For deep wells and wells larger than 4 inches, the diameter is usually chosen to accommodate the size of pump required. The diameter should be 2 nominal sizes larger than the pump to allow sufficient clearance for ease of installation and operation. If the screened interval is to be at much lower depths than the pump, the casing below the pump may be of a smaller diameter. For wells smaller than 4 inches it is satisfactory to select casing which is only 1 size larger than the pump.
In shallow wells where the pump is directly connected to the top of the well casing or connected to a suction pipe inside the well, the casing diameter should be selected in relation to the diameter of the suction inlet of the pump.
One departure from casing to the bottom of the aquifer is when the bottom of the aquifer is known to contain poor quality water. In this case the well should be completed to a depth which will avoid this water.
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