Pumps are used to transfer fluid around a system of pipework. The fluid enters the pump at the inlet (suction) and exits the pump at the outlet (discharge). They are usually driven by an electric motor, although engine take off drive pumps and manually operated pumps are also available.

 

 

There are many different types of pump designs and can be classified as either dynamic pumps or positive displacement pumps.

 

A centrifugal pump is an example of a dynamic type. Fluid enters at the central axis, whereupon a multi vaned impeller rotating at high speed causes the fluid to accelerate radially from the centre to the outer limits of the inner casing and therefore exit at a higher velocity and pressure.

 

A rotary peristaltic pump is a positive displacement type example. This incorporates a casing with a centrally located rotor onto which are attached compression rollers. These rollers act against a flexible hose containing fluid in a manner to draw the fluid through the hose from the inlet to the exit.

 

A mechanical design engineer or a pump manufacturer’s technical advisor will be able to assist in the selection of a pump to suit a customer’s requirements. The information needed from the customer includes the specific fluid to be pumped and the environment it is used in. Pressures, flow rates and distances of fluid transfer etc will also need to be considered.

 

Flow rate is the volumetric speed of the fluid through a pipe or channel and is also referred to as capacity or rate of discharge. Units include litres/sec or gallons/minute.

 

A commonly used formula to determine the flow rate requirement for a pump 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.

 

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.

 

The pressure rating for a particular pump is a performance value relating to its ability to push fluid through a system. The pressure performance value of 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 in an application.

 

In addition to elevation changes and frictional resistance within a system, losses can occur 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.

 

 

 

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

 

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