Tapping a new port into an air tank

[strantor]

A guy I know, well he's a real big idiot. He knows better than to weld on pressure vessels but in a moment of weakness he convinced himself it would be alright just this once, so he drilled a hole in his air compressor tank. Then he stood back and looked at it, and knew that he had made a mistake.

I was doing some math. A 3/4" NPT fitting has a radius of 0.5".
Pi*r^2 * 150PSI, that's 117 lbs of force on the fitting, pushing it outwards.
That's not really a lot, but I think the concern with welding it, isn't that your weld will fail, but that the heat you introduce while welding will un-do the heat treatment of the tank, and the tank itself will fail (explode).
So, what if you just don't introduce any heat?
What if a guy (an idiot) were to just tap that hole he already drilled, with a 3/4" NPT tap, and screw a fitting into it?
I think (please correct me if I'm wrong) then the only risk is that of the fitting being violently ejected from the tank at high pressure, which is less unsafe than the risk of the tank rupturing.

The fitting could be built up around the protruding area with some of that "steel putty" stuff to distribute any side load that might be experienced from pulling on the fitting, and hopefully mitigate some of the risk of the fitting being ejected. Then the tank could be positioned with that untrustworthy fitting pointed in a "safe" direction, like the back corner of the shop, where if it ever does turn into a projectile it can only kill some old scrap pallet wood.

Does that sound like a less than 100% stupid solution?

 

[Jeff.64]

Does that sound like a less than 100% stupid solution? Ever so slightly.

The wall thickness of the tank would most likely be too thin for enough threads.

If you must, maybe try something like this:

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

If it were me, I would get an estimate from a pressure vessel certified welder. Compare that with the cost of a new tank.
Then go with the less expensive option.

 

[Jeff.64]

Have your friend look here:

air compressor explosion - Google Search

Select Videos in the search. Here is one:

 

[strantor]

The wall thickness of the tank would most likely be too thin for enough threads.

I thought it was already too big but but just found that the hole was small enough to go with 1/2" NPT. By the math, a 1/2" NPT fitting would only see 79lbs of force instead of 117 for 3/4" NPT.
I just tapped it to see, and there are 2 complete threads. Fitting feels firm, I would trust it to hold way more than 79lbs of force as long the force is only acting straight out on it. Any lateral leverage applied to it, I think could pull the threads out. Hence the putty stuff.

If you must, maybe try something like this:

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Those weld-in bungs are great and that's what I was planning (I was going to machine my own) but the whole welding thing... I'm not that brave.

If it were me, I would get an estimate from a pressure vessel certified welder. Compare that with the cost of a new tank.
Then go with the less expensive option.

I already know which would be cheaper and if I can't convince myself that this won't kill me then I will be buying a new tank.

 

[FOMOGO]

To my knowledge compressor tanks are not heat treated. Personally I would have no issues welding on a tank, and have done so in the past. Where your at now I would open up the hole to the o.d. of a steel 3/4 inch coupling and weld it in. If you are not confident in you welding skills have some one do it for you. Mike

 

[strantor]

Have your friend look here:

air compressor explosion - Google Search

Select Videos in the search. Here is one:

Thanks. After watching that I have decided that the tank will live outside anyway, even if there were no shenanigans done to it. So I am less concerned now. The compressor itself will live in the shop and air will be plumbed out to the tank at the edge of my junk yard about 30ft away from the shop. I don't have any windows to explode outwards and relieve the pressure if a compressor tank (any compressor tank) were ever to rupture in here. So instead it would take out my walls, bay door, eardrums, and any other sensitive organic membranes.

 

[rabler]

I think (please correct me if I'm wrong) then the only risk is that of the fitting being violently ejected from the tank at high pressure, which is less unsafe than the risk of the tank rupturing.

I certainly don't know for sure, but my concern would be that the drilled and tapped hole could introduce a weak point for the steel to tear. Just like a cut in a piece of fabric can make it easier to tear. Think of the tank circumference like a series of straps/rings. A 3/4" diameter hole supports the integrity of that entire 3/4" band around the tank, so the pressure is not just the hole, you're dealing with the force on that entire band/ring/strap. By drilling a hole you've made the two adjoining straps pick up the load in addition to their own inherent load.

 

[matthewsx]

I certainly don't know for sure, but my concern would be that the drilled and tapped hole could introduce a weak point for the steel to tear. Just like a cut in a piece of fabric can make it easier to tear.

Stress riser

 

[strantor]

I certainly don't know for sure, but my concern would be that the drilled and tapped hole could introduce a weak point for the steel to tear. Just like a cut in a piece of fabric can make it easier to tear.

That's a good point (I think?) - maybe it would indeed be better if the fitting were "sewn in" to tank by some method. Maybe silver solder? If the bulk of the torch heat were put into the brass fitting, and removed just as soon as the solder flowed in the joint, maybe that much heat wouldn't be enough to cause an issue with the tank's heat treatment.

To my knowledge compressor tanks are not heat treated. Personally I would have no issues welding on a tank, and have done so in the past. Where your at now I would open up the hole to the o.d. of a steel 3/4 inch coupling and weld it in. If you are not confident in you welding skills have some one do it for you. Mike

I have read several times that they are heat treated. Maybe for very large tanks which would be costly to put into a giant heat treatment oven they just make them thick enough to not matter? But for small the small tanks of consumer grade compressors I am pretty sure they are heat treated. Otherwise I wouldn't have any qualms about welding on it.

 

[rabler]

Stress riser

Yep. I hear that term thrown around a lot more often that explained, so I was trying to put some reasoning behind it specific to this case.