How a Pneumatic Displacer level sensor is used to Control Liquid Level ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Pneumatic Displacer level sensor is used to Control Liquid Level

Although we have covered the principle of operation of the displacer level sensor before, here we attempt to introduce you how a pneumatic displacer level sensor is applied in liquid level control in a process plant. The anatomy of a typical pneumatic displacer level sensor is exemplified by the fisher 25000 controller/displacer level sensor assembly:
Fisher 2500 Pneumatic Level Sensor. Photo Credit : Fisher

Principle of Operation of the Displacer Level Sensor/Controller 

Liquid Level Controller Using a Displacer Level Sensor

As shown above, changes in the level of liquid in the vessel whose level is being controlled exerts a buoyant force on a displacer which causes the rotation of a torque tube shaft. The rotation of the torque tube shaft is converted into a proportional pneumatic output signal by the pneumatic controller attached to the displacer level sensor. Typically, this pneumatic output is 3 – 15psig. The output signal from the controller drives a dump valve open to evacuate liquid from the vessel. When liquid level rises, the buoyant force on the displacer increases leading to increasing output from the controller if it is a direct acting controller. When the liquid level falls, the buoyant force on the displacer decreases resulting in decreasing output to the controller. If the controller is set at 25% of vessel level for example, then as soon as liquid in the vessel reaches 25%, the controller outputs a signal to completely open the dump control valve to release the liquid to a dump or safe area in the case of a hazardous liquid e.g hydrocarbon condensate.



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