This paper proposes a practical methodology to assist the definition of optimal centrifugal pump impeller diameter, for an existing process water pipeline subject to an upgrade of capacity, and with an uncertain equivalent roughness evolution through time. This approach was essential in the owner ' s decision making process reconciling technical and economic aspects of the project and keeping at a high priority the energy efficiency of the system. Over a plot of impeller diameter versus equivalent roughness of the pipeline, limiting lines for maximum and minimum allowable roughness for a given impeller diameter were traced, and hence a feasibility area was defined for a given equivalent roughness range. This methodology was developed for a detail engineering project where a single pump station, with a maximum of 6 operating parallel vertical axis centrifugal pumps, feeds a 50-kilometer, 812.8 mm of outside diameter, FBE internally lined high resistance carbon steel pipeline. Sensitivity analysis was also performed to assess the impact of the flowrate capacity requirement variations, and on maximum power consumption allowed per pump.