kenn-
The family business was propane back in the day. Literally the first thing I drove as a little kid was a Wheel Horse with a 20# bbq tank behind the seat that it ran on, so you get the idea...
Anyway, I have been looking at the close ups of your tank, and I am not really sure what it's original application was. Not stationary, it is some sort of a vehicle tank. Not motor fuel, there is no liquid withdrawal valve or opening. It may be an RV tank, but the relief valve position is a little odd for that. Possible for some sort of equipment like a steam jenny? Either way it looks to have all the proper guards and valves for your application. The pic you showed of the valve in the bottom is the pressure relief valve, not a liquid withdrawal opening. A liquid opening would be labeled "liquid" just like your service valve is labeled "vapor". Normally on a vehicle tank it would be positioned on the end with the other valves or on top of the tank. The pressure relief valve has to go into the vapor portion of the tank (top), so the opening must have an internal tube to the top of the tank. A little unusual setup to have the relief in the bottom like that, that's why I'm wondering if it is for some sort of equipment.
First, you cannot take out the relief valve, that is definitely the most important safety feature on tank. Second, if you are planning on enclosing the tank, you will need to pipe that relief valve to the outside of the compartment. There should be an internal thread on the valve (just inside the hex) where you can tread in an adapter for a remote exit, typically a stainless flex pipe (they make them for propane relief valves specifically) hooked to a pipe to get outside. If the tank gets hot (like in a fire, or if the tank is overfilled) that relief valve vents excess pressure to keep the tank from rupturing, and it would just make matters worse to vent inside a compartment. I'm thinking it is positioned where it is to make that remote plumbing easier on what ever they designed the particular tank for. Don't try to cover it, it needs to be clear if it needs to vent excess pressure. I see it is recessed into the tank, which is not typical, and that is done specifically to protect the relief valve from damage like you describe, which should be adequate. I suppose you could fashion some sort of guard in the flight path of a rock from the tire if it worries you, just be sure to leave the opening in the valve unrestricted if it needs to open. Remember that you cannot weld directly to the tank, only on the brackets, and that should only be done on a brand new tank that has not had fuel in it, or has had the valves removed and purged with nitrogen, for the obvious safety reasons.
Not sure how you are planning on designing the rear of your truck for access to the tank for filling, but you can use a remote fill valve setup to plumb the filler and bleeder to somewhere convenient like a small filler door. Any local propane company should have or can order the valves and hoses for that, also I found the valves and mounting bracket for that at Bontrager's for $20 (without the hoses) and they may have the full setup for cheap as well. Just thinking...
Nice job on the sturdy looking brackets and cradle for the tank.
Dave
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