by Sarah Parker, Applications Manager, Emerson Process Management, Rosemount division
The emergence of radar has been an important advance in the level measurement field. Radar represents a cost effective, accurate solution that is immune to density and other process fluid changes as well as most vapour space conditions.
Radar level measurement systems are available in contacting and non-contacting versions. Contacting is generally a good fit for small spaces, and is an easy replacement of older technology such as displacers and capacitance probes. Non-contacting is generally a better fit for dirty, viscous and/or corrosive applications and when agitators are present. Currently, contacting devices, called guided-wave radar (GWR), are slightly more prevalent primarily because they are capable of providing interface level measurement (e.g. oil and water), as well as standard direct level measurements. But both formats are now widely accepted by the process industry.
While GWR works in many conditions and is not dependent on reflecting a signal off a flat surface, some precautions need to be taken with respect to probe choice. Several probe styles are available and the application, length, and mounting restrictions influence their choice. Unless a coax-style probe is used, probes should not be in direct contact with a metallic object, as that will impact the signal. Twin and coaxial probes are susceptible to clogging and build up. If the application involves liquids that tend to be dirty, sticky or can coat, then only single lead probes should be used. For such applications, devices offering signal quality diagnostics can help the user determine if the probe needs to be cleaned and allows maintenance to be scheduled only when needed. In general, GWR is not suitable for extremely viscous products where fluid flow is minimal. If GWR is used with very viscous fluids and is installed in a bypass chamber, then the chamber should be heat traced and insulated to ensure fluidity. Furthermore, the connections from the tank to the chamber and the chamber’s diameter should be sufficiently large enough to allow good fluid flow. Applications such as asphalt, where heavy coating is likely, are more suitable for non-contacting radar.
With non-contacting radar, process conditions and installation constraints all need to be carefully considered. Non-contacting radar requires a clear, unobstructed view of a liquid surface. It is also important that there is an unrestricted mounting nozzle. The measured surface needs to be relatively flat, not slanted. Non-contacting radar gauges can handle agitation, but their success will depend on a combination of the fluid properties and amount of turbulence. Dielectric properties will also impact the measurement. With low dielectric process fluids, much of the radiated energy is lost to the fluid, leaving very little energy to be reflected to the gauge. If the liquid surface is turbulent, whether from agitation, product blending, or splashing, more of the signal is lost. So a combination of low dielectric fluid and turbulence can limit the return signal to a non-contacting radar gauge. To overcome this, bypass pipes including self-contained chambers, or stilling wells can be used to isolate the surface from turbulence.
High temperature and pressure applications
In applications with extreme temperature and pressure conditions, it is important to select a heavy-duty process seal with multiple layers of protection and a flexible assembly to handle the stresses and the forces induced. This is to prevent leakages and ensure the safety and efficiency of your plant.
When measuring liquids at very high temperatures in a chamber, it is important to insulate and heat trace the chamber. Fluctuations in temperatures alter the density and volume of the product which then affects the level in the chamber. Maintaining the temperature of dirty liquids such as heavy oil also helps to avoid clogging and sticking within the chamber and enables adequate flow-through.
Although radar technology is not affected by density changes, dielectric changes can have an impact. For boiler and feed water systems, where boiling water and high pressure saturated steam vapours are present, the returned signal from the surface becomes weaker as water temperature increases. If this is not taken into account, the saturated steam alters the propagation velocity of the radar signal and generates an error in the level reading proportional to the measured distance. The dynamic vapour compensation method dynamically compensates for the changes in the vapour space dielectric and reduces the incorrect distance caused by varying pressure and or temperature to less than 2%.
Level and interface measurement has been successful with GWR but presents a separate set of challenges. The fluid with the lower dielectric must always be on top. The two liquids must have a dielectric difference of around 10, and the upper layer dielectric value must be known. Certain thicknesses of layers are also required for effective measurement to take place. Typical successful applications have a hydrocarbon-based fluid with a dielectric around 2 on the top layer and water-based fluid with a dielectric over 40 at the bottom.
Interface measurement applications where the density of the two fluids is very close, or where emulsifier chemicals are used, can produce fairly large emulsion between the products. This may make the interface indistinct. Heavy and thick emulsion layers or liquid layers with similar dielectrics can pose a problem for GWR as the technology requires a distinct dielectric difference to detect the interface. GWR devices have proven to work, but success is difficult to predict. A manual adjustment of the interface threshold on the radar device may be required and a chamber/stilling well can help to minimise the emulsion.
In applications with large emulsions, a displacer device, which relies on a buoyancy effect rather than any dielectric value, tracks the midpoint of an emulsion layer and may provide a better solution. However, the great disadvantage of this technology is that it has 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 tank applications
Radar often works well in open air or non metallic tank installations. However, in some cases outside disturbances may interfere with the radar signal. Here it is important to select a radar device with high resistance to EMI such as a GWR with a smart galvanic interface. For most open sump and well installations, an ultrasonic meter is a more cost effective solution. However, should vapours be present, then a low frequency radar device is the preferred solution.
In critical level applications it is necessary to use a minimum of two level technologies or devices and if the same technology is used, 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 using technologies such as Dual Port and Direct Switch Technology. Enhanced echologics – the ability to ignore false echoes – and smart software functions also improve the performance of the radar. Some GWR devices incorporate Reference Pulse Elimination software, which improves measurements in the near zone (high level areas), especially for low reflective targets. However, above a certain level the surface echo may not be visible at all in the waveform, but the echologics are used to monitor the changes in the signal as the level gets close to the top, adding 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, in some GWR devices, Signal Quality Metrics inform the user in real-time if the probe gets coated. This provides the opportunity to schedule proactive maintenance.
A good installation is key to success with radar. When installing a new radar device it is usually on an existing nozzle. This nozzle can sometimes be too tall or narrow for the instrument. It is recommended that users try to minimise the height of the nozzle used. Ideally nozzles should be at least two inches, but no more than six inches high for GWR. For non-contacting radar, it is preferable that the end of the antenna extends slightly beyond the nozzle. Longer nozzles can be used with high frequency non-contact radar, but they need to be clear of obstructions and smooth.
Another installation problem is when the nozzle is positioned directly over a pipe, baffle or some other obstruction. The obstruction interferes with the radar beam and becomes the level measurement rather than the process medium in the vessel. Similarly if the fluid stream coming into the tank falls into the path of the radar beam or on the probe, then this will impact the reliability of the measurement.
As with all instrumentation, radar devices must be configured correctly in accordance with application needs. Special care must be taken 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 there are good set-up guides and functions such as Measure and Learn, so in most cases configuration is easily achieved in just a few steps.
The correct selection and implementation of process level solution is crucial to obtain accurate and reliable measurements.