Analysis on Selecting Level Switch in Flour Processing

 

Stainless steel storage tank for raw material

Cement storage tank for wheat

Top-mounting on cement storage tank

At present, the most commonly used level switches for raw material (wheat) barn and finished product (flour) silo in flour mills are rotary paddle level switch and RF admittance level switch. Radar level switch and ultrasonic level switch are also used in a small amount.

Rotary paddle level switch was favored by many manufacturers in early days because of its low price and simple installation. However, they found that the motor in rotary paddle level switch was easily damaged and stopped operation. It caused production suspension, affected production efficiency and even completely stopped normal production sometimes. Further, because rotary paddle level switch contains moving parts which need to be lubricated, it is easy to contaminate materials in raw wheat barn, in silo for wheat moisturizing and in silo for flour storing, and causes enormous economic loss.

Because the humidity of moisturized wheat and finished product-flour are different, the dielectric constant of these materials often varies. RF admittance level switch fails to work properly in these applications. In addition, the probe of a RF admittance level switch is long and thin, it is easy to be bent or damaged to affect normal production.

Although radar level switch works well for level measurement in flour mills, it is too expensive and not cost effective. Ultrasonic level switch is not so expensive, but it could not adapt to the application conditions. Because of heavy dust in level measurement of finished flour, inaccurate level measurement often happens and causes false alarm.

In order to ensure safe and stable automatic production operation in long-term, improve production efficiency, lower labor cost and maximize profit in flour mills, considering process conditions onsite, a level switch for application in flour mill should meet the following requirements:

（1）Suitable for food industry. Because flour is a food product, it is related to people's health. In order to avoid secondary pollution to materials in raw materials barn, wheat moisturizing silo, or finished product silo due to sealing problems of the equipment in the process, the selected level switch must be a food grade instrument. That means: the level switch must not contain lubricated moving parts; the probe must be made of 304 stainless steel (at least); and surface finish of its mechanical parts must be less than 0.8μ.

（2）High reliability. Detailed flour processing includes raw material receiving, storing and cleaning of raw wheat, wheat flour milling, post-processing of flour, as well as packing, storing and distribution of finished product and its by-product. Working conditions for applications in flour processing are very complicated. The selected level switch must be highly reliable without false alarm and meet the application requirements for interference resistance, redundancy for material buildup and long service life etc.

（3）Dust explosion protection. During the process of flour production, a large amount of fine particles of flour drift in the air and form flying dust. If the concentration of these particles reaches to the level of explosion, once there is a flame, spark, electric arc or even when the temperature reaches to combustible level, these fine particles of flour will be ignited instantly and burned up rapidly to cause a violent explosion. Therefore, the selected level switch must be certified for dust explosion protection to ensure safety of production process in flour mills.

（4）Easy for installation. In order to spend less time on installation and test, lower maintenance cost and improve implementation efficiency of the project, the selected level switch should be easy for installation and maintenance-free.

Different level meters should be used in level measurement for different materials or in different working conditions. The common level meters at home and abroad are: rotary paddle level switch, fork level switch, RF admittance level switch, radar level meter and Vibrating rod level switch etc. Because different types of level meters are based on different measuring principles, the specifications of the level meters are also different. Each of them suits different applications. How to choose the most suitable instrument according to the real situation of production process is very important.

（1）Rotary paddle switch.  Because the rotary paddle level switch contains lubricated moving parts, it does not meet the food-grade requirements for flour mills.

（2）Tuning fork level switch. When common tuning fork level switch made in China was used for level measurement of fine powdery material, the material tended to aggregate and form a “hole” around the probe which led to false alarm. Obviously it is not suitable for level measurement for flour.

（3）RF admittance level switch. RF admittance level switch could not meet the dust explosion-proof requirements because of its structure.

（4）Single rod vibrating level switch. It is often used in food processing. But because of the single tube structure, its ability to tolerate material buildup is poor and false alarms often happen. Further, it is expensive, not cost effective.