There are many kinds of flow measurement methods and instruments, and there are many classification methods, including rotameter, throttling flowmeter, slit flowmeter, volumetric flowmeter, electromagnetic flowmeter, ultrasonic flowmeter and weir. Among various flow meters, each product has its specific applicability and limitations. According to the measurement object, there are two categories: closed pipeline and open channel; According to the purpose of measurement, it can be divided into total amount measurement and flow measurement. Its instruments are called total amount meter and flowmeter respectively.
The total meter measures the flow through the pipeline over a period of time, which is expressed as the quotient of the total flow in a short period of time divided by that time. In fact, the flowmeter is usually equipped with a cumulative flow device for use as a total meter, and the total meter is also equipped with a flow transmitting device. Therefore, it has no practical significance to divide the flowmeter and the total meter in a strict sense.
According to the measurement principle, it can be divided into mechanical principle, thermal principle, acoustic principle, electrical principle, optical principle and atomic physics principle.
1 ultrasonic flowmeter
When ultrasonic wave propagates in the flowing fluid, it carries the information of fluid velocity. Through the received ultrasonic wave, the fluid velocity can be detected and converted into flow. According to the detection method, it can be divided into different types of ultrasonic flowmeter, such as propagation velocity difference method, Doppler method, beam offset method, noise method and correlation method. Ultrasonic flowmeter is a kind of fluid metering method that has been applied only with the rapid development of integrated circuit technology in recent ten years.
Ultrasonic flowmeter is a non-contact measuring instrument, which is suitable for measuring fluids that are not easy to contact and observe and large pipe runoff. It is linked with the water level gauge to measure the flow of open water flow. The use of ultrasonic flowmeter does not need to install measuring elements in the fluid, so it will not change the flow state of the fluid and produce additional resistance. The installation and maintenance of the instrument will not affect the operation of the production pipeline, so it is an ideal energy-saving flowmeter.
All kinds of ultrasonic flow meters can be installed outside the pipe, which is non-contact flow measurement. The cost of the instrument is basically independent of the diameter of the measured pipe; The cost of other types of flow meters increases significantly with the increase of diameter. Therefore, the larger the diameter, the better the function price ratio of ultrasonic flow meter than other types of flow meters with the same function. It is considered to be a better large pipe diameter flow measurement instrument. Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used for the measurement of sewage flow such as sewer and sewage. In power plants, it is much more convenient to use portable ultrasonic flowmeter to measure large pipe runoff such as water inflow of hydraulic turbine and circulating water of steam turbine than pitot tube flowmeter in the past. Ultrasonic flow measurement can also be used for gas measurement. The applicable range of pipe diameter is 2cm ~ 5m, which can be used from open or closed channels several meters wide to rivers 500 meters wide.
In addition, the flow measurement accuracy of ultrasonic measuring instrument is hardly affected by the temperature, pressure, viscosity, density and other parameters of the measured fluid, and can be made into non-contact and portable measuring instruments. Therefore, it can solve the flow measurement problems of highly corrosive, non-conductive, radioactive, flammable and explosive media that are difficult to measure by other types of instruments. In view of the characteristics of non-contact measurement, coupled with reasonable electronic circuits, one instrument can adapt to a variety of pipe diameter measurement and a variety of flow range measurement. The adaptability of ultrasonic flowmeter is unmatched by other instruments. Ultrasonic flowmeter has some of the above advantages, so it has attracted more and more attention, and is developing towards product serialization and generalization. It has been made into standard, high-temperature, explosion-proof and wet instruments with different sound channels to adapt to the flow measurement of different media, different occasions and different pipeline conditions.
2 electromagnetic flowmeter
Electromagnetic flowmeter is a new type of flow measurement instrument developed rapidly with the development of electronic technology in the 1950s and 1960s. Electromagnetic flowmeter is made according to Faraday’s law of electromagnetic induction, which is used to measure the volume flow of conductive liquid. Because of its unique advantages, it has been widely used in the flow measurement of various conductive liquids in industry and agriculture, such as the flow measurement of various corrosive media such as acid, alkali and salt and various slurry, forming a unique application field.
In structure, the electromagnetic flowmeter is composed of electromagnetic flow sensor and converter. The sensor is installed on the pipeline. Its function is to linearly convert the volume flow value of the liquid flowing into the pipeline into an induced potential signal, and send the signal to the converter through the transmission line. The converter is installed not far from the sensor. It amplifies the flow signal sent by the sensor and converts it into a standard electrical signal output proportional to the flow signal for display, accumulation, regulation and control.
Application of flowmeter in farmland irrigation
In the national economy, the vigorous construction of farmland and water conservancy is an important measure for China’s agricultural development. The construction of water conservancy projects is inseparable from irrigation, especially it is very important to correctly grasp the method of measuring flow in the irrigation system. At present, there are two main methods, namely velocity area method and hydraulic method. Each measurement method has its unique flow sensing (detection) part.
The velocity-area method measures flow by multiplying the average velocity of the water flow by the cross-sectional area of the channel, and the velocity of the water flow is measured at specific points by a flow meter. There are generally three different ways to measure flow by this method: one is the velocity distribution method; the other is the 0.6h method (h is the distance from the water surface to the bottom of the canal); the third is the 0.2h-0.8h method. The velocity distribution method is to measure the flow velocity of several points between the water surface and the bottom of the canal, and then determine a velocity curve of the vertical water surface from these measuring points, and divide the area defined by the velocity curve by the depth to obtain the average velocity; 0.6h method The flow velocity of a single point at a depth of 0.6h below the water surface is measured, and it is assumed that this value is the average value of all the velocity; The average of the readings can be estimated as the average flow rate value.
All three methods calculate the average speed within any particular measurement area. The number of soundings along the entire width of the channel and the number of points measured on a single vertical plane varies. Many European countries require 5 points for sounding a single surface, while the United States considers 2 points sufficient. The number of soundings depends on the width of the channel. The number of soundings in Europe is 5 to 15, and in the United States it is 8 to 18. The measurement error is determined according to the number of soundings and the measurement time at each point. The average error of the velocity-area method can be less than 6%.
There are two techniques for measuring flow by the velocity-area method. Two points should be paid special attention to: one is to use an ultrasonic flowmeter, and the other is to use an electromagnetic flowmeter.
Ultrasonic technology calculates the flow rate by measuring the ultrasonic wave propagation time across the channel. The transmitter and receiver are placed on both sides of the channel, so that the angle between the path of the wave and the direction of the water flow is in the range of 45° to 60°. This geometric relationship creates a time difference between the sound pulses in the upstream and downstream directions. The shape of the channel determines whether a single-path system or a multi-path system will be used, that is, using one or several transmit-receive pairs.
The electromagnetic technique of current measurement uses an artificially created magnetic field in a conductive water stream, and then measures the resulting potential difference across both sides of the water stream. The electromagnetic flowmeter is mainly composed of a water channel, an excitation coil and a pair of electrodes.
4 Selection of flowmeters
The selection of the flowmeter refers to the safety, accuracy and economy of the measurement based on the actual supply of the instrument product according to the production requirements, and the method and method of the flow sampling device are determined according to the nature and flow of the measured fluid. Measuring instruments, and the form and size of measuring instruments.
The safety and reliability of flow measurement is first of all a reliable measurement method, that is, the sampling device will not cause an accident due to mechanical strength or electrical circuit failure during operation; secondly, the measuring instrument will not affect the safety of the production system under normal production or failure conditions.
On the basis of ensuring the safe operation of the instrument, we strive to improve the accuracy and energy saving of the instrument. To this end, not only the display instrument that meets the accuracy requirements should be selected, but also a reasonable measurement method should be selected according to the characteristics of the measured medium. For the measurement of the main steam flow of the power plant, because it is very important to the safety and economy of the power plant, a mature standard throttling capacity and a differential pressure flowmeter are generally used. The sewage and fuel oil treated by chemical water belong to dirty flow and low Reynolds number viscous flow respectively, and neither standard throttling parts are applicable. For dirty flow, non-standard throttling parts such as circular orifice plate are generally used with differential pressure gauge or ultrasonic Doppler flowmeter, while for viscous flow, volumetric flowmeter, target type or wedge flowmeter can be used respectively. The inlet water volume of the turbine, the circulating water volume of the condenser and the regenerative steam of the regenerative unit are all flow measurement parameters of large pipe diameter (above 400mm). Due to the difficulty in processing and large pressure loss, standard throttling devices are generally not used. According to the characteristics of the medium to be measured and the requirements of measurement accuracy, the flow rate can be measured by using plug-in flowmeter, speed measuring element with differential pressure gauge, ultrasonic flowmeter or non-destructive methods such as marking method and simulation method. For more information, please visit: http:/ /www.ybhrun.com.
In order to ensure the service life and accuracy of the flowmeter, the anti-vibration requirements of the instrument should also be paid attention to when selecting the model. In humid and hot areas, choose a humid and hot meter. The correct selection of the specifications of the instrument is also an important part of ensuring the service life and accuracy of the instrument. Special attention should be paid to the selection of static pressure and temperature resistance. The static pressure of the instrument (that is, the degree of pressure resistance) should be slightly greater than the working pressure of the measured medium, generally 1.25 times, to ensure that no leakage or accident occurs. The selection of the measuring range is mainly the selection of the upper limit of the scale of the instrument. If it is too small, it is easy to overload and damage the instrument; if it is too large, it will hinder the accuracy of the measurement. Generally selected as 1.2 to 1.3 times the maximum flow value in actual operation.
For long-term contact meters installed on production pipelines, the energy loss caused by the flow measuring element should also be considered. In general, multiple measuring elements with large pressure loss, such as throttling elements, should not be used in the same production pipeline.
5 Conclusion
In short, there is no single measurement method or flow meter that is suitable for every fluid and flow situation. Different measurement methods and structures require different measurement operations, usage methods and usage conditions, and each type has its own unique advantages and disadvantages. Therefore, on the basis of a comprehensive comparison of various measurement methods and instrument characteristics, the best type that is suitable for production requirements, safe and reliable, economical and durable should be selected. With the development of my country’s agricultural modernization and the realization of precision agriculture, flowmeters will play an increasingly important role in farmland irrigation.
Post time: 23-02-22