Flow rate is the volume of liquid pumped or transferred per unit of time, such as litres/sec or gallons/minute. A commonly used formula to determine the flow rate is Q=Av where A is the cross-sectional area of a pipe and v is the liquid velocity through the pipe. Some applications will start with a predetermined flow rate value based on what is to be achieved. Flow rate is also referred to as capacity or rate of discharge.

Pressure performance values for a pump must overcome all of the frictional losses in a system of pipework and valves as well as the height difference or elevation change from input to output.

Additional losses can occur in a system such as leaks for example, it is therefore essential to calculate total frictional losses in a system and apply an additional service factor to overcome all eventualities of performance degradation and system resistance.

Pressure values are often stated as psi (pounds per square inch) or bar. Pressure can be referred to as head (equivalent height of water column) with units in metres or feet.

Pump manufacturers often supply technical data including performance graphs which show head v capacity. Study these charts to determine suitability for an application and note that as head increases in a system, the pump’s capacity to deliver that requirement is reduced and vice versa.

Always check the pump manufacturers specification to ensure other service criteria are met, for example temperature limits, fluid compatibility, electrical supply, operation cycles and recommended application etc.

A mechanical design engineer or a pump manufacturer technical advisor will be able to assist better.

Typical examples of motor driven centrifugal pumps are shown below.