Drill & tap compressor tank?

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 it has, it would be adequate for a 1/4" pipe thread, but marginally. Best policy 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 welding is concerned. With luck, he'll chime in.
Is there a better n.g. to ask this The one he frequents most, which I don't recall. I think it's scientific welding and joining, or some such. Others will probably provide the proper group for you.
Harold

It is nowhere near 1/4" thick. Probably less than 1/8". I don't recommend welding on it. But if you're going to anyway, why not just weld on a coupling that is already threaded?

Dave
Since this is a drain, and you are concerned about the integrity, maybe you'd be less concerned with a 1/8th pipe valve, or petcock. But, if the tank pressure never gets higher than 125 psi, the force on 1/2 inch diameter area is only about 25 pounds. Besides, the valve is on the bottom so there is minimal probability of any failure injuring anyone.
Jerry

"Harold and Susan Vordos" wrote: (clip) Others will probably provide the proper group for you. ^^^^^^^^^^^^^^^^ It's sci.engr.joining.welding Ernie follows this group pretty closely as well--he will probably read your post.

It won't be thick enough to tap. Have a plumbing fitting welded or brazed in. I'd braze it. That's what I did on my tank. Plumbing fittings are often cast iron which can be welded to steel -- but brazing is a lot easier, would have ample strength and is less likely to have pinhole leaks or cracks.
There are rules for damned near everything, but if it's for your own use I wouldn't worry about it. As Jerry noted, there isn't much hydrostatic force on a small fitting like that at 125 PSI.

| | >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, | | It is nowhere near 1/4" thick. Probably less than 1/8". I don't | recommend welding on it. But if you're going to anyway, why not just | weld on a coupling that is already threaded?
If the coupling stuck up inside the tank, there'd be a puddle that the drain could not get to. I wonder how well a flange would work brazed to the tank. Drill the hole, shape the flange to match very closely, then braze in place. The low heat of brazing will not affect the heat treatment of the tank wall that much. The flange ought not to be small, though. Weld a coupling to the flange before brazing and connect that to the drain valve.
Does this sound safe?

snip----
The low heat of brazing will not affect the heat treatment of the Not true, Carl. Anything above 700 F will change any heat treat present in carbon steel for the worse, but I'm not convinced tanks are heat treated, so it's a non-issue. Brazing, even silver brazing, occurs at a temperature over 1,000 F.
Harold

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

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

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

Thus spake Lloyd E. Sponenburgh:
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:
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?

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.
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. LLoyd

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

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

Thus spake Lloyd E. Sponenburgh:
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,

Thus spake Lloyd E. Sponenburgh:
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,

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.

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.