I'll look to see if I can find a 2 1/2 NPT and a bushing 2 1/2 to 1/2 in metal. However, the water system used plastic fittings and they're rated for potential house pressure of 100 PSI. They MUST be tested to a higher standard than that.

Also, the fiberglass tank has a plastic fitting at the top of it. The only differences I could find regarding conduit and PVC is that the conduit shouldn't be used for potable water because of hazardous chemicals that can leach out. Now that I think of it - - - what happens if a wire becomes overheated inside a conduit? Wouldn't that release hazardous gasses?

Anyway, what you see is what I did. Smart ? ? ? Maybe not. Does it work? Well, "Empirically" it has so far. Yes, I know, that doesn't mean it won't fail. That's why I don't leave it pressurized.

 

I'll look to see if I can find a 2 1/2 NPT and a bushing 2 1/2 to 1/2 in metal. However, the water system used plastic fittings and they're rated for potential house pressure of 100 PSI. They MUST be tested to a higher standard than that.

Also, the fiberglass tank has a plastic fitting at the top of it. The only differences I could find regarding conduit and PVC is that the conduit shouldn't be used for potable water because of hazardous chemicals that can leach out. Now that I think of it - - - what happens if a wire becomes overheated inside a conduit? Wouldn't that release hazardous gasses?

Anyway, what you see is what I did. Smart ? ? ? Maybe not. Does it work? Well, "Empirically" it has so far. Yes, I know, that doesn't mean it won't fail. That's why I don't leave it pressurized.

One item not mentioned is that the fittings are not what bursts, worst case they will just leak. So the hazard is not with any fittings that you use. Any possible burst hazard is in the pipe itself, and possibly in the joints.

 

Also, the fiberglass tank has a plastic fitting at the top of it. The only differences I could find regarding conduit and PVC is that the conduit shouldn't be used for potable water because of hazardous chemicals that can leach out.

Also don't forget that your water tank never sees anywhere near the pressure your putting on it in its real purpose.
https://www.plumbingsupply.com/residential-water-pressure-explained.html

 

@shortbus My typical water pressure is 65 PSI. I've seen my pressure as low as 40 PSI and as high as 95 PSI. I know this because I have an RO (Reverse Osmosis) drinking system with a pressure gauge. There's also a prelate pump which will boost the RO pressure up to full line pressure. If pressure drops back down later the tank pressure remains constant until someone uses the water.

I've heard of people who regularly see 100 PSI water pressure. It's not very common but it happens. Manufacturers of water softeners must account for this possibility, and the possibility of even higher pressures. As you have seen in an early post, the operating pressure is rated for 150 PSI @ 120˚F (48.89˚C). One standard practice I've seen when rating a system is to under-rate it. If the tank is designed for 150 PSI then it is tested to 133% to 150%. Likely in the case of commercial water softeners, I'd say 133%, meaning it's probably tested - and withstood a pressure of nearly 200 PSI. Plastic threads and all.

While I admit I'm nervous running my system, I'm also relatively confident it won't burst on me. And should the plastic decide to give way, with the tank in the upright position and a nozzle of 2 1/2 inches on a tank that is probably 9 inches diameter, it's release of energy will be very short lived. The greatest danger would be to stand in the path of flying parts. They WILL be moving at extreme velocity.

Back in 2009 I was hydro-testing a tank of approximately 12 foot diameter and 40 feet length. After the test we used compressed air to drive the water out of the tank under test and send the test water back to a storage tank. There were numerous fittings on the tank ranging from 3/4 inch to 24 inch. When the pressure dropped down to just 2 PSI I decided to hasten the bleed-off by opening a 2 inch port. I unscrewed the plug. When the last thread cleared, the sudden pressure blew my arm to the side. I didn't get hurt, but even at 2 PSI with a volume of 50,420 cubic feet at 2 PSI, there was a lot of energy still in the tank to be bled off. I quickly learned to not do that again. The tank in question is approximately 9 inch diameter and about 48 inch tall. A volume of 3,052 cubic inches. Yes, that's a lot. But venting through a failed 2 1/2 inch outlet will result in a rapid reduction of stored energy.

I realize everyone's concern for safety here. I appreciate it as well. I too am a bit skittish around the tank. After all, I've never hydrotested it myself. I COULD should I choose to, all I'd need is a pressure gauge that can read up to 250 PSI. Since it's rated for 150 PSI I would test it to at least 200 PSI. Full of water, if it should fail there would be next to zero energy stored in the tank. Perhaps one day for peace of mind I'll do that. For now I'll seldom use it. In the past 4 days I've used it each of those days. Two days for spray painting using a HVLP sprayer running at 29 PSI, and the last two days at 40 PSI to operate an air hammer with chisel (for removing construction adhesive from decorative bricks I've been moving around. That makes 4 times this year I've used it. Last year I used it 3 or 4 times to operate a pin-nailer gun at 100 PSI. I have yet to experience any kind of failure. Also used the tanks to blow down my sprinklers last year, also at 100 PSI. But NEVER above that.

Geez! Now you got me thinking. I'm probably going to order a gauge big enough to do the test. Then I can test it to 200 PSI. If it holds at that then I will never lack confidence in my tank again.

 

At work I saw and heard aSCH 80 12" diameter by 40' long PVC shop built tank blow when they were testing it for leaks, with 90PSI shop air. The tank was to be used as a vacuum tank for a row of rubber molding presses, but the engineer said it would be faster to check with the pressure instead of pumping the vacuum.

Luckily no one got killed. when it blew pieces of plastic the size of dinner plates but all jagged shapes went clear across the plant over about 65 feet. And buckled the angle iron and expanded metal cat walk that was over it. This was into and over an assembly area that would have had around 75 people working if it hadn't been lunch time. Ever since then when someone says PVC and air pressure I try to talk them out of it. Not trying to pick on you just wanting to keep you alive.

 

testing it for leaks, with 90PSI shop air

That is the exact reason for hydrostatic testing. They should have at least filled it with water before putting air pressure on it.

 

At work I saw and heard aSCH 80 12" diameter by 40' long PVC shop built tank blow when they were testing it for leaks, with 90PSI shop air. The tank was to be used as a vacuum tank for a row of rubber molding presses, but the engineer said it would be faster to check with the pressure instead of pumping the vacuum.

Luckily no one got killed. when it blew pieces of plastic the size of dinner plates but all jagged shapes went clear across the plant over about 65 feet. And buckled the angle iron and expanded metal cat walk the was over it. This was into and over an assembly area that would have had around 75 people working if it haden't been lunch time. Ever since then when someone says PVC and air pressure I try to talk them out of it. Not trying to pick on you just wanting to keep you alive.

12" dia x 40 ft = 54,259.2 cubic inches of air. That's a lot of air that has to go somewhere when the container fails. That, versus my 3,000 cubic inch tank is a far cry apart. Still, even 3,000 cubic inches is a lot of energy. Put it another way: 54,259 cu. in. is equal to 31.4 cubic feet. My 3,000 tank is 1.74 cu. ft.

The more I hear theses horror stories the more inclined I am to do a hydro test on my tank(s).

 

You forgot to divide 90 psi by 14.7 psi (atmos) ~15 so multiply the ~30 ft^3 by 6 = ~180 ft^3 It decompresses and expands to reach atmospheric pressure,