Bolt Design Stresses - ASME B31.3 2

[reichertc]

Alright, somebody please correct me if I am wrong here.

ASME B31.3 302.3.1 states "The allowable stresses defined...shall be used in design calculations unless modified by other provisions of this code." It then goes on to state that the design stresses for bolting materials are as per Table A-2.

For A193-B7 studs at room temperature, Table A-2 lists a design stress of 25,000 psi. This is much lower than "industry standard" bolt stresses used when bolting up flanges.

For example, ASME PCC-1-2000 "Guidelines for Pressure Boundary Bolted Flange Joint Assembly" Table 1 "Target Torque Values for Low-Alloy Steel Bolting" contains a general note stating that the values shown are for a target prestress of 50,000 psi (root area). I haven't looked at why they have used the root area rather than the tensile stress area, but for a 1/2" bolt 50,000 psi applied to root area corresponds to about 44,300 psi on the tensile stress area. This still substantially exceeds the 25,000 psi that ASME B31.3 would seem to permit for this bolt.

That is just one example, but my experience indicates that virtually all the owners and gasket manufacturers recommend/require bolt loads higher than the code design stresses. Is there a section of B31.3 that permits the use of these bolting materials at such "high" stresses?

Or does everyone just say it's easier to just crank up the torque on the bolt than to perform accurate calculations for every boltup out there, and ignore the code.

Maybe I'm way out to lunch here, but when I talk with some of our senior folks about the issue, I get a lot of blank stares...

 

[bvi]

The allowable stress for bolts is used in determining the required bolt area, when doing detailed flange design calculations such as provided in Section VIII, Div 1, Appendix 2. This is a force over area type calc.

This has nothing to do with the stress the bolts are torqued up to during installation. This concept is described in Section VIII, Div 1, Appendix S.

If you search, you will find other threads on this topic.

 

[deanc]

Your stress tables are where the safety factors come into play. Check the difference of seamless or ERW pipe-same material.

 

[reichertc]

So are there no code limits for stresses applied during assembly? If so, where is it stated?

 

[bvi]

There are no Section VIII or B31.3 limits for the stress in the bolt resulting from flange bolt-up torque, but clearly you do not want to start necking the bolt, or overloading the flange. A generally recommended bolt pre-stress for low alloy high strength bolting in B16.5 type flanges is 50 ksi.

 

[reichertc]

Thanks bvi. Very much appreciated

 

[deanc]

Check these:

http://www.boltscience.com

Really intrested?

http://www.hexagon.de/dose/dose1e.htm

 

[rickylee]

I share your sentiment on the confusion regarding bolt stress. There is conflicting information within the code. This is the way I have handled it.

Based on experience, 25 ksi bolt stress is low if you are looking for a leak free joint using B7 studs, however how high do you go to ensure a leak free joint and not damage your equipment? Appendix S suggests that higher bolt stresses may be required at times.

A reasonable bolt stress limitation to consider is the flange stress calculated using Appendix 2 methods. What I have done extensively is substitute 90% of the amb. yield strength of the flange as the allowable material limit and backward calculate the corresponding bolt stress. You will find that the bolt stress for most B16.5 flanges will work out to be 30-45ksi using this method. I have witten bolting specs for entire petrochemical plants using this logic in conjuction with controlled bolting field practices. I rationalize using greater than ASME material allowable stress when examining the intent of Appendix S. Where this method trips up is when you try to calculate bolt stress using App. 2 methods on some of the alternative flanges, such as ASME B16.47B flanges. You will find that the bolt stress may be in the low 20 ksi range, which is intolerable to me.

ASME PCC applies across the board bolt stress values to a bolt size. This is unreasonable considering the gasket area to bolt area on B16.5 flanges is all over the map. Gasket manufacturers wind gaskets within a class to a similar density, which means that gaskets within the same class will seat at the same stress. The bolt stress to achieve that same gasket stress between any two flanges is different, which is why ASME PCC fails for me ginving target stress by bolt size with no consideration for gaskets or flange material strength.


Another thing to consider is bolting methods, and to me, the methods of bolt-up is as important as the stress.

Hope this creates some thought, Rick

 

[xhpipe]

Well Im late on this thread but perhaps I can help....

First the codes are design codes not assembly codes perse. therefore the stress values stated are for for design purposes.

However between design and the real world lies many considerations. I strongly suggest a read on this book

http://www.amazon.com/exec/obidos/A...41/sr=2-1/ref=pd_ka_b_2_1/103-1467971-0448634

In it Dr. Bickford discusses the situation quite clearly. The bottom line is bolt-up stresses are not the same as design stresses and may be quite higher in nature.

Regards,
XHPIPE

 

[Xianglu]

If anybody revisite this topic again.

Q: What load should be used in FEA, calculated bolting load from the code or the bolting pre-load?

 

[bvi]

if you are trying to realistically model the joint and get its behavior, you would put in the expected bolt preload. However, I suppose if you were trying to qualify the flange design, something not usually done with FEA, you would have to decide based on the criteria you are using to determine if the joint is acceptable.