I know, I know. You have been tossing and turning at night, wondering what's inside your RV propane tank. Well, I am here to relieve your suffering, because I just found a cut-open tank behind the Roadtrek factory here, and can provide you with the photos to show you what the various devices are and how they work.
Let's look at all the gadgets, and their accompanying fittings on the outside of the tank that you are more familiar with. Left to right, we have the fuel level gauge, the relief valve, the fill port, the bleed, and the tank main valve.
First a little dreary technical background so we can understand the job each of these has to do. Propane tanks are filled to 80% of their actual volume with liquid propane, so there's a pocket of propane vapor at the top of the tank all the time. Your RV system runs on propane vapor, not liquid propane – if it gets out of the tank and into your RV plumbing, liquid propane will ruin your pressure regulators and cause them to leak. The goal is to fill the tank to 80% and draw vapor from the top of the tank until it's time to refill.
Let's start with the fill port. Like all of the fill ports installed in the last ten years it has what's called an overfill protection device (OPD). This is the yellow hot dog on the end of the valve, attached to a moveable arm. This yellow plastic thing is a float, and as the liquid propane level rises in the filling tank, the float follows it up and moves the arm. When it has moved far enough, the arm closes a valve which pinches off the orifice through which the propane is entering the tank.
That's the theory, anyway. If the arm is bent or the float is broken or they installed it wrong, well, it's not going to work correctly. The “protection” in the device name is protection for the propane filling station so you don't blow up in their driveway – it's not designed to give you an exact 80% fill every time, or to protect your RV propane devices.
That brings us to the bleed, the device to the OPD float's immediate right. It is a vent connected to the pipe inside which goes up to the height associated with a precise 80% fill level, and opens inside the tank. If the tank is below 80%, the end of the pipe is in the propane vapor, and clear propane vapor will come out of this bleed. If the liquid level is at or above 80%, a white mist comes out – this is liquid propane. Notice the number of moving parts? There aren't any. The bleed is foolproof – it will always ensure that you don't overfill the tank. If a fueling technician refuses to use the bleed to fill your tank and just relies on the OPD, immediately stop him and find another place to buy propane. He isn't going to buy you new regulators.
If I just filled up and some bozo had overfilled my tank, what I do is leave the main valve shut, go out into a field somewhere, turn off all sources of ignition in the vehicle, park with the tank on the downwind side of the vehicle, and vent the liquid propane until nothing but vapor comes out. Then you can safely open your main valve and use your propane. Better yet, get a propane place to do this – maybe the one that just overfilled your tanks.
There's another black plastic float on the far left – that's the fuel level gauge. This one looks like a little black paint roller. Same principle as the OPD float – the float rises and moves an arm. The arm moves a magnet, which slides across a flat place on the tank. The outside portion of the gauge senses the position of the magnet inside the tank and creates a reading for the gauge on your tank and the one inside your RV. The magnet setup ensures there are no electrical connections that go inside the tank, so this gauge is safe (although usually not very accurate).
The relief valve is to the right of the fuel level gauge, and is a valve that opens at above 200 psi connected to a big pipe going up into the vapor zone. It prevents tank explosions in the event of a major fire or other overheating event, at the cost of venting unburned propane into an already unsafe environment. This is the big plume of flame you see coming from the bottom of burning RVs, and is the effect, not the cause, of the fire. Almost all RV fires are either electrical or people doing dumb things with candles, campfires, etc. Fires at gas pumps where people leave their propane appliances running and spray gasoline on their vehicle which the propane appliances ignite aren't propane fires, they're idiot fires. If this relief valve ever opens (and it won't unless you seriously overfill your tank, park in a paint oven, or catch on fire), turn off all sources of ignition and leave the area immediately until it stops venting. If the propane is already ignited, skip the first step 😉
The pipe connected to the main tank valve is similar to the relief valve's but bent instead of straight – big and going all the way up to the vapor area of the tank. Open this valve by SLOWLY turning it counterclockwise, otherwise the other safety device – the slam-shut valve – will misinterpret your rapid tank opening for a leak, and shut. To reset it, close the valve, wait a few minutes, and try opening it more slowly. Close this valve by turning it all the way clockwise and turn off your battery disconnect switch if you smell a propane leak, or are going to refuel. Also, turn the valve off to drive the vehicle. That's all there really is to it. Follow the rules, and propane is as safe as any other form of fuel.