somebody asked me this question:

what effect does wall thickness have on exp stresses & i replied the below:

1. With the inc in thickness, the system stiffness get increased (less flexibility), hence thermal force will inc & exp stress will increase.Am i right in general?

2.my doubt is with the inc in wall thickness,SIF will get reduced.Hence exp stresses will be less.

Which one is correct?

Experts Kindly reply

For a specific load a thicker pipe will have lower stresses. But (depending on how your system is restrained) it is possible that the extra stiffness will produce higher restraint loads which may result in higher stresses.

I think MaatR we cannot make generic statement wrt thickness effect on exp stress.
Let's see what is the opinion of other experts.

Dave/Richard

waiting for your reply.....

As vick said if the thickness is increased, Pipe flexibility will be decreased.

But as thk increased, Section modulus(Z=π*r^2*t) also increased, hence Expansion stress(Mc*i/Z) will be decreased. We need to consider this one.

If you work through the theory of the Guided Cantilever Method, you will see that the equation for the Expansion Stress Range reduces to:

SE = 6 * E * R * dl / L^2

where:
E - modulus of elasticity
R - outer pipe radious
dl - the change in length (thermal growth)
L - length of the leg absorbing the growth

Note that pipe wall thickness is not in this equation. Therefore, increasing (or decreasing) the pipe wall thickness is usually not an adequate solution to an expansion stress problem.

MR. Richard, I have a question.
Subtracting corrosion allowance in pipe wall thk doesn't have any effect on Expansion stress calculation? Despite it'll affect the Sustain stress, Occ stress as well.

The various Piping Codes treat corrosion differently with regards to load cases. B31.3 by default corrodes only SUSA and OCC, not EXP. However, CAESAR II has a Configuration Directive that will permit the consideration of corrosion in all loads cases. Check out this option.

Thank you sir

Dear Richard

i think guided cantiliver is an ideal case but in real piping configuration (2D/3D plane), Is it true that exp stress will not have any effect with change in thickness?
Please advise?

Build a small 3D model in CAESAR II and see for yourself.

Wall thickness affects your flexibility characteristics/factors and stress intensification factors from appendix D, so it will have an effect if you have any of these components in your model.

Increase in pipe thickness, though reduces stress acting on the pipe increases the loads, forces acting on nozzles..