Why RF Admittance Level Switch and Rotary Paddle Switch Are Improper for Level Measurement of Pressurized Silo

RF Admittance Level Switch and Rotary Paddle Switch are commonly used in automated measurement industry to detect the level of bulk solid or powder. The typical application is the level measurement of ash silos in the dedusting system of power plant. Due to the technical principle and structural limitations, RF Admittance Level Switch and Rotary Paddle Switch are generally not applied to the level measurement of the pressurized silo, for example, the level measurement of the pressurized silo in the ash conveying system is rarely used.

From the aspect of detection principle, Rotary Paddle Switch operated by the micro motor, through a clutch, drives the rotation of a transmission shaft on which a paddle is assembled. When the spinning paddle is not contacted with measured material, the motor is in normal rotation; when the paddle is covered by measured material, the motor stops rotating and its power is switched off automatically. At the same time, a switching signal is generated by a detection device to control the process of material transport /delivery. Rotary paddle switch is widely used in modern industrial process and storage.

For RF Admittance Level Switch, a probe is designed to detect the change of capacitance and impedance between probe and vessel wall to fulfill material level measurement and control. In its electronic unit, reactance between the measurement electrode and the wall of an empty vessel constitute a balanced bridge circuit which outputs a stable oscillation signal. As medium level rises, measured medium covers measurement electrode, reactance between electrode and the wall of vessel changes. It causes imbalance of bridge circuit and stops the oscillation as a result. A post-circuit is designed to detect the change of output signal from bridge circuit and generate an alarm signal. In addition, oscillation signal at radio frequency is not only applied to the measurement electrode but also applied to a shield pole through a 1: 1 voltage-follower, so measurement electrode and shield pole have the same electric potential, phase, and frequency, but these two electrodes are insulated from each other. Even if there is medium buildup on probe, because there is no electric potential difference between measurement electrode and shield pole, and the reactance change on shield pole has no effect on the detection of post-circuit. Therefore, only reactance change on measurement electrode induced by the filled medium between measurement electrode and the wall of vessel will be detected by post-circuit. It eliminates the effect of medium buildup on level measurement.

From the perspective of structure, in a pressurized silo, the fine powder can inevitably seep from the gap between the main shaft, bearing and the end cover through the meter housing, causing the main shaft to get stuck, or even the powder accumulate in the housing and damage the electronic module.

The probe of RF Admittance Level Switch adopts a precision rolling process of Teflon tube and stainless steel tube with strong tightness. However, the non-integral structure and low stiffness also prompt the thermal ageing of Teflon under high temperature conditions, which gaps Teflon and stainless steel tube, resulting in the powder gradually penetrating into the meter housing. In high pressure silos, this leakage is particularly serious, even leading to overflow from the gland of electrical interface. This results in many serious problems for sites, like powder escaping into the environment, damage to instruments and buckling of the silo.

Therefore, RF Admittance Level Switch and Rotary Paddle Switch are improper for the level measurement of pressurized silo, at least not the ideal solutions. Powder overflow and leakage resulting in pollution and product loss and can even put lives at risk. However, this can be avoided by implementing proper instrument selection.