Pipe Allowable Stress

[Tankman650]

Allowable pipe stress for A106 material is 20000psi at 400 deg. F.

Is there an allowable increase (ANSI B31.3)due to mechanical or thermal loading like ASME SEC VIII Div. 1 ????

The pipe is well overdesigned for pressure but the thermal loading is a big problem.

Thanks

 

Hello,

No change in the allowable stress (Sc or Sh) as of yet. Perhaps B31.1 will look at makinga change in the mid-term future (there is NO near-term future for Code committees).

Some things to remember, B31.1 allowable stress (at temperature below the creep threshold) is based upon 2/3 of yield or 1/4 of ultimate strength (whichever is less). B31.1 (ike Section VIII, Div. 2) is based upon 2/3 of yield or 1/3 of ultimate strength (whichever is less). That is why MOST B31.3 allowable stresses are greater than B31.1 allowable stresses.

The "thermal expansion stress RANGE" (or in B31.3, the displacement stress RANGE) is a secondary stress. Don't overlook that the calculated eapansion stress RANGE, SE, is to be compared to the allowable stress RANGE.

Best regards, John.

 

[Tankman650]

John,

For 31.3 piping, it seems as though the requirements for thermal are an after thought. Rather than include a loading condition with pressure, it is addressed as an additional check. That is, rather than limit pressure to S, and P+ T to 2*S, the code says limit P to S and T to S, separate conditions.

Thanks for your comments.

 

[prex]

First of all you must remember that design requirements for flexibility and support are based onto a check for the longitudinal stress in the pipe, whereas pressure design is based onto circumferential stress.
Now if you look at the alternate definition for S_A in B31.3 you'll see that the longitudinal stress due to pressure is deducted from the allowable stress for flexibility and support, so that in fact the effect of pressure is (OK, indirectly, but indeed is) included in the check for thermal stresses.
And please don't say that thermal stresses are an after thought for the code! prex
motori@xcalcsREMOVE.com

http://www.xcalcs.com

Online tools for structural design

 

Hello,

Well said Prexy.

Tankman, perhaps I was ambiguous in my previous posting. The B31 Codes address circumferential stress due to internal pressure in the requirement of minimum allowable wall thickness. The calculated (sustained) LONGITUDINAL stresses due to internal pressure and bending are primary stresses and they are added and the sum is compared to (Appendix A) allowable stresses at temperature Sh. The Thermal expansion/contraction stresses are secondary stresses and they are addressed differently - the stress range from the pipe at its coldest temperature to its hottest temperature. The Code intends to allow the entire range of stress from cold yield to hot yield (with a little of that range taken away as a safety factor)to be used for the combination of all the loadings (thermal plus weight plus longitudinal pressure). So, the Codes have you calculate the sustained longitudinal pressure stress and add it to the bending stress(additive stresses)and compare the sum to Sh from Appendix A. Then you have to calculate the "occasional stresses" and compare them to an allowable stress (1.33 x Sh for B31.3, and 1.2 Sh for B31.1). Then you have to calculate the thermal expansion/contraction stress range from coldest to hottest (displacement stress range for B31.3) and compare that to the the allowable stress range (SA = f (1.25 x Sc + 0.25 Sh, and if you have any "unused" Sh from your allowable sustained ongitudnial stress you can add it to the 0.25 Sh term). The Codes also include rules for accounting for "partial cycles" (and most cycles will be partial cycles since you do not go from coldest to hottest very often).

None of these calculations are an "after-thought". These rules have been in the Code as a "package" since 1955 and the only real expansion of these rules came in the Summer 1974 addenda to B31.1 when they gave us explicit equations for each of the stresses (stress ranges) that they want us to calculate.

Regards, John.

 

TANKMAN, go to the COADE discussion board. Go to Ceasar II discussions ands especially go here:

http://www.coade.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=000072

 

[Tankman650]

Thanks for all the comments.

I now have a better understanding of how to handle the thermal loads.