Tricky Scratch fix
Tricky Scratch fix
(OP)
We are trying to fix a scratch on the sealing surface of the head of a stainless steel pressure vessel. It needs to be blended to within 1 mil/inch. Now comes the difficult part. The vessel is huge, and due to schedule restraints, we are trying to fix it while it is on its stand which is 15" above the ground. There are multiple scratches that need to be fixed, and for various unrelated reasons, the workers can not be working for a long time near the vessel head. We made a mockup and have tried various grinding/buffing methods to try and remove scratches we put in the mockup, but are having difficulties blending out the scratch within 1 mil/inch. Any ideas would be greatly appreciated. We may have to do a weld buildup on a few of the scratches.





RE: Tricky Scratch fix
Each will have to be addressed as a separate case: what will work for a few will not work for all. What will be needed (potentially a pre-heat, weld-buildup, PWHT (maybe) and then machining) for the worst "scratches" will not be needed for the easiest.
Have you done a blue-check for the cover to know exactly where the low spots are?
What is your cover and how do you seal the surface? (Gasket material is?) What torque on the fasteners?
Radiation area?
RE: Tricky Scratch fix
We have not done a blue-check, and I am not sure if we can or not because of the possibility of getting some of it in the primary system. The studs are pre-tensioned. I just took a look at the vender manual, and they want the studs stretched 77.6 mils which I believe is a force of just under 2,000,000 lbs per stud.
The main issue we are trying to work out now is how to blend out the surface to a smooth taper for 1" for each mill of indication.
RE: Tricky Scratch fix
Ted
RE: Tricky Scratch fix
RE: Tricky Scratch fix
RE: Tricky Scratch fix
See if you can find an old machinist to teach your people how to properly draw file a surface to flatness.]
Mike Halloran
Pembroke Pines, FL, USA
RE: Tricky Scratch fix
1 thou per inch doesn't sound too onerous, but that's on a workbench, not 15 feet in the air and glowing in the dark.
Cheers
Greg Locock
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RE: Tricky Scratch fix
RE: Tricky Scratch fix
RE: Tricky Scratch fix
Use a long flat stone (10 inch long x 3 or 4 inch wide, 1 inch "deep" or more to be stiff against the bending pressure as you push it down against the metal). The problem will be the sealing surface: If you use a file it will produce hundreds of small parallel scratches as you wear off the metal.
Unfortunately, both stones and files (any grinding method for that matter) doesn't remove scratches - they remove the high parts and wide areas between the scratches so the net average metal height is slowly reduced to the bottom of the scratch. But that "slowly" term means greater radiation exposure.
Weld buildup put a very thick "bump" into the "scratch" - which then means that new "mountain" then has to itself be ground down so the final surface is more nearly flat. But any undercut or edge effect at the sides of the weld to the base metal at the weld will form yet another groove in the surface.
Of course, any scratch that does not cross the whole surface - anything that does not provide a leakage path from inside to outside of the pressure boundary - will not (theoretically) leak.
RE: Tricky Scratch fix
Ted
RE: Tricky Scratch fix
This might appear to come from somewhere in loonie-land, but once rigged, it could be left to run with only remote observation.
I'm envisioning some sort of drive spider made to fit the cover holes, a motor and gearbox either adding its weight to the cover or suspended from a crane hook, and with a torque arm (or arms) restrained by chains or links to nearby structural supports.
Okay; I'll go back to my corner and talk to myself.
RE: Tricky Scratch fix
Do these scratches structurally affect the tank. If not - BONDO!!!!
RE: Tricky Scratch fix
I don't know if you were being serious or not, but I don't think that jakejas should be using BONDO on a nuclear reactor.
Patricia Lougheed
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RE: Tricky Scratch fix
Bruce
www.accuratus.com
RE: Tricky Scratch fix
potteryshard's lapping idea might have some merit, if possible.
Regards,
Mike
RE: Tricky Scratch fix