Rotary paddle switch in level measurement for materials that is moisturized or containing moisture is a typical complicated application.
Rotary paddle switch is a common level switch for level measurement of granular and powdery solids. With simple working principle, convenient operation and lower price, it is widely favored by customers. However, working conditions in level measurement applications are complicated and different. For each application and its working conditions, there are specific requirements for rotary paddle switch and its installation accordingly. Rotary paddle switch in level measurement for materials that is moisturized or containing moisture is a typical complicated application. That´t what we are going to talk about in this article.
Rotary paddle switch is generally used in level measurement for dry materials. It is not recommended for material with strong hygroscopicity due to its physical structure. We know that normal operation of rotary paddle switch relies on mechanical precision fitting between motor and transmission shaft, transmission shaft and bearing, and fittings in oil sealing mechanism. Moisturized material or material containing moisture can easily "scar" at the base of spindle. Over a period of time the material accumulated around the shaft. It would increase resistance to spindle rotation, affect normal rotation of the blade and eventually motor would fail to drive the spindle and false switching signal would be generated. In addition, moisturized solid materials are easy to agglomerate. That means it can agglomerate to a large volume or heavy weight of blocks or dough. These blocks will impact rotary blade and transmission shaft, cause serious deformation of the blade and transmission shaft, damage the instrument and stop the normal operation of the whole equipment.
So is it true that rotary paddle switch cannot be used in level measurement for material that is moisturized or containing moisture? The answer is certainly “No”, not absolutely true. The characteristics of the measured material have a great impact on the service life of the instrument. What we could do is to find out how to avoid adverse effects. After going through a large number of tests and analysis, engineers in Jiwei found following measures could facilitate the applications of rotary paddle switch in level measurement for moisturized material.
1. Install rotary paddle switch at 20°to a horizontal plane.
Compared with horizontal installation, inclined installation can reduce material accumulation on the base of spindle during material filling process, reduce the amount of moisture to enter into the instrument and avoid overfilling due to false detection or “scar” phenomenon.
2. Install a V-shaped baffle
Rotary paddle switch is a contact level switch. The basic requirement for its installation is to avoid installing the probe on the way of material filling. In order to avoid the agglomerate material direct impact on the blade, it is necessary to install a V-shaped protective baffle above the rotary paddle switch. Due to inadequate consideration, no V-shaped baffle was installed on silos at the beginning. Unlike the shaft which was protected by a protective sleeve, the blade was not rigid enough to bear the impact from moisturized and easily agglomerated materials (such as sludge etc.). The blade was deformed almost 90 °due to the impact.
In addition, when the V-shaped protective baffle is installed, one must consider whether the space between protective baffle and spindle is too narrow or small because material containing moisture usually has poor fluidity and easy to agglomerate. It is easy to accumulate in a narrow space (dead zone) and form “scars” on the shaft.
3. Use a silo with an alcove instead
Because silo and level measurement instrument are two independent devices, protecting the measuring instrument usually is not in the consideration when selecting a silo. It undoubtedly increases the risk of being damaged for the rotary paddle switch. In addition to installing a baffle, using a silo with an alcove can also effectively avoid the risk. Let´s take a real application as an example to make a further explanation.
Fig.1 shows a sludge discharging device. When the silo is full, the sludge will be transported away by the spiral conveyor underneath. Because of the constricted space, when sludge is filled into the silo, it directly faces the probe of a rotary paddle switch and causing huge impact on the probe. Ideally if the silo has an alcove, the rotary paddle switch can be installed in the recess area to avoid material impact on the probe. When the filling material accumulates slowly from the bottom of the silo and contacts the blade, the rotary paddle switch will send an alarm signal. Therefore, it is recommended to use a silo with an alcove (as shown in Fig.2) to avoid material direct impact on the blade and material buildup in dead zone.
Furthermore, besides of taking the above measures and precautions, regular cleaning and maintenance of level meters is also necessary in order to ensure the rotary paddle switch to function well and extend its service life as much as possible.