Analysis on the application of flow meter in farmland irrigation

There are many types of flow measurement methods and instruments, and there are many classification methods, including rotor flowmeter, 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 major categories: closed pipes and open channels; according to the measurement purpose, it can be divided into total measurement and flow measurement, and its instruments are called total meter and flowmeter respectively. The

total meter measures the flow rate flowing through the pipeline in a period of time, which is expressed as the quotient of the total amount flowing in a short period of time divided by the time. In fact, the flow meter is usually also equipped with a cumulative flow device for use as a total meter, and the total meter is also equipped with a flow signaling device. Therefore, it is no longer meaningful to strictly divide the flow meter and the total meter.

According to the measurement principle, there are mechanical principles, thermal principles, acoustic principles, electrical principles, optical principles and atomic physics principles.

1 Ultrasonic flowmeter

When ultrasonic waves propagate in a flowing fluid, they carry information about the flow velocity of the fluid. The flow velocity of the fluid can be detected by the received ultrasonic wave, and then converted into flow. According to the detection method, ultrasonic flowmeters can be divided into different types such as propagation velocity difference method, Doppler method, beam deviation method, noise method and correlation method. Ultrasonic flowmeter is a fluid measurement method that has only been applied in the past decade with the rapid development of integrated circuit technology.

Ultrasonic flowmeter is a non-contact measuring instrument suitable for measuring fluids that are difficult to touch and observe and large-diameter flow. It can measure the flow of open water flow in conjunction with a water level meter. When using an ultrasonic flowmeter, there is no need to install measuring elements in the fluid, so the flow state of the fluid will not be changed and no additional resistance will be generated. 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 types of ultrasonic flowmeters can be installed outside the pipe, which is a non-contact flow measurement. The cost of the instrument is basically unrelated to the diameter of the measured pipeline; while other types of flowmeters increase in cost as the diameter increases, so the larger the diameter, the better the function-price ratio of ultrasonic flowmeters than other types of flowmeters with the same function, and is considered to be a better large-diameter flow measurement instrument. Doppler ultrasonic flowmeter can measure the flow of two-phase medium, so it can be used to measure dirty flow such as sewer and sewage. In power plants, portable ultrasonic flowmeters are used to measure large pipe flow such as turbine water intake and turbine circulating water, which is much more convenient than the previous peel tube flowmeter. Ultrasonic flowmeters can also be used for gas measurement. The applicable range of pipe diameter is 2cm~5m, from open channels or culverts with a width of several meters to rivers with a width of 500m.

In addition, the flow measurement accuracy of ultrasonic measuring instruments is almost unaffected by parameters such as temperature, pressure, viscosity and density of the measured fluid, and can be made into non-contact and portable measuring instruments, so it can solve the flow measurement problems of highly corrosive, non-conductive, radioactive and 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 the measurement of multiple pipe diameters and multiple flow ranges. The adaptability of ultrasonic flowmeters is also unmatched by other instruments. Ultrasonic flowmeter has some of the above advantages, so it is increasingly valued and developed towards product serialization and generalization. It has now 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 that has developed rapidly in the 1950s and 1960s with the development of electronic technology. Electromagnetic flowmeter is made according to Faraday's law of electromagnetic induction and is used to measure the volume flow of conductive liquids. Due to its unique advantages, it has been widely used in the flow measurement of various conductive liquids in industry and agriculture, such as various corrosive media such as acids, alkalis, salts, and various slurry flows, forming a unique application field.

In terms of structure, the electromagnetic flowmeter consists of two parts: an electromagnetic flow sensor and a 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 this signal to the converter through the transmission line. The converter is installed not too 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 and regulation control.

3 Application of flow meter in farmland irrigation

In the national economy, the vigorous construction of farmland water conservancy is an important measure for the development of agriculture in China. Water conservancy project construction is inseparable from irrigation, especially the correct mastery of the method of measuring flow in the irrigation system is crucial. There are currently two main methods, namely the velocity-area method and the hydraulic method, each of which has its own unique flow sensing (detection) part. [page]

The velocity-area method measures flow by multiplying the average flow velocity of the water flow by the cross-sectional area of ​​the channel, and the water flow velocity is measured at some specific points by the flow meter. There are generally three different ways to measure flow using this method: one is the velocity distribution method; the second is the 0.6h method (h is the distance from the water surface to the bottom of the channel); the third is the 0.2h~0.8h method. The velocity distribution method measures the velocity at several points between the water surface and the bottom of the channel, and then determines a velocity curve perpendicular to the water surface from these measuring points. The average velocity can be obtained by dividing the area defined by the velocity curve by the depth; the 0.6h method measures the velocity at a single point 0.6h below the water surface, and assumes that this value is the average of all velocities; the 0.2h-0.8h method measures the velocity at two points 0.2h and 0.8h below the water surface, and the average of these two readings can be estimated as the average velocity value.

All three methods calculate the average velocity in any specific measurement area. The number of depth measurements along the entire channel width and the number of points measured on a single vertical plane are different. Many European countries require 5 points to be measured when measuring depth on a single plane, while the United States believes that 2 points are sufficient. The number of depth measurements depends on the width of the channel. The number of depth measurements in Europe is 5-15, and in the United States it is 8-18. The measurement error is determined by the number of depth measurements and the measurement time at each point. The average error using the velocity-area method can be less than 6%.

There are two technologies for measuring flow using the velocity-area method that should be paid special attention to: one is the use of ultrasonic flowmeters, and the other is the use of electromagnetic flowmeters.

Ultrasonic technology calculates flow velocity by measuring the propagation time of ultrasonic waves across the channel. The transmitter and receiver are placed on both sides of the channel so that the angle between the wave path and the water flow direction is within 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, one or more transmitter-receiver pairs will be used. The

electromagnetic technology of measuring flow uses the magnetic field artificially established in the conductive water flow, and then measures the synthetic potential difference across the two sides of the water flow. The electromagnetic flowmeter is mainly composed of a water channel, an excitation coil and a pair of electrodes.

4 Selection of flowmeter

The selection of flowmeter refers to the comprehensive consideration of the safety, accuracy and economy of measurement in accordance with production requirements, based on the actual supply of instrument products, and determining the mode of flow sampling device and measuring instrument, as well as the form and specifications of the measuring instrument according to the properties and flow conditions of the measured fluid.

The safety and reliability of flow measurement first depends on the reliability of the measurement method, that is, the sampling device will not have mechanical strength or electrical circuit failures during operation to cause accidents; secondly, the measuring instrument will not affect the safety of the production system regardless of 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, we should not only select display instruments that meet the accuracy requirements, but also select reasonable measurement methods according to the characteristics of the measured medium. For the measurement of the main steam flow of power plants, mature standard throttling devices with differential pressure flowmeters are generally used because it is crucial to the safety and economy of power plants. The sewage and fuel oil of chemical water treatment belong to dirty flow and low Reynolds number viscous flow respectively, and standard throttling devices are not suitable. For dirty flow, non-standard throttling devices such as circular orifice plates are generally used with differential pressure gauges or ultrasonic Doppler flowmeters, while viscous flow can be respectively used with volumetric, target or wedge flowmeters. The water volume at the turbine inlet, the circulating water volume of the condenser, and the heat recovery steam of the heat recovery unit are all large-diameter (400mm or more) flow measurement parameters. Due to the difficulty of processing and creation and the large pressure loss, standard throttling devices are generally not used. According to the characteristics of the measured medium and the requirements of measurement accuracy, the flow rate is measured by insertion flowmeter, velocity measuring element with differential pressure gauge, ultrasonic flowmeter or non-energy loss 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, attention should also be paid to the vibration resistance requirements of the instrument when selecting. In hot and humid areas, wet and hot instruments should be selected. Correct selection of instrument specifications 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 (i.e., pressure resistance) of the instrument should be slightly greater than the working pressure of the measured medium, generally 1.25 times, to ensure that no leakage or accidents occur. The selection of the range is mainly the selection of the upper limit of the instrument scale. 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, it is selected as 1.2 to 1.3 times the maximum flow value in actual operation. For

contact instruments installed on production pipelines for long-term operation, the energy loss caused by the flow measurement element should also be considered. In general, multiple measuring elements with large pressure loss, such as throttling elements, should not be selected in the same production pipeline.

5 Conclusion

In short, there is no measurement method or flow meter that can adapt to all fluids and flow conditions. 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 agricultural modernization and the realization of precision agriculture in China, flow meters will play an increasingly important role in farmland irrigation. (end)