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 that you remove to have filled. Those nifty little containers – properly called cylinders – 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?
What’s an OPD device on an LP cylinder?
Most states require an LP cylinder to have an OPD, or “overfill protection device.” If the cylinder has an OPD, it has a 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. This can happen, 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. At least that’s what some LP filling station operators say.
Inside of a cylinder equipped with an OPD valve is a small float. The float 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.
What to do with a “hung up” LP cylinder
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. By that, we mean, “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 and disconnect the gas connection. 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.”
What’s a “pigtail” on an LP cylinder?
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.
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.
Safety protocols in ACME fittings
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. 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.
Another safety feature on the OPD valve
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. The first one is 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.
How we discovered a pigtail problem
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?
Related:
Sticky LP problem: Valve not passing gas
LP pigtail propane hoses for RVs on Amazon
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Nice job as many people can have issues and not know why. I will also add a good policy is, “Look, Listen and Smell”. LP has a “smell” added to help identify it. Leaks “hiss” and looking at fittings, connections and pigtails will provide indications that they are in good working condition and safe. I have had to replace a regulator and both 5 year old pigtails because of leaks (age related). Found them by smell and sound (listening). Simple and safe way to check for issues. Be safe.
Excellent article. Between it and the article on “Sticky LP problem: Valve not passing gas” one can conclude that there are 2 locations, one in the pigtail ACME nut and one in the bottle’s valve that can cause flow disruption. And very good to know how both can be addressed.
This is great information that’s definitely share-worthy!
Russ, Good write up! You reminded me of a few things I had forgotten like where the term POL came from. However I need some clarification when you state “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,” How can you get liquid propane into the gas line when you have to take the cylinder loose from the pigtail to invert it?
Dave: Great question. Perhaps we should have clarified. Yes, one would need to disconnect the pigtail in order to invert the cylinder. The issue, to our understanding, comes in that in rare cases, it’s thought that liquid LP make its way to the valve. Then when you reconnect the pigtail fitting and open the valve, the liquid LP could make its way into regulator. Frankly, I’ve never heard of it happening, but that means nothing–particularly if you’re the guy who actually had it happen!
Russ, Thanks for the clarification. I once had to dump liquid propane by inverting my cylinder when an attendant at a chain rental truck business seriously overfilled my cylinder and I was unable to bleed enough off using the 10% screw. I guess in this case no liquid entered the regulator either.
Great article, learned something new (didn’t realize there was a check valve in the pigtail itself). I hope to see more like this from RVTravel!
I’ve recently run into one other reason for a no-flow in an ACME fitting. If I screw the pigtail nut onto the ACME fitting, and tighten it down firmly – sometimes FIRMLY is not quite enough. There is a pin deep inside the pigtail nut which depresses the release valve inside the ACME fitting. If the nut isn’t screwed down quite enough, the pin doesn’t depress the valve, and no gas flows. The solution is simply to grasp the nut BY HAND and tighten it just a bit more – DON’T USE A WRENCH! Since this has happened to me on two new pigtails, I think the problem may not be wear or age, but that the new rubber seal may be slightly over-thick. At any rate, it is easy to try.
Thank you for this article, this goes into the ‘learn something new everyday’ file in my brain!
FYI, It is called ACME probably because of the style of the threads in the nut.
https://en.wikipedia.org/wiki/Trapezoidal_thread_form
I recently had a leaking problem with one of my 11 year old, recently re-certified, 40 lb cylinders, which I thought was a pig tail problem, even though I’d replaced the pigtails 2 years ago. The leak appeared to be at the twist on connector at the end of the pigtail. I bought a new pigtail, which didn’t fix the leak, so I switched tanks, & that proved the problem was with the brass fixture on the top of the tank. $55 to replace the valve, which I assume included the internal float & mechanisms. That fixed the leak.
Great article. One thing “they” forgot to address is keeping the pigtail connection clean. Need a cap that mimics the ACME valve when the appliance is not being used.
As a former rv technician I often pass over tips like this one, simply because normally I have encountered the same problem with a customer. I read today’s article out of curiosity as to what would be the advice. I found a well written, well researched article. Cudos Russ and Tina
“Heat caused expansion of LIQUID” is why it is only filled to 80 %