Calculating pump flow
Learn how to calculate pump flow.
To calculate the flow needed from the pump, let us first analyze the aspects of pump sizing.
Pump sizing refers to selecting the right pump, which can meet the required flow and head at its highest efficiency.
To get relevant data for the operating conditions, the pump supplier normally selects a pump with the highest efficiency for a given set of operating conditions. To find the operating conditions, you need to calculate the relevant data. Let us understand how to do that.
For the calculation, you need to determine the amount of water that should be applied per day during the peak periods. This can be calculated from the size of the field multiplied by the amount of water,
typically in millimetres, that must be applied. The result is the total amount of water that must be applied per day, and by dividing that with the time in hours the pump will run (24 hours), we get the pump flow in m3/h. This number is the minimum size of the pump. To allow for down time and/or service we normally only run the pump between 12 and 18 hours a day, and therefore select a correspondingly larger pump.
For example, let us consider that you want to grow sweet corn on a 20 hectare field with water coming from a nearby well. The Evapo-Transpiration or ETP in the area for sweet corn is 11.4 mm per day. Crop efficiency Kc is 1. Water loss and water
used by crop is ETP× Kc. In this case, 11.4 x 1 or 11.4 millimetres per day. Let’s assume irrigation efficiency or IE is 90 percent. Water required per day is then a bit more that is 11.4 × 100/90 or 12.7mm/day. 1 ha = 10,000m2. It is, therefore, necessary to apply 10,000m2 x 0.0127m or 127m3/day per hectare. The 20 hectare field, therefore, needs 20 x 127m3 or 2,540m3/day. That is the same as 106 cubic meters per hour. If a pump with best efficiency at 106 cubic meters per hour is chosen, it should pump 24-hours a day. To allow for system downtime and power cost windows, a larger
pump will therefore always be chosen, and a runtime of between 12 and 18 hours per day is typical. If you choose 12 hours runtime per day, the pump must be able to pump double the amount that is 212m3/h. The Grundfos submersible pump type SP215 can give 212m3/h, which is close to the efficiency required. This pump is, therefore, selected for the current scenario.
This chart shows the efficiency and the Net Positive Suction Head, or NPSH, curves for the Grundfos submersible pump type SP215. The pumps type designation typically notes the pump flow at best efficiency, in this case 215m3/h.