Measuring bulk solids: selecting the right technology — Part 1
Measuring the level and volume of bulk solids and powders in a vessel is challenging, so choosing the right measurement technology for the application is critical.
When it comes to the measurement of level and volume for bulk solids, there are drawbacks in using traditional mechanical measurement devices, in contrast to the advantages of automated technologies such as non-contacting radar, guided wave radar, and acoustic phased-array antennas. Each technology has its relative strengths and weaknesses, and so will be more or less suited to each application.
Measuring bulk solids
Measuring the level and volume of powders or bulk solids within a vessel is more complex than measuring liquid level, and the most fundamental challenge is the uneven and shifting nature of the material surface. When determining liquid level, there is only the need to measure a single point on the surface, because the level at that one location will be the same as at any other point within the vessel. However, the same is not true in solids applications, where the material surface is rarely flat. Instead, the surface consists of a multitude of peaks and troughs that constantly change as the vessel is both filled and emptied. Depending on how much the material in question can pile before sliding, the number of filling and emptying points, and the width of the vessel, the difference between the level of a peak and a trough can be very large. The particles of the materials can also vary from very fine powders to large rocks. And these variations can also have an effect on the effectiveness of measurement devices.
Additionally, since solids are dry materials, there is typically dust in the space above them, especially during filling. This dusty environment can cause issues with different measurement technologies. And dust mixed with moisture can create build-up on the surfaces of measurement devices.
Solids can also have application conditions which change significantly over time in a way that’s not obvious from outside the vessel. For example, the wood chips, ore or grain received in spring can be completely different from what is delivered in the summer, and can sit completely differently in the vessel with a different angle of repose. Perhaps there is changing moisture content, humidity or particle size, or different settling, or compaction. Unlike liquids, solids applications can be difficult to cycle from 0–100% during commissioning to find any reflective ‘surprises’ under the surface. Because of this, solids applications may need the configuration checked or tweaked periodically, especially on larger vessels. Having a remote connection to the devices via a mobile service or a network connection can be extremely beneficial.
Solids vessels are often quite large, and measurements are made from the tops of them. Automating measurements is an important key to keeping workers on the ground instead of requiring them to climb to the top of the large vessels, which raises safety concerns and wastes valuable time.
Upgrading from mechanical or manual methods
Mechanical devices such as yo-yos have been used for many years to perform solids level measurement. These devices lower a weight onto the surface of the material. By measuring the length of wire required for the weight to touch the surface, the level of the material can be determined. These systems require regular maintenance — putting personnel at risk by being exposed to hazardous conditions on tall silos — and they have limitations in terms of accuracy, reliability and repeatability of measurement. Additionally, the weight can continue to move past the top surface, so you are not getting an accurate point measurement. If the measurement is made during the filling cycle, the weight and cable could experience pull forces. Also, if the weight breaks free for any reason, it can cause damage to machinery downstream. All of these issues affect the reliability of these systems.
Measurements of this type can be made by automated means or done manually. Some silos have no automated systems. Manual measurements require operators to climb the silo to take the measurement and may have an impact on the repeatability of the measurement. They may have operators using a yo-yo or another method to take manual measurements from the top of the vessel. Many manual measurements methods continue to be used because that is the way it has always been done or because there were issues with using or implementing an automated system. Much progress has been made on alternative automated systems for solids measurement. Consequently, many operators of modern production plants have chosen to upgrade to continuous automatic measurement and control technology, helping to improve safety, reliability and repeatability and enabling accurate and reliable continuous measurements to be accessed from remote locations such as a control room.
Some of the most common technologies for solids measurement include non-contacting radar, guided wave radar, acoustic phased-array antennas, ultrasonics and load cells. Although ultrasonics and load cells have traditionally been used for solids measurement, ultrasonics can have issues with dust, and load cells need regular calibration and have limitations for vessel sizes. Non-contacting radar, guided wave radar and acoustic phased-array antennas are newer technologies and have quite a few advantages for solids measurement, although selecting the appropriate technology depends on the application.
Solids measurement applications can be divided into two main types. The first is continuous level measurement in smaller vessels used within the production process. The main benefit of a continuous level measurement is that it can be used to control a process, or make sure that there is material available. In such applications, quick changes in surface level can occur due to the speed at which material enters and exits the vessel. This requires technology that can perform very fast level measurements and respond to these changes quickly.
The second application type is volume measurement in larger vessels or warehouses used either in the production process or for bulk storage, which is often related to a demand for inventory control. In these applications, there is a much greater surface area to monitor and inaccurate measurement can lead to a huge discrepancy in product volume; there are relatively big vessels in the production process, as well. Here, the surface level changes at a much slower rate and, therefore, device speed is less relevant — but greater accuracy is required to support better inventory management. When selecting a measurement device, it is important to understand whether volume or level is the desired primary measurement, as this will influence the technology choice.
Guided wave radar
Guided wave radar level transmitters provide continuous level measurements, based on microwave technology. Low energy microwave pulses are guided down a probe, and when the microwaves are reflected from the material surface back to the transmitter, the level can be measured. If the dielectric constant of the material is low, a proportion of the emitted pulse continues down the probe, which allows the probe end to also be detected.
Guided wave radar level transmitters are especially well suited for smaller vessels with a diameter of less than 10 m containing powders and small granular materials, and where the installation area is restricted. These devices can use probe-end projection functionality to allow for measurements when the surface pulse is too weak to be detected. This commonly occurs when the material dielectric constant is very low, especially in combination with a long distance to the surface, or electromagnetic interference. When the dielectric constant of the material being measured is low, only a portion of the electrical signal is reflected off the top of the material. The rest of the signal continues down the probe. When the signal reaches the end of the probe, there is a strong reflection. Since the microwave signal propagates more slowly in the material than it does in air, this echo is seen at a distance further than the actual probe end. With probe-end projection technology, the actual probe length, the probe end reflection echo location and the dielectric of the material can be used to calculate the level of the material when the initial reflection from the top of the material is not strong enough to make a direct reading. This function is recommended for solids with a dielectric constant less than or equal to two (eg, perlite or plastic pellets).
In solids applications, the material can cause down-pull forces on vessel roofs, so the vessel roof must be able to withstand the maximum probe tensile load, which depends on silo size, material density and the friction coefficient. Forces increase with the buried length, the vessel width and the probe diameter. A flexible single lead probe is the most suitable choice for guided wave radar in solids applications, but the tensile load should be calculated and the most appropriate cable thickness should be used. And an anchored probe will increase the forces on a probe by two to ten times.
Non-contacting radar level transmitters also provide continuous level measurements, but there is no contact with the material surface. Pulse radar or frequency modulated continuous wave (FMCW) techniques are used to perform the measurement. FMCW devices can improve the measurement of solids compared to pulse transmitters because they continuously send out microwave energy, meaning that the total amount of energy sent to the surface will be much higher. In practice, this means that FMCW transmitters are much better than pulse devices at determining weak echoes within a noisy environment. FMCW devices also have much higher resolution than pulse radars. With pulse radar, microwaves are emitted towards the material and reflected to the sensor, with the level being directly proportional to the time taken between the microwave signal being transmitted and received. In FMCW devices, the device transmits a continuous signal sweep with a constantly changing frequency. The frequency of the reflected signal is compared with the frequency of the signal transmitted at that moment. The difference between these frequencies is proportional to the distance from the radar to the surface, which enables the level to be measured. Inclining or sloping surfaces deflect energy away from the radar and can generate several small reflections.
By using a dedicated solids algorithm, some non-contacting radar level transmitters can provide high reliability even with very fast changes in level. Non-contacting radar devices still see only the portion of surface within their beam angle. Like guided wave radar, this makes them a suitable choice for applications using smaller vessels or silos, where fast movements are possible but accurate volume measurements are seldom needed. Unlike guided wave radar, there are no restrictions with respect to the weight of the material and pull forces. If the instrument is sufficiently energy efficient that it requires only two wires for power and communication, then additional cabling infrastructure will not be required.
In Part 2
In part 2 of this article we will look at the benefits of acoustic phased-array antennas, as well as the various challenges of bulk solid level measurement for different technologies.
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