So why all the tales of accidents and propane lines spewing? Actually it was true at one time, but times have changed along with the equipment.
Prior to 1977 the majority of connections between the regulators and the propane containers (pigtail) in RVs were made of copper tubing, and were wound in a coil to take up excess tubing creating the "pigtail" coil appearance, from which they got their name. Copper, being malleable, when strained to breaking will many times reduce the inside diameter of copper tubing, either from stretching, or bending. This can restrict the flow to the point that excess flow check valves, which reduces the pressure in the propane line in case of a break or leak in the line, might not detect excess gas flow. Thus full pressure flow could occur.
In 1977 the code was changed mandating that the connections, or pigtails, between the propane container and the regulator be made of rubber and then thermoplastic tubing, which would not change diameter in a mechanical "accident," and would activate the excess flow valves reliably when torn apart.
It is important at this point to clarify terms. A tank is not a cylinder and vice versa. A propane cylinder, is a removable container like used in 5th wheels, TTs, and BBQs. It is not permanently mounted. A Tank refers to a permanently mounted propane container, like in a motorhome. My fiver uses a cylinder; your motorhome uses a tank.
Here is the very important distinction. A tank has a permanent connection to the Motorhome's propane system, and has an extra fitting for it to be connected to at fill stations. The connection between the tank and the MH's propane system is never disconnected, except by a professional technician in most cases.
With removable cylinders, the owner, a non-professional disconnects the cylinders, whenever a fill is needed. Thus the new valve design, and extra safety features, to prevent accidents by the end user while removed, during filling, and during removal and reinstallation of the pigtail from/to the cylinder.
The MH tank is an ASME (American Society of Mechanical Engineers) design and does not require the new OPD (Overfill Protection Device) valves because it already has a check valve and overfill protection, albeit a different design than cylinders.
The trailer cylinders are DOT (Dept. Of Transportation) and have additional safeguards in the OPD and the pigtails because, as said earlier, they are disconnected completely on a regular basis by the user. These are in the 4-40 pound capacities. (Usually 30 pounds on fivers, and 20-pound cylinders on some TTs.)
Several changes have been mandated for DOT cylinders since the pigtail mandate. In 1998 it was mandated that hand screw on Acme nuts be used on the pigtails in place of the older POL (Named after the company that designed them, the Prest-O-Lite Company) connectors. The Acme nut is the part that screws into the cylinder, and takes the place of the older brass POL connectors that screwed into the cylinder valve with a reverse thread requiring a wrench to tighten and seal. The Acme nut is the new Green one that screws onto the OPD's outside thread, uses a normal right hand twist to tighten, and which also tightens to seal by hand only, not requiring a wrench. This is significant because two safety features are built into the Acme nut itself. First, Acme nuts contain a thermal bushing, which will melt in a fire, and prevent the escape of gasses from the tank. When it melts, it allows the new OPD spring activated shut off valve, which will only allow the flow of gas if a fitting is attached, to pop out and completely shut the flow down. They also have an improved excess flow check valve, compared to the valve in the old POL fitting-more on that later. The new Acme nut also adds an O-ring to further help prevent leaks at the connection.
With the advent of the requirement for all 4 to 40 pound DOT cylinders to change over to OPD valves in 2001, the safety features, in addition to the spring loaded shut off valve, have been further enhanced by insuring that the cylinders cannot be filled over 80% of capacity. This allows for vapor expansion, without building up excessive pressures from overfilling, and venting by way of the pressure relief valve. The old overfill device was a tube that connected to a bleeder valve that was observed during filling for liquid emission which indicated 80%. But if the person filling it did not shut off the propane it could overfill anyway. They used a scale for weight to fill also. Now DOT cylinders have an automatic stop fill valve built into the OPD valve.
The ASME Tanks for MHs did not require a change in valve on the tank because they already had sufficient check valves for excess flow, should there be a leak or sudden break in a line. And for more than 17 years, ASME tanks have used automatic stop fill valves to limit filling to 80%. Why so long before DOT cylinders? Because you can't remove them to verify the fill by weight! More importantly, since the connection between the tank and the MHs system is not broken, there is no need for the spring-loaded valve that the DOT cylinders had added with the new OPD valve. That valve prevents the gas from coming out of the cylinder unless there is a tight connection made from the POL or Acme nut depressing the valve. In other words, you can turn on the valve of the DOT cylinder, and no gas would come out unless there was a connector depressing the inside valve. Not necessary for MH connections that are never removed. Thus they were exempted from the OPD changeover rules.
Now to the new style excess flow check valve on the Acme nut that connects to the DOT cylinder's OPD valve. It is really a simple device. Inside the Acme nut, there is a ball bearing check valve that almost shuts off momentarily when full pressure is released by opening the cylinder valve. You should hear a click at that point. It doesn't shut down completely, as that would negate its ability to detect leaks and excess flow from a leak or major line break, and no gas could flow. It closes just enough to allow a bypass of gas that is very slight, about 10 cubic feet or less per hour (cf/hour) flow, as opposed to the max flow of 200 cf/hour. The bypass gas goes into the propane system, and if there are no leaks or broken pipes, it backs up and builds pressure in the gas lines in the RV. When it equalizes on both sides of the valve, the valve opens to allow full pressure up to about a max of 200 cf/hour at 100 psi. Then if you were standing there, you would hear a second click. Using all of your propane appliances should not exceed 100cf/hour. If there is a leak or a broken pipe, the pressure can't build up and the excess flow valve doesn't open up all the way thus limiting the gas output to about 10 cf/hour, or less. It is important to note that propane, like gasoline vapor, has narrow ignition requirements as far as air fuel mixture is concerned. So if you lit the leak (Don't try this at home!) it would show a flame at the leak in the air, but the concentration is too high in the line for it to ignite. In other words the fire can't run up the line to the cylinder or tank to ignite it. Want to see it in action? Watch your gas range work. Why doesn't the flame travel up the tube it comes out of to the burner? Same thing.
In fact, it is pretty difficult to actually blow up a propane tank or cylinder. What does occur when the pressure relief valve blows, which occurs at 312 psi in ASME tanks, and 375psi in DOT cylinders, is that the gas is vented and makes a lot of noise. I have had one overfilled tank vent and it really got my attention! But there was no flame or problem, as it dissipated outside very quickly. Should there be a fire or source of ignition when it vents, it won't explode either, instead it makes a torch. It does not ignite inside the gas line and run up it to the tank or cylinder and "explode." The only exception to that, is if it is punctured and ignited simultaneously, and then the same external ignition occurs only more rapidly. Or, if a leak is allowed to build up inside an enclosed space like our RVs, and then is ignited, there is one heck of a fire. But oxygen must be present in the right mixture within the ignition range, which is fairly narrow. The important thing is that with the garlic odorant (Ethyl Mercaptin) that is added, you would smell it in most cases of a small leak or break, with time to take appropriate action. It can be smelled by the human nose and your detectors at about 1/5th of the lower limit of combustibility or about 1/2 % propane to air. (Propane combusts at 2.15% gas to air to 9.6% gas to air - not above and not below.) Since most RVs have propane detectors near the floor now, they would alert long before the levels reach dangerous concentrations, if they are in good working order.
An interesting side note is that there are two types of pigtail connector designs to attach to a DOT Cylinder. Appropriately called Type 1 and Type 2. Type 1 is the Acme nut on the pigtail and the OPD valve on the cylinder. Type 2 is a quick-disconnect (QD,) which requires the type 2 OPD Valve that matches it on the DOT cylinder. What makes the Type 2 different from a plain quick disconnect, aside from needing the valve that matches on the cylinder, is that the Type 2 QD has all the safety items built into the pigtail end as the Type 1. This includes the excess flow check valve, and the thermal bushing.
For some reason, unknown to the industry, when the new code was formulated, RVs were left out of approval for the Type 2 valve and pigtail design for a quick disconnect at the cylinder. It is important to note that this part of the code limits only the main connection from cylinder to pigtail to regulator to a Type 1 in RVs. It is acceptable to use a plain propane approved QD for a tap in the line after the regulator in an RV, for example to run a line to a BBQ grill or Catalytic heater.
In summary, while some people have experienced propane leaks and breaks in the lines, there are safety devices that you never see, and most folks are unaware of, that stop or limit the flow in a mechanical leak, fire, or breakage. With today's built in safety systems, propane detectors, and the final back up, our own noses, it is much safer to use and live with propane than ever before. Should you have a sudden low-pressure condition in your RV appliances, it may be that the excess flow check valve has kicked in and is telling you that you have a leak or break in the line because it would not equalize. Of course to test that, all you have to do is turn the propane off at the cylinder or tank, wait a few minutes and turn it back on to full. If it then equalizes and runs OK, it just had a piece of trash or contaminant that temporarily hung up the excess flow check valve. That click you hear is the check valve operating when you first open the cylinder valve all the way. It is possible to turn it on so slowly that you won't hear it, but in most cases you will with the new equipment.
If you plan to run your fridge on the road, and still have the old design pigtails and valves on your DOT cylinders, you might want to consider changing over for maximum safety. (Yes, in some states like Louisiana, you can still fill non-OPD DOT cylinders.) Now that you know how the check valves work, you know that it is very important to keep the inside of the cylinder valve clean, to avoid getting grit and road grime from fouling them and possibly getting into the excess flow check valve when reconnecting. Looking at the valves that are exposed when transporting your DOT cylinders for refilling, it is very important to use the plastic plugs for the old style valves, and the caps supplied with the OPD valves on the cylinders when transporting them to and from the refueling station, to keep contaminants out of your safety devices. Also keep them upright at all times. This keeps them from possibly leaking liquid propane. I use an old plastic milk crate for transporting that keeps them upright and stable enough not to fall over with reasonable driving. The 20, 30, and 40-pound cylinders all fit in them snugly. For the taller tanks, an additional strap or bungee cord might be necessary.
None of these precautions or items will make it safe to have an open flame (propane appliance running) while refueling with any fuel, especially around gasoline vapors. Also remember that it is not necessary to turn off the propane before fueling, assuming your system is in good shape with no leaks, but to turn the propane appliances off electrically, by using the on/off switches inside. With the electronic igniters used today, should one go into auto ignite mode, either by design or malfunction, that could ignite gasoline fumes as well as the flame. If it makes you feel more secure to shut the propane valves off as well, that is fine. Just remember that the excess flow check valve insures no leaks, if the fridge was operating properly at the time of shut off.
I have not even gone into all of the propane and ignition safety devices that cut off the gas supply at the appliance for under and over pressures, and failure of ignition etc. Every one of them has those built in as well.
Also remember that all propane appliances generate CO, or Carbon monoxide. That is why they vent their combustion products to the outside, and draw their air for combustion from the outside. They must be sealed from the inside compartment or we would all have CO inside. Should there be a leak outside on the road, the outside vents would probably keep it from reaching any concentrations that would ignite. And the gas should not be able to get inside. If there is a leak inside, it should not be able to be ignited by the appliance flame outside. Any leak is dangerous and should be dealt with immediately by turning off the propane at the source and shutting down any electrical devices that might create a spark.
Understanding all of the above should make it plain why I said the risk in running a reefer on the road is not one of catastrophic danger. And why I say that there is only a slight increase in risk, which is negligible from a fire standpoint based on propane ignitions causing less than 1/10th of one percent of all fires. The chance of a fire from your engine leaking fuel, or an electrical fire, are magnitudes greater. But we start our engines and use our electrical systems.
Again, running the reefer on the road does not include while refueling. Choosing not to run your propane while in transit is a personal preference made with all the facts in hand. Not running it while in transit does add some extra measure of safety. Which is rational, and may make for peace of mind for many folks. But running it or not is not the big risk, or lack thereof, many make it out to be. Whatever decision makes your travel and comfort levels best, is the right one for you. Can mechanical devices fail? Of course! Which ones fail most often has been covered.
I would like to thank the kind folks at Marshall Gas Controls for spending so much of their valuable time with me to provide the history, facts, and figures used in this article. And for finally getting this RVr to understand how that excess flow valve and thermal bushing work.