Pump sizing and installation in domestic groundwater applications
Learn the basics of sizing and installing submersible pumps for domestic groundwater extraction and why correct sizing is so important.
In this tutorial you will learn the basics of sizing submersible pumps for domestic groundwater extraction. We will also briefly touch upon best practise when it comes to installing the pumps.
Domestic groundwater extraction is typically relevant in remote areas where the cost of extending the municipal water supply to just a few houses is too high.
Instead, a well is drilled close to the individual homes and a submersible pump is installed, providing the household with groundwater for all domestic purposes.
The most important thing when sizing a submersible pump is to make sure that it is able to deliver the required pressure and flow. The required flow depends on end-user requirements. If there are two bathrooms, a washing machine and a dishwasher in the house and an irrigation system in the garden the pump must be able to deliver a higher flow than if there is just one bathroom and a kitchen tap.
The pressure – or head – is found by determining the following parameters: Head and loss from dynamic water level to diaphragm tank – the dynamic water level is the level of the water when the pump is in operation. Head and loss from diaphragm tank to highest tap in the house. Minimum pressure at highest tap.
Once you have determined these parameters you can select the optimum pump for the groundwater extraction job. Simply find the desired flow and head on the pump curve chart. In this example, you will need a pump that is able to deliver 2 m3/h at 100 m, e.g. an SQ2 pump.
But does sizing really matter that much when it’s just one house being supplied by the pump? The answer is yes. An undersized pump results in lower flow or pressure, which means that it will not be able to deliver the required capacity per hour at peak loads or the required head.
An oversized pump, on the other hand, will reduce energy efficiency and give unnecessarily high operational costs. So for the family living in the house, sizing matters.
Now we’ve determined the optimum size of the submersible pump, let’s turn to look at a few important installation points. As a rule of thumb, the nominal pump diameter should be the same as the borehole diameter.
If the borehole is too big there’s a risk that the lack of velocity past the motor will cause the water to stand still in the borehole and potentially overheat the motor. The motor is very sensible to the water’s temperature so always make sure that the minimum flow past the motor is 0.15 m/s if the groundwater temperature is above 30°C.
In harsh conditions, the pump can be equipped with a cooling sleeve to optimise the motor cooling.
Another important aspect is the installation depth. The pump unit must be installed below the dynamic water table to make sure there is no risk of dry-running during operation.
Finally, let’s look at the pressure tank in a submersible system. The size of the tank depends on the choice of pump, which is a valuable thing to keep in mind – especially if the space allocated for the tank in the house is relatively confined.
In a conventional system where the pump is controlled by a pressure switch the pressure tank must typically be able to hold between 50 and 200 litres of water to ensure sufficient pressure in the taps.
If instead we install a more advanced pump with a built-in frequency drive a small 8 litre tank is enough. This is because the pump itself automatically adjusts its speed according to demand to deliver the exact required pressure in all taps at all times.
Which means more comfort for the people living in the house.