Land reclamation pump

Low-lying swampy areas can be turned into agricultural land by draining. The catchment area determines the size of the land reclamation pump. In round figures the capacity is usually about 1.5-2 l/s for every hectare. The material for the dyke is taken from the neighboring ground and should be of clay to seal off the water. If clay of good quality is not to be found in the vicinity, plastic sheeting provides good protection against leakage through the dyke.

Pumping stations

Pumping stations constructed before the sixties were cast in situ and were large and expensive. The pumps had extended shafts with electric motors attached to foundations. During the sixties submersible pumps with capacities from 50 to 1000 l/s were developed for draining purposes. The stations were usually constructed as wells made of concrete rings. See figure.

Pump

The pump is usually driven by an electric motor, in exceptional cases by a diesel motor with hydraulic power transmission. The pump is of propeller or mixed-flow type. The delivery heads are between 1 and 3 metres and with capacities currently up to 2 m3/s. Since the water flow varies during the year, sometimes two or more pumps are installed and put into operation when required.

Pipes and valves

The pipes are welded steel structures or made of plastic reinforced with glass fibre. The valves are of reverse-acting flap type and insensitive to rubbish.

Control

The pump or pumps are started and stopped by level control. The levels are sensed by rockers, floats or electrodes. Propeller pumps can be flow-regulated by automatic adjustment of impeller blades. Mixed-flow pumps can be regulated with regard to flow by change of speed or by pre-rotation of the water before the impeller.

Pitfalls

From the point of view of power demand, propeller pumps are sensitive to suction head variations. The power required increases very sharply with suction head, unlike centrifugal pumps where the effect is the opposite. The regulation of levels must be carefully adjusted. If the stopping level is too low, the pump can suck in air at constant surface levels without stopping. Too low a starting level may lead to perpetual switching on because the water rises rapidly when it flows backwards through the outlet pipe. Excessive switching on and off causes fatigue of the reverse flap valve and there is a risk that it may work loose. Contactors in the electric control, seals and bearings are subjected to more rapid wear. It is therefore an advantage to construct large water storage in the form of drainage ditches or large diameter feed pipes. Alternatively pump flow can be regulated so that they work continuously.