What Size Pump Do I Need?

This question is one of the most often asked of us. It may seem that there is a baffling array of things to be considered in answering that question but these can easily be worked through. Let us look at the steps along the way to choosing the right size pump for a typical “stock and domestic” installation.

1. How Much Water

The first step is really to decide how much water you need to pump. This will depend upon your circumstance (you can find a guide here for the water required by stock) but lets say you wish to pump 20,000 litre per day for the average weather conditions to water stock by pumping from a bore to a header tank that then flows out to troughs in the paddocks.

Let us assume that this is a new bore and that the pump is going to be powered by a standalone off-grid solar pumping inverter. The pump is going to operate only during daylight hours and for the average time of year we can count on 5 hours of operation per day.

 2. Site Data

The second step is to collect some site data of how deep is the bore, what is the water level in the bore, what height is the tank inlet so the height difference from water to top of tank can be estimated. You will also need an estimate of the length of the water line from the pump to the top of tank.

Height difference. For our case study here, let us say that the height difference between the top of water and the outlet into the tank is 73m. (Height differences are easily measured using phone apps such as GPS Essentials). When pumping, the water level in the bore will be drawn down so add 2m to the height difference to compensate. For our study the height difference we will assume is 75m.

Water line. The length of the water line, how many valves, elbows and other restrictions, along with the size of pipe and what it is made from all feed in to what pressure loss the pump will have to overcome. All sounds complicated but we can make some appropriate assumptions to ease the way.

  1. Pump Volume. The pump volume is simply the daily demand divided by the operational hours per day. In our case 20,000 l ÷ 5 hrs = 4000l/hr or 4m^3/hr (approximately 880 gallons per hour).
  2. Pipe Size. Use the Head Loss Calculator to estimate the head loss. For our example
  3. First Guess. Now we need to find a bore pump that will comfortably deliver 4m^3/hr with a total head of 85m or so.
    From our shop pages select a bore pump 4SD4 pump (4″ bore pump series designed to deliver 4m^3/hr). You can find here.
  4. Using Pump Curves.
    Select the “pump curve” tab wher you will find the series of curves for the different pump configurations and powers.
    Step 1. We know that we want a pump capacity of 4m^3/hr, or 67litres/minute. Using the Head Loss Calculator we have estimated that the total head loss of 85m. Enter the pump curve chart at a flow of 67l/min and go up to the line line 85m.
    Step 2. Now go to your right along the 85m (red) line until you intersect a pump characteristic curve, in this case it is for model 4SD4/22.
    Step 3. Now estimate the flow volume for where the total head loss line (85m line at this time) intersect the pump chracteristic curve, in this case we estimate 82l/min or 4.9m^3/hr.
    Step 4. Using the Head Loss Calculator estimate the total head loss at this higher flow rate. The estimated Total Head Loss for this higher flow rate is now 89m.

    Step 5. This new estimate of the Total Head Loss (the blue line) moves us off the pump characterist curve so we must adjust the estimate of the flow volume.
    Step 6. We do this by moving along the Total Head Loss estimate line (the blue line) until we intersect with the pump characteristic curve.
    Step 7. We can now adjust our estimate of the flow volume by along the bottom axis at 78l/m or 4.7m^3/hr.
    Step 8. We could continue to go around and around this loop making our estimates of head loss and adjusting the flow volume up or down accordingly. However one or two interations, as we have done in this example, will generally provide a reliable estimate of the pump performance and whether your initial selection was good or not-so-good and you need to consider a different model.
  5. Final Selection. Our final selection is for 4SD4/22 pump. However there are other possible pumps that could fulfil the requirements of our example.
    However, an important consideration is that pumps are most efficient around the middle of their performance curve. In the example here a final estaimated flow of 3-5m^3/hr and the selected pump will comfortably deliver the required 4m^3/hr.

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