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Water Pump Flow Rates Explained

Water Pump Flow Rates Explained

When installing a rainwater tank pump, the first thing you need to know is what pump is best. You do this by identifying your pump parameters, which usually means volume flow rate, pressure, and the number of outlets or heads. Flow rate, also referred to as capacity, is the volume of liquid that moves through the pump and is measured in litres per minute. Anyone can get a good understanding of how flow rate works just by just turning on a tap. Flow rate describes the mass of liquid pouring through. In contrast, your pressure, outlets and heads are all related parameters. The head refers to the height of the tap above the inlet, the pressure refers to how high the pump can lift the fluid above and over obstacles, and the outlet is what finally releases the flow of water.

Determining Your Pump Flow Rate

To work out the best pump for your system, you need to determine how many outlets your pump will service, and the pressure required to get the water through the outlet at a satisfactory flow rate. The pressure is determined by the head or height of the tap above the inlet in millimetres. The flow rate is determined by how many litres of water per minute (LPM) should be coming out of your tap. Next, you need to count up all the taps, compare their heights, and determine the average head. From there, you can take a look at the pumps on offer. Every pump has a different pressure capability and will come with a unique flow chart. On this flow chart you can find the meeting point between your ‘flow rate’ and ‘head’.

It goes without saying that the higher the flow rate, the more water will need to be supplied from the tank. A higher flow rate will equal a bigger tank and more powerful pump. Bear in mind, a good flow rate for most taps is around 15 LPM. However, it is a requirement of Australian Standards that the flow rate for a shower, household basin, kitchen sink or laundry trough does not exceed 9 LPM. This chart is known as a ‘pump performance curve’. The pump curve illustrates the best flow rate for the head. Generally, flow rate decreases as the head increases. Manufacturers display the best efficiency point (BEP) for their pump along the curved line. All you have to do is line up your parameters on the pump curve and match the BEP to the pump model.