Why bother?

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

Is a propane tank safe for air?

There are several basic questions:

• Does the basic construction of a propane tank give it enough strength to safely store compressed air at the pressures used for pneumatic props?
• Can the conversion be done safely?
• Does the conversion compromise the integrity of the tank?

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

• At a temperature of 70° F, propane's vapor pressure is 124 PSI.
• At a temperature of 162° F, propane's vapor pressure is 390 PSI.

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?

• freon

  I have seen kits sold for the purpose of converting old freon tanks for compressed air storage. I don't know whether or not this conversion is safe.

• disposable "balloon" helium tanks

  I have seen balloon kits for sale that some with a quantity of balloons and a disposable tank of helium mix. I don't know whether or not this conversion is safe, or even if anybody has tried it.

• dry chemical fire extinguishers

  In the spring of 2003, the Halloween mailing lists made frequent discussion of "air cannon", which require local air storage. One suggestion for air storage was used fire extinguisher tanks.

  On 4/17/2003, "mutant" posted to Halloween-L

  Yesterday at work I found somebody in my parking space. It was the guy who had come to recharged the 500 some odd fire extinguishers that our city has. I asked him about the threads used for the extinguishers hoping to find a answer for those of you who have been playing with these. When I explained what the uses was for he rolled his eyes, and informed me that these might not be the best items to use as a pressurized reservoir. He stated that the only use about 150 psi in them, and that they are not designed for multiple charges and discharges.

  He strongly recommended not using these for props, however in our discussion I mentioned the use of propane bbq tanks, he said that those are designed to withstand much higher pressure, and work more than likely work fine.

  I suspect that he was referring to dry chemical extinguishers.

• CO2 fire extinguishers

• just buy an air storage tank

  Well, they are for sale.

Detailed Procedure

Getting A Propane Tank

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

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

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.