As a result of conservative engineering practices, pumps are often substantially larger than they need to be for an industrial plant’s process requirements. Centrifugal pumps can often be oversized because of “rounding up,” trying to accommodate gradual increases in pipe surface roughness and flow resistance over time, or anticipating future plant capacity expansions. In addition, the plant’s pumping requirements might not have been clearly defined during the design phase.
Because of this conservative approach, pumps can have operating points completely different from their design points. The pump head is often less than expected, while the flow rate is greater. This can cause cavitation and waste energy as the flow rate typically must be regulated with bypass or throttle control.
Over sized and throttled pumps that produce excess pressure are excellent candidates for impeller replacement or “trimming,” to save energy and reduce costs. Trimming involves machining the impeller to reduce its diameter. Trimming should be limited to about 75% of a pump’s maximum impeller diameter, because excessive trimming can result in a mismatched impeller and casing. As the impeller diameter decreases, added clearance between the impeller and the fixed pump casing increases internal flow re circulation, causes head loss, and lowers pumping efficiency.
For manufacturing standardization purposes, pump casings and shafts are designed to accommodate impellers in a range of sizes. Many pump manufacturers provide pump performance curves that indicate how various models will perform with different impeller diameters or trims. The impeller should not be trimmed any smaller than the minimum diameter shown on the curve.
Net positive suction head requirements (NPSHR) usually decrease at lower flow rates and can increase at the higher end of the pump head curve. The NPSHR at a given flow rate will normally be greater with a smaller impeller, but engineers should consult with the pump manufacturer to determine variations in NPSHR before trimming the impeller. Manufacturers can often provide trim correction charts based on historical test data.