tailieunhanh - Mechanical Science HandbooksMechanical Science Handbooks 20000 Part 6

Tham khảo tài liệu 'mechanical science handbooksmechanical science handbooks 20000 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | VALVE ACTUATORS DOE-HDBK-1018 2-93 Valves Electric Motor Actuators Electric motors permit manual semi-automatic and automatic operation of the valve. Motors are used mostly for open-close functions although they are adaptable to positioning the valve to any point opening as illustrated in Figure 33. The motor is usually a reversible high speed type connected through a gear train to reduce the motor speed and thereby increase the torque at the stem. Direction of motor rotation determines direction of disk motion. The electrical actuation can be semi-automatic as when the motor is started by a control system. A handwheel which can be engaged to the gear train provides for manual operating of the valve. Limit switches are normally provided to stop the motor automatically at full open and full closed valve positions. Limit switches are operated either physically by position of the valve or torsionally by torque of the motor. ME-04 Page 46 Rev. 0 Valves DOE-HDBK-1018 2-93 VALVE ACTUATORS Pneumatic Actuators Pneumatic actuators as illustrated in Figure 34 provide for automatic or semiautomatic valve operation. These actuators translate an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are used in throttle valves for open-close positioning where fast action is required. When air pressure closes the valve and spring action opens the valve the actuator is termed direct-acting. When air pressure opens the valve and spring action closes the valve the actuator is termed reverseacting. Duplex actuators have air supplied to both sides of the diaphragm. The differential pressure across the diaphragm positions the valve stem. Automatic operation is provided when the air signals are automatically controlled by circuitry. Semi-automatic operation is provided by manual switches in the circuitry to the air control valves. Figure 34 Pneumatic Actuator Hydraulic Actuators Hydraulic actuators provide for .