Electromagnetic flowmeter electrode maintenance and cleaning method

Electromagnetic flowmeter electrode maintenance and cleaning commonly used methods are the following:

(l) Electrochemical methods

The metal electrode has an electrochemical phenomenon in the electrolyte fluid. According to the electrochemical principle, there is an interface electric field between the electrode and the fluid, and the interface electric field between the electrode and the fluid is caused by the double electric layer existing between the electrode/fluid phase. The study of the electric field at the interface between the electrode and the fluid reveals that the molecules, atoms, or ions of the material have an enriched or lean adsorption phenomenon at the interface, and that most of the inorganic anions are surface-active substances that have typical ion adsorption laws and the surface of inorganic cations. The activity is very small, so the electrochemical cleaning electrode only considers the anion adsorption. Adsorption of anions is closely related to the electrode potential. Adsorption occurs mainly at a potential range that is more positive than the zero charge potential, ie, the surface of a telegraph with an opposite charge. On the surface of the electrode with the same charge, when the residual charge density is slightly larger, the electrostatic repulsive force is greater than the adsorption force, and the anion desorbs quickly. This is the principle of electrochemical cleaning. Some companies add a negative voltage of about 1.2.1.4V to both electrodes by applying the voltage drop of two forward diodes to the signal loop. Because the voltage applied across the two electrodes is a negative dc common-mode voltage, there is no amplifier saturation. The dc common-mode voltage is superimposed on the tiny alternating flow signal. The capacitor isolates the dc and the pre-amplifier rejects the common-mode voltage. The dc common-mode voltage does not affect the flow measurement. The negative DC voltage applied to the electrodes forms a negative electric field that can repel substances that adhere to the electrodes and reach the date of cleaning the electrodes. This method effectively, automatically and continuously performs electrode washing in AC excitation. However, for low-frequency rectangular wave excitation, the effect is not necessarily good due to the high amplitude of the polarization voltage, so it has rarely been seen recently.

(2) Mechanical removal method

The mechanical removal method is achieved by installing a special mechanical structure on the electrode to achieve electrode removal. There are currently two forms:

One is to use a mechanical scraper. A hanging knife with a thin shaft was made of stainless steel, and the hanging knife was led out through the hollow electrode. A mechanical seal was used between the thin shaft and the hollow electrode to prevent the medium from being outside, so that it became a mechanical scraper. When turning the thin shaft from the outside, the hanging knife rotates against the plane of the electrode tip to scrape off dirt. The scraper can be manually or automatically scraped by a motor-driven fine shaft. The other is in the tubular electrode, equipped with a wire brush to remove dirt, and the shaft is wrapped in a sealed “O” ring to prevent fluid leakage. This type of cleaning device requires someone to constantly pull the wire to clean the electrode.

(3) Ultrasonic cleaning method

Ultrasonic voltage of 45-65 kHz generated by the ultrasonic generator is applied to the electrode to increase the energy concentration of the ultrasonic wave. The applied voltage is almost concentrated on the attached matter. The high voltage breaks down the attached matter and is then washed away by the fluid. From the viewpoint of safety, using the electric breakdown method must be a direct connection of the AC (50Hz or 60Hz) high voltage signal to the sensor signal output terminal in the event of a flowmeter interrupt measurement, disconnection of the signal line between the sensor and the converter and the converter, and power outage. Clean it.

(4) Increase the average flow rate in the measuring tube and use the tip small area electrode

In the measurement of media that are prone to fouling and adhesion, sensors that are smaller than the diameter of the process can often be selected to increase the flow rate. Experience shows that the average flow rate in the pipe is higher than 2m/s, and the sedimentation is generally less likely to occur. There is also an instantaneous increase in flow rate of 3-5m/s (depending on the attachment condition) to flush the adhesive layer. The tip of the electrode protrudes into a pointed shape, and the force of flushing by the fluid is large (because the flow velocity of the tube wall is equal to zero, and the tip is separated from the boundary layer of the tube wall and enters the flow velocity layer), so the adhesion contamination is less likely. In addition, since the internal resistance of the small-area electrode itself is large, the influence of the change in the internal resistance of the signal after the adhesion of the electrode is small, so the influence on the measurement of the instrument is also small.