Flow Control – Positive Displacement Pump Part 2

When using a positive displacement pump in flow control, then the minimum flow which may be achieved using a variable speed drive is limited by the motor low speed limit. It is possible to achieve a wider range of flow control by regulating flow using a fixed speed motor and a throttling valve in combination with a backpressure regulator – as illustrated below.

The backpressure regulator maintains the pump discharge pressure constant by automatically regulating the spillback to maintain a balance between the pump flow and the total flow through the valve and spillback. As the control valve opens then the spill back flow is automatically reduced. If the motor isequipped with a variable speed drive, then the speed may be set as a function of flow demand to minimize the spillback flow.

In some applications it is necessary to control the flow in multiple streams downstream of the positive displacement pump. It is easy to meet this requirement when  a back pressure regulator is used in combination with throttling valves as is illustrated below.