Understanding and Optimizing Accuracy for Silo Inventory Monitoring Systems

The feed, ethanol, seed, and grain industries have been increasingly requesting greater inventory accuracy. Continuous production and the financial performance of the industries rely on the value and the amount of inventory.

Accuracy can be defined as the quality or state of being precise or correct. Technically, the term refers to the degree to which the outcome of a calculation, specification, or measurement meets the standard or correct value. Accuracy can be measured in terms of level, mass, and volume.

Figure 1. Technically, accuracy means the degree to which the outcome of a calculation, specification, or measurement meets the standard or correct value and is measured in terms of mass, volume, and headroom.

Level

The level of a device is the distance between the device and the material surface. The dead zones and measuring ranges vary with different device, and it is the area where the device cannot “sense” the material.

The space at the top of the vessel where the material is not sensed is known as the upper dead zone and it is not measured. The distance is measured from the bottom of the dead zone. Some devices have lower dead zones, where the measurement of the material is stopped when it reaches a certain point at the bottom or the output of the vessel.

Figures 2 and 3 show the Bob and Open Air Radar level sensoring devices by BinMaster.

Figure 2. BinMaster’s Bob

Figure 3. BinMaster’s Open Air Radar

Volume

The total amount of three dimensional space that is taken up by the material is known as volume. The volume of device is calculated from the vessel’s internal dimensions and the distance to the material’s surface.

An incorrect calculation of bin dimensions can result in inaccurate volume calculations. Similarly, material flow, bridging, and buildup, and the location of filling and discharge points can also influence the volume calculations. Even if the level is accurate, volume calculation can be incorrect.

Figure 4. Correct dimensions are vital for volume calculations to be accurate

Mass

Mass is the weight of the material in the bin calculated in terms of pounds, tons or metric tons. The bulk density - the weight of a cubic foot of materials - impacts the mass of a device, and bulk density (lb./ft.³) can vary significantly.

Greater bulk densities can be created at the bottom of the bin compared to the top due to compaction. Such fluctuations or errors in bulk density can affect mass estimation. Moisture is another factor that affects weight.

Figure 5. Grain compaction

Single Point Measurement

Devices such as open air radar, bobs, ultrasonic, and guided wave radar (Figure 6) can be used to measure a point in the vessel, directly beneath the device. The device measures the distance between the sensor on the top of the tank and the material surface, excluding the dead zone.

When installed properly, the accuracy is ± 0.25% of the total distance measured, as stated in the literature. Using these automated devices for measurement eliminates risk, as they are similar to dropping a tape measure without climbing the vessel (Figure 7). The distance between the top of the vessel and the material is known as headroom.

Figure 6. Guided Wave Radar

Figure 7. Automated devices eliminate risky climbing

Angle of Repose

The angle of repose is the angle that forms when the material is filled into or emptied from the vessel (Figure 8). In a center-fill, center-discharge bin, flowing material tend to pile symmetrically, and this is generally referred to as “cone down” or “cone up” (Figure 9).

To calculate the angle of repose, a horizontal line from a point 1/6 from the outer perimeter must be drawn. Taking the material at its peak and filling in the voids will flatten the angle.

The best volume estimation can be obtained by mounting a single point device 1/6 from the outer perimeter. The volume accuracy of free flowing material in small bins can be measured quite well by employing a single point device.

Figure 8. Angle of repose

Figure 9. In a center-fill vessel flowing material generally piles symmetrically

Single Point Device Accuracy

As a user progresses from level to volume to mass, numerous factors can result in diminishing accuracy. Level - the distance to the material at that point - will be highly accurate if allowances for the dead zones are followed.

The accuracy of volume can be affected if the bin dimensions are inaccurate, or if the material is not free flowing, or if the device is not installed in the mentioned optimal point. Mass can be affected if compaction occurs or if the bulk density is incorrect. All of these factors have an impact on accuracy (Figure 10).

Figure 10. Technically, the glass is completely full.

Multiple Point Measurement

Multiple point measurement devices include bob systems and 3D scanners. The devices measure the level of a material at multiple points in the bin, and the volume can be calculated from multiple measurements.

These devices take into consideration the differences that are present across a material’s topography. They are often employed while measuring the statistics of bins that have multiple filling and discharging points or in bigger bins.

Multiple point measurement devices can help to obtain greater accuracy in materials that often form irregularly in the vessel (Figure 11), such as soybean meal or in powders such as flours.

Figure 11. Multiple point measurement devices can help obtain greater accuracy in materials that often form irregularly in the vessel.

Multi-Bob System

Using the multi-bob system, 2 to 32 bobs can be installed on the top of a building or bin (Figure 12). The sensors will take measurements directly below their mounted points at scheduled time intervals.

The level measurements taken by the sensors will be reported by software (Figure 13) that will also calculate the average level and full percentage. Compaction can be adjusted by employing the strapping table for input of “weight to distance” data.

Figure 12. Multi-Bob system

Figure 13. A software will report the level data collected by the sensors, and calculate average level and full percentage.

Although these devices can collect more data than a single bob system, they are not ideal when the mass and volume need to be accurate. They can provide an economical alternative when high levels of mass and volume accuracy is not required.

A sample multi-bob configuration involving two bob systems that detect cone down or up is shown in Figure 14. Figure 15 shows the corresponding data reported by the software.

Figure 14. Two bob system detects cone up or down

Figure 15. Data from two bob system reported by the software

Figures 16 and 17 show a sample multi-bob configuration involving five sensor systems that is suitable for 105’, 132’ bins, or bins where more data is required and the data of the software, respectively.

Figure 16. Five sensor system ideal for 105’, 132’ or any bin where more data is desired

Figure 17. Data from multiple sensors reported by the software

Single Scanner System

Single scanner system is a non-contact, acoustics-based technology that measures numerous points in a 70° beam angle (Figure 18). The system uses advanced algorithms that assign a weight to each measurement depending on its relevance. The system is ideal for bins that have a diameter of less than 45 feet. A single scanner can be used to measure wider bins, but with less accuracy.

Single system scanners require minimal maintenance, and perform and provide more accurate measurements in high dust. In addition to providing average, minimum, and maximum levels, the device also offers an optional visualization of a materials surface.

Figure 18. Single system scanner

Multiple Scanner Systems

Using additional scanners will increase the amount of surface area that can be covered. The increase in measurement points provides increased accuracy.

Additional scanners are essential to detect irregular topography in large vessels. Additional scanners are generally used in vessels that are over 45 feet in diameter, especially in vessels with multiple filling and emptying points and where high levels of accuracy are required.

The budget, desired accuracy, and the size of the bin determine the number of scanners required. Bins with 90’ and 105’ diameter generally require two or three scanners, while four scanners are required for high accuracy in bins that are 132’ and 160’ in diameter. Users can start with one scanner and expand to include additional scanners when needed.

Figure 19. Scanner #1 mounted near the center of the bin

Figure 20. Scanner #2 mounted halfway between the center and sidewall

Multiple Point Device Accuracy

Multiple point devices are recommended when the bins are large or have multiple filling and discharge sites, when the material is not free flowing and generally piles up in a random manner, or when high levels of accuracy are required.

The levels will be highly accurate as multiple point devices take into account the variations in the topography across the material. Measurement of the distance to the material at multiple points and ability to provide reports on average, minimum and maximum levels ensures high accuracy levels.

Accuracy in volume can be ensured if the vessel dimensions are correct; when a device is installed at the optimal location; and when the number of sensors is adequate to cover the full material surface. Multiple point devices can ensure a volume accuracy of up to 1% and 3% of total vessel volume.

Mass accuracy can be obtained when compaction is accounted for and bulk density is calculated correctly. It is ideal to calculate bulk density when a known quantity of material is filled in the vessel.

Ethanol Plant Case Study

The ethanol plant required increased inventory accuracy in highly used dusty corn bins. The plant was using single MV scanners to visualize the topography of two 75’ corn vessels. A MV scanner on concrete DDG silos was also added.

In order to upgrade the system, MVL-2 systems were installed on two new 105’ diameter steel vessels and MV scanner on concrete silos were upgraded to MVL-2 systems. MultiVision software was installed to obtain network visibility of all vessels in real-time. The software observes levels during filling and discharging, and reports mass and volume.

This information has been sourced, reviewed and adapted from materials provided by BinMaster.

For more information on this source, please visit BinMaster.