Solid bowl scroll discharge decanter centrifuges

A solid bowl decanter centrifuge comprising a solid, generally cylindrical bowl having a liquids outlet at one end and a solids outlet at the other end, means for rotating the bowl at a first speed, inlet pipework for introducing influent to the interior of the bowl, a feed valve in said inlet pipework for controlling the rate of flow of influent therethrough, a scroll conveyor mounted for rotation within the bowl at a second speed, a hydraulic motor whose body is connected to the bowl and whose output shaft is connected to the scroll conveyor whereby the motor speed determines the differential speed of the conveyor relative to the bowl, a hydraulic drive system for the hydraulic motor which includes a pump for supplying hydraulic fluid to the motor, a primary control system adapted to monitor the hydraulic pressure in the hydraulic drive system for the hydraulic motor and to regulate the displacement of the pump so as to maintain a predetermined relationship between conveyor speed and the pressure in the hydraulic drive and hence between the conveyor speed and the conveying torque, and a secondary control system which includes control valve means having a control slide, restriction means disposed in one of the hydraulic lines connected to said motor, and power cylinder means for controlling the opening state of said feed valve, said power cylinder means having a differential piston, said control valve means being coupled to said one of the hydraulic lines such that the position of said control slide of said control valve means is determined by the pressure drop across said restriction means and said control valve means being coupled to said power cylinder means to control the variation of the supply of pressurised fluid to said differential piston of said power cylinder means for controlling the opening state of said feed valve.