From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
Date: Thu, 7 Apr 2005 11:05:04 -0500
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
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
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