Solids Level Measurement Technology Selection Considerations

Emerson's Lydia Miller

Solids, unlike liquids, can have an uneven surface, making level measurement very difficult. In a Control Engineering magazine article, How to choose the right level instrument for applications, Emerson’s Lydia Miller shares tips on selecting the right level measurement technology for your application.

solids-level-measurementThree primary factors you must consider include:

  • Specific material characteristics
  • Installation considerations
  • Measurement needs

Lydia opens noting level characteristics for solids:

Solids, on the other hand, form piles. If a pipe or chute is sending granules into a vessel, the highest point will be directly under the pipe. The difference between the highest and lowest point in the vessel may be great, or it may be quite uniform, depending on the material and other factors.

The angle of repose or steepness of the pile determines when the solids material will fall down the sides:

Spherical plastic pellets will not pile up very high because they roll down the sides of their own hill. Other products, even if they aren’t sticky, can form higher piles due to the particle shape or natural cohesiveness. Under most circumstances, few products have an angle of repose below 30 deg or above 45 deg. Even wet sand, if allowed to move freely, will not pile up steeper than 45 deg.

Three of the most common technologies for solids level measurement include:

  • Guided-wave radar using a probe to direct the pulse travel
  • Noncontacting radar
  • Acoustic

Guided wave radar (GWR) measurement includes a rigid or flexible probe down which the radar signal travels until reflected back by the solids material. Lydia highlights some considerations when using GWR technology:

Some probes are made from flexible cable while others are rigid rods. Flexible probes are better for solids because when large amounts of material start moving, the forces can bend or even break a rigid probe due to the unevenness of movement, particularly when the vessel is filling or emptying.

Probes can be yanked out of the instrument housing, or worse, a well-reinforced probe can simply pull the top of the vessel down. But the tensile force can be calculated to avoid these situations. Depending on the material, frequent movement close to the probe can also cause abrasion and premature wear. Still, in the right application, a guided-wave radar instrument can be a very accurate and economical choice.

Noncontacting radar does not use the probe but rather an antenna to read the radar signals bouncing back off the surface. Accuracy:

…depends on the strength and repeatability of the return signal. The characteristics able to create a strong signal make for major differences between radar and acoustic methods. Radar instruments depend primarily on the dielectric constant (DC) of the material, while acoustic instruments depend on bulk density.

This measurement technology is good for production silos up to 20 meters tall, handles rapid level changes and is a good fit for narrow silos or where obstacles are present.

The angle of solid off which the radar signals reflect can pose a challenge:

…if the pulse hits a slope, part of it can be reflected to the side of the vessel and not captured… In most situations, enough of the signal is returned to get a usable measurement, but if the material has a low DC and a high angle of repose, it makes for a difficult combination. Special solids algorithms in the instruments and parabolic antennas can help with measurements in solids applications.

Solids level and volume measurement reflections

Acoustic 3D Solids Scanner level information:

…can create a topographical map of the surface inside the vessel. In very large vessels, individual pictures from multiple instruments can be knit together into a very precise larger image covering the surface of the material from wall-to-wall…

Acoustic level measurement is preferred when larger surface areas need to be measured or precise volume measurements are required.

Read the article for more on considerations in selecting the right measurement technology for your solids level measurement application. You can also connect and interact with other level measurement experts in the Level group of the Emerson Exchange 365 community.

Posted Tuesday, December 13th, 2016 under Level.

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