Dear Srs.,

I'm analyzing a flare system with the following conditions:

Operation: T=37ºC and P=0,5kg/cm2
Design: T=175ºC and P=3,5kg/cm2

There is a rule here that for flare system we have to perform a modal analysis and ensure that all frequencies are above 2Hz. This is because of a possible two phase flow or other vibration problem.

Since modal analysis (eigenvalue) is a linear calculation, sometimes it is very difficult to get all frequencies above 2Hz. For example, in the ".c 2" attached when the frequencies are extrated without friction effect the first mode is equal to 0,4Hz. In the other hand, if it is considered the friction effect through a "stiffiness factor for friction" equal to 2 (CAESAR II's manual suggest a value beetwen 50 to 100!!) the first frequency increases to 2,5Hz and meets the requeriment (above 2Hz). It's important to note that it's a 50inch pipe diameter and the friction forces are relevant!

Not considering the friction effect will result in criating several other supports (guides and stops) just to meet the frequency requerement.

Appreciate any sugestion or advice in this issue.

Regards,
Roberto Liberato