FEA Computed Flexibilities and Modeling Vessels

A recent debate came up between a friend and I regarding whether it is considered double dipping to computer nozzle flexibilities with an FEA program such as NozzlePro, and then in CAESAR also model the vessel itself as a piece of pipe to simulate the flexibility of the vessel.

I argue that it is not double dipping, my coworker says it is, since you specify a length and end condition in NozzlePro that affects your nozzle flexs.

My reasoning is that the FEA calulates LOCAL nozzle flexs, based on where it is located on the vessel, but you can still model your vessel as a piece of pipe in CAESAR. My coworker only uses rigid elements to model equipment, and assigns flexibilities at the nozzle-shell junction.

What says you all? Any professional opinions?

In general, I agree with you. You flexibilities coming out of NozzlePro or another FEA should be based on the junction itself.

As for the nozzle projection itself, I would model that as pipe. Really, once you are outside of the nozzle shell interface we as pipe stress really don't care where the purchasing folks put the flange connection that separates the "pipe" from the "vessel". It all behaves like pipe to us. Though, to be fair to your coworker, that stub of pipe between the shell and flange is probably not going to make a big contribution to system flexibility compared to the rest of the system.

As for the vessel itself, we tend to model a rigid element from shell point to the centerline of the vessel. I then use pipe elements to model the vessel for graphical clarity, but I will use an absurdly large wall thickness to minimize it's contribution to the system flexibility. (if the drawing shows a 5/8" shell, I might code it as 5" thick) My reason for this is not a concern for "double dipping" per se as to avoid dealing with the uncertainty - internal trays, tube sheets, etc are going to affect the stiffness is ways that are too difficult to adequately predict for consideration in Caesar, so I code them out of the design altogether.

Once I've considered local flexibility of the nozzle, if I still can't get the system to work, some unknown extra bit from the rest of the vessel is not likely to negate the need to flex up the overall layout some more.

Yes with vessels with internals such as Columns and Heat Exchangers, it makes sense to model those as rigid elements.

Also, yes I do in fact model a pipe for the nozzle itself (including the nozzle flange).

Heat exchanger - yes, rigid elements

Column - maybe, it deflect on wind, seismic action. Perhaps a combination of rigids and pipe would be more appropiate.