Can any body explain the theory of neglecting the stresses calculation for rigid elements in CAESAR ?

You are not required to specify a wall thickness for a rigid element, so it is assumed you are not interested in stresses. Of course lacking that information, stresses cannot in any case be calculated.

If you need stresses in a valve body for instnace, model in a pipe element with appropriate properties.

But also, the cross section varies along the length of the rigid body, so a single beam element is not capable of determining stresses.

So we are saying that we are neglecting the stresses because we do not have the full information about the element , rather than saying that such elements can be hardly overstressed and need not to be analyzed!! is this really acceptable technically!!
If some body asked what about the stresses in the valves and the flanges, simply i will say it may be overstressed or under-stressed and i do not know .I think what i should say that the stresses on the rigid elements are usually negligible due to X , Y , Z reasons!!

If what we call rigid elements in CAESAR II were failing in the field I believe we would have engineering standards in place to prevent such failures.
The Codes have little to say about mechanical strength of such piping components. Existing standards focus on (hoop) pressure strength.
There have been recent changes to B31.3 to ensure mechanical strength of expansion joint assemblies, but, again, to handle pressure thrust.

Ahmad Akila, you must to note that all the rigids have been selected by Piping Materials Departemnt, selecting the rating, the range temperature, etc. for each service and system of piping. Of course that is not like a stress analysis, but the experience shows us that rigid elements are not criticals.

My problem here that saying " experience , or the practice " to people who do not have design background. I know already that such things should not be checked (if the element is appropriately supported below or near by ), and if i have doubts i can apply flange leakage check method (if the item is flanged), but after that it is very hard to explain such things to the end users who do not have design background.