100lb Propane to Air Cylinder Conversion

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Old 04-26-19, 08:41 PM
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100lb Propane to Air Cylinder Conversion

I'm looking to convert a older horizontal 100lb propane cylinder into a air storage tank to add capacity to my smaller compressor. I believe its also a little larger then the average #100 cylinder. This one has a 80% fill capacity and after measuring the length and diameter i estimate its a little larger then 30 gallon. My concern is the same obviously as most people that do this. Cleaning it out so its no longer a explosive/fire hazard. Anyone have experience cleaning these out? From what i have heard it seems to truly clear them they need to be flashed out. I have not seen anyone on YouTube try it DIY style so i'm going to assume its not very safe. Others have said to fill it with water; but other then allowing you to safely drill or cut into it i still don't see that completely evacuating the gases. I have the valve currently removed and the tank upside down to clear it for now the best i can. I would eventually like to be able to weld a bung into the tank for the valves and want to make sure its safe to do so. Any suggestions?
 
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Old 04-26-19, 09:06 PM
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Do you mean flushed out ?
An empty propane tank is no longer an explosion hazard. Propane is a liquid under pressure.
Once it's no longer contained under pressure it evaporates.

I don't think I'd flush the tank with anything.
Keep in mind that an air tank needs to have a bottom drain as it will take on water from the compressed air.
 
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Old 04-26-19, 09:36 PM
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Do you mean flushed out ?
An empty propane tank is no longer an explosion hazard. Propane is a liquid under pressure.
Once it's no longer contained under pressure it evaporates.
Are you positive about that? I have read a few people say they have torched the open hole after removing the valve just to make sure and they have caught fire and shot flames out the hole.
 
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Old 04-26-19, 11:03 PM
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There will be residual gas in there just after removing the valve. After several days it should be gone. It would dissipate even faster if it were warm/hot outside. You could use your air compressor to put in 30-50psi of air and let it blow back out. Several light fill ups should purge any remaining fuel. The ethyl mercaptan residue (smell additive) will be around for a good while.

On edit: It is possible for some gas to remain in the tank for an extended period. To use it as it is for an air compressor tank would be fine. If you intend to weld on the tank..... that would require the tank to be filled with something non volatile. Engine exhaust can be used as well as a gallon or two of warm water and some dry ice.
 

Last edited by PJmax; 04-26-19 at 11:42 PM.
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Old 04-27-19, 03:53 AM
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There would most definately be a hazard if the tank were not properly purged to remove all traces of propane.
DIY'ers have welded pressure vessels and got away with it but we would be negligent if these forums supported it.
There are regulations about who can do this and specific methods on how it should be done.
There is a consiberable amount of energy in a tank when even under as little as 100 psi and temendous forces and damage can be released if something went wrong and the vessel failed.
Don't do it!
Purchase a certified tank approved for air or better still purchase a bigger compressor which is what you really need if you are trying to get more air..

These forums promote themselve as profesionals offering advice and DIY welding on a pressure vessel would not be something a pro who knows better would suggest.
 
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Old 04-27-19, 04:04 AM
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You can do what you said. Flash the tank and let the gas burn itself out. The amount and strength of flame will depend on how long you leave the tank opened up. When it's first opened there won't be much/any flame as there is no oxygen in the tank. Wait a while with a couple fittings open and some air will get inside and you can have more flame.

When I've cut a large tank where I was afraid to burn off the gas I filled the tank with CO2. You can do it by simply dropping dry ice into the bottom. You can also use a bottle of CO2, regulator and hose. Put the hose into the tank so the end is a the bottom of the tank. CO2 and propane are nearly the same density so you want to introduce the CO2 into the bottom of the tank (which is why dry ice works so well) so as it sublimates/fills it will push the propane out the openings in the top of the tank. I know of no safe gas that is heavier than propane so some form of CO2 is the only gas you can use.

You can take a similar but opposite approach. Turn the tank upside down so the openings are at the bottom. Insert a air hose from your compressor into the tank so the hose is near the top. Then pump in air until you no longer smell any propane coming out.
 
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Old 04-27-19, 07:58 AM
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Apparently there have been accidents from people doing this very thing, even if they washed, pumped or forced out the propane.

From what I can find, the best and safest answer is that you don't cut into it or weld on it without having it tested with a calibrated LEL, and the monitor reading is zero.
 
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Old 04-27-19, 08:42 AM
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Originally Posted by SkyWil
I'm looking to convert a older horizontal 100lb propane cylinder into a air storage tank to add capacity to my smaller compressor.
...
I have the valve currently removed and the tank upside down
With valve out, I'd set the tank right-side up, fill with water from a garden hose, then tip upside down to drain. I would't cut any new holes in the tank, but would mount it upside down with and a four arm "+ fitting" that allows condensation to drain out. Left side of + is compressed air in, top of + goes to tank, right side of + is compressed air out, bottom side of + is an automatic water drain.
 
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Old 04-27-19, 09:08 AM
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There would most definately be a hazard if the tank were not properly purged to remove all traces of propane.
That's why i initially thought flaring it would yield the best overall results. The only way i think you can truly eliminate traces from the pores of the metal would be to burn it off. Also do you think a poor weld for example on a air tank could actually cause a explosion and or part of the tank to fire off like a bullet? I don't want to say its impossible but i strongly feel worst case scenario i would have a air leak and not a catastrophic rupture.

ou can do what you said. Flash the tank and let the gas burn itself out. The amount and strength of flame will depend on how long you leave the tank opened up.
Have you ever done this or seen someone do it? I would feel a lot more confident giving it a shot if i could see a YouTube video so i know what size fireball to expect.

ft side of + is compressed air in, top of + goes to tank, right side of + is compressed air out, bottom side of + is an automatic water drain.
I thought about that too but how well do you think it would drain off water from the air if the outgoing line sits near where it puddles up in the bottom?

Basically the only thing i would weld on it if i do would just be a bung in the bottom to property thread in a drain valve. The tank is not that thick and i feel if i just drilled a hole and tapped threads it would not have enough depth to properly secure itself. No matter what the danger factor is i'm probably going to attempt it anyways. I have another option that i could possibly do as well. With the main valve removed i could probably drill a drain hole on the opposite side of the tank and run a extension through the valve hole and use a nut on the inside of the tank to hold the drain valve in appose to welding in a bung.
 
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Old 04-27-19, 10:01 AM
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I don't think I'd be worried about gas in the pores in the metal. The pores could only hold a very, very, very tiny amount of gas.

Yes, an improper weld could cause the tank to fail. That's why they are pressure tested.

I've flared small tanks. It's not a very reliable method. The fire will only burn if there is oxygen. So, even though there is propane in the tank it might not burn and you are left with a tank still containing flammable gas. Filling the tank with water or purging well with air or an inert gas is more reliable.
 
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Old 04-27-19, 10:09 AM
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If you made a bad weld on the tank it is unpredictable what a rupture would look like. This could occur just from the compressed air, with no propane igniting.

I like the fill with water method to purge the tank. If you are really paranoic you could let the water sit for an hour before draining it out and doing the process a second time.
 
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Old 04-27-19, 01:23 PM
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Originally Posted by SkyWil
Originally Posted by Hal_S
left side of + is compressed air in, top of + goes to tank, right side of + is compressed air out, bottom side of + is an automatic water drain.
I thought about that too but how well do you think it would drain off water from the air if the outgoing line sits near where it puddles up in the bottom?
Shouldn't be a problem with an auto drain. Add a 2" section of pipe to accomodate any liquid water.

Quick-tip, I replaced the annoying-to-use "water-drain stopcock" on the bottom of my 5HP / 60 Gallon tank with an easy-to-use air gun on a short section of hose. (And an auto drain). Instead of crawling underneath the tank to open the drain every few days, just use the air gun, gets used all the time for blowing dust out of the shop.

Any air tank WILL accumulate condensation inside as the compressed-heated air cools.
Fewer holes in the tank means fewer failure points. Easier to have a tank with a single port than cutting and and mounting multiple ports.
 

Last edited by Hal_S; 04-27-19 at 01:39 PM.
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Old 04-28-19, 02:34 AM
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apples to oranges but I've brazed up a few gasoline tanks over the yrs and what I always did was to first rinse them out with water, sometimes I'd leave the water in the tank until the torch work was done.

I replaced the annoying-to-use "water-drain stopcock" on the bottom of my 5HP / 60 Gallon tank with an easy-to-use air gun on a short section of hose
What I did on mine was remove the petcock and replaced it with an elbow and short piece of pipe. On the end of the pipe I screwed in a ball valve. I still have to bend down to bleed off the water but that's easier than having to reach under the tank.
 
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Old 04-28-19, 06:27 AM
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Quick follow up -

Since you've already GOT a compressor, flush the tank with compressed air.

Propane, while still in the tank, is too rich a mix to burn well, so I'd just shove an air hose in, let the compressor run for 15 minutes to vent the air. The propane itself if non-polar, odorless and very volatile: it will evaporate quickly. The ethyl-mercaptan added to create that pungent "gas smell" is a chemical cousin of methanol, is polar, has a strong odor, and is much less volatile: it will stick around in the tank for a while.

I'd avoid cutting any extra holes in the tank.
IIRC the horizontal tanks often have several ports, to take advantage of that, I'd consider welding additional brackets onto the head-collar and foot collar so that one port faces down for draining.

Aside from that, I'd look for adapter fittings instead of trying to re-tap the existing fittings.
 

Last edited by Hal_S; 04-28-19 at 06:44 AM.
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Old 04-29-19, 06:32 AM
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It does indeed have multiple ports on the tank. Output valve, filler valve, Release Valve and the gauge embedded in the tank. I had the tank upside with the valve off like i said and quite a bit of oily residue drained out over a few days into a bucket. I filled it with water last night and was gonna drain it out but after realizing how bad the mercap smell was spreading everywhere i'm not sure i want to dump that out in my yard lol. So its till sitting with water in it until i find a nice place to dump it....maybe dig a hole and bury it with soil?

Also due to the high price of a good compressor i convinced myself i was going to do this. I don't know if its a sign....but out of nowhere a guy on craigslist posted a Husky branded Campbell Hausfeld last night for only 150$. Its single stage, 60 gallon, 7 HP and rated at 10.2 CFM at 90 PSI / 11.8 CFM at 40 PSI. Hes also throwing in a 25' hose and a Grizzly branded tool oiler / air filter regulator. This is a no brainer right lol? I'm thinking i should skip this propane cylinder venture and go pick up this Husky this morning....
 
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Old 04-29-19, 06:41 AM
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Sounds like a good deal, listen to it run and check out the tank. I have a 60 gal 6hp DeVilBliss compressor that I've been more than satisfied with for 25+ yrs.
 
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Old 04-29-19, 08:55 AM
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Agreed, just grab the 7 HP Husky 60gallon/CH compressor & tank.

However, I MIGHT keep the propane tank around as a 2nd pressure tank, if you have an additional workspace that is far from the tank.

I've got a 5HP 60gallon compressor is in the barn/garage/workshop, about 150' away from the house. House has an unfinished basement with a utility area that is useful for winter projects, it can be useful to have access to air tools when working in the basement.

Problem is, at 150+ feet the pressure drop is significant, HVLP spray gun works nicley, but forget running an air-sander for any length of time, and impact wrench has no real power.

The solution was to run 150' of air hose at full pressure to a 10 gallon air tank in the basement, then run power hungry tools from a 4' whip hose attached to the 10 gallon tank. The tank stores enough compressed air that you can work with high flow tools.
 
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Old 04-29-19, 10:58 AM
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I would jump on it at that price.
Call and go now so you don't loose it.
If you have a compressor that meets your needs and a proper size airline there is no need for extra air volume.

The idea about DIY welding on a pressure vessel is really quite insane!
 
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Old 04-29-19, 06:53 PM
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Well i went and picked up the Husky. It looks to be in decent shape. The tank looks to be rust free and it seemed to run as expected. It also has "recon" and a number carved into the top of the tank. I was told that it was a open box item and they do that anytime a compressor is returned and they resell it. That sound about right? I have no idea myself... It does have one issue though... Something is wrong with the pressure switch. It wont shut off once it reaches the set PSI and will keep going until the safety valve kicks and then drains the tank. Is this something that can be cleaned and fixed or better off to replace it? And if so will any universal do or one from another tank?
 
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Last edited by SkyWil; 04-29-19 at 08:16 PM.
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Old 04-30-19, 02:28 AM
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If I remember correctly there is a pressure regulator switch that is similar to a well switch with a cut on and cut off contacts. You might be able to adjust the high side. It should have a screw but on some you just bend the tab a little. Worst case scenario you'd replace the switch.
 
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