Make the Most of Radar

Following some best practices can ensure accurate level measurement.

By Sarah Parker, Emerson Process Management

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The emergence of radar has significantly advanced level monitoring. Radar provides cost-effective and accurate measurements that are immune to density and other process fluid changes, as well as most vapor-space conditions.

Radar level measurement systems come in contacting and non-contacting versions, both of which are widely applied in the process industries. Contacting generally is a good fit for small spaces; it easily replaces older technologies such as displacers and capacitance probes. Non-contacting usually is better for dirty, viscous or corrosive applications and when agitators are present. Currently, contacting devices called guided-wave radar (GWR) are slightly more prevalent, primarily because they can measure interface level (e.g., oil and water) as well as standard direct level (Figure 1).

There are some important considerations when applying both types of radar technology. For example, getting accurate measurements in steam applications of more than 400–500 psi (30–35 bar) requires GWR systems that have dynamic vapor compensation. Similarly, in applications where signal reflection is weak, you must select a device with technology that minimizes losses in the returned signal. In cases -- including some solids applications and those with low dielectric fluids and turbulence due to boiling or entrained air -- where the return signal is so weak that it occasionally disappears, the device should be able to provide a level measurement via alternative methods such as probe end projection (which uses a combination of the known length of the probe and an online reading of dielectric to calculate level).

While GWR works in many conditions and doesn't depend on reflecting a signal off a flat surface, probe choice requires care. Probes come in several styles -- the application, length and mounting restrictions influence the correct choice. Unless coax-style units are used, probes should not directly contact a metallic object because 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 use single-lead probes. For such applications, devices offering signal quality diagnostics can help determine if the probe needs cleaning and allow scheduling of maintenance only when needed.

In general, GWR doesn't suit extremely viscous products where fluid flow is minimal. If GWR is used with very viscous fluids and installed in a bypass chamber, heat-trace and insulate the chamber to ensure fluidity. In addition, check that the connections from the tank to the chamber and the chamber's diameter are large enough to allow good fluid flow. For applications such as asphalt where heavy coating is likely, go with non-contacting radar and mount directly on the vessel, not in a chamber.

Non-contacting radar (Figure 2) demands careful consideration of process conditions and installation constraints. It requires a clear unobstructed view of a liquid surface. An unrestricted mounting nozzle also is important. The measured surface must be relatively flat, not slanted. Non-contacting radar gauges can handle agitation -- but success depends on the fluid properties and the amount of turbulence not limiting the return signal too much. Low dielectric fluids reflect very little radiated energy back to the gauge. Additional signal loss occurs if the liquid surface is turbulent, whether from agitation, product blending or splashing. To overcome this, you can use bypass pipes -- including self-contained chambers or stilling wells to isolate the surface from turbulence.

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