Use of welded pipes according to ASME sect.1 1

[GBENARD]

Hello,

One of our boiler supplier have used welded pipe for the manufacturing of the reheater outlet Headers. Instead of A335 P91, he has used A691 Gr91 (according ASTM 98 seems). As per the ASTM/ASMEII spec., A 387 91 plates were used to manufacture the welded pipes.
Up to now everything is clear as :
-ASME I does not recognize A 691 as usable materials in BEP or boiler proper.
-ASME II clearly indicates that A 691 is intended for use for component under the juridiction of ASME III.
-there is no stress values in ASME II table 1A for A 691 for temperatures above 700°F (little bit low for headers designed at 570°C, that's why they used the stress values for A 387 91 for thickness calculation).

But, ASME I accepts A 387 material for plates. Who can prevent me to manufacture from A 387 91 two cylindrical shells and use them as headers, while complying with the code?
In other words, I do not really understand what is the advantage of going through the process of manufacturing a new "material", i.e. A691. What is the gain and for who?

If somebody has an idea of the reason why, it would be really helpfull.

 

[metengr]

GBenard;
SA 691 is a specification for carbon and alloy steel pipe electric fusion welded, and lists Grade 9 not Grade 91. Grade 9 is a 9 Cr-1Mo material - completely different animal.

SA 387 provides for both Grade 9 and Grade 91 plates, however SA 691 does not list Grade 91. Grade 91 is NOT the same material, and cannot be assumed for use under SA 691. Grade 91 is an upgraded material, and contains vanadium and niobium alloy additions.

 

[metengr]

As a follow-up you can still fabricate a seam welded header from Grade 91 rolled plate under the SA 387 specification using Section I design guidelines and NDT requirements. SA 691 is not an approved welded pipe specification for Section I applications.

 

[GBENARD]

Hi,
Thanks a lot for your answer. I forgot to write one thing in my thread. The supplier does not refer to ASME II specification for its material but to ASTM 691 98. It seems that ASME II (at least the revision I have, 1998 if I am right) is referring to ASTM 691 93. I agree with you the available grade is 9CR and not a modified 91 alloy. Moreover I was not able to find any stress value for this grade in table 1A. But it seems that Gr91 is available in ASTM spec. A691 98. The point is so to know if my ASME II version is up to date. If it is the case, that would mean that, even better, they used a material not recognized by ASME BPVC.

 

[davefitz]

They are using welded plates probably because there is not available a seamless F91 forging for the diameter in question. As I recall, the max diameter P91 or F91 pipe by Mannesman is 48" OD.

The ASME II stresses for A387 gr 91 plate are identical to that of seamless SA 335 P91, but they cannot be used directly without incorporatig a weld strength reduction factor for rolled and welded pipes. For the design metal temp of 570C, a weld stregth reduction facor of about .6-.7 would be required, to account of the "soft zone" - an overtempered band of parent material in the intercritical zone of the HAZ that has about a 30% reduction in creep strength. This weld strength reduction factor might be overshadowed by the need to provide thickness reinforcement due to ligament efficiency for a perforated header, depending on how the penetrations are aligned relative to the longitudinal seam weld.

P91 is a tricky material, so you should obtain all the recorded data on the plates's history, including preheat temp, electrode documentation, PWHT times and temps, andhardness test results. Thismight be the reason the supplier used an oddball spec- the A691 spec might have fabrication procedures that are better suited to P91 than the 387 spec . Read both specs, and contact the authorized inspector for his interpretation.

 

[GBENARD]

Thanks again to everybody for your advises,
In fact the Header is 24" and 30mm/1,25" thick. The problem was not availability but delays to get A335 P91.
The main concern is that the boiler is supposed to be supplied according to ASME I for evrything (design, material, tests, etc..) but NOT stamped, as it is installed in Mexico. So, the efficiency factor cannot be discussed with an AI.
Ligament efficiency is most probably not really a problem (HRSG with finned tubes bundle).
In the absence of an AI to review this, how should I interpret this material spec. out of ASME I requirements? What could be the real concern, apart from contractual and maybe regulatory matters?

 

[davefitz]

The main issue of concern is to confirm that a weld strength reduction factor of 0.65 was used. I would also obtain a hardness test reading of the header material, since it was likely PWHT's twice.

The fabrication procedure should be carefully reviewed, since the fact that they are using an off-spec material suggests they don't know what they are doing. Issues with fabbing p91 include:
a) using documented low hydrogen low nickle electrodes
b) preheat temp - proof via montored thermocouples
c) post weld hydrogen bakeoff followed by cool down to 200F for martensite traasformation followed by NDT for weld integrity of seam weldd
d) PWHT time and temp - to be confirmed by multiple thermocouples
e) ditto procedures for weldign tubs to header, implies 2 PWHT's and possibly overtempered.

 

[metengr]

I would agree with davefitz. P91 material is something that is not to be taken lightly. This is an extremely sensitive material to PWHT. The optimum range for PWHT is 1380 to 1400 deg F for a minimum of 4 hours at this thickness. More importantly is the Mn and Ni contents of the weld filler material. In some cases, I have heard of Mn and Ni contents that exceed 1.5% (combined) resulting in a depressed lower critical transformation temperature and causing untempered martensite to form during PWHT cooling.

 

[GenB]

Youshold have specified: ASME compliance (that includes inspection/stamping). If the mat is notincluded in PG9.1 PG9.1.1, PG9.2 or procedures to procure under PG10 are not done, the mat can not be used in sec I construction.
Specification to ASME w/o stamping means: nothing to a Mfr.
you must specify compliance to The Codeto asure of inspection/stamping/reporting), the mfr may get away without registration tothe NB but the Inspector gets to signthe data anyways and you can ask for a copy ofit.
PS Most of the equipment I send to Mexico (even small US$500.00 vessels are stamped)
Regards,
ER

 

[davefitz]

After reading the A 691 spec, it is possible that it was used because it has an optional requirement for normalizing and tempering and 100% X-ray, provide the class of material was defined accordingly.

If it turns out the vendor did specify the class which requires N+T and X-ray, then there is a good chance the header is OK, but I would request a hardness reading anyway.