Allowable stress of B7 stud 2

[facilityengr]

We have a Sec VII, Div 1, Appendix 2 designed flange. The allowable stress for the bolting material should be per Div. 1 Table UCS-23 (we are using a 3 1/2" B7 - 23,000 psi allowable). Can we use twice the allowable stress (46 ksi) per Sec VIII, Div. 2, Appendix 4, 4-141? Does this require a full stress analysis of the stud to conform?
Also, is a R101 RTJ good for CL 1500 service?

 

[prex]

Of course not: those allowables are for preload + pressure + differential thermal expansion stresses! prex
motori@xcalcsREMOVE.com

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[rickylee]

What level of bolt stress do you need to seat the gasket as per the gasket manufacturers recommendations?
Consideration must be given to all three components: flange, gasket, studs. Although I am not condoning working outside the code, it is not uncommon to preload studs to the stress you suggest. Read addendix S for further insight.

 

[facilityengr]

The gasket seating stress is 26,000 psi, but the gasket seating case is not our limiting factor, its the operating case. Using Appendix 2 formulas we are getting 27-28 ksi bolting stress (allowable 23,000). The Rigidity from Appendix S is J=0.4976 for operating and J=0.4110 for seating.

The company designing the flange, input 50,000 psi as the inital bolt tightening stress. The tensioning tool manufacturer told us B7 bolts are typically tensioned to 45,000 psi. How can they tension these bolts to these loads (per Div II 4-141?).

prex-
We have already included the thermal expansion in the form of a moment and axial force on the flange.

 

[prex]

What I meant is that you can not use that allowable for operating conditions. With your figures you cannot comply with code: however if you don't need compliance...
Note also that under Div.1 you don't need to include thermal expansion stresses in flange calculation. If you want to anyway check those stresses you should first comply with Div.1 without expansion stresses, then make an analysis per Div.2 with expansion stresses included. prex
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[rickylee]

I suspect that the tensioning tool manufacturer nor tensioning contractor should not be the one to consult unless they support their recommendations with engineering analysis.

I assume you arrived at 26 ksi gasket stress using the m factor. Now consider the residual gasket stress the manufacturer specifies and work back to calculate a bolt stress. Is this bolt stress allowable to App 2 flange stress? Is this bolt stress reasonable to UCS limits? If it is a SS spiral wound with graphite filler (i.e. Flexitallic CGI) then you may target approx 15 ksi, and for a Low Stress gasket target you may target approx 10 ksi.

 

[mikehill]

1. We have external bending moments on our flange due to pipeline thermal expansion....this load causes the bolt stresses to be over the allowable per Div. 1 Table UCS-23 (we are using a 3 1/2" B7 - 23,000 psi allowable). We then refer to Division 2, Appendix 4 Section 4-141 and it states that we can go to 2x the allowable for a combination of preload, pressure and thermal expansion...this section does not specifically address external loading but it does indicate that this is the maximum allowable service stress in the bolts...please explain when and how this section can be used?

2. Based on Prex comment above we should calculate the bolt loading due to the internal pressure and external loading based on division 2 calculations. In doing this, it will allow us to use the 2x allowable criteria? Please comment

3. Please explain the core differences between a div 1 and div 2 calculation. Why would one be chosen over the other. What are the differences in calculations between Div1 and Div 2 for the bolt stresses? In looking at the Code they are the same equations for bolt stress????

 

[facilityengr]

FYI...Mike and I are working on the same project.

 

[StressGuy]

If you're overstressing your bolts due to bending from thermal expansion, I'd say you've got bigger problems. If possible, I would recommend moving your flange connection, to get some flexibility between the connection and the anchor point that is causing the bending.

Out of curiousity, what kind of stress are you getting in the piping system at the flange? And, have you checked the flange for leakage under the bending load you are seeing? From what I've seen, a flange will leak before it will overstress. Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[mikehill]

The stress in the bolts is slightly (26000 psi) over the allowable of 23000 psi per ASME Section 2 part D Tabel 3. The flange and leakage has been checked and passes. We are getting about 500,000 ft*lbf at the connection. We cannot move the flange.

What I need to know is what defines the criteria of when and how to use ASME Section VIII division 2 Appendix 4 section 4-141. It states that we can exceed the allowables in Table 3 by 2x and 3x for thermal expansion.
It is not clear if this thermal expansion can be a result of thermal expansion from the pipeline thermal expansion on the flange or if it is soley a result of external temperatures that may cause the bolts to expand. It appears to me to be the latter....but I do not know as the Code is not very clear on defining specifically. This leads me to conclude that it is at my discretion....Where are my Code experts?

Please also explain what is the difference in the flange calculations between Div. 1 and Div 2...the bolt stress calc appears to be the same....

 

[prex]

Let me try to explain what would be the correct procedure IMHO:
1) Assume you need to comply with ASME VIII Div.1
2) Check your flange design per Div.1 App.2 with no account of expansion stresses from external piping
3) Expansion stresses give rise to secondary stresses and these are not required to be accounted for in Div.1 (this is by the way the main and very basic difference between Div.1 and Div.2): se also below, however.
4) As your expansion stresses seem a bit high (though for sure not really excessive) you can check them, still staying within the frame of Div.1, hence using Div.1 allowables, but going to Div.2 for the checking method, as Div.1 doesn't provide one.
5) App.3 of Div.2 is of no use here: it is in fact the same as App.2 of Div.1 and uses the same type of loads (pressure only).
6) App.4 of Div.2 is what you need, and with 4-141 you should be OK. However note that the service stress in bolts should be determined including the effect of preload (of course by accounting also the effect of preload stress going towards zero under flange loads) and the effect of torsion in tightening, if any.
7) Of course under the provisions of Div.2 the material could plasticize (though not really in your case it seems) and this is not normally a good thing for flanged joints. The limit set forth by Div.2 insures that no accumulation of plastic strain will occur across load cycles, but in my opinion this is not fully insured if you repeatedly retighten the bolts in service (though of course you could replace them when needed).
8) Now a much more complicated consideration need be made. Secondary stresses are by definition self limiting: so the classification I made above for expansion stresses to be secondary is based onto the assumption that, when the bolts will eventually deform under load, causing the flange to act as a partial hinge, the expansion load should go to zero (or, better said, when the expansion loads are increased due to increased temperature, the loads in the flange calculated as a plastic hinge should stay stationary). If this condition is not met (and may well not be, especially if the expansion load is mainly an axial load, not a bending moment), then point 3 above is no more true: you should classify the stresses coming from the external loads as primary, hence transform those loads into an equivalent pressure for the flange, and fully check it per Div.1 (I understand that in this case you wouldn't satisfy the limits, but that's it...).
9) As a closing comment I should also recall that the normal procedure to account for external loads, be them of mechanical or thermal origin, is as at the end of point 8: this is safe, and does not require any special consideration on the interaction between the piping and the vessel. I mentioned the procedure above (points 2 to 7) only because you have a problem and this could well be your escape lane. prex

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