Ground flushing welds on pressure vessel internal surface

[balamurugan02]

Greetings!

Is it necessary to ground flush welds on internal surfaces when internal coating is required. If so, can you please explain why?

P.S: Client specification doesn't call for ground flushing.

Thanks,
Bala

 

[Dhurjati Sen]

Dear balamurugan02,

I guess you are talking about pressure vessels.

If the weld procedure mentions back gouging, then it needs to be done.

After hydro-test, there could be flash rusting of the weld. Surface preparation as per the painting procedure is sufficient.

The above-mentioned two things are entirely different and need not be mixed at all.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[weldstan]

Depends on the coating system and the internal weld profile.

 

[balamurugan02]

Thank you Dhurjati. (It was for a pressure vessel)back gouging is not required as per WPS
Weldstand, all welds are from outside only (Single V).

I was told that, it is a standard practice to flush welds on the inside when internal coating is required. But nobody was able to explain why.

 

[SnTMan]

If the coating manufacturer or applicator requires it, it's required. Part of proper surface prep I'd think. Ask 'em.

Regards,

Mike


The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[weldstan]

Check with the coating manufacturer. I suspect that you will need to grind nearly flush to provide a suitable profile for coating adherence.

 

[r6155]

See NACE SP0178 for more information.

Regards

 

[Dhurjati Sen]

Dear balamurugan02,,

Which customer / client / inspection agency would give hot job clearance after hydro-test of the vessel?

If at all welds need to be flush ground as per painting procedure, the same has to be included in the welding procedure.

And this has to be incorporated during the engineering / feed stage of the project as rarely the vessel manufacturer would carry out the painting.

Hope this makes my earlier post clearer.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[balamurugan02]

Thank you all for your valuable advice.

 

[Compositepro]

Many paints and coatings rely on liquid surface tension for leveling to obtain a uniform coating thickness. A convex bump on the surface of the substrate will result in significant thinning of the coating. Also, any tiny pit in the weld surface will trap air which will often result in a hole in the coating.

 

[DekDee]

If client specification does not require it and it is not a code requirement it is not required.
However, if the client requires it ground flush then they have to open their cheque and pay for the extra work,
Cheers,
Shane

 

[meltedEng]

Dhurjati Sen, "If at all welds need to be flush ground as per painting procedure, the same has to be included in the welding procedure."

Would you mind linking to this requirement for the WPS?
Also this is not my specific area, but grinding flush an ID weld as we are talking about does not interfere with weld calculations to my knowledge? Weld finish is optional and outside code Div 1 consideration I think?

balamurugan02, I agree with the group here, ask your client or the vendor doing the coating. Depends on the coating what is required. If you quoted not flush ground, maybe cost adder for doing so.

 

[r6155]

DekDee: I disagree.

It is widely known that the substrate must be well prepared for all kinds of material before applying the paint (surface protection).
Thickness of paint shall be specified and measured.
Any imperfection in the substrate will cause the painting to fail,
Sharp edges shall be ground to a smooth radius.
All rough welds shall be ground to remove sharp edges and other such irregularities. All weld spatter and arc strikes must be removed. Chipping may be used if followed by grinding or the use of an abrasive disc.

Regards

 

[balamurugan02]

Gents,

Just an update! the most common reason I've heard so far for ground flushing before internal coating is as "Compositepro" mentioned tiny air pockets may form at the weld reinforcement edges that bursts latter in service exposing bare metal to corrosive fluid. So we are ground flushing all welds now! Sincere thanks to all your advice

Regards,
Bala.

 

[david339933]

I agree with DekDee. Grinding sharp edges and rough welds are part of the Code (UW-35). Grinding spatter and arc strikes are also done whether there is internal coating or not. Most lining specs state to stripe coat the welds...even twice if need be. Grinding all welds flush is extremely costly and by no means required...unless stipulated by customer specification prior to.

 

[marty007]

I agree with DekDee and david339933, unless the customer specs or paint system specs stipulate grinding flush, it is not required. Many internal coatings will operate fine with properly finished welds.

 

[weldstan]

While it is true that coatings can be effectively applied to welds without grinding; however, the OP stated that the welds in question were made from one side only. One assumes that the root passes were made with either the GTAW or GMAW process, in which case it is highly doubtful that an acceptable weld profile would be obtained without considerable grinding preparation.

 

[r6155]

Good Engineering Practice: the paint company shall inspect all surfaces prior to hydrostatic test.

Regards

 

[balamurugan02]

Thank you all