Flow Control, Flow Control November 2017

Cover Series Pump Efficiency Centrifugal pump hydraulic rerate considerations Practices for long term reliability By Randy Riddell CMRP CLS Essity S ometimes in the life of pumping equipment in a plant the pump needs to be rerated for a different operating point and process condition This may be caused by process changes poor reliability or energy savings projects The evaluation and execution of these ideas can be critical to the long term success of such a rerate These rerates typically involve an impeller change motor change speed change or combination of all three to achieve some desired new process operating point Since most pumps are electromechanical devices many areas must be investigated Some of these key areas that must be carefully evaluated when rerating a centrifugal pump for a different operating point are outlined in this article Hydraulic If the rerate is to meet a new process condition then where the pump is operating on its curve is primary concern Bad things happen as the pump operates away from its best efficiency point BEP Several minimum flows must be satisfied on the pump curve The pump manufacturer will usually have a hydraulic minimum flow for the pump The minimum continuous stable flow is present to eliminate recirculation followed by the minimum thermal flow For best overall performance it is best to keep the operating point above the hydraulic minimum flow Horsepower is probably the first variable with a design point change that will be evaluated An increase in flow head or specific gravity of the fluid Figure 1 Pump curves at 1200 and 900 rpm feed pump All graphics courtesy of Essity 8 November 2017 Flow Control