SOLVIA CleanMix is a system of automatic self-cleaning for the mixers of wastewater treatment works. A rise in the current or the power of the motor serves the control unit as a sign of fibre wound onto the impeller, and the answer is a smooth reversal whose depth follows the size of the excess.
A mixer at a treatment plant works in a medium where the wastewater carries rags, textile, synthetic threads, polymer fibres and wet wipes. Such inclusions wind onto the shaft, the blades or the propeller, and the phenomenon is known worldwide as ragging. SOLVIA CleanMix follows the load on the motor, recognises the winding by its rise and takes the fibre off by reversing the impeller, without waiting for cleaning by hand.
It is meant for sites where mixing runs in a medium at risk of fibrous ragging: sewage pumping stations, biological treatment plants, balancing tanks, aeration tanks, anoxic and denitrification zones, sludge holding tanks. The development suits new mixers and the retrofitting of working ones alike: to installed equipment with a three-phase motor a current sensor, a control unit and a variable-frequency drive are added.
Fibre caught on the impeller does not stop the mixer at once. It goes on turning, but the mixing leaves its design regime, while the motor overcomes an added resistance and draws more energy. The trouble builds up gradually: small windings appear first, then the motor current rises. Further on come the heating of the drive and the wear of the bearings and seals, and the hydraulic efficiency falls until the matter reaches overload and an emergency stop.
An ordinary mixer runs until the winding either impairs the work or leads to a failure, and then it is lifted out of the tank, switched off, cleaned by hand and started again. For a treatment plant this means downtime, added labour and the risk of upsetting the process regime. Where servicing is done irregularly, fouled equipment becomes a cause of odours, malfunctions and emergency call-outs.
The sign of winding is taken from the electrical side: the sensor passes the current or the power of the motor to the control unit continuously, and the present load is compared with three threshold levels. An excess over the norm of 5–15 % belongs to initial fouling, of 15–30 % to moderate fouling, of 30–50 % to heavy fouling. The level settles not the fact of intervention but its depth.
Light fouling is answered by a short reversal of 0.5 of a turn, moderate fouling by 1–1.5 turns and a longer run, heavy fouling by a series of 3–5 reversals, each of up to 2 full turns. Too weak a reversal does not remove the winding, while too strong a one at light fouling puts needless mechanical loads on the machine, and for that reason the angle of reversal is held within 0.5–2 turns. A fixed programme draws no such distinction: it fires where there is no fouling at all and stays weak where the winding is already heavy.
The change in the direction of rotation is carried out through a variable-frequency drive: the mixer brakes smoothly, stops, accelerates smoothly the other way, passes the set angle and returns to the main direction. The braking and acceleration ramps take 2–5 seconds, and the starting current at such a reversal does not exceed 1.5 of the rated current of the motor. The materials of the development set it against a direct start by contactor, where the starting current may reach 5–7 rated values.
The second occasion for a reversal is not the current but the time. A timer with a period set from 5 minutes to 4 hours starts a light reversal regardless of the present load, taking off nascent windings before they give a noticeable rise in current. Such a regime is meant for sites where rags, threads and wet wipes are present in the flow at all times, and it works beside the adaptive reversal rather than in place of it.
Automatic cleaning does not always cope. If after a set number of attempts the current does not return to the norm, the control unit raises an alarm: a light indication, an audible warning and a message to the dispatching system or to SCADA. The operator receives it not at the moment of a full stop with the process regime already upset, but once the self-cleaning has spent its means and the intervention of staff is called for.
The make-up of the unit is small: a mixer with a three-phase motor, a current sensor, a control unit, a variable-frequency drive and a timer. On a working site that allows retrofitting to serve — the motor and the mixer itself stay, and measurement, control and a drive of adjustable frequency are added. The threshold values of current, the parameters of the reversal, the period of prevention and the logic of the alarm are tuned to the particular mixer and to the character of the flow.
SOLVIA CleanMix is SOLVIA's own technological development in the field of the operation of treatment plant equipment; the technology holds patent protection in Russia. The solution brings together the mixer, a sensor of current or power, a control unit, a variable-frequency drive, a timer, an adaptive algorithm of reversal and the passing of an alarm to the dispatching system. It is applied both in new mixers and treatment works and in the modernisation of existing equipment with three-phase motors. The particular settings of the thresholds, the parameters of reversal, the control algorithms and the integration with SCADA are chosen for the site individually.
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