Shell patch Plate

[blawn]

On what side should a patch plate be installed on a shell?I have a tank to be repaired.There's a crack on the shell about
1 meter below the roof.Im planning to grind crack wide open then weld it(build up)and patch it on the product side.
Im not so sure if this method is allowed.I cant find provisions in API 653 regarding this kind of issue.
Any word from anyone is a great help.Thanks!

 

[valleytank]

Are you generally speaking when you say the shell as a whole or are you specifying shell plate directly? Is this on a weld seam?

Depending which one, some of your weld repair criteria will change. Is a lap patch totally necessary or is it a remaining thickness issue?. Api 653 does permit the use of interior patches(see 9.3) unless they will interfere with an internal floating roof or be too close to the critical zone. Seeing as you're only a few ft down from the least stressed area on a tank and probably above any roof travel(if any)you should be fine but remember 'lap' patches follow spacing criteria.

Once you determine you removed enough material have someone visual and mpi the prepared area prior to welding, backgouge/grind from the the oposite side, mpi check, then fill it out and blend both sides with the rest of your shell. MPI the final ground areas and you should be okay. If this is on a weld seam similar rules would apply with the addition of RT or (UT if permissible by client). If it's fine and everything is still within its design thickness I don't see the need for an additional lap patch on the interior unless the client specifically requests that you do it.

 

[blawn]

Its on a direct plate actually.Its on the last course.I'm just afraid the crack might propagate so I decided to grind the crack then
weld it then flush grind it.Yes NDT shall be carried before and after the repair.I don't see any thickness issue,its just the plate sustained crack(the reason I haven't investigated it yet) so its leaking and they reduced the fill height.Patch may not be necessary because there's no metal thinning,and its the least stressed area as you said,and I agree on that.
So if my client insist on patching it,then I will still grind and weld the crack,then patch it inside.Its a fixed cone roof so I don't see any problem doing interior patching.
I hope this method would be fine.