Air compressor

[babyseal]

Picked up an old compressor dirt cheap. Wanting to hydrotest the tank before I expend a bunch of effort/money into it. Do I need to remove this (I think it's an unloader valve?) and cap off the fitting first? It's not wanting to budge, and I don't want to force/break it since the compressor works just fine. Tank looks fine inside, very heavy gauge steel. Powerwashed/flushed it out, stuck a scope in it, some rust/scaling but nothing alarming visually.

Thanks!

 

[babyseal]

Got the unloader valve (or whatever it is) off. Heated the pipe ******, cranked right off with a little persuasion. Pulled out all the ports, ran a pressure washer around inside where I could really get to the walls...lots of rust busted loose. Lots. Far more than I expected. 30 minutes of flushing it out before I had it all out and water running clear.

How much rust is "too much"?

Passed a 300 lbs hydro test no problem, left it under pressure for 4 hours.

 

[GeoBruin]

I think you're going to have a hard time getting a firm answer to this particular question. When it comes to issues of safety (which let's face it, this is) no one can tell you how safe is safe enough.

Good luck.

 

[babyseal]

I think you're going to have a hard time getting a firm answer to this particular question. When it comes to issues of safety (which let's face it, this is) no one can tell you how safe is safe enough.

Good luck.

Been stewing on it, I'm going to run it till I can find another tank. It's old, made of very heavy gauge steel, so I seriously doubt it's going to rust through overnight. It's a 60 gallon tank that's so heavy I've got to move it around with an engine hoist rather than just hossing it around. It passed the hydro, so it's "safe enough" in the short term. Heavy as the steel is, more likely to get a pinhole than fail catastrophically.

That said, I don't trust it a bit. Be sorta silly to get blown up in my garage at this late date to save a couple bucks. I'll put a 145 pop-off on it, set the pressure switch for about 125, and start hunting for a better tank. I've got a nice 33 gallon/3HP compressor that's fine for 99% of my needs, just needed a bigger compressor to run a sandblaster and media booth. Which I don't need at the moment, so I'll drag my feet putting it back together. Might find another tank first.

Got a very nice 5hp motor out of the deal, already cracked it open, went through it, replaced the bearings "just because", and it's in excellent shape...so I'm ahead of the game. Tank looked fine with a visual inspection, plan was to buy it for the tank and motor, slap a HF compressor on it, end up with a decent budget compressor. So...at least I've got a motor!

TANNSTAFL. You'd think I'd learn at my age.

 

[mark-NJ]

You cannot hydrotest this yourself. Hydrotesting requires equipment that you do not own.

In a hydrotest, there's a great deal more than filling the vessel with water & pressurizing. The vessel being tested needs to be placed inside ANOTHER (water-filled) vessel. When the vessel under test is pressurized, it expands. This causes displacement of the water in the outer vessel. This displacement is measurable. Measured & recorded.

The vessel under test is typically over-pressurized to 5/3 (five thirds) it's rated working pressure......and the resultant displacement of the outer jacket is recorded. When the 5/3 pressure is released, the tested vessel returns to it's original size....sort of. By looking at the displacement, the operator can see if the tested vessel returned elastically to it's original size within 5-10% [pass] or if the tested vessel had plastic deformation resulting in it not returning to it's original size (>10%) [fail].

(I used to test SCUBA tanks...this is the DOT process)

I assure you that you do NOT have the equipment to do this testing....don't even try.

 

[babyseal]

I assure you that it's too late. I've not only tried it, I did it. It's an imperfect world, and needs must when the devil drives.

Now, I can't afford to go plunk down multi-thousands of dollars on a new system. That means "used". Any used tank has rust inside, and you're rolling the dice. So you do what you can. Home hydro testing puts the tank under a load, safely, without running the risk of catastrophic failure. No more, no less. Is it by the book, does it measure elasticity of the tank, sonic testing, blah blah blah? Nope. But I'm not running a certification shop here, it's not a SCUBA tank running 3500 psi. and I can be reasonably confident that when I crank it up and put it under pressure for the first time, it's "probably" not going to explode. Doesn't speak to 6 months from now...or 6 years. Anytime you're using a used pressure vessel, with a questionable history, you're taking a risk. So you do what you can to mitigate that risk.

Probably, I'm going to cook it with electrolysis to remove the rust, so I can get a better look inside. If I don't see anything alarming, I'll coat the inside with rust converter/sealer, re-pressure test it...and use it till I find something better.

 

[Steve_P]

Agree that what the OP did isn't perfect, he knows that, but he did what you can do at home, and he did more than what 99.99% of people would do- which would be to just run it and hope for a pinhole leak to alert them to an impending failure. Pretty much no one that buys a used home or shop compressor is going to go thru the trouble of having the tank legally tested and certified; they'd just buy a new tank.

Decades ago, I worked with a guy that previously worked for a company that formed giant steel pipes by rolling sheet and then welding the seam. These weren't pressure vessels, but they were required pressure test them like the OP did- cap off both ends, fill with water, add air. Bang on welds with a sledgehammer. Yes, I know this is not the same thing as certifying a pressure vessel.

 

[LopezBart]

Agree that what the OP did isn't perfect, he knows that, but he did what you can do at home, and he did more than what 99.99% of people would do- which would be to just run it and hope for a pinhole leak to alert them to an impending failure. Pretty much no one that buys a used home or shop compressor is going to go thru the trouble of having the tank legally tested and certified; they'd just buy a new tank.

Decades ago, I worked with a guy that previously worked for a company that formed giant steel pipes by rolling sheet and then welding the seam. These weren't pressure vessels, but they were required pressure test them like the OP did- cap off both ends, fill with water, add air. Bang on welds with a sledgehammer. Yes, I know this is not the same thing as certifying a pressure vessel.

Agreed.

Annual (if operated) testing by filling w/ water and pressurizing to 150% of MWP is what steam hobby people do with their boilers.