Displacer Switches

In a typical displacer switch design, a spring is provided which is burdened with weighted displacers. The displacers having weights greater than the process fluid gets submerged in the liquid resulting in a buoyancy force change. This will cause a variation in the net force operating on the spring. In general, the spring will compress with the raise in buoyancy force. Just like the float level switches, a magnetic sleeve and a non-magnetic barrier tube is also incorporated in displacer switches. The magnetic sleeve is attached to the spring and it moves according to the spring movement resulting in activation of switching mechanism. An in-built limit switch is provided in the design which proves useful in level surge conditions since it keeps a check on the over stroking of the spring. The operating principle of a typical Displacer switch is illustrated in the figure below.



Displacer switches are most commonly employed in oil and petrochemical fields as level transmitters and local level controllers. These switches offer extremely correct and consistent measurement results in applications where clean liquids having stable densities are concerned. They are particularly not appropriate for slurry or sludge type applications since coating of the displacer causes a change in its volume and a resulting change in its buoyancy force. Temperature adjustments should also be done for these switches, specifically in areas where changes in process temperature can significantly affect the density of the process liquid.

The performance of displacers can be influenced by non-stability in process density in view of the fact that the displacement i.e. the weight loss of the material is equivalent to the weight of the liquid dislocated. As soon as the specific gravity of the process varies, the weight of the displaced material also varies accordingly, resulting in a change in the calibration. Due to this, one can specifically face problems in cases of interface level detection between two liquids having different densities, where the relative signal depends upon the difference between two densities. An important requirement while working with displacers is that even after commissioning, the liquid being detected must retain its density for getting good repeatability.


Advantages

Following are the major advantages associated with the use of floats and displacers:

• They perform extremely well with clean fluids.
• Use of these level sensors proves to be very accurate.
• They are flexible to extensive changes in density of the medium.