MAWP vs DP for calculating minimum required thickness

[cespi82]

Hello everyone,

What pressure value (Design Pressure or Maximum Allowable Working Pressure) should be used to calculate vessel's post construction minimum required thickness (Tmin)?

If proving an example helps the understanding of the question, let's say that after reviewing the vessel documentation I was able to find DP=250 psi and MAWP=275 psi. The vessel has been in service for several years and it has general loss of thickness due to corrosion in the shell at a well known rate. I want to calculate the remaining life of the corroded area of the shell, and therefore I need the Tmin as an input for the remaining life calculation.

I'd go ahead and use UG-27 formula to calculate Tmin, but what pressure value should I use? DP or MAWP?

Thanks!

 

[pdiculous963]

Often manufacturer's will stamp vessels for the highest calculated MAWP, not the design pressure. What is the relief device set pressure on the vessel? If it is at the design pressure, it may be acceptable to use that to calculate tmin, else the MAWP should be used. If you do base it off the design pressure/relief setting, it is important to document that internally so that in the future no one decides that they can then operate at the stamped MAWP.

 

[r6155]

In formula UG 37 use actual thickness and calculate pressure.

Regards
r6155

 

[MrPDes]

End note 36 at the back of ASME VIII Div 1 might help you.

 

[marty007]

The MAWP should be used, because that is the pressure that is stamped on the nameplate and documented in the MDR.

If you were to accept a thickness based on the design pressure, then you can no longer safely operate the vessel up to the MAWP, and you're creating a dangerous situation for anyone that isn't aware of your approach.

If you perform calculations based on the design pressure and your future inspections are based on this, then the vessel should undergo a formal re-rating to lower the MAWP to equal the design pressure. This would be deemed an alteration, would require the attachment of an alteration nameplate, and would be documented on an R-1 form (assuming NBIC jurisdiction).

 

[cespi82]

Thank you pdiculous963 and marty007, I agree with you.
r6155 Thank you for your reply, however I do not get your point. Can you please clarify? What exactly do you mean?

Regards,

 

[r6155]

Uhhhh, Sorry my typing error: please read UG-27 instead of UG-37

Calculate P in UG-27 using actual thickness (corroded), S, E and R (corrosion =0)
If P is greater than DP= 250 psi (your example) then nothing you need to do: continue in service until next inspection.
If P is < than DP= 250 psi you need to re-rate this pressure vessel.

Regards
r6155

 

[r6155]

See more in ASME PCC-2, API 510, API 579 and API RP 571

Regards
r6155