ASTM A335 Gr.P91 EXPERIENCES 2

[jonesey]

Anybody have any experience with use of P91 (9Cr-1Mo-V)grade materials in piping plants? As part of a visbreaker revamp we've been recommended by our engineering study contractor to change out some of our existing ASTM A335 Gr.P9 piping to P91. Reasons are better resistance to creep in the higher temperature range and easier welding than P9. Lines in question have design temp of 507degC and design pressure of 97.6barg. What about valve materials ? I reckon we'll keep to the same material type for our socket weld valves with A182 Gr.F9 so we don't get problems with welding dissimilar materials. What about flanged valves. Could these just be 'plain' P9 with ASTM A217 Gr.C12 or would they also have to be P91 material with ASTM A217 Gr.C12 .

 

[athomas236]

You could try

thread378-106055
thread794-110710
thread794-109161
thread330-997767
thread330-93804
thread794-92534
thread330-91127

To name but a few

 

[davefitz]

see the links recommended by Athomas236.

The modified cast version of P91 is C12a, not C12.

P91 is not easier to weld properly than P9. The key word is properly. You should review the recent ASME proposals to modify the required fab and weld procedures of P91 before proceeding any further on the changeout. If your contractor has based his recommendation on past experience of simply meeting the current ASME code min requirements for P91 , then he hs not reached the correct conclusion.

 

[Raaden]

There is an interesting article in the latest Combined Cycle Journal that deals with the problems that has been associated with P91 and it also deals with the proposed code changes.

http://www.psimedia.info/Industry Alert.pdf

 

[athomas236]

Very useful Raaden.

It was Mr Henry that made an oral presentation to the 2004 ASME P&PV conference that sparked my interest.

Mr Henry will be presenting a paper at a conference here in the UK in a couple of weeks.

Regards,

athomas236

 

[athomas236]

According to Mr Henry's article it appears that ASME have adopted new rules for PWHT and are suggesting that it would be better industry practice to install P91 piping with cold pull.

Does anyone know when amendments to the Codes will be available.

Regards,

athomas236

 

[fvincent]

Jonesey

Please correct if I am wrong, but it seems that P91 is a best choice for a temperature and for a pressure operating ranges a little bit higher than yours.
I was involved in the selection of P91 for a petrochemical plant in Brazil in 1990 and I remember that P91 was worth selecting only for temperatures higher than 530oC. If not P22 was the best choice.
Well, maybe relative costs are more favourable today.

fvincent

 

[davefitz]

raaden:

regarding the article, It is my opinion that there was a technical error in the review of the P91 failure between the P91 piping and the 1.25Cr 1 Mo V stop valve. The original weld detail most definitely did NOT meet ASME B31.1 code requirements, as that code requires that, for butt welds, the thickness transition at the OD nto have a slope greater than 2:1 ( 30 degree max), while the original detail had a 70 degree slope. Thre were also several other areas in which the Code was not met, but that one specific area is unambiguous.

 

[MJCronin]

jonesey, davefitz, fvincent

I fully agree with the observation and recommendation about material selection made by fvincent.....with the maximum temperature and pressure described above I suggest that you stay away from A335 Gr.P91 and use P9 or even P22 materials.

Unless your piping is very large diameter, you see no real advantage with P91.

At these temperatures and pressures, I would suggest the safety and cost savings of butt welded valves.

davefitz, I worked on a Main Steam piping failure at the TAMPA Electric Company Gannon Station some years back. (1010F - ~2600psig -- A335 - P22) There was a ~2" thickness difference between an "out of spec" elbow and pipe that was not properly tapered internally as per ASME B31.1.

The pipe cracked during a "turbine trip" injured two fitters, blew grating, steel and concrete around the plant.

The fix was to repair the pipe with the proper elbow, internally tapered to meet the piping code....

Moral: They make these rules for a reason.....


MJC

 

[tprimaterial]

Dear sirs,
While welding the P91 pipe, the postheating maybe carried out directly after the welding, or firstly cooled to 80-100 degree then reheating to 300-350 degree, which is better? does it influence the microstructure and properties of the weldment?
another question, anybody has studied the influence of cooling rate of PWHT on Charpy impact properties of the weld metal?
thanks in advance!

 

[davefitz]

tprimaterial:

as per the Mannesman- Vallourec guidelines, it is better to cool to 100 C prior to PWHT. This ensures that all martensite is formed prior to PWHT. The other choice of PWHT directly after welding without cooldown will result in a more bittle weld area, as additional martensite will form after PWHT and not be tempered. There are results of hardness testing of weld nones for both choices, see papers online by Eurweld and their parent company .

The notch toughness and impact properties are recognized in European codes but ASME does not seem to indicate much concern. I can tell you they are a real concern, as I am aware of at least 6 prompt ( ie short term fatigue) major piping weld failures of P91 piping in the last 2 years at one single utlity, and incorrect PWHT and weld geometry played a key role in those failures, and I suspect the failures were hastened by poor notch toughness. These properties are also in the Eurweld papers, and are also a function of weld electrode trace alloy content.

 

[tprimaterial]

dear davefitz,
how to find the recent ASME proposals to modify the required fab and weld procedures of P91, is it included in the 2004 version?

 

[davefitz]

tprimaterial:

see the link by Raaden above, or contact ASME at

http://www.asme.org.

Also, someof the linkes by athomas236 above will identify the web based documents.