Positive displacement (PD) pumps aren’t as common as centrifugal pumps at most plants. So, engineers typically are less familiar with how to control the flow of these pumps. Yet, many applications of PD pumps may benefit from flow control — e.g., direct control to meet process objectives or a cascade from level, temperature or other control loops.
Flow control on most centrifugal pumps involves varying the discharge pressure to alter the pump flow rate. A control valve supplies the variable pressure drop for the system. As long as the pump has a continuously rising head to zero flow, this system usually works dependably.
In contrast, changing flow resistance on a PD pump doesn’t vary the flow rate. In an ideal PD pump with incompressible fluids the flow rate is independent of both the suction and discharge pressures. Real pumps may vary from the ideal due to internal leakage. A control valve on a PD pump doesn’t control the flow rate. A control valve on a PD pump discharge controls the discharge pressure.