Recycling an LP tank for pressurized air
By Mechlogs.com
Pennsylvania
Published: Fri, Aug 3, 2007

If you have an old LP tank kicking around and don't know what to do with it, I've found they can be easily and cheaply converted for use as an air storage tank.

First, I want to express in no uncertain terms:

1. You perform the procedures listed in this article at your own risk.
2. By following the instructions listed here, you agree to absolve the author, site owner and everyone else of all liability, financial, medical or otherwise.
3. There is a fire hazard when working with these tanks. They must be emptied under normal use, such as a propane grill.
4. DO NOT OPEN THE VALVE ON A PARTIALLY FULL TANK TO VENT IT! If you have a partially full tank and can not empty it under normal use, you should consult a propane refilling station or other propane service center.
5. Under no circumstances should you use cutting tools which create sparks or flames, welding equipment or open flames on these tanks at any time. Even after the conversion.
   

Ok, now that that's out of the way, let's get on with it.

A little bit about propane:
Propane in it's liquid form is under pressure at temperatures above -44 degrees F. At 70 degrees F, it is at 124 PSI. Normal propane cylinders, such as those used with home grills have pressure relief valves (PRV) set to 375 PSI. PRV settings for ASME (American Society of Mechanical Engineers) tanks are 250 PSI. DOT engine fuel cylinders use PRV settings of 312.5 PSI. Well over your normal air pressure system. These tanks can handle 200 PSI without any problems.
Old style propane tanks were discontinued in the US in 1998. After 2002, they can no longer be refilled because they must retrofitted with an Overfill Protection Device (OPD). Tanks must be recertified every twelve years. But it's cheaper to buy a new one than recertify it.
A 20lb LP tank has a capacity of roughly 5.75 gallons. (3 gallons LP @  80% capacity) So they are a decent size storage vessel for compressed air for filling tires and such.
But they have a left-hand thread on the shut off valve, so you can't just connect an ordinary pipe adapter on it and use it to fill tires or what ever you'd need a portable tank for, with good reason. The manufacturer wouldn't want some tool trying to fill his car tires with propane.
Based on what other people have done, recycle them as outdoor fire pits, use them as the base for art, even create musical instruments from them, there is no doubt that just because they are past their useful life as a propane tank, that they can't be used for other things.

So the next question is how?
First, you'll need an old tank. If you don't have one kicking around, I'm sure you can find one at a scrap metal yard, propane filling station or through friends or relatives.
If you do not have an old one, it's a waste to take a tank fitted with a OPD valve to do this, it will be refillable until at least 2010, probably longer. Once you start this process, you will not be able to refill this tank without having to recertify it.
Next, since it's empty, open the valve, if any amount of gas escapes after a second or so, it's not empty, connect it to a grill, fire up the grill. get out some steaks and wait for it to empty itself, or see item number four above.
With the valve open, you'll need to remove the old valve. There is a special tool to do this, but a large pipe wrench will work. You'll need brute strength and possibly a friend to help you hold it. I used a large rubber mallet to strike the end of the pipe wrench to get it going. I swear it was in a dozen turns, After about four or five half-turns, it can be undone by hand.
Once the valve is removed, turn the tank upside down and let it sit for a week or so. This will help cut down on the propane gas "smell", which is actually provided by another chemical, ethyl mercaptan. After a week, the tank will still smell like propane, but will not be as strong.
Now that you're certain that the tank is completely empty of propane, it's time to assemble it. Shake it while upside down, mine had some granular debris inside. This could be rust.
You can reduce the odor and increase the life of the tank by coating the inside with paint. Do not use an oxidizer, such as bleach to remove the tank, as it will cause it to rust quickly. This is an optional step, to do this: screw a 3/4" male PVC threaded fitting in the top of hole where the valve was. Pour about a pint to a quart of any type of paint through a funnel into the tank. Swirl, slosh and everything else you can think of to coat the inside of the tank. Pour the remaining paint out, leave upside down for several days to dry. You can speed up this process by repeatedly blowing low pressure air in the tank to purge the paint fumes. Once dry, remove the PVC male end, the only real purpose of the PVC is to keep the threads paint free.
Next, you'll need:

1. 3/4" to 1/4" black pipe bushing. These are available at Lowe's for less than $1.
2. You'll need a 1/4" MPT air male-male air coupling. A 1/4" X close black pipe will work if the above is not available.
3. 1/4" female thread quick disconnect female air hose coupling.
4. 1/4" male thread male quick disconnect air coupling.
5. 1/4" female thread male quick disconnect air coupling
6. Last on the list is pipe dope or teflon plumbers tape.

Items 2-5 can be purchased from Wal*Mart in a single package for less than $5. I believe the brand is Campbell-Hausfeld similar to this one.
Apply the teflon tape to the 3/4" bushing, turn in as tight as possible with a wrench. Apply teflon tape to the 1/4" coupling, tighten. Be careful as to how tight, I snapped the first one off while testing this idea. And last add the quick disconnect coupling. Tighten. You will need to use extra tape between the black pipe thread and the MPT air thread to make it seal. Extra is probably around a foot or more.
To fill, screw the two treaded male quick disconnect couplings together (one NPTm one NPTf), you do not need to worry about teflon tape. Connect one in the tank's quick disconnect, the other in the female quick disconnect of your air supply line. Allow to fill, no more than 150 PSI. (This is the maximum pressure rated on the black pipe bushing) Check for leaks using water. If bubbles appear, you need to tighten the connections more.

You may want to remove the air-supply line female quick disconnect, leaving the double ended male fittings in the tank, turn the tank upside down and let it vent. Do this several times to minimize the propane odor. Eventually it will go away, but how long is anyone's guess.

Two last optional steps, so your neighbors do not think you're filling your car tires with propane, paint the outside of the tank a different color. I went with black so when the sun warms the tank, increasing the pressure. You may want to use a different color, since a hot tank will hold less air than a cool one when filling. Last, affix a DOT Diamond Hazmat Non-Flamable Gas #2 label. I got one from the local welding supply shop (GTS Gas Services) at no charge. I can't see paying more than one dollar for a sticker like this at even the most overpriced store that would have one. Another possibility is big rig stores.

And you're done. A cheap and easy portable air storage tank for about $6, and 15 minutes of work. Although if you're like me, you probably have a lot of the couplings needed laying around the shop, so it may cost you less.

One more note, if you're thinking this is a good idea but do not already own an air compressor, supply lines and tire-filling adapters, don't bother, it will cost you more than buying a similar 5-gallon portable tank from Wal*Mart (about $20), but less than an equivalent tank from anywhere else (starting at $30).