Drill & tap compressor tank?

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,

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I got a great find last year in a nice two-stage pump that only needed the valves re-seated to make it cherry again. I was also given a beautiful, hot-dip galvanized upright propane tank of 100gal capacity. Hmmmm..... I think I see a picture here.
I didn't want to compromise the tank's integrity because I'm NOT a good weldor. So, I looked at things from a Rube Goldberg point of view.
Here were: fill port, output port, and gauge port. The fill and output were both 1" NPT bosses in good shape. The gauge port was a huge 2-1/4" machined flange with the gauge bolted down onto a gasket.
Ok... fill will be fill; just screw in a check valve. Output was output; natch. Took off the gauge and extracted the float assembly. Machined a manifold that sat where the gauge used to.
In the manifold is a: Pressure relief port (tested... it does keep ahead of the pump), a pressure gauge port, and... and... a dip-tube drain. It's just a copper pipe that kisses the bottom of the tank, and a petcock valve to open it to ambient pressure. Air pressure pushes the water up the tube.
I welded the pump base to the top flange and the feet to the bottom flange -- never once touched a torch to the tank itself.
Since I'm not a weldor, I feel safer, and I've got a perfectly servicable upright that has a drain valve I can actually reach without aggravating my knees.
LLoyd

Thus spake Lloyd E. Sponenburgh:
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This sounds like a very creative idea, but I'm having difficulty picturing what your set-up is. (A compressor with a "float assembly"? Water actually accumulates enough to require a float?)
I've got 2 ports on this tank: input (small boss fitted with 1/4 copper tube connecting to compressor output), and output (large (2-inch?) threaded boss with 1/4ntp threaded adapter (plug) connecting to 1/4 pipe with tees for gauge and quick disconnect hose fitting).
Hmm... I could adapt the input to also act as output, and put the dip-tube drain on the large boss.
How, exactly, is a dip-tube drain constructed? How do you fit a tube internally and connect that to the backside of a 1/4ntp (or such) output?
I like the idea. It would fit in nicely with the large boss at the front of the tank.
Thanks,

Thus spake DaveC:
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What's the purpose of a pressure relief port? Is this an "open at x psi" valve to keep pressure below the tank's limit? But this is what an electrical pressure switch does.
What's a relief port?

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Dave, pressure switches weld shut in storms, or with age, or with mud-dauber nests inside them.
EVERY compressor must have a pop-off valve to protect the tank from over-pressure.
LLoyd

Thus spake Lloyd E. Sponenburgh:
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Well, it looks like several things need to be added to this old compressor.
I looked closely and discovered a third port, down low on the end where the handle attaches. It is a 3/4 inch (?) threaded boss with a square-head plug threaded in. A 'manual' drain valve, I guess.
This old compressor has the basics, but needs some safety items. Drain valve and pop-off valve to be added.
Thanks for your help,

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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It was a propane tank in real life. It had a float-actuated gauge. The gauge port was BIG, so it was a nice place to combine some functions.
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The manifold had two holes straight through into the top of the tank: one for the relief valve and one for the new pressure gauge. It had a THIRD hole that had a petcock on the outside, and a copper compression fitting on the inside to accept the dip tube.
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LLoyd

Thus spake Lloyd E. Sponenburgh:
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So I guess without some custom "manifold" or such, there's no way to fit a dip tube, internally. All I have is threaded holes in the tank for 1/4 inch input, and 3/8 inch output (previous message said 1/4 output, which was an error).
Ideas?
Thanks,

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don't all compressors already have a drain, unless this is a homebrew job?

Thus spake Charles Spitzer:
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no, this one doesn't. Craftsman brand, old.

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I am confused. Without a drain, where does the water go?
i

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    That sounds very strange. I have an old Craftsman -- horizontal tank, belt-driven pump, two wheels at one end and a handle at the other.
    Between the wheels, on the underside, is the drain valve. It was rather small -- perhaps 1/4" NPT thread. It had a T-handled valve, which was hollow for the water to flow out. It was rather prone to jam, so I replaced it with one with a knurled knob on the end, and a silicone rubber-gasketed seat on a wider portion on the inside. That one remains controllable with just my fingers.
    If there were room, I would replace it with a timer controlled solenoid valve, but there is not on this one.
    You might consider looking up the parts list for your model on Sears' website, to see whether it lists a drain valve as part of what it should have.
    I could imagine taking the output fitting and replacing it with a concentric fitting, with the inner part being a dip tube to drain from near the bottom, and the outer part passing air to the regulator.
    Enjoy,         DoN.

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Are there rules for such stuff? I would hope there are, in your country.
If you don't know the answer without asking don't mess with air or gas filled pressure systems. There are some very scary answers to this question.

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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.

DaveC wrote:
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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