Valve Automation With Pneumatic Actuators-ddrtys

Pneumatic Actuators The Pneumatic Actuators are energized by .pressed air, or non-aggressive gas, this means that a pressure supply system is required to operate the actuators. For Actuators with larger stroke volumes the European Union requires CE-certification according to the PED. Quarter Turn Pneumatic Quarter Turn Actuators are mainly used for automation of ball Valves, Butterfly Valves, Plug Valves and Dampers. The advantages for this type of Actuator are that they are less sensitive for intensive cycling than the Electric Actuators and also very reliable thanks to a minimum of moving parts. These actuators are more or less considered as maintenance-free. Therefore these types of Actuators are the absolute most popular solution for process automation in all kinds of industries such as chemical, petrochemical, water treatment, oil and gas, off-shore, district heating, steel and metal, pulp & paper, food & brewery etc, etc. The Pneumatic Quarter Turn Actuators are available both as Double Acting and Single Acting with spring return. Dominating Designs The two dominating designs are the Rack&Pinion design and the Scotch-Yoke design. Rack&Pinion Rack & pinion design actuators have pistons with Rack and a Drive shaft with Pinion giving a linear torque output over the 90 travel for double acting actuators. The single acting spring return actuators have a torque output that decrease at the end of the travel in both directions. With a constant air supply flow capacity the turning speed of the shaft is constant over the full 90 travel. This can affect the service life of the actuator-valve package in a negative way because of hard acceleration and retardation in the end positions. The rack & pinion design is mainly used for actuation of smaller valves. Scotch-Yoke Scotch-Yoke design has pistons with piston rollers aligning in the slot of the scotch-yoke connected to the drive shaft. The power transmission works as a variable lever over the 90 travel, starting with a long lever, in the mid position a short lever and in the end position a long lever. This gives, for both double acting and single acting actuators, a high torque output at the end positions where most valves require higher operating torque. With a constant air supply flow capacity the turning speed of the shaft is starting smoothly, then accelerating and at the end of the travel slowing down again. This favourable turning speed has a positive effect on the service life of the actuator-valve package. Scotch-Yoke actuators are available with canted yoke and with symmetric yoke. Canted Scotch-Yoke Canted Scotch-Yoke gives a higher Break- away torque than end of travel torque and covers most butterfly valves and ball valves. Symmetric Scotch-Yoke Symmetric Scotch-Yoke gives same torque in both end positions and is used for plug valves and metal seated ball valves. Double Acting A Double Acting Pneumatic Actuator is pneumatically operated in both turning directions and will by loss of Air Supply Pressure stay in position. Depending of how the Solenoid Valve or the Positioner that Controls the Actuator is set up the actuator can set the valve into a decided end position Open or Closed by electrical signal failure. Single Acting Spring Return A Single Acting Pneumatic Actuator is pneumatically operated in one direction and by a spring return function in the other direction. A spring return Actuator is normally used for applications where a Fail Safe function is required. The function can be Fail Close or Fail Open. This means that if there is a major air supply failure the safety function of the actuator will operate the Valve to the decided safety position by the spring return function. Multi Turn Multi Turn Pneumatic Actuators are used for Gate Valves, Globe Valves, Plug Valves, Dampers and other applications where multi turn function is required, for example quarter turn valves with reducing gear boxes. The multi turn pneumatic actuators are only available as double acting but can be equipped with air bottles for emergency operation in case of air supply failure. Controlling and Signalling Pneumatic Actuators are often fitted with Control and signalling accessories as Limit Switch Boxes, Solenoid Valves and Positioners. The Actuators can be connected to almost any digital Control System or Monitoring system such as AS-Interface (ASI-bus), Profibus, Foundation Fieldbus, Interbus, Modbus, Device Net, HART-protocol, etc, etc, by the use of suitable .munication accessories. 相关的主题文章: