I got a new and empty big propane tank from a neighbour (he works at the propane distribution in the city) and I wanted to convert it to an air tank. I know even thought it looks like it, the tanks are never empty so my plan would be taking off the valve and filling it with water in order to remove any gas in the tank left. After that I was thinking about adding rust proof paint inside and rolling it around so it got in every place and let it dry for like a week or something, I read somewhere you shouldn't do this because it would contaminate the air line?! The last problem would be the drain valve, what should I do? I know I am not supposed to weld to the tank so that option is out, I was thinking about drilling and tapping a hole but I read somewhere that the tank is too thin to do that and the last option would be turning it upside down but the problem is that it would contaminate the air line with water. What should I do?

1 Answer 1


You have the right approach with filling it with water, especially if you can ensure that you've removed all gas bubbles during the filling. If the tank is going to be used by connecting it to a conventional oil-lubricated compressor, you can empty the water and slosh some protective oil inside. There are preservative oils for rust prevention which work. You've not indicated the use of the air, and contamination is certainly a consideration.

With an oil-lubricated compressor, one has an air-dryer and oil-separator device in the outgoing air line, which would eliminate contamination in general.

The drain option is a bit more complex. My conventional shop compressor as a plastic hose in which one end is placed in the tank low point and the other exits the top of the tank through a fitting. After the compressor reaches cut-off pressure, the valve on the hose opens, causing some of the air to pass through the plastic hose, pulling water from the tank.

This particular tank has a conventional drain and I have yet to experience any water within the tank when performing a manual drain, but I also have a Franzinator installed between the compressor head and the tank, removing virtually all the water and oil before it reaches the tank.

You'd want the ability to perform a periodic manual drain with this conversion project.

If your tank is small enough to flip over and use the fill valve opening to drain, that's one option. If you can remove the fill valve opening and drop a suction line into the tank to ensure cleaning, that's another.

My large propane tanks are cylindrical with the long axis parallel to the ground. There would be no single low point unless one end was tipped up slightly. Even a half inch would provide a single collection point.

You could modify the fill valve opening "cork" to take a hose fitting and drop the open end into the low point. Some form of retention for the hose at the low point would be a good idea, of course. On the outside of the fill valve cork, a valve with a hose to direct the water would give you the "pressurized" drain capability described above.

All of the above does not imply safety or lack of safety in the practice of converting a propane tank to air storage. I'm not qualified to make that judgement and have not performed research specific to that discussion.

EDIT: additional information for clarification.

enter image description here

This is a poor image, but saves typing. Consider that the rubber tube is the opening from which the water will drain. The drawing shows the tube in the stopper to be deeper into the bottle, but it should be flush with the plug. The "thermometer" in this drawing should be a hollow tube, extending some distance, just as shown in the image. When turned upside down, the "thermometer" tube will be above any collected water and will provide air to your use, while the other tube will allow water to be ejected when open.

The water ejection port does not need to be a tube, as a hole in the plug/cap will serve, and the outside will have a valve to keep the air in when not draining the water.

The clean air tube will have to be some hollow item. The equivalent of a long hollow bolt that threads into place would work, but I can't picture such an item as being off-the-shelf.

If you have fabrication skills, soldering or welding and have pipe to fit the opening, you should be able to make something that works, while maintaining the integrity of the tank.

I would hope your current valve has a threaded opening of about 20-25 mm when the valve is removed. That should give you enough working room to build an assembly for clean air.

The following images are different views of my interpretation of your drawing. The varying perspectives should provide easier understanding and/or correction.

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.

Can you tell me if these views represent correctly your current concept? 3d image of sorts

another view yet another view

  • First of all thank you so much for all the information. Couldn't I dump a rust protective paint bucket inside the tank and roll it around and turn it and then remove the extra content? Wouldnt that be better than oil because it would dry, that way there wouldn't be oil contamination?
    – Diogo
    Commented Jan 2, 2017 at 9:58
  • "If your tank is small enough to flip over and use the fill valve opening to drain, that's one option. " When I said having the tank flipped over was always have it flipped and the main connection in the tank would divide in two, one for draining another for hose, my concern is that it would contaminate the air line?
    – Diogo
    Commented Jan 2, 2017 at 10:00
  • If you have sufficient time to ensure the rust protective paint is dry, and the tank is clean enough, paint should work. You'd want to have some forced air into the tank to exhaust the fumes as the paint dries. Your idea of having the fill opening on the bottom as a permanent placement would work if it is the lowest point. To prevent water and oil from being pulled into the air line, you'd have a rigid extender tube inside the fill cap with sufficient height to stay clear of the puddle/pool.
    – fred_dot_u
    Commented Jan 2, 2017 at 12:25
  • Yeah I have plenty of time for the paint to dry since I dont have money yet to buy all the fitting and things like that :P Wouldn't I need two holes for the rigid extender tube? I was thinking of connecting the drain and air hose to the same filling hole but then it would have those 1 male to 2 females fittings, one for draining another one for air hose. How would I add a rigid extender tube? Sorry I am new at these compressor things
    – Diogo
    Commented Jan 2, 2017 at 12:58
  • Maybe I could drill another hole and tap it and the internal tube in that hole connecting to the outside air line (I think the top part of the propane tank is thicker) but I still dont know how I can put a internal tube inside and seal it from the outside except on the air fitting. If you were able to explain it to me I would really appreciate it, sorry if I am not making myself clear
    – Diogo
    Commented Jan 2, 2017 at 14:24

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