In applications with large emulsions, a displacer device (which relies on a buoyancy effect rather than any dielectric value) can track the midpoint of an emulsion layer and, so, may provide a better solution. However, that technology relies on moving parts that require frequent cleaning and replacement, thereby reducing the reliability of the measurement and incurring greater maintenance costs.
Open air and non-metal tanks. Radar often works well in such applications. In some cases, though, outside disturbances may interfere with the radar signal -- and you must select a device, such as a GWR with a smart galvanic interface, with high resistance to electromagnetic interference. For most open sump and well installations, an ultrasonic meter is a more-cost-effective solution. However, should vapors be present, choose a low-frequency radar device instead.
Overfill protection. In critical applications, you must install a minimum of two level technologies or devices; if several measurements rely on the same technology, you must employ a voting scheme. Using technologies less influenced by process conditions, such as radar in combination with vibrating fork switches, is a good step to more accurate and reliable measurements.
For radar, most failure modes relate to a loss of signal. High sensitivity normally results in high availabilities. High sensitivity is achieved by increasing the signal-to-noise ratio, e.g., via Dual Port and Direct Switch Technology. Enhanced echologics -- the ability to ignore false echoes -- and smart software functions also improve radar performance. Some GWR devices incorporate software that improves measurements in the near zone (high level areas), especially for low reflective targets.
However, above a certain level the surface echo may not appear at all in the waveform. Using echologics to monitor signal changes as the level gets close to the top adds an extra layer of protection. This supports the basic level signal and gives informative warnings that the tank is full, even if the level signal is lost.
Advanced diagnostics is another step in the right direction for safe measurement. For example, some GWR devices provide real-time notification if the probe gets coated, providing the opportunity to schedule proactive maintenance.
Today, vibrating fork switches continuously monitor corrosion of the forks, external damage to the sensor, internal wire disconnect or breakage, and over-temperature. This results in a fault indication and fail-safe operation of the switch output.
A good installation is key to success with radar. A new radar device usually is installed on an existing nozzle. This nozzle sometimes may be too tall or narrow for the instrument. Try to minimize the height of the nozzle used. Ideally, nozzles should be at least two inches in diameter but no more than six inches high for GWR. For non-contacting radar, the end of the antenna should extend slightly beyond the nozzle. High-frequency non-contact radar can use longer nozzles but they must be smooth and clear of obstructions.
Positioning the nozzle directly over a pipe, baffle or other obstruction can cause problems. The obstruction interferes with the radar beam and it, rather than the process medium in the vessel, becomes the level measurement. Similarly, a fluid stream coming into the tank that falls into the path of the radar beam or on the probe will impact the reliability of the measurement.
As with all instrumentation, you must correctly configure radar devices in accordance with application needs. Take special care when inputting thresholds for the radar signal -- these will change depending on the medium being measured. For example, oil appears very different to a radar device than water and, therefore, requires very different threshold settings. However, today good set-up guides and functions in most cases enable easy configuration in just a few steps.
SARAH PARKER is level application manager for Emerson Process Management, Chanhassen, Minn. Email her at Sarah.Parker@emerson.com.