Field application of orifice flow meter in power industry

Abstract: The on-site application information of orifice flow meter in the power industry is provided by excellent flow meter and flow meter manufacturers. When the steam-water pipeline of the power plant does not need to adjust different pressures according to the requirements of the system, but the pressure difference between the front and rear of the pipeline is large, the method of increasing the throttling orifice is often used. The principle is: when the fluid flows in the pipeline, due to the Local resistance makes fluid. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following is the details of the on-site application of orifice flowmeters in the power industry. When the steam-water pipeline of the power plant does not need to adjust different pressures according to the requirements of the system, but the pressure difference between the front and rear of the pipeline is large, the method of increasing the throttling orifice is often used. The principle is: when the fluid flows in the pipeline, due to the The local resistance reduces the pressure of the fluid and loses energy. This phenomenon is called throttling in thermodynamics. This method is simpler than using a regulating valve, but it must be selected properly, otherwise, the liquid is prone to cavitation, which affects the safe operation of the pipeline. 1. Cavitation phenomenon The function of throttling orifice flowmeter is to reduce the aperture in the appropriate place of the pipeline. When the liquid passes through the constriction, the flow beam will become thin or shrink. The smallest cross-section of the stream appears downstream of the actual constriction and is called the systolic cross-section. At the systolic flow section, the flow velocity is the largest, and the increase of the flow velocity is accompanied by a great decrease in the pressure at the systolic flow section. As the beam expands into a larger area, the velocity drops and the pressure increases, but the downstream pressure does not fully return to the upstream pressure, as a result of greater internal turbulence and energy consumption. If the pressure pvc at the cross-section of the shrunken flow drops below the saturated vapor pressure pv at the temperature corresponding to the liquid, steam and gas dissolved in the water will escape from the flow stream, forming small bubbles where steam and gas are mixed. More bubbles. If the pressure p2 downstream of the orifice is still lower than the saturated vapor pressure of the liquid, bubbles will continue to be generated in the downstream pipeline, and the two phases of liquid and vapor will be mixed. This phenomenon is flash evaporation. If the downstream pressure is restored to higher than the saturated vapor pressure of the liquid, the bubbles will rapidly condense and rupture under the action of high pressure. At the moment when the bubbles burst, local cavities are generated, and the high-pressure water flows to these original bubbles at a very high speed. The occupied space forms an impact. Because the gas and steam in the bubbles are too late to dissolve and condense in an instant, they are divided into small bubbles under the action of the impact force, and then compressed and condensed by the high-pressure water. The noise of the sand flowing through the pipeline, this phenomenon is called cavitation. Under the action of water hammer pressure, the surface of the runner material is fatigued and seriously damaged. We call the entire process of the formation, development and rupture of gas bubbles that damage the material as cavitation. The main difference between flashing and cavitation is whether or not the bubble bursts. In the system pipeline with flash evaporation, because the medium is a two-phase flow of steam and water, the specific volume and flow rate of the medium are doubled, and the scouring surface is very worn, which shows that the scouring surface has a smooth and polished appearance. Flash evaporation also produces noise and vibration, but the sound level is generally below 80 dB, which does not exceed the allowable range specified in the specification. Cavitation is not the case. Bubble burst and high-speed impact will cause serious noise, and the pipeline will vibrate greatly. On the extremely small area of ​​the flow channel surface, the pressure formed by the impact force can be as high as hundreds or even gigapascals, and the impact frequency can reach Tens of thousands of times per second may cause serious damage to the scouring surface in a short period of time, which is manifested as a rough and rough surface similar to cinders on the scouring surface. Moreover, reactive gases such as oxygen escaping from the liquid will release heat when condensing with the help of bubbles, which will also have a chemical corrosion effect on metals. Whether it is flash evaporation or cavitation, it will cause different degrees of damage to the pipeline, which is detrimental to safe operation. Therefore, the occurrence of these two situations should be avoided when choosing a throttling orifice flowmeter. Since the pressure downstream of the orifice is often higher than the saturated vapor pressure of the liquid, the most important thing when choosing a throttling orifice is to prevent cavitation. 2. Methods to prevent fluid from cavitation For cavitation, replacing the scouring surface with advanced materials is not a complete solution to the problem. Controlling the pressure pvc at the systolic flow section and keeping the pressure not lower than the saturated vapor pressure pv of the liquid is the only way to solve the problem. A fundamental measure to prevent cavitation. For pipelines with large pressure drop, multi-stage decompression can be used to ensure that the pressure of the medium passing through each shrunken flow section is greater than the saturated vapor pressure of the liquid. 

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