1. Overview of flowmeter
Flow is an important parameter in the process of modern industrial measurement. Human beings have a long history of fluid measurement. From the ancient Romans to measure the residents of water flow, the ancient Egyptians to measure the river flow to the modern people of various industrial process flow measurement, are fully system of human infinite wisdom. The 20th century with the energy measurement, environmental protection, transportation and other applications on the flow measurement needs of the traction, making the rapid development of flow meters. In particular, the rapid development of microelectronics technology for the flowmeter manufacturing technology to provide a variety of new components, and further promote the flow meter from the mechanical to intelligent, modular development. New technology, new devices, new materials and new technology and new software development and application, making the measurement accuracy of the flow meter is getting higher and higher, the flow measurement range is more and more widely. At the same time the requirements of the flowmeter on the measurement medium is reduced, the scope of application is also wider, intelligent degree and reliability has been greatly improved.
2. Classification and principle of flowmeter
Flow meter classification has a variety of methods, such as the principle can be classified according to measurement, but also by the instrument function classification and structure type classification and so on. This paper describes the principles and characteristics of various flow meters.
2.1 differential pressure flow meter
The differential flowmeter is a meter that calculates the flow rate based on the differential pressure generated by the flowmeter installed in the pipe, the known fluid condition, and the geometry of the detector and the pipe. Differential pressure flowmeter by the detection device (a device) and differential pressure conversion, flow display instrument (secondary device) composition.
The advantages of differential pressure flowmeter are: solid structure, stable and reliable performance, long service life; wide range of applications, there is no any class of flowmeter can be compared with; detectors, transmitters, display instruments were different Manufacturers produce, easy to scale generation.
Differential flowmeter shortcomings are: measurement accuracy is generally low; narrow range; field installation conditions require high.
Differential flow meter The most widely used flow meter, in all types of flow meters in the use of the first. In recent years, due to the advent of a variety of new flowmeter, the percentage of its use in the gradual decline, but is still the most important class of flow meter. It is used in the industrial sector accounts for about 1 / 4-1 / 3 of the total flow meter.
2.2 float flowmeter
The float flowmeter, also called the rotor flowmeter, is a kind of variable-area flowmeter. In a vertical upwardly extending vertical cone, the weight of the circular cross-section of the float is supported by the liquid power, Can freely rise or fall within the cone tube. Especially in the small, micro-flow has a pivotal role.
Float flowmeter advantages are: glass cone tube float flowmeter simple structure, easy to use;
Float flowmeter shortcomings are: low pressure, broken glass tube and so on.
Float flowmeters are suitable for small diameter and low flow rate fluid measurements.
> 2.3 Volumetric flowmeter
Volumetric flowmeter, also known as the displacement flowmeter, measurement accuracy is very high. Which utilizes the mechanical measuring element to continuously divide the fluid contact into a known volume portion and measure the total volume of the fluid volume according to the number of times the volume of the volume portion is repeatedly filled and discharged by the measurement chamber.
Volumetric flowmeter according to its measuring elements can be divided into oval gear flowmeter, scraper flowmeter, dual rotor flowmeter, rotary piston flowmeter, reciprocating piston flowmeter, the original flow meter, liquid drum drum flow meter, wet Gas meter and membrane gas meter and so on.
The advantages of the volumetric flowmeter are: high measurement accuracy; installation of pipeline conditions on the measurement accuracy has no effect; can be used for high viscosity liquid measurement; wide range; direct reading instrument without external energy can be directly obtained the cumulative total, clear , Easy to operate.
The shortcomings of the volumetric flowmeter are: the structure is complex, the volume is huge; the type of the measured medium, the caliber, the medium working condition is limited; does not apply to the high and low temperature occasions; most of the instrument only applies to the clean single phase fluid; Larger.
2.4 Turbine flowmeter
The turbo flowmeter uses a multi-blade turbine rotor to sense the average velocity of the fluid, deriving the flow or total amount of the meter. It consists of two parts of the sensor and display instrument.
Turbine flowmeter advantages are: the most accurate flow measurement; good repeatability; no zero drift, anti-interference ability; wide range; compact structure.
Turbine flowmeter shortcomings are: can not maintain long-term calibration characteristics; fluid properties of the flow characteristics have a greater impact.
Turbine flowmeters are widely used in the measurement of petroleum, organic liquids, inorganic liquids, liquefied petroleum gas, natural gas and low temperature circulation.
2.5 electromagnetic flowmeter
The electromagnetic flowmeter measures the flow rate with Faraday's law of electromagnetic induction, and then uses some method to measure the flow level of the flow section, so as to obtain the flow area and multiply the flow rate. Electromagnetic flowmeter has a series of excellent features, can solve other flowmeter difficult to solve the problem, such as dirty sewage, corrosion flow measurement. The seventies and eighties of the last century, electromagnetic flowmeter has made a major breakthrough in technology, making it a widely used class of flow meters, the use of rising.
The advantages of the electromagnetic flowmeter are: the measurement channel is a smooth section straight pipe, will not block, suitable for measuring solid particles with liquid-solid two-phase fluid, such as pulp, mud, sewage, etc .; does not produce flow detection caused by pressure loss, Energy flow is good; the measured volume flow is not affected by the fluid density, viscosity, temperature, pressure and conductivity changes significantly; flow range, wide caliber range; can be applied to corrosive fluids.
Disadvantages of electromagnetic flowmeter: can not measure the conductivity of very low liquid, such as petroleum products; can not measure the gas, steam and contain large bubbles of liquid; can not be used for higher temperatures. Electromagnetic flowmeter widely used, large diameter meters are widely used in water supply and drainage engineering; small and medium caliber meters commonly used in high demand or difficult to test occasions, such as steel industry blast furnace outlet cooling water control, paper industry measuring pulp and black liquor, chemical industry Of small corrosive liquid, non-ferrous metallurgical industry pulp; small caliber and small diameter meter commonly used in the pharmaceutical industry, food industry, biochemistry and other health requirements of the place.
2.6 vortex flowmeter
Vortex flowmeter is placed in the fluid in a non-streamlined swirl generator, the fluid on both sides of the body alternately separated from the release of two strings of regularly arranged staggered vortex of the instrument. According to the frequency detection method can be divided into stress, strain, capacitive, thermal, vibration, photoelectric and ultrasonic, etc., they are the youngest class of flow meter, but the rapid development, has become a common A class of flowmeters.
Vortex flowmeter advantages are: simple and solid structure; applicable to the type of fluid; high precision; wide range;
Vortex flowmeter shortcomings are: not suitable for low Reynolds number measurement; need a longer straight pipe; instrument coefficient is low (compared with the turbine flowmeter); instrument in the pulsating flow, multiphase flow is still lack of application experience.
2.7 ultrasonic flowmeter
The ultrasonic flowmeter, which is suitable for natural gas storage and delivery metering, is less efficient because of the low propagation efficiency of the ultrasonic wave at the solid and gas interface. It is difficult for the ultrasonic transducer (probe) to deliver enough sound energy from the pipe wall. Clamped gas ultrasonic flowmeter. Gas ultrasonic flowmeter began in the early 1980s, most of the measurement of short tube and inserted into the tube wall transducer in one form of integration, due to lower measurement accuracy, in the past can not be expensive in the field of natural gas trade settlement measurement Occupy a place in recent years, there are a variety of models of high precision gas ultrasonic flowmeter. Ultrasonic flowmeter using ultrasonic waves in different liquids in different speed of physical properties, in the measurement of flow at the same time identify the liquid type in the pipeline. Such as Europe in the ship unloading oil storage when the ultrasonic flowmeter commonly used to measure the flow of storage, and to determine the transport of liquid oil or oil tankers bottom of the water. An ultrasonic flowmeter is a meter that measures the flow by detecting the effect of fluid flow on the ultrasound beam (or ultrasonic pulse). According to the principle of signal detection, ultrasonic flowmeter can be divided into propagation velocity difference method (direct time difference method, time difference method, phase difference method and frequency difference method), beam shift method, Doppler method, space filter and noise method Wait. Ultrasonic flowmeter and electromagnetic flowmeter, because the instrument flow channel does not set any obstructions, are unimpeded flowmeter, is suitable to solve the problem of flow measurement problems of a class of flowmeter, especially in large diameter flow measurement is more prominent The advantages of rapid development in recent years, one of the flow meter.
The advantages of ultrasonic flowmeter are: do non-contact measurement; no flow obstruction measurement, no pressure loss; measurable non-conductive liquid, non-blocking measurement of electromagnetic flowmeter is a supplement.
Ultrasonic flowmeter shortcomings are: the propagation time method ultrasonic flowmeter can only be used to clean liquid and gas; and Doppler ultrasonic flowmeter can only be used to measure a certain amount of suspended particles and bubbles of liquid; Doppler ultrasound Flowmeter measurement accuracy is not high.
2.8 Coriolis flowmeters
Coriolis flowmeter uses the measuring tube part of the vibration frequency phase difference is proportional to the mass flow to measure the flow rate, using the measuring tube resonant frequency and the measured density of the tube as a function of density. The Coriolis flowmeter also derives the volumetric flow qv from the two basic parameters mass flow qm and the density ρ. If the measured liquid is a mixture of two liquids with a certain density difference, it is also possible to obtain a liquid by density Concentration in the mixture.
Due to the different measuring principle of different flowmeter, the use of flowmeter is not the same, so far there is no flow meter is a common. Each flowmeter will always be limited in its use due to the limitations of its measurement principles, the ability to adapt to the type of measurement medium, the measurement medium properties and the flow characteristics, and the ability to adapt to environmental conditions. Because of this, in the new flowmeter emerging at the same time, some applications have been very long history of the traditional flow meter is still widely used.
3. Domestic flow meter market distribution
In recent years, from the needs of China's industrial development, the use of domestic flow meter with the economic development, industrial applications grew rapidly, industry associations to provide statistical data in 2003 and 2005 summarized in Table 1. From Table 1 we can see that in 2003 and 2005 a variety of flowmeter market demand has increased greatly, of which the largest differential pressure transmitter, followed by the float flow meter. In recent years, the overall annual growth rate of flow meters in more than 15%.
4. Flowmeter development trend
Flowmeter with the development of modern industry is necessary to gradually improve its performance, and technological progress also makes the flow meter perfect possible. The development of the flowmeter will improve the reliability of the flowmeter, improve the adaptability of the flowmeter to the media, and adapt to the environment. At the same time with the new signal processing technology and the application of new signal transmission technology, flowmeter will be highly intelligent direction.