case-1 : I just built a dummy vertical vessel with 1 shell & two 2:1 ellipsoidal dish ends. now after the "run" i have got the vessel's natural frequency in operating, empty condition.

case-2 : Now i have added a misc weight (in operating condition) using weight dialog box. And again ran the file. Interestingly, both the natural frequency in operating as well as natural frequency in empty condition got changed.

My ques is that if we are adding some weight in operating condition, then why natural frequency in empty condition is getting changed?

In the Element and Detail Weights of PV Elite output, check whether the weight has been considered in the Fabricated Condition or Operating Condition. Misc weights are considered in the fabricated/shop/empty condition. Hence your natural frequency is getting changed.

If you wish to add weight only in the operating condition, then use option like Packing for adding weight. The Packing can be considered only in Field weight of the vessel - (see the installation options of Load Cases).

Circular freuency = Squareroot[Stiffness/Effective Mass}

If you play with one of the them and keep one of the the same, the third one will change.
You change the mass, and stiffness is the same. Therefore, the frequency changes.

Regards,

Ibrahim Demir

Mr Mech Engg,

i have entered the load in operating condition only. as written in my question itself its very clear that adding the weight in any one of the condition , the nat. freq gets changed for all other condition.

Hello Ibrahim,

i would just check this out.

M.Shukla,

Can you attach the output of your PV Elite ?

Dear Members,

I am new to PV-Elite software usage. I am in need to calculate the natural frequency of the closed drain drum (Horizontal vessel) in both empty and operating conditions. How to check it. Please help me.