This paper describes how CFD simulations were used to help evaluating a centrifugal pump performance. The simulated pump was designed totally from scratch. Many aspects of pump performance characteristics were predicted based on the geometry generated using a pump design software. Especially the effects of the free spin of the 3rd rotor in a 3 stage pump assembly on overall performance were evaluated. Pump models, simulation techniques, and simulation approaches will be presented in detail. Simulation results will be discussed and compared with available test data.