Flow Control – Centrifugal Pump

Centrifugal pumps can be used to transfer large volumes of liquid between vessels in an industrial process. When a constant speed motor is used to drive the pump then the pump flow rate may be controlled by adjusting a throttling valve that is installed after the pump discharge, as illustrated below.

The pump discharge pressure drops as the flow increases through the pump. Conversely, the pressure drop through the piping and other downstream restrictions increases proportionate to the square of the flow rate. The flow rate established through the process is that which results in a pump discharge pressure that equals the downstream pressure drop at that flow rate. Thus, the maximum flow rate that may be achieved is determined by the pump characteristic curve and the downstream system restriction with the valve fully open. As the valve is closed then downstream system restriction increases resulting in a decreased flow rate, as illustrated below.

Since the change in centrifugal pump discharge pressure with flow rate is fairly small, it is possible to use a throttling valve to regulate flow over a wide range of operation. Centrifugal pumps are commercially available to address a variety of flow rates. However, when working with processes that require a flow rate of  2 GPM or less it is often necessary to use a positive displacement pump. In such cases a different approach must be taken when implementing flow control – as will be addressed in next week’s blog.