Air Compressor - Hydrostatic Tank Testing

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Post by dirtcrasher » Sat Oct 18, 2008 6:46 pm

When I was 16yo I found a broken compressor with a tank and it was OLD!! I put a gas powered motor on it and that was a big pain, remember - I was just a kid! But finally I got a 3/4 HP electric on there. I was just so ecstatic to have an air gun, I always watched the garages with such envy :D

Anyhow, my dad warned me about having tanks tested every so often and I never listened. I didn't know how to wire the pressure switch so my procedure was to unplug it at 130PSI.

That worked well for a couple of years until one day I FORGOT that it was running. By the time I got to it it was at 200PSI and believe me when I say that this thing was old and struggling, I mean it!

Anyhow, not much of a crazy blown up ending to this story other than dad made me cut it up and throw it away after I told him about this experience :roll:

Now I buy the barely adequate Craftsman 150PSI compressors for about 275$ with free air tools every 5 years just to be safe. Funny thing is, I never get much condensation out of the tank when I drain it. And I drain it slowly as if you do it quickly the water turns to ice with the hifgh pressure to lower pressure quickly stuff.

I have to invest in a dual stage 50 gallon unit sometime soon now that I sandblast and powdercoat!
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Post by STRR » Sun Mar 08, 2009 4:52 pm

Hello All,

I realize the last post in this thread was several months ago. Being fairly well versed in air systems and hydro testing, I would like to add my two cents.

Oil in air tanks is not a bad thing. The coating of the interiors helps prevent the moisture caused rust. The bad thing about oil in pressurized air is DO NOT use the air for breathing air. The oil will cause severe health problems.

Draining air receivers. Drain, Drain, Drain! This should be done daily when you find moisture in the drain output. The draining period can be extended in areas where you don't find moisture in your drain output. I suggest you set up your own drain periods as follows. Start on a daily basis. If you get no moisture move to a weekly basis. If you get no moisture, move to a monthly basis. Never go more than a month between drains. It is important to get moisture OUT of the receiver. Various times of the year, and different weather patterns will also affect your drain periods.

Storage. Many professionals keep pressure in their tanks at all times. They do not keep the compressors on, they just "lock" the pressure in, by isolating the tank, when the compressor is off. The idea is: If there is already pressure in the tank, the compressor runs less, and thus less moisture in the receiver.

Hydro testing. With ALL types of tanks, we are talking about energy storage. Compressed gasses will store more energy than liquids. Thus the stories of great damage from failures involving compressed gasses. Using liquids to hydro test, is much safer since a failure will allow pressure to drop almost instantly.
Safety: Most manufacturers will recommend no more than 150% of maximum operating pressure for hydro testing. Most of the newer receivers will have a max. pressure listing stamped into the tank. Some will even have the test pressure stamped in. If you are concerned about failure consequences, cover the receiver with a tarp or blanket. This will minimize the spread of liquid and movement of the receiver in the event of a failure.
Use a fluid pump to pressurize. Two gauges to verify pressure is a good idea. Ensure there is a way of venting pressure and a relief valve should be included in the hydro rig. Testing should be done with the vessel in it's normal position i.e. standing up or laying down. The most likely failure point will be where rust has weakened the vessel. This will usually, but not always, be at the bottom where the moisture collects.
Duration: The majority of hydro testing lengths are 1/2 or 1 hour. If the vessel does not fail in that time period it is safe for use. Hydro testing spans (time between hydros), are usually 5 or 10 years. Use your judgment. If you are concerned about the receiver safety, conduct a hydro test.

One thing that wasn't addressed in previous posts: Dry the receiver as best possible prior to placing it back in service.

Also, as mentioned before, it is perfectly fine to put some oil back into the tank prior to return to service. I recommend adding oil, to a dry tank, and rolling the tank around to coat as much of the interior as possible. Then drain the excess oil.

Do not ignore inspection of relief valves, control switches, connecting piping, and any cooling coils or fins. All should be clean and lubricated, as appropriate, for proper operation. Relief valves should be tested for proper operation and set points. Use your hydro rig and fluid to do this. Cover the relief valve as appropriate to control any spraying.

One last comment: WEAR protective gear when conducting hydro testing. Face Shield, gloves, heavy clothing, will provide protection from any jets or sprays and flying debris.

I hope this helps clarify things and makes everyone be and feel safer.

Good Luck,

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Re: Air Compressor - Hydrostatic Tank Testing

Post by mac.doogle » Mon Nov 14, 2011 9:58 pm

Steve I have watched them test blend tanks just that way. No matter how many times I see it that scares the crap out of me.
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Post by magic9r » Sun Feb 08, 2015 12:41 pm

steamin10 wrote: FYI: it is the same reason you cannot use a CO2 pressure bottle for oxygen under any circumstances.

I think it's actually because the Vapor Pressure of CO2 doesn't get a whole lot above 800psi (Circa. 50 Bar) whereas Oxygen is normally stored at 230Bar or greater so the bottle doesn't have test for the required pressure.

Even if you tried then the burst disk (which is usually present in a CO2 cylinder to protect against Hydraulic Lock in the event of over-filling with liquid CO2 and subsequent exposure to high ambient temperature) would simply vent the cylinder.

Oil/Grease is a possible explosion, filling to over 3 times the SWP is a failure to communicate or comprehend!

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Re: Air Compressor - Hydrostatic Tank Testing

Post by redneckalbertan » Mon Feb 09, 2015 2:12 pm

Oxygen valves have a burst disk as well. Just for a different pressure.

I have seen bottles originally certified for oxygen service in the 30 ' s and 40 ' s still in use for CO2. Last time I saw one that old was about 10 years ago with a swastika stamped on the bottle. Must have been considered spoils of war, or something like that.

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Re: Air Compressor - Hydrostatic Tank Testing

Post by ken572 » Mon Feb 09, 2015 2:42 pm

Steve_in_Mich wrote: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.

Excellent write up, great detail, and about
as safe of a procedure that you can get. 8)

Thank You. :wink:
Ken. :)
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Re: Air Compressor - Hydrostatic Tank Testing

Post by choprboy » Thu Feb 12, 2015 5:46 pm

Interesting... I recently devised my own pressure testing method which nearly identical to Steve's method with one important (and probably cleaner) difference. Instead of using a grease gun to build pressure, I used an old hydraulic jack and water.

In my case, I was testing a 60gal tank from a 1975 Quincy compressor. After thoroughly cleaning the inside of the tank and checking for any defects/rust bubbles/dents (if there is any question... just dump the tank, its not worth the risk), I positioned the tank upright with a port open at the highest point. All of the other tank ports were then plugged with the exception of 3: one on the bottom a pump would be connected to, a second with a pressure gauge, and the third a top filling port. The tank was then completely filled to the top with water, rocked/etc to as many bubbles as possible, and then sealed tightly.

For the high pressure pump, I grabbed an old hydraulic with failed seals out of my scrap pile. The pump was disassembled, the ram removed, a piece of threaded pipe welded on to the end of the cylinder, and then connected into the tank. A small bolt was drilled out and turned into a barbed fitting on the inlet side of the hydraulic jack to accept water for pressurizing. Once setup, it was simply a matter of pumping the hydraulic jack to build pressure in the tank. As I recall, it took about 18-20 ounces of water to pressurize the 60gal tank to 300psi. (It can be hard to remove all the air and I suspect I still had a bubble inside the jack). Once the test is completed the tank can simply be drained out, no mess of oil or grease in the tank/on the ground.

My research indicated that most manufacturers seem to test new tanks at 50% over the rated pressure. Like Steve, I decided that a 2x safety factor seemed like a good test for a used tank. In my case, the tank was originally rated for 200psi, I intend to only run it at 150psi, so a 300psi test satisfies both conditions. Like Steve, I filled the tank and left it at pressure for about 16 hours. I also rapped the tank solidly with a large rubber mallet several times to try and provoke any possible cracks (it makes a very satisfying solid thud).

You will most likely have to add a small of water to compensate for leaks back thru the pump and pipe threads. The important thing is to look for any leaks where there shouldn't be; around welded fittings, seams and corners, and anywhere there are attachments to the tank. In my case I filled the tank with cold water in the evening, overnight it dropped to about 40F. The following morning I pumped the tank back up to 300psi and within a couple hours it went over 340psi just from the warming of the sun.

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Re: Air Compressor - Hydrostatic Tank Testing

Post by liveaboard » Wed Dec 16, 2015 4:58 pm

How about a high pressure cleaner pump? clean water at plenty of pressure, and you probably have one gathering dust in the back of your shed.
Of course, you'd have to be very careful with the trigger; mine does 200 bar, which is 2,900 psi.

I guess I should check my compressor tank. The old thing lives in the same steel sea container where I work.

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Post by Magicniner » Mon May 30, 2016 3:04 pm

STRR wrote:Oil in air tanks is not a bad thing. The coating of the interiors helps prevent the moisture caused rust. The bad thing about oil in pressurized air is DO NOT use the air for breathing air. The oil will cause severe health problems.
Any air fed mask designed for use with shop air will have a replaceable filter pack incorporating a charcoal filter to remove compressor oil rendering the air safe for breathing.

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Re: Air Compressor - Hydrostatic Tank Testing

Post by steamin10 » Tue Jun 07, 2016 6:54 am

Being a Diver and having been around air plants, I would caution anyone from assuming that filtered air from a common compressor is safe. Breathing air is easily contaminated by any vapor, and that includes lube oil vapors form the compressor section. Diving air from a high pressure compressor uses oils that are rendered harmless in the final filter stage, formerly vegetable oil based lubricants that are serviced regularly in breathing air. While the final filter uses charcoal and such designed filters, caution is needed to not assume air quality, as foreign gasses and vapors can be harmful in breathing air. That includes supply hoses that can get moisture and junk in use. Once dirty there is virtually no way to clean them.
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Re: Air Compressor - Hydrostatic Tank Testing

Post by tomjaksa » Sat Apr 01, 2017 1:27 am

A couple of years ago my friend lost his life to a compressor.
He was busy in his workshop with his son.
The compressor was outside. He heard his compressor motor straining. He went outside to check.At that moment the compressor blew up.
The pressure switch failed to switch off the motor.
The safety valve was for a 20bar system!!!

I now run my compressor ouside of the workshop. I have two safety valves .....they are not expensive.
Pressure switch also has a backup.
My routine is to check the safety valves once every week it takes 5 sec but could save a life.

Safety devices are like parachutes......if they not there when you need them............well.....

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Re: Air Compressor - Hydrostatic Tank Testing

Post by BigDumbDinosaur » Sat Apr 01, 2017 7:08 pm

My shop compressor has a drain valve that is held open for a couple of seconds each time the compressor starts. The valve is rated at 100 CFM at 150 PSI and is piped to the exterior of the shop through 1/2 inch copper pipe. Needless to say, it makes quite a blast when open and never fails to startle anyone in the immediate vicinity. :D

The (ASME rated) tank is 27 years old and was inspected last year for signs of internal corrosion. There were none, which shows the value of regular draining. There is also a filter and water trap on the tank discharge to keep crud out of the piping. Hence none of my air tools is water-cooled. :shock:

The small automatic drain valves that some use will eventually get plugged up with the smegma that all air compressors exude while running. That is why I have that large drain valve on mine. As it is electrically operated, it will always cycle on each startup, regardless of the existing air pressure.
I'm an old guy. What's your excuse? ☻

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