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:
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| Fisher 2500 Pneumatic Level Sensor. Photo Credit : Fisher |
Principle of Operation of the Displacer Level Sensor/Controller
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| 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.
