ASME Sec VIII Div 1 Flange stress calculation 1

[AARAR]

ASME Sec VIII Div 1 App 2 calls for stress verification for operating condition bolt load being Wm1 and for seating condition bolt load being Wg, recommended bolt load in place of Wm2. As per the code Wm1 & Wm2 are the minimum required bolt load for operating and seating condition. So we can select Maximum of Wm1 and Wm2 (or Wg) and apply either in the form of torque or direct tension.

What I am not understanding is that why code calls for stress calculations based on individual bolt load rather than maximum applied bolt load?

As per code Hg for seating condition is Wg (which is fine) whereas for operating condition is Wm1-H and moments / stresses are calculated accordingly.

For me, bolt load should be same either in seating condition or operating condition ignoring some minor change based on stiffness. Considering this , Hg for seating condition should be Wg ( assuming applied load is Wg) and for operating condition should be Wg-H.

Am I missing something? Can anyone explain??

 

[TGS4]

I'll leave the explanation of the rationale regarding the selection of specific loads to specific stress calculations to others, although Modern Flange Design -
Taylor Forge Bulletin 502 has some good explanations (it's out of print, but you can find free copies on the internet).

I will take umbrage with your statement

So we can select Maximum of Wm1 and Wm2 (or Wg) and apply either in the form of torque or direct tension.

No. See Appendix S and PCC-1. The bolt loads calculated in Appendix 2 are to design the flange, not to be used in actual operation.

 

[DSB123]

RREDYN,
Yes you are missing something. Read and understand the procedure as TGS4 indicates.

 

[SnTMan]

Appendix 2 gives a pretty good discussion of the two distinct loading cases. It also give the designer the option of using a flange design bolt load greater than both Wm1 and Wm2, namely Ab * Sa.

Regards,

MIke

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[AARAR]

TGS, Thank you vey much for your response. I am aware of the Appendix S & PCC-1 loads. PCC-1 provides step by step procedure for calculating bolt load based on the gasket, bolt and flange rotation and finally suggesting checking of flange stresses either by elastic closed-form analysis or FE analysis. Again we are coming back to Appendix 2 ( closed-form elastic analysis) for flange design for loads selected based on Appendix O of PCC-1.

Your statement "Appendix 2 loads to design the flange not to be used in the field" is confusing. Is it not required to design the flange for the field applied loads???

 

[AARAR]

Mike, Thank you...

Yes, Appendix 2 gives option to select higher bolt load and clearly recommends flange design accordingly...

My clarification is that If I select bolt load equal to Ab*Sa or more than this, what would be the Hg value for operating condition??

To my knowledge Hg is the residual load on the gasket and it should be always more than or equal to Wm1. Considering this, Hg for operating condition is W (selected pre-load) minus H ( total hydrostatic load) not Wm1 always as stated in Appendix 2. Correct me if I am wrong.

 

[BJI]

There is no correlation between Appendix 2 and Appendix O of their respective standards. S8D1 App2 is used to design the flange only, using the calculated flange design bolt load, which has nothing to do with the actual assembly load applied in the field. It purely designed to adequately size the flange to protect against operating loads and assembly damage. After the flange is designed and constructed, you refer to the post construction guides (PCC) to determine the appropriate assembly bolt stress (PCC-1 Appendix O). You should read S8D1 appendix S again if you are familiar with it, it acknowledges that the assembly bolt loads will be higher than that used for design.

You would not want to go higher than Ab*Sa for flange design loads, and in general should not need to use this increase in flange design load. If the bolts apply the load to your gasket, then your gasket load would have to be equal to or less than your bolt load (Hg = Wm1 - H, assuming operating is limiting). W is not the selected pre-load, please refer to Appendix 2. When you consider that the majority of the ASME flanges do not pass the App2 calculation at full pressure rating, you are being too conservative in your flange design. Even if you used closed form solutions to assess actual assembly loads, the stress limits in Appendix 2 are not applicable, so you should not be going back to Appendix 2. Actual analysis of flange assembly stresses accept local yielding as non-injurious, which contradicts the limitations of the Appendix 2 calculations which assumes no flange plasticity.

 

[TGS4]

Your statement "Appendix 2 loads to design the flange not to be used in the field" is confusing. Is it not required to design the flange for the field applied loads???

No. Appendix 2 is a method to design flanges using standard procedures. It is a fictitious approach that has proven robust for 80+ years. But 80+ years of experience also says that you don't use the calculation-basis loads for sealing.

Confusing? Indeed. But use the calculation method to design the flanges, and then PCC-1 to actually seal them. Simple?

 

[SnTMan]

RREDDYN, "...what would be the Hg value for operating condition??" Hg is always equal to Hp. It is related to the operating case only, it has no meaning for the gasket seating case. It is the compression load remaining on the gasket when the joint is pressured up, required to maintain a seal. Makes no difference what value is used for W.

Hg is always a non-negative quantity, so it cannot be larger than W. Consider a self-energizing gasket, m = 0, y = 0. Hp, Hg and Wm2 are also equal to zero. Wm1 becomes equal to H. W may be larger, if the Ab*Sa value is used.

Hope this is helpful.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[AARAR]

Mike, "Hg is always Hp" and "it is the compression load remaining on the gasket..." are contradicting each other. Hg can't always be Hp if it is the remaining compression load, it is against simple statics, sum of forces equal to zero. If the gasket seating load is W=Ab*Sa, then during operation, the remaining load on gasket, Hg must be equal to W-H ( If I go by your statement 'compression load remaining') unless seating case and operating case are independent checks...

I am going by simple statics and load bolt load propagation from seating case to operating case...

I hope I have explained my problem clearly....

 

[SnTMan]

RREDDYN, no offense intended, but I infer you are not overly familiar with Code Appendix 2 and the design method. First, it is not an analysis of an operating flange. It is a process to design the flange.

It goes like this: After establishing the flange geometry and gasket material, the first thing to do is select the bolting to be used. Two cases are examined. The operating case at design pressure (Wm1) and the gasket seating case at zero pressure (Wm2).

For the operating case Wm1 = H + Hp is used. H is the hydrostatic end force due to design pressure acting at the gasket mean diameter, Hp is the contact load applied to the gasket to maintain a seal. Hp is a function of the gasket property "m".

Wm2 is the load required to initially seat the gasket at zero pressure. It is a function of the gasket property "y".

These are minimums. The larger value is used to select the diameter and number of bolts to be used.

The flange applied moments and resulting stresses are then calculated for both the operating and the gasket seating case. For the operating case, W = Wm1 is used, for the seating case the quantity W = 1/2(Am + Ab)Sa is used. Alternatively the quantity W = Ab*Sa may be used, I assume, for both.

The flange moments for the operating case result from the hydrostatic end force acting at the flange ID, Hd, the gasket contact load Hg and the difference between H and Hd, Ht = H - Hd. The moments are totaled giving the operating moment Mo.

For the operating case, I can say that Hg = Hp because Hg is defined as Hg = Wm1 - H, and Wm1 is defined as Wm1 = H + Hp. Of course if W = Ab*Sa is used Hp and Hg will not be equal.

For the seating case the applied flange moment is the result only of the design bolt load W, equal to either W = 1/2(Am + Ab)Sa or W = Ab*Sa.

The calculated stresses must be within the allowables for both cases.

Again, Appendix 2 explains fairly well. You might find Taylor Forge Bulletin 502 helpful as well.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[DK44]

Mike, the link is not working. Please advise.

 

[SnTMan]

Huh, works for me...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[david339933]

Couldn't get it to work either.

 

[SnTMan]

Don't know what to say. Any problems with other Engineering.com file shares?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[TomBarsh]

link did work for me. today.

 

[XL83NL]

Link works fine