Guided Wave RadarPros and Cons

By David Land, P.E.ConocoPhillips

Field Instrumentation Network Lead

TimeGWR ‘Cons’ First

• GWR is a “contacting” technology – the probe is in the process fluid. If the fluid is particularly corrosive or there are material compatibility issues this can become a limiting factor. There are alternate metals available for probes and Teflon coating is an option, but with Teflon coating you run into Temperature issues.

• The maximum process temperature for most units is around 750 F. Temperatures over this require a different technology.

• GWR is unpredictable in foaming applications. In light foam the GWR will ignore the foam and reflect back the liquid level. In heavy foam, the signal can be absorbed and reflect back the top of the foam as level and/or weaken the overall signal.

GWR more ‘Cons’• Highly viscous fluids should be approached with caution. They

tend to coat the probe. Single probes work well with coating, but heavy coating will weaken the signal. Dual probes and coax probes can strengthen the signal but can also “bridge” in coating service – meaning the fluid will bridge across the two probes or inside the pipe of a coax and cause the signal to lock on the bridging as the level.

• Tanks with agitators or extreme turbulence should be approached with caution. If the agitator hits the probe it will cause a false signal. Extreme turbulence can either break a rigid probe or cause a flex probe to hit against the nozzle, causing a false signal. A coax probe or bridle is a good solution for turbulence as long as the process media does not plate out or coat metals.

• Interface applications where the product dielectrics are not at least “10” apart will not work with GWR, since GWR depends on a difference in dielectric to reflect back the interface level. Oil/Water is the ideal interface application – water with a dielectric of “80”, oil with a dielectric of approx. “2’. They are much more that “10” apart.

GWR more ‘Cons’• Fiberglass or plastic tanks should be approached with caution.

The GWR requires a solid ground for the microwave signal on the probe. We have successful installations on Fiberglass, but dual probes or a metal flange at least 2” OD for a mounting surface is recommended.

• In interface applications, if the dielectric constant of the upper product varies there can be an error in the measurement. The dielectric of the upper product must be lower than the dielectric of the lower product to have an interface reflection. The upper product cannot have a dielectric greater than “10” for the single probe and “5” for a dual probe. Depending on the probe, the upper product must be at least 4-8” for the radar to distinguish the echoes between the two fluids.

• Emulsion layers between two liquids can cause some inaccuracies.

• Longer probe lengths may require a crane to extract from the vessel.

GWR Pros• Direct level measurement• No moving parts• Handles changing density, dielectrics,

conductivity, temperature, pressure, pH and viscosity

• Handles small tanks, difficult tank geometry, interfering obstacles

• Easy swap (small openings) • Virtually unaffected by dust, vapor and

turbulence

GWR Pros

• Solids, powders, granules• Level and Interface Level from one

transmitter• Guided Wave Radar technology is virtually

unaffected by probe coating» Recalibration not necessary » Recalibration cost dramatically reduced

• Measurement availability improved

GWR Pros

• Reducing CAPEX Cost Reducing CAPEX Cost – Low installation cost– Installation while tank is in

service– Easy Commissioning

• Pre-configured transmitters• No re-calibration required• User-friendly Configuration tools

GWR Pros

• Reducing OPEX Cost Reducing OPEX Cost – Increased Availability

• No re-calibration• Entire tank range• Advanced Signal Processing• No moving parts – less down-time

– Increased Safety, Health & Environment• Dual Compartment• Configuration adjustments without opening tank• One point entry on top of tank

GWR Pros

• Reducing OPEX Cost Reducing OPEX Cost – Reduced Operations & Maintenance

• No matching parts• No moving parts, – less spare parts, less

maintenance• Remove electronics while tank is in service• Easy troubleshooting

– Higher Quality• Reliable, accurate level measurement• Virtually unaffected by process conditions