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Tuesday, April 3, 2001

Not Your Father's Pigtail!

Not Your Father's Pigtail!
By Gary Bunzer

Back in April 2002, a new industry standard was adopted for upright DOT cylinders. Instead of the tried and true, left-handed, POL-type service valve many old-timers may remember, the new RV Type I CGA 791 service valve was introduced. The new pigtail connector, which mates to the new valve features a wrench-less, green swivel nut that can be tightened and loosened by hand. You may be familiar with a similar set-up on your home gas barbeque grill. Additionally, the large, 1-5/16 inch Acme threads are right-handed threads. The new valve contains an internal, spring-loaded component that prohibits all gas flow from the container until a positive, leak-free connection has been made. Since April 2002, propane gas retailers will only refill DOT cylinders with the Type I service valve.

Let’s fully explore the Type I valve fitting used with current DOT cylinders; there are two versions of the Type I valve connector now in use. The older version will have a black thermal sensitive bushing located between the green nut and the flexible hose.

When this bushing is exposed to temperatures over 240 degrees F, it will melt and allow this brass nipple inside the nut to shift about 1/4-inch. The forward movement of the nipple will close a small piston inside the cylinder valve and all propane gas will stop flowing.

The newer Type I valve fitting does not have this visible black bushing. As seen in this photo, the thermal sensitive device is located behind the four slits located at the rear of the green nut. Still, the flow will be stopped when the brass nipple moves forward and trips the piston located in the service valve on the cylinder.

The flow-limiting device, machined into the brass nipple, is located inside the green nut.

The purpose of the flow-limiting portion of the ACME connector nut is to restrict the flow of propane gas should an excessive leak exist anywhere in the propane system. Just about every time the DOT cylinder service valve is opened, this small ball bearing in the nut assembly is moved towards the piping system and into a brass seat, restricting the flow of gas. It does not, however, fully shut off the entire gas flow; the ball bearing only restricts it. By design, a small amount of gas is allowed to bypass the ball and continues to flow into the distribution piping system. As long as all the appliances are turned off and there are no propane leaks anywhere in the system, this small amount of bypass flow quickly builds up enough backpressure that it eventually equals the incoming bypass flow pressure coming through the device. When that happens, the small spring pushes the ball bearing back and off the seat and permits unrestricted flow of gas into the system. This whole process takes about five seconds and is designed to work in this fashion.

If, however, a burner is inadvertently left on somewhere in the system or if there is a small leak in the piping or at any connection, here’s what happens that leads many RVers and service technicians to erroneously believe the main pressure regulator is faulty.

The cylinder valve is opened, the ball bearing shifts into its seat, the bypass flow passes through into the system, but this time the back-pressure cannot equalize because of the escaping gas through the open burner or leak. Without this pressure equalization, remember, unrestricted flow will not be permitted through the flow limiting device. The bypass flow, however, may be sufficient to operate one or two stove burners, so you may think everything is operating normally. That is until you try to light a larger appliance like the furnace or the water heater. Quite rapidly the volume of available bypass propane is used up and any burner flame at that appliance will be quickly drawn down to virtually nothing. In other words, there is not enough propane to support furnace or water heater operation and one would presume the regulator is not functioning properly. The fact is, the flow limiting safety feature in the ACME nut is working properly and signifying a leak somewhere in the system.

To reset the flow-limiting device, completely turn off all propane-burning appliances and close the service valves on the cylinders. Wait about thirty-seconds and re-open the service valves fully. Light a stove burner and try to ignite the furnace. If the stove burner flame shows evidence of fuel starvation, it means the backpressure still has not equalized the bypass flow and a leak still exists somewhere in the piping system. Further troubleshooting is in order.

You may notice that some Type I ACME cylinder valves have, not only external ACME threads, but also internal, left-hand POL threads. All RVs using DOT cylinders have now switched to the new Type I valve, but many RVers still use the older flexible POL pigtails in an effort to save money. It is highly recommended to switch to the newer Type I ACME pigtail connector, (the green nut), with the above built-in safety features, including the thermal sensing bushing and internal flow-limiting device. Without the new connector, these safety features are null and void when using older POL pigtails.

Remember, RVing is more than a hobby, it’s a lifestyle!


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If you are in doubt or do not feel comfortable about a procedure, do not continue. Simply call your local RV service facility and make an appointment with them. The advice, recommendations and procedures offered by the RV Doctor are solely those of Gary. They do not necessarily reflect the opinions, procedures and recommendations of our sponsors or advertisers.