They have gotten more expensive since I bought one but here is a typical Freon tank to air tank conversion kit. It has a gauge and a overpressure relief. Fill by shraeder valve and tank comes with open/close valve. Only problem I ever had was the rubber dry rotted and had to be replaced and tank eventually (after 30+ years) developed a pinhole leak from rust. Never painted it or it may have lasted longer. Worked great for bicycles, yard tractor tires and low car tires.

https://www.ebay.com/itm/Tool-Aid-9...ADAPTER-KIT-/251278903428?hash=item3a8163c884

The pinhole leak from rust was actually a pretty dangerous situation. Well before the pinhole appeared, the rust was weakening the integrity of the sidewall and it could have blown out.

This is actually a big concern with steel tanks that were never meant to be air receivers are used as air receivers. Often they were intended for propane or some other gas that could be counted on to be dry, but the air coming out a an air compressor usually has quite a bit of water in it, even in arid climates, that unless removed with a dryer of some kind, condenses in the tanks. If the tank doesn't have a purge valve located at the lowest point, that water collects and corrodes the tank. Tanks designed as air receivers not only have those valves, but the interiors of the tanks are usually coated to protect them.

EDIT: Corrected the good pointed out by Tonyr1084.

 

Which is why there are Instrument Air Dryers for pneumatic control system. Most compressor systems have a tank drain valve which sometimes has a float valve connected to blow the water down when it accumulates. Early instrument air dryers at out plant had a small tank to hold a Kotex napkin to merely soak up the water. Later regenerative desiccant dryers were added to the plant compressed air system. Eventually a dedicated Instrument Air supply system which was also used for pneumatic actuating cylinders.

 

the air coming out a an air compressor usually has quite a bit of air in it

I'll bet it does.

 

Tell you what would be a real hoot! Filling a bottle half full of water and then pressurize it and turn it upside down then crack open the bottom end and turn it into a giant water rocket. Remember those when we were kids?

Water Rocket

I produced just such a "water rocket bottle launcher" for a Cub Scouts group many years ago. With a 2-liter bottle half full of water and 100PSI from a serious tire pump the height was very impressive. What was more impressive was that the first version worked, given that I had not seen one and never saw the drawings and the person who described it had no concept of the words to use to describe the mechanism. So the only parts of my creation that were similar were the tire pump and the pop bottle.

 

Thing about my tanks - I only pressurize them when planning on using them. When not in use I blow them down. On the one tank with the fittings, opposite the curly-q hose is a standard hose bib. It's completely open. And when I blow it down the tank wants to push me over. Loud as heck too when blowing down. But it's very short lived, the noise. Then next time I want to run air I charge them up.

My fun creation is an air cannon made with a CO2 fire extinguisher tank, rated 3AA1800, meaning 1800PSI working pressure. So at 120 PSI it is very safe. Then a 1 inch port ball valve and a 5 foot barrel of 2 inch heavy wall aluminum conduit. It can shoot a "spit wad" made of wet newspaper stuffed in a single plastic bag quite a distance, about 100 yards. It is by no means a kid toy.

 

A friend made a cannon once. Details will not be discussed.

I made a projectile. Details will not be discussed.

We had been sanding and sealing his boat for much of the day when he got the idea to show me his cannon. I got the idea to show him my projectile. Perfect fit! We decided to go out to a field that was about 300 yards long. It was just after sunset when we set it off. We didn't expect to launch the projectile very far so we boosted the power. Then boosted it a little more. Then a little more. We ended up nearly clearing the field. At the end of the field were hedges. Beyond the hedges was a Winchel's Doughnut. A place frequented by the Ontario (California) law. That projectile rolled into the hedges, then there was a bright flash of light. Then the boom. Then the lights and sirens. We grabbed the cannon and ran like heck. Just got out of sight before the police helicopter cruised overhead.

We got busy stuffing the cannon and the rest of the projectiles up under the bow of the boat and hiding them under blankets. Covered in dust from all the sanding, we returned to the former activity. Minutes later a police officer appeared asking if we knew anything. He was leaning with his arm up on the bow of the boat, less than 20 inches away from the "Stuff".

We laughed about that for years. I wonder what ever happened to J.

 

How did water dockets get into this post- but here's another. Wasn't the Grand Canyon crossed with a water rocket or superheated water that turned to steam ?

 

The nasty thing about steel tanks bursting is that a fracture can spread at the speed of sound. Not the speed of sound in air but the speed of sound in a dense medium like steel. When a tank rusts out, it may start as a pin-hole. If you get a pin hole and you continue to use that tank then - I dare say - you may be a "Pin Head" for doing so. Pits and sharp corners are stress risers in steel. That's the point where fractures can occur. And like lightning and electrical current, the fracture will likely follow the path of least resistance. Given that the development of a pin hole is a sign of a compromised interior, there are likely far more points ready to fail. And when they do - they can do so catastrophically, leading to the evacuation of the bottom (or end) of a tank leaving all that compressed air to expand, and thus send the tank hurtling across the room.

Water in a tank can be a bad bad thing. That's why when operating the compressor I leave the purge valve ever so slightly open. Sure, the compressor comes back on sooner than later, but the tank doesn't sit full of water until I remember to purge the tank. Small price to pay. I'd rather ruin the pump than the tank. The pump can't hurt me like the tank can.

 

Unless I've missed this in the thread, I have not seen any mention of hydrostatic testing of pressure cylinders. This is done by filling the cylinder with water, immersing it into a sealed container also full of water that has a measuring pippet at the top. A small high pressure hand pump then adds watter to the cylinder, and as water is virtually incompressible, just a tiny amount will raise the pressure in the cylinder. As the tank expands, it displaces some of the water in the surrounding jacket which is measured in the pippet.
The manufacturers information on the cylinder will show the maximum amount of displacement allowed at the test pressure. Done this way, if there is a rupture, all that will happen is the pressure in the cylinder will drop abruptly and the small ammount of water that was added will simply enter the containing jacket with no risk to life and limb.
Anyone foolish enough to pressurise an unknown cylinder with air without knowing the safe working pressure and it's current inspection for corrosion etc, is either insane or has a death wish.
Part of my job required me to be a certificated inspector for high pressure vessels and associated equipment.

Something else that needs to be considered is condensation. As air is drawn into the compressor, so is the ambient humidity. As the air is compressed, so the water is litteraly "wrung out" and then lies in the bottom of the pressure vessel. There needs to be a valve/tap that allows this to be frequently drained to avoid rust forming and weakening the container.
We used very large de-humidifiers at the input to the compressors, and impingement water separators on the output of each stage, these had automatic blowdown valves that would expell the condensate periodically.
If you google "impingement water separators" you will see how they work. Originally used on steam engines to keep only dry steam from entering the cylinders to prevent hydraulic lock and consequent damage.

 

I would only worry about catastrophic rupture in 100 psi tank if the tank was exposed to extreme heat such as a building fire. I would be more concerned about a fitting being broken. Which is why OSHA requires pressurized tanks to be restrained.

Hola @SamR What the part in bold, does it mean in this context?

 

@atferrari see post #9. Hydro - testing = hydrostatic testing. Also see post #12 for similar testing.

 

does it mean in this context?

For fixed tanks bolted to the floor and for portable oxyacetylene tank carts usually chained to the cart to prevent tip-over. For portable air tank carts bolted to the cart. In our high schools Agriculture Shop, one of the boys yanked on the oxyacetylene hose to get some slack and the oxygen tank fell snapping the regulator and piping off. it flew about the shop caroming off pieces of shop gear before somewhat lodging into the brick wall of the shop. Luckily no one was injured.

 

@atferrari see post #9. Hydro - testing = hydrostatic testing. Also see post #12 for similar testing.

For fixed tanks bolted to the floor and for portable oxyacetylene tank carts usually chained to the cart to prevent tip-over. For portable air tank carts bolted to the cart. In our high schools Agriculture Shop, one of the boys yanked on the oxyacetylene hose to get some slack and the oxygen tank fell snapping the regulator and piping off. it flew about the shop caroming off pieces of shop gear before somewhat lodging into the brick wall of the shop. Luckily no one was injured.

Hola, thanks to you both and sorry. I should have asked what was the actual meaning of "restrained" here. By rereading several times I think I've got it.
Thanks again.

Workshops coming on board to cut stoppers /sea fastening of cargoes, fail sometimes to secure bottles properly, resulting in spectacular incidents with so many regulators brutally severed on the pier after falling.

 

Perhaps it is time to knot this thread?

 

Plastic 2 liter Coke bottles can hold 170psi ... You can make up a large pressure tank with a string of these using plastic water pipes , if one should fail it's only 2 liters of pressurized gas , the other bottles that make up the storage will in a controlled way , discharge through the pipe network out of the the ruptured bottle ...

Or you could just use large bore (100mm) plastic water pipe , very easy to construct and will go much higher than 170psi ....

Bottle water rocket constructors are not pushing the pressure high enough!

 

This is done by filling the cylinder with water, immersing it into a sealed container also full of water that has a measuring pippet at the top.

That may be how they do it where your from but not around here. Filling the cylinder with water would in many cases ruin it. They, around here, leave the cylinder full of what ever gas it is supposed to hold, with a gauge attached. And the tank is immersed in a water tank that gets pressurized, the gauge on the tank is then read and compared to the pressure on the water.

 

The top of the bottles (2 of 3) are threaded with NPT 2 1/2 threads. I've not been able to find PVC or ABS in that size but I have found electrical conduit that size.

Tony, I hope you don't mean you're using plastic electrical conduit fittings? They are not pressure rated like the standard plumbing piping fittings.

 

PVC schedule 40 at 73˚F is rated for 150 PSI (threaded); 180 PSI (socket) (not sure what socket means). I'm limiting myself to 100 PSI.

The issue of compressor air heat was raised earlier. Yesterday I ran my air chisel and used my tank at 100 PSI. The 1/2 inch fittings (black steel) were cool too cold. Another page suggests schedule 40 PVC 2 1/2 dia. is good for 300 PSI @ 73˚F. Working on an image.

 

I would not recommend using plastic pipe for a pressure container at any time under any condition. The reason is that regardless of what some labels say, the process control is not adequate to provide a safety factor that I would accept. The damage from a burst, or, more likely, a failed joint glue job, is not worth the risk. Discarded tanks whose condition can be readily evaluated are available, at least I have no problems getting them. Pressure testing at a safe distance does not involve water filling, just putting them on the other side of a suitable barrier. Having dismantled some pipe installations it is clear that the glue joints are not always good penetration, even if they have lots of glue applied.

 

PVC schedule 40 at 73˚F is rated for 150 PSI (threaded); 180 PSI (socket) (not sure what socket means). I'm limiting myself to 100 PSI.

True, but that is pipe use PVC, the electrical conduit(gray) is not rated that I could ever find. The water piping is made from "virgin" material, the conduit is not. Since it doesn't have to meet any standards that is why it is cheaper.