Repair of pitting in a tank bottom.
Repair of pitting in a tank bottom.
(OP)
Dear Forum,
We are inspecting an atmospheric tank of 15 mtr x 15 mtr dimension.
Service is AGO.
Tank wall thickness : 5.4 to 7.6 mm in 6 course.
Tank Bottom Thickness : 7.4 mm
Tank Material : A 283 C
Service Period : around 14 years
Obersvation:
We are observing pitting of maximum depth of 4.2 mm in the tank bottom alongside the HAZ of bottom plate weld joints.
Query:-
1. What could cause these pitting in the bottom plate?
2. Any suggestion for the best method to inspect the whole bottom plate? We are planning for DPT of bottom welds and the Vacuum Box Testing.
3. How to repair the pitted areas at the bottom plate?
Thanks in anticipation.....!!
We are inspecting an atmospheric tank of 15 mtr x 15 mtr dimension.
Service is AGO.
Tank wall thickness : 5.4 to 7.6 mm in 6 course.
Tank Bottom Thickness : 7.4 mm
Tank Material : A 283 C
Service Period : around 14 years
Obersvation:
We are observing pitting of maximum depth of 4.2 mm in the tank bottom alongside the HAZ of bottom plate weld joints.
Query:-
1. What could cause these pitting in the bottom plate?
2. Any suggestion for the best method to inspect the whole bottom plate? We are planning for DPT of bottom welds and the Vacuum Box Testing.
3. How to repair the pitted areas at the bottom plate?
Thanks in anticipation.....!!





RE: Repair of pitting in a tank bottom.
RE: Repair of pitting in a tank bottom.
AGO stands for "A"utomitve "G"as "O"il
RE: Repair of pitting in a tank bottom.
(1). If the pitting attack is confined to the base metal heat affected zone on the tank bottom this is obviously local corrosion attack from some type of contaminant that resides along the tank bottom. If you had significant undercut from the original shell course welds this could be creating a natural crevice and the initial cause for the local corrosion attack.
(2). I agree with having an inspection performed to evaluate tank bottom thickness away from welds. There are inspection vendors that provide in-situ tank bottom inspections without complete removal of contents. The device is a crawler that is outfitted with UT capability and fiber optic for visual inspection.
(3). For repairs, it depends on the extent of damage found during the inspection performed as in (2) above. You have several options; local tank bottom weld repairs by splicing in new sections or wholesale replacement of the tank bottom.
RE: Repair of pitting in a tank bottom.
I shall enadevor to locate some vendor to inspect Bottom, however the vacuum box testing , thorough thickness measurement and DP test should be able to give a good result alongwith close Visual inspection.
Any further advice ...? I shall uppdate as the job progress.
Thanks
RE: Repair of pitting in a tank bottom.
Cheers,
gr2vessels
RE: Repair of pitting in a tank bottom.
Steve Jones
Materials & Corrosion Engineer
http://www.linkedin.com/pub/8/83b/b04