The environmental protection industry has increasingly higher requirements for the accuracy and reliability of the measurement and control of sewage flow. Both theory and practice expound their respective characteristics.
1. Electromagnetic flowmeter selection design
Electromagnetic flowmeters have been used and developed rapidly in flow measurement in the 1970s and 1980s since the domestic industrial application in the late 1950s. 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 a direction perpendicular to the medium flow and the magnetic line of force. ), the electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, and severity. There is no contraction or pressure loss in the protruding part of the measuring tube. In addition, the initial signal detected by the flow element is an average of the fluid The flow rate is a voltage with a precise linear change, which 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 corrosivity, and certain conductivity, electromagnetic flowmeters are a good choice for measuring the flow of sewage. It has compact structure, small size, convenient installation, operation, and maintenance. For example, the measurement system adopts intelligent design and the overall sealing is strengthened, which can work normally in a relatively inferior environment. An electromagnetic flowmeter with neoprene lining and stainless steel electrodes containing molybdenum can be selected to meet the requirements of sewage flow measurement.
During the production of a certain refinery, due to the needs of the production process, a large amount of industrial sewage will be produced. The sewage treatment branch must monitor the flow of sewage. In the past design, many flow meters used vortex flowmeters and orifice flowmeters. In actual applications, it was found that the measured flow display value had a large deviation from the actual flow rate, and the deviation of the electromagnetic flowmeter was greatly reduced.
2. Selection and design of 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 it is more and more widely used in modern flow measurement. In China, the use of vortex flowmeters for flow measurement has also received more and more attention. At present, my country has a product series with excellent performance and independent intellectual property rights. The vortex flowmeter is developed based on fluid vibration. According to the different vortex, the detection method has gradually developed from the hot wire type and the heat sensitive type to the stress type, the magnetic sensitive type, the differential switch capacitive type, and the ultrasonic type. The vortex flowmeter can be used in almost all occasions where vortex rows 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 meter constant is stable; compared with the orifice flowmeter, the vortex flowmeter has a large measuring range and small pressure loss, high accuracy, Installation and maintenance are simple. However, there are many environment-related parameters of vortex flowmeters, which are easy to be ignored at the use site and affect the correct performance of the flowmeter.
The principle of the vortex flowmeter is to install a stagnant part in the flowmeter pipe. When the fluid flows through the stagnant part, due to the stagnant effect on the surface of the stagnant part, two rows of asymmetrical flow will be generated downstream of the stagnant part. The vortices, these vortices are separated on the side and rear of the stagnation part, forming the so-called Karman vortex series. When the width between the columns, L is the distance between two adjacent vortices), the vortex column is a stable Reynolds number Re is a dimensionless number that characterizes the flow characteristics of the viscous fluid. The ratio of stagnation. Therefore, the flow state of the fluid also has a certain impact on the use of vortex flowmeters. If the environmental parameters have an impact on the fluid flow state, it will also affect the performance of the vortex flowmeter.
Through 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 relatively large, generally 10:1, but the lower limit of measurement is limited by many factors: Re>10000 is the basic ZUI condition for the vortex flowmeter to work. In addition, it is also affected by the vortex energy. The limitation is that if the flow rate of the medium is low, the strength and rotation speed of the vortex are also low, and it is difficult to cause the sensing element to generate a response signal. 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 (for example, the magnetic sensor generally does not exceed 400Hz) and the frequency of the circuit. Therefore, the flow rate range must be calculated and accounted for during the design, and the selection should be made according to the flow rate of the fluid. The environmental conditions at the use site are complicated, and electromagnetic interference should be considered in addition to environmental temperature, humidity, atmosphere and other conditions when selecting models. In strong interference, such as high-voltage power transmission stations, large-scale rectifier stations, etc., magnetic sensitive, piezoelectric stress and other instruments cannot work normally or cannot accurately measure.
(2) Vibration is also a major enemy of this type of instrument. Therefore, attention should be paid to avoid mechanical vibration when using, especially the transverse vibration of the pipe (vertical to the pipe axis and vertical vortex to generate the vibration of the body axis). This effect cannot be suppressed and eliminated in the flowmeter structure design. Since the vortex street signal is also sensitive to the influence of the flow field, it is not suitable to use when the length of the straight pipe section cannot guarantee the necessary flow conditions for the stable vortex street. Even the capacitive and ultrasonic types with strong vibration resistance can not be ignored to ensure that the fluid is a fully developed unidirectional flow.
(3) The temperature of the medium also has a great influence on the performance of the vortex flowmeter. For example, the pressure stress type vortex flowmeter cannot be used for a long time at 300℃, because its insulation resistance will change from 10MΩ at room temperature.
~100MΩ drops sharply to 1MΩ~10KΩ, and the output signal also becomes smaller, which leads to deterioration of the measurement characteristics. For this, a magnetic sensitive or capacitive structure should be used. 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. The vortex flowmeter is a relatively new type of flowmeter, which is in the development stage and not very mature. If it is not selected properly, the performance will not be able to play well. Only after reasonable selection and correct installation, it is necessary to earnestly and regularly maintain during use, and 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. Selection and design of throttling flowmeter
Throttling flowmeter is a kind of metering device used in large quantities in the early days to measure flow. It has the longest history and uses a lot of ZUI. Now the common ones are round orifice plate type and tapered inlet plate type. The working principle is to add an orifice plate throttling piece in the fluid pipeline, and introduce the pressure difference transmitter through the pressure guiding tube to measure the upstream and downstream of the throttling piece. The pressure difference is calculated according to the measured pressure difference to obtain the instantaneous value of the flow. Due to the immobility of the water in the pressure guiding tube, in colder regions, the orifice plate pressure tube installed outdoors in winter is easy to freeze and crack (frozen), making the differential pressure instrument unable to work normally. When measuring dirty sewage, the orifice plate needs to be cleaned frequently. If it is not cleaned in time, the measurement accuracy will be reduced, the pressure tube is often blocked by dirt, and the instrument cannot be used. There are disadvantages such as large pressure loss and large maintenance amount when using the orifice method to measure the flow rate. Therefore, changing the pressure method, such as the diameter pressure method, can reduce the influence of the orifice plate dirt.
Among the above several types of flowmeters for sewage flow measurement, electromagnetic flowmeters have better performance and a wide range of applications for throttling flowmeters, while vortex flowmeters are relatively new and are constantly being developed. Only by understanding the performance of these types of flowmeters, can the flowmeter selection and design be well, so that the measurement and control of sewage flow can meet the requirements of accuracy and reliability.