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Compressed Air Tank

06/04/2010 9:16 PM

Any reason why I couldn't use either a large propane tank or old water heater to store compressed air. The pressure would be 120PSI. I know; before you start quoting ASME regulations, the tank would be located outside my shop with no one around in a rural area. I'm not concerned if the thing blows up. There wouldn't be any danger to people

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#1

Re: Compressed air tank

06/04/2010 10:30 PM

Ron, it's good for propane, it's DEFINITELY good for air, i.e.: regulation is more stringent for the former.

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#21
In reply to #1

Re: Compressed air tank

06/07/2010 6:42 PM

A propane tank may be OK when band new but definitely have it hydrostatically tested well above the working pressure you intend to use it for, if it's a used one. They become unsafe after a certain amount of use. Make certain your pressure relief is set below the test pressure. You'd be well advised to be in town on errands whenever the tank is up to working pressure.

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#2

Re: Compressed air tank

06/04/2010 11:23 PM

I suspect the internal corrosion allowance on air tanks might be greater than on propane tanks, even at the same rated pressure. In other words, it is possible that if your repurpose a propane tank for air, it might not have enough corrosion allowance. This is similar to the reason that used refrigerant jugs should not be reused as air tanks.

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#3

Re: Compressed air tank

06/05/2010 8:57 AM

You need to check the design pressure of the vessel and confirm it's greater than 120psi.

Propane has vapour pressure 120psig at about 75°F. I don't know, but I suspect the regs will want it designed for a higher temperature than 75°F, hence higher design pressure, so you should be OK. But I'd find out for sure.

For the old water heater it's anybody's guess. If it's only designed for hot water to sinks etc it could easily be a lot less than 120psi.

Cheers...........Codey

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#4

Re: Compressed air tank

06/05/2010 11:07 AM

Well Ron, I've asked similar questions here, and typically discover that while it may be possible to repurpose some things, it is often not advisable, or cost effective. My recent question regarding how to clean steel or plastic drums resulted in my judgement that while it could be done, there were enough drawbacks to it as a backyard project, to only do such a thing as a last resort. Certainly I got the message that typically water collected from roofs into repurposed vessels, was highly suspect as far as drinking water.

As far as adapting a propane tank for use as storage for compressed air, I do wonder if there is the potential of its blowing up. We can pretty much figure out a way to break most anything.

(Air compressor tanks are fitted with water drains, and are supposed to be drained daily. They do end up with a fair amount of water in them, especially on humid days.)

You are a people as well, and we don't want you to get tore up by some explosion. There are limits to our ability to see the future.

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#5

Re: Compressed Air Tank

06/05/2010 11:54 PM

Guys, we don't know anything until we can talk about the comparative wall thicknesses in each type of tank and compare that with the ASME section 8 requirements. And then we must cope with the intangible of rusting from water in an air tank and how we can manage to prevent it if we adopt some other type of tank. Also note that water tanks cannot store anywhere near the energy when full at pressure as an air tank.

Another note here; this is an engineering forum and I see the difference between engineers and artisans as being the ability of engineers to base their decisions on numbers and relevant calculations. We often slough that off; but when dealing with something that can turn your old bod into hamburger the numbers may well be worth looking at.

A second comment I can offer is something I learned from my recent sojourn into buying a 500 gallon propane tank. Apparently in the last 10-20 years the competition in that business has forced the builders of the big tanks to use less steel in them to remain competitive. I guess the regulations may have changed or whatever. Wish I knew more about the subject.

Ronnseto --- What's the exterior wall of your shop look like? Is there a lot of rebar in it? ..............Ed Weldon

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#6

Re: Compressed Air Tank

06/06/2010 1:11 AM

Definite NO on both. Propane tanks have a bleed-off valve. Water pressure tanks are rated at far less than than 120 psi. Used ones of either type can be rusted inside. More significant, you obviously have never seen a compressed air tank blow up. The fun part with a water tank is when the welded-in fittings launch at 200 mph... they rust more at the welds. Having it where humans are EVER around it when it is charged is prosecutable as gross negligence if someone were hurt. And just to save a few bucks. There are automatic drains that leak a little when the air pressure cycles.

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#7

Re: Compressed Air Tank

06/06/2010 6:26 AM

If ya do, I suggest you put a blast screen between the shop and the tank. Blowing air tanks or boilers is gruesome!

I've used refrigerant tanks for portable air tanks, but not for continuous use. If you turn them upside down and open the valve, you blow out all the water accumulated from the atmosphere, so internal rusting is sorta minimized.

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#8

Re: Compressed Air Tank

06/06/2010 6:34 AM

I suggest it might be safer to re-purpose an old high pressure cylinder that has failed a hydro. You will have a much improved safety margin. These tanks are available from gas supply and SCUBA shops. You may have to do some sweet talking because they don't normally let them go.

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#9

Re: Compressed Air Tank

06/06/2010 6:59 AM

If you are able to bury the chosen vessel under 2 feet minimum of dirt, AND to fill it plumb full of water and pump it up to 250 PSI, You might be OK. Definitely do not attempt to use an air tank that has not been hydrotested to twice your maximum working pressure.

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#10

Re: Compressed Air Tank

06/06/2010 7:43 AM

Hi Ron, If you are still there.

I have a good mate who tried the same thing.

He used an old hot water tank to act as the reserviour for a pressure pump.

He knew that he would need to keep it about 1/2 full of air to reduce the cycling of the pump during partial usage. (start/stop/start/stop = burnt out pump)

After tapping the side of this very tight drum (with his ear against it) and not being to tell how full it was, he headed up to the house to get some boiling water to use to pour down the side and thus find the level.

This almighty BOOM reverbrated up the valley, shook the windows of his house and caused his wife to ask (quite casually of course) "What the heck was that!"

Well, my mate, feeling like jelly, went down to see what happened and found his pump shed blown to bits.

My advice is ...Well ... Don't let me tell you what to do, its your shed.

Using a sound propane bottle might be a better proposition.

Cheers Tim

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#11

Re: Compressed Air Tank

06/06/2010 12:55 PM

You would be far better off scrounging old truck air brake tanks and manifolding them together. They will take the pressure and usually have several ports built into them for your convenience.

Propane tanks don't have all that much pressure on them in use; to fill a large one with compressed air is to create a time bomb with an indeterminate fuse.

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#13
In reply to #11

Re: Compressed Air Tank

06/06/2010 1:22 PM

"Propane tanks don't have all that much pressure on them in use", Jerry New Hampshire

More numbers (GROAN!!)

http://www.propanecarbs.com/propane.html

Scroll down the page to the chart showing liquid propane pressure at various temperatures. Examples: 80F, 128 PSIG; 100F, 172 PSIG; 130F, 257 PSIG

Of course dry propane doesn't rust steel as has been discussed already.

Ed Weldon

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#12

Re: Compressed Air Tank

06/06/2010 1:19 PM

Propane Tanks can be used up to 120 psi. The saturated pressure of propane is ~ 110 at ~ 100F and they are built for higher pressure for safety...(?)

As it has been said, you need to bleed the water from it at regular intervals and therefore you need to add a drain valve. Make sure that the tank is empty and has been cleaned from any residual propane before working on it. There is allways a residual gas in empty tanks...

As far as the safety concern: Whether in rural or not or whether no one is arround or not, the safety rules apply and should not be overlooked. There is allways a proper way to do things without taking un-necessary risks.

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#14
In reply to #12

Re: Compressed Air Tank

06/06/2010 4:45 PM

Dear LAA Lucke, I agree wholeheartedly that the safety rules apply all of the time.

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#15

Re: Compressed Air Tank

06/06/2010 8:24 PM

Propane tanks can be used as air pressure tanks! Propane tanks have many connections. One is the fill valve port, which is used to fill the tank with liquid propane. Another is the service valve gas port, which is where the user connects a regulator to draw off gaseous propane for use for some type of heating.

There is a safety pressure release valve port, which has a pressure regulator set for 250psi. Otherwise on a hot summer day we would be releasing thousands of gallons of propane all over the USA.

And there is a liquid valve withdrawal port, which is connected to a pipe to near the bottom of the tank to draw off liquid propane. This port could be used to draw off water condensation once it reaches the bottom of the liquid pipe. This could be extended to minimize the depth of the water. Or you could add a port to the bottom of the tank with a valve on it to drain the water.

On every propane tank for residential or commercial use in the USA there is a nameplate with the mfg name, date of mfg, pressure rating, etc. http://www.propane101.com/manufacturersnameplate.htm

You will some tanks have been tested to up to 300psi when initially mfg. Change the pressure relief valve to just a few pounds over what you need and you should have a considerable safety margin.

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#20
In reply to #15

Re: Compressed Air Tank

06/07/2010 6:40 PM

Agreed, I don't see any great problem in using a propane tank. I looked at a friend's propane tank over the weekend and design pressure was 17.2 bar = your 250 psi. OP could fit a PRV with set pressure say 150 psi, or adjust the one on the tank if it goes that low. That gives a huge safety margin, equivalent to a generous corrosion allowance that somebody mentioned.

I still think using a water heater would need careful checking though.

Cheers..........Codey

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#26
In reply to #20

Re: Compressed Air Tank

06/13/2010 11:15 AM

Correction to this - I looked at it again yesterday and PRV set pressure is 17.2 bar. Tank design pressure is 18.68 bar, test pressure 23.35 bar.

Cheers........Codey

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#24
In reply to #15

Re: Compressed Air Tank

06/07/2010 9:14 PM

Just as a story of fact about a propane tank I lived with in an old Airstream, one day the top blew off and vented all the propane. Was a small Airstream, and a small tank. I was down in Fort Lauderdale living in a condemned area under a flight path where some jet engines had come apart and rained down turbine blades.

It has been at least twenty years since this event, but as I remember there were plastic parts on the tank to stove and heater feed for the Airstream that were the culprits in the popoff and gas venting.

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#16

Re: Compressed Air Tank

06/07/2010 11:08 AM

ASME aside you'll be playing with fire regardless of your remote location from both a liability standpoint and the potential for injury to yourself and others, however remote. It's not a smart way to economize.

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#17

Re: Compressed Air Tank

06/07/2010 11:46 AM

"There wouldn't be any danger to people" is one of the most idiotic things I have ever read.


The answer is "Yes." There is a reason to not use tanks for other than their designed purpose.

It's called safety. You screw up, you risk a blow up.

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#18
In reply to #17

Re: Compressed Air Tank

06/07/2010 6:18 PM

It appears the regulatary thinking has overcome the practical one.Why do so many people have to comment that if the label doesn't say you can do it, then you should not.

Is it that instead of being progressive and forward thinking, they will not take personal responsibility but want a scape goat for any eventuality?

You could have just grabbed an old, and maybe rusty, aircompressor tank and banged it out side and it would have been perfectly complying, and because the label said so, it would be perfectly safe! NOT!

Good on you for asking the question and seeking informed comment. I would hope that those who are wet towels will come up with scientific principle and data which show why a propane bottle with adequate safety relief valves and drains, (just as air tanks do) is unsafe. I'm sure that when they opened their minds to the details of science, they would have to agree. (Unless of course their job depended on not agreeing)

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#19
In reply to #18

Re: Compressed Air Tank

06/07/2010 6:34 PM

I had an old boss who used to tell me "Say all that again but this time pretend your in court and put "your honour" at the end of it."

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#22
In reply to #19

Re: Compressed Air Tank

06/07/2010 7:31 PM

Good point Chris, But can you more confidently stand before "your Honour" and say that you used an underated or old rusty air tank, or one that did not have working safety valves, just because the label said "Air tank"?

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#23
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Re: Compressed Air Tank

06/07/2010 7:54 PM

No, I agree that you can't. Pressure equipment needs to be maintained to the appropriate standards.

I was really responding to your point about "progressive and forward thinking".

I would not want become a scape goat because someone else wants to be "progressive and forward thinking" by giving someone this advice.

By the way, I suspect this tank wont see too much maintenance unless something stops working.

Bye

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#25

Re: Compressed Air Tank

06/12/2010 12:37 AM

Do you really need 120 #'s ?

I suppose you will be using it for air tools?

having a much larger tank will increase both the run & off times.

another way is to install some 2" pipe around the perimeter of the shop

pipes usually good for 300#

I only suggest 2", because a threader for 3" is much less common.

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#27

Re: Compressed Air Tank

06/13/2010 4:19 PM

Refineries repurpose vessels all the time, and operate them safely. BUT they never fail to hydrotest them to way over the application pressure. 100 pound compressed air systems got 300 pound hydrotest and 125 pound relief valves for one customer. Use a freshly calibrated gauge, and bleed out all the air and pump it up to midscale on your 0-600 psig calibration lab certified gauge. If it holds, you are good for now. How often this procedure is repeated is up to you and applicable local codes. Corrosion is certainly a consideration, and borrowing a boroscope to look for pitting would give comfort. But nothing beats a recent hydrotest.

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#28
In reply to #27

Re: Compressed Air Tank

06/14/2010 6:53 PM

Good observations. And in the case of a do-it-yourselfer who doesn't have the testing capabilities of a refinery, nothing beats buying a new, rated tank to start with. A used tank isn't worth the considerable risk.

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#29
In reply to #28

Re: Compressed Air Tank

06/20/2011 2:50 PM

I've found the compressor air tank doesn't "explode" when it rusts through, instead it begins to leak through a pin sized hole. Now if you had a new tank and over pressured it, I could see it exploding, but rusting out is completely different. The rusting is not uniform and causes deep pits hence tiny weak spots that begin to leak.

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#30
In reply to #29

Re: Compressed Air Tank

08/30/2011 11:57 PM

I hope you're right, charley. I welded shut a leak in the tank of our our water heater months ago due to not being able to get another at the time. The leak started really slow like a drip, eventually graduating to the flow of something like a garden hose. It my belief that since water heaters like your average garden variety 30, 40, 50 gallon units have far too soft and thin metal in them to ever consider using one for anything but water. However, when you get into some of the older water heaters between 75 to 100 gallons (such as the rheem night and day), you begin to see a galvanic coated metal of at least twice the thickness of the smaller counterparts, so it would stand to reason that you could put more pressure into this thicker metal. How much pressure is not up to me as I didn't follow through with my plans for the rheem tank I picked up... I was thinking about using the tank again...raising it to a height of 10 feet and putting a very large water valve at the bottom....so that I could water each of my garden rows quickly so I didn't have to babysit the garden hose...then I started thinking air, but read this site just now instead. @@@@@ This is a little off topic, but I bet if someone poured one of those leaky old water heaters (say about a 50 gallon or so) clear full of gunpowder, tamped it down, put a long fuse in the top and lit it perched on the edge of some deserted canyon, that'd make one HELL of a racket. It woudn't go to waste that way, it would be recycled...into mineral dust.

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