Run a water trap between the tank and the line? Would remove the water issue.
I don't know if emptying the tank further is needed. And, don't know what to think about rust proofing. But, I would mount it upside down and connect it to a vertical line that is terminated at the bottom with either a petcock bleeder valve or an air compressor separator to drain off the water. Input and output branches can be on an angle so they also drain into the vertical line.
It can be welded to if you fill it with water first to disperse the propane i would say the best result would be to have a drain at the low point and pull and fill from the top or side.
Hello, first of all thank you for all of your replies, by the time you answered I already had done the project
Here are some pictures and I will explain how I did it.
First I got a new (validation date is until 2031) empty propane tank and coated the inside with rust proof paint and waited really long for it to dry.
Basically it is like this image:
The propane tank is the "Vacuum bottle" The drain is like the rubber tube (but without the part that goes inside, it will be at the lowest point of the propane tank) and the "thermometer" is like the air line, (it has a tube that goes to like the middle of the propane tank, that way water won't get in the air line).
I made this piece on a lathe:
This other images will explain the setup better:
The green disk represents the existing tank wall. The blue cylinder represents a threaded (?) plug inserted in the valve opening. The blue cylinder does not need to extend higher than the tank wall surface, but is exaggerated here. The red cylinder represents the air exit line and should extend above the tank wall, to prevent water "ingestion" when in use. The white cylinder should be as low as possible on the bottom of the blue cylinder, in order to eject as much water as possible. Of course, there would be valves on both exit cylinders to control air and water flow.
This is how the setup turned out:
Far away from the place where I work so if it decides to ever blow up (which I don't think it will) it won't hurt me.
This way I didnt have to do any welds and it works flawlessly. If you want to do this project I think you should, because it tripled the storage of my compressor and these can stand safely 30 bar (all of them in my country are tested to this pressure) and some of them are even tested to 100 bar. If you have any questions about this ask and I will answer