PROJECT: MOTOR SPEED CONTROL USING VARIABLE SPEED DRIVES (VSD)
Drying Plant VSD Replacement
A VSD controls the sludge feed pumps to the sewerage drying plant based on the MLSS (mixed liquor suspended solids). The pumps must run slower on high MLSS, because higher solid concentrations increase the water content and viscosity that must be processed.
Based on the sludge feed, a polymer is dosed to bind the sludge. The dosing speed is controlled by another VSD, to ensure that the dosing speed matches the incoming sludge feed speed.
A third VSD controls the speed of the drying plant, to remove as much water as possible from the sludge.
When either of the three VSDs do not function, the entire plant is jeopardised, because the motors will continue to run at full speed regardless of the sludge viscosity.
At a local municipal sewerage plant, the drying plant could not function due to faulty VSDs. Operators consequently had to make use of traditional drying beds to dry out the waste active sludge. This is a labour intensive, not to mention unpleasant, task to manually clean out the drying beds once the sludge is dried out.
We replaced the VSDs and the control system in the drying plant, and reprogrammed them so that the drying plant could function normally.
