UPDATE 2014-01-15: The attached .zip file now contains a Mathcad Prime 3.0 worksheet (.mcdx) and—for those of you who are still using earlier verisions of Mathcad—an Adobe Acrobat printout (.pdf) of the worksheet so can see how it is put together.
A centrifugal pump's performance curve is fixed for a given motor speed and a given impeller diameter. However, by applying equations known as similarity (or affinity) laws, pump performance can be predicted for any speed and any impeller diameter, with certain caveats. These caveats will not be covered here, but can be found in any good fluid mechanics or hydraulics textbook or pump design handbook. The key point to remember is that these equations cannot be applied indiscriminately and real-world performance may differ.
This worksheet applies the similarity laws to predict the performance of a centrifugal pump by changing the diameter of its impeller. A similar worksheet (Lusk_Pump Sim_Speed.mcdx) handles the case of changing pump speeds. The calculations can be combined as needed.