Shenzhen Jiwei Automations Ltd.
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How to Select Level Switch for Level Measurement of Fumed Silica
Fumed silica is an important nano-scale inorganic chemical material. Because of its properties such as low density, fine particle, larger surface area, strong adhesion and strong moisture absorption, etc., it is used to enhance strength, increase viscosity, and also used for thixotropic and extinction effect. It is widely used in agriculture pesticide, medicine, food, cosmetics, silicone rubber etc. For example, one often mixes fumed silica with fertilizer to improve fluidity of powdery fertilizer for its transportation, storage and avoid agglomeration due to moisture. In industrial production process, level meters used to monitor and control level of fumed silica must be carefully selected. Let´s start from fumed silica production process flow and problems in the level measurement.
1. Fumed silica production process flow
Fumed silica is an ultrafine silica powder yielded by gas phase hydrolysis from methyltrichlorosilane under high temperature of an oxygen-hydrogen flame. The production process flow is shown in Fig.1.
Fig.1 Fumed silica production process flow chart
Usually level switch is needed for storage tanks in level measurement and level control of fumed silica to prevent environmental pollution and material waste due to material overflow.
2. The difficulty in level measurement of fumed silica
Level measurement for fumed silica has been always a difficult problem. That is because the yielded fumed silica is usually stored in a vertical silo roughly 10m tall in the production process. It is lightweight with very low density between 0.01～0.03g/cm³ and low dielectric constant. In order to avoid material compaction due to gravity and adjust cl- which is attached to the surface of fumed silica to control PH value of the product, usually nitrogen gas is filled into the storage silo from its bottom. Filling with nitrogen introduces rolling and waving of the material and causes flying “dust” in the silo. To meet the environment protection and safety standard and avoid air pollution due to overfill of fumed silica requires reliable level measurement. Therefore, it is very important to choose an appropriate level switch for the application.
3. Level Switch selection for level measurement of fumed silica
At present, there are many kinds of level switches available in the market. Rotary paddle switch, capacitive level switch, RF admittance level switch and vibrating level switch are commonly used level switches. Among them, vibrating level switch is categorized as vibrating rod level switch and fork level switch. It is mainly used for granular and powdery bulk solids. Rotary paddle switch is not suitable for measuring level of fumed silica because the density of fumed silica is too low to stop the motor spinning and trigger the micro switch. Measuring principle of both capacitive level switch and RF admittance level switch is based on detecting changes of the capacitance between probe and vessel when material level changes. The dielectric constant of fumed silica is too low to be effectively measured by this type of level switches. Obviously, because of properties of fumed silica, level switch for fumed silica must be non-capacitive level switch and must be able to reliably measure ultra-low density powdery solids.
Measuring principle of fork level switch is based on detecting state of harmonic resonance of the fork bodies. Vibration of fork body is driven by piezoelectric ceramics and not related to dielectric constant of the measured material. After it is precisely adjusted for complete harmonic resonance, the fork level switch reaches to the maximum sensitivity and can be used to measure medium with ultra-low density. Therefore, resonance well pre-adjusted fork level switch meets the requirement for this special application. In fact, fork level switch which is precisely pre-adjusted and reaches to the highest sensitivity is particularly designed for measuring level of material with ultra-low density, such as fumed silica.
Jiwei Fork-11 is designed with relatively wider and thinner fork body compared with other similar products in the market. It is precisely adjusted to reach to complete resonance. Its sensitivity can be pre-adjusted according to the density of measured material and the minimum measurable density can be as low as 8g/l (0.008g/cm3). Up to now, Jiwei Fork-11 has been successfully applied in multiple projects for level measurement of fumed silica.
Communicating with our customers, we found that customers often concerned if the level measurement accuracy could be affected when large amount of fumed silica drifting above the material surface adheres to the highly sensitive fork bodies. Actually there is nothing to worry about because the resonance of our Fork-11 is pre-adjusted so well that the instrument has larger redundancy for material buildup. Fork-11 works well as long as there is no “scar” formed on the fork bodies. In some severe conditions when the instrument is measuring level of strong adhesive material in long term, it is possible that the material can attach to the fork bodies continuously and form “scars” in a long run. In this case, a nitrogen purge device is recommended to clean fork bodies before filling the material and prevent heavy material buildup or “scar” phenomenal.
Jiwei focuses on designing and manufacturing level meters for production process in automation industry. We meet challenges and offer our customers professional solutions for various problems in industrial level measurement. Our level meters are comparable to brand products in global market in high reliability and high cost-effective ratio. Our products have been demonstrated in various complicated application conditions and very well recognized by more and more customers in their high quality and reliability.
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