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technical article
How to choose a flow meter for sewage flow measurement
2011-7-11 17:15:30

Electromagnetic flowmeter
Since the first industrial application in China in the late 1950s, the electromagnetic flowmeter has 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 measured medium flows perpendicular to the direction of the magnetic field lines. ) Is proportional to the electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, gravity, etc., there is no pressure loss in the measuring tube inside the contraction or protruding part. In addition, the initial signal detected by the flow element is an The flow rate is a voltage that changes precisely and linearly. It has nothing to do with other properties of the fluid and has great advantages.
According to the characteristics of sewage with large flow changes, impurities, low corrosiveness, and certain conductivity, electromagnetic flowmeters are a good choice. It has a compact structure, small size, and convenient installation, operation, and maintenance. For example, the measurement system uses an intelligent design and the overall seal is strengthened to work normally in harsh environments. An optional electromagnetic flowmeter with neoprene lining and molybdenum stainless steel (OCrI8Ni12Mo2Ti) electrodes can be used to meet the requirements of sewage flow measurement.
During the production of a smelter, a large amount of industrial sewage will be produced due to the needs of the production process. The sewage treatment plant must monitor the sewage flow. In previous designs, many flow meters used vortex flowmeters and orifice flow meters . However, in actual application, it is found that the measured flow display value has a large deviation from the actual flow rate, and the use of an electromagnetic flowmeter greatly reduces the deviation.
2. Vortex flowmeter
As a new type of flowmeter, the 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. In China, the use of vortex flowmeters for flow measurement is gaining more and more attention. At present, China has a product series with excellent performance and independent intellectual property rights. Vortex flowmeters are developed based on fluid vibration. According to different vortices, the detection methods have gradually developed from hot wire type, thermal type, stress type, magnetic type, differential switch capacitor type, and ultrasonic type. Vortex flowmeters can be used in almost all situations where vortex rows can be formed, not only for closed pipes, but also for open trenches. 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 measurement range, small pressure loss, and high accuracy. , Simple installation and maintenance. However, there are many environmental related parameters of vortex flowmeters , which are easily ignored at the site of use and affect the correct performance of the flowmeter.
The principle of the vortex flowmeter is to set a stagnation member in the flowmeter pipe. When the fluid flows through the stagnation member, due to the stagnation effect on the surface of the stagnation member, two rows of asymmetry will be generated downstream. Vortex, these vortices are separated at the side of the stagnation part to form a so-called Karman vortex train. The rotation direction of the two vortex trains is opposite. Carmen theoretically proves that when h / L = 0.281 (h is two vortexes The width between the columns, L is the distance between two adjacent vortices), the vortex array is a stable Reynolds number Re is a dimensionless number that characterizes the flow characteristics of viscous fluids, and its physical meaning is the inertial force and viscosity of the fluid flow The ratio of lag. Therefore, the flow state of the fluid also has a certain impact on the use of the vortex flowmeter. If the environmental parameters have an impact 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 the vortex flowmeter , and these problems should be analyzed.
(1) The measurement range of the vortex flowmeter is large, generally 10: 1, but the lower measurement limit is limited by many factors: Re> 10000 is the most basic condition for the vortex flowmeter to work. In addition, it is also affected by the vortex energy. Limitation, the medium flow velocity is low, the intensity and rotation speed of the vortex are also low, it is difficult to cause the sensor element to generate a response signal, the vortex frequency f is also small, and it also makes signal processing difficult. The upper limit of measurement is limited by the frequency response of the sensor (such as magnetic type generally does not exceed 400Hz) and the frequency of the circuit. Therefore, it is necessary to calculate and calculate the flow velocity range during design, and choose according to the flow velocity of the fluid. The environmental conditions at the application site are complex. In addition to paying attention to conditions such as ambient temperature, humidity, and atmosphere, you must also consider electromagnetic interference. In the case of strong interference such as high-voltage transmission power stations, large rectifiers, etc., the magnetic sensitive and piezoelectric stress meters cannot work normally or cannot be accurately measured.
(2) Vibration is also a great enemy of this type of instrument. Therefore, pay attention to avoid mechanical vibration, especially the lateral vibration of the pipeline (the vibration that is perpendicular to the pipeline axis and the vertical vortex generator body axis). This effect cannot be suppressed or eliminated in the flowmeter structure design. Since the vortex signal is also sensitive to the influence of the flow field, it is not suitable to use it when the length of the straight pipe section cannot guarantee the necessary flow conditions for stabilizing the vortex street. Even the capacitive and ultrasonic types with strong vibration resistance can ensure that the fluid is a fully developed unidirectional flow, which cannot be ignored.
(3) The temperature of the medium also has a great influence on the performance of the vortex flowmeter . For example, the piezoelectric stress type vortex flowmeter cannot be used at 300 ° C for a long time, because its insulation resistance will drop from 10MΩ ~ 100MΩ at normal temperature to 1MΩ ~ 10KΩ, and the output signal will become smaller, which will cause the measurement characteristics to deteriorate. Should choose magnetic sensitive or capacitive structure. 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. It is in the development stage and is not very mature. If it is not selected properly, the performance will not play well. Only after reasonable selection and correct installation, it is necessary to carefully and regularly maintain during use, continuously accumulate experience, improve the predictability of system failures, and the ability to judge and deal with problems, so as to achieve satisfactory results.
3. Throttling flowmeter
Throttle flow meter is a kind of metering device used to measure flow in the early days. It has the longest history and the most usage. Commonly used now are round-hole plate type and conical inlet plate type. The working principle is to add an orifice plate throttle in the fluid pipeline. The differential pressure transmitter is introduced through the pressure guiding pipe to measure the upstream and downstream of the throttle. Differential pressure, the instantaneous value of the flow is calculated according to the measured differential pressure. Due to the immobile nature of the water in the pressure guiding tube, in colder regions, the orifice plate pressure tube installed outdoors in winter is easy to freeze (freeze), making the differential pressure instrument unable to work normally. When measuring dirty sewage, the orifice plate should be cleaned frequently. If the cleaning is not timely, the measurement accuracy will be reduced, the pressure taking tube will often be blocked by dirt, and the instrument will not be used. There are also disadvantages such as large pressure loss and large maintenance when measuring the flow rate by the orifice plate method. Therefore, changing the pressure taking method, for example, using the radial distance pressure taking method, can reduce the influence of the dirt on the orifice plate.
4. Conclusion
Among the above several types of sewage flow measurement flowmeters , electromagnetic flowmeters have better performance and throttle flowmeters have a wider range of applications. Vortex flowmeters are relatively new and are constantly developing. Only by knowing the respective performances of these types of flowmeters can we properly design and select the flowmeters so that the measurement and control of sewage flow can meet the requirements of accuracy and reliability.