Corrosion allowance required for Steam piping 1

[JohnBrittoUGL]

Can anyone please clarify the following?
We have selected SA335 P11 pipe material for the superheated steam piping (for the operating temp of 470 deg C). The piping was designed to ASME B31.1. The question is, what should be the minimum corrosion allowance considered while selecting the pipe thickness?

 

[metengr]

Based on your stated information, you do not need to worry about corrosion resistance. Instead, I would make sure you have nominal wall thickness on the +12% side above minimum wall thickness.

 

[davefitz]

As Meteng says, in most cases no add'l corrosion allowance needed for large diameter piping that has a 12.5% assumed variance between nominal average wall thickness and calculated required minimum wall thickness ( refer to the ASTM spec for the grade used for actual variance).

For power piping, there may be 2 exceptions to that rule- the thin wall piping downstream of teh HP bypass spray attemporator may suffer erosion if not provided with an inconle liner, and for feedwater piping downstrem of the BFP dishcarge nozzle and at elbows that may suffer FAC . Those exceptions also need to incorporate routine ( bi-annual) UT of wall thickness to confirm the erosion rate has not removed excessive wall thickess.

 

[JohnBrittoUGL]

Yes, we have used a mill tolerance (or) a negative tolerance of 12.5% while calculating the thickness.

Thanks a lot for your responses. Based on your feedbacks, I have justified to the Client.



JB

 

[EdStainless]

It is common in steam piping to use the "nominal - 12.5%" for design and then buy "min-wall" for service.
It has become common to see the use of higher Cr grades in mild FAC locations and Ni alloy ID weld overlay in places where it may be serious.


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