In another thread, the discussion was about a small 12 volt air compressor. It was recommended by someone to use an old propane tank as an air tank. I won't get into that part of it, but I did comment that I've used water softener tanks as air pressure reservoirs. This thread is to describe the methods I've used to achieve just that.

First thing: Safety. In the other thread I believe I stated my tanks were rated for 200 PSI. Upon closer examination I see that I was wrong. Photos below will show the tank is rated for 150 PSI at 120˚F. I never pump my tanks higher than 100 PSI, so I'm within a reasonable pressure without fear of rupturing the tank - provided I don't drop them and crack them. Safety should be your first and utmost concern and should be a constant concern throughout the entire build - should you decide to pursue the same avenue I've taken.

More photos will follow.

First; the rating:

 

I'm having to save pictures as CR2 files then convert them to PDF, then to JPEG in order to upload them. As PDF's they're too big to load. So bear with me.

 

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.

 

As mentioned; the conduit PVC 2 1/2 thread to slip fitting

 

Finding a PVC bushing reducer in 2 1/2 has been impossible. I had to find something that fit and then modify it so that I could then insert the bushing. It's been many years since I built this and I don't remember all the steps. But if you're creative enough you should be able to find a way. If you have a bottle that is held closed by clamshells I'll post pictures of one of those bottles currently in process and using all PVC. Be patient.

The reducer in the photo is a 2 inch to 1/2 inch bushing type. It doesn't fit in a 2 1/2 inch slip fitting. I had to find something that fit - like I said, I don't recall exactly what I used. Sorry. But you can see between the conduit and the PVC is another pipe of some sort. I don't remember if it was a 3 inch PVC cut, heated and shrunk down to fit then welded in using PVC glue - probably lots of it.

 

The short bottle has clamshells holding what was once the water softener brain and Control Valve Assembly (CVA). I removed the dip tube and cut away the filter screen, though that might not have been necessary. I then found a PVC Union that would fit over it. Using some original O-Rings I sealed the bottle. This project is as yet unfinished, but at this point I have a 2" PVC slip joint I can easily install a 2" to 1/2" reducer and then, as with the other tank, do the plumbing.

 

By now you've probably noticed all the debris on the o-rings. Yes, that will need to be cleaned, and it's recommended the rings be greased before final assembly.

If you have any questions, comments or concerns please feel free to express your opinion(s) or ask questions.

Finally, SAFETY! Like I started out - you're building a pressure bomb. Safety needs to be your first, all encompassing and final concerns. IF you decide to proceed with a build similar to this - YOU DO SO AT YOUR OWN RISK. Though I've done it - I wouldn't recommend it. I'm nervous every time I run a bottle.

Next I'm working on a YouTube video. I'll post that when I've got that done.

 

In another thread, the discussion was about a small 12 volt air compressor. It was recommended by someone to use an old propane tank as an air tank. I won't get into that part of it, but I did comment that I've used water softener tanks as air pressure reservoirs. This thread is to describe the methods I've used to achieve just that.

First thing: Safety. In the other thread I believe I stated my tanks were rated for 200 PSI. Upon closer examination I see that I was wrong. Photos below will show the tank is rated for 150 PSI at 120˚F. I never pump my tanks higher than 100 PSI, so I'm within a reasonable pressure without fear of rupturing the tank - provided I don't drop them and crack them. Safety should be your first and utmost concern and should be a constant concern throughout the entire build - should you decide to pursue the same avenue I've taken.

More photos will follow.

First; the rating:
View attachment 178916

What is the temperature of the air entering that tank from the compressor? Put your hand on the discharge port and you are likely to find that it is well above 120 °F. Typical discharge temperatures from small single-stage compressors are above 180 °F. From multi-stage, high-pressure compressors can exceed 350 °F, though that doesn't apply in your case. Do you have enough tubing in between to be an effective heat exchanger? Even local heating of the tank near the point of entry can weaken it, both immediately and over time.

 

FOLLOWUP:

In steel tanks, when they're given their rating their designed to withstand at a minimum of 1 1/2 times the rated pressure. So a tank rated for 150 PSI service max will have been hydro-tested to 225 PSI. If the tank does not burst it's then rated safe for use by general population. This has been true of industrial water tanks as well as fuel storage and cracking tower tanks. I know - I've done the actual hydro testing on quite a few tanks. The largest tank I've tested was 12 foot diameter 100 feet long with 3 inch steel wall construction. If memory serves, the test pressure was 275 PSI. I also tested a small stainless steel tank for medical research. It's test pressure was something like 1500 PSI. Talk about a nervous cat in a dog house!

I'm generally confident with 100 PSI in my tanks, though I still always worry. If you want to hydro test your own system you'll need a pressure gauge and probably an electric high pressure washer, one you can easily control, then fill the tank with water leaving an inch or two at the top for compression, then pump the tank up using the pressure washer. But be careful not to over pressure it. Like all things electronic - exceeding the rating of a device can cause it to fail. In the case of electronics, usually it just leaks smoke. In this case, a tank full of water, should it fail, will go FOOP! The energy will be short lived and nothing much will happen, other than the ruination of your tank. But if you opt for air pressure, understand you're putting a lot of energy into the tank. Should it fail you're asking for a big explosion. That's why I'm harping on safety first and foremost.

 

What is the temperature of the air entering that tank from the compressor? Put your hand on the discharge port and you are likely to find that it is well above 120 °F. Typical discharge temperatures from small single-stage compressors are above 180 °F. From multi-stage, high-pressure compressors can exceed 350 °F, though that doesn't apply in your case. Do you have enough tubing in between to be an effective heat exchanger? Even local heating of the tank near the point of entry can weaken it, both immediately and over time.

Excellent point for concern.

As in the photos, there is a short curly air hose. Not exactly a great heat sink, but the air coming out of the compressor is not hot. The air has been compressed and stored in the 4.5 gallon tanks before exiting into the storage bottles. Given that as expanding gasses tend to cool, I've found more issues with water condensation than heat. Nevertheless, it's an excellent point to be made for anyone who may directly compress air into these tanks.

[additionally] PVC can't handle high temperatures without softening. As far as temperature goes - I've had my hands on the upper valve assembly during filling of the bottles. I've never noticed any heat to speak of.

 

I think I would go only with using old propane tanks and not a fiber-glass tank.
But I'm conservative that way.

 

FOLLOWUP:

In steel tanks, when they're given their rating their designed to withstand at a minimum of 1 1/2 times the rated pressure. So a tank rated for 150 PSI service max will have been hydro-tested to 225 PSI. If the tank does not burst it's then rated safe for use by general population. This has been true of industrial water tanks as well as fuel storage and cracking tower tanks. I know - I've done the actual hydro testing on quite a few tanks. The largest tank I've tested was 12 foot diameter 100 feet long with 3 inch steel wall construction. If memory serves, the test pressure was 275 PSI. I also tested a small stainless steel tank for medical research. It's test pressure was something like 1500 PSI. Talk about a nervous cat in a dog house!

I'm generally confident with 100 PSI in my tanks, though I still always worry. If you want to hydro test your own system you'll need a pressure gauge and probably an electric high pressure washer, one you can easily control, then fill the tank with water leaving an inch or two at the top for compression, then pump the tank up using the pressure washer. But be careful not to over pressure it. Like all things electronic - exceeding the rating of a device can cause it to fail. In the case of electronics, usually it just leaks smoke. In this case, a tank full of water, should it fail, will go FOOP! The energy will be short lived and nothing much will happen, other than the ruination of your tank. But if you opt for air pressure, understand you're putting a lot of energy into the tank. Should it fail you're asking for a big explosion. That's why I'm harping on safety first and foremost.

I used to stress relief titanium hydraulic lines after they had been fabbed and those had to be taken to 25,000 psi. We were VERY careful to make sure there was no air in those lines (which was actually a bit easier said than done) as even a small amountmade the ones that failed very impressive, even under the shield enclosure we used when we could. Unfortunately, some of the larger lines were just too long to go in there. Since we had to reach over the line in order to run the hydrostatic pump (not the best test stand layout the world has ever seen) we took extra precautions in purging the air from those.

 

those had to be taken to 25,000 psi.

YIKES!

Here's my hookup: I use the main pressure regulator to set the tank to no more than 100 PSI. The smaller regulator is used just before the air dryer to set the desired pressure on the line.

 

Some 40+ years ago, when I was a kid, I bought a C02 tank from a junk yard. You can't do that anymore. It does have a rupture disc. That was/is my portable air tank. The compressor has been dead for a while. It probably only has a 1/2 gallon tank on it.

A male-male quick disconnect was used to fill the bottle. Usually to about 160 PSI. There was a valve to atmosphere and a gauge on the compressor. I'd turn off the compressor, turn off the external tank and control vent the line to atmosphere. I could take the tank outside and add some air in a tire or two. The compressor tank had almost no volume.

 

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

 

@Tonyr1084

Did you ever consider wrapping the fiberglass tank with fiberglass packing tape for safety?

 

No, I hadn't. And I probably won't.

 

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 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. I have seen the result of a 50,000 gallon insulated tank of hot resin @ 300°F when condensate was blown into it while purging a line which flashed into steam. The top separated and flew like a frisbee for about 200 feet from the tank and completely cleared the intervening power lines to land in what was the only safe open area in a crowded industrial plant.

 

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.