Working principle of vortex flowmeter

Under certain flow conditions, part of the fluid kinetic energy will be converted into fluid vibration, and its vibration frequency has a certain proportional relationship with the flow rate (flow rate). This is the principle of the vortex flowmeter. Today, the Automation Channel will introduce you to the application of vortex flowmeters. Features The vortex flowmeter has the following characteristics: 　　①The output is pulse frequency, whose frequency is proportional to the actual volume flow of the fluid being measured, and is not affected by fluid composition, density, pressure, and temperature; 　　②Wide measuring range, general range The degree can reach 10:1 or more; 　　③The accuracy is in the upper-middle level; 　　④No moving parts, high reliability; 　　⑤Simple and firm structure, convenient installation, low maintenance costs; 　　⑥Wide range of applications, applicable to liquids, gases and steam.　　Application in the field　　Vortex flowmeter is suitable for a wide range of fluids, but it is not suitable for low Reynolds number (Re D ≤ 2 × 10 4) fluids. Because when the Reynolds number is low, the Strouhal number changes with the Reynolds number, and the linearity of the meter becomes worse.　　 At the same time, the scouring of the vortex generator by the fluid containing solid particles will produce noise, and the short fibers contained in the vortex generator will change the meter coefficient if it is wound on the vortex generator. The applications of vortex flowmeters in mixed-phase fluids are as follows: 　　①It can be used for gas-liquid two-phase flow with dispersed and uniform microbubbles, but the volumetric gas content should be less than 7%-10%. If the volumetric gas content exceeds 2 % The meter coefficient should be corrected.　　②It can be used for gas-solid and liquid-solid two-phase flow containing dispersed and uniform solid particles with a content of not more than 2%.　　③It can be used for the two-component flow of insoluble liquid (such as oil and water). Pulsating flow and rotating flow will have a serious impact on the vortex flowmeter. If the pulsating frequency coincides with the vortex frequency band, it may cause resonance, destroying normal operation and equipment, and causing the vortex signal to 'lock-in'. At this time, the signal will be fixed at a certain frequency. 'Lock-in' is related to the pulsation amplitude, the shape of the vortex generator and the blocking ratio.　　The accuracy of the vortex flowmeter is approximately between ±0.5%R～±2%R for liquids, and between ±1%R～±2%R for gases, and the repeatability is generally 0.2%～0.5%. Because the meter coefficient of the vortex flowmeter is low, the frequency resolution is low, and the larger the diameter, the lower the accuracy, so the meter diameter should not be too large (below DN300).　　 The wide range is the characteristic of the vortex flowmeter, but the important point is the flow value at the lower limit of the range. Generally, the lower limit of the average flow velocity of liquid is 0.5m/s, and that of gas is 4～5m/s. The normal flow rate of the vortex flowmeter is preferably at 1/2～2/3 of the normal measurement range. The biggest advantage of 　　 vortex flowmeter is that the meter coefficient is not affected by the physical properties of the measuring medium, and can be extended from a typical medium to other mediums. However, due to the large difference in the flow rate range of liquid and gas, the frequency range is also very different. In the amplifier circuit that processes the vortex signal, the passband of the filter is different, and the circuit parameters are also different. Therefore, the same circuit parameter cannot be used to measure different media. In addition, the density of gas and liquid is very different, and the signal intensity generated when the vortex is separated is proportional to the density, so the signal intensity is also very different. The gain and trigger sensitivity of the liquid and gas amplifier circuits are different, the piezoelectric charge is very different, and the parameters of the charge amplifier are also different. Even if it is the same gas (or liquid, steam), as the medium pressure, temperature, and density are different, the flow range used is different, the signal strength is also different, and the circuit parameters also need to change. Therefore, it is not feasible to change the used medium or change the gauge diameter without hardware or software modification for a vortex flowmeter.　　Installation precautions　　Vortex flowmeter is a flowmeter sensitive to pipeline flow velocity distribution distortion, rotating flow and flow arterial motion. Therefore, full attention should be paid to the on-site pipeline installation conditions and strictly follow the instructions for use.　　 Vortex flowmeter can be installed indoors or outdoors. If it is installed in a ground well, in order to prevent it from being submerged by water, a saliva-type sensor should be used. The sensor can be installed horizontally, vertically or obliquely on the pipeline, but in order to prevent the interference of bubbles and droplets when measuring liquids and gases, pay attention to the installation position (see Figure 4).　　 Vortex flowmeter must ensure that the upstream and downstream straight pipe sections have the necessary length (see Figure 5). In Figure 5, a is a 90° elbow, b is a concentric expansion tube, c is a concentric contraction full-open valve, d is two 90° elbows in different planes, e is a regulating valve half-open valve, and f is the same plane Two 90° elbows. The connection between the sensor and the pipeline is shown in Figure 6. When connecting with the pipeline, pay attention to the following issues. ① The inner diameter D of the upstream and downstream piping is the same as the inner diameter of the sensor D', and the difference meets the following conditions: 0.95D ≤ D'≤ 1.1D; 　　② The piping should be concentric with the sensor, and the coaxiality should be less than 0.05D'; Protruding into the pipeline, its inner diameter can be 1～2mm larger than the sensor inner diameter; 　
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