Chapter 17 - When trimming a centrifugal pump impeller can save energy and increase flow rate

Abstract

The possibility of trimming the pump(s)’ impellers to allow a higher single pump flow, before motor overload, exists where the control valve is partially open at the maximum demanded flow rate, during dual (parallel) pump operation. This also requires that the control valve flow characteristic be known to assess the real degree of system dynamic pressure relief through further opening of the control valve. Before a trimmed impeller can be installed in each pump, the permissible control valve opening at maximum demand flow must be determined. This allows the calculation of the minimum pump head which must be available at the maximum demand flow. This minimum pump head then determines the trimmed diameter for the impeller. Further, in assessing the minimum required pump head, care must be taken to evaluate the effects of system fouling, which increase the system dynamic resistance. It is equally essential to assess the potential effects of erosion or corrosion, which may require that the pump be periodically overhauled to recapture original performance. In determining the trim diameter, it is not advisable to use the affinity laws, but rather to consult with the pump manufacturer to obtain actual predicted performance curves for various impeller diameters. After the trim has been selected it is better to add marginally to the selected diameter. Any surplus head generation may be adjusted, if necessary, by further trimming during subsequent pump overhaul.