Introduction
A water flow meter is a flow meter that measures and displays the flow rate of water. Water flow measurement is important for industrial applications such as wastewater treatment, and food & beverage. In principle, the types of water flow meters are electromagnetic (magnetic), turbine, ultrasonic and differential pressure. Coriolis and oval gear flow meters are also available for water flow measurement. These water flow meters are available with a choice of digital display, battery, analog or pulse output. The material can be 316 stainless steel or a special material.
Flow meters are one of the few instruments that are harder to use than to make. This is because the flow rate is a dynamic quantity, and there is not only viscous friction in the liquid in motion but also complex flow phenomena such as unstable vortices and secondary flows. The measuring instrument itself is affected by many factors, such as pipeline, caliber size, shape (circular, rectangular), boundary conditions, physical properties of the medium (temperature, pressure, density, viscosity, dirtiness, corrosion, etc.), fluid flow state (turbulence state, velocity distribution, etc.) and the influence of installation conditions and levels.
Faced with more than a dozen types and hundreds of varieties of flow meters at home and abroad (such as volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic, and thermal flow meters that have been developed successively), how to Reasonable selection of factors such as flow state, installation requirements, environmental conditions, and economy are the premise and basis for a good application of flow meters. In addition to ensuring the quality of the instrument itself, the provision of process data and whether the installation, use, and maintenance of the instrument are reasonable are also very important.
Electromagnetic flow meter
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 force line, so an induced electromotive force EX is generated in the direction perpendicular to the medium flow and the magnetic force line when the magnetic field intensity B and the distance between the two poles d When constant, the induced electromotive force EX is proportional to the measured medium flow rate (flow rate).
The electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, and gravity, and there is no pressure loss in the measuring tube inside shrinkage or protrusion. In addition, the initial signal detected by the flow element is an accurate linearity with the average flow velocity of the fluid. The varying voltage, which is independent of other properties of the fluid, has great advantages. According to the characteristics of sewage with large flow changes, impurities, low corrosion, and certain conductivity, electromagnetic flowmeters are a good choice for measuring the flow of sewage. It has a compact structure, small size, and convenient installation, operation, and maintenance. For example, the measuring system adopts an intelligent design, and the overall sealing is strengthened, so it can work normally in harsh environments. When choosing a model, pay attention to the following points:
The liquid to be measured must be a conductive liquid or slurry;
Caliber and range, it is best that the normal range exceeds half of the full range (generally 4 to 8 times the normal flow rate), and the flow rate is between 2-4 m/s;
The operating pressure must be lower than the pressure resistance of the flowmeter;
Different temperatures and corrosive media use different lining materials and electrode materials.
Advantages of electromagnetic flowmeter: no throttling parts, so the pressure loss is small. It is not affected by the temperature, pressure, density, and viscosity of the fluid; it is only related to the average velocity of the measured fluid, and the measurement range is wide; it can measure other media only after being calibrated with water without correction, and is most suitable as a metering device for settlement use. Due to the continuous improvement of technology and process materials, the continuous improvement of stability, linearity, precision, and life, and the continuous expansion of pipe diameter, the measurement of solid-liquid two-phase media adopts replaceable electrodes and scraper electrodes to solve the problem.
Due to the measurement problems of high pressure (32MPa), corrosion-resistant (anti-strong acid, alkali lining) medium, as well as the continuous expansion of caliber (up to 3200mm caliber), the continuous increase of service life (generally greater than 10 years), electromagnetic flowmeters are getting more and more It is widely used, and its cost has also been reduced, but the overall price, especially the price of large pipe diameters, is still relatively high, so it plays an important role in the purchase of flow meters.
Disadvantages: Electromagnetic flowmeters cannot be used to measure gas, steam, and liquids containing a large amount of gas, cannot be used to measure liquid media with low electrolysis rates, and cannot measure high-temperature and high-pressure fluids. The installation and debugging of electromagnetic flowmeters are more complicated than other flowmeters. And the requirements are more stringent; when used to measure viscous liquids with dirt, sticky substances or sediments adhere to the inner wall of the measuring tube or electrodes, causing the output potential of the transmitter to change, resulting in measurement errors, and the dirt on the electrodes reaches a certain thickness, may cause the meter to fail to measure.
Ultrasonic flowmeter
Ultrasonic flow meters are instruments that measure flow by detecting the effect of fluid flow on ultrasonic beams (or ultrasonic pulses). Ultrasonic flowmeters used in closed pipelines are classified according to measurement principles: time propagation method, Doppler effect method, beam offset method, correlation method, and noise method.
When the testing pipeline flow, in order to improve the accuracy of water flow measurement, it is required to select the part with a uniform fluid flow field when selecting the measurement point. Generally, the following principles should be followed:
The fluid in the pipeline under test must be full.
The material of the pipe to be tested should be uniform and dense, easy for ultrasonic transmission, such as a vertical pipe section (fluid from bottom to top) or horizontal pipe section (the lowest point in the entire pipeline is better).
The installation distance should be greater than 10 times the straight pipe diameter upstream, and greater than 5 times the straight pipe diameter downstream (Note: The distance required by different instruments will be different, the specific distance is subject to the instrument manual used) without any valves or elbows, variable diameter, and other uniform straight pipe sections, the measurement point should be sufficiently far away from interference sources such as valves, pumps, high voltage electricity, and frequency converters.
Fully consider the fouling situation in the pipe, and try to choose a pipe section without fouling for measurement.
Advantages of ultrasonic flowmeter: It is a non-contact measuring instrument, which can be used to measure fluid flow and large pipe diameter flow that is not easy to touch and observe. It will not change the flow state of the fluid, will not cause pressure loss, and is easy to install. It can measure the flow of strong corrosive media and non-conductive media; the ultrasonic flowmeter has a large measurement range, with a pipe diameter ranging from 20mm to 5m, and is not affected by thermal and physical parameters such as temperature, pressure, viscosity and density of the measured fluid; Available in bundled, piped and portable forms.
Disadvantages: The temperature measurement range is not high, and generally only can measure fluids with a temperature lower than 200°C; poor anti-interference ability; susceptible to interference from bubbles, scaling, ultrasonic noise mixed in by pumps and other sound sources, affecting measurement accuracy; requirements for straight pipe sections Strict, the first 20D, the last 5D, otherwise the dispersion is poor and the measurement accuracy is low; the measurement pipeline will seriously affect the measurement accuracy due to fouling, resulting in significant measurement errors, and even the instrument has no flow display in severe cases; reliability and accuracy levels Not high (generally about 1.5 to 2.5 grades), poor repeatability.
Vortex flowmeter
As a new type of flowmeter, the vortex flowmeter has developed rapidly since the mid-1980s. It has many advantages and strengths in flow measurement, and it is more and more widely used in modern flow measurements. The use of vortex flowmeters for flow measurement in China has also been paid more and more attention. At present, our country has a series of products with excellent performance and independent intellectual property rights. The vortex flowmeter is developed based on fluid vibration. According to the difference of the vortex, the detection method gradually develops 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 principle of the vortex flowmeter is to install a flow-blocking piece in the flowmeter pipeline. When the fluid flows through the flow-blocking piece, due to the blocking effect on the surface of the flow-blocking piece, two asymmetric flow lines will be generated downstream of it. Vortex, these vortices are separated at the side and rear of the baffle to form the so-called Karman vortex column, the rotation direction of the two columns of the vortex is opposite, when the vortex column is stable, the frequency f of the vortex is related to the fluid flow velocity υ in the flowmeter pipeline linear relationship.
Advantages of vortex flowmeter: It can be used in almost all occasions where vortex columns can be formed, not only for closed pipes but also for open grooves. 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.
Disadvantages: (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 most basic condition for the work of the vortex flowmeter. In addition, it is also affected by the vortex Due to the limitation of energy, the medium flow rate is low, the strength and rotation speed of the vortex is also low, and it is difficult to cause the sensing element to generate a response signal, and 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 (such as the magnetic sensitive type generally not exceeding 400Hz) and the frequency of the circuit. Therefore, the flow rate range must be calculated and calculated during design, and the selection should be made according to the flow rate of the fluid. The environmental conditions of the use site are complex.
In addition to paying attention to environmental temperature, humidity, atmosphere, and other conditions when selecting models, electromagnetic interference must also be considered. In places with strong interference such as high-voltage transmission power stations and large-scale rectification stations, the magnetic-sensitive and piezoelectric stress-type instruments cannot work normally or measure accurately.
(2) Vibration is also a great enemy of this type of instrument. Therefore, pay attention to avoiding mechanical vibration during use, especially the lateral vibration of the pipeline (vibration perpendicular to the axis of the pipeline and the axis of the vortex generator), this effect cannot be suppressed and eliminated in the structural design of the flowmeter. Since the vortex street signal is also sensitive to the influence of the flow field, it is not suitable to directly select the straight pipe section when the length of the straight pipe section cannot guarantee the necessary flow conditions for a stable vortex street (it is necessary to install a rectifier). Even for the capacitive and ultrasonic types with strong vibration resistance, it cannot be ignored to ensure that the fluid is a fully developed unidirectional flow.
(3) The medium temperature also has a great influence on the performance of the vortex flowmeter. For example, the piezoelectric stress vortex flowmeter cannot be used for a long time at 300°C, because its insulation resistance will drop from 10 MΩ to 100 MΩ at room temperature to 1 MΩ to 10 KΩ, and the output signal will also become smaller, resulting in measurement characteristics. For deterioration, it is advisable to choose a magnetic-sensitive or capacitive structure. In the measurement system, the sensor and converter should be installed separately to avoid long-term high temperatures from affecting the reliability and service life of the instrument.
The vortex flowmeter is a relatively new type of flowmeter. If it is not selected properly, its performance will not be able to play well. Only after reasonable selection, and correct installation, it is necessary to carefully and regularly maintain during use, accumulate experience, and improve the predictability of system failures, and the ability to judge and deal with problems, so as to achieve satisfactory results.
Throttling flowmeter
The throttling flowmeter is a measuring device with a long history of use and relatively complete experimental data. It is a flow meter that displays the flow rate by measuring the static pressure difference generated by the fluid flowing through the throttling device. The most basic configuration is composed of a throttling device, differential pressure signal pipeline and differential pressure transmitter. The most commonly used throttling device in the industry is the "standard throttling device" that has been standardized. For example, standard orifice plates, nozzles, Venturi nozzles, and Venturi tubes. Now the throttling device, especially the flow measurement of the nozzle, is moving towards the direction of integration, and the high-precision differential pressure transmitter and temperature compensation are integrated with the nozzle, which greatly improves the accuracy.
The throttling device can be calibrated online by pitot tube technology. Nowadays, some non-standard throttling devices are also used in industrial measurements, such as double orifice plates, circular orifice plates, annular orifice plates, etc. These instruments generally require real-flow calibration. The structure of the standard throttling device is relatively simple, but due to its relatively high requirements for dimensional tolerance, shape, and position tolerance, the technical difficulty of processing is relatively high. Taking the standard orifice plate as an example, it is an ultra-thin plate-shaped part, which is prone to deformation during processing.
Larger orifice plates are also prone to deformation during use and affect accuracy. The pressure-taking hole of the throttling device is generally not opened too large, and deformation will occur during use, which will affect the measurement accuracy. Due to the friction of the fluid on the standard orifice plate during use, it will also wear the structural elements related to the measurement (such as acute angles) and reduce the measurement accuracy.
Although the differential pressure flowmeter was developed earlier, with the continuous improvement and development of various other forms of flow instruments, and with the continuous improvement of industrial development requirements for flow measurement, the position of differential pressure flowmeters in industrial measurement has gradually increased. Replaced by advanced, high-precision, and convenient flow meters.
Turbine flowmeter
A turbine flowmeter is the main type of velocity flowmeter. It uses a multi-blade rotor (turbine) to sense the average flow velocity of the fluid, thereby deriving the flow or total amount of the instrument. The turbine flowmeter first converts the flow rate into the rotational speed of the turbine and then converts the rotational speed into an electrical signal proportional to the flow rate. Generally, it consists of two parts, the sensor, and the display, and it can also be made into an integral type. It has been widely used in some measurement objects, such as petroleum, organic liquid, inorganic liquid, liquefied gas, natural gas, a cryogenic fluid, etc.
Turbine flowmeter, positive displacement flowmeter, and Coriolis mass flowmeter are known as the three types of products with the best repeatability and precision among flowmeters. As one of the top ten types of flowmeters, their products have developed into a variety of The scale of serial mass production.
Advantages: Vortex flowmeter has high precision, high repeatability, no zero drift, strong anti-interference ability, wide measuring range, and compact structure. The pressure loss is small, and the impeller has an anti-corrosion function; it is easy to maintain, has a self-rectifying structure, is small and light, has a simple structure, and can be assembled and disassembled in a short time. The turbine flowmeter produced at the present stage also adopts a full carbide (tungsten carbide) shielded cantilever beam structure bearing, which integrates the rotating bearing and the pressure bearing, which greatly improves the life of the bearing, and can be used in places with a small amount of sand and dirt in the medium. Moreover, the intelligent flow display with a nonlinear accuracy compensation function has the accuracy of the correct formula better than ±0.02%.
Disadvantages: It is not suitable for long-term use, it cannot maintain the calibration state for a long time, and the physical properties of the fluid have a great influence on the flow characteristics. ; It is required that the length of the upstream pipeline should have an equal-diameter straight pipe section not less than 2D; it is not suitable for dirty media.
In addition to the above types of flow meters, there are Rotameters, Coriolis mass flowmeters, thermal (gas) mass flowmeters, positive displacement flowmeters, etc.
To sum up, the selection of a flowmeter refers to the fact that according to the production requirements, starting from the actual situation of instrument product supply, comprehensively considering the safety, accuracy, and economy of measurement, and determining the flow sampling device according to the nature and flow of the measured fluid. The method and the type and specification of the measuring instrument. On the basis of ensuring the safe operation of the instrument, strive to improve the accuracy and energy saving of the instrument. For this reason, it is necessary not only to choose a display instrument that meets the accuracy requirements but also to choose a reasonable measurement method according to the characteristics of the measured medium. In order to ensure the service life and accuracy of the flowmeter, the anti-vibration requirements of the meter should also be paid attention to when selecting the type. In hot and humid areas, choose a damp heat instrument. Correctly selecting 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.
In short, there is no measurement method or flowmeter that can adapt to various fluids and flow conditions. Different measurement methods and structures require different measurement operations, methods of use, and conditions of use. Each type has its own advantages and disadvantages. Therefore, the best type that is safe, reliable, economical, and durable should be selected on the basis of a comprehensive comparison of various measurement methods and instrument characteristics.