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Modulating control valves each with 4-20mA I to P converter linked to pneumatic a valve positioner controlling a diaphragm actuator. An air-operated valve, also known as a pneumatic valve, is a type of power-operated pipe valve that uses air pressure to perform a function similar to a solenoid.
Specific example of a smart valve positioner used. Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable, such as pressure, flow, level, or temperature, within a required operating ...
The basic principle behind partial stroke testing is that the valve is moved to a predetermined position in order to determine the performance of the shut down valve. This led to the adaptation of pneumatic positioners used on flow control valve for use in partial stroke testing. These systems are often suitable for use on shutdown valves up to ...
The second type of positioner is an analog I/P positioner. Most modern processing units use a 4 to 20 mA DC signal to modulate the control valves. This introduces electronics into the positioner design and requires that the positioner convert the electronic current signal into a pneumatic pressure signal (current-to-pneumatic or I/P).
Air pressure acts on a piston or bellows diaphragm creating linear force on a valve stem. Alternatively, a quarter-turn vane-type actuator produces torque to provide rotary motion to operate a quarter-turn valve. A pneumatic actuator may be arranged to be spring-closed or spring-opened, with air pressure overcoming the spring to provide movement.
The example shown is pneumatic. At sub-millimeter distances, a small movement of the flapper plate results in a large change in flow. The nozzle is fed from a chamber which is in turn fed by a restriction, so changes of flow result in changes of chamber pressure.
Pneumatic field actuators are still widely used because of the advantages of pneumatic energy for control valves in process plant environments. Showing the evolution of analog control loop signaling from the pneumatic to the electronic eras Current loops used for sensing and control signals. A modern electronic "smart" valve positioner is shown ...
Pneumatic logic is a reliable and functional control method for industrial processes. In recent years, these systems have largely been replaced by electronic control systems in new installations because of the smaller size, lower cost, greater precision, and more powerful features of digital controls.
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