ASME R- Stamp for Vessel with Pits 1

[nickypaliwal]

We have a sour water stripper column 9mm thick on very top section and 18mm for the remaining section. The design corrosion allowance (CA) is 3mm.

Pits are mainly found on the 18mm thick section.
-Pits with depth about 1mm or less are spread across the shell for which plan is to use metal putty.
-However there are few pits (about 6) with depth 2-4.5mm which are planned to be repaired by machining and weld buildup as per NBIC-3 to get R-Stamp.

ASME Inspector suggests to update the column drawing stating no CA is left due to pits spread across the column else to machine entire area and repair by weld buildup. However, if the build up is already planned for the 6 pits with higher depth, it does not seem to make sense to update the drawing stating no allowance is left.

Is it a general procedure to update the drawing with no CA? Also it does not make sense to consider that only thickness equivalent to 3mm is lost, what is the logic? why is it not 3.5 or 4.5. If we consider 3mm is lost then vessel should be considered retired or will need to do FFS.



Nickypaliwal

Materials & Corrosion Engineer

 

[KevinNZ]

I agree, seems not logical to remove the CA. The drawing states as-built thickness and the CA. Any inspector will know to look for difference between the design thickness and as found thickness.

A design with no CA will be condemned as soon as there is any corrosion.

If have repaired the pits to code that you are as good and new.

 

[Gr8blu]

The way the OP worded the ASME Inspector's response indicates the OP has a choice.
1) Machine the ENTIRE PITTED AREA REGARDLESS OF PIT DEPTH AND RE-WELD or
2) Modify drawing to indicate there is no corrosion allowance (CA) left, due to size/number of pits.

It is not a recommendation to "fix AND update".

Converting energy to motion for more than half a century

 

[nickypaliwal]

@Gr8blu: Yes, in sort the options are as you summarized. It is impractical to machine entire surface and do a weld buildup. We are however already planning to coat entire surface with high temp. coating to control pits.

Is there a clause in NBIC which mandates repair of all pits? Can we not just coat it with high temp coating system.

Even if pits are left as it is, how can the assumption of not having corrosion allowance justified?

Nickypaliwal

Materials & Corrosion Engineer

 

[MUHAMMAD FAHAD RANA]

Dear Nicky,

Better to talk and convince Authorized Inspector by taking UT Thickness survey of different location with mapping.

Otherwise, put email to his AIS for better opinion.


Fahad