The National Science and Technology Nomenclature Validation Committee defines it as: indication of the measured flow rate and/or the total amount of fluid in a selected time interval instrument.Simply put, it is a meter used to measure the flow of fluid in a pipe or open channel.
Flowmeter is also divided into a differential pressure flowmeter, rotor flowmeter, throttle flowmeter, slit flowmeter, volumetric flowmeter, electromagnetic flowmeter, ultrasonic flowmeter and so on.Classification by medium: liquid flowmeter and gas flowmeter.
Products
Measurement is the eyes of industrial production.Flow measurement is one of the components of measurement science and technology, it has a close relationship with the national economy, national defense construction, scientific research.Do a good job of this work, to ensure product quality, improve production efficiency, and promote the development of science and technology have an important role, especially in the energy crisis, the degree of automation of industrial production is getting higher and higher in today's era, the flow meter in the national economy, the status and role of the more obvious.
Engineering commonly used unit m3 / h, which can be divided into instantaneous flow (Flow Rate) and cumulative flow (Total Flow), instantaneous flow that is the amount of time per unit of time over the closed pipe or nullah effective cross-section of the material flowing through the gas, liquids, solids; cumulative flow that is, for a certain period of time within the interval (one day, one week, one month, one year) of the fluid flow through the closed pipeline or nullah effectiveCumulative flow is the cumulative amount of fluid flowing through the effective cross-section of a closed pipe or open channel within a certain time interval (one day, one week, one month, one year).The cumulative flow can also be obtained by integrating the instantaneous flow over time, so instantaneous and cumulative flow meters can also be converted into each other.
History Broadcasting Editor
As early as 1738, the Swiss Daniel Bernoulli to the first Bernoulli equation based on the use of differential pressure method to measure water flow.Later, the Italian G.B. Venturi research with the Venturi tube to measure the flow, and in 1791 published the results of the study.
In 1886, the American Herschel application of the venturi made the measurement of water flow of practical measuring devices.
Early to mid-20th century, the original measurement principle is gradually mature, people will no longer be limited to the original measurement method, but began a new exploration.
To the 30s, and appeared to explore the use of sound waves to measure the flow rate of liquids and gases acoustic wave measurement of the flow rate of the method, but until the Second World War did not get a lot of progress, until 1955 there is the application of the acoustic cycle method of the Markson flowmeter was introduced for the measurement of the flow of aviation fuel.
After the 1960s of the 20th century, measuring instruments began to precision, miniaturization and other directions.
With the rapid development of integrated circuit technology, with phase-locked loop technology ultrasonic (wave) flowmeter has also been widely used, the widespread use of microcomputers to further improve the ability to flow measurement, such as laser Doppler flowmeter after the application of microcomputers can handle more complex signals.
Growing trend
In industrial sites, instruments that measure fluid flow are collectively called flow meters or flow meters.It is one of the most important instruments in industrial measurement.With the development of industry, the accuracy and range of flow measurement requirements are becoming higher and higher, in order to adapt to a variety of uses, various types of flow meters have been introduced, widely used in oil and gas, petrochemical, water treatment, food and beverages, pharmaceuticals, energy, metallurgy, pulp and paper and construction materials and other industries.
The global market for flowmeters reached $2.83 billion in 2008, an increase of about 3.9% from 2007.
Development status
Due to the lack of strong support for the institutional mechanism of the country in this industry, resulting in a lack of high-level composite talents in China's instrumentation industry, a lack of familiarity with, and proficiency in, the cross-disciplinary comprehensive talents.Low capacity for independent innovation, lack of independent intellectual property rights.Specifically manifested in the domestic high-end automation products are scarce, market competitiveness is not strong.
Modern measurement is optical, mechanical, electrical, computer and many basic disciplines highly integrated product, more sensitive to new technologies, is one of the modern industrial products in the speed of replacement of one of the fastest products, every year there will be new products, especially in today's information technology to attack the rapid development of the times, the competition is becoming more and more fierce, the development of a slightly slower pace will be left far behind.Although China has entered the 21st century, but many things remain at the level of the early 80's, large-scale and high-grade instruments and equipment almost entirely dependent on imports, and even some specialized instruments in the country is still in a state of blank, low-end product quality assurance there are still many problems need to be overcome.
Therefore, there must be a high degree of national strategic planning and related resources to support the opportunity to continue to narrow the gap.If the test instrument enterprises can not do a relatively large scale, then it is difficult to have and international giants and the strength of a long time, and the growth of the early stage of the need for a huge amount of long-term capital investment.
Scope of Application
Flow measurement is widely used in industrial and agricultural production, national defense construction, scientific research, foreign trade and people's lives in various fields.In the petroleum industry, from oil extraction, transportation, refining and metallurgical processing until trade and sales, flow measurement throughout the entire process, any one of the links are inseparable from the flow measurement, otherwise it will not be able to ensure that the normal production of the petroleum industry and trade exchanges.In the chemical industry, inaccurate flow measurement will cause the chemical composition of the distribution ratio is out of proportion, can not guarantee the quality of products, serious production safety accidents will occur.In the power industry, the liquid, gas, steam and other media flow measurement and regulation occupies an important position.The accuracy of flow measurement is not only to ensure that the power plant in the best parameters under the operation has great economic significance, and with the development of high-temperature and high-pressure large-capacity units, flow measurement has become an important link to ensure the safe operation of power plants.Such as large-capacity boiler instantaneous feedwater flow interruption or reduction, may cause serious dry boiler or pipe burst accident.This requires that the flow measurement device should not only do accurate measurement, but also in a timely manner to send an alarm signal.In the steel industry, steelmaking process of circulating water and oxygen (or air) flow measurement is to ensure product quality is one of the important parameters.In light industry, food, textile and other industries, are also inseparable from the flow measurement.
More applications of the transducer is the external clip and insertion type.Monoacoustic ultrasonic flowmeter structure is simple, easy to use, but this flowmeter on the flow distribution changes in adaptability, microelectronics and computer technology, the rapid development of greatly promote the instrumentation upgrades, new types of flowmeter such as the emergence of a spring.So far, it is said that there are hundreds of flow meters to the market, the field use of many difficult problems can be expected to be solved.Our country carries out the work of the recent flow measurement technology relatively late, is provided at the upstream end of the measuring flow channel 6 and relative to the orifices 11 and 12, for reducing the flow of the measured fluid into the orifices 11 and 12; the measuring control member 19, for measuring the propagation time of the ultrasonic waves between the ultrasonic transducers 8 and 9; and the calculation member 20, for calculating the flow rate on the basis of the signals of this measuring control member 19.
Flowmeter try to avoid ferromagnetic objects and equipment with a strong electromagnetic field (such as large motors, large transformers, etc.), so as to avoid the magnetic field affects the sensor's operating magnetic field and flow signal.Sensor spoon converter between the flow signal line and the excitation line.However, from the lightning failure in the analysis of damaged parts, caused by the failure of the induced high voltage and surge current most of the foot from the control room power lines introduced, the other two ways less.As the electromagnetic flowmeter measurement contains suspended solid phase or dirty body opportunities far more than other flow meters, the probability of failure generated by the inner wall of the adhesion layer is relatively high.If the adhesion layer conductivity and liquid conductivity is similar.Common commissioning period failures are usually caused by improper installation.
Common Types
There are many types of flow measurement methods and instruments, and there are many ways to categorize them. before 2011, there were as many as 60 types of flow meters available for industrial use.Before 2011, there were 60 types of flow meters available for industrial use. The reason for such a large number of varieties is that there is no flow meter that is suitable for any fluid, any range, any flow condition and any use condition, but with the progress of the times, in this era of technological explosion, there is finally a new product - Mass Flow Meter, Mass Flow Meter is suitable for any fluid, any range, any flow condition and any use condition, but the price is relativelyThe mass flow meter is suitable for any fluid, any range, any flow condition and any use condition, only the price is more expensive, can not be popularized in so industry.
The old 60 kinds of flow meter, each product has its specific applicability, also has its limitations.According to the measurement of the object is divided into two categories of closed pipes and open channels; according to the purpose of the measurement can be divided into total measurement and flow measurement, the instrument is called total meter and flow meter.
In addition, they can be divided into the following broad categories according to the measurement principle:
1, the principle of mechanics: belonging to such principles of instrumentation are the use of Bernoulli's theorem of differential pressure, rotor type; the use of momentum theorem of the impulse type, movable tube type; the use of Newton's second law of the direct mass type; the use of the principle of fluid momentum target; the use of the angular momentum theorem of the turbine; the use of the principle of fluid oscillations vortex type, vortex type; the use of the total static pressure difference between the Pitot tube type, as well as the volumetric type, and the weir, tank type and so on.
2, the principle of electricity: for such principles of instrumentation are electromagnetic, differential capacitance, inductive, strain-resistive and so on.
3, acoustic principle: the use of acoustic principles of flow measurement are ultrasonic.Acoustic type (shock wave type) and so on.
4, thermal principle: the use of thermal principle of flow measurement of heat type, direct calorimetric type, indirect calorimetric type, etc.
5, the optical principle: laser type, photoelectric type, etc. is such a principle of the meter.
6, the principle of atomic physics: nuclear magnetic resonance, nuclear radiation, etc. are such principles of the instrument.
7, other principles: the principle of labeling (tracer principle, the principle of nuclear magnetic resonance), the principle of correlation and so on.
In this paper, according to the most popular and widespread classification to explain the principle, characteristics, application profile and domestic and international play in that situation:
Target flowmeter
Target flowmeter is based on the principle of mechanics of a flowmeter, which has been developed and applied in industry for decades.The new SBL target flowmeter is based on the traditional target flowmeter, with the development of new sensors, microelectronics technology developed into a new type of capacitance force sensing flowmeter, which has both orifice plate, vortex and other flowmeter has no moving parts of the characteristics of the flowmeter, at the same time has a very high sensitivity, and volumetric flowmeter with the accuracy of comparable to the range of a wide range.
China in the 1970s to develop electric, pneumatic target flow transmitter it is electric, pneumatic unit combination instrumentation detection instrument.Because of the force converter at that time directly using the differential pressure transmitter force balancing mechanism, this flowmeter use can not help but bring the force balancing mechanism itself caused by the many defects, such as the zero position is easy to drift, measurement accuracy is low, poor reliability of the lever mechanism and so on.As a result of the poor performance of the force balance mechanism drag, target flowmeter itself, many of the advantages have not been able to effectively play, so far, users of the old target flowmeter has not been eliminated the bad impression of the flowmeter.
The force converter of the new SBL target flowmeter adopts strain type force converter, which completely eliminates the shortcomings of the above force balancing mechanism, the new target flowmeter also applies microelectronics and computer technology to the signal converter and display part, the flowmeter has a series of advantages, and it is believed that in the future, it will play an important role in the many flowmeters.
Differential pressure flowmeter
Differential pressure flowmeter is installed in the pipeline based on the flow detection device and the fluid interaction generated by the differential pressure, known fluid conditions and the geometry of the detection device and the pipeline to calculate the flow rate of the instrument.
Differential pressure flowmeter by the primary device (detection device) and secondary device (differential pressure converter and flow display meter).Usually to detect the form of differential pressure flowmeter classification, such as orifice plate flowmeter, venturi flowmeter, homogeneous tube flowmeter, Pitot tube principle - Bitoba flowmeter and so on.
Secondary device for a variety of mechanical, electronic, electromechanical integrated differential pressure gauge, differential pressure transmitter and flow display instrumentation.It has developed into three (serialization, generalization and standardization) degree is very high, the types of specifications of a large class of instruments, it can measure both flow parameters, but also can measure other parameters (such as pressure, level, density, etc.).
Differential pressure flowmeter test pieces according to its principle of action can be divided into: throttling device, hydraulic resistance type, centrifugal type, dynamic pressure head type, dynamic pressure head gain type and jet type several categories.
Test pieces can be further divided into two main categories according to their degree of standardization: standard and non-standard.
The so-called standard test piece is as long as in accordance with the standard document design, manufacture, installation and use, no need to be calibrated by the real flow can determine its flow value and estimate the measurement error.
Non-standard test pieces are less mature test pieces that are not yet included in international standards.Differential pressure flowmeter is a class of the most widely used flowmeter, in all kinds of flow meter in its use of the first place.As a result of a variety of new types of flowmeter, it is the use of the percentage of the gradual decline, but is still the most important type of flowmeter.
Float flowmeter
Float flowmeter, also known as rotor flowmeter, metal rotor flowmeter, Chengfeng glass rotor flowmeter, is a kind of variable area type flowmeter.In a vertical cone tube expanded from below, the gravity of the float of circular cross-section is borne by the liquid power, so that the float can rise and fall freely in the cone tube.
Float flowmeter is second only to the differential pressure flowmeter application of the widest range of a class of flowmeter, especially in small, micro-flow has a pivotal role.
In the mid-80s, Japan, Western Europe, the United States, the amount of sales accounted for 15% to 20% of the flow meter.China's output in 1990 was estimated at 12~14 million units, of which more than 95% were glass cone tube float flow meters.
Features:
(1) glass cone tube float flowmeter structure is simple, easy to use, the disadvantage is the low pressure resistance, there is a greater risk of glass tube fragile;
(2) for small pipe diameter and low flow rate;
(3) lower pressure loss.
The volumetric flowmeter
The volumetric flowmeter, also known as a constant displacement flowmeter, or PD flowmeter for short, is one of the most accurate types of flow meters.It utilizes a mechanical measuring element to continuously divide the fluid into individual known volume sections, and measures the total volume of fluid based on the number of times the chamber is repeatedly filled and discharged with that volume section, one at a time.
Electromagnetic flow meter
1, Advantages
(1) Electromagnetic flowmeter can be used to measure industrial conductive liquid or slurry.
(2) No pressure loss.
(3) Large measurement range, electromagnetic flow transmitter caliber from 2.5mm to 2.6m.
(4) Electromagnetic flowmeter to measure the volume flow rate of the measured fluid in the working state, the measurement principle does not involve the influence of the temperature, pressure, density and viscosity of the fluid.
2, Disadvantages
(1) electromagnetic flowmeter applications have certain limitations, it can only measure the liquid flow of conductive media, can not measure the flow of non-conductive media, such as gases and water treatment better heating water.In addition, its lining needs to be considered under high temperature conditions.
(2) Electromagnetic flowmeter is by measuring the velocity of the conductive liquid to determine the volume flow rate of the working state.In accordance with the measurement requirements, for liquid media, the mass flow rate should be measured, the measurement of media flow rate should be involved in the density of the fluid, different fluid media have different densities, and changes with temperature.If the electromagnetic flowmeter converter does not take into account the fluid density, only to give the volume flow at room temperature is not appropriate.
(3) electromagnetic flowmeter installation and commissioning than other flowmeter complex, and more stringent requirements.Transmitter and converter must be used in conjunction with the two can not be used with two different types of meters with.When installing the transmitter, from the selection of the installation location to the specific installation and commissioning must be carried out in strict accordance with the requirements of the product specification.There should be no vibration and no strong magnetic field at the installation site.The transmitter must be installed in such a way that there is good contact between the transmitter and the piping and good grounding.The potential of the transmitter is equipotential to the measured fluid.When in use, the gas stored in the measuring tube must be exhausted, otherwise it will cause a large measurement error.
(4) Electromagnetic flowmeter used to measure viscous liquid with dirt, viscous material or sediment attached to the inner wall of the measuring tube or electrode, so that the transmitter output potential changes, bringing measurement error, electrode on the dirt to reach a certain thickness, may lead to the instrument can not be measured.
(5) water supply pipe scaling or wear and tear to change the size of the inner diameter, will affect the original flow value, resulting in measurement error.Such as 100mm caliber meter diameter change 1mm will bring about 2% additional error.
(6) transmitter measurement signal is a very small millivolt potential signal, in addition to the flow signal, but also mixed with some of the signals that have nothing to do with the flow, such as phase voltage, quadrature voltage and common mode voltage.In order to accurately measure the flow, it is necessary to eliminate all kinds of interference signals and effectively amplify the flow signal.The performance of the flow converter should be improved, and it is best to use a microprocessor-type converter, with which the excitation voltage can be controlled, and the excitation mode and frequency can be selected according to the nature of the fluid to be measured, which can exclude in-phase and quadrature interference.However, the structure of the improved instrument is complicated and the cost is high.
(7) Higher price
Ultrasonic flowmeter
1, Advantages
(1) ultrasonic flowmeter is a non-contact measurement instrument, can be used to measure the flow of fluids that are not easy to contact, not easy to observe the flow rate and the flow of large pipe diameter.It will not change the fluid flow state, will not produce pressure loss, and easy to install.
(2) can measure the flow of strong corrosive media and non-conductive media.
(3) ultrasonic flowmeter measurement range, pipe diameter range from 20mm ~ 5m. (4) ultrasonic flowmeter can measure a variety of liquid and sewage flow.
(5) ultrasonic flowmeter to measure the volume flow rate is not subject to the measured fluid temperature, pressure, viscosity and density and other thermal and physical parameters.Can be made into fixed and portable forms.
2, Disadvantages
(1) ultrasonic flowmeter temperature measurement range is not high, generally can only measure the temperature below 200 ℃ fluid.
(2) Poor anti-interference ability.Susceptible to air bubbles, scaling, pumps and other sound sources mixed into the ultrasonic noise interference, affecting the measurement accuracy.
(3) Strict requirements for straight pipe section, for the first 20D, after 5D, otherwise poor dispersion, low measurement accuracy.
(4) The uncertainty of installation will bring large errors to the flow measurement.
(5) Measuring pipeline due to scaling, will seriously affect the accuracy of measurement, bring significant measurement error, and even in serious cases the meter has no flow display.
(6) Reliability, accuracy grade is not high (generally about 1.5 ~ 2.5 grade), poor repeatability.
(7) Short service life (general accuracy can only be guaranteed for one year).
(8) ultrasonic flowmeter is to determine the volume flow rate by measuring the fluid velocity, the liquid should measure its mass flow rate, the instrument to measure the mass flow rate is obtained by multiplying the volume flow rate multiplied by the density set by man, when the fluid temperature changes, the fluid density is changing, set the value of man-made density, can not guarantee the accuracy of the mass flow rate.Only in the measurement of fluid velocity at the same time, but also measured the fluid density, in order to get the real mass flow value through the operation.
(9) Higher price.
Other common types
Ultrasonic flow meter
Ultrasonic flow meter is through the detection of fluid flow on the ultrasonic beam (or ultrasonic pulse) in order to measure the flow rate of the instrument.
Advantages:
(1) non-contact measurement;
(2) no flow obstruction measurement, no pressure loss;
(3) non-conductive liquids can be measured, no obstruction measurement of electromagnetic flowmeter is a complement.
Disadvantages:
(1) propagation time method can only be used for clean liquids and gases; and Doppler method can only be used to measure a certain amount of suspended particles and bubbles in the liquid;
(2) Doppler method of measurement accuracy is not high.
Application Overview:
(1) The propagation time method is used for clean, single-phase liquids and gases.Typical applications are plant effluents, strange liquids, liquefied natural gas, etc.;
(2) For gases, good experience has been gained in the field of high-pressure natural gas;
(3) The Doppler method is suitable for two-phase fluids with a low content of heterogeneous phases, e.g., untreated wastewater, plant effluents, dirty process fluids; it is not usually applied to very clean liquids.
Thermal flow meter sensors
Thermal flow meter sensors contain two sensing elements, a velocity sensor and a temperature sensor.They automatically compensate and correct for gas temperature variations.The electrically heated part of the instrument heats the velocity sensor to a constant value above the operating temperature, creating a constant temperature difference between the velocity sensor and the sensor that measures the operating temperature.When the temperature difference is kept constant, the energy consumed by the electrical heating, which can also be described as the heat dissipation value, is proportional to the mass flow rate of the gas flowing through it.
Characteristics:
1, high reliability, good repeatability, high measurement accuracy, low pressure loss;
2, no moving parts, wide range ratio, fast response speed, no need for temperature and pressure compensation.
Applications:
1、Gas mass flow measurement in industrial pipeline
2、Flue gas flow rate measurement in chimney discharge
3、Flue gas flow measurement in calciner
4、Air flow measurement in gas process
5、Compressed air flow measurement
6、Gas flow measurement in the process of semi-channel chip manufacturing
7、Gas flow measurement in wastewater treatment
8、Gas flow measurement in heating ventilation and air-conditioning system
9、Gas flow measurement in the melt recovery systemMeasurement
10、combustion boiler combustion gas flow measurement
11、 gas flow measurement of natural gas, flare gas, hydrogen and other gases
12、 carbon dioxide gas flow measurement in the process of beer production
13、 gas mass flow measurement in the process of production of cement, cigarette, glass factory
Fault analysis
(1) flow control instrumentation system indication value reaches the minimum, first check the field detection instrument, if normal, then the fault in the display instrument.When the field detection instrument indication is also the smallest, then check the regulator valve opening, if the regulator valve opening is zero, it is often the regulator valve to the regulator between the fault.When the field test instrument indication is minimum, the regulator valve opening is normal, the cause of the failure is likely to be insufficient system pressure, system piping blockage, the pump is not on the amount of media crystallization, improper operation and other reasons.If the instrumentation failure, the reasons are: orifice plate differential pressure flowmeter may be positive pressure induced pressure conduit plug; Differential Pressure Transmitter positive pressure chamber leakage; mechanical flowmeter is the gear jammed or filter mesh plug and so on.
(2) flow control instrumentation system indicator value reaches the maximum, then the detection instrument will often indicate the maximum.At this time, you can manually remote control valve to open or close the small, if the flow can be reduced is generally caused by the process operation.If the flow value does not fall, it is the cause of the instrumentation system, check the flow control instrumentation system regulator valve action; check the instrumentation to measure the lead pressure system is normal; check the instrumentation signal transmission system is normal.
(3) flow control instrumentation system indicator value fluctuations are more frequent, the control can be changed to manual, if the fluctuations are reduced, it is the instrumentation or instrument control parameters PID is not appropriate, if the fluctuations are still frequent, it is the process of operation caused by the reasons.
