Thread Content
Preface As the level of automation in flour mills increases, the careful selection of scientific monitoring and control instruments for their production processes becomes increasingly important. The processing process in a flour mill involves: after the raw wheat is transported to the factory, it goes through sorting and soaking (for 24 hours), after which it is sent to the processing area where it is processed (including hulling and grinding). Once the wheat has been hulled, it is ground into flour, which is then stored in silos or packaged. The storage bins for moistened wheat, the flour storage tanks, and the packaging holding bins all require level control in order to ensure efficient production operations. Domestic flour mills currently use a wide variety of brands of level switches, and the quality of these switches varies greatly. False level readings caused by failures in these switches lead to production interruptions on a regular basis. How flour mills can choose level switches in a scientific and rational manner is a common issue in the industry. By examining the specific requirements of flour mill operations as well as conducting a comparative analysis of major level switches available both domestically and internationally, this article aims to provide guidance and insights for selecting appropriate level switches for flour mills. http://www.jiweimeter.com/uploads/image/20181112/1542012634.png Stainless steel storage tanks for raw materials Wheat-cement storage tanks Top-mounted level switches for cement storage tanks Current status of level switch technology At present, the most commonly used level switches in the raw material warehouses (wheat) and finished product warehouses (flour) of flour mills are rotatory level switches and radio frequency admittance level switches; radar level gauges and ultrasonic level switches are also used to a limited extent. Due to their low cost and simple installation, anti-rotation level switches were highly favored by many manufacturers in the early days. However, many manufacturers have found that after using anti-rotation level switches, the motors connected to them tend to get damaged, which prevents them from functioning properly and affects production efficiency or even makes production impossible to carry out ； Furthermore, since it contains components that lubricate moving parts, it can easily contaminate the materials in the raw material bin, wheat moistening bin, and finished product bin, resulting in economic losses. Due to the varying humidity of the wheat after moistening and that of the flour, the dielectric constant changes frequently, which can easily prevent the RF admittance level switch from functioning properly ； Furthermore, the probe of the radio frequency admittance level switch has an embedded structure (long and thin), which can easily lead to the probe being bent or broken, thereby damaging the radio frequency admittance level switch and disrupting normal production. Although radar level gauges perform well in measuring levels in raw material silos, their high cost results in poor cost-effectiveness ； Ultrasonic level switches are reasonably priced, but due to the high dust levels in flour mills, measurements taken in the finished product silos are often inaccurate, resulting in frequent false alarms. How to select level switches appropriately for level measurement in flour mills, in order to ensure the safe and reliable storage of materials and maintain continuous and stable production, has become a major challenge that flour mills need to address urgently. Requirements for selecting level switch technology In order to ensure the long-term, efficient, safe, and stable operation of automated flour processing plants, improve production efficiency, reduce labor costs, and maximize the economic benefits of such plants. Considering the actual production processes in flour mills, the selection of level switches should meet the following requirements: (1) Food-grade. Since flour is related to people’s health, and in order to prevent secondary contamination of the materials in storage areas such as raw material bins, wheat moistening bins, and finished product bins due to issues with product sealing, the level switches used must be of food-grade quality. This means that the devices cannot contain any moving parts that require lubrication, and the material of the probes should be at least 304 stainless steel, with a surface finish of less than 0.8μm. (2) High reliability. The flour processing process involves multiple steps such as the receipt of raw grains, storage and cleaning, wheat milling, post-processing of flour, as well as the packaging, storage, and distribution of the finished products and by-products. Given the complexity of these operations, the level switches used must meet high reliability requirements, including the ability to avoid false alarms, resistance to interference, immunity to material buildup, excellent performance, and a long service life. (3) Dust explosion prevention. During the production of flour, a large amount of dust is generated. When this dust remains suspended in the air and reaches an explosive concentration, it can catch fire instantly upon contact with flames, sparks, arcs, or sufficient heat, resulting in a violent explosion. Therefore, the level switches used must meet dust explosion prevention requirements to ensure safe operation in the factory. (4) Easy to install. To reduce installation and commissioning time as well as subsequent maintenance costs, and to improve construction efficiency, measurement solutions that are easy to install and require no maintenance should be considered during the selection process. (5) High cost-performance ratio. Once the above requirements are met, price will be an important factor to consider when selecting a product. Radar level gauges have a very low cost-performance ratio due to their high price. Comparison of the performance of major level switches at home and abroad Different level measurement instruments should be used for measuring various materials under different operating conditions. The main level measurement instruments available at home and abroad include rotator level switches, fork-type level switches, radio frequency admittance level switches, radar level gauges, and vibrating rod level switches. Due to different principles, the performance of level measurement instruments varies, and it is therefore very important to choose the most suitable measuring instrument in actual production. (1) Anti-rotation level switch. Since the anti-rotation level switch contains lubricated moving parts, it cannot meet the food-grade requirements of flour mills. (2) Tuning fork level switch. Domestic ordinary fork-type level switches tend to cause puncturing when measuring powdered materials, which leads to false alarms; therefore, they are also not suitable for use in flour mills. (3) RF admittance level switch. Due to its structural design, the radio frequency admittance level switch cannot meet dust explosion prevention requirements. (4) Single rod level switch. It is often used in food processing, but due to its single-bar structure, it has poor resistance to residue accumulation, tends to cause false alarms, and is also expensive, resulting in a low cost-performance ratio. (5) Double-bar level switch. The double-bar vibrating level switch overcomes the weakness of single-bar level switches in terms of their resistance to material buildup; it operates more stably and reliably. Moreover, its circuit and structural design meet the requirements for dust explosion prevention, making it highly suitable for measuring materials in flour mills. For a comparison of the performance of different level switches, please refer to the table below. Comparison Table of Performance Among Different Level Switches: Level measurement instruments that meet food-grade standards; performance is related to the dielectric constant of the medium; ability to resist material accumulation; ease of installation and calibration; tendency to generate false alarms; dust explosion protection; cost-effectiveness. Rotary level switch: No, No, Poor, Easy; Yes, Yes, Low. Domestic fork-type level switch: Yes, No, Average, Easy; Yes, Yes, Medium. Radio frequency admittance level switch: No, Yes, Average, Difficult (requires calibration); Yes, No, Medium. Single-rod level switch: Yes, No, Poor, Easy; Yes, No, Low. Imported double-rod level switch: Yes, No, Strong, Easy; No, Yes, Medium. Domestic double-rod level switch: Yes, No, Strong, Easy; No, Yes, High. From the comparison in the table above, it is clear that the double-rod level switch is the most suitable instrument for level measurement in flour mills. At present, imported double-bar level switches are expensive and offer poor cost-performance; in contrast, domestic double-bar level switches have reasonable prices and perform on par with foreign ones. Since 2017, COFCO Group has installed 36 extended Tube-11 dual-rod vibrating level switches manufactured by Shenzhen Jiwei Automation Technology Co., Ltd. in raw material warehouses, finished product warehouses, etc., to replace the rotary obstruction type level switches that were in use previously. The double-rod vibrating level switch has proven to be stable and reliable once it is in use. It effectively solves the problems associated with other level switches used in flour mills in the past, such as complex installation and calibration, frequent false alarms, the probe being easily bent, the occurrence of hole-punching, and a short service life. It eliminates the disruptions caused by frequent production halts, reduces labor costs, and improves production efficiency. Conclusion This paper systematically examines the current application status and existing problems of level switches in flour mills. By comparing the performance characteristics, advantages, and disadvantages of various level switches, it aims to provide a reference for selecting appropriate level switches in such facilities. This approach helps ensure the long-term, efficient, safe, and stable operation of automated production processes in flour mills, reduces financial and labor costs as much as possible, and maximizes the economic benefits of these factories.