Taking control of a slurry tank
07 May 2013
Key requirements for a custom control system developed and supplied by Ralspeed for the UK’s largest slurry tank, included simplicity and reliability.
The tank is 40m in diameter, almost 6m high and is capable of holding over 7 million litres of slurry (farm animal excrement mixed with water) which is stored in the tank at certain times of year as an environmental protection measure, and then used as a natural fertiliser on farmland at other times of year.
Pumps are used to control the flow of slurry into and out of the tank, which is also equipped with motorised agitators to periodically stir the slurry during storage.
Ralspeed was approached to provide the control system for the pump and agitator motors. This had to take into account the limited capacity of the mains supply at the tank installation site. Arrangements had to be made to limit the inrush current when starting the pumps and agitators, and also to ensure that no more than two agitators could run at the same time, and that the agitators could not run when the pumps are running.
The agitators also needed to be have approximately equal usage in order to equalise wear and tear, thereby maximising their operating lives. Further requirements were that the control system should be as simple as possible to facilitate maintenance, and that the user controls should be straightforward and intuitive.
Ralspeed used 18.5 kW Torq-master soft starters from its own range to control the motors. These units provide control over the motor starting current, ensuring that the capacity of the supply is never exceeded, and also ensure that the agitators and pumps start and stop smoothly.
Because no more than two of the motors on the tank could ever be running simultaneously, the Ralspeed engineers realised that substantial cost savings could be achieved by using only two soft starters, and connecting them, via main contactors, to the appropriate motors as needed to meet operational requirements.
A single selector switch is used to configure the operation of the system. This switch allows the user to rotate the duty of the agitators, and to select operation of the pumps instead of the agitators when necessary. With careful design, it has been possible to implement this straightforward and effective form of control without needing to use a PLC or smart relay
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