21st Nov 2019
An ‘Operating Head’ describes a pump’s flow-rate at varying heights. The amount of water a pump will produce falls as the head increases; the higher the pump has to push the water, the lower the flow-rate at the outlet.
Pump operating heads are calculated by the manufacturer and are usually displayed on the packaging or in the instruction manual.Most pumps display their maximum flow-rate. This is the amount of water leaving the pump directly at it's outlet. Some pumps have their flow-rate displayed at a 1 metre head. This is the maximum amount of water the pump will deliver directly up a 1 meter length of pipe.Operating heads vary between pumps. Using two different pumps the graph below demonstrates how the flow-rates fall as the head increases.At a 1 metre head the Lotus Maximus 9000 has a flow-rate of 8,800 Lph, whereas the Draper 144 has a lower flow-rate of 6,600 Lph. The flow-rate of both pumps falls as the head height increases.At a 5 metre head the Maximus flow-rate has fallen to zero, but the Draper is still pumping at 2,250 Lph, this is because the Draper pushes the water at a higher pressure, and the pump’s impeller is designed to drive the water further.The Draper’s flow-rate finally reaches zero at a head of about 7 metres, this is the total height that the pump will push water, or the pump’s ‘Maximum Head’.Although the Maximus has a higher flow-rate at a 1 metre head, the Draper has a higher maximum head rating of 7 metres compared to the Maximus, which has a maximum head rating of 5 metres,A pump’s operating head isn’t just determined by vertical distance. Horizontal distance will also reduce the flow-rate, but only by around a tenth of that of the vertical distance.To calculate the ‘total head’ that a pump has to send the water, the following distances need to be measured in metres;
- The vertical distance from the surface of the pond to the highest point on the water-way (H) and,
- The horizontal length from the pump to the water-way outlet (L).
(The vertical distance is measured from the ponds surface, rather than the bottom of the pond where the pump is likely to be positioned because there will be no pressure within the submerged hose to reduce the flow-rate).
The formula for calculating total head is; Total Head = H + (L ÷ 10 )E.g. a 3 metre tall waterfall, which will require a flow-rate of around 3000 Lph, is set at the top of a stream, 15 metres from the pump Total Head = H + (L ÷ 10)= 3 + (15 ÷ 10)= 3 + 1.5= 4.5 metresTherefore, a pump is required with a flow-rate of 3000 Lph at a 4.5 metre head.This calculation should also be used if working out the required flow-rate for a filter which is positioned quite far from the pond.It is worth noting that the operating heads given by manufacturers are calculated under ideal conditions. Ribbed hose, such as polyhose, will agetate the water inside the pipe, increasing the pressure and reducing the flow-rate further, as will bends, fittings and any appliances in the pipe-work such as UVCs.Using a smaller hose will dramatically reduce the flow-rate.