Not so fun news

This is second hand, but I edited a number of articles from the AWS on pressure vessels, so that's where this comes from:

The HAZ is not the central issue; tank-mounted compressors are the issue. The AWS says don't mount compressors on tanks but everybody keeps doing it.

It's an invitation to fatigue cracks and eventual failure. The HAZ provides a possible weakness, relative to the parent metal, along which the crack may propagate. But it's years of vibration that starts the crack, initiated at the hard spot located where the pump bracket attaches to the tank.

Once you have a fatigue failure, the tank is done for. Fatigue is a cumulative effect and a fatigue crack in one place doesn't necessarily relieve the stress, anyway. The fatigued area can surround the crack by a large area; the rest is just waiting to go over the fatigue limit and crack. If you weld it, you've just introduced another hard spot, which will transmit the vibration to the surrounding area.

This may be -- probably is -- true fatigue. True fatigue occurs at loads well below the elastic limit, after many millions of load cycles, for steel. It's actually a pretty difficult phenomenon to analyze, because, except for actual cracks, it's difficult to judge how far it's progressed without destructive testing.

Put that into your hopper; I'm not claiming to have a deep knowledge of it. But the AWS put out press releases and articles every few years about this very issue, and I read several of them.

-- Ed Huntress

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Hydrotesting is more than looking for leaks - it's also looking for permanent deformation (the tank under test is also submerged in water, and the amount of water that is displaced when the tank is pumped up is checked against when the tank is depressurized afterwards - too much permanent deformation is cause to reject a tank).

While we are a newsgroup full of folk that like to do things themselves, the only way this would be remotely acceptable (IMNSHO) would be to have a real hydrotest facility check it out and stamp it. But frankly, given one failure on an old tank, I'd be wary of others - and I'd leave welding on pressure vessels to folks certified for that specific, rather demanding, job. In short, I suggest looking for a new/used tank. Keeping the compressor separate from it might make it live longer.

Go with option 1 and you're good.

If you plan to resell your end assembly, that is the way to go.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

This is the kind of tank I need for my plasma project, one that is NG for significant pressure use, but would still be fine repaired and operated at the perhaps 10psi I need.

80 Gal is a bit too big for my coolant tank need, but if the diameter is within spec I could cut a section out of the middle and weld it back together shorter.

Pete, if I find a replacement and would want to get rid of this tank, I will let you know.

Iggy, I happen to have a like-new IR 80-gallon compressor tank in storage here in Texas.

Do you have any good plumbing supply houses around town? If so see if they have any old/warranty bladder tanks lying around. My Dad got hooked on these (old bladder tanks) years ago. We must have had 50+ in all shapes and sizes around. He couldn't pass up free.

They were kind of hard to get rid of back then, decent amount of metal, but they had a thick rubber bladder trapped inside. That shouldn't be a problem for you. Split it open and pull that bladder out. For some images (in case you don't know what these are) see:

Just one make, lots of others around too.

Pete For that low pressure usage a water heater tank would work fine and they are cheap. Steve

I need the not-like-new ~40gal tank in TX too.

Good idea, as is the water tank idea. I also thought of the 40gal LP tanks and the fact that one that is past it's hydro date would be just fine for this application too.