Air Compressor - Hydrostatic Tank Testing

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Steve_in_Mich
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Air Compressor - Hydrostatic Tank Testing

Post by Steve_in_Mich » Sat Sep 20, 2008 2:05 pm

Air Receiver Hydrostatic Test

This is the way I test a tank, it is not a standard test procedure and you proceed at your own risk.

I position the tank so I have a port at the highest point for filling. All other ports plugged. Fill as full as possible with water (get all the air out). Install fittings as necessary (bushings) and install a nipple and cross fitting. One port for the test gage, another for pressurizing, and a bleed. Now fill the fittings also with water thru the bleed port if you can . I'll give you what I used years ago as I think you are more likely to have a grease gun than a portapower type pump and reservoir. I think a grease gun can be used to pump oil (be it automatic transmission fluid or all-trans hydraulic fluid) if held with the plunger end up but I haven't really tried it so I can't say for a fact. I have used general purpose grease and a standard grease zerk fitting in the pressurize port of the cross. The purge can be any kind of valve you have handy. Needle, ball, etc., I have a tire valve stem that threads into 1/8" pipe I have used. Need I say that all fittings should be rated for the test pressure you will be using. I remember once using a compression tester gage that had the little purge valve built right into it - I think that was an automobile tire valve stem also. I will leave it up to you what factor of safety you desire to test your tank to. If your compressor is rated for 135 psi maximum for instance a factor of safety of 2 would mean testing the tank at 2 times this max operating pressure, or 270 psi. The temperature of water and tank during the test is of lesser importance but I like to perform the test outside and exposed to a good hot sun - 100 degree day is fine. Remember compressed air pumped to the tank will be warmer than ambient temp. unless you are using a cooler/chiller. With the tank full of water you can now raise the pressure to the test pressure pumping grease (oil) in the zerk. A 60 gal. tank usually takes about one and 1/4 tubes of grease to reach a test pressure of 275 psi.. Sounds messy doesn't it. Pump until you reach the test pressure, wait an hour or so (assuming you didn't get wet already) and you will probably see the pressure has dropped a little. Tank expansion, grease compression I don't know but I usually see a few pounds drop and pump it back up to test pressure. I let it sit at pressure for 24 hours for all to come to equilibrium - nothing magic about 24 hours. First time I did a hydro test it was late in the day and I thought the test ought to have benefit of a good hot sun beating down on the tank and contents so I waited 24 hours for that to happen. I've been doing it that way every since. Once the pressure test is complete, crack the purge and expect a little squirt of water from it. Now you get to retrieve the GLOB of grease by opening the biggest bung on the tank to let the water out. I tried catching the grease using an old tee shirt rag as a strainer but water gushing out can push grease thru a rag at near 100 degrees F so I have since performed this tank draining in a sandy portion of what will someday be a driveway (maybe). The grease stays pretty much together in a big GOB or GLOB, sometimes 2 or 3 GOBS but that is as much as I've seen it separate. I chalk it up to, gob 1 the first tube pumped in, gob 2, change to the second cartridge of grease and gob 3, the final top off of pressure after the tank reaches steady-state. This seems to be supported by the size of the gobs that exit the tank. I scoop it up and smear the tops of fence post (rot protection) and/or save it for wiping down equipment to keep it from rusting (plow points and other ground engaging equipment). That's it, nothing fancy but it does give the best indication of the receivers ability to handle the job safely, at least for now. Hydrostatic testing is generally done on a schedule - every so many years. I won't say much about the frequency of testing as the only reference to that I've seen in print was for 4,000 psi air receivers we used in the Air Force to start Jet Aircraft. If you drain your tank often and monitor the condition of purge water from it you will have some idea of how much rust is free in the tank.

If you do use a non synthetic oil to pressurize, remember it floats on water and can be purged from the top of a tank if you leave yourself a means of supplying low pressure water to tank at some lower port. The idea is to catch the oil for proper disposal.

If anyone has another procedure for hydrostatically testing air receivers and suggested frequency of testing please don't keep it a secret, I for one would be interested to see how others do their testing.
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steamin10
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Post by steamin10 » Sat Sep 20, 2008 2:15 pm

Steve. While that is inventive, why go to the trouble? I have successfully used the grease pump, usually directly coupled to the tank being tested, and use water in a tub to pump it up. Plumbed to a gauge, a grease gun head can easily exceed #1000 lbs. No goo in the water, no problems. I had to find a better way to get good pressure, as one of those lab type hand pumps is over a hundred bucks as I recall, and I dont have money to collect dust on the shelf.

As an operating check there is absolutely no benefit from going over 150% of service pressure. 135 lb service, (mine) 200 psi test. (close enough)

Most likely, any problem, will be from pinhole corrossion in a 'pressor tank, and a hydro will blow it out. A visual with a small lamp dropped inside will give a good view of the bottom where the most action occurs.

A friend ruptured an air tank in his pickup, used for airing semi tires, it was not pretty, His service pressure was 150.

I suppose annual spring cleaning is the time to do the inspection, but I dont. I am lax in that area. I blow down all the time and squirt some 30 weight oil in the relief hole, to slow corrosion, even tho it vents out pretty fast.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
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magic9r
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Post by magic9r » Sat Sep 20, 2008 2:34 pm

I have a cheap hydraulic jack with a tin can as a larger reservoir and a pipe fitting brazed on in place of the jack mechanism, I water fill the tank and use cheap hydraulic oil in the pump, it's very quick, easy to reclaim the oil.
I didn't build it I inherited it and have had no reason to use anything else,
Regards,
Nick

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ronm
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Post by ronm » Mon Sep 22, 2008 9:05 am

I bought an old 7-1/2 HP Ingersoll Rand at a yard sale once, don't ask me why the guy had it, it was sitting in his front yard...my (then)BIL was in the pressure-washing business, & had 3ph. power at his shop. We wanted to hook up the comp. to see how it worked, but didn't want to risk it w/o a test, so we hooked his big Hotsy hose to the tank. filled it to the top, & put a 1000# gauge on it. The big washer pump had no trouble making 650#, we closed it off, & it held for 24 hr., no drop at all, no leaks. This was a 100 gal. tank. I "loaned" him that comp. for about 5 years, finally sold it to him, & as far as I know, it's still working away. That was a nice compressor, ticked over slow & quiet, & you could almost see the needle jump up every stroke.

Jose Rivera
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Very informative

Post by Jose Rivera » Mon Sep 22, 2008 11:41 am

This thread is extremely informative to me.

I have a question to those that have far more knowledge on these issues than me.

Should compressor tanks be pressure checked periodically?

I have two small ones, one is a Sears 12 gallon and the other that was given to me a Ingersol Rand 20 gallon.

I know the age of mine more or less which is about 28 years.

The larger one though I seldom use and although in good condition still, I don't know how old it is.

Reading this thread and steaming10 description of his friend tank makes me wonder if due to corrosion a tank can explode instead of just starting a leak.

Thanks
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steamin10
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Post by steamin10 » Mon Sep 22, 2008 12:18 pm

Jose: the truck driver situation, ( he is a Jose too). The 40 gallon tank was under a gas driven heavy compound compressor, bolted into a 3/4 ton pickup bed, with a light aluminum cap over the bed. Given the service and kind of guy he was, I cannot vouch for relief valves or anything else. Kind of "air it up and GO' kinna thing. So while he fretted over some issue the compressor ran and the tank ripped along a corrosion line at the bottom. The release drove the compressor into the cap making a humpback out of it, and the lightweight bolts ripped out of the truck bed, probably rust weakend. All told it was an impressive failure that literally totalled the bed of the truck. The tank itself was bloated to a basketball shape, and could not resume a stance on 4 legs.

Lightweight portable air tanks, for tire inflation, usally fail by holing just behind the weld, where the body overlaps the end cap. These are quite thin, and inexpensive to replace, I have lost 2 over the years. I tried to spot weld one but the thining process made it a challenge to weld holes together. -scrap-
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

Jose Rivera
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tanks

Post by Jose Rivera » Mon Sep 22, 2008 12:39 pm

Thank you steaming10

I have never heard of home use compressor tanks failing by rupture, not even by small holes created by corrosion.

Mine although always under pressure stays pretty dry. I just drained it and although that last time I check for water built up was some five years ago or more, all it came out was no more that three or four spun fulls, California weather I guess.

Seeing this I assume that these tanks are still pristine inside.
There are no problems, only solutions.
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Retired journeyman machinist and 3D CAD mechanical designer - hobbyist - grandpa

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steamin10
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Post by steamin10 » Mon Sep 22, 2008 12:52 pm

All compressed air carries water. The dew point is the key. Here in the Midwest we are having foggy nights at 45 degrees, in the flatlands. 70-80% humidity at 85 summer temps are not uncommon here. Making air in the shop can create gallons of water in a week for me. (80 gal vert tank) I plumbed a copper line outside to weep all the time to control it, as auto drains and such fail too often, from the oily rust/dust mud that gets out. Certainly climate has alot to do with the variable needs.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

10 Wheeler Rob
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reguarding testing

Post by 10 Wheeler Rob » Mon Sep 22, 2008 7:49 pm

I would think they are a lot like propane tanks:

New ones good for 10 to 12 years of normal use. Probably not a bad idea to hydro when new, unless it's an ASME Stamped tank, then it's already been done.

Then hydro ever year or two, at 1 1/2 times you relief valve rating.

If your around salt, then hydro ever year.

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steamin10
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Post by steamin10 » Tue Sep 23, 2008 1:40 am

ASME code is kinna BS, as it is mostly a safty spec standard for as built design, meaning air recievers are built to minimum 4x strength standard. I dont have my OSHA book right in front of me for 29cfr's, but I think an air reciever is on a ten year life check for inspection. There is a followup interval. I will have to check that. Be carful when you refer to asme code, as it is standards agreed to by American Society of Mechanical Engineers, and is a spec book for pressure vessels of many kinds, and standards that may be diferent for other purposes, like boilers. For example FRA ignores many asme standards and invokes their own as it applies to tank cars especially, and railcars in general. They spec material , weld proceedure and inspection for all rail services. Once design is established, verified prcedure must be followed.

Rob: That is apples to oranges. LP tanks have a long life as they are not exposed to water internally. LP turns liquid at something like 40 below but has a low vapor pressure. The water is frozen out in terminal process storage. Also the liquid carries a trace amount of compressor oil from processing, into the tank. It generally coats the inside of the tank, surface pores of the steel, and carries the ID smell injected into the liquid LP also. It is also why you should not use an LP tank for air service unless you heat the open tank to 400+ degrees to drive off the oils from process. I used a 100lb cylinder for backup storage for years, and had stinky air. I got warned about the conflict of air storage later. I took it out of service. I had 6, 20 lb tanks, when they changed the non-full valve rules. The tank is fine, the valve is diferent, now tricorn shaped handle. A retail valve will cost almost as much as a new tank.

FYI: it is the same reason you cannot use a CO2 pressure bottle for oxygen under any circumstances. The bottle is stamped CO2 service on the neck, and has oil that escaped from delivery pumps and compressors coating the inside of the bottle. It is bad news when pressured with Ox. It can explode. Oxygen and other industrial bottles (scuba too) must be hydroed every 5 years, and the bottle service stamped for checking date, and company. Lp tanks are low pressure bottles and fall under diferent rules.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.

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ronm
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Post by ronm » Tue Sep 23, 2008 10:00 am

I've told this story numerous times on various forums, prob. this one before, but: My Uncle had one of those little 2HP roll-around compressors, it was prob. 20 yrs. or so old, & one summer night they had forgotten to unplug it; it kicked on & the pressure switch apparently stuck...the tank opened up like a banana peel, & went through the metal wall of the shop...made a hole a man could crawl through. They found the tank about 50 ft. away, the pump 100 ft. out, & NEVER DID find the motor... :shock: my cousin was sleeping on the back porch, & he thought WWIII had started. :D Since then I've been a fan of hydro-testing & draining water once in a while...

magic9r
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Post by magic9r » Wed Sep 24, 2008 5:52 pm

My current main compressor

Image

is a 1950's job and the receiver has not yet failed or failed test, I have a suspicion it probably never will, it was built before some gimp with a calculator could work out just how little metal he could get away with :D
Regards,
Nick

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