Air tanks

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Air tanks

Postby Bus Driver » Sat Oct 15, 2011 1:18 pm

This will no doubt bring out differing opinions. About 35 years ago, I encountered my first tank, 30 gallon, with rusted-through pinholes. I welded with the stick, the only electric welder I had at the time. Tested the tank to 410 PSI with water and a grease gun. My external fitting holding the gauge cracked at that pressure.That tank later developed another hole at a different location and I welded it with MIG. Still in use at 125 PSI.
Lately, I have welded up pinholes in three tanks in the 8 to 20 gallon size. All the holes were in the bottom. All three are Sears "portable" compressor units, probably 30 to 40 years old. Two were built by Campbell-Hausfeld, supplier prefix 106, and the third was built by Devilbiss, supplier prefix 919. All are belt-driven compressors. These are limited to about 100 to 120 PSI. Scott and Fetzer may have owned Campbell- Hausfeld at some time.
New replacement tanks of similar size are currently available from Grainger at incredibly high prices, in the $300.00 range.
Over the years, I have cut open several tanks to find that significant rusting is almost always very localized with pitting only in small areas.
Automobile cooling systems now range up to 20 PSI with aluminum and plastic radiator components.
And if the tank was going to explode, why did it not do so just before the pinhole developed? Why would it be more likely to do so just after a welded repair? And how often are explosions of air tanks reported?
So I test with the pointed end of a chipping or pick-type body hammer for soundness of the tank. My other, and perhaps best, test is to set the MIG welder to penetrate about 75% of the thickness of 16 gauge sheet and start welding around the pinhole. When the hole can be fully encircled without burn through, one more pass around the outside of that bead and then start filling in the inside of that bead "circle".
I bought two of the compressors complete and running with motors for scrap price. The third belongs to a friend who was going to junk his and had bought new Harbor Freight oil-free unit. The new was so disappointing that he asked me to look at it to see if it was operating properly. He took the new one back for refund and likes the repaired one better.
Good as new after welding? I doubt it. But my confidence in the repair is high and I now have an abundance of available compressors for my use at minimal cost to supplement the 80 gallon 5 HP unit in the shop.
Your opinion may vary.
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Re: Air tanks

Postby beaconlight » Sat Oct 15, 2011 6:01 pm

I had a much younger cousin do a science fair project that involved steam. It scared the living @$&** out of me. It had a 15 pound relief valve. His dad , my uncle was a tug boat captain and gotten the tank, gauge and relief valve off a scrap barge. I convinced them of the danger finally so they brought the combo to me for test. First I removed the relief valve and plugged the hole. Next I filled the whole apparatus with water and then applied 125 pound air from my air compressor. Fortunately nothing leaked, if it had so what a little stream of water and no one hurt. Next I replaced the relief valve and slowly worked up the pressure till the relief popped off. It, the gauge and the one on my air tank agreed. I felt comfortable with it at that point. I caused Eugene to make a brass plate engraved " Hydro-statically tested to 125 lb" and what ever the date was. He won a prize. One of the judges was from Marsh and Mclennan or how ever you spell it. The commercial insurance company. They awarded Eugene an extra award that was not part of the original contest for the Hydro test plate. I was proud of him and he of me for making him do it. At first they were going to disqualify him because of the steam. Oh he had a CO2 extinguisher with a favorable test date at the display too. You bring back happy memories of a family working together.
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Re: Air tanks

Postby Bus Driver » Sat Oct 15, 2011 7:32 pm

Steam accidents happen when a dry boiler is overheated and then water is added which then flashes into steam volumes far faster than any safety vent can handle. Pressure instantly rises to incredible numbers and disaster results.
I do not think that 120 PSI will rip open long places in a tank.
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Re: Air tanks

Postby John *.?-!.* cub owner » Sat Oct 15, 2011 7:55 pm

I noticed something unique about my 4 year old Coleman 60 gallon vertical tank compressor that bothers me. The bottom drain screws into a bushing that is welded into the bottom if the tank. The bushing appears to actually extend an inch or so up into the bottom of the tank, and leaves an area that holds an inch or so of water in the bottom and never drains out. What made me notice it was that I used the compressor for several days before any water came out when I opened the drain. After that time water comes out every time open the drain.
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Re: Air tanks

Postby Bus Driver » Sun Oct 16, 2011 7:40 am

Oxygen causes rusting. Pressure in the tank might "reoxygenate" that trapped water. But if it does not, then the trapped water does not further cause rusting once the dissolved oxygen in it is depleted.
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Re: Air tanks

Postby Bill V in Md » Sun Oct 16, 2011 9:24 am

I do a lot of work with ASME pressure vessels and am constantly amazed at how much air pressure a very thin-wall vessel will contain. As long as the tank is round, the forces remain uniform and they seem to last forever. Having said that, I would be afraid of welding pinholes on thin gauge steel, without a "backing-bar", and even then I am not sure I would trust it completely. I think you are doing the right thing by hydro testing first, because it is safer than a pneumatic test. Good luck with your repairs.
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Re: Air tanks

Postby John *.?-!.* cub owner » Sun Oct 16, 2011 11:57 am

Bus Driver wrote:Oxygen causes rusting. Pressure in the tank might "reoxygenate" that trapped water. But if it does not, then the trapped water does not further cause rusting once the dissolved oxygen in it is depleted.
I figure that if and when it rust through there will be a circle of pin holes all the way round tight at the top of the water, where the water and air meet.
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Re: Air tanks

Postby jwl » Sun Oct 16, 2011 1:30 pm

Bus Driver wrote:
I do not think that 120 PSI will rip open long places in a tank.



Oh really, the guy in one of these pictures was killed. The rest just got hurt a lot. Making up a tag and claiming somethings certified is against the law. Unless you have an R stamp for welding pressure vessels, you should leave them alone.
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Re: Air tanks

Postby RaymondDurban » Sun Oct 16, 2011 2:26 pm

jwl wrote:Oh really, the guy in one of these pictures was killed. The rest just got hurt a lot. Making up a tag and claiming somethings certified is against the law. Unless you have an R stamp for welding pressure vessels, you should leave them alone.

I can certainly appreciate the fact that something bad happened in the photos you posted, but you can't come into a conversation and claim such things with out giving us the story behind the pictures. For all we know, someone filled the tank with hydrogen, opened a valve and then had a smoke break. What happened, chain of events, pressures involved, heat, and on and on.....
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Re: Air tanks

Postby Bus Driver » Mon Oct 17, 2011 7:00 am

The photos show multiple compressors and tanks. This was a service facility for compressors? What service/modifications were done to the exploded unit? Suppose that the controls were changed and no changes made to the tank? Would one then blame the explosion on the tank?
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Re: Air tanks

Postby Landreo » Mon Oct 17, 2011 9:15 am

I have one of the green Sears compressors, 106 prefix, 1 hp, 2 cyl, still has the original ribbed belt. Belonged to my father, maybe 40 years old. Sat in my BIL's basement unused for 20 years or so until I grabbed it. Had a hole that I brazed 15 years ago. Still used the compressor until this spring when I got a much larger unit. Seemed like a good idea to braze the hole closed at the time but maybe not. I have no idea what the common failure mode is for tanks in an explosion, over pressure from a bad relief valve, metal fatique, rust weakening the metal?

In general I would say it is a bad idea to weld/braze the holes but I admit that that is exactly what I had done.

What is the technique for hydrotesting with a grease gun? Fill the gun with water?
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Re: Air tanks

Postby danovercash » Mon Oct 17, 2011 9:40 am

Where I used to work we had a vertical compressor suppling air to a car was machine. It developed a 1 1/2" crack at the top of the tank under the compressor mounting frame. Glad I wasn't there! I just replaced the whole unit, with plans to cannabilize the compressor later. BTW my shop compressor was made from a propane tank, it was originally used in the local post office to air up tires, probably over 50 years old,very heavy tank and Ingersol Rand 2 cyl. compressor.
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Re: Air tanks

Postby Bus Driver » Mon Oct 17, 2011 4:30 pm

Since water is essentially incompressible ( it does compress significantly at 50,000 PSI!) one fills the tank completely with water with a threaded opening at the highest point. Then a grease fitting is installed in that opening ( with tee and pressure gauge if desired) and start pumping in the grease. Grease guns are said to be capable of developing 10,000 PSI. I think testing to twice the operating pressure is adequate. If the tank bursts with water, less than a cup full will be discharged while all the pressure is relieved. No danger. One tube of grease was all that was required for the 30 gallon tank which was a former propane tank. Obviously any tank can be destroyed with enough pressure applied.
Removing and plugging high pressure relief valves, miswiring pressure switches so that they do not shut off the compressor, several other things are all setups for disaster. Compressors drawing over about 15 amps should be on a magnetic starter for long life of the pressure switch- and correct wiring of those is essential for safety.
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Re: Air tanks

Postby ricky racer » Mon Oct 17, 2011 6:09 pm

I went through some of the same issues with my compressor. The condensation collects at the bottom of the tank and that promotes the rust. With horizontal tanks the sidewall is made of much thinner material then the end bells. Because of that when I replaced mine I purchased an upright so it should take a longer time to rust through. I didn't want to replace mine either. I wanted to repair it too, but in the long run due to the potential for injury, I decided that the cost of a new compressor wasn't so bad after all.
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Re: Air tanks

Postby Eugene » Mon Oct 17, 2011 8:54 pm

jwl wrote:Oh really, the guy in one of these pictures was killed. Unless you have an R stamp for welding pressure vessels, you should leave them alone.

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#1: Damage appears to be caused by something other than the pressure most air compressors can produce.

#2: What is the source of the information?

#3: There would have been an investigation conducted in the case of an accidental death. Local newspaper would have published information on the accident and subsequent court cases?
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