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What is Pneumatic Controller?

10/31/2013 4:13:26 PM | by Anonymous

Pneumatic Controls

Pneumatic controller is a comparative device that receives an input signal from a measured process variable and compares this value with a predetermined control point value as well as determines the appropriate amount of output signal required by the final control element to provide the corrective action within a control loop. This controller circuit can operate on air pressure and can use a mechanical means such as a temperature sensitive bellows or bimetal to perform control functions which include actuating a flapper and nozzle or switching relay. The controller output is usually positions or operates a pneumatic actuator. Although, switches and relays are often in the circuit.


The trerice pneumatic controller can operate through a coordination of its thermal or pressure sensing system and the air signal relay system. Furthermore, the controller’s sensing pressure or bulb connection is installed within the process application and senses the change within the measured variable as well as relays this input signal to the controller. The controller mechanically compares the signal against a predetermined set point and sends a corrective air signal to a pneumatic control valve which modulates the process flow and thereby returning the application to the desired condition.


The pneumatic controllers can be specified for both proportional, off or on control and the processes which are characterized by stable load conditions and can be controlled using off or on control with a “quick opening” control valve by providing a full corrective response to a minimal change in the measured variable. These applications with unstable conditions or those requiring precise control are the best maintained using the proportional control and have an equal percentage valve trim design which gives a corrective response to the changes in the measured variable.


Off/on control can recognize only a deviation that exists. Any deviation between the measured process variable and set point will produce a full corrective signal whereas the proportional control can react to the size of the deviation from set when sending a corrective signal.


The size of the corrective signal can be adjusted in relation to the size of the error by changing the width of the proportional band. However, a narrow proportional band will cause a large corrective action in relation to a given amount of error while a wider proportional band will cause a smaller corrective action in relation to the same amount of error.


Pneumatic controller temperature range from -100°F through 600°F and the pressure ranges from 30PSI to 100PSI are available. The set point is the actual pressure or temperature required within the process. This set point indicator and a process temperature or indicators are read against the range plate. The controller is equipped with two internal pressure gauges to indicate the air supply pressure and the pneumatic output signal pressure.


In addition, the pneumatic controller can be furnished in either reverse or direct action. A direct acting controller will increase the output signal as the pressure or temperature of the application increases. The reverse acting controller can reduce the output signal as the input of the application increases. This action must be specified when ordering but can be easily reversed in the field.


As for the thermo well, all applications in which the process media may be corrosive or contained under pressure, the use of this thermo well is required to prevent damage to the sensing bulb. Thermo well can also facilitate the removal of the sensing bulb from the operating process and are available in a variety of lengths, materials and connections.

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