A displacer is used to measure and control the level of a tank. The displacer is used in industries where the level tank, for which the level should be controlled is in a remote area or at a long distance from the control room.
Displacer work on the base of Archimedes principle. “The weight of a body immersed in a liquid depends on the weight of the volume of liquid displaced”. The other words, if the displacer displaces a particular volume of liquid, the displacer will seem to lose the particular weight of the displaced liquid than it weighs when it’s not in the liquid.
As displacer moves up when the level increase. The displacement is converted to electrical signal using secondary transducers, such as LVDTs, Torque tube etc.
Displacer control system:
The control loop for the level control using displacer is shown above.
Here a Torque tube is used as the secondary transducer. As the displacer moves, it makes the torque tube twist or untwists.
The twisting motion of the torque tube moves a flapper against a nozzle. This sends a control signal to the pneumatic control valve
And the pneumatic signal is given to the control valve, to take the proper control action. The pneumatic control valve opens or closes to keep the level constant at the setpoint.