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LP Gas Safety Features

Several people have reported delays in being able to light LP Gas appliances on their campers and many times it appears to be the safety features now incorporated in our gas systems.

There have been several changes in the way RV propane is conveyed from the high pressure cylinder to the low pressure piping in the RV. Originating with the portable BBQ market, a 1 5/16" external right hand acme thread Type 1 valve and mating black plastic pigtail connection nut was required on all 20lb. cylinders starting in 1994.


This replaced what was universally used on high pressure vapor LP cylinders, the left hand thread CGA 510 (POL) valve.  The photo to the right shows the old style POL connector.  Remember needing a wrench to hood these up?

In 1998, a modified Type one cylinder connection was introduced to RV's. The main modification was to increase the flow rate allowed through the pigtail to meet the RV's typically higher BTU demand, but additionally there were two new safety features incorporated.   The first safety feature deals with fires at the connection to the tank.  The connection nut contains a thermoplastic sleeve that, when heated to between 240 - 300 degrees F, will fail in such a way as to back the pigtail connection out of the cylinder valve, causing cylinder pressure to close the cylinder check valve, interrupting the flow of gas. The RV connection nut will be green as seen in the photo on the right.

The second new safety feature is called a “flow-limiting device,” and 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 seems to cause the most problems among campers.  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 system with the cylinder pressure and with the help of a small spring, pushes the ball back off 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 every-thing is normal.

Now to the problem - From the Marshall Type 1 Excess Flow Pigtail instructions: "Under certain conditions, such as when a cylinder valve is opened, this flow control device will activate, causing a restriction in the supply of fuel to appliances. The flow control mechanism will automatically reset ... (once the pressure has equalized in the system) provided there are no leaks in the system and all appliance have been turned off." 

Let’s look at the LP gas system, but with a burner left on at the range top. Again, the cylinder valve is opened and 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 to cook 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 this? Turn off all the appliances, make sure everything is closed and in the “off” position, and wait about fifteen seconds. If the system is closed and tight, the backpressure 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. 

Symptoms of "excess flow" valve activation are no gas flow to system, low gas flow, low or yellow flame at burner. To help prevent this problem Marshall recommends: "Important: Be sure all gas appliances, including their pilot lights, are off. Open the LP gas cylinder valve slowly. DO NOT SNAP OPEN. Test all fittings to insure that they are leak free. Wait at least 15 seconds before lighting appliances." 

Once the "excess flow" check has activated, the pressure on both sides of it MUST be equalized. If any appliances or pilots are open, or if there are leaks, the pressure will not equalize. Also, if the check is fouled with oil or other foreign material, it may not readily reset. The high pressure at the tank side of the check valve will hold it firmly seated against any gas flow. It also seems that the check (or as some describe it - safety) valve inside the tank valve (not the "excess flow" check) to be pressurized closed also, creating a double blockage.

Now that you know the secret of the safety devices, 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

Thanks to: fairfaxjim, RV.net Forums as well as BaltimoreBayside, PUT Forums


   Revised: May 08, 2007


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