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Re: Pressure Vessel Repair

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Thanks for the vote of confidence, Bill ;->

On Apr 7, 2005, at 12:03 AM, Daryl Richardson wrote:

I have been asked to participate in discussions regarding the best way to go about repairing a pressure vessel which seems to have problems with hydrogen embrittlement.

Frankly this project makes me a little nervous. I visited a refinery once where they were hoisting vessels around like that, until one day they dropped something--crane malfunction. It only just missed a big cracking tower which, if I recall, was still in operation. If they'd busted something it would have really spoiled everyone's whole weekend. The way this job was handled was pretty stupid, certainly in hindsight, but I suspect an experienced heavy lift would have recognized the hazards right away.

First, figuring the stress is the easiest part but you'll need to know how they intend to support the vessel off the crane. The implications of what they want are pretty tricky, though, and it goes a lot further than wind loading. If I understand the letter they want to cut pieces out of the shell so as not to cut the thing actually into two pieces. That's easy enough to figure, except the vessel will distort visibly as they continue cutting, and they're going to run into serious fit-up problems with the new piece. In effect as you make the cut the dead weight load path will shift away from the cut and you'll have a moment you didn't have before, and the vessel gradually changes shape. You can estimate the stresses on the back of an envelope, but you'll probably need FEA to figure how the vessel changes shape.

The mechanics of the lift is pretty interesting too, made all the more so by the 150 ton load involved. I think leaving a load like this on the hook for 3 weeks is generally a bad idea, but the person to ask is a big contractor with experience in plant construction. A good place to start is the outfit that built the plant in the first place. You need to know whether you can even get such a crane in position, and how the service crane which will handle the cut out and repair pieces will also fit. Handling sections of 3 5/8 plate (145 lb/sq ft) is no walk in the park, especially since they'll need to be supported firmly when cut-out or you'll risk tossing your workers off the vessel. The replacement pieces need to be held properly for fit up, too. This is the kind of thing they do in ship-yards, and requires enormous and expensive expertise to plan and execute.

There's also the matter of supporting the vessel internals during the burning operation and protecting them from slag and grinding dust, then cleaning up everything later. Presumably the vessel is also full of some nasty petroleum-based vapor, and it'll be a fire hazard for welding. BTW 1300F is too hot for the stress relief; normal PWHT temperature is 1100F. Speaking of which, that job alone is hugely complicated with the vessel in place. But not as tricky as getting the replacement pieces fit up properly.

I'd also be a little curious about the belief that they have hydrogen embrittlement and the extent of the problem. The use of the term 'rot' or 'lamination' to describe the problem doesn't sound like hydrogen embrittlement, which causes cracks, not general wastage. It's also very difficult to detect after the fact, and doesn't normally occur in low carbon steel like A-212 unless the welding technique was very poor. And it usually doesn't take 40 years to show up. If they haven't already they should do a comprehensive metallurgical exam to figure out what they actually have. Normally hydrogen embrittlement is pretty much terminal, because you won't know the extent of the damage without cutting up every single weld. In any event every weld will have to be cleaned and inspected for similar damage. No point doing a partial fix. I know the vessel is expensive, but your guys ought to consider replacement. It's been in service 40 years, and such things don't last forever. Repair under these circumstances is only cost effective if there are no mistakes, either in the repair or the diagnosis. If the damage isn't completely fixed and the replacement is perfect, they'll be fine. If not, they may be in for replacement anyway.

I don't get into heavy wall vessel repair much, so it's a good idea to contact someone in the tower construction business and an experienced ASME Code fabricator. As I mentioned, the best people to talk with are the original fabricators if they're still around. People who do power plant construction (if they're still around) like Parsons or Sargent & Lundy might be good places to start. This isn't a job for the plant engineering staff.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw/


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