as far as we know, no relation to the company that supplied Wile E. Coyote all his stuff to go after the roadrunner. ACME nuts are plastic, right-hand threaded fittings that go over the outside threads on an OPD valve. They should be tightened by hand to prevent damage, and are easy to use.

ACME fittings are equipped with two safety protocols. First, they have a heat sensitive thermal bushing that if overheated, shuts down the flow of gas. Great for barbecue grills. Second, there’s an excess flow check valve. When the flow of gas is first allowed through the ACME fitting, the check valve closes, allowing just a small amount of gas through the fitting. This gas pressure builds up in the lines on the far side of the ACME fitting, and provided there aren’t any major leaks or broken lines, pressure builds up in the line, backing up against the check valve. When that happens, the check valves open fully to allow the maximum flow of gas.

If you open the cylinder service valve too quickly, the check valve will sometimes hang up and not allow the free flow of LP. Hence, the recommendation to close the valve, disconnect the fitting, reconnect, then slowly open the valve. Why disconnect the fitting? If you get a spurt of gas when you disconnect the fitting, you’ll know two things: First, that gas is actually getting through the cylinder service valve, and that you also had completely installed and tightened the ACME fitting.

The latter point is because there’s another safety feature in OPD valves. This safety prevents any gas from flowing out from the cylinder unless there is a completely installed fitting on the valve. This ensures you have a good seal between the fitting and the valve to prevent leaks, and also precludes the possibility of thermal injury. How’s that? Imagine your neighbor’s kid comes over, finds your filled LP cylinder just waiting to be hooked up, and opens the valve while the output is pointed at his face? ‘Nuf said.

We had to replace an ACME pigtail when the check valve apparently decided to quit working. To diagnose the problem we tried reconnecting the ACME fitting a couple of times and banged the cylinder a couple of times, all to no avail. Then the “light came on.” We disconnected the second LP cylinder and moved the freshly filled cylinder to that “position.” Flow was immediately good from the fresh cylinder—the problem was the pigtail. Once it was replaced, our problem was solved.

Another preventable problem is the chance of clogging the pigtail check valve. While not highly likely, here’s a tip: When you have your cylinder refilled, always use the valve dust cap to keep bugs and crud out of the valve. Foreign material could get into the valve mouth if you leave the cylinder disconnected and laying about in storage for a long period, but hey, why take the chance?

##RVT764 ##RVDT1237

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Thomas Becher
1 year ago

American Congress of Mechanical Engineers.
ACME. Easy

Kenneth Fuller
1 year ago

Great article. Thanks.

1 year ago

I used to work for a FUEL Company in Louisiana. We delivered Fuel, Gas and Propane. You had to be certified to work on and fill Propane Tanks.

People who fill propane are supposed to know the things described in this article, but many times they do not. Propane technicians are also supposed to check for the Date of Manufacture on the Tanks. Propane tanks need to be recertified every 10 years and then again every 5 years after that initial 10 year inspection.

I would never try dropping the tank on a hard surface to loosen a stuck valve! Take the tank to a Propane company and have it inspected! Most Trailers and 5th wheels usually have 2 propane tanks on them and you litterally have 2 small bombs on board.

You should also, turn off the propane when you are traveling. If you have a properly cooled Gas/Electric Fridge it will stay cool and keep your food cold for up to 8 hours.

Just about everything we do in RVing is Safety Related! Unfortunately, too many RVers don’t think about safety, until it’s too late!

Jeff Arthur
1 year ago

Oddly enough I just replaced a pigtail yesterday & it did fix my problem.

4 years ago

Because of the excess flow check-valve, I continue to advocate it’s okay to drive down the highway with your tanks open. If the hose or line is ruptured, gas will cease to flow from the tank. This is a never ending discussion just like being in deer camp and arguing which is the best deer rifle.