The Best Kept Secret In Your RV’s Propane
If your new RV has those nice new propane cylinder valves with the big ACME threads on the ends you’re the owners we want to talk to. If your cylinders still have thee old style valves with the left-hand wrench connections, it won’t be long before you will be a candidate for this information too. As of April 1, 2002, no one should fill propane cylinders that don’t have the new OPD feature (Overfill Prevention Devise) and those new OPD cylinder valves will all have the big ACME threads on the outside. The old left-handed, wrench type, connections have been replaced with a very user friendly, non-wrench right-handed, big green plastic nut that threads right on to the end of the new valves. The big green nut is on the end of a flexible hose called a “pigtail.” The pigtail connects to the RV’s propane regulator.
Let’s talk about a couple of the features on these new pigtails. Both features are parts of the big green nut.
The first is a black thermal-sensitive bushing found just behind the green nut. If this bushing is ever in a fire with temperatures between 240°F and 300°F, it melts and allows the brass nipple inside the nut to move back about ¼ inch, which closes a small piston inside the cylinder valve totally shutting off the flow of propane. This innovative feature will help mitigate fires or explosions.
The second new safety feature is called a “flow-limiting-device.” Its purpose is to restrict the flow of escaping gas if there is an excessive leak in the RV’s gas system. It’s this second “flow-limiting-device” feature and how it works, that forms the core of the secret! Almost every time the cylinder valve is opened, a small ball in the center of the brass nipple (inside the green nut) is pushed forward into a brass seat. This seat doesn’t totally shut off the gas. By design, it allows a small amount of gas (by-pass flow) to go into the RV’s gas system. If everything in the gas system is closed and in the off position and if there are no leaks, the by-pass flow builds up a back pressure that equalizes the pressure in the ball back of the seat and allows unrestricted flow through the system. All this happens in about five seconds and the owner doesn’t even know it’s happening, appliances light, furnaces and water heaters run and everything is normal.
Now lets look at the same system but with a burner left on at the range top. Again the cylinder valve is opened, the ball is pushed into the seat, the by-pass flow goes out through the system but this time it can’t equalize because the flow continues out through the open range top. The owner goes inside and starts cooking dinner. He lights two burners on the range top and everything seems OK until the furnace comes on. At that point, the flames on the range top are drawn way down and the furnace won’t ignite, everything seems starved out. The reason? We don’t have full-unrestricted cylinder flow to run our system, we only have the small by-pass flow which is far too small to feed the appliances in our gas train.
HOW DO WE FIX THE SITUATION? Turn off all the appliances, make sure everything is closed and at the off position, wait about 15 seconds. If the system is closed and tight, the back pressure builds up, equalizes and feeds unrestricted cylinder flow for normal operation. Keep in mind if your system has a cracked fitting or split tube somewhere in the gas system the by-pass flow will not equalize until that leak is found and fixed.
Now that you know the secret, tell your friends, share the new safety features and help them understand why the design is there and how it helps protect them in the event of fires and leaks. Like any change, the new features bring new benefits and protection and without proper understanding new frustrations. It’s no secret, when it comes to Rving the more you know the easier you go.
For more information on your RV’s gas fundamentals you can call 1-800-877-2495 to receive a copy of “What You Should Know About Your LP Gas System.”
March 22, 2001, Revised April 11,2001
Dave Shearer Marshall Gas Controls
Back to Motley RV Repair Home Page Back to Motley RV Repair RV Care/ Use Articles