03/16/2012 | Autor / Editor: Kevin Cullen / Jörg Kempf
Learn how the diagnostics built into Emerson’s latest generation vibrating fork level switches provide information that enables operators to resolve problems before they impact productivity and quality.
Vibrating-fork level switches are widely used in the process industries and are suitable for virtually all liquid applications. Operating on the principle of a tuning fork, an internal piezoelectric crystal oscillates an external fork at its natural frequency. The frequency changes depending on the medium in which it is immersed — the denser the liquid, the lower the frequency. Thus, the frequency varies depending on whether the fork is immersed or dry. Changes to this frequency are monitored.
A short fork with a high natural frequency — approximately 1,400 Hz — is used to avoid interference from other plant vibrations which otherwise could cause false switching. The fork is shaped in such a way that liquid quickly drips off as the level subsides. By monitoring the frequency it is also possible to determine the condition of the fork.
Vibrating-fork level switches can be used in applications with liquids and slurries, including coating and aerated liquids. Sensing is virtually unaffected by flow, turbulence, bubbles, foam, vibration, solid particles, build-up, or fluid properties. Applications include high and low level detection in liquid tanks as a backup to a continuous level transmitter, activating pumps based on level, and starting or stopping mixers based on level around the blades. Unlike many other level switch technologies, vibrating-fork technology does not have parts that can get stuck and therefore is less prone to failure.
Opportunities to deploy this technology are common. For example, many tanks around a plant may initially not have been fitted with instrumentation connected to the control system. Similarly, coolant and lubricant levels in various machines may have not been monitored continuously.
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