Repair of pitting in a tank bottom. 1

[ak1965]

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.....!!

 

[metengr]

For a slow learner like myself what is AGO service???

 

[ak1965]

Good Question ..infact i ahsould have explained it ealrier....!

AGO stands for "A"utomitve "G"as "O"il

 

[metengr]

ak1965;
(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.

 

[ak1965]

Thanks Metengr


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

 

[gr2vessels]

I believe that a good cleaning and Belzona coating of the bottom will give you many years of happy usage of your tank. Keep the coat in place to avoid corrosive fluid coming in contact with the metallic bottom of your tank.
Cheers,
gr2vessels

 

[SJones]

Once an analysis shows that repairs are required then allowable repairs are detailed in API STD 653. Water in the stored liquid, either directly introduced or from condensation, is probably the cause of the metal loss. Coating, possibly supplemented by sacrificial anode cathodic protection, would serve as an effective corrosion control measure when applied in accordance with API RP 652. If the analysis shows that the observed corrosion is not detrimental to tank integrity, the pits can be filled with an appropriate filler material to facilitate coating application.

Steve Jones
Materials & Corrosion Engineer

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