Bulk Materials in Level Measurement and Monitoring

Bulk materials refer to granular or powder media types that are handled and processed in large quantities and stored in vessels, like silos.

Level measurement and monitoring for bulk materials is used in many applications and is a crucial part of many processes.

Some examples industries include:

• Agriculture
• Mining
• Concrete
• Pharmaceuticals
• Food and Beverage
• Manufacturing

While the terms "level monitoring" and "level measurement" are often used interchangeably, there are subtle differences in their meaning.

• Bulk Material Level Measurement: The purpose of bulk material level measurement is to ascertain the distance from a reference point to the surface of the bulk material in the vessel. It is used to obtain a numerical value that can also be turned into a measurement of volume based on the known geometry of the vessel.
• Bulk Material Level Monitoring: The purpose of bulk material level monitoring is to track, and sometimes control, the level of bulk material within a vessel. It may not provide a measurement function but allows for trend observation and some degree of level control if paired with a relay that can operate start/stop fill functions. They can also provide a simple alarm via the switch point when a certain level has been reached.

There is a variety of level instrumentation technology that can be used for bulk materials. When working with bulk materials, it's essential to consider factors such as material characteristics, vessel geometry, temperature, and environmental conditions to choose the most suitable level measurement technology for the application.

Many bulk level applications employ a switch or switches installed within the vessel. The challenge with measuring level from the “top” is that many technologies require a reflective surface to operate. The other problem is that, unlike liquids, bulk materials flowing into a container do not create a flat surface.

Depending on where the materials are being inserted into the vessel, there will be a “cone” of extra material above what could be called the “main surface” of the bulk material. Trying to ascertain the level of an uneven surface, using a single point of reference, does not provide an accurate picture of the volume of material in the vessel.

Some level technologies that are sometimes used for “top down” level detection are ultrasonic, guided wave radar, and capacitance. However, these technologies are more commonly used with liquid level applications and not bulk material.

Not all level applications for bulk materials require level measurement. Some simply require level monitoring and control. If measurement is not required, simple level switches offer a much more economical option than level measurement instrumentation. They all operate by mechanical means. Common level switch technology for bulk materials include:

Tilt/Float Level Switches

Float/Tilt Level switches are commonly used in simple liquid level applications. There are also tilt switches built for bulk materials, which KOBOLD offers.

Diaphragm Level Switches

Diaphragm Bulk Material Level Switches are triggered when the material level exerts enough pressure on a membrane. KOBOLD Bulk Material Diaphragm Level Switches include:

Rotary Paddle Level Switches

Rotary Paddle Level Switches employ a rotating paddle that constantly revolves in a circular manner. When the level reaches a certain point, the media exerts enough pressure to stop the paddle’s rotation, and this triggers a switch. KOBOLD offers a superior solution to comparable models because of the ability to customize the paddle shape to best fit the material and application. It is designed to counteract the motor burnout prominent in other options in the market.

Vibrating Rod Bulk Media Level Switches

Vibrating rod level switches consist of a vibrating probe and an electronic switch. When the probe vibration is damped by contact with rising media, the resulting change in resonant frequency is converted to a switching signal.

Capacitive Level Switches for Bulk Materials

The sensor of a capacitive level switch along with the wall of the tank is a capacitor. The dielectric constant of this capacitor is the air when the material doesn’t reach the sensor. When the product covers the sensor, an electronic circuit detects this change in the dielectric constant and activates an output relay.

Pendulum Bulk Level Switches

A pendulum bulk level switch consists of two parts: the aluminum connection housing with the microswitch, and the connection terminals and the pendulum rod with the measuring cone. They are installed vertically on the side of the holding vessel. When the vessel is filled with bulk solids, the rising level causes the measuring cone of the level monitor to swing to the side. This movement is directly transferred from the pendulum rod to a microswitch.

Measuring and monitoring bulk material level presents several challenges due to the diverse nature of these materials and the unique environments in which they are often stored and processed. Choosing the best bulk material level instrumentation is dictated by the specific challenges found in the application. Some of the key challenges in level measurement for bulk materials include:

• Material Characteristics: Bulk materials vary widely in terms of composition, density, particle size, and flow properties. These variations can affect the accuracy and reliability of level measurements if the instrument chosen is not optimally compatible with the material.
• Dust and Vapor: Many bulk materials generate dust or vapors during handling and processing. This can interfere with certain level measurement technologies, such as optical sensors which will provide false readings if confused by the presence of excessive dust above the material surface. Certain types of sensor technologies can function in the presence of dust but will require regular maintenance to keep them clean.
• Buildup: Some bulk materials can adhere to the walls of containers or to the sensors, leading to material buildup. This buildup can affect the accuracy of level measurements and may require frequent cleaning to remove the buildup.
• Vessel Geometry: The geometry of storage vessels or silos varies significantly. Irregular shapes and large sizes may render certain bulk material level measurement methods incompatible.
• Temperature and Environment: Extreme temperatures, high humidity or corrosive atmospheres can impact the performance and longevity of level measurement equipment more than others.
• Measurement Range: Bulk material surfaces can experience significant level changes, especially in large storage vessels. Selecting a measurement technology with an appropriate measurement range is crucial to be able to measure or monitor the entire depth of the application. Sometimes this can be remedied in monitoring situations by installing more than one switch in the column depth where monitoring is required.
• Safety: Safety can be a concern in certain applications. For example, in hazardous areas where dust or vapors may pose an ignition risk, it is best to choose an option that will minimize this risk by the nature of how it operates or by features inherent to the design that offer protection from electrical sparks or arcs.
• Maintenance: Regular maintenance is often required to keep level measurement systems functioning correctly. This can involve cleaning, calibration, and replacement of components that are subject to wear for level options operating by a mechanical principle.
• Accuracy/Reliability: To control processes and costs, accuracy is essential. Factors such as signal interference, calibration drift, and sensor stability can impact the overall reliability of the level measurement system. Balancing cost with actual accuracy needs is also advisable. Some applications require minimal accuracy and buying a more expensive option with higher accuracy will not provide any added benefit, just extra cost.
• Cost: The initial cost of equipment, the cost of installation, the cost of maintenance, and the expected lifespan of the instrument should all be considered when figuring out what the most economical option is for the long haul.

The accuracy required for bulk level applications depends on the specific application and the components of its processes. Here are some considerations when picking a bulk material level instrument in the light of both accuracy and application components:

• Safety and Compliance: High accuracy level instrumentation is essential in industries where safety is a concern or where there are regulations. For example, in the pharmaceutical and chemical industries, precise level measurements are essential to ensure the correct dosage and to prevent spills.
• Process Control: Industries that require high levels of control for manufacturing/production usually require moderate to high accuracy. This includes applications like food processing, cement manufacturing, or plastics production, where accurate inventory management is crucial.
• Storage: For simple bulk material storage in vessels like grain silos or coal bunkers, only moderate precision may be necessary for demands of inventory management and replenishment.
• Process Optimization: In certain applications, achieving a high level of accuracy is essential for optimizing processes and ensuring efficiency. This is often seen in industries where material flow rates and inventory turnover are closely monitored.

KOBOLD offers many different bulk material level monitoring and measuring solutions. For free advice about what the best option is for your application and your budget, please speak with one of our experts at 412-788-2830 or info@koboldusa.com.