Method and apparatus for an electrohydraulic control system of a steam

Abstract

Pilot valves used in electrohydraulic control systems are driven by either an electromagnetic electromechanical actuator or an electromotor electromechanical actuator. A drawback of electromagnetic actuation is that on brief interruptions of electrical power the actuator causes a trip response to the pilot valve, whereas a drawback of electromotor actuation is that on complete interruptions of electrical power the actuator cannot independently cause a trip response of the pilot valve on demand. The proposed modification, involving the pilot valve, provides a means to effect a trip response regardless of the electromechanical actuator type used, together with overcoming the drawback of electromotor actuation. To realize the trip response, an additional piston (connected to the electromotor actuator's stem) is positioned between the actuator stem and the pilot valve. A surface area of the additional piston and one surface area of the pilot valve are loaded by pressure from an oil trip line in the hydraulic system. The loaded surface area of the pilot valve causes a force in the direction opposite to that of a trip, while the force on the additional piston is in the direction required for a trip. When a trip is required, hydraulic pressure in the trip line is reduced causing (1) the force on the actuator stem to go to zero; and (2) the other force, intrinsically found on the pilot valve, to actuate the pilot valve into a trip condition. The decrease of trip line pressure is carried out by three solenoid drain valves (each equipped with two solenoid coils) manipulated by three electronic overspeed trip devices operated by a two-out-of-three voting scheme. Consequently, this invention not only negates the disadvantage of using electromotor actuators, but it also provides overspeed protection and an under-load test of various control system elements.