Abstract: To address excessive shaft power caused by fixed-speed throttling of condensate pumps under low-load conditions, a three-dimensional representative-stage model was developed for a KNL350-400×6 pump. Twenty operating conditions covering four speed ratios and five flow rates were simulated using the SST k-ω turbulence model and the multiple reference frame (MRF) approach. The head difference between the ultrafine and verification meshes was 0.366%, and the efficiency difference was 0.174 percentage points. Under the design condition, the six-stage-equivalent head was 254.96 m, with a relative deviation of 5.57% from the published rating. At load-demand ratios of 0.8 and 0.7, variable-frequency operation reduced shaft power by 44.64% and 62.45%, respectively, compared with fixed-speed throttling. Reducing the rotational speed can eliminate excess head; however, deep variable-frequency regulation requires checks of pump efficiency, minimum delivery pressure, and system stability margin.