Vibrating fork level switches are widely used in the process industry and are suitable for virtually all liquid applications. Operating on the principle of a tuning fork, an internal piezo-electric 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 of approximately 1400 Hz is used, avoiding interference from other plant vibration 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 a pump based on level, and starting or stopping a mixer 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 level in various assets have not been monitored continuously.
Diagnostics to suit every application
Many switches now provide basic diagnostics which allow the status of the device to be accessed locally. For example, a visible heartbeat LED flashing at different rates can be used to indicate various status conditions. These include output status, calibration, internal failure and load fault.
Some devices offer enhanced diagnostics to detect conditions such as a damaged fork, sensor, media build up on fork and excessive corrosion on the fork. In the event of a fault condition, the load is handled in a safe manner and the LED is pulsed at a special rate to show that a fault has been detected.
A recent innovation in vibrating fork level sensors is the introduction of a fully integrated wireless level switch. The WirelessHART Rosemount 2160 from Emerson Process Management offers advanced diagnostics which continuously monitor the status of the device and perform instrument health diagnostics of the fork and sensor.
The device features HART diagnostics that can be accessed using Emerson’s AMS Suite predictive maintenance software. The diagnostics can be accessed directly, by connection to the HART communication terminals on the device, or in the control room and maintenance workshop using the WirelessHART network and AMS Suite.
AMS Suite enables users to troubleshoot WirelessHART devices like the 2160 directly from the maintenance workshop desktop. This eliminates unnecessary trips to the field, optimises maintenance, and minimises downtime by enabling users to identify faults and take action before potential problems impact productivity and quality.
User configurable alerts
The 2160 features four user configurable device variable alerts: Output State (PV), Sensor Frequency (SV), Device Temperature (TV), and Supply Voltage (QV). These may be configured for a rising or falling value and a limit or threshold can be set to trigger the alert. There is also the provision to apply a dead band or hysteresis.
Level measurements and diagnostics in remote locations
Installing level measurement devices in remote locations has traditionally been a problem because of the need for power and data cabling. Digging trenches and running cabling is both costly and time consuming. However, the introduction of wireless networks overcomes these issues making it possible to install level instrumentation in locations where previously it would have been considered impractical or too expensive.
Wireless communication presents an obvious alternative to instrumentation cabling and self-organising mesh networks based on the IEC 62591 (WirelessHART) standard are starting to make a significant impact in these remote monitoring applications.
Wireless enabled vibrating fork level sensors with inbuilt wireless adaptors make connection to an existing wireless network quick and easy. Emerson’s Rosemount 2160 WirelessHART vibrating fork liquid level switch has the same features as wired level switches in the Rosemount 2100 series, but without the complication and cost of wiring.
For more information contact Mark Tapson, Emerson Process Management, +27 (0)11 451 3700, [email protected], www.emersonprocess.com
| Tel: | +27 11 451 3700 |
| Fax: | +27 451 3800 |
| Email: | [email protected] |
| www: | www.emerson.com |
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