Measuring bulk solids: selecting the right technology — Part 2

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.

Measuring the level and volume of bulk solids and powders in a vessel is considerably challenging. In Part 1 of this article we examined the automated versus manual methods and looked at radar technologies. An alternative technology to radar comes in the form of the latest 3D solids scanners, which use acoustic phased-array antennas to deliver multiple point volume measurement and enable remote visualisation of a surface, therefore providing an unmatched degree of inventory control.

Acoustic phased-array antennas

3D solids scanners, based on acoustic technology, include an array of three antennas, which generate a mix of three audible dust-penetrating low-frequency acoustic signals, and receive multiple echo signals from the contents of a storage vessel. Using these integral antennas, the scanners continuously measure the direction and distance of each echoed signal, and generate a coordinate for each of the echoes inside the vessel. The built-in digital signal processor digitally samples and analyses the echoed signals and produces accurate measurements of the level and volume of the stored contents by mapping all of the signals across the entire surface within the beam angle of the device.

The wide beam angles produced by acoustic phased-array antenna devices are especially suited to very large vessels, which are typically used in bulk storage inventory applications. In these large vessels, the angle produced by the three frequencies used (2.7, 4.5 and 7 kHz) allows more of the surface to be ‘seen’ giving the greatest measurement accuracy with the fewest number of devices.

Figure 1: Example of an acoustic phased-array solids scanner.

When the correct number of devices are installed in the right locations on a vessel, this enables the entire surface to be seen by the devices. This allows for optimisation of vessel storage capacity and can greatly improve optimisation of production efficiency.

3D mapping

Acoustic phased-array antenna devices not only provide continuous online volume measurement but also offer visualisation of the various peaks and valleys within vessels. The ability to visualise the formation of the material is important because uneven sidewall loading caused by uneven filling and emptying can cause a bin or silo to collapse, with potentially catastrophic consequences. 3D mapping provides a better understanding of how the material is distributed within the vessel, and therefore helps to prevent the threat of structural damage to the container. Also, as the 3D mapping is displayed on remote computer screens, it eliminates the need for personnel to be put at risk by climbing vessels and being exposed to harsh conditions.

Matching the received data with known vessel dimensions allows 3D solids scanners to calculate product volume, enabling the immediate and continuous accurate listing of inventory value for accounting and financial reports. It also allows for annual or rolling inventory measurement, which is a primary requirement in preventing the over-purchasing or under-purchasing of products.

Efficient inventory management allows companies to have the right amount of stock in the right place at the right time, and ensures that capital is not tied up unnecessarily. Replacing a level measurement device with a 3D solids scanner can immediately save between 8% and 13% of the on-hand inventory cost. Based on an annual inventory carrying cost of between 25% and 52% of on-hand inventory, this translates into a lot of money and a quick return on investment.

Figure 2: 3D mapping using a solids scanner.

Advanced features of acoustic phased-array antennas

New features allow for creating individual virtual sections within vessels and monitoring the average, minimum and maximum level within those particular sections. The virtual sections feature allows for use of a controls system to keep overall levels across the vessel even.

An additional new feature is the ability to set the centre of gravity per silo. Using this feature, when the centre of gravity moves outside the designated zone, an alarm is given. This provides a warning that the centre of gravity has shifted and there may be some structural concerns for the vessel itself. These additional features add even more safety and controls capability for solids monitoring.

Application characteristics

Whichever technology you select, there are certain variables that can affect accuracy and reliability in solids measurement. Therefore, it is important to select the right technology to best overcome each of these variables discussed below.

Uneven surface

Most technologies for measuring level or volume of solid materials are top-down measurements and depend on a signal reflecting from the surface back to the device. Guided wave radar is less affected than non-contacting radar by uneven surfaces since the microwave signal is more compact and guided by the probe. But that advantage can be quickly lost by the realisation that a probe is required and less of the surface is seen. Non-contacting radar is affected by uneven surfaces since much of the signal is not reflected directly back and instead may be redirected away from the device. For best results, the device needs to gather several smaller echoes concentrated in an area and then merge them into a single echo and have an effective way to decipher between the surface echoes and noise. Some newer non-contacting radar devices take advantage of advanced algorithms which do this step seamlessly inside the device.

Acoustic phased-array technology is not affected by the uneven surfaces and it triangulates all the measurement over a wide surface area, resulting in high accuracy volume and average level calculations.

Dielectrics and bulk density

The dielectric constant of some solids might be fairly low. For radar technology, this is a key indicator of the amount of signal that will be reflected back to the gauge and therefore the possible measuring range. Devices based on radar technology are not affected by bulk density. The acoustic technology is not affected by dielectric properties but can be affected by bulk density, although most solids materials do not absorb enough of the acoustic signal. In general, a bulk density below 0.2 g/cm³ should be looked at carefully.

Dust

There is often a considerable amount of dust created during the fill cycle in solids applications. The amount of dust depends on the type of filling and the material. Radar and acoustic phased-array antenna technologies can both handle dust in the vapour space fairly well. However, a heavy layer of dust on the antenna can block the signal. With non-contacting radar, an air purge system may be required. However, some users may be reluctant to use air purging due to the cost of maintaining the airflow, or the fact that air purging may disturb the process. An alternative solution is provided by devices with a process seal antenna. With guided wave radar, the natural flexing of the probe can knock off excessive dust build-up. The fact that acoustic devices operate at lower frequencies is an advantage in dusty applications, because low-frequency soundwaves are absorbed less as they travel through dust than high-frequency soundwaves. Also, the vibrating membranes in acoustic devices drive out any dust particles that coat the horns, making them inherently self-cleaning. In applications where the dust is especially sticky, non-stick antenna materials may be necessary.

Figure 3: Non-contact radar air purging.

Condensation and sticky build-up

Condensation is present in many solids applications — with the vessel ceiling being a common location, as this is normally the coldest spot. Unfortunately, this is also where top-down measurement devices are located. Condensation can also tie up dust and create a layer on the wetted parts that may cause problems if no action is taken. Even without condensation, some materials may just inherently create a sticky build-up. Guided wave radar is not affected by condensation, so is a good choice for applications where there is extreme condensation. But build-up on the probe may need to be monitored and devices that can monitor the signal strength using advanced diagnostics are helpful in these situations. Non-contacting radar may need air purging or a process seal antenna to cope with condensation-related issues, and sticky build-up can be minimised with these solutions as well. Diagnostics for signal strength can help determine if and when additional cleaning might be needed. Acoustic phased-array antenna technology includes self-cleaning functionality, which reduces the need for maintenance, but caking build-up can affect signal strength. A regular maintenance schedule may be required for extreme instances. Using a PTFE antenna can help reduce maintenance requirements in these cases.

Mounting location

The mounting location of a measurement device in relation to the vessel’s filling location is important for most measuring technologies, as dust and the actual stream from the filling can disturb the measurement to a large extent. The closer the device is mounted to the filling point, the larger the risk of measurement interference. There are also cases where the material is blown into a silo through a pneumatic process. Due to the nature of acoustic phased-array antenna technology, measurements can be affected during such filling but the effect decreases with increased silo size.

With guided wave radar devices, the probe should be mounted as far away as possible from filling and emptying ports, to minimise wear and help avoid disturbances from the incoming product. Also, the probe should be regularly inspected for damage. The minimum recommended probe distance to silo wall or disturbing objects is 50 cm and the probe should not be able to touch the wall of the silo during operation. Best practice is to have a free-hanging probe but an anchored probe is sometimes needed for application reasons. The probe end should not be fixed for 30 m or longer probes. The probe must be slack when anchoring, to reduce the risk of breakage.

Non-contacting radar devices should not be mounted in the centre of the vessel or very close to the silo wall. General best practice is to mount non-contacting radar devices at 2/3 silo radius from the silo wall. The inlet stream of the product will interfere with readings if it is in the path of the non-contacting radar beam. Cone antennas and parabolic antennas should be mounted perpendicular to the ground and should protrude at least 10 mm into the vessel.

Acoustic phased-array antenna devices should also be mounted perpendicular to the ground to ensure highest accuracy. Devices should be mounted with the antenna at least 10 mm below the standpipe of the nozzle. It is also important to know the location of any obstacles in the vessel, as some obstacles may affect the measurement. If an obstacle can’t be avoided by relocating the device, a neck extension can be used to extend the antenna past the obstacle. To get the highest accuracy, it is important to make sure the distance to walls and filling points is at least 600 mm and to use the recommended device quantities and locations as provided by the manufacturer.

Noise

Many bulk solids applications are in noisy environments. The noise can be generated by running engines, conveyor belts or during filling/emptying, depending on the method. Sound has no effect on radar-based devices. Acoustic devices might be impacted by noise around the 2.3, 4.5 and 7 kHz frequencies. However, it is rare that all three frequencies are disturbed at the same time and an acoustic phased-array antenna device can work even if two frequencies are compromised. The effect can often be mitigated with a different location of the device or possibly a different configuration.

Electrostatic discharges

In some applications, such as plastic pellets, electrostatic charges can build up and eventually discharge. Guided wave radar devices are most suitable for these applications. While their electronics can tolerate some static charge, providing a good earth ground for the electronics by anchoring the end of the probe to the vessel will create ground paths for discharge away from the electronics. If the product can build up static electricity, the probe should be properly grounded.

Open air applications

Open air applications include measurements on piles and distance control between conveyor belts and the pile. These types of applications have different properties compared to standard vessel applications. There are no walls or roof on which to install instruments, so the biggest challenge in these types of applications is to find an installation point. Protection from external factors like wind and rain can also be a challenge. Non-contacting radar or acoustic phased-array antenna technology is recommended in these types of applications. Non-contacting radar devices are not affected by outdoor conditions, though can only deliver a single-point measurement, while acoustic phased-array antenna devices will not be affected if the wind speed is less than 30 km/h.

Summary

Bulk solids measurement involves many different variables. With the variety of technology choices, this information should help you be better prepared to evaluate these technologies for your specific application.

Top image: ©stock.adobe.com/au/Chris Van Lennep