Improving Level Measurement for Solids

Selecting the right technology to ensure accurate and reliable level measurement can help solve the unique challenges posed by storing and processing solid materials

Advanced measurement solutions are helping to enhance process control, operational efficiency and safety throughout the chemical industry. That is certainly the case in measurement applications involving solids, which can be notoriously challenging, especially when it comes to accurately and reliably determining levels in tanks and vessels. Having continuous insight into the level of solid materials, such as powders or pellets, enables organizations to optimize their processes, suffer fewer interruptions and achieve higher levels of site and worker safety. For example, knowing the amount of material contained in a vessel enables it to be filled to its capacity, thereby helping to ensure material availability and prevent any adverse impact to the process. Monitoring the level in the vessel also helps in preventing overfills, which could cause a safety incident, and run-dry situations, which could damage equipment and cause costly process interruptions.

Technology selection

When selecting a level measurement device for use in a solids-handling application, it is important to understand the purpose of the measurement. For example, is it for detecting when the material level has dropped below a predefined minimum, so that it can be determined when the material needs to be replenished, and a run-dry situation can be avoided? Is it for detecting when the material is nearing — or has exceeded — the capacity of the vessel, to avoid overfilling and spills? Or is the aim to continuously monitor the level, to optimize inventory management?

There are also very specific process characteristics to consider in solids applications, and these present challenges that can often best be met by level measurement devices with special features.

First and foremost, it is important to note that measuring the level of solids is typically more challenging than measuring liquid levels (Figure 1). Whereas liquids settle to an even level and can therefore be measured at any single point on the surface, solids surfaces are usually uneven, peaks and troughs constantly changing as vessels are filled and emptied. Solid materials often have a low dielectric constant (DC), making them hard to measure, and applications usually involve high levels of dust, which can impact level-measurement accuracy when using certain technologies. Furthermore, solids often have very abrasive tendencies, and as with many liquid chemicals, they can be corrosive.

No single level-measurement technology is a perfect fit for all solids applications, so the decision of which technology to select typically depends on the application in question.

Continuous level measurement

Radar is widely recognized as the best technology to provide continuous level measurements, either in the form of a non-contacting transmitter or a guided-wave device. Non-contacting radar is an all-round technology that is suitable for use in a large variety of applications. However, the latest non-contacting radar transmitters based on frequency-modulated continuous wave (FMCW) technology have overcome the problem of high processing power requirements using radar-on-chip technology, which replaces the traditional circuit board. As a result, these devices only require two wires for power and communication, enabling end users to benefit from the superior accuracy and sensitivity of FMCW technology without needing to install costly additional infrastructure.

Modern FMCW non-contacting radar level transmitters can provide robust measurements of both a small surface area and in large storage silos. When paired with a parabolic antenna, these devices can be used in silos as tall as 295 ft.

Radar level transmitters are more reliable and versatile than, for example, ultrasonic level transmitters. They are able to maintain their measurement accuracy despite fluctuations in temperature, pressure, or the presence of dust. The latest devices feature superior signal processing. This helps them to disregard false echoes from internal obstructions and inconveniently placed nozzles, thus helping to ensure accurate measurements. Devices are also available that feature specific solids algorithms that enable an average level to be calculated from a footprint instead of one point. These devices also respond rapidly to level changes, making them suitable for process applications and those involving smaller vessels.

Guided-wave radar

Guided-wave radar devices guide low-energy microwave pulses down a probe, which is submerged into the process medium. When the pulse hits the process medium, a significant proportion of the energy is reflected up the probe to the device, which enables the level to be measured. Guided-wave radar provides a higher signal return than non-contacting radar due to direct switching technology, which increases measurement reliability in very tall silos containing low-reflective materials.

Guided-wave radar can be used in many different applications. It is ideal for measuring the level of powders and small granular materials with low DC, for smaller vessels with diameters of less than 33 ft, where the installation area is restricted, and levels are changing rapidly. Guided-wave radar is virtually unaffected by dust, moisture, density changes and temperature (Figure 2). Although suitable for long ranges, as the vessel height increases, wear on the probe becomes more of a factor in the suitability of its use. Devices utilizing probe-end projection functionality help to expand the viable range and suitability for media with a low DC. Guided-wave radar is less appropriate for heavier, large-particle solids, as the stress from the weight of the materials may cause the probe to break.

Ultrasonic

Another top-down non-contacting technology that can be used for continuous level measurements is ultrasonic. Ultrasonic level devices work in a similar way to radar transmitters, with the time taken for an ultrasonic wave to reflect off the surface of a solid material being used to calculate distance and level. The technology will work when the reflected echo is strong enough, but ultrasonic waves tend to dissipate over long distances, which means that they have a limited effective range and struggle with applications involving tall tanks and vessels, windy open-air applications and those with high noise levels. Ultrasonic sensors are also not appropriate for use with materials that absorb the sound, as this impacts the quality of the received echo.

Dusty conditions, large particle sizes, internal vessel obstructions and the formation of clumps on internal vessel surfaces all impact the way the sound wave reflects off the surface and can generate false echoes, making it very difficult for them to detect the true level signal. Environmental factors, such as mechanical vibration, can affect ultrasonic signals, thus impacting measurement accuracy. The measurements provided by ultrasonic devices are unaffected by media properties, such as dielectric constant and density, but ambient temperature and humidity can impact their accuracy.

Point-level sensors

Solids level switches, also known as point-level sensors, enable full, demand or empty detection for all bulk media in all types of vessels. These devices are robust and reliable, very simple to use and install, and insensitive to dust, electrical charge and adhesion. They are designed to support simple process-control applications and cope with extreme conditions, such as high temperatures, pressure, mechanical stress and tensile forces. To ensure a safe process, multiple devices can be installed in a vertical, horizontal or oblique position.

Solids level switches utilize several different measurement technologies, each suitable for specific application requirements.

Rotating paddle switches utilize a simple electromechanical measurement principle and have a robust design which makes these switches suitable for extreme process conditions, such as high pressures and temperatures. Paddle switches are a popular low-cost option for use in small process vessels and applications involving most bulk solids. They are widely applied in applications involving granular, pelletized and powdered dry products.

Vibrating-fork switches are highly reliable and have extremely low maintenance requirements. They perform well with low bulk density and fine-grained and fine-powdered products. In addition, vibrating-fork switches can withstand high mechanical loads due to their short extension length. For applications with high levels of vibration, fork switches are available with special electronics and remote housing options. Short response times ensure their suitability for fast-flow applications.

Vibrating rod switches have a simple but robust design that makes them extremely reliable. The technology is unaffected by the dust that can be created during filling cycles and has good resistance to media that are susceptible to caking and clogging. This technology is ideal for measuring fine-grained and powdered products. Some vibrating rod switches have approvals to use in hazardous and explosive environments.

Capacitance switches operate by measuring the capacitance between the probe and container wall. These devices are a cost-effective option that can tolerate a variety of challenging process conditions, including variable density, low dielectric values, high temperatures and high pressures. This makes them a good all-around technology, suitable for use with most bulk materials, regardless of particle size. The latest devices offer functionality that ensures reliable monitoring even if there is media buildup or caking caused by moisture.

Measuring uneven surfaces

Selecting the appropriate level measurement device for the application is essential, but no matter which technology is chosen, all level measurement instruments are affected by the uneven surfaces encountered in solids applications. The characteristics of the material and the size of the vessel will affect the structure of the solid’s surface, which subsequently impacts the choice of level measurement technology and its preferred installation location.

As previously mentioned, most technologies for measuring the continuous level of solid materials involve top-down devices that depend on a signal reflecting from the surface back the transmitter. Guided-wave radar is less affected by uneven surfaces since the microwave signal is more compact, guided by the probe and reflected from the contact point on the surface.

Non-contacting radar is impacted by uneven surfaces, as some of the signal may not be reflected directly back to the transmitter and instead redirected away from the device. To address this, the latest advanced non-contacting radar transmitters have a specific setting for solids and collect echoes from a larger footprint and process it into a representative measuring value. As the antenna size increases, the radar signal becomes more concentrated. While the overall surface area is reduced, the return signal is strengthened. Larger antennas are best suited for long measurement ranges. For larger solids piles and silos, it may be necessary to deploy several radars to ensure sufficient data is available.

Low-DC materials

Materials with a low dielectric constant and low density may attenuate signals, so that they become insufficient for accurate and reliable measurements. The dielectric constant of many solids is low. For radar technology, this is a key indicator of the amount of signal that will be reflected to the gauge and thereby the possible measuring range. For level switches, capacitance technology is affected by the bulk-solid dielectric constant. Both radar and capacitance can handle low dielectrics with ease. Radar is unaffected by bulk density, but this is not the case for some solids switches. Bulk density therefore impacts the selection of point level measurement devices.

An example comes from applications involving the direct loading of pellets of common polymers, including polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC) and polypropylene (PP), into storage silos from trucks. For compounding production, these materials are transferred from the silos to the molding area via smaller batch silos. The pellets and additives are then molded into the final product. Both PP and PE pellets have a low dielectric constant, creating challenges in achieving accurate level measurement. Varying surface characteristics and static charges also must be considered in such cases.

Previous monitoring solutions proved to be unreliable, causing false alarms during filling and discharging. These false alarms created production interruptions. Incorrect readings caused significant drifts between reported and actual level measurements. Guided-wave and non-contacting radar technologies perform expertly and reliably under these conditions. In this case, a guided-wave radar level transmitter was recommended and installed. The immediate outcome was the elimination of the false alarms, which helped to increase production, while accurate inventory management also helped to increase control of production orders.

Dust challenges

There is often a considerable amount of dust created during the fill cycle of solid materials, and level switches can handle dust in the vapor space. Ultrasonic devices are less suitable, since their signal is significantly impacted by dust. Should dust form a heavy layer on the radar antenna, it can block the signal. This can be mitigated using non-stick antenna materials and air purging.

A good example of this can be found within PVC production. To closely monitor inventory, INEOS Inovyn wanted to continuously measure the level of PVC powder in a 23-meter-high silo at its plant in Stenungsund, Sweden (Figure 2). As with all solids level measurement, the varying surface reflection made this a challenging application, but this was amplified by the fact that PVC has a small particle size. Dust generated during filling was a major issue. Non-contacting radar had been used to provide the measurement, but buildup was impacting the reliability of the measurement. Regular maintenance (cleaning) was therefore required, which interrupted the production process at often inopportune times. A non-contacting radar level transmitter with a process seal antenna was utilized, with air purging that prevented buildup on the antenna. The transmitter has an algorithm for solids, which has enabled a stable reading, and the new solution has significantly reduced maintenance interruptions.

Another challenge is when condensation may be present in the vessel, especially at the coldest point — the vessel ceiling where top-down measurement devices will be located. Condensation can tie up dust and create a layer on wetted parts which can cause problems if no action is taken.

Positioning devices

Best practice is to implement both continuous and point level measurement instrumentation, with multiple devices installed in different locations, to ensure a safe and reliable operation.

It is important to consider where in the vessel the level will be changing, depending on the material characteristics, vessel size and filling location. The mounting location of measurement devices in relation to the filling location is important. The closer the device is mounted to the filling point, the larger the risk of measurement interference. Non-contacting radars should not be mounted in the center of the silo or very close to the tank wall.

For guided-wave radar, it is recommended to mount the probe as far away as possible from the filling and emptying ports. This will minimize load and wear and will help to avoid disturbances from the incoming product. The best practice is to have a free-hanging probe, although in some applications anchoring may be required. If anchored, the probe should be slack to reduce the risk of breakage. For applications such as pellets, electrostatic charges can build up and eventually discharge. By anchoring the probe, this provides a good earth ground, helping to protect the guided-wave-radar transmitter electronics.

Open-air level measurement applications, such as monitoring the amount of materials on conveyor belts, the size of piles and distance control between conveyor belts and the pile, have different characteristics from standard bin or silo applications. There are obviously no walls or roof to install instruments onto, so the biggest challenge in these applications is usually to find an installation point. Protection from external factors like wind and rain can also be a challenge. Non-contacting radar would be the appropriate technology (guided-wave radar is not suitable) for these applications, but it is essential that free propagating radar complies with the national radio directives when installed in open air.

Concluding remarks

Measuring the level of solids can be a challenge, but obtaining accurate and reliable measurements can be achieved through correct technology selection and installation. Once the specific characteristics of the application are known, including material type, particle size and DC, vessel size and shape, speed of filling and so on an instrumentation vendor can recommend the appropriate solution.

Edited by Scott Jenkins

Author

Jenny Leion is global chemical industry manager at Emerson (8027 Forsyth Blvd., St. Louis, MO 63105; Phone: 888-889-9170; Email: jenny.leion@emerson.com; Website: www.emerson.com/rosemountsolidslevel) She has extensive work experience in the chemical industry, spanning both operations and development. Her background includes working on automation solutions, process optimization and capital expansion projects, equipping her with the expertise to deliver practical, hands-on solutions for chemical industry applications. She holds a M.S.Ch.E. with engineering physics from Chalmers University of Technology.