Ralspeed has developed a custom control system for use on the UK’s largest slurry tank, which is 40m in diameter, almost 6m high and capable of holding over seven million litres of slurry. The tank was designed, manufactured and installed by slurry handling specialist, Storth.
Slurry – which is essentially farm animal excrement mixed with water – is stored in the tank at certain times of year as an environmental protection measure, and then used on farmland as a natural fertiliser. Pumps are used to control the flow of slurry into and out of the tank, which is also equipped with motorised agitators to stir the slurry periodically during storage.
The design of the control system for the tank had to take into account the limited capacity of the mains supply at the tank installation site. This meant 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. Storth also wanted to be sure that the agitators had approximately equal usage, in order to equalise wear and tear; and the control system should be as simple as possible.
To meet these special requirements, Ralspeed designed and built a fully customised control panel. The company used 18.5 kW Torq-master soft starters from its own range to control the motors, which provide excellent 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.
Ralspeed was able to provide a single selector switch to configure the operation of the system. This switch allows the user to rotate the duty of the agitators, and also to select operation of the pumps instead of the agitators when necessary. With careful design, it proved possible to implement this straightforward and effective form of control without needing to use a PLC or smart relay, thereby satisfying the end user’s request for simplicity.
