Centrifugal pump systems with flow throttled by a control valve are ubiquitous at process plants. Unfortunately, the majority of pumps are oversized, which both poses problems regarding controllability and wastes energy.
Pump oversizing occurs for two major reasons. First, a system’s pressure drop must be calculated rather early in planning using estimates for pipework and its fittings. Unfortunately, values taken from the literature, for example of flow resistance coefficients of valves, may vary widely. Consequently, dynamic pressure losses are somewhat shaky. Static pressure differences usually are better defined. Hence, planning engineers must add some safety margins. Correcting a pressure drop calculation much later when pipework has been designed does not really solve this problem. Some margin also might be added to the rated flow. Second, the pump’s manufacturer adds safety margins, too, to ensure the pump meets guaranteed performance, in particular if the liquid handled is not water. So, it is nearly impossible to avoid installation of oversized pumps. Hence, the only realistic option is to adjust a pump based on experience once the plant is operating.