ASME B16.5 4

[19750412]

Good day Everyone
I have a question referring to flanges acc. to ASME B 16.5. According to point UG-44 of ASME VII Div.1, if flanges, gaskets and bolts fulfilled requirements of listed there standards, is enough to choose flanges only based on calculated pressure-temperature rating in referring to design parameters and assumed flange material. And in may particular case for flange 2 inch class 150, for design parameters 0.82 MPa and 343 oC, calculated rating 150 is good enough. But if I make detail strength calculations of this flange with spiral wounded gasket with dimensions acc. to ASME B 16.20 with coefficients m and y taken from Table 2-5-1 of Annex 2, I reach overload in Radial Stress. Same situation I have for blind flange for above described flange, offcourse there is a problem that nominal thickness for 2 inch blind flange is not enough. For flanges chosen material is SA-350 gr.FL2 Cl.1 and corrosion allowance is 6 mm. Such problem, I have also for example for LWN nozzles 20 and 24 inch class 150. There are overloaded stress in hub. Therefore, I would like to ask you whether I could accept such flanges chosen based only on rating thought that strength calculations gives unsatisfactory results. And how I shall do in future.
Thanks for advice.

 

[TomBarsh]

As you have indicated, ASME Code accepts "standard" flanges per their ASME B16.5 pressure-temperature ratings. You need not do any further analysis if the flange including bolting and gasket meets the requirements of B16.5.

It is true that many standard flanges will not meet their B16.5 pressure-temperature rating when analyzed by ASME Section VIII Div 1, App 2. ie: The flange MAWP found per App 2 may be lower than the published rating.

On the other hand, many "stockier" standard flanges will have an MAWP per App 2 that is greater than their B16.5 rating.

I am surprised that the 2" Class 150 wasn't one of these that falls into the "stockier" category (a 24" Class 150 would certainly be the "non-stocky" type with lower MAWP).

 

[TGS4]

One note with respect to the pressure -temperature rating in B16.5. They do NOT include any allowance for corrosion. So, the fact that your application has a non-trivial corrosion allowance of 6mm in the NPS 2, Class 150, should be reason for pause.

I fully agree with TomBarsh that it is well-known in industry that many B16.5 flanges fail the Appendix 2 calculation. From a strength perspective, I would recommend Div. 2, Appendix 4 analysis. From a leakage perspective (and aren't all flanges designed so that they won't leak???), you're going to have to look farther afield than the ASME Code.

My recommendation - EN-1591-1.

 

[19750412]

Thanks for Your answers
From my designer experience, I have always checked even standard flanges. And if the result of analysis has been unacceptable I have choose flange from upper rating class. But now my client request only design base on rating calculation, therefore I have some objections to this. But now I can accept such solution, especially that I have made strength analysis with using of calculation software, which include in strength calculation corrosion allowance automatically, what gives in result thicker wall.

 

[deanc]

Make sure you use the acceptable year. See Table U-3.

 

[TomBarsh]

TGS4 and deanc both make good points.

I hear no end of questions about how to specify corrosion for standard flanges.

And now that ASME Section has finally adopted the 2003 Edition of B16.5 (per Table U-3) we can toss out that decade-old copy (well, better keep it around).

Another point: The pressure-temperature ratings may (and do) vary with the Edition. Some materials in the new Edition have little changes in their pressure rating at given temperature, like + or - 5 psi here and there. So flanges that passed in older Editions may no longer pass with new Edition.