Dear Dave:

Thanks for your reply.

Regarding my 3rd query and your reply of it, i applied the forces on all elbow of piping system after the source of slug flow (i.e.) downstream of PCV in my case. The way i defined the slug forces are such that the forces are not cancelling each other case (i.e. i used the available 9 forces defining provisions in the input spread sheet on each elbow so that one force shall not cancel other force) and forces that cancel each other.

When i started the modal analysis, i checked the 1st mode natural frequency (5.2 Hz) of piping system with the load case of slug forces on all elbows at a time. When i changed the load case to only one force at a time, the first natural freq. is 3.6 Hz. So i conclude the slug force has an effect on the natural frequency of the system to a great extent and while doing modal analysis for checking the frequency it is best to check on all load cases the 1st natural frequency is more than the required one.

Finally, in modal analysis, there is input sheet for lumped mass analysis, under what case, we must use the lumped masses. Please clarify....

Thankyou...