Make the Most of Radar

Following some best practices can ensure accurate level measurement.

By Sarah Parker, Emerson Process Management

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Such bypass pipes (Figure 3) provide a calmer surface in case of turbulence. They offer external mounting with valves, allow for easier servicing of level devices and enable radar measurement in tanks such as towers that only offer side connections. Always provide multiple holes or slots on one side of the pipe to ensure good fluid flow-through.

Both GWR and non-contacting radar work well for pipe or stilling well applications, but GWR is far simpler to install and maintains accuracy and sensitivity independently of the pipe. For these types of applications, correctly dimensioning the chamber and selecting the appropriate probe are essential.

Choose 75-mm or 100-mm chambers. Smaller ones raise risks of build-up and flow-through problems, and increase the chances of the probe contacting the chamber wall.

Select a single probe, as it is less susceptible to build-up. Most chamber installations rely on rigid probes but flexible ones also may also be used. Ensure the probe is suspended vertically and does not touch the pipe wall. A "centring" disk can keep the probe properly positioned away from the wall.

For shorter pipes, use GWR with rigid probes. GWR with flexible probes can work in long pipes, but the probes must remain taut and away from the wall. A combination of weights and centring disks can achieve this. For longer pipes or when head space for installing probes is limited, opt for non-contacting radar.

Several types of applications raise particular issues.

High temperature and pressure. Applications with extreme temperature and pressure conditions demand 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, you must insulate and heat-trace the chamber. Fluctuations in temperatures alter the density and volume of the liquid, which then affect the level in the chamber. Maintaining the temperature of dirty liquids such as heavy oil also helps avoid clogging and sticking within the chamber and enables adequate flow-through.

Although radar technology is not affected by density changes, dielectric variations can have an impact. For boiler and feed-water systems where boiling water and high-pressure saturated steam vapors are present, the returned signal from the surface becomes weaker as water temperature increases. In addition, the saturated steam alters the propagation velocity of the radar signal and generates an error in the level reading proportional to the measured distance. Dynamic vapor compensation can address the changes in the vapor-space dielectric and reduce the incorrect distance caused by varying pressure or temperature to less than 2%.

Interfaces. The fluid with the lower dielectric must always be on top and you must know its dielectric value. The two liquids must differ in dielectric value by at least around ten. Effective measurement requires certain thicknesses of layers. Typical successful applications have a hydrocarbon-based fluid with a dielectric around two as the top layer and water-based fluid with a dielectric over 40 at the bottom.

Applications where the densities of the two fluids are very close or where emulsifiers are used can produce fairly large emulsion between the liquids. This may make the interface indistinct. Heavy and thick emulsion layers or liquid layers with similar dielectrics can pose a problem for GWR because it requires a distinct dielectric difference to detect the interface. GWR devices have proven to work in interfaces with emulsions but success is difficult to predict. The interface threshold on the radar device may require manual adjustment.

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