I've got an old 20 gal (?) compressor tank in which I want to install a drain. I'm guessing it to be 1/4-inch (6.3 mm?) thick steel. Tank is rated to 125 psi. Can I drill and tap this for 1/4 or 3/8-inch pipe? Or should i get a "plug" welded to it that I can drill & tap?Are there rules to follow for such stuff? Is there a better n.g. to ask this question in?Thanks,

I'd be totally surprised if your tank has the wall thickness you speak of.They typically are thinner, especially if it's rated for 125 PSI. If ithas, it would be adequate for a 1/4" pipe thread, but marginally. Bestpolicy is to weld in a bushing, assuming you have the capability.Pressure vessels are generally built to the pressure vessel code, so, yes,there are rules. Ernie is our chief cook and bottle washer where weldingis concerned. With luck, he'll chime in.Is there a better n.g. to ask thisThe one he frequents most, which I don't recall. I think it's scientificwelding and joining, or some such. Others will probably provide the propergroup for you.Harold

Drill & tap compressor tank?

Aug 26, 2005 23 Replies

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Bob Engelhardt 20 years ago

I assume that your 3/8 threaded hole is NPT. In which case, use a npt-compression adapter/union, 3/8 NPT to 1/4 (OD) compression. Such as McMaster-Carr #50915K316:

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also generally hardware-store available.

Drill out the adapter to 1/4" to allow the tube to pass through it. The fitting has plenty enough "meat" to do this, but as-made it stops the tube for ease of assembly.

Screw the adapter to the tank, pass the tube through it, tighten the compression sleeve/nut, add a compression-fitting valve to the tube. If you file a notch on the in-tank end of the tube, you can lower it all the way to the bottom and still have an opening for the water.

If you first fit a 3/8 NPT tee to the tank, with the straight-through vertical, you can take your tank output from the horizontal leg, while fitting the dip tube adapter to the top. The dip tube will be co-axial with part of the output path, but with enough room for air flow.

HTH, Bob

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B.B. 20 years ago

You could try avoiding modifying the tank altogether. Get a pipe cross, mount the tank above so the pipe has to go up to get into the tank, then have a six to eight inch (assuming it's 3/4 or 1"--longer for thinner pipes unless you don't mind draining more often) leg of pipe go out the bottom of the cross to collect water and gunk. Stick a drain at the bottom of that pipe. Then use the remaining two sides of the cross for inlet and outlet. Not pretty or very mobile, but works OK.

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Boat_dreams 20 years ago

A welded iron half-coupling is the correct way to go. Do it yourself if you can. Your tank isn't worth what a shop would have to charge--about $80 or their one hour minimum. good luck, Boat_dreams

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Chipper Wood 20 years ago

A 'Pressure relief port or valve' is a redundant safety feature. (Required by most codes for any pressure vessel.) Electrical pressure switches do fail.

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