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Painting of Storage Tank Underside

Painting of Storage Tank Underside

Painting of Storage Tank Underside


Would please explain how the Storage Tank underside is painted in accordance to API 650.


A steel welded tank with 18m dia and bottom plate thickness is 10 mm.
The Tank bottom underside spcs is as follow :

For Underside Tank


Mid Coat : 1 x 250 MICRON Coal tare epoxy
Final Coat : 1 x 250 MICRON Coal tare epoxy

Total Coat Thickness : 550 MICRON



RE: Painting of Storage Tank Underside

Good morning AlFareh,

Normally underside bottom plate, including bottom sump painted with 400 micron DFT Coaltar Epoxy. But off course its depend on the client specification.

After cutting the bottom and annular plate, proceed with painting for the under side of these bottom plate and bottom sump per the approved painting procedure.

Normally 50 mm from edge of the plate is un-painted. This must be specified in the painting procedure.

Also we understand during construction some painted area will be damaged due to permanent and temporary welding. These damages can not be repaired. I always stated this issue in the procedure and again explain during kick of meeting. This must be clear before construction stage. We do not someone from the client raising this issue during construction.


RE: Painting of Storage Tank Underside

Thanks for your explanations,

1. Do you mean that we shall paint all coat layers (Primer, Intermediate, and Final coat) prior to installation and welding, if so, will it affect the welding, is there a standard that explains this process like API 650?

2. Are the tank bottom plate welded only from top (no welding in the underside)?

3. why do we leave the 50 mm unpainted and I didn't understand its location, would you send me a photo to clear it?

4. what areas are affected by welding and can not be repaired, and how client can be convinced on it (is there a clause in API 650).


RE: Painting of Storage Tank Underside


As far as I remember there is no standard covering this issue. This is based on experiences only.

1. Yes we have to paint completely till top coat before putting the plates on the tank foundation. The reason 50 mm from each edge un-painted is to avoid problem during
welding. If we painting all surface completely, this paint will be melt during welding and effected the welding quality.

2. Underside painting is applied on the surface which is contact to tank foundation.

3. A plate have 4 edges. From each edges applied 50 mm masking tape. This area should not be painted.

4. During construction we may weld temporary tools (nuts, finger bar etc) on the top of the plate for fit up purpose. When we weld this tools, the painting underside the
plate will be melted. When we weld the shell plate to annular plate or bottom plate, the painting underside the bottom plate or annular plate will be melted. There is
no way to repair this paint damage. API 650 does not cover this issue.

That the reason why the above cases shall be clearly specify in the painting procedure then again explain during kick off meeting.


RE: Painting of Storage Tank Underside

Tahnks Syah,

I really appreciate your elaboration. it is clear now.

Best Regards,

RE: Painting of Storage Tank Underside


Just to clarify, on underside bottom plate there is a part contact to tank foundation. This part must be fully coated. There is also about 25 mm not contact to tank foundation. This part is contact to other bottom plate as overlap. This area no need to be coated.


RE: Painting of Storage Tank Underside

Dear Syah,

1. do you mean by tank foundation (Ring beam and the back filling inside it or only ring beam)?
2. where is the part in contact to tank foundation that shall be fully coated, this point is not clear, what about melting the paint while welding in this case, and isn't it welding affected by paint melting?
3. is the tank bottom plate overlap 25 mm?


RE: Painting of Storage Tank Underside

OK. SO, why not prime, paint and coat the underside plates as you indicate in your answers - as I was going to originally propose as an answer (but that would have been only a unexperienced guess) - but add a short vertical ring plate (10 - 14 inches, 1/2 meter in metric) to each bottom plate so the entire bottom plate and its ring weld can be coated up the side? Yes, the joint welds underneath would be damaged by the heat, but the ring welds (horizontal welds around the side plates) can be finished and inspected from both sides all the way up before priming, cleaning and painting out.

RE: Painting of Storage Tank Underside

Where are you working? Is it on a fab shop or site? Do you have lifting or jacking facilities for tank? If you donot paint the bottom, then the things are going to be severe. Complate your welding and then paint the bottom by lifting the tank. And hope you have a good CP system for your tank. All the best.

RE: Painting of Storage Tank Underside


1. Tank foundation meaning the foundation where bottom plate / annular plate rest on it.

2. If the design of the bottom plate is over lap may be 40 or 50 mm (depend on the design) so the underside (50 mm) of top bottom plate no need to be painted. Because it would be melt during welding.

3. The bottom plate could be overlap could be butt weld. It's depend on the design or project specification. Mostly overlap.

I attach the sketch of the bottom plate over lap joint for your info.


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