Caesar - II default for anchor stiffness is 1.0E12.Since we normally simulate the pump nozzle as an anchor point, I would like to know what is the acceptable value of the pump nozzle stiffness that I can use instead of the default value. I had tried using the stiffness of 1.0E6 and it gives me a considerable change on the nozzle loads but I am not sure if my data is correct. I would greatly appreciate if you could provide me something in this regard.

Perhaps you can squeeze out a stiffness term for your pump using data from Appendix F of API 610. It states there that, if you remain within Table 2 limits, your shaft displacement will be less than 0.010 inches (or 0.015 inches should you opt for more than Table 2). If you want good pump stiffness numbers, consult your pump vendor.

If you start looking for better numbers here where do you stop? Do you have the right preload on springs? what about other support stiffness and location? what about initial alignment? To ensure reliability in your design you should not use such a sharp pencil in your analysis if you do not have tight control over the final installation.

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).]