Flange torque values for stainless steel and duplex SS bolts 1

[engrom]

ASME PCC-1 provides comprehensive guidelines on bolt torque values with target bolt stress 50ksi. Does this apply for Stainless steel and DSS flanges as well or just limited to CS flanges? Any useful information is appreciated.

 

[C2it]

Provided that the material you have in mind has simlar Young's modulus value and similar or better (as DSS) allowable stress value than CS, the only question is bolting. Provided you use high yield bolts(not for instance 8M) with a smys around 100 - 105ksi then PCC-1 guidance should be directly applicable.

 

[salmon2]

I feel your real concern is what is the relation between shear strength and tensile strength. If that is the case, there is a constant factor between tensile/shear strength since your design is by yield streng plus design factor and both CS and SS are ductile. Depending which yield criteria you choose, shear yield strength = 0.577 or 0.5 * tensile yield strength. 0.577 and 0.5 corresponds to von Mises or Tresca models, respectively.

 

[RVDallas]

I agree with C2it's answer with a caveat.

I would like to add that if you use Stainless Steel bolting with your stainless steel flange (a common scenario), the torque values in Table 1 of PCC-1 2010 won't be directly applicable. Those are based on Coefficients of Friction of 0.16 and 0.12, which by the guidelines of Appendix K give nut factors of 0.2 and 0.16 respectively. Those nut factors are for carbon steel bolting.

Footnote 2 in Appendix K mentions that A193-B8M bolts generally have a nut factor 30% higher than A193-B7 bolts. By that math, the nut factor for stainless steel bolting would be 0.26, which would result in higher torque values to get the 50 ksi target pre-stress, per the short form torque equation given in Appendix K.

 

[Duwe6]

As discussed in several earlier posts, those formulas are VERY dependant on finish of thread surfaces and the 'slipperyness' of the lubrication. Renders calc's only accurate to an Order of Magnitude.

You have to "ask the parts". If you cannot borrou the use of a Skidmore-Wilhelm tester, use Load Indicating Washers. They are available in A325 and A490 structural "fully pretensioned" values.

For a 3/4" A325 washer, when torqued to spec [read the accompanying mfr's literature for "Refusals' vs Go's"] it will give you a nominal 29-30kips 30,000 pounds of tension. Just slowly torque up a couple of samples, checking the washer at each stage. This will give you an ACTUAL torque value for YOUR bolts/studs combined with YOUR nuts, using YOUR lube.

Engineers always want to calculate torque vs. tension, and it is not reasonably calculateable. Too many unknown variables that you are forced to make assumption on top of assumption with.

 

[engrom]

Thanks for all your feedback.

ASME PCC-1 is a good starting point, but not really elaborate on SS and DSS bolts apart from the point what RVDallas mentioned on the nut factor.

Given the math in this standard, the SS A193-B8M Class 2 bolt will have higher Torque values than A193-B7 bolts to attain 50ksi. Is this a fair comment and in line with practical scenario? Given the SMYS for A193-8M is significantly lower than A193-B8M, is it not true that the required bolt stress (not 50ksi) is lower resulting in lower torque values for 316SS? How does this change for DSS bolts as well? What are the scenarios if dissimilar materials are torqued, say Carbon steel to Stainless Steel?

 

[engrom]

Please read: ***Given the SMYS for A193-8M is significantly lower than A193-B7, is it not true that the required bolt stress (not 50ksi) is lower resulting in lower torque values for 316SS?***

 

[Duwe6]

Only if the material finishes are very similar. Again, torque is proportional to stress, but *not directly equal*

To achieve the required 28,000 lbs of preload tension on 3/4" A325 bolting, I have seen torque values with a HUGE range of ft-lbs.
Highest I have seen is 355 ft-lbs. Lowest I have seen is 270 ft-lbs. Median value runs about 300-330 ft-lbs. The 270 ft-lb value was on *hot-dip galvanized* bolts! With the poor surface finish of galv bolts, I had anticipated a value of at least 350 ft-lbs! The reason for the very low value was that the nuts had been given a very light coat of teflon by the mfr. So, with the roughest finish, these bolts gave the lowest torque due to the lubrication. Lube plus surface finish have about the same influence on torque as threads-per-inch. I know you wouldn't ignore TPI in your calc's, so please take in the "Lube plus Finish" also.

The deviation on the above 3/4" A325's was about *25-percent*. Unless this is an acceptable deviation from your required bolt stress, you have to put down the pencil and "ask the bolt".

Just go to any industrial fastener supply and get a handful of Load Indicating Washers. A Skidmore-Wilhelm device is very nice, but most companies don't want to buy one. The washers work fine.