Guided-wave radar gets attention for more level measurements

A common observation about today’s automation practices is that we’re all dazzled by the digital and computer advances but the technology behind the primary sensors and final control devices doesn’t change, or at least not nearly at the same pace.

GWR was specified for this feedwater heater to provide reliable, accurate level measurement to protect against water induction into the steam turbine

GWR was specified for this feedwater heater to provide reliable, accurate level measurement to protect against water induction into the steam turbine

At last year’s Ovation Users Group (OUG) meeting, Paul Yankello and Bill Pace of Emerson Process Management’s Rosemount Div, challenged that assertion. Their presentation covered general level-measurement applications and technology selection, but the expanding applications for guided-wave radar (GWR) technology were a highlight of the message. Keep up with advancements in I&C technology by attending the upcoming OUG conference, July 26-30, in Pittsburgh’s Westin Convention Center Hotel (users only).

The 2013 edition of Emerson’s Engineer’s Guide to Level Measurement for Power and Steam Generation, available online, gives the basics of GWR. It works this way:

“The GWR (source) is mounted on the top of a tank or chamber, and the probe usually extends to the full depth of the vessel. A low-energy pulse of microwaves, traveling at the speed of light, is sent down the probe. At the point of the liquid level (air/liquid interface) on the probe, a significant proportion of the microwave energy is reflected back up the probe to the transmitter. It measures the time delay between the transmitted and received echo signal and the onboard microprocessor calculates the distance to the liquid-level surface.”

GWR, note the authors, is independent of liquid density but the steam dielectric constant (DC)—important in many powerplant applications—can cause up to 20% error and will vary with pressure. For this reason, a technique called dynamic vapor compensation (DVC) is employed. A reference reflector measures the steam DC. The distance to reflector is fixed. As microwaves travel through steam, the DC impacts the speed, making reflector distance appear further away. The distance between the physical and “electrical” distance is used to measure the DC and automatically correct for surface distance. DVC can limit the error rate to less than 2%.

The guidebook lists the following benefits of GWR:

      • It is good for tanks of all sizes and tight geometries.

      • Advanced versions work in turbulent and low-DC fluids.

      • Can be installed directly in the tank or in a bypass chamber.

      • No compensation is required for density or conductivity.

      • Changes in temperature and pressure have no impact, nor do most vapor-space conditions.

      • There are no moving parts; minimal maintenance is required.

      • A direct, “top down” measurement is provided, as opposed to prevalent indirect methods.

The authors claim that GWR is “best practice” for the following applications:

      • Condenser hotwell (vacuum-to-positive pressure range, density changes of water during condensation).

      • Lube-oil tanks (clean, low-DC fluid; typical location next to turbines can pose vibration issues for alternative instrumentation; works in skid-mounted and small tanks with limited access).

      • Boilers, steam separators, and feedwater systems (steam and water density and DC changes with temperature and pressure, magnetite buildup issues with other options). Photo shows a GWR installed on a feedwater heater.

GWR works well in solids level measurement, too, although there are few applications in combined-cycle plants.

Probe selection is the principal caution in GWR applications, according to the guidebook. Coaxial style probes are the most versatile, especially because, unlike other types, the limitation of not being in contact with any metallic object is lifted. Single-lead probes are best for fluids which pose stickiness or coating issues.

Advanced versions of GWR can include the ability to detect buildup on the probe. Like all state-of-the-art microprocessor-based instrumentation, they can be equipped with the latest digital automation diagnostics and health monitoring. Wireless-based products are also available. Other industry sources suggest GWR, though a relatively young technology, presents a formidable challenge to more conventional differential pressure (DP) measurement.

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