Sewage measurement flow meter selection attention

Abstract: Sewage measurement flow meter selection attention information is provided by excellent flow meter and flow meter manufacturers and quotation manufacturers. A flow expert, F.C. King horo, once said that a flowmeter is one of the few meters that is harder to use than to manufacture. This is because flow is a dynamic quantity, and there is not only viscous friction inside the liquid in motion, but also unstable rotation. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of the article selection of sewage measurement flowmeters. A flow expert, F.C. King horo, once said that a flowmeter is one of the few meters that is harder to use than to manufacture. This is because the flow is a dynamic quantity, and there is not only viscous friction inside the liquid in motion, but also complex flow phenomena such as unstable vortex and secondary flow. The measuring instrument itself is affected by many factors, such as: pipeline, caliber size, shape (circle, rectangle), boundary conditions, physical properties of the medium (temperature, pressure, density, steam flowmeter viscosity, dirt, corrosiveness, etc.), fluid The flow state (turbulent flow state, velocity distribution, etc.) and the influence of installation conditions and levels. In the face of more than a dozen types and hundreds of varieties of flow meters at home and abroad (the types of volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic and thermal flowmeters developed successively), how to Reasonable selection of factors such as flow state, installation requirements, environmental conditions, and economy is the premise and basis for the application of flow meters. In addition to ensuring the quality of the instrument itself, it is also very important to provide process data and whether the installation, use and maintenance of the instrument are reasonable. 1. Vortex flowmeter selection and design As a new type of flowmeter, vortex flowmeter has developed rapidly since the mid-1980s. It has many advantages and advantages in flow measurement, and is more and more widely used in modern flow measurement. . The use of vortex flowmeters for flow measurement in my country has also been paid more and more attention. At present, there are product series with excellent performance and independent intellectual property rights in China. Vortex flowmeter is developed based on fluid vibration. According to the difference of vortex, the detection method has gradually developed from hot wire type, thermal type to stress type, magnetic sensitive type, differential switched capacitive type, ultrasonic type, etc. Vortex flowmeters can be used in almost all occasions where a vortex column can be formed, not only in closed pipes, but also in open grooves. Compared with the turbine flowmeter, the vortex flowmeter has no moving mechanical parts, the maintenance workload is small, and the instrument constant is stable; compared with the orifice flowmeter, the vortex flowmeter has a large measuring range, small pressure loss and high accuracy , easy to install and maintain. However, there are many environment-related parameters of the vortex flowmeter, which are easily ignored in the use site and affect the correct performance of the flowmeter. The principle of the vortex flowmeter is that in the flowmeter pipeline, the steam flowmeter is provided with a stagnation piece. When the fluid flows through the stagnation piece, due to the stagnation effect on the surface of the stagnation piece, two columns will be generated downstream of the flowmeter. Asymmetric vortices, these vortices are separated at the side and rear of the stagnation member, forming the so-called Karman vortex row, the rotation direction of the two rows of vortices is opposite, Karman theoretically proved that when h/L=0.281 (h is the width between two vortex rows, and L is the distance between two adjacent vortex rows), the vortex row is stable, in this case, the frequency f of the vortex generated is related to the flow velocity of the fluid in the flowmeter pipeυThe relationship is: f =sv/d (5) v=df/s (6) where d——Diameter of cylindrical stagnation; s——A dimensionless constant, called the Strouhal number, is related to the Reynolds number Re of the fluid flow state. The Reynolds number Re of the circular section pipe of the flowmeter is: (7) in the formulaυ——Fluid velocity, m/s;ρ——The density of the fluid, kg/m3;μ——Dynamic viscosity of fluid, kg?m-1/s. And the flow rate of fluid: Q=A *v It can be seen from the above formula that after the selection and design of the vortex flowmeter, the flow rate Q of the steam flowmeter is not only related to f, but also to the Reynolds number Re. The Reynolds number Re is a dimensionless number that characterizes the flow characteristics of viscous fluids, and its physical meaning is the ratio of the inertial force to the viscous force of the fluid flow. Therefore, the flow state of the fluid also has a certain influence on the use of the vortex flowmeter. If the environmental parameters have an influence on the fluid flow state, it will also affect the performance of the vortex flowmeter. After practice, the following aspects have an impact on the use of vortex flowmeters, and these problems should be analyzed. (1) The measurement range of the vortex flowmeter is large, generally 10:1, but the lower limit of measurement is limited by many factors: Re>10 000 is the most basic condition for the vortex flowmeter to work. In addition, it is also affected by the vortex energy. If the medium flow rate is low, the strength and rotation speed of the vortex will be low, and it is difficult to cause the sensor element to generate a response signal, and the vortex frequency f is also small, which will also make signal processing difficult. The upper limit of measurement is limited by the frequency response of the sensor (such as the magnetic sensitive type generally does not exceed 400Hz) and the frequency of the circuit. Therefore, the flow velocity range must be calculated and calculated during the design, and selected according to the flow velocity of the fluid. The environmental conditions of the use site are complex. In addition to the environmental temperature, humidity, atmosphere and other conditions, electromagnetic interference should also be considered when selecting models. In strong interference such as high-voltage power transmission stations, large-scale rectifiers, etc., magnetic-sensitive, piezoelectric stress-type and other instruments cannot work normally or cannot measure accurately.

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