Hello,

Dave's point is well taken. In a static analysis it will always be conservative to use the CAESAR II default stiffness for pump nozzles. Just a small (extremely small) amount of rotation will result in an appreciable reduction in moment but as Dave says, "where do you stop".

There are MANY unknowns in piping stress analysis. The model is really an approximate representation of the piping system. The conservatism is the reason the B31 rules (and in the case of the pump nozzles - the guidelines) have been providing safe and reliable systems. I think that most of us try to make our piping systems a little more "inside the margin" by ignoring pump nozzle flexibilities and for that matter vessel nozzles flexibilities. If the piping system design results in acceptable forces and moments on the strain sensitive equipment without employing these implicit flexibilities we can take comfort in the thought that it will maybe account for one of the "unknowns". I would ask, would you want to determine the flexibility of the pump baseplate, ....the anchor bolts, ....the flange bolts. As Dave says, where do you stop.

I also think that it would not be prudent to try to "reverse engineer" flexibilities for pump nozzle stiffnesses. The API shaft displacement tolerance data is based upon a very limited amount of FEA work done on relatively few pump designs. It would be a daunting task to try to make representative FEA models of all the pump designs available for all sizes. I think the data in the API Standard is conservative but that ain't bad.

Just a trhought.

Best regards, John.


[This message has been edited by John Breen (edited May 01, 2001).]