How Advanced Automation Technologies Meet Solids Level Measurement Challenges

The ability to accurately and reliably measure and control the level of solids materials in process vessels and storage silos is essential in a broad range of applications across many industries, including chemical, minerals, metals and mining, and power.

This not only helps organisations to optimise their inventory management, but also increases safety when protecting against overfills. However, measuring the level of solids is not straightforward.

Whereas a liquid level will typically be identical throughout a vessel and can therefore be measured at a single point on the surface, solids surfaces are uneven, with peaks and troughs constantly shifting as vessels are filled and emptied.

In addition, solids applications often involve challenging environments where vibration, large amounts of dust, and materials with low dielectric constants can affect the accuracy of certain level measurement technologies.

Manual or automated measurements

Mechanical devices, such as yo-yos – where a length of wire lowers a weight onto the surface of the material – were once typically used to measure the level of solids. However, these devices are rudimental by modern standards and require workers to climb tall vessels to take manual measurements, thereby exposing them to risk and introducing the potential for human error. Also, as with all mechanical devices, they need regular maintenance to prevent them from failing at inopportune moments.

In contrast, modern digital automation technology enables personnel in a control room to immediately access data from remotely located tanks, thereby enhancing plant efficiency. Moving away from the use of manual measurements and mechanical devices to automated technology increases the accuracy and reliability of the measurement. It also reduces maintenance costs, increases worker safety and is less time-consuming, enabling personnel to focus on other tasks.

Solids level measurement technology selection

When considering which automated technologies to implement for solids level measurement, organisations first need to consider whether the application requires point-level detection or continuous measurement.

Point-level switches provide limit detection to prevent overfills, spills and run-dry/empty situations, which could result in costly unplanned downtime and potentially hazardous clean-up and repairs. Continuous level measurement is performed to provide uninterrupted access to data, providing visibility of tank levels at all times and thereby enabling improved tracking and control of the materials.

Point-level switches can handle extreme conditions such as high temperatures, pressure, mechanical stress, dust and caking. There are several different point-level switch technologies available that are well proven in solids applications, including vibrating fork switches, vibrating rod switches, capacitance switches and paddle switches.

Because of the diverse nature of the materials and process conditions involved in these applications, there is no ‘one size fits all’ solution. The selection of switch technology is typically based on the size and space constraints of the vessel, and the application conditions.

Non-contacting radar and guided wave radar (GWR) transmitters are the most widely applied continuous level measurement technologies, as they provide the most accurate and reliable readings. When using radar transmitters, it is not necessary to compensate for changes in density, dielectric constant or conductivity, thereby reducing complexity and supporting ease-of-use.

In addition, the considerable amount of dust typically created during the fill cycle of solids does not generally impact the accuracy of radar transmitters. This is not the case with other technologies such as ultrasonic and laser devices, where the signal can be significantly affected by dust. Also, because radar devices do not have moving parts, their maintenance requirements are minimal.

Non-contacting radar and GWR transmitters both perform level measurements through reflected microwave signals, or echoes. GWR devices send a pulse down a metal probe that extends to the tank bottom, whilst non-contacting radar transmitters send their signal through the open head space with nothing contacting the product.

GWR transmitters

GWR transmitters are especially well suited for smaller vessels with a diameter of less than 10 metres (33 feet) containing powders and small granular materials, and where the installation area is restricted. The latest GWR devices, such as the Rosemount™ 5300 Series Guided Wave Radar Level Transmitter from Emerson, can use probe end projection functionality to allow for measurements when the surface echo is too weak to be detected. This commonly occurs when the dielectric constant of the material is very low, especially in combination with a long distance to the surface, or electromagnetic interference.

When the dielectric constant is low, only a portion of the electrical signal is reflected from the material surface. 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. 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.

The transition from normal measurement to probe end projection is seamless and occurs only when required. The dielectric properties of the media are measured and stored when the device measures the level in normal mode, ensuring that the dielectric value that is used when in probe end projection mode is always fresh and that the measurement is therefore accurate.

In bulk solids applications, the material can cause down-pull forces on vessel roofs, so the 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 GWR in bulk solids applications, as long as the tensile load is properly calculated, and the most appropriate cable thickness is used.

Non-contacting radar transmitters

Non-contacting radar transmitters can be used in a large variety of applications. There are no restrictions regarding the weight of the material, so these devices can be used in applications where GWR transmitters may not be appropriate because of pull forces or concerns about probe breakage.  

The latest advanced non-contacting radar devices can suppress process noise, which maximises the amplitude of the signal reflection from the surface, enabling measurements of solids with low dielectric values over long ranges in the toughest applications.

Whereas GWR transmitters measure level at a single point on the surface, the latest non-contacting radar devices, such as the Rosemount 5408 Non-Contacting Radar Level Transmitter from Emerson, use a measurement algorithm that merges the peaks of an uneven surface within the radar footprint, enabling them to provide high accuracy and reliability, even with rapid changes in level.

The high processing power requirements of radar transmitters based on frequency modulated continuous wave (FMCW) technology have in the past led to them typically being four-wire devices. This could require additional cable infrastructure, which is costly and time-consuming to install.

However, recent developments with energy-efficient radar chips have made the latest FMCW devices less power-hungry. As a result, they require only two wires for power and communication. This enables users to benefit from the high sensitivity and accuracy of FMCW technology without having to install additional infrastructure.

Lime processing plant – a challenging environment

Due to the production processes involved and the nature of the products, operating conditions in plants producing lime, limestone and clay create a challenging environment for automation technology. At a large lime processing plant in the US, there were a number of applications in which the production team had tried several level measurement technologies to measure solids, with varying degrees of success. Here we will describe two of these applications and how the challenges faced were met by implementing advanced radar transmitters.

Hydrated lime surge bin

The first application involved a hydrated lime surge bin, where finished product passes through before being conveyed to large storage silos. The company wanted to run a screw conveyor to carry the material out of the surge bin at a steady rate.

To do this, an optimal level height was needed, but getting a stable reading was challenging. Within the small surge bin, the material tended to compress in some areas, and create gaps and bridges in other areas. To prevent this, pneumatic vibrators are used to shake and redistribute the material.

Previously, a capacitance probe had been used to perform the level measurement, but its measurements were erratic and slow to respond to level changes. In addition, the high vibration shortened the lifespan of the capacitance unit to only a few months. As the surge bin is only four feet tall, this made GWR the ideal choice.

A Rosemount 5303 Guided Wave Radar Level Transmitter from Emerson was selected to replace the capacitance probes. A critical advantage of the Rosemount GWR transmitter is its ability to provide accurate measurements in small tanks with rapidly changing levels, which is essential for this application and enables a steady level to be maintained. Configuration of the device was very straightforward using the easy to use software interface.

Since the probe was installed parallel to the slope of the bin, the horizontal level measurement needed to be corrected for the slant. This only required the angle of the probe to be entered into the software. Critically, the Emerson device is available with a remote housing extension to enable the transmitter head to be mounted away from the probe.

This protected the electronics from the vibration, helping to extend the life of the device. Since it was installed, the device has proved to be very reliable and the improvement in measurement accuracy has allowed the process to become much more stable.

Coal silos

Coal used to heat the kilns at the plant is stored in 34 metres (112 feet) tall silos. Level measurements are required for inventory monitoring purposes, to ensure that the kilns are always supplied with fuel. Coal is pulled out of the two silos simultaneously and the measurements are used to determine when to order more coal. With no place to store surplus fuel, good inventory management is essential.

Previously, an ultrasonic level sensor had been used to perform the measurements in the silos. However, the device would occasionally lock up and despite several work orders to investigate and eradicate the error, the cause was never determined. Because of the unreliability, there was a reluctance to empty the silo to less than 40%.

The production team decided to replace the device with a Rosemount 5408 Non-Contacting Radar Level Transmitter. Due to the height of the silo, a parabolic antenna was installed to direct the radar signals. An air purging system would normally be applied to prevent dust from blocking the antenna, but since the coal was often wet, dust was not considered a problem.

Unfortunately, as the weather warmed, the radar started to act in a similar way to the ultrasonic device, with short periods where it would lock up. However, one of the key features of the Rosemount 5408 is its built-in data historian, which automatically collects and stores data for up to seven days, making troubleshooting much easier.

Using the data historian and its accompanying tank radar echo curve, large signal peaks were found close to the antenna area that corresponded to the times it locked on high readings. This insight to the change in process conditions had not been available with the ultrasonic unit.

Inspection of the device during one of these peaks revealed that moisture was condensing on the antenna and causing the high-level readings. During the summer, the damp coal and high heat created a very humid environment in the vessel, with condensation occurring on surfaces.

Air lines were installed to blow off the moisture and this eliminated the condensation issue. The radar has subsequently worked well, providing accurate and reliable measurements. Ultimately, a second radar was installed on the second silo with both a parabolic antenna and an air purge system.

To learn more about Emerson’s portfolio of solids level measurement solutions, visit Emerson.com/SolidsMeasurement.