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Thread: Rust inside compressor tank
05-14-2008, 03:08 PM #1
Rust inside compressor tank
Got a small ( 150l) compressor. The tank seems to be rusted badly inside - forgot to drain it for a few years. I wonder if I could cut the ends on the welds, sandblast it, paint and TIG weld it back. At least I could check how bad it is. How serious is an explosion of one of those ( I run aprox 6 bar ) ? Or do they tend to crack and leek air firstly ? How critical is the weld quality ? I am an innept TIG welder.
05-14-2008, 03:26 PM #2
Buy a new tank!
Compressed air explosions can be pretty darn serious a blast.
Maybe you can weld it up fine, maybe not. The price of failure in this case is having your arm or head ripped off in a blast if the tank lets go. Go spend a little cash buying a new tank instead.
05-14-2008, 03:33 PM #3
I have a friend that had an air tank explode in his shop. They are very much like a grenade when they go, throwing shrapnel everywhere. Pieces of the tank took at least a 3" by 5" section of my friends forearm right to the bone. His arm was nearly destroyed. After tons of reconstructive surgery and taking muscle tissue from other places on his body, they have made his arm usable again, but it looks horrible.
I would be safe and get rid of the tank.
05-14-2008, 03:50 PM #4
I got the point.
05-14-2008, 05:32 PM #5
05-14-2008, 05:50 PM #6
I would like to see some slow film footage of a tank blowing. Sounds like a job for the Mythbusters team?
Our setup at work comprises two 40hp 8bar screw compressors which run alternately giving about 5cu m/min, a dryer, oil/water seperator and an air receiver that is 7 feet tall by 3 feet diameter. All pipe work is 3" galvanised.
We use a lot of air! 90% of it goes to running four custom built air-over-hydraulic printing presses. Each one generates a stamping pressure of 20,000kg.
It has been said that if the tank blew, it would remove the side of the building easily. I would guess that the pressure wave in the workshop could cause some injury as well ( the tank stands in the corner ) the heat from the compressors is ducted to heat the workshop in the winter and to the outside in the summer.
05-14-2008, 06:16 PM #7
I'll have to concur with some of the earlier comments.
Air receiver 'explosions' may be quite rare, in comparison to tank failure by simple leakage due to pitting, but they have happened, so the use of a known defective air tank represents an unacceptable risk.
A new ASME grade air receiver is 'cheap insurance', all things considered.
There is another safety issue with compressors which may be too easily forgotten, that being to throw the disconnect switch to 'off' when leaving the building for the day. There was a major fire in a plant here, some years ago, for which the probable cause was a 'freak accident'. The compressor had been left 'on' when the workers left at the end of the day, and, sometime during the night, the line pressure leaked down to the point at which the compressor started........or, rather, attempted to start.
The compressor malfunctioned, either seizing up due to some internal problem, or being unable to start due to an unloader malfunction, and its motor ran with the belts slipping. Eventually, the belts got so hot from friction that they threw small glowing or flaming particles of rubberised cord, which set light to some adjacent flammable material.
That may be an extremely 'long odds' situation, but its a good reminder to be certain that the compressor is turned off at its disconnect switch before leaving the building.
(I know of another instance in which an air hose which had been left connected to the air line blew out (old or cheap air hose, doubtless), sometime during a week-end. The shop owner had forgotten to turn off the compressor, and came back to the shop on Monday morning to find that his compressor had been running continuously for some great length of time.......the compressor had gotten pretty hot, running continuously, but, fortunately, after an oil change, was alright.)
05-14-2008, 06:42 PM #8
We have some guy from the state that comes by every year or so and checks the thickness of our tanks. He uses some non destructive device to check the wall thickness.
We also use automatic tank drains. Not cheap but keeps the water out.
05-14-2008, 06:56 PM #9
He's testing the wall thickness of the tank using an ultrasonic thickness tester. They are OK but it's only a spot test. It can miss areas of pitting. There is an "area" utrasonic tester but as far as I know it only works on flat surfaces. Recently had a sub use one on the flat bottom of a 350,000 gallon fuel oil tank.
I do a lot of professional engineering work with compressed air systems and everything I specify is ASME pressure vessels, with a healthy corrosion allowance.
However, I can assure you that the typical small compressors, especially the imports, are not ASME pressure vessels and there sure ain't no corrosion allowance.
In my home shop, I always turn off the power to the compressors when I'm done and I always shut the outlet valves on the tank-always.
We had a construction worker killed a few years ago when his employer, a contractor, pressure-tested 24" polyethylene water pipes with compressed air. When the line blew up, it killed one guy and severly injured several more.
Ed in Florida
05-14-2008, 06:57 PM #10
Why don't you just hydro-test it to 9 bar (1.5 times your working pressure)? If it holds up, it may be good for a few more years.. Whatever you do, don't cut it and reweld it.
05-14-2008, 07:07 PM #11
Avoid welding it at all.
I avoid welding anything that is gonna hold high pressure like that. Ive heard of people doing some crazy stuff welding. Gas tanks, Heating oil tanks. You name it. Ive welded since I was a teenager and would never try it.
Just head off and get yourself a new tank, and kinda make a habit of draining it before you leave the shop for the day. It will take a couple of times till you get it in your head. But, like everything you do habitually it will become second nature.
05-14-2008, 07:17 PM #12
It's quite clear - I'll get a new tank. I'll mount it vertically that should help with the draining as well. Years ago I was quite close to an exploding truck tire - not a pleasant experience.
05-14-2008, 09:45 PM #13
Hydro testing the shop air tanks is an interesting idea and would be time and effort well spent on a weekend. Where would one learn more about "proper" (from adequate safety standpoint) if not officially excepted methods of said testing?
Aside from that........nearly 25 years ago at about age 22, I "made" the receiver on my current small (2HP) shop compressor. 100 lb propane tank, welded on inlet, outlet, and drain fittings (malleable iron pipe fittings, nickel rod) plus legs, and the pump/motor platform.
Tank completely full of water during the process. This was a 'new' propane tank, has never had LP in it....
It's been running most every day since.....drained weekly. 110 max PSI, used to be 125.
I bet the "authorities" would frown on it's presence in this day and age....
I should look for an ASME replacement.....
05-15-2008, 02:26 AM #14
I've read posts on this topic in the past, and it always makes me think. I have a compressor from the 40's or 50's. Original tank. I originally cleaned a LOT of rust out of the bottom, and now have a length of pipe on the bottom of the tank, to hold water prior to a ball valve for weekly or monthly discharge. ( The pipe thing is a nice trick, it holds the water, and lets you put the ball valve wear you can just kick it open, then back closed. )
So a question, has any one ever wrapped a tank? Fiber Glass or Carbon Fiber? I want to see how long my current tank will last, and think this could be an option for the OP as well? I am fairly confident that my tank will last 4 of my life times. However, if she does let go... the top 3/4 of the tank is 1/4" (or thicker). I'll be dead. I would assume that if it was wrapped it shouldn't "let go", it should just leak. Right? I know they use "wrapped" vessels in boiler systems.
Another idea. Maybe? Box it in a corner, then make a "releaf" or "blowout wall". Those are common in big transformer vaults, knowing that if she goes you won't contain it, so aim it.
If / When it blows up, no one will be hurt, and you can post the pictures.
05-15-2008, 02:48 AM #15
I'd inspect it.
Ok, I've PERSONALLY WHITNESSED an air tank explosion, it's SCARY!!! It sounds like a STICK OF DYNAMITE, and coule EASILY KILL if you were close to it. All that considered, I's still take a look inside the tank, and see how pitted it is. You can probably stuff a camera, ro get a good look inside with a mirror or something. If it looks like it has some bad pitting, CHUCK IT!, it's NOT WORTH the risk. But if it's just a thin layer of surface rust, maybe put some phosphoric acid in there and dissolve the rust. If in doubt CHUCK IT, it's JUST NOT WORTH THE RISK OF FAILURE! However, we ride around in rolling steel coffiins EVERY DAY, and there are things that are MUCH MORE RISKY than an air tank that's been given a good once over. So, there's no harm in looking inside to see if it might be a bunch of minor surface rust. I've seen tanks that were just surface rusted inside, and would clean up OK with some phosphoric, but I've also seen ones that I'd chuck in a HEARTBEAT!!!