Hi Richard,

Is CAESAR excludes the corrosion allowance, for calculating fundamental natural frequency in modal analysis.

Please advice

Thanks
Manjunath

Natural frequencies are based on nominal wall.

But, rather than asking this question, why not do a simple test with a caltilever of a few elements. Run it with and without corrosion and prove it to yourself ?

The Piping Codes (well almost all of them) state that corrosion is to be removed only from the "section modulus", just before the stress calculation is made. So, as stated above, the FEA solution (either static or dynamic) is performed using nominal dimensions.

Hi Moverz,

Thanks.

If natural frequencies are calculated based on nominal wall thickness, then keeping the natural frequency above the limit (e.g., above 4hz), not hold good for design life of 20 years. As line thickness keep on decrease due to corrosion.

Thanks
Manju

If you like then do two calculation...
Beginn of life and end of life with two different thickness.

Manju,

Just remember that pipe may be supplied with the actual wall +/- 12.5% of nominal, so actual mass and stiffness may be quite different to nominal. Also, corrosion rarely causes a uniform loss of material ... it's often most effective at top or bottom of the inner wall.

I have been able to monitor real piping systems and model the same using Caesar. The results were very close, both shape and frequency. That said, unless you are absolutely sure of all data and in particular, boundary conditions such as restraint stiffness, your modal analysis can only be approximate.

Moverz,

Thanks for valuable advice.

Rgds
Manju