Even though I've got my little compressor set-up complete, I am tinkering a little bit at a time with building a good, big shop compressor. I'm either going to use two 20# propane tanks like you use on a gas grill, or a large propane tank that a local dealer may have just sitting around collecting dust. I've already done some pricing and I can build a really heavy duty compressor for less than I can buy one.
My question is how do you purge the propane out of the tank to insure you don't have a trace left? One spark from the compressor motor could blow that tank... and me... to kingdom come. When I worked in the fertilizer plant, we used to pressure them up with nitrogen (if I remember correctly), or another inert gas, then slowly relieve the pressure. This was done a few times over a few days, then the tank was left open to the atmoshphere for a few days before putting the tank back in service.
I don't have access to gas, and I'd also like to do a high-pressure test to be sure my tank will handle at least 150 PSI. Is a welding or machine shop the kind of place I can get these two tasks done? Or how about a welding gas supplier? Or is it something I can do myself?
I'm in no rush. This is just a project to tinker with a piece at a time.
First bleed any remaining propane. Then with the tank at zero pressure, remove the valve. Invert the tank and let it sit for a few days. All the propane should be gone. Air has 65% the density of that of propane. (1.285 g/l vs. 1.964 g/l assuming Propane is C3H8.) In a day or two there should be no propane left in the tank.
The other thought is to ask the dealer to purge the tank for you. Tell the dealer what your plans are and he may be able to help with the project.
Have you thought of using a hot water tank? They are rated at 150 pounds per square inch too. My Dad had a 30 gallon one he used for about 30 years & I have a 50 gallon one that I've used for about 5 years. I set mine up in the regular way with the rounded top up. If I were to do it again I would invert the tank so it would drain the moisture out the bottom of the tank better.
All the above is assuming that you can find a hot water tank that is still good & doesn't leak.
You could, I suppose, bleed all the propane out, then fill the tank with water. Then you'd only have the problem of how to turn a tank full of water that weighs a million pounds over to drain. Let it drain for a few days and it should be dry enough.
If you have a contact at a propane dealership, they might have a suggestion for how to safely purge a tank. Better yet, they might have one that's never had propane in it, therefore might not need to be purged.
One other consideration: install a drain for the inevitable condensation that will build up in the tank. I don't know how thick a propane tank is, so I wouldn't hazard a guess as to how this should be done.
The easiest solution to the condensation issue might be to simply mount the tank upside down so that all the valves and fittings are on the bottom of the tank instead of the top. Then, the condensation will run into your air plumbing, and you can add a condensation trap in a convenient location.
I've thought of doing this myself, but never got around to it yet. Let us know how it turns out.
My one concern with bleeding of the remaining propane (from the weight, I don't think it is much) is that in this town the faintest whiff of propane or natural gas will have someone calling the fire department and city utilities out to investigate.
But I may just check and see how much there is, and do it during the rain that is supposed to hit us again this evening.
Yeah, I'd given a water heater tank a lot of consideration, but the problem is that just about every one I find has been replaced because it busted at the seam, or rusted out. A 20 gallon from an RV would be great, but those are even harder to find.
>You could, I suppose, bleed all the propane out, then fill
>the tank with water. Then you'd only have the problem of
>how to turn a tank full of water that weighs a million
>pounds over to drain. Let it drain for a few days and it
>should be dry enough.
I hadn't thought of the water idea.
>If you have a contact at a propane dealership, they might
>have a suggestion for how to safely purge a tank. Better
>yet, they might have one that's never had propane in it,
>therefore might not need to be purged.
I've only got $2 invested in this one. A new one would be about $20 or $30.
>One other consideration: install a drain for the inevitable
>condensation that will build up in the tank. I don't know
>how thick a propane tank is, so I wouldn't hazard a guess as
>to how this should be done.
>The easiest solution to the condensation issue might be to
>simply mount the tank upside down so that all the valves and
>fittings are on the bottom of the tank instead of the top.
>Then, the condensation will run into your air plumbing, and
>you can add a condensation trap in a convenient location.
You just solved the one problem I hadn't figured out yet.
>I've thought of doing this myself, but never got around to
>it yet. Let us know how it turns out.
Well, I finally got an answer from a local propane company, as well as from one of their authorized dealers. Here is their advice:
If the valve will match (propane tanks now have a different type valve and thus a different type connection), I am to have our daughter use up the propane via her gas grill. If that won't work, I am to bleed off the propane a little at a time, and then leave the valve open until all the pressure is relieved.
After the tank is fully de-pressured, I am to remove the valve to allow any remaining vapors to escape. After the tank has had a few hours for the vapors to escape, I am to fill the tank with water, let it sit for an hour or so, and then drain the tank. That will completely remove any trace of propane from the tank and make it perfectly safe for use. I can even repeat the water step a few times if it makes me feel better, but the manager of the propane company said that wouldn't be necessary.
The final step is to let the tank set upsidedown for a day or so to let all the moisture drain out. Then I'm ready to use it to build my compressor. I'll have to check and see if the second propane tank I have in the garage is the same height so it will match the first. If not, I have already located a couple of possible sources for good used tanks for a couple of bucks.
I've already got the plans in my head for my compressor, so I can start tinkering right away. It could take me 6 months to a year to complete the project, but I should have a compressor that will allow me to hit Sonny with a spitball way out there in Cal-eee-forn-ya. :)
Try using a little Simpley Green cleaner with water, cap off and slosh it around then rinse with water. It does an excellent job when servicing scuba tanks used for enriched air and no after taste. So it should work for your application..