Rusty air compressor tank

[WyEast]

I just discovered that our shop air compressor is rusty on the inside. I had put one of those automatic drains on it about a year ago, and this stopped working properly, so I would drain the tank manually (not as often as I should). This is a 5 h.p. compressor mounted on a vertical tank with a 175 psi. in the tank. I discovered today that the reason the auto drain wasn't working because it was clogged with rust granules. The compressor is 10-15 yrs old. Is there any way to determine the condition of the tank, or are the rust granules that clogged my autodrain enough to merit getting a new tank now?

[Forrest Addy]

Don't screw around with it. Air receivers are a pressure vessel having a finite life. Yours is a commercial shop therefore subject to bysybody inspections.

Strip all the hardware off that rusty air receiver, send it out, and get it hydro-ed. If it's OK put it back in service. If not replace it.

[Ian B]

Just my personal opinion on this one:

It doesn't take a lot of rust to block an autodrain - so the tank may not be too bad.

Does the tank have an inspection hole, that you can get your arm inside? If it does, clean the tank and then hydrotest it - I think 1.5 times working pressure is the usual, which would put the test pressure at around 265psi (make sure there's no air in the tank when you test!).

Check the bottom of the tank (inside & out) for deep rust pits - if it has serious ones, even with passing a pressure test, I'd probably junk it.

If it passes the above, remove any surface rust, properly clean & degrease inside and coat the inside with something to stop further corrosion - I used epoxy, which worked fine.

If the tank doesn't have an inspection hole, I'd be tempted to junk it anyway.

While you're at it, verify that the safety relief valve is working properly, and check its setting.

hth,

Ian

[Duct Taper]

Just curious... what happens if the tank bursts when hydro-testing? Does it violently explode, or just split and stop moving because the pressure has been instantly relieved?

[elbryant]

The idea of hydro-testing is that water doesn't compress (much), so if it breaks, there is nothing to become uncompressed explosively. That is why you want NO air in the tank during the test.
Commercial outfits are able to measure the "swelling" of the tank to detect weakness without a rupture.

Ed

[tattoomike68]

by all means have it tested for safetys sake.

my friend has a junk yard with a tank that blew up, it turned inside out and has a nice imprestion of the pully with the fan shaped ribs on it and all.(scarry stuff)
It blew up from the bottom.

[Forrest Addy]

Under hydro the tanks is solid full of water If it fails catastrophically (like a weld splits) you'll see a little squirt of water as the gage drops to zero.

A pressure vessel hydro is a bit more sophisticated than pumping water into at a specified gage setting. A good pressure vessel is perfectly elastic; it will "inflate" a trifle under pressure and return to it's original volume when the pressure it relaxed. Water will compress a trifle in proportion to the pressure it's subjected to thanks to its bulk modulus. Thus you have to pump in a small volume of extra water to pressurize a tank.

In a pressure vessel hydro a known amount of water is used to pressurize the tank. If after the test less water reappears in the graduated column than the pre-test amount that's proof the tank has yielded in some way and thus it failed the test.

Most small home shop and commercial air tanks can get by with a simple pressurize with water and hold test. This involves only a throrough clean up of the tank so pinhole leaks, cracks, and other seepages can be located. This is not an approved pressure vessel test but it will "prove" an air tank for a short peroid of time.

Most air tanks fail by "pinholing". The tank rusts through at the bottom of the pits. When the first pinhole appreas tell the boss to start making decisions because there's a thousand pits not far behind. Before long the tank will leak like a showerhead. You can surface weld the pinholes to patch them but sooner ot later you'll have weld zits all ofer the surface.

I've down quick and dirty pressure tests in the shop by plumbing the tank with plugs and piping with apppropiate connections, filling it solid with water, tapping it with a hammer to disloge evolved air, venting the air completely, then connecting a cheap grease gun filled with water and using it to pressurize the tank to a gage setting.

[gunsmith]

Short Story
A friend (elcheepo) decided to use a water tank for a compressor tank. The bottom, concave instead of convex rusted along the inner groove on the bottom. After about a year of use she blew. It lifted the compressor head the motor and all of the tank except the bottom which remained on the floor. The rest launched 15 feet through a steel roof and continued until it came to rest on the neighbors truck two doors down (about 280' horizontal). My wife seen it as it left the shop, roof trailing behind it. She said it was just like the movies as it hit the roof of the truck and all the window blew out. Damage to the truck, $21,000 and no insurance for "falling objects". Truck two weeks old.

[WyEast]

Hey guys,
Thanks for the input, I am going to replace the tank.

[wierdscience]

I consider all air recievers to be bombs reguardless of condition.I have seen the results of a 40 gallon tank exploding,not pretty at all.

BTW,if you replace the tank,you may cosider an after cooler to remove the moisture,BEFORE it sees the tank.

I don't like the newer compressors either,the tanks are paper thin and I don't see much of a corrosion factor on them.I wonder how many tank failures there are every year.

[This message has been edited by wierdscience (edited 01-06-2005).]

[Duct Taper]

I have often thought of pumping about a gallon of phosphoric acid into the tank to remove the accumulated rust and to give it the rust-resistant coating. Then do the same with baking soda to neutralize the acid, then do the same with some STP diluted with naptha, then let it thoroughly dry out before refilling so the STP sticks to protect the tank further.

Is this a sound idea, or just another of my crackpot off-the-wall ones?

[firbikrhd1]

Duct Taper, perhaps I'm just another crack pot, but about 18 to 20 years ago my compressor with 20 gallon horizontal tank developed a rust hole in the bottom. Being frugal with little expendable income at the time, not knowing better and having a new welder I decided I was going to fix it. I got a new piece of steel the same thickness as the original, cut out a section of the tank about 8" wide where the tank was most rusty and eroded and welded the new metal in place after forming it to the shape of the tank. It was overlapped and welded on the outside. Then I used inhibited acid, which is used to clean heat exchangers on water cooled A/C units, to remove ALL the rust in the tank. The tank was then coated with epoxy inside by pouring it in and rolling it around to cover all surfaces. The stuff I used is called Steelflex and is a perfect material for this application. It is used to coat the bottom of Air Boats so it's very tough, adheres extremely well and is flexible. End result, no more rusty water when I drain the tank and for 18 to 20 years it has held up.
I wasn't knowledgable enough to know about hydrostatic testing back then, or didn't consider it's merits, so I took the compressor out in the back yard, turned it on and let it pump up past it's normal 120 PSI shut off to about 150 PSI. No boom so I put it back in service.
The idea of an "exploding" tank does concern me though as I've seen the results when performing duties on a rescue call since that time. Not at all pretty! If I had to do it again, I'd buy a new tank and coat it inside right from the beginning to prevent the rust from ever forming. That may be in the near future anyway as the compressor developed a rod knock a few years ago and I figure it can't go on much longer.