(2) Supersonic plasma spraying. The principle of supersonic plasma spraying is to input the main gas (nitrogen, with a small flow rate) from the rear gun body, and a large amount of secondary gas (nitrogen or the mixture of nitrogen and hydrogen), under the action of the gas body cyclone ring, will be emitted from the Laval tubular secondary nozzle together with the main gas (tungsten electrode is connected to the negative electrode, and the primary nozzle is connected to the positive electrode when striking the arc), and then conduct high-frequency arc striking in the primary gas, The positive pole is transferred to the secondary nozzle, that is, an arc is generated between the tungsten electrode and the inner wall of the secondary nozzle. Under the strong effect of the rotating secondary gas, the arc is compressed in the center of the nozzle and stretched to the outer edge of the nozzle, forming a high-pressure extended plasma arc. The high-power extended plasma arc effectively heats the main gas and the secondary gas, thus obtaining a stable and concentrated supersonic plasma jet from the nozzle. The sprayed powder is added into the supersonic plasma flow through the powder feeding nozzle to obtain a very high temperature and kinetic energy, and it strikes the workpiece surface to form a coating. The main feature of this method is that it has extremely high heat source temperature (plasma arc temperature up to 16000 ℃) and power, so it can heat the ceramic powder above its melting point in a short time to obtain high-quality coatings. It has the characteristics of high heating temperature of plasma arc spraying and high flying speed of spraying materials of gas explosion spraying and HVOF spraying.
2.3 The actual application of thermal spraying technology in the power industry In the mechanical equipment manufacturing industry, advanced thermal spraying technology is widely used, such as spraying corrosion resistant coating, spraying wear-resistant coating, spraying high-temperature coating, spraying functional coating, spraying molding, etc. These technology applications can improve the leakage and wear resistance of equipment, Especially when the reliability and safety of automatic control in the power industry and the requirements of high-temperature and high-pressure working conditions are more stringent, it provides a broad space for the application of thermal spraying technology. The application of thermal spraying technology in electric power industry mainly includes the following aspects: restoration of plane dimension of generator cylinder; For hydraulic turbine components, HVOF is mainly used to quickly repair hydraulic turbines of hydropower stations damaged by cavitation and sediment abrasion; Application in fan equipment and key components; Thermal power plant boiler; Four pipes; The application of anti-corrosion and erosion resistance (water wall tubes, superheater tubes, reheater tubes and economizer tubes), and the spraying and strengthening repair of generator rotor journal, electric brush, plunger, shaft sleeve, ball mill journal, valve stem and valve seat of high-temperature and high-pressure valves; Spraying of high and intermediate pressure rotor oil shield bearing of steam turbine. It can be seen that thermal spraying technology is widely used in the power industry.

It is better to measure and record the electrode contact resistance of the electromagnetic flow sensor at the time of commissioning of the newly installed instrument, and then measure it every time during maintenance (the resistance values measured by analysis and comparison must be measured by using the same type of multimeter and the same measurement gear). Analysis and comparison will help to judge the instrument fault in the future, and the flow sensor will be omitted from the pipeline for inspection.