Bottom Plates External Protection 5

[Inchtain]

Dear Engineers,

For the external protection of the bottom plates of a crude oil above ground fixed dome roof tank against soil corrosion, is it still advised to apply underside painting, whether a CP system has been established or not?

In the past, the popular type was epoxy coal tar, but I heard that it has been abandoned? Is that true? and why?

What other recommended paint generic types to use?

Thanks

 

[JStephen]

Is this the existing tank in your other question?
Shop-built tanks may be coated on the underside of the bottom, but this is uncommon for field-erected tanks.
Usually, some combination of doing nothing, using cathodic systems, adding corrosion allowance, using pad below the bottom.

 

[HTURKAK]

Epoxy tar or other paints can be effective for shop erected tanks. In your case, the paint of underside of bottom plates will deteriorate during welding.

I looked to your member profile and internet country domain is (LY).. If this is the case, I will suggest the bitumen sprayed compacted sand or the better asphalt layer.
If you are asking for the existing tank in your other question, i will suggest the use of 100 mm RC spacer to avoid CP also.

 

[r6155]

You can use Carboweld 11 (Carboline) and weld over this primer. This primer shall be included in the Welding Procedure.

Regards

 

[Inchtain]

Thanks all,

Yes, this topic is related to my previous one.

Actually, our new plates were required with Carboweld 11.

As a more preventive measure, our staff are thinking of using an epoxy glass flake as a top coat for the plates underside, do you think that will be suitable and effective?

Regards,

 

[IFRs]

In my opinion, paint and coatings are bound to fail - any holiday at all and the corrosion goes right there with a vengance.

You can't weld the bottom without disturbing a coating and coating after installation is painfully expensive unless these are small tanks.
Dragging coated plate over the foundation and each other during erection ruins the coating.
Thick coating may reduce your ability to measure thickness during future inspections.

Keep the chime dry with proper drainage, do routine inspection and have a realistic expectation of service life.

Additional measures:
Use 5/16" bottom plates to give yourself additional life.
Use cone up concrete under the bottom with leak detection grooves.

Again, these are my opinions only and remember - you read it on the World's Wild Web.

 

[SJones]

Coal tar epoxy may have been "abandoned" only because of the health implications, but coating hasn't been abandoned. Cathodic protection is then the backup to the coating with the latter serving to cut down the amount of CP current required, and helping its distribution. In deciding what to do, it is first necessary to ascertain whether the pad soil will be corrosive by appropriate tests and/or by operating experience with adjacent tanks on similar pads.

Coating selection will depend on a number of factors, mainly temperature. For up to 80 deg C, here are a number of example coatings that might be applied (bearing in mind that the coating has to be proven resistant to environments generated by the CP):

High build epoxy, 1500 microns - examples Carboguard 102, Interzone 485, Sigmaline 2500

Glass flake vinyl ester, 3 x 500 microns - example Polyglass VEF

Glass flake epoxy, 2 x 750 microns - example Marathon 1000 XHB

Leave a 50 mm cutback at weld preparations, and your ISO 15257 Level 4 certificated cathodic protection specialist can design a CP system accounting for the bare metal and factoring in other handling and age related degradation.

If CP is decided to be necessary, don't use bitumen sand, or oiled sand, as it will shield the CP: link to Matcor

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[IFRs]

The OP is asking about underside coating.

I still don't understand how you achieve anything useful coating the underside of a bottom. If you coat the plates before they are welded, the heat of welding and movement of plates during setup will ruin the coating and corrosion will zoom there. If you coat them after they are welded, well, I don't see that realistically happening unless it is a shop built tank.

Please help me understand!

 

[SJones]

The quoted coatings are for the underside. A cutback is left from the weld edges to allow for welding heat. Corrosion won't zoom, it will occur at the same or reduced rate, just more localised. The cathodic protection is deployed to protect the bare areas of cutback and damage. For sure, it takes careful handling to get the coated plates in place, but this isn't some brand new idea. I haven't yet come across an oil and gas operator that doesn't coat the external bottom, and not deploy CP to go with it. The consequences of a leak would appear to convince them of the necessity. The coating also serves to cut down the CP current required and also helps it distribute over the tank bottom. There's a whole API RP 651 on the subject.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[IFRs]

Steve - we must work in different areas - I've worked on hundreds of API 650 tanks and none had a coating on the underside of the bottom plates. I guess I need to go learn some more about this, thanks for the info!

 

[SJones]

@IFRs - no problem. The Matcor link in my previous post gives a similar view of the current approach of owners.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.