Re-Rate existing Vessel (lower the MDMT) by PWHT 2

[LittleEd]

Just wondered if this was acceptable: Can you re-rate a vessel MDMT by performance of PWHT of the vessel in order to get temperature reduction allowed by UCS-66.2? If a vessel was not built with PWHT and therefore welding procedures used to fabricate the vessel did not include PWHT, can you (40+) years later perform PWHT of the vessel to lower the MDMT?

 

[metengr]

No.

 

[LittleEd]

Thank you for responding, that was my answer as well! I did not see how this could be done, but owner (large oil company) sure had described this as their Alteration Plan and was looking for R stamp holder to re-rate vessels for them.

 

[metengr]

Being a member of the NBIC main committee, this becomes a real NO.

 

[LittleEd]

Below is from the owner's plan. As I understand it, they are wanting to PWHT vessels (40+ yrs old) to try to lower the MDMT (additional 30 deg.F). I am not familiar with this FFS-1 standard

Several vessels have been evaluated for risk of brittle fracture using API 579-1/ASME FFS-1 Fitnessfor-
Service Level 2 analysis. Per the FFS evaluation the Minimum Allowable Temperature (MAT)
curve was calculated from atmospheric pressure to maximum allowable working pressure (MAWP) for
each vessel to provide an allowable pressure/temperature operating envelope. The review of the
scenario of autorefrigeration has resulted in a recommendation to perform post weld heat treatment to
lower the minimum allowable temperature (MAT) of these vessels. The FFS standard allows for a
reduction of MAT under some cases if the component was subjected to post weld heat treatment
(PWHT). By definition the minimum design metal temperature (MDMT) is the MAT at the MAWP.
Lowering the minimum temperature of the vessel is an alteration per the NBIC. Based on these code
requirements the application of PWHT to lower allowable operating temperatures will be an alteration
and will require a re-rate of the vessel.

Contractor is to perform rerating of the vessels without performing impact testing, following the
guidelines set forth in ASME Section VIII Division 1 and / or API 579-1 / ASME FFS-1 and/or
ASME BPVC Section VIII. The contractor is to re-rate these existing vessels for the desired low
temperature operation. The requested analysis is to include review of the available
documentation, performance of the applicable stress calculations and development a scope of
work to perform post weld heat treatment to meet the desired reduced MDMT.

 

[LittleEd]

"The FFS standard allows for a reduction of MAT under some cases if the component was subjected to post weld heat treatment(PWHT)."

Yes, if the PWHT was done at the time of construction, you could possibly take credit for that later (if needed). But you cannot PWHT 40+ yrs later to take the credit to reduce the MDMT.

 

[metengr]

This situation is similar to trying to use increased radiography from original code of construction to justify increasing the efficiency factor of original welds to re-rate , and this is clearly not permitted by the NBIC.
The only way out that I see for re-rate under the NBIC would require original vessel material and a demonstrated benefit in lowering of the MDMT having vessel material subjected to PWHT and impact tested. How's that?

 

[Duwe6]

Both rerating by PWHT and increased volumetric examination of welds [RT %] seem to be allowed per
API-510; as are true 'Repairs'. An NBIC 'repairs' requires that the vessel be returned to original condition - more correctly called "Rework to Original Spec", rather than Repair. Two very different philosophies; NBIC wants it to be exactly like the day it was Code-stamped, not more or less. API-510 allows anything that you can show good enginering justification for, and can prove with calculations.

Sounds like LittleEd's folks are going the API route.

 

[metengr]

How can you predict that PWHT is going to enable you to reduce MDMT without notch toughness testing to back-up the approach? There are just too many variables - original grain size, heat to heat variability, how was the orignal welding performed? You may have API-510 as wiggle room, but if the pressure vessel is within a Jurisdiction in the US that regulates pressure vessels, the NBIC will be the final say in addition to the Jurisdiction.

 

[jtseng123]

LittleEd,
As Metengr says, you shall recommend the owner to cut a patch, PWHT and impact test it. If failed, no more saying.
40 years old, the material may be carbonized and deteriorated. You shall also recommend to run a tensile test as well as micro structural examination to make sure it is still weldable and has the properties.
Big oil company shall not save this little money. If they don't listen, you have done your job and let them run their own risk.

 

[Bocephus50]

When did vessels begin to get MDMT ratings?

If a vessel was built in the early 1960's using SA-285-C, but was not stamped with an MDMT rating, could you now "re-rate" this vessel (give it an MDMT rating) as -20 deg.F using current ASME VIII-1 edition (UG-20f/UCS-66) without having to do anything other than thickness testing to verify min. thicknesses for MAWP rating are still good?

If a vessel is rated for -20 deg.F, can the ASME FFS-1 Service Level analysis of a vessel reduce its rating (i.e. to 0 deg. F)?

 

[metengr]

No, not for vessels built prior to 1968. This is stated in both API 510 and NBIC, and the reson for this is because of changes in steel making practice. This is why re-rating old vessels is not cut and dry or textbook.

 

[LittleEd]

Like Bocephus was asking, if a vessel is rated for -20 deg. F, how can it loose that rating (without being re-rated)? The owner had one vessel that was stamped MDMT -20 deg. F, but their Report / Plan said the ASME FFS-1 analysis had reduced it "MAT" to -3 deg.F and now they were wanting to re-rate it to -33 deg. F.

 

[stanweld]

The Owner appears to be asking the Contractor to Rerate the vessel and take responsibility. This conflicts with the intent of API 510, which is stated in 1.1.2 thereof. It is the Owner/User's responsibility.