Saturday, February 4, 2023



LP cylinders: Tips, tricks and safety

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 line on the far side of the ACME fitting, and provided there aren’t any major leaks or a broken line, pressure builds up in the line, backing up against the check valve. When that happens, the check valve opens 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: 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 feature 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?



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4 years ago

Believe ASME propane valves referred to “American Society of Mechanical Engineers”

5 years ago

Excellent information – thanks!

5 years ago

Among common usage (such as Wile E. Coyote cartoons), ACME stands for All Corporations Making Everything. It became popular in the 20s to refer to generic items as ACME (ie “non-“)branded. So, an ACME thread on an LP cylinder is saying it’s a ubiquitous generic item (“Just give me an ACME nail…”).

Some etymologists believe it derives from the greek word ekme for “peak” or summit, but this makes very little sense since it conversely is used to say “not very superlative.”

I paid $200K for twin English degrees. Does it show?

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