Hydrostatic testing air compressor tank

[Tony Wells]

Ever thought about how much energy is stored in a large compressor receiver? Enough to be fatal if it were to rupture near you, and cause severe property damage in your shop. I was given a 1964 two stage compressor that has been out of service for a while, and I am working on getting it back to operating condition again. I worried about the safety of the reservoir since I had no idea how well it had been taken care of as far as keeping it drained. They tend to rust out the bottom after a few years of neglect. So......I wanted to test this tank. It had originally been certified to run at a working pressure of 200 PSI. What I did was to bring it into the shop, fill it with water and plug all the holes, save one. See the picture for what I put in that hole.



Here I have a Festo air cylinder with a tee for the gage hosed (high pressure hydraulic hose) to the tank. On the tank drain, I had a 1/2" quarter turn ball valve, and bushed it down for a male quick connect air fitting. With the system full of water, I pressured it up gradually with the shop air, and then applied pressure with the arbor press to get it up to about 300 PSI, and bounced it a few times to simulate a bit of shock loading. All I found was a 1 1/2 X 1" NPT reducer bushing in one of the holes had split from someone over-tightening it and there was a stream of water out of it. Fixed that and it's all good now. I will proceed with the blast, prime and paint, etc. It pays to be cautious and never assume something is safe when it could potentially kill you.

 

[Bill Gruby]

Thank you for sharing that with us. I would have expected no less caution from you. Well done Tony.

"Billy G" )

 

[November X-ray]

For anyone else considering doing this, a few words of caution. First, at a minimum spend some money if you do not already have and get at least a 50 foot hose to get a further distance from the vessel to the gauge (a pressure washer hose works fine). Second, if you can place something substantial between you and the vessel, like a vehicle or a building, this will offer some added safety should the vessel give up. Third, make sure there are no persons, pets or others nearby and barricade the area off if possible. Fourth, I would not use shop air to make up pressure as air will compress making an already potentially dangerous situation even more deadly due to the stored energy. Fifth, NEVER NEVER NEVER Attempt to do this with a pressure washer if you do NOT have the proper equipment and experience.

I can tell you from first hand experience that witnessing someone get killed from a component failure associated with a hydrostatic test is way up on the list of the worst things one can experience!!!

 

[Tony Wells]

Actually, NX, there is very little danger of a catastrophic rupture using this method. Since the vessel is full of water, as far as possible, the volume of air is less than 3 cubic inches by my estimates. Even if seam split, the pressure of that small volume would drop so fast that not much would be ejected, and that would have a very steep pressure drop off. That's the entire reason for using water for this test. It would, however, make a mess! NEVER use just air for this. Besides, the additional compression from the cylinder would have very little effect on the air pressure of the 120 gallon volume of the tank. In other words, you could not raise the vessel pressure much at all above where the shop air had it.

I have used this method in design verification many times with new products in the fluid filtration business, sometimes deliberately to the point of failure. 'O' rings can fail due to poor design of the seal surface, or incorrect materials selection, or a dozen other causes. Materials can be stretched until retaining rings let go. I have tested vessels up to 3000 PSI, and had several fail, but contrary to instinct, it's not very spectacular. All you usually get is a little "tink" when something moves enough to relieve the pressure. Kind of disappointed me the first time. The key is to eliminate any airspace in the system. Of course, this requires eliminating a source of additional pressure being fed into the tank. Once I had pressured the tank up to shop air pressure, it was valved off. I have even pulled a vacuum on some vessels to ensure that there is no voids. Since water is virtually incompressible, if there is any failure, the pressure falls off so sharply that there is little energy to eject either the water, or any component that may fail with much energy.

Of course, reasonable caution must be observed around any pressurized container, regardless of what it is filled with, but hydrostatic pressure testing has been a standard method for a long time and is considered to be safe.

 

[4GSR]

Tony is right, if it was to leak or burst, it's just going to shoot out a stream of water for a couple seconds and quit. In fact, it won't burst. It'll create a pin hole leak at the weak point, once the pressure drops off a bit, will stop leaking. I know I would, for peace of mine, as others had said, use a longer hose, just to keep from getting wet from a stream of water.

 

[Tony Wells]

Well Ken, it was the only handy hose I had with 1/8 NPT on one and and 1/4 NPT on the other. I wasn't worried about getting a little wet. Those I have tested to a much higher pressure I filled with oil though, and used a porta-power pump, so with oil, I wouldn't want to get soaked. Kinda makes for a miserable day if you don't have a shop shower and fresh uniform.

 

[xalky]

That's absolutely correct. Liquids are not compressible, therefore they do not store much energy. Even if the seem in the tank failed completely, you would have nothing more than a wet mess, no projectiles. That's a great way to test pressure vessels. :thumbzup:

 

[Tony Wells]

OK, next on the agenda for this air compressor is addressing the years of rust accumulated inside the tank. Much of it is loose, and would likely wash out. I have had a look inside with a remote camera, so I have a pretty good idea of what the tank looks like inside. My quandary is deciding on which method to use to remove the rust from inside. One extreme is to build a set of rolls to tumble a pile of gravel and sand for a week or so. Doesn't sound all that practical or fun, but different, nonetheless. Doubt I'll do that.

How about the molasses treatment? Simple, straightforward. Lengthy. But time I have. This is for me and not a customer. Worth some thought I believe.

How about electrolysis? Sounds interesting. 120 gallons of washing soda solution and with 2 1 1/2" NPT openings on the top near each end, I could get pretty good coverage, but as I understand the process, it's sort of line-of-sight, so not sure how well suspending electrodes in those holes would work.

Lastly, the phosphoric acid conversion method. I have set up a manganese phosphate tank that hold pretty close to enough to fill the tank, and can be heated to near 200°f, then pumped into the tank. As much rust is there, I'd probably not want to use the solution for phosphating my parts any more, so I'd have to dispose of it.

Should I take a poll? Have a missed a possible method?

 

[12bolts]

OK, next on the agenda ...... is to build a set of rolls to tumble a pile of gravel and sand Have a missed a possible method?

Nope,
Go with the tumble method. it is tried and proven for loosening rust, and revealing problem spots. I doubt a week is required. And I am sure a clever bloke like yourself will come up with a way of tumbling a cylinder on the cheap. Afew ball bearings should do a good job

Cheers Phil

 

[Tony Wells]

Oh, I had already thought of a way. 4 casters on a wooden (4 X 4) frame and a belt drive. I wasn't relishing the idea of getting all the rocks and sand back out.:lmao: