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MONITORING BULK SOLIDS INVENTORIES - · PDF fileremote inventory management system can transform ... Monitoring Bulk Solids Inventory 6 Conclusion Bulk Solids Inventory exists in a

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  • Bulk Solids Inventory exists in a wide range

    of industries including agriculture, grain

    processing, feed production, plastic

    processing, concrete production and many

    more. Reliable monitoring of these

    inventories and efficient management of

    the corresponding data requires a choice of

    sensing technologies such as weight or

    level sensors as well as up-to-date, efficient

    data management systems.

    Monitoring

    Bulk Solids

    Inventory

    Sitara Hultman

    Marketing Manager

    Bindicator and Kistler-Morse

  • Monitoring Bulk Solids Inventory

    1

    Background Mass production involving bulk solid materials such as grains, plastics or concrete requires reliable management of bulk inventory levels. Unlike traditional parts inventory, bulk solids cannot be easily counted one particle at a time. That would be virtually impossible and somewhat ridiculous. In addition, rather than being purchased or sold by the piece, bulk solid materials are traded by volume or weight. Therefore, appropriate methods in which to measure and monitor bulk solids inventory are by volume or weight. Not all bulk solid storage vessels can be weighed, such as flat bottom corrugated grain bins or vessels storing relatively lightweight material. Cost restrictions may also cause many common storage vessels to be installed without weighing systems. If weighing systems are not possible, the alternative method has long been to measure the height or level of the material pile within a vessel and translate that measurement to volume and then to weight. Monitoring bulk inventories is often done locally at the storage vessels where it is recorded on inventory tally sheets and possibly entered manually into an ERP or other management system. But, in recent years, alternatives with greater flexibility and efficiency have been developed. This white paper discusses the sensor measuring technologies available as well as the data management technologies that exist.

    Measuring Inventory There are two primary methods for measuring bulk solid inventory: 1) Weighing the material 2) Measuring the level of the material pile Here, we briefly examine both methods. Measuring bulk material inventory by weight Since most inventory measurement applications for bulk solids need to express quantity in terms of weight (pounds or tons) then weighing the vessel contents seems the most logical approach. Two types of weighing systems exist, the stationary load cell weighing system and the bolt-on strain gauge system. The primary difference between the two types is that the stationary load cell is installed directly

    under each leg of the silo or bin and the bolt-on sensors are installed on the leg or cross member surfaces.

    Figure 1: Load stand installed under

    each leg of the storage vessel

  • Monitoring Bulk Solids Inventory

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    Load Cells: Smaller bins typically use a smaller load cell package than large silos that will use a load stand as shown in Figure 1. A typical installation of stationary load cells on a smaller bin is shown in Figure 2. These smaller load cells are typically available to measure up to 25,000 lbs per sensor, while the load stand sensors will be able to measure up to 1,000,000 lbs per sensor. Load cell sensors output a variable

    voltage in response to changing force or weight on the sensor, increasing and

    decreasing. Weight measurements are typically calculated and indicated on a weight controller. A variety of different weight controllers can be used. The weight controller/indicator processes the signals from all the load sensors for the vessel, allows

    for calibration of the system, indicates the weight measurement and interfaces to other devices with a variety of outputs. A weight controller/indicator is shown in Figure 3. Bolt-on strain gage systems: Ideal for vessels that were not installed with weighing systems during initial installation. The reason for this is that the bolt-on systems measure strain in the cross members that support the vessel rather than being installed

    Figure 3: Wight controller/indicator underneath each leg. This makes the bolt-on sensors ideal for retrofitting existing vessels. However, while easier to retrofit vessels and typically lower in cost than stationary systems, the tradeoff is measurement accuracy. Stationary weigh systems are capable of accuracy with no more than 0.5% system error. Bolt-on weigh systems typically provide system error of 3% to 5%.

    Figure 2: Small bin load cells mounted beneath legs of bin

  • Monitoring Bulk Solids Inventory

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    Measuring bulk material inventory by level Measuring bulk solids inventory by weight can be costly ($5,000 to $50,000 or more) and retrofitting existing vessels can be complicated and costly. For these reasons using sensors to measure the height of the inventory material pile in the vessel is a common method for bulk solids material inventory measurement. There are several types of level measurement sensors used for bulk solids inventory measurement, including weight & cable, acoustic, contact radar and non-contact radar, ranging from $1,500 to $4,000 with error rates by weight of from 5-10% or more. Each has its strengths and weaknesses. Table 3 compares these technologies. Table 3: Most commonly used level sensor technologies for bulk solid inventory monitoring.

    TECHNOLOGY

    NON CONTACT

    SENSITIVE TO DUST

    SENSITIVE TO ANGLE OF REPOSE

    DIELECTRIC SENSITIVITY

    PRICE

    Weight & Cable Periodic and

    Momentary

    No No No Low

    Acoustic Yes Yes Yes No High

    RF Capacitance

    No Yes No Yes Moderate

    Guided Wave Radar

    No No No Yes Moderate

    Through-Air Radar

    Yes Some Some Yes High

    Most of the level sensing technologies in Table 3 measure the material level at one spot on the surface of the material pile. This is a common characteristic that must be considered when choosing a sensor. If the material pile has an irregular surface shape, errors might occur in the accuracy of the measurement. The acoustic level sensing technology accounts for irregular surface shapes by performing 3D surface mapping. A 3D level scanner is shown in Figure 4. This level scanner uses three acoustic transducers to measure distance in three axes. With sophisticated signal processing the sensor is able to accurately map the surface of the material and determine the volume of material present to within typically 1.5-2.5% error on volume when properly applied.

    Figure 4: Surface mapping level

    measurement scanner

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    Monitoring Options To discuss bulk solids inventory data monitoring we first need to look at the objectives that might exist:

    1. Materials and production planning: Accurately understand and analyze material usage and trends. Ensure production efficiency and minimize inventory shrinkage.

    2. Material procurement: Ensure just-in-time material availability.

    3. Event notification: Inform appropriate personnel of various inventory and inventory monitoring system conditions including, empty, re-order, full and sensor error/alarm conditions.

    4. Financial balance sheet management: Ensure timely acquisition of bulk solids inventory data and values.

    5. Efficiently integrate inventory data into an ERP system

    These are only some of the possible objectives that could exist for a bulk solids inventory monitoring system. Some or all of these may exist, and perhaps others. The typical options for monitoring bulk solids inventory quantities include 1) local standalone display and control, 2) PC-based software and 3) web browser local/remote monitoring. Local Standalone Display/Controller The weight controller shown in Figure 3 is an example of a local standalone device for a weight system. A typical level measurement system remote control and indication device is shown in Figure 5. These level measurement remote control devices typically communicate with the inventory sensors and can

    perform a variety of control and calculations including converting the level measurement to a calculated volume and weight. Other remote display devices could be simple digital meters that accept 4-20mA signals from a single or multiple standalone sensors.

    Figure 5: Remote display/controller for weight &

    cable type sensor system

  • Monitoring Bulk Solids Inventory

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    PC-Software Monitoring Systems PC-software monitoring systems provide similar functionality to the local standalone controllers but with the added graphical interface. These systems use dedicated software programs that must be installed on a PC. Access to the graphical interface and PC software control functions is limited typically to the PC that the primary software is installed on. Some brands offer client software to be installed on other PCs that share a network with the PC that has the primary software installed. These systems tend to be limited to local inventory monitoring.

    Web browser based monitoring systems Web browser based monitoring systems offer true remote monitoring and management of the inventory data with flexibility. They allow this to be done at single or multiple locations very easily. A remote inventory management system can transform your inventory and process data into valuable information that increases productivity and reduces supply chain costs. These local and remote monitoring systems utilize an input/control unit web server such as that shown in Figure 6, and the simplicity and consistency of your web browser. Whether at your PC in your office, at home or at any other location; so long as you h

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