Converting A Propane Tank For Compressed Air

We like to give each pneumatic effect in our haunt its own local air storage. We do this by converting old propane tanks into air storage tanks.

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Why bother?

In order to activate a pneumatic prop, you need a way to feed it a supply of compressed air. The usual setup requires a compressor, a bit of hose, and a valve that can be used to turn on and off the air flow as desired.

But when a prop is activated, it consumes some of our compressed air and the compressor must work to replace it.

The compressor makes all the pressurized air that we need, but it might be tempted to run its motor all night long. This consumes power, generates heat, and makes sound that might distract from the effects. In order to keep the compressor motor from running, we provide air storage tanks. The motor runs only until the storage tanks are filled, then shuts off until the storage tanks run low again. Larger compressors have storage tanks of their own, and you can easily add more air storage with additional external tanks.

We have elected to give each air-powered haunt effect a little air storage of its own; as you add props, you add air storage to operate them.

 

General Procedure

[photo] Construction is simple: get empty propane tank; make sure it's empty; unscrew and remove the two valves; wash out with soap and water; drill and tap for 1/4-inch NPT; clean; paint; replace the pressure-relief valve.

 

Is a propane tank safe for air?

Thanks for asking! We take safety quite seriously.

There are several basic questions:

The latter two issues are personal, and depend on the exact materials, tools, and skills of the person doing the conversion. We will delve deeper into the first issue - of the design limits of the propane tank.

How much pressure does propane generate? It depends on the temperature:

Propane tanks are equipped with pressure relief valves that vent excess pressure. If the tank gets too hot, the tank will fart instead of explode. :-)

Normal propane cylinders, such as those used with portable grills have pressure relief valves 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.

During manufacture of propane cylinders for engine fuel storage, each propane cylinder must be exposed to two times its service pressure, and one out of every 500 cylinders is exposed to four times its pressure, or about 960 PSI.

It would seem that propane tanks are built to withstand at least 250 PSI. I think it logical to assume that any propane cylinder you are likely to encounter can withstand at least 200 PSI virtually indefinitely.

So, what kind of load will we be putting on them? Regulations for home and shop air compressors limit them to 125 PSI.

I think propane tanks can safely handle the pressure that a home compressor can dish out.

 

Are Other Tanks Suitable?

There are plenty of different kinds of tanks out there intended for use under pressure. You might be tempted to use one of them instead of a propane tank.
 

Detailed Procedure

 

Getting A Propane Tank

Here is an assortment of old propane cylinders. We use the little tanks, intended to power camping lanterns, soldering torches, and camp stoves.

Technically, these tanks can be refilled, but it is illegal to transport them after refilling. Effectively, empty tanks are worthless.

If you don't use propane, I suggest asking friends, especially those who camp out, for an empty. These are usually thrown away, and finding another use for them is a good thing. You might also ask at stores that sell camping equipment. Perhaps, as a service to customers, they accept old tanks. You might also check your local dump or recycling depot.

I wouldn't try to empty a full propane tank. That's wasteful. But if you really want to start with a full one, it is possible.

The top of the tank should look like this.

The large externally threaded opening in the center is where the propane-using appliance hooks to the tank. This is also where the propane is pumped in at the factory.

The smaller hole to the left contains a pressure relief valve. If the pressure inside the tank gets too high, it dumps some from this valve.

The center hole contains a check-valve that will only dispense propane when a properly designed appliance is screwed on.

 

Finish Emptying The Tank

Even when the tank runs so low that it is no longer useful, there is probably still some propane inside! We must finish emptying the tank.

The small propane tanks used for this project don't have a valve you can just open to let the gas out. They have a safety connection that only lets out gas when the tank is attached to something that uses it. In order to empty the tank, you have to connect it to some gadget that uses propane. This might be a lantern, stove, or torch.

In my case, I attached a torch, turned on the gas, and didn't light it. And out goes the gas. It is important to do this outside, and away from all sources of ignition. Since my tank was nearly empty, venting the flammable gas to the atmosphere wasn't much of a safety hazard.


If you want to empty a full tank, I suggest using it up: hook it to a stove or something, light it, and let it burn out.

After the tank is empty, remove the tank.

Now, there are still two things in the tank: first a little bit of extra gas that doesn't have enough pressure to push out; second, some of the "odorant" that they add to make the gas smell.

 

Propane Tank Preparation

The next step is to allow free flow of air in and out of the tank. There are usually two valves on the tank: One big one in the middle, where you hook it up to something; one small one to vent excess pressure.

Both valves have removable spring "cores". Just unscrew them and they fall out. The valves cores themselves are similar to those used in automobile tires, and they unscrew the same way.

Sometimes the screw-on cover on a tire valve has a little tool on the top that can be used to remove the cores. I guess the tool can be bought at auto supply stores. We made one by filing a notch in the end of a screwdriver.

Keep the pressure-relief core for later - you will be putting it back. The core from the center connection can be thrown away.

Now you can flush out the last of the gas and the "odorant" that is used to give it the "gas" smell. Squirt soapy water in one valve, shake it around a bit, and dump it out. The soapy water washes out the smelly stuff. It might take a couple of washes. Then rinse it out, and fill the tank entirely with water - to be sure that there is no gas left inside.

Dump out the water, and it is time to drill out the big valve and tap it.

Wash the tank again, to get rid of the cutting oil you used during the drill and tap. Dry the inside thoroughly. Then I suggest painting the inside to protect the metal from rusting from damp compressed air. I did this by pouring paint into the tank, swirling it all around, and pouring out the excess. Make sure you don't get paint on the new threads, or the fine threads of the small valve.

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