Hardware Punching Machine,Metal Automatic Punching Machine Equipment,Metal Automatic Punching Machine,Metal Automatic Punching Equipment Shenzhen Yonglun Intelligent Equipment Technology Co., LTD , https://www.foyooo.com
1. Electromagnetic Flowmeter Selection Design Since the first industrial application in China in the late 1950s, electromagnetic flowmeters have been used and developed rapidly in the flow measurement in the 1970s and 1980s. The working principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction, that is, the medium to be measured flows perpendicularly to the direction of the magnetic force lines, and thus generates an induced electromotive force EX in a direction perpendicular to the medium flow and the magnetic force lines. 
(1)
Where Ex - EMF, V;
B - magnetic induction, T;
d - distance between two electrodes, mm;
Ï… - Mean flow rate of measured medium, m/s.
The flow Q of the measured medium is related to d and Ï…: 
(2)
From equation (2): 
(3)
When the magnetic field strength B and the distance d between two poles are constant, then:
Ex=K*Q(4)
Among them, K=4*10-4/Ï€d=constant.
From equation (4), it can be seen that the induced electromotive force EX is proportional to the measured medium flow rate (flow rate), and the electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, and severeness, and the pressure inside the measuring tube is not constricted or protruded. Loss, in addition, the initial signal detected by the flow element is a voltage that varies linearly with the average flow velocity of the fluid, and it is independent of the other properties of the fluid and has great advantages.
According to the sewage has the characteristics of large flow variation, impurities, low corrosion, certain conductivity, etc., to measure the flow of sewage, electromagnetic flowmeter is a good choice. Its compact structure, small size, easy installation, operation, maintenance, such as the measurement system using intelligent design, the overall seal to strengthen, can work in harsh environments. Selectable neoprene lining, electromagnetic flowmeter containing molybdenum stainless steel (OCrI8Ni12Mo2Ti) electrode, can meet the requirements of the sewage flow measurement.
A smelter in the production, due to the needs of the production process, will produce a large amount of industrial sewage, sewage treatment plant must monitor the flow of sewage. In previous designs, many flow meters use vortex and orifice flowmeters. In practice, it is found that the measured flow rate deviation from the actual flow rate is large, and the deviation of the electromagnetic flowmeter is greatly reduced.
2. Vortex flowmeter selection design Vortex flowmeter as a new flowmeter, has developed rapidly since the mid-80s, it has many advantages and strengths in the flow measurement, more and more widely used in modern flow measurement. . The use of vortex flowmeters for flow measurement in China has also received increasing attention. At present, China has a series of products with excellent performance and independent intellectual property rights. Vortex flowmeter is developed based on fluid vibration. According to the different vortices, the detection method is gradually developed from hot wire type, thermal type, stress type, magnetic sensitivity type and differential switched capacitor type, ultrasonic type and so on. Vortex flowmeters can be used in almost any application where a vortex array can be formed, not only for closed pipes, but also for open trenches. Compared with the turbine flowmeter, the vortex flowmeter has no movable mechanical parts, and the maintenance workload is small. The meter constant is stable. Compared with the orifice plate flowmeter, the vortex flowmeter has a large measurement range, low pressure loss, and high accuracy. , Easy to install and maintain. However, vortex flowmeters have a lot of environment-related parameters, which are easily overlooked in the use site and affect the proper performance of the flowmeter.
The principle of the vortex flowmeter is to set a stagnation element in the flowmeter pipeline. When the fluid flows through the stagnation element, due to the stagnation effect of the surface of the stagnation element, two columns of asymmetry will be generated downstream of the stagnation flow element. Whirlpools, these vortices are separated laterally behind the stagnation element, forming a so-called Karman vortex array. The rotation direction of the two vortexes is reversed. Karmen theoretically proves that when h/L=0.281 (h is two vortices. When the width between the columns, L, is the distance between two adjacent vortices), the vortex array is stable. In this case, the relationship between the frequency f of the generated vortex and the fluid velocity Ï… in the flowmeter pipeline is:
f=sv/d(5)
v=df/s(6)
Where d is the diameter of a cylindrical stagnation;
s - dimensionless constant, called the Strouhal number, related to the Reynolds number Re of the fluid flow state.
The Reynolds number Re of the circular section of the flowmeter is: 
(7)
In the formula, the flow rate of the fluid, m/s;
Ï - fluid density, kg/m3;
Μ—dynamic viscosity of the fluid, (kg·m-1)/s.
And the flow of fluid:
Q=A*v
As can be seen from the above formula, the vortex flowmeter selection design is completed, and the flow Q is not only related to f, but also related to the Reynolds number Re. The Reynolds number Re is a dimensionless number that characterizes the flow characteristics of a viscous fluid. Its physical meaning is the ratio of the inertial force to the viscous force of the fluid flow. Therefore, the fluid flow state also has a certain influence on the use of vortex flowmeters. If the environmental parameters have an effect 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 vortex flowmeter is relatively large, generally 10:1, but the lower limit of measurement is limited by many factors: Re> 10000 is the most basic condition for vortex flowmeter work. In addition, it is also affected by vortex energy. The limitation is that if the flow velocity of the medium is low, the strength and rotation speed of the vortex are also low, and it is difficult to cause the sensor element to generate a response signal, and the vortex frequency f is also small, which also makes signal processing difficult. The upper limit of the measurement is affected by the frequency response of the sensor (such as the magnetic sensitivity type generally does not exceed 400Hz) and the frequency limit of the circuit. Therefore, the flow rate range must be calculated, calculated, and selected according to the flow rate of the fluid. The use of on-site environmental conditions is complex. In addition to the environmental temperature, humidity, and atmosphere conditions, electromagnetic interference must be considered when selecting a model. In strong interferences such as high-voltage power transmission stations, large-scale rectifiers and other occasions, magnetic-sensitive, piezo-electric stress and other instruments cannot work properly or cannot be accurately measured.
(2) Vibration is also a major rival of this type of instrument. Therefore, when using, pay attention to avoid mechanical vibration, especially the lateral vibration of the pipe (the vibration of the vertical vortex generating body axis perpendicular to the axis of the pipe). This effect cannot be suppressed and eliminated in the structure design of the flowmeter. Since the vortex street signal is also sensitive to the influence of the flow field, the length of the straight pipe section cannot be used to ensure the necessary flow conditions for a stable vortex street. Even a capacitive and ultrasonic type with strong vibration resistance ensures that the fluid is a fully developed unidirectional flow that cannot be ignored.
(3) The medium temperature also has a great influence on the performance of vortex flowmeters. For example, pressure-stressed vortex flowmeters cannot be used at 300°C for a long period of time, because the insulation resistance will suddenly drop from 10MΩ to 100MΩ at room temperature to 1MΩ to 10KΩ, and the output signal will also become smaller, resulting in deterioration of the measurement characteristics. Use magnetic or capacitive structure. In the measurement system, the sensor and converter should be separated and installed so as to avoid long-term high temperature affecting the reliability and service life of the meter.
Vortex flowmeter is a relatively new type of flowmeter and it is in the development stage. It is not yet very mature. If it is not properly selected, the performance cannot be played well. Only after a reasonable selection and correct installation, but also need to maintain regularly in the use of the process, and constantly accumulate experience to improve the predictability of the system failure and ability to judge, deal with the problem, so as to achieve satisfactory results.
3. Throttle flow meter selection design Throttle flow meter is a large amount of measurement equipment used in the early measurement of flow, its longest history, the largest amount. Commonly used are round hole plate type and conical inlet plate type. Its working principle is to add a orifice plate throttling element in the fluid pipe and introduce a pressure difference transmitter through the pressure pipe to measure the upstream and downstream of the throttling element. The pressure difference, calculated according to the measured pressure difference, yields the instantaneous value of the flow. Due to the non-flowability of the water in the pressure guiding pipe, in the colder regions, the orifice plate for outdoor installation in winter is prone to freezing (frozen), making the differential pressure instrument unable to work properly. When measuring dirty sewage, the orifices need to be cleaned frequently. If the cleaning is not timely, the measurement accuracy is reduced, the pressure tube is often blocked by dirt, and the instrument cannot be used. There are also disadvantages such as large pressure loss and large maintenance when measuring the flow rate with the orifice plate. Therefore, changing the pressure mode, for example, using the span pressure method, can reduce the influence of the orifice dirt.
4. Conclusion In the above several kinds of sewage flow measurement flowmeter, electromagnetic flowmeter has good performance, throttling flowmeter has a wide range of applications, and vortex flowmeter is relatively new, and is continuously developing. Only by understanding the respective performances of these types of flowmeters can the design of the flowmeters be well designed, so that the measurement and control of the sewage flow rate can meet the accuracy and reliability requirements.
Flowmeter Selection and Discussion in Sewage Flow Measurement
Keywords: Sewage flow measurement, flow meter, accuracy One foreign flow expert, FC Kinghoro, once said that flow meters are one of the few that use more difficult to manufacture than meters. This is because the flow is a dynamic quantity. In the liquid in motion, there is not only viscous friction, but also unstable vortexes and secondary flow and other complex flow phenomena. The measuring instrument itself is subject to many factors, such as: pipeline, size, shape (circular, rectangular), boundary conditions, physical properties of the medium (temperature, pressure, density, viscosity, dirtiness, corrosion, etc.), fluid flow state (turbulence conditions, speed distribution, etc.) and the effect of installation conditions and levels. Faced with more than a dozen or more types of flow meters at home and abroad (volumetric, differential pressure, turbo, area, electromagnetic, ultrasonic, and thermal flow meters, etc.) Reasonable selection of flow conditions, flow conditions, installation requirements, environmental conditions, and economic factors are prerequisites and foundations for the application of flow meters. In addition to ensuring the quality of the instrument itself, it is also very important to provide the process data and the installation, use, and maintenance of the instrument. This article describes the electromagnetic type, vortex type, throttling type flow meter selection design.