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Case study

Providing flush pumps with flow control

The flush pump system at the Lith drinking water production plant, a division of Brabant Water, recently underwent an overhaul. This system comprises two 55 kW centrifugal pumps driven by electric motors. They produce a combined output of 0 – 2000 m3/hour.

[29 June 2005]

Flush pumps are used to rinse filter beds clean. In fact, here is a brief description of what the whole process involves.

Groundwater is pumped through a number of well fields and collected in a pair of pipes. The water is then channelled to the filter station. Groundwater is rich in dissolved iron and manganese. The first stage of the purification process involves groundwater being intensively exposed to air. As a result of this, the dissolved metals are converted into insoluble flakes.

This aeration process also removes a number of gases from the water, which do not feature in drinking water. The second stage of the process involves catching the iron flakes in the upstream filter, a concrete tank filled with filter gravel. The water is passed through the gravel in this filter and the iron flakes are left behind. Once this first filter process is complete, the water is passed through a second gravel bed, which, this time, mainly catches the manganese particles. The water which emerges from this downstream filter is then drinking water.

Both gravel beds become contaminated and must therefore be cleaned at regular intervals. This is done by blowing a volume of water upwards from under the gravel bed. The volume of water blown through the gravel bed is extremely critical, as this volume must be selected so that the gravel is just lifted up enough for the impurities to be rinsed away from it, but ensuring that the gravel does not swirl about.

Previously, Brabant Water only had the choice of operating 1 or 2 pumps and the volume of water was controlled by an electrical control valve, which was used to cut off any surplus volume. Unfortunately, this also produced a commensurate energy loss. In the new set-up, however, the pumps are started up using an Emotron FDU frequency inverter and an Emotron MSF softstarter respectively and are controlled by a pump control fully integrated in the frequency inverter. This means that the 0 to 2000 m³/hour range can be steplessly controlled.

The volume of water required by Brabant Water is converted into a control value between 4 - 20 mA. The pump control compares this required value with the actual flow by feeding back a flow measurement from the pipe and decides what the rotational speed of the first pump must be and whether the softstarter for the 2nd pump motor must be switched on or off . During the commissioning phase, the control was even able to adapt without any problem to rapid adjustments to the required value.

 

Photo: In the new set-up the pumps are started up using an Emotron FDU frequency inverter and an Emotron MSF softstarter respectively and are controlled by a pump control fully integrated in the frequency inverter.

Given that the pump control is fully integrated in the frequency inverter and that all the motor protection devices are integrated as standard in both the frequency inverter and softstarter, it was possible to convert the existing control cabinet using few additional components. Emotron supplied the mounting plate with the components already assembled on it, which meant that Brabant Water simply swapped the old mounting plate for the new one.

The Emotron pump control is suitable for nearly all processes where a number of pumps or fans (up to a maximum of 7) must automatically achieve and maintain a flow volume, pressure or other process values. Its standard features include continual process monitoring, automatically switching to another pump when there is a fault with the original pump and equally distributing the number of hours the pumps operate for. During operation, the incidences of sudden pressure and flow fluctuation affecting the frequency inverters and softstarters are added up and water hammer is prevented. In addition, there is no external PLC system required any longer, as the process used now can ease the load on the existing PLC system.

The pump control comprises an option card, which is inserted in the Emotron FDU frequency inverter and the settings automatically appear in the menu structure as soon as the card is inserted.

The additional investment is therefore so small that there is no need to purchase or programme any PLC beforehand.