Welding feet to a compressor tank

Ignoramus25760 fired this volley in news:1_2dnRUwmJMI_WfbnZ2dnUVZ snipped-for-privacy@giganews.com:

When I first saw your post, I assumed you were asking about how to weld feet to a tank, and I was going to suggest "saddle straps" instead -- basically large band clamps around the tank, with the feet welded to the clamps.

But what you have is potentially a bomb. No. Don't use it. Certified (or certifiable) tanks are not that expensive. Soft tissues are. Hard body parts more so.

LLoyd

Lloyd, that's exactly my thoughts as well. Thanks a lot. I would never weld anything to a compressor tank. A 120 gallon tank at, say, 200 PSI, would probably take down more than one home in my neighborhood.

i

take it home and use it.

I'm glad you don't live near me!

That would be the safe course.

That said, how do you suppose those tanks are made? They're welded! I have welded on compressed air tanks. No problems. But you don't know if the guy that did the welding on the tank in question was any good, or if he tested it after welding. Or, for that matter, that they used it as a pressure vessel after welding since it was separated from the pump.

Welds near the bottom can burn the galvanizing off the inside, creating a site for condensate-caused rusting from inside out.

Don, I thought that compressor tanks are heat treated after welding. That would relieve stresses and such. Since you welded compressed air vessels, maybe you know the answer, I would lova to know myself.

i

It might be okay. It might not. But it's very hard to know, and it's unlikely that anyone in this newsgroup will be able to give you a definite answer. Personally, I would not use the tank.

The danger isn't so much that the tank will fail immediately. The danger is more that the welds will act as initiation sites for fatigue cracks, which will grow ever so slightly with each cycle of pressurisation and de-pressurisation, until eventually the tank fails. This is a situation in which the quality of the welds is very important.

I can't think of a major disaster which was caused in exactly this way, but the Alexander Kielland oil rig disaster was very close. A bracket for mounting an underwater instrument was welded onto one of the bracing members of the rig. It was done without the knowledge of the rig's designer and by all accounts it was done badly. The waves caused the stress in the bracing member to vary cyclically, and a crack began to grow. Eventually the bracing member broke during a storm, allowing one of the platform's legs to break off, and the platform to capsize.

This isn't the exact explanation of the disaster given in the Wikipedia article, but it's the explanation given in a book I have on the disaster, which is probably more trustworthy.

Best wishes,

Chris

If you know the person who welded the PV or did it your self, thats one thing, but to take a unknown 120 gal PV that had been modified and fill it to 150psi+_ is risking allot. PV's are for the most part not stress relieved, but they are welded by certified welders (or should have been). and they are hydro checked. And do you know what was in it ? It may have been highly corrosive. gary

Take the tank home and use it.. infact rig it up so you can put about twice the pressure in that it was designed for. When it blows you have one hell of a law suit against Fermilab for allowing you to take a defective tank from their dumb. If they were going to scrap it as unsafe they should have taken a shotgun or an airhammer to it . LLB

I don't know. Some might be. I knew a boilermaker who made big tanks out of 5/8" steel plate that he pressure tested to 200 PSI. I'm sure he didn't heat treat them. These suckers were 8 or 10 feet in diameter. Tawk about a slip roll! They were stick-welded. Rollers turned the tank as it was being welded, the welder just held the stick and watched the puddle. He could probably control the roller speed, but that probably didn't change once set to his satisfaction.

Although possible it would seem inefficient to heat treat a pressure vessel after welding. While a heat treated vessel could be made from thinner material the additional cost of heat treating would probably result in a higher manufacturing cost that if simply rolled and welded from "as delivered" stock. Can you imagine the cost of an oven to take a 10 ft. diameter vessel? Or the amount of energy necessary to heat it?

Bruce in Bangkok (brucepaigeATgmailDOTcom)

I am 100% positive that it is an air tank from the compressor.

i

Interesting. A while back I was talking to a guy who was welding the tyres of railway wagon wheels as part of a repair process. He said he tried welding them by putting the wheelset in a huge lathe and setting it turning very slowly, but that it was actually not a great deal of help. He said it was much harder than it looked to lay a bead at precisely the same speed as the wheel turned, so instead he preferred to weld a section with the wheel stationary, then rotate the wheel before continuing. He did an exceptionally neat job too; you couldn't see the restarts.

Best wishes,

Chris

snipped-for-privacy@gmail.com wrote: o

Postweld heat treat for large pressure vessels is done all the time. It is not done within an oven unless the vessel is pretty small. What is normally done is wrap the weld area and the HAZ with thermal blankets that have electric heating elements in them and the area is brought up to the specified temp for the the required time. The largest that I've personally seen done were some reactors that my dad was the materials and welding engineer on. They were 20+ feet in daimeter and over 200' tall when stood up. Vessel wall thickness was

11". When completed they were barged out of the US to Trinidad and installed in a refinery there.

You know you are working with big stuff when someone considers vessel material under 1" to be sheet metal!

I've made a few jigs in the past so I could turn stuff by hand while welding them. It isn't terribly hard to adjust your speed that way.

One of the problems is that it needs to be pretty smooth (the turning) or you run the risk of liquid metal (puddle) running away. A good type of bearing helps a lot with that. Vibration is no good for the quality of the weld either if it happens while the puddle is setting up. All of this is doable, but it takes some planning and practice for it to work well.