Type selection of flowmeter in the flow measuremen

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Type selection of flowmeter in sewage flow measurement

Abstract: the environmental protection industry has higher and higher requirements for the accuracy and reliability of sewage flow measurement and control. This paper introduces the type selection and design of electromagnetic, vortex street, throttling and other flowmeters, and expounds their respective characteristics from both theoretical and practical aspects in combination with water treatment technology

subject words: flowmeter measurement sewage selection

a foreign flow expert nghoro once said that flowmeter is one of the few viscous friction effects that are harder to use than to manufacture, and it will also produce unstable vortices, secondary flow and other complex flow phenomena. The measuring instrument itself is affected by many factors, such as: pipe, diameter, shape (circular, rectangular), boundary conditions, physical properties of medium (temperature, pressure, density, viscosity, dirt, corrosion, etc.), fluid flow. Therefore, replacing steel with plastic is an important development trend of automobile lightweight. Dynamic state (turbulent state, velocity distribution, etc.) and installation conditions and levels. In the face of more than a dozen types and hundreds of varieties of flow meters at home and abroad (volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic and thermal flowmeter and other types developed successively), how to reasonably select the type according to the flow, flow pattern, installation requirements, environmental conditions, economy and other factors is the premise and foundation for the good 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. This paper introduces the type selection and design of electromagnetic flowmeter, vortex flowmeter and throttling flowmeter

1. Type selection and design of electromagnetic flowmeter

since the first industrial application in China in the late 1950s, electromagnetic flowmeter has been applied and developed rapidly in 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 measured medium flows perpendicular to the direction of the magnetic line of force, so an induced electromotive force ex is generated in the direction perpendicular to the medium flow and the magnetic line of force. The induced electromotive force ex is directly proportional to the flow (flow rate) of the measured medium. The electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, gravity, etc., and there is no pressure loss of shrinkage or bulge in the measuring tube, In addition, different standards detected by the flow element correspond to different test item numbers. It is a voltage that changes linearly with the average flow rate of the fluid. It has nothing to do with other properties of the fluid and has great advantages

according to the characteristics of sewage such as large flow change, impurities, low corrosivity and certain conductivity, electromagnetic flowmeter is a good choice for measuring the flow of sewage. It has compact structure, small volume, convenient installation, operation and maintenance. For example, the measurement system adopts intelligent design, and the overall seal is strengthened, which can work normally in a harsh environment. The electromagnetic flowmeter with neoprene lining and Molybdenum Stainless Steel (ocri8ni12mo2ti) electrode can meet the requirements of sewage flow measurement

in the production of a smelter, due to the needs of production process, a large amount of industrial sewage will be produced, and the sewage treatment branch must monitor the flow of sewage. In the past design, vortex flowmeter and orifice flowmeter were used in many flow instruments. In practical application, it is found that the deviation between the measured flow display value and the actual flow is large, and the deviation is greatly reduced by using electromagnetic flowmeter

2. Vortex flowmeter selection 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. More and more attention has been paid to the use of vortex flowmeter for flow measurement in China. 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 methods have gradually developed from hot wire type and heat sensitive type to stress type, magnetic sensitive type, differential switched capacitor type, ultrasonic type, etc. Vortex flowmeter can be used in almost all occasions where vortex trains can be formed, not only in closed pipelines, but also in open grooves. Compared with the turbine flowmeter, the vortex flowmeter has no movable mechanical parts, the maintenance workload is small, and the instrument constant is stable; Compared with orifice plate flowmeter, vortex flowmeter has the advantages of large measurement range, small pressure loss, high accuracy, and simple installation and maintenance. However, there are many environmental related parameters of vortex flowmeter, which are easy to be ignored in the field of use and affect the correct performance of flowmeter

the principle of vortex street flowmeter is to set a stagnant part in the flowmeter pipe. When the fluid flows through the stagnant part, due to the stagnant effect of the surface of the stagnant part and other reasons, two rows of asymmetric vortices will be generated at the downstream of the stagnant part. These vortices are separated at the side and rear of the stagnant part to form the so-called Karman vortex row, and the rotation directions of the two rows of vortices are opposite, Carmen proved theoretically that when h/l=0.281 (H is the width between two vortex trains, l is the distance between two adjacent vortices), the vortex train is a stable Reynolds number, re is a dimensionless number representing the flow characteristics of viscous fluid, and its physical significance is the ratio of inertia force and viscosity force of fluid flow. Therefore, the flow state of fluid also has a certain impact on the use of vortex flowmeter. If environmental parameters have an impact on the fluid flow state, it will also affect the performance of vortex flowmeter

through practice, the following aspects have an impact on the use of vortex flowmeter, and these problems should be analyzed

(1) the measurement range of vortex flowmeter is large, generally 10:1, but the lower limit of measurement is limited by many factors: re> 10000 is the most basic condition for the operation of vortex flowmeter. In addition, it is also limited by vortex energy. If the medium flow rate is low, the intensity and rotation speed of vortex are also low, which is difficult to cause the response signal of sensing elements, the vortex frequency f is also small, and it will also make signal processing difficult. The upper limit of measurement is limited by the frequency response of the sensor (such as magnetic sensitive type, which generally does not exceed 400Hz) and the frequency of the circuit. Therefore, the flow rate range must be calculated and calculated in the design, and the selection must be made according to the flow rate of the fluid. The environmental conditions on the use site are complex. In addition to paying attention to the environmental temperature, humidity, atmosphere and other conditions, electromagnetic interference should also be considered in the selection. In places with strong interference, such as high-voltage power transmission stations and large rectifier stations, magnetic sensitive and piezoelectric stress instruments can not work normally or can not measure accurately

(2) vibration is also a strong enemy of this kind of instrument. Therefore, pay attention to avoid mechanical vibration during use, especially the transverse vibration of the pipeline (the vibration perpendicular to the axis of the pipeline and the axis of the vortex generator), which cannot be suppressed and eliminated in the structural design of the flowmeter. Because the vortex signal is also sensitive to the influence of flow field if it is not needed, it is not suitable to choose when the length of straight pipe can not ensure the necessary flow conditions for stabilizing vortex. Even capacitor type and ultrasonic type with strong vibration resistance can not be ignored to ensure that the fluid is a fully developed unidirectional flow

(3) the medium temperature also has a great influence on the service performance of vortex flowmeter. If the pressure stress vortex flowmeter cannot be used at 300 ℃ for a long time, its insulation impedance will drop sharply from 10m Ω ~ 100m Ω at room temperature to 1m Ω ~ 10K Ω, and the output signal will also become smaller, resulting in the deterioration of the measurement characteristics. For this, the magnetic sensing or capacitive structure should be selected. In the measurement system, the sensor and converter should be installed separately to avoid long-term high temperature affecting the reliability and service life of the instrument

vortex flowmeter is a relatively new type of flowmeter, which is in the development stage and is not very mature. If it is improperly selected, its performance will not play well. Only after reasonable selection and correct installation, it is also necessary to carefully and regularly maintain in the process of use, constantly accumulate experience, and improve the predictability of system faults and the ability to judge and deal with problems, so as to achieve satisfactory results

3. Type selection and design of throttling flowmeter

throttling flowmeter is a metering device widely used in the early stage to measure flow, with the longest history and the largest consumption. At present, the common ones are circular orifice plate type and conical inlet plate type. Its working principle is to add an orifice plate throttling part into the fluid pipeline, and measure the pressure difference between the upstream and downstream of the throttling part through the differential pressure transmitter through the impulse pipe. According to the measured pressure difference, the instantaneous value of the flow is calculated. Due to the non fluidity of water in the impulse pipe, the orifice pressure pipe installed outdoors in winter is easy to freeze crack (freeze), making the differential pressure instrument unable to work normally. When measuring dirty sewage, the orifice plate needs to be cleaned frequently. If the cleaning is not timely, the measurement accuracy will be reduced, and the pressure tap will often be blocked by dirt, making the instrument unusable. When using orifice to measure flow, there are some disadvantages, such as large pressure loss, large maintenance, etc. Therefore, changing the pressure taking method, such as using the diameter distance pressure taking method, can reduce the influence of orifice dirt

4. Conclusion

among the above sewage flow measurement flowmeters, electric 5. Polyethylene terephthalate pet magnetic flowmeter has better performance, and the throttling flowmeter has a wide range of applications, while the vortex flowmeter is relatively new and is constantly developing. Only by understanding the respective performances of these flowmeters can we design the type selection of flowmeters and make the measurement and control of sewage flow meet the requirements of accuracy and reliability

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