Dear All,

I am a beginner in learning stress. I have some basic questions i hope you will help me to resolve my confusions.
My first question is when the software calculate the stress for thermal expansion & compare it with code equation f(1.25sc+0.25sh) then the calculated stress is for the complete cycle or from ambient/installation to operating point. If it is for cycle then it seems to be ok as per texts but if it is the other way then the basic idea that stress should not exceed the yield in hot or cold condition is not valid.

My second question is are theories of failure such as rankine von n tresca are applicable for thermal failure of pipe. or above theories are for primary stress only.

The Expansion load case is considered a range between two "states" of the piping system. You define those two states: perhaps the range is between operating and installed. In a different system you could have several ranges: OPE1 to installed, OPE2 to installed, and OPE1 to OPE2. The allowable stress applicable to the range is based on the hot allowable Sh (at the maximum temperature of the range) and the cold allowable Sc (at the minimum temperature of the range).

Primary stresses (determined in the Sustained load case) must remain below the hot allowable Sh, since yielding must be avoided in all positions (conditions) of the piping system.

The typical power and process Codes (B31.1 & B31.3) adhere to the Tresca (maximum shear) theory.

For this particular query of yours, have a look at this posts.

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=230
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=6771
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=1462#Post1462

Dear Sirs,

Thanks for the reply sir. But my question how come tresca theory is applied for thermal & longitudinal stress both independently as dave sir quoted

"The diameter is easier to come by (S1-S3), so use that number and compare to double the allowable (using Sy rather than Sy/2 - another easy number - Sy). Simplification (for a slide rule)." I agree to the above statement.

However S1 is hoop & S3 is longitudinal stress so where comes the thermal stress.

If i have to evaluate combined three stress thermal hoop & longitudinal then is there any equation for tresca which i can apply.

As i can understand that calculated stress in thermal stress is much higher than actual but it is still at yield point. So what happens to shear thermal stress when both longitudinal & thermal or displacement stress is applied simultaneously. If tresca would have predicted theory of failure as it is applicable for primary stress only then there would have been no confusion. But i understood it is applicable to both independently but then what is tresca significance.

The Markl papers would help in describing the B31 philosophy in stress evaluation.
That, and many years of safe design and simplification, allows us to separate the evaluation of piping systems into unique categories based on type of load (force-based v. strain-based) and their respective modes of failure.