In service inspection - nozzle t-min

[marty007]

Hey everyone,

I was reviewing some in-service inspection t-min values for an ASME VIII-1 vessel. Currently we are setting the initial t-min value for piping nozzles as t-nom - CA. For a 12"-sch.120 piping nozzle with 1/8" CA for example, this would be 1" - 0.125" = 0.875". When the nozzle neck nears 0.875" during inspections either the nozzle is repaired, replaced, or a more detailed FFS analysis is performed.

What this doesn't consider however is the 12.5% under tolerance on piping components. Per code, this nozzle could have a maximum under tolerance of 1" x 12.5% = 0.125", meaning the wall thickness could already be 0.875" in the new condition. This would be flagged immediately during the very first inspection for repair/replacement, which doesn't make sense.

Would it be appropriate instead to set the initial t-min value for nozzles fabricated from piping components to: (t-nom * 87.5%) - CA? In the case of the example provided, this would set the initial t-min to: (1" * 87.5%) - 0.125" = 0.75".

Thanks!
Marty

 

[SnTMan]

marty007, is (t-nom * 87.5%) - CA not the thickness considered for MAWP calculations? I think your proposal would be entirely justified.

Regards,

Mike

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

 

[GD2]

marty007,
For after service, the minimum required thickness becomes the design thickness adjusted for mill tolerance and corrosion allowance.
therefore, your calculation of 0.75 inch will be correct.

Once it reaches below 0.75 inch you there are couple of options you can still continue in service:
1. Reduce the MAWP or you may be already operating at reduced MAWP. in this case, calculate the new design thickness at the reduced MAWP.
2. Do a remediation to bring the thickness to the original design conditions, or
3. Do a FFS (Level 2 by API 279 for Type B Class 2 component), Part 4 if there is general metal loss.

GDD
Canada

 

[marty007]

Thank you both SnTMan and GD2, it is very much appreciated.

 

[KevinNZ]

You would have to sure the original designer used 12.5% mill tolerance in the calculations. 12.5% Applies to pipe. The nozzle may bee made from rolled plate or some other material type.