Hello Guys,
According to LC Peng book 'Piping stress engineering' Chapter 8, There is strain follow up in control valves with smaller size of the main line .
How can I check this in Caesar II ?

Could anyone help please?

If you model your sensitive elements as pipe (and not as "rigids") the program will strain the elements properly. But like Peng says, the best thing to do is avoid strain follow up rather than design for it. Drop your allowable stress for these components.

Dear Dave
thank you for your reply but I have two question,
How can I drop the allowable stress for element in caesar 2? and do you mean that I model valve as a normal pipe and drop its allowable stress ?

In other words, you should allow the pipe enough flexibility to grow away from the valve instead of into it.

If you want to estimate strain using CAESAR, you have to be able to estimate the valve's rigidity, which can only be done in CAESAR as a regular piping element.

However, the accuracy of that strain will only be as accurate as the assumption you make. Dave suggests lowering the allowable - either by manually putting your eyes on the stresses at that location, or by swapping it for a material with a lower allowable stress, or a user-defined material.

If you want actual strain, you'd need to perform an FEA on the valve, with forces as reported from CAESAR, but with the resistances as calculated from the FEA.

Nice explanation Dave and Michael. This is one of the check points that a stress engineer should look out for in the system he is analyzing. And I agree it's better to avoid it as much as possible rather than design for it, since proving it by calculation is not a straightforward. If I am not mistaken (correct me if I'm wrong fellow stressers) this is somewhat related to the elastic follow up phenomenon where due to excessive strain in the system the secondary stress is acting in a primary state (this is somewhat similar to Dave's suggestion of using lower allowable stress as one way of tackling it). In the EN code I believe 1/3(M/Z) of the thermal expansion stress is being added to the primary stress to take into account for this. And if it fails, an elastic plastic analysis of the component failing can be carried out to prove that it will not collapse in the given loading (which is a tedious undertaking).

And I think this is somewhat related to my old post where an SIF is being introduced to a valve (just like a strain concentration in the ASME VIII Div. 2).

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Main=13753&Number=60185#Post60185

Corrections and other opinions are greatly appreciated.

Cheers!!!