A meter used to measure fluid flow in a pipeline (volume of fluid passing through a unit of time). There are rotameters, throttle flowmeters, slit flowmeters, volumetric flowmeters, electromagnetic flowmeters, ultrasonic flowmeters and helium.
There are many types of flow measurement methods and meters, and there are many classification methods. Up to now, there are as many as 60 types of flow meters available for industrial use. The reason for so many varieties is that no flow meter has been found for any fluid, any range, any flow conditions, and any conditions of use.
These more than 60 flow meters, each with its specific applicability, also have its limitations. According to the measurement object, there are two types of closed pipelines and open channels; according to the measurement purpose, it can be divided into total measurement and flow measurement. The meters are called the total meter and the flow meter.
The total meter measures the flow through the pipeline over a period of time. It is expressed as the quotient of the total amount of flow over a short period of time divided by the time. In fact, the flowmeter is usually equipped with a cumulative flow device for use as a total meter. The meter is also equipped with a traffic transmitter. Therefore, it is of no practical significance to divide the flow meter and the total meter in a strict sense.
According to the measurement principle, there are mechanical principles, thermal principles, acoustic principles, electrical principles, optical principles, and atomic physics principles.
According to the current most popular and widely classified classification, it is divided into: volumetric flowmeter, differential pressure flowmeter, float flowmeter, turbine flowmeter, electromagnetic flowmeter, vortex flowmeter in fluid oscillation flowmeter, quality Flowmeters, plug-in flowmeters, and probe flowmeters are used to illustrate the principles, features, application profiles, and developments at home and abroad.
The differential pressure flowmeter is a meter that calculates the flow rate based on the differential pressure generated by the flow detecting member installed in the pipe, the known fluid condition, and the geometrical dimensions of the detecting member and the pipe.
The differential pressure flowmeter consists of a primary device (detection member) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified in the form of test pieces, such as orifice flowmeters, venturi flowmeters, and uniform velocity flowmeters.
The secondary device is a variety of mechanical, electronic, electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments. It has developed into a large-scale instrument with a high degree of categorization (series, generalization and standardization) and a wide variety of specifications. It can measure flow parameters as well as other parameters (such as pressure, level, density, etc.). ).
The detection parts of the differential pressure flowmeter can be divided into three types according to the principle of operation: throttling device, hydraulic resistance type, centrifugal type, dynamic pressure head type, dynamic pressure head gain type and jet type.
Test pieces can be divided into two categories according to their standardization: standard and non-standard.
The so-called standard test pieces are designed, manufactured, installed and used according to standard documents, and the flow value and estimated measurement error can be determined without actual flow calibration.
Non-standard test pieces are those that are less mature and have not been included in the international standard.
The differential pressure flowmeter is one of the most widely used flowmeters, and its use ranks first among all types of flowmeters. In recent years, due to the advent of various new flowmeters, its percentage of usage has gradually decreased, but it is still the most important type of flowmeter.
advantage:
(1) The most widely used orifice plate type flowmeter has firm structure, stable and reliable performance and long service life;
(2) A wide range of applications, so far no class of flowmeters can be compared;
(3) The test piece, transmitter and display instrument are produced by different manufacturers, which is convenient for economies of scale.
Disadvantages:
(1) Measurement accuracy is generally low;
(2) The range is narrow, generally only 3:1~4:1;
(3) The requirements for on-site installation are high;
(4) Large pressure loss (refer to orifice plate, nozzle, etc.).
Note: A new type of product: a balanced flow meter developed by the US Aerospace Administration. This meter has a measurement accuracy of 5-10 times that of a conventional throttling device and a permanent pressure loss of 1/3. The pressure recovery is 2 times faster, and the minimum straight pipe section can be as small as 1.5D, which is easy to install and use, and greatly reduces the power consumption of the fluid.
Application overview:
Differential pressure flowmeters are widely used in a wide range of applications in flow measurement of closed pipes, such as fluids: single phase, miscible, clean, dirty, viscous flow, etc.; working conditions: atmospheric pressure, high pressure , vacuum, normal temperature, high temperature, low temperature, etc.; pipe diameter: from a few mm to a few m; flow conditions: subsonic, sonic, pulsating flow. Its use in various industrial sectors accounts for about 1/4 to 1/3 of the total flow meter.
3.2 Float flowmeter The float flowmeter, also known as the rotameter, is a type of variable area flowmeter. In a vertical cone that is enlarged from bottom to top, the gravity of the circular cross section of the float is absorbed by the liquid power. So that the float can freely rise and fall within the cone.
The float flowmeter is the most widely used type of flowmeter after the differential pressure flowmeter, and it plays an important role in small and microflow.
In the mid-1980s, sales in Japan, Western Europe, and the United States accounted for 15% to 20% of flow meters. China's output in 1990 was estimated at 12 to 140,000 units, of which more than 95% were glass cone tube float flowmeters.
Features:
(1) The glass cone tube float flowmeter has a simple structure and is convenient to use, and has the disadvantages of low pressure resistance and large risk of fragile glass tubes;
(2) Suitable for small pipe diameters and low flow rates;
(3) The pressure loss is low.
3.3 Volumetric flowmeter Volumetric flowmeter, also known as fixed displacement flowmeter, referred to as PD flowmeter, is the most accurate class in flowmeters. It utilizes a mechanical measuring element to continuously divide the fluid into a single known volume portion, and the total volume of the fluid is measured based on the number of times the measuring chamber repeatedly fills and discharges the volume of the fluid one by one.
Volumetric flowmeters can be classified into elliptical gear flowmeters, scraper flowmeters, dual-rotor flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disk flowmeters, and liquid-sealed rotary flowmeters according to their measurement components. , wet gas meter and membrane gas meter.
advantage:
(1) High measurement accuracy;
(2) Installation of pipeline conditions has no effect on measurement accuracy;
(3) It can be used for the measurement of high viscosity liquids;
(4) The range is wide;
(5) Direct-reading instruments can be directly accumulated without external energy, and the total amount is clear and easy to operate.
Disadvantages:
(1) The results are complex and bulky;
(2) The type of the tested medium, the caliber, and the working state of the medium are more limited;
(3) Not suitable for high and low temperature applications;
(4) Most instruments are only suitable for clean single-phase fluids;
(5) Noise and vibration are generated.
Application overview:
The volumetric flowmeter and the differential pressure flowmeter and the float flowmeter are listed as the three types of flowmeters with the largest usage, and are often used for the total measurement of expensive media (oil, natural gas, etc.).
In recent years, the sales volume of PD flowmeters (excluding domestic gas meters and household water meters) in industrialized countries accounted for 13% to 23% of flow meters; China accounted for about 20%, and production in 1990 (excluding household gas meters) was estimated at 340,000. The table, in which the oval gear type and the waist wheel type account for about 70% and 20% respectively.
3.4 Turbine Flowmeters Turbine flowmeters are the main type of velocity flowmeters that use a multi-bladed rotor (turbine) to sense the average flow rate of the fluid and derive the flow or total amount of the meter.
Generally, it consists of two parts, a sensor and a display, and can also be made into a unitary type.
Turbine flowmeters and volumetric flowmeters, Coriolis mass flowmeters are called three types of repeatable and accurate products in flowmeters. As one of the ten types of flowmeters, their products have been developed into many varieties and more. The scale of series mass production.
advantage:
(1) High precision, among all flow meters, is the most accurate flow meter;
(2) good repeatability;
(3) Yuan zero drift, good anti-interference ability;
(4) The range is wide;
(5) Compact structure.
Disadvantages:
(1) The calibration characteristics cannot be maintained for a long time;
(2) Fluid properties have a large impact on flow characteristics.
Application overview:
Turbine flowmeters are widely used in some of the following measurement objects: petroleum, organic liquids, inorganic fluids, liquefied gases, natural gas and cryogenic fluids. In Europe and the United States, turbine flowmeters are second only to the natural metering of orifice flowmeters. Instrumentation, the Netherlands only uses more than 2,600 gas turbine flowmeters of various sizes and pressures from 0.8 to 6.5 MPa on natural gas pipelines, which have become excellent natural gas meters.
3.5 Electromagnetic flowmeter The electromagnetic flowmeter is a meter for measuring conductive liquid based on Faraday's law of electromagnetic induction.
The electromagnetic flowmeter has a series of excellent characteristics that can solve the problems that other flowmeters are not easy to apply, such as the measurement of dirty flow and corrosion flow.
In the 1970s and 1980s, electromagnetic flow had a major breakthrough in technology, making it a widely used type of flowmeter, and its percentage of usage in flow meters continued to rise.
advantage:
(1) The measuring channel is a smooth straight pipe with no obstruction. It is suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, mud, sewage, etc.
(2) The pressure loss caused by the flow detection is not generated, and the energy saving effect is good;
(3) The measured volume flow is virtually unaffected by changes in fluid density, viscosity, temperature, pressure, and conductivity;
(4) The flow range is large and the diameter range is wide;
(5) Corrosive fluids can be applied.
Disadvantages:
(1) It is not possible to measure liquids with very low conductivity, such as petroleum products;
(2) It is not possible to measure gases, vapors and liquids containing large bubbles;
(3) Cannot be used for higher temperatures.
Application overview:
Electromagnetic flowmeters are used in a wide range of applications. Large-diameter instruments are used in water supply and drainage projects. Small and medium-sized calibers are often used in high-demand or difficult-to-measure applications, such as steel industry blast furnace tuyere cooling water control, paper industry measuring pulp and black liquor, chemical industry. Strong corrosive liquid, pulp of non-ferrous metallurgical industry; small caliber, small caliber is often used in medical industry, food industry, biochemistry and other places with hygienic requirements.
3.6 Vortex Flowmeter The vortex flowmeter is a non-streamlined vortex generator placed in the fluid. The fluid is alternately separated on both sides of the generator to release two strings of regularly staggered vortex.
The vortex flowmeter can be divided into stress type, strain type, capacitive type, thermal type, vibrating type, photoelectric type and ultrasonic type according to the frequency detection mode.
The vortex flowmeter is one of the youngest types of flowmeters, but it has developed rapidly and has become a general-purpose flowmeter.
advantage:
(1) The structure is simple and firm;
(2) There are many types of fluids to be applied;
(3) high precision;
(4) The range is wide;
(5) The pressure loss is small.
Disadvantages:
(1) Not applicable to low Reynolds number measurements;
(2) Longer straight pipe sections are required;
(3) The meter factor is low (compared to the turbine flow meter);
(4) The instrument still lacks application experience in pulsating flow and multiphase flow.
3.7 Ultrasonic Flowmeters Ultrasonic flowmeters are meters that measure the flow by detecting the effect of fluid flow on the ultrasound beam (or ultrasound 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 offset method, Doppler method, cross correlation method, spatial filtering method. And noise law, etc.
Ultrasonic flowmeter is the same as electromagnetic flowmeter. It is an unobstructed flowmeter because it does not have any obstruction parts. It is a kind of flowmeter suitable for solving difficult flow measurement problems, especially in large-diameter flow measurement. The advantage of this in recent years is that it is one of the fastest growing types of flow meters.
advantage:
(1) can be used for non-contact measurement;
(2) for no flow obstruction measurement, no pressure loss;
(3) It can measure non-conductive liquid, which is a supplement to the electromagnetic flowmeter with no obstruction measurement.
Disadvantages:
(1) The propagation time method can only be used to clean liquids and gases; the Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles;
(2) Doppler method measurement accuracy is not high.
Application overview:
(1) The propagation time method is applied to clean, single-phase liquids and gases. Typical applications include factory effluent, blame, liquefied natural gas, etc.
(2) Good experience in the application of high pressure natural gas for gas applications;
(3) The Doppler method is applicable to two-phase fluids with a low heterogeneous content, such as untreated sewage, factory effluent, and dirty process fluid; generally not suitable for very clean liquids.
[Edit this paragraph] 3.8 Coriolis mass flowmeter Coriolis mass flowmeter (hereinafter referred to as CMF) is made by using the fluid in the vibrating tube, produced by the Coriolis force principle proportional to the mass flow A direct mass flow meter.
The application of CMF in China started late. In recent years, several manufacturing plants (such as Taihang Instrument Factory) have developed and supplied the market by themselves; there are also several manufacturing plants that form joint ventures or use foreign technology to produce series instruments.
Thermal Gas Mass Flow Meter The thermal flow meter sensor consists of two sensing elements, a speed sensor and a temperature sensor. They automatically compensate and correct for gas temperature changes. The electrically heated portion of the meter heats the speed sensor to a certain value above the operating temperature temperature to create a constant temperature difference between the speed sensor and the sensor that measures the operating temperature. When the temperature difference is kept constant, the energy consumed by electric heating can also be said to be a heat dissipation value proportional to the mass flow rate of the gas flowing through.
The thermal gas mass flow meter is Mass Flow Meter (abbreviated as MFM), which is a new type of instrument in gas flow metering. It is different from other gas flow meters without pressure and temperature correction. It directly measures the mass flow of gas. One sensor can Do the range from very low to high range. It is suitable for the measurement of single gas and fixed proportion multi-component gases.
Thermal gas mass flow meters are new instruments for measuring and controlling gas mass flow. It can be used for the monitoring of air, hydrocarbon gas, flammable gas and flue gas in petroleum, chemical, steel, metallurgy, electric power, light industry, medicine, environmental protection and other industrial sectors.
Features High reliability, high repeatability, good measurement accuracy, high pressure loss, no moving parts, fast range, wide response, no need for temperature and pressure compensation applications, gas mass flow measurement in industrial pipes, smoke flow rate measurement from chimney discharge, calciner flue gas flow measurement • Air flow measurement during gas flow • Compressed air flow measurement • Gas flow measurement during semi-channel chip manufacturing
• Gas flow measurement in wastewater treatment • Gas flow measurement in heating ventilation and air conditioning systems • Gas flow measurement in flux recovery systems • Measurement of combustion gas flow in combustion boilers • Gas flow measurement in natural gas, flare gas, hydrogen, etc.
• Measurement of carbon dioxide gas flow during beer production • Measurement of gas mass flow during production of cement, cigarettes and glass plants
Such as: US SIERRA
China DSN
3.9 The open channel flowmeter differs from the previous ones in that it is a flow meter that measures the free flow of the free surface in a non-full tubular open channel.
A waterway that is not full of tubular flow is called an open channel, and an open channel flowmeter that measures the flow of water in an open channel is called an open channel flowmeter.
In addition to the circular shape, the open channel flowmeter has various shapes such as U-shape, trapezoidal shape, and rectangular shape.
All urban water supply diversion canals for open channel flowmeter applications; diversion and drainage channels for thermal power plants, sewage treatment inflows and discharge channels; water discharge for industrial and mining enterprises; and channels for water conservancy projects and agricultural irrigation. It is estimated that 1995 units account for about 1.6% of the total flow meters, but there is no estimated data for domestic applications.
4, research and development of new working principle flow meter
4.1 Electrostatic flowmeter
The Tokyo Institute of Technology in Japan developed an electrostatic flowmeter suitable for the measurement of low-conductivity liquid flow in petroleum pipelines.
The metal measuring tube of the electrostatic flowmeter is connected to the pipe system insulatively, and the electric charge in the measuring tube can be known by measuring the static charge on the capacitor. They performed real-flow tests on metal and plastic measuring tube instruments with inner diameters of 4~8mm copper and stainless steel. The tests showed that the flow rate and charge were close to linear.
4.2 combined effects meter (combined effects meter)
The working principle of the meter is based on the deformation of the fluid induced by the momentum and pressure of the fluid, and the deformation of the composite effect is measured to obtain the flow. This instrument was developed by the GMI Engineering and Management School of the United States and has applied for two patents.
4.3 Tachmetric flowrate sensor
It was developed by the Industrial Instrumentation Company of the Russian Science and Engineering Center and was developed based on the theory of suspension effects. The instrument has been successfully applied in several sites (for example, installing more than 2,000 measuring hot water flows in nuclear power plants for 8 consecutive years) and is still improving to expand the application area.
5, several flow meter applications and development trends
5.1 Coriolis mass flow meter (CMF)
Foreign CMF has developed more than 30 series. The technical focus of each series development is: design innovation on the structure of flow detection and measurement tube; improving the stability and accuracy of the zero point of the instrument; increasing the deflection of the measuring tube, improving the sensitivity; improving the stress of the measuring tube Distribution, reduce fatigue damage, and strengthen the ability to resist vibration.
5.2 Electromagnetic Flowmeter (EMF)
Since the EMF entered industrial applications in the early 1950s, the field of use has expanded. Since the late 1980s, it has accounted for 16% to 20% of the sales volume of flow meters in various countries.
China has developed rapidly in recent years. In 1994, sales were estimated at 6500-7500 units. The domestic ENF has a maximum diameter of 2~6m, and has the capability of real-flow calibration with a diameter of 3m.
5.3 Vortex Flowmeter (USF)
The USF entered industrial applications in the late 1960s. Since the late 1980s, it has accounted for 4% to 6% of the sales volume of flow meters in various countries. In 1992, the world's estimated sales volume was 3.548 million units, and domestic products were estimated to be 8,000-9000 units in the same period.
5.4 Powerbar Flowmeter The Veolibar Flowmeter uses a fully aerodynamic engineering design and is a sensing component that achieves unparalleled accuracy, efficiency and reliability.
6. Conclusions As can be seen from the above, although the flowmeter has developed to the present day, its type is still extremely diverse, and there is no flowmeter suitable for any occasion.
Each flow meter has its scope of application and has limitations. This requires us to:
(1) When selecting the instrument, be sure to be familiar with both the instrument and the object under test, and take into account other factors, so that the measurement will be accurate;
(2) Strive to develop new types of instruments to make them more perfect on the existing basis.
Differential pressure flowmeter Differential pressure flowmeter (hereinafter referred to as DPF or flowmeter) is a gauge that measures flow according to the differential pressure generated by the flow detection component installed in the pipeline, the known fluid condition, and the geometrical dimensions of the test piece and the pipe. . The DPF consists of a primary device (detection) and a secondary device (differential pressure conversion and flow display instrument). The DPF is usually classified by the type of the test piece, such as a hole pull flow meter, a venturi flow meter, and a constant velocity tube flow meter. The secondary device is a variety of mechanical, electronic, electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display and calculation instruments. It has developed into a series of specifications with high degree of serialization, generalization and standardization. A large class of instruments. The differential pressure gauge can be used to measure flow parameters as well as other parameters (such as pressure, level, density, etc.).
DPF can be divided into throttling type, dynamic pressure head type, hydraulic resistance type, centrifugal type, dynamic pressure gain type and jet type according to the principle of its detecting parts. Among them, throttling type and dynamic pressure head type application The most extensive.
The detectors of the throttle type DPF are classified into two types according to their standardization degree: standard type and non-standard type. The so-called standard throttling device refers to the design, manufacture, installation and use according to the standard documents. It can determine the flow value and estimate the flow measurement error without real-current calibration. The non-standard throttling device is poorly mature and has not been included in the standard. The test piece in the file.
The development of the standard throttling DPF has gone through a long process. As early as the 1920s, large-scale throttling device experiments were started in the United States and Europe. The most common throttling devices - orifice plates and nozzles - began to standardize. One type of standard ISA l932 nozzle for standard nozzles is now standardized in the 1930s, and the standard orifice plate was also known as the ISA l932 orifice plate. The standardization of the structure of the throttling device has far-reaching significance. Because only the structural form of the throttling device is standardized, it is possible to bring together many international research results, which promotes the depth and breadth of the theory and practice of the test piece. This is beyond the reach of other flow meters. In 1980, ISO (International Organization for Standardization) officially passed the international standard ISO 5167, and the first international standard for flow measurement and throttling devices was born. ISO 5167 summarizes the theoretical and experimental research results of several limited throttling devices (orifice plates, nozzles and venturis) for decades, reflecting the state of the art in contemporary science and production of such test pieces. . However, since the official promulgation of ISO 5167, it has exposed many problems that need to be solved. These problems mainly include the following aspects.
1) The old data of ISO 5167 test data The data used in ISO 5167 are mostly the results of the test in the 1930s. Today, regardless of the throttling device manufacturing technology, the flow test equipment and experimental technology have made great progress, re-test systematically to obtain Higher accuracy and more reliable data are necessary. In the 1980s, large-scale trials were conducted in the United States and Europe, laying the groundwork for the revision of ISO 5167.
2) The issue of the length regulation of straight pipe in ISO 5167 When the ISO voted to pass ISO 5167, the United States voted against it. The main reason is that there are different opinions on the length of the straight pipe. This issue should be the main problem of the revision of ISO 5167. One.
3) The scientific problems of ISO 5167 affect the outflow coefficient of the throttling device. There are many factors, such as the ratio of the aperture to the diameter of the pipe diameter β, the pressure device, the Reynolds number, the eccentricity of the throttle installation, and the front and rear flow. The type of the piece and the length of the straight pipe section, the sharpness of the inlet edge of the orifice plate, the roughness of the pipe wall, the turbulence of the fluid flow, etc., many factors affect the intricacy, and some parameters are difficult to measure directly, so some provisions in the standard are not scientifically determined, but Consensus has to be determined artificially. The famous traffic expert Spencer (E.A. Spencer) proposed a series of issues that should be reviewed, such as plate flatness, concentricity, sharp edge sharpness, pipe roughness, upstream flow velocity distribution and the role of flow regulators.
4) Regarding the improvement of the accuracy of the throttling DPF measurement, in view of the importance of the throttling DPF in the flowmeter, it is of great significance to improve the measurement accuracy. Previous international academic conferences have found that traffic measurement workers, fluid mechanics and computer technology workers must work closely together to solve this problem.
In the 1980s, the United States and Europe began large-scale experimental research on orifice flowmeters. Europe is the EEC Experimental Program and the United States is the API Experimental Program. The purpose of the test is to conduct a new round of extensive experimental research using the latest modern test equipment and statistical processing techniques of test data, laying the technical foundation for the revision of ISO 5167. In 1999, ISO issued a revised version of ISO 5167 (ISO/CD5167-1-4), which is a draft of the committee. It has been greatly changed in terms of technical content and editing, and is a brand new standard. It was originally scheduled to be reviewed as a DIS (Standard Draft) at the ISO/TC30/SC2 meeting held in Denver, USA in July 1999, but the meeting considered that there were still details that should be discussed and failed. It is not known when the new ISO 5167 standard will be officially promulgated. The new ISO 5167 standard has substantial changes in the two core contents of the standard. One is the equation for the outflow coefficient of the orifice plate. The Reader-Harris/Gallagher calculation formula (RG type) is used instead of the Stolz calculation formula, and the other is the upstream of the throttling device. The length of the side straight pipe section and the use of the flow regulator.
We usually refer to the throttling devices listed in ISO 5167 (GB/T2624) as standard throttling devices, others are called non-standard throttling devices. It should be noted that non-standard throttling devices not only refer to those throttling devices. If the standard throttling device is different, if the standard throttling device works under deviation from the standard conditions, it should also be called a non-standard throttling device. For example, the standard orifice plate works under the critical flow in the mixed phase flow or the standard venturi nozzle. .
At present, there are some types of non-standard throttling devices:
1) 1/4 round orifice plate, tapered inlet orifice plate, double orifice plate, double inclined orifice plate, semi-circular orifice plate, etc. for low Reynolds number;
2) Round median plates, eccentric orifice plates, annular orifice plates, wedge-shaped orifice plates, elbow throttles, etc. for dirty media;
3) Low pressure loss with Roros tube, Dow tube, Dow orifice plate, double venturi nozzle, universal venturi tube, Vasy tube, etc.;
4) The whole (built-in) orifice plate for small pipe diameter;
5) end orifice device end orifice plate, end nozzle, Borda tube, etc.;
6) Wide-range throttling device elastically loaded variable area variable head flowmeter (linear orifice plate);
7) capillary throttle flowmeter;
8) pulsating flow throttling device;
9) a critical flow throttling device sonic venturi nozzle;
10) Mixed phase flow throttling device.
The continuous expansion of the throttling DPF field application will inevitably require the development of non-standard throttling devices. For more than ten years, ISO has been continuously developing technical documents on non-standard throttling devices, and published as technical reports before they can become official standards. . It is foreseeable that in the future it will be possible for some of the more mature non-standard throttling devices to be promoted to standard.
In the mid-to-late 1990s, the sales volume of various types of DPF in the world accounted for 50%-60% of the total volume of flow meters (about one million units per year), and the amount accounted for about 30%. The number of sales in China accounts for about 35%-42% of the total flow meter (excluding domestic gas meters and household water meters and glass tube float flowmeters) (60,000-70,000 units per year).
2 Working principle 2.1 Basic principle The fluid filled in the pipeline, when it flows through the throttle in the pipeline, as shown in Figure 4.1, the flow rate will form a local contraction at the throttle, so the flow rate increases, the static pressure decreases, so the section A pressure difference is generated before and after the flow piece. The larger the fluid flow rate, the greater the pressure difference produced, so that the flow rate can be measured by the pressure difference. This measurement method is based on the flow continuity equation (the law of conservation of mass) and the Bernoulli equation (the law of conservation of energy). The magnitude of the pressure difference is not only related to the flow rate but also to many other factors. For example, when the physical properties (density, viscosity) of the fluid in the form of the throttling device or the pipe are different, the pressure difference generated under the same flow rate is also different.
Figure 4.1 Flow velocity and pressure distribution near the orifice plate 2.2 qm--mass flow rate in the flow equation, kg/s;
Qv--volume flow, m3/s;
C--outflow coefficient;
Ε--expansion coefficient;
---diameter ratio, β=d/D;
D--the aperture of the throttle under working conditions, m;
D--the inner diameter of the upstream pipe under working conditions, m;
P--differential pressure, Pa;
Ρl--upstream fluid density, kg/m3.
It can be seen from the above equation that the flow rate is a function of six parameters of C, ε, d, Ï, P, and β(D). These six parameters can be divided into real measurements [d, Ï, P, β(D)] and statistics. Two types of quantity (C, ε).
(1) Real measurement
1) d, D (4.1) where d is proportional to the flow rate, and its accuracy has a great influence on the total flow accuracy. The error value should generally be controlled at ±0.05%, and the operating temperature should be calculated. The effect of thermal expansion of the material. The standard stipulates that the inner diameter D of the pipe must be measured. It is necessary to perform multiple measurements on several sections of the upstream pipe section to obtain the average value, and the error should not exceed ±0.3%. In addition to the high accuracy requirements for numerical measurements, it should also be considered that the deviation of the inner diameter will have a serious impact on the abnormal throttling of the upstream passage of the throttle. Therefore, when it is not a complete supply of throttling devices, the piping at the site should pay sufficient attention to this problem.
2) Ï Ï is in the same position as P in the flow equation, that is to say, when pursuing the high-precision level of the differential pressure transmitter, never forget that the measurement accuracy of Ï should also match. Otherwise the increase in P will be offset by the decrease in Ï.
3) The accurate measurement of P differential pressure P should not be limited to the selection of a high precision differential pressure transmitter. In fact, whether the differential pressure transmitter can accept the true differential pressure value is also determined by a series of factors. The manufacture, installation and use of the correct pressure tapping and pressure piping are the key to ensuring the true differential pressure value. Many influencing factors are difficult to quantify or qualitatively determined. Only by strengthening the standardization of manufacturing and installation can the goal be achieved.
(2) Statistics
1) C statistic C is an amount that cannot be measured (refers to the standard design and manufacture, and is not used for calibration). The most complicated situation when used in the field occurs when the actual C value does not match the C value determined by the standard. Their deviations are caused by a number of factors in the design, manufacture, installation and use. It should be clarified that all the above-mentioned links strictly follow the standards, and the actual values ​​will be consistent with the values ​​determined by the standards. It is difficult to fully meet this requirement on site.
It should be noted that deviations from standard conditions, some can be quantitatively estimated (can be corrected), and some can only be qualitatively estimated (magnitude and direction of uncertainty). But in reality, sometimes it is not only a conditional deviation, but it brings a very complicated situation, because the general information only introduces the error caused by a certain conditional deviation. If many conditions deviate at the same time, the lack of relevant information can be found.
2) ε The expansion coefficient ε is a correction for the change of the outflow coefficient caused by the change of the density of the fluid through the throttle. The error is composed of two parts: one is the error of ε under the common flow rate, that is, the standard determination value. The error; the second is the error caused by the fluctuation of the ε value due to the flow change. Generally, in the case of low static pressure and high differential pressure, the ε value has a non-negligible error. When P/P ≤ 0.04, the error of ε is negligible.
3 Classification of differential pressure flowmeters is shown in Table 4.1.
Table 4.1 Differential Pressure Flowmeter Classification Table Classification Principle Classification Type
According to the principle of generating differential pressure 1) throttling; 2) dynamic head; 3) hydraulic resistance; 4) centrifugal; 5) dynamic pressure gain; 6) jet
Classification by structure 1) standard orifice plate; 2) standard nozzle; 3) classic venturi tube; 4) venturi nozzle; 5) tapered inlet orifice plate; 6) 1/4 circular orifice plate; Plate; 8) eccentric orifice plate; 9) wedge-shaped orifice plate; 10) integral (built-in) orifice plate; 11) linear orifice plate; 12) annular orifice plate; 13) Daoer tube; 14) Roros tube; Elbow; 16) replaceable orifice throttling device; 17) critical flow throttling device
Classification by use 1) Standard throttling device; 2) Low Reynolds number throttling device; 3) Dirty flow throttling device; 4) Low pressure loss throttling device; 5) Small pipe diameter throttling device; 6) Wide range Throttle device; 7) critical flow throttling device;
3.1 Classification according to the principle of generating differential pressure 1) The throttling type works according to the principle that the fluid converts part of the pressure energy into kinetic energy through the throttle to generate differential pressure. The detection part is called the throttling device and is the main type of DPF. .
2) The dynamic pressure head type works according to the principle that the dynamic pressure is converted into static pressure, such as a constant velocity tube flowmeter.
3) The hydraulic resistance type works according to the principle of pressure difference generated by fluid resistance. The detecting part is a capillary beam, also called a laminar flow meter, which is generally used for small flow measurement.
4) Centrifugal pressure differential work based on the principle of centrifugal force generated by a curved tube or a ring tube, such as a curved tube flow meter, a ring tube flow meter, and the like.
5) The dynamic pressure gain type works according to the dynamic pressure amplification principle, such as the Pito-Venturi tube.
6) The jet type works according to the principle of fluid jet impact generation, such as a jet type differential pressure flow meter.
3.2 Classification by structure 1) Standard orifice plate is also called concentric right-angle edge orifice plate, and its axial section is shown in Figure 4.2. The orifice plate is a thin plate with sharp, right-angled edges that are machined into a circular concentric shape. The upstream side edge of the orifice opening should be a sharp right angle. The standard orifice plate has three pressure-receiving methods: angle joint, flange and DD/2 pressure; as shown in Figure 4.3. In order to measure the flow from either direction, a symmetrical orifice plate can be used, and both edges of the orifice conform to the characteristics of the upstream edge of the right-angle edge orifice, and the total thickness of the orifice does not exceed the thickness of the orifice.
Figure 4.2 Standard orifice plate Figure 4.3 Three pressure-receiving methods for orifice plate 2) Standard nozzles are available in two configurations: ISA 1932 nozzle and long-diameter nozzle.
a. ISA 1932 nozzle (Fig. 4.4) A nozzle consisting of a constricted section, a cylindrical throat and a groove defined by two arcs perpendicular to the plane of the shaft and contoured by the circumference. The pressure-receiving method of the ISA 1932 nozzle is only one type of pressure.
Figure 4.4 ISA 1932 nozzle b. Long-diameter nozzle (Fig. 4.5) The upstream surface consists of a plane perpendicular to the axis, a 1/4 elliptical contraction section, a cylindrical throat and possibly grooves or bevels. nozzle. The pressure-receiving method of the long-diameter nozzle is only one type of DD/2.
3) The classic venturi consists of the inlet cylinder section A, the conical section B, the cylindrical throat C and the conical diffusion section E, as shown in Figure 4.6.æ ¹æ®ä¸åŒçš„åŠ å·¥æ–¹æ³•ï¼Œæœ‰ä»¥ä¸‹ç»“æž„å½¢å¼ï¼šâ‘ å…·æœ‰ç²—é“¸æ”¶ç¼©æ®µçš„ï¼›â‘¡å…·æœ‰æœºæ¢°åŠ å·¥æ”¶ç¼©æ®µçš„ï¼›â‘¢å…·æœ‰é“æ¿ç„ŠæŽ¥æ”¶ç¼©æ®µçš„。ä¸åŒç»“æž„å½¢å¼çš„L1ã€L2ã€R1ã€R2与Dã€d的关系如表4.2所示。
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1 ±0.25D(100mm＜D＜150mm)
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3 R1=1.375D+20% R1＜0.25D R1=0,ç„Šç¼é™¤å¤–
4 R2=3.625d至3.8d R2＜0.25D R2=0,ç„Šç¼é™¤å¤–
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