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Allowable stresses during pressure test.
6

Allowable stresses during pressure test.

Allowable stresses during pressure test.

(OP)
According ASME B31.1, during the pressure test the circumferential stresses shall not exceed 90% of the yield strength.

For a piping systm with the following data:

Pipe Material: ASTM A335 P91
Outside Diameter / Thickness : 273.1 mm / 21.44 mm
Design Pressure: 120 bar(g)
Design Temperature: 570
Pressure test : 280 bar appr. (According PED 97/23 EC)

how can i calculate these stresses;




RE: Allowable stresses during pressure test.

3
HI Pandil,

Exactly as BigInch stated - it is all in the Code.

Specifically, You must determine the required test pressure in accordance with B31.1, paragraph 137.4.5.  Then you must calculate the combined longitudinal stresses (paragraph 104.8.1) due to weight (dead weight and live weight) additive to the longitudinal pressure stresses (test pressure - see paragraph 102.3.3(B).  This combined stress may not exceed 90 percent of the yield stress of the material (you MAY need to add temporary supports for the test).  Then you calculate the  circumferential (hoop) stress for all diameters of piping in the system and the greatest circumferential stress may not exceed 90 percent of the yield stress of the material.  The circumferential pressure stress can be calculated by rearranging equation 104.1.2 (3) to give you approximately P*Do / 2*tn (see B31.1 for the definitions of the variables).

I hope that you are aware of "the issues" involved in ASTM A-335 P91 material.

Good luck with youe project.

Regards, John.

RE: Allowable stresses during pressure test.

Hi BigInch and all,

ASTM A-335, P91 is a wonderful material.  But there have been some problems - sadly some involved fatalities.

thread794-176859: P91 pipe failure in China

thread292-143071: New ASME Code case for P91/T91 ???

www.steamforum.com/steamforum2003/_discstm/000007a7.htm

www.iaea.org/inis/aws/htgr/fulltext/htr2004_e13.pdf

www.babcockpower.com/pdf/rst-164.pdf

Apparently there have been at least three categories of problem with application of P91 material: "mislabled" material, inappropriately heat treated material and contaminated material.  

Some people will always try to "beat the system".  There have been documented reports of "ASTM A-335, P91" material being delivered on site that did not conform to the ASTM Specification - it was deficient in one or more of the alloy's constituents (some were blatant attempts to pass off lesser materials as P91).  After some investigations of failures in P91, it has been determined that the material structure had be changed by inappropriate heat treating (in every case that I am aware of it was too high a heat for too long or some variation).  Some boiler tube failures were traced to the material being not properly protected from contaminants while being stored or shipped - these materials were then welded without proper cleaning and the welds were defective.  The welding of this material seems in general to be sensitive to vargaries in the welding process.

Perhaps Metengr will weigh in on this topic?

Regards, John.

RE: Allowable stresses during pressure test.

3
Hi John;
Per your request. I concur with your opening statement regarding Grade 91 material. This material, like other creep strength enhanced ferritic steels (CSEF), was not completely or least understood by many fabricators in the US and abroad when it was endorsed by Section II Subcommittee of the ASME Boiler and Pressure Vessel Code. Part of this misunderstanding had more to do with past practices using the work horse Cr-Mo materials, which as you know are very forgiving in terms of handing and heat treatment. I believe when Grade 91 material was introduced, the Code committees fell short on providing specific guidance for handling this material (John, this is not meant to slam any of our code colleagues, which includes both you and I. On the bright side, there has been significant movement to add technical guidance in ASME Section I for heat treatment after bending and limits for cold strain).

The recent problem with Grade 91 material is installation of pipe and fittings on heat recovery steam generators (HRSG’s). Several of my metallurgical associates have reported inspection results related to fittings and pipe spools for HRSG's that were commissioned over the last 5 years. Part of the stepped up inspection was due to actual failures (one being on a Grade 91 fitting for an HRSG at FP&L) due to an improper heat treatment.

The Grade 91 material inspections have centered on field hardness testing and subsequent surface replication to evaluate the microstructure. The results have been quite disturbing, namely soft regions in fittings and spools unrelated to field welding. Field welding presents its own set of problems in terms of post weld heat treatment.

In my opinion, the Grade 91 material was mishandled and installed in this condition. For those that are not aware, a “soft region” is anything below 200 Hv. However, there is some debate as to the validity of comparisons between portable hardness testing techniques (Equotip versus Microdur versus …). This is presently being sorted out. I believe one needs both field hardness testing and replication for this type of evaluation.

At this point in time, the cause of the soft regions in the heat treated Grade 91 material has been lack of control during fabrication of bends and fittings.

The soft spots in Grade 91 material can be caused by several factors, which I ranked in occurrence (my professional opinion);

- inadequate quenching; meaning the microstructure of the Grade 91 contains ferrite and carbides versus 100% martensite, prior to tempering (even if the tempering was controlled per the ASTM or ASME specification, the soft region would remain because it was not martensite prior to tempering). Fabricators must understand that when heat treating this material, very tight temperatures and controls are required, and must be implemented. The furnace used for austenitizing and tempering should be checked for calibration with temperature surveys to assure uniform heating. Any stacking of material that can result in temperature spreads of 100 deg F or more will result in an inferior microstructure either during quenching or even during tempering.

- lack of heat treatment. Here we have a complete breakdown in the understanding of what the desired microstructure needs to be after hot bending. Most fabricators with no experience in handling Grade 91 assumed it would respond similar to Grade 22 material. In other words, I can heat it up to 1900 deg F, bend it and air cool. There is my so called normalization heat treatment. This is obviously incorrect and dangerous to assume. This practice will result in very soft microstructures (less than 175 Hv).


- last is possible segregation of alloy elements during original fabrication of the pipe spool or fitting. There have been sporadic reports on possible segregation of alloying elements that resulted in hardness variability. Still being evaluated.

Closing comment regarding post weld heat treatment or tempering of Grade 91 material; over-shooting the tempering temperature will generate fresh martensite that upon cooling will result in excessive hardness and increased susceptibility to stress corrosion cracking upon exposure to moisture. This is what I believe caused the rupture of the Grade 91 main steam line in China.



RE: Allowable stresses during pressure test.

(OP)
Thank you all for your responds.

It was my first post in eng-tips. You qave me not only the answer to my question but additional valuable info also.
Thanks for your time.

Regards
Nick

RE: Allowable stresses during pressure test.

In addition to metengr's and John's commens, failures have occured due to poor designs, and poorly designed weldments at attemperators, weld fabricated wyes and at dissimilar metals welds with highly dissimilar thicknesses. Lastly, numerous fittings have simply been mis-heat treated (by the manufacturers); e.g., annealed at P22 annealing temperatures, annealed rather than air quenched from the appropriate normalizing temperature (recommend that all fittings be hardness tested at receipt). Failures have also been due to tempering (by the manufacturer) above the lower critical temperature.  

At least one or 2 failures have been attributed to stress corrsion cracking.  

RE: Allowable stresses during pressure test.

Some other good information:

http://www.thefabricator.com/TubePipeFabrication/TubePipeFabrication_Article.cfm?ID=268

http://www.eprictcenter.com/infocenter/cc_o_and_m/hrsg/FAQ.php3

and the topic of dissimilar metal joints is being studied by several research organizations.

http://www.muni.cz/research/publications/566312

stanweld,

Do you have any documents or websites to share describing the failed attemporators, fabricated "wyes" and other problematic P91 weldments ?

I would like more details......

I am very curious about when the lawyers will begin to smell blood in this matter...

-MJC

  

RE: Allowable stresses during pressure test.

Hi Pandil

I am not familiar with the AMSE codes but just check that
the code formula cater for the ratio of your vessel o.d. to
wall thickness, it would be classed as a thick walled vessel if it was designed to BS500 and the formula used for
hoop stress would not be valid.
For thick walled vessels the radial stress across the wall
as to be accounted for and for this case you would use Lame'
equations which can be found in any strength of materials text.

regards

desertfox

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