Flowmeter type, development and characteristics

1, the development of flow meters The development of flow measurement can be traced back to ancient water conservancy projects and urban water supply systems. In the ancient Caesar era, orifice plates were used to measure the amount of drinking water of residents. Around 1000 BC, ancient Egypt measured the flow of the Nile River by means of the rake method. China's famous Dujiangyan water conservancy project Baokou mouth water level observation of water size and so on. In the 17th century, Torrey Setle laid the theoretical foundation for differential pressure flowmeters, which is a milestone in flow measurement. Since then, prototypes of many types of instrumentation for flow measurement in the 18th and 19th centuries have begun to form, such as helium, tracer methods, pitot tubes, venturi tubes, volume, turbines, and target flow meters. In the 20th century, due to the rapid increase in demand for flow measurement in the process industry, energy metering, and urban public utilities, rapid development of instrumentation was promoted. The rapid development of microelectronics and computer technology drastically promoted the replacement of meters. New flowmeters have mushroomed. . So far, hundreds of flowmeters have been reported to be on the market, and many difficult problems in field use are expected to be solved.
The work of modern traffic measurement technology in China was relatively late, and the flow instruments needed in the early days were imported from abroad.

Flow measurement is the science to study the change of the quality of objects. The law of mutual change of quality is the basic law of the development of the relationship between things. Therefore, the object of measurement is not limited to the traditional sense of the pipeline liquid, and wherever there is a need to grasp the quantitative change, there is a problem of flow measurement. Flow, pressure, and temperature are listed as three major test parameters. For a certain fluid, as long as these three parameters are known, the energy it has can be calculated, and these three parameters must be detected in the energy conversion measurement. Energy conversion is the basis of all production processes and scientific experiments, so flow, pressure, and temperature instruments are the most widely used.
2, flow meter application areas Flow measurement technology and instrumentation applications generally have the following areas.

Flow Meter Type A meter that measures the flow of fluid in a pipe (the volume of fluid that passes in a unit of time). Rotary flowmeters, throttling flowmeters, slit flowmeters, volumetric flowmeters, electromagnetic flowmeters, ultrasonic flowmeters, and helium.

There are many kinds of flow measurement methods and instruments, and there are many classification methods. So far, there are more than 60 types of flow meters available for industrial use. The reason why so many varieties are so far is that they have not found a flow meter that is suitable for any fluid, any range, any flow conditions, and any conditions of use.

These more than 60 flow meters, each with its own specific applicability, also have their limitations. According to the measurement object, there are two types of closed pipes and open channels; according to the purpose of measurement, they can be divided into total measurement and flow measurement, and the meters are called total meters and flow meters, respectively.

Aggregate meters measure the flow through a pipeline over a period of time, expressed as a quotient of the total amount of flow in a short period of time divided by this time. Actually, a flow meter is usually equipped with an accumulative flow device for use as a total gauge. The total volume table also has a traffic signalling device. Therefore, dividing the flow meter and the total amount table in a strict sense has no practical significance.

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

According to the current most popular and most widely used classification method, it is divided into volumetric flowmeter, differential pressure flowmeter, floater flowmeter, turbine flowmeter, electromagnetic flowmeter, vortex flowmeter in fluid oscillation flowmeter, and quality. Flow meters and plug-in flow meters, probe-type flow meters, to explain the various flow meter principle, characteristics, application overview and development at home and abroad.
3.1 Differential pressure flowmeter Differential pressure flowmeter is based on the differential pressure installed in the flow detection device installed in the pipeline, the known fluid conditions and the geometry of the test piece and the pipe to calculate the flow of the instrument.

The differential pressure flowmeter consists of a primary device (test piece) 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 equal velocity tube 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 specification, serialization, generalization and standardization. It can measure flow parameters as well as other parameters (such as pressure, level, density, etc.) ).

Differential pressure flow meter detection parts according to its role 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.

Detectors can be divided into two categories according to their degree of standardization: standard and non-standard.

The so-called standard test pieces are designed, manufactured, installed, and used as long as they are in accordance with standard documents. They do not require real-flow calibration to determine their flow values ​​and estimate measurement errors.

Non-standard test pieces are of poor maturity and have not been included in international standards.

Differential pressure flowmeter is one of the most widely used flowmeters, and it ranks first in the use of various types of flow meters. In recent years, due to the advent of various new flowmeters, the percentage of its use has gradually declined, but it is still the most important type of flowmeter.

advantage:

(1) The most widely used perforated plate flowmeter has a strong structure, stable and reliable performance, and long service life;

(2) It has a wide range of applications. No flowmeter has yet been compared with it.
(3) The detection parts, transmitters and display meters are produced by different manufacturers respectively, which is convenient for economies of scale production.

Disadvantages:

(1) The measurement accuracy is generally low;

(2) The range is narrow, generally only 3:1~4:1;

(3) High requirements for on-site installation conditions;

(4) Large pressure loss (finger plates, nozzles, etc.).

Note: A new type of product: Balanced flowmeters developed with the introduction of NASA. These flowmeters measure 5-10 times the accuracy of conventional throttling devices and have 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 convenient to install and use, and greatly reduces the capacity consumption of fluid operation.

Application Overview:

Differential pressure flowmeters have a particularly wide range of applications. They can be applied to various objects in closed-circuit flow measurement, such as fluids: single-phase, mixed-phase, clean, dirty, viscous flow, etc.; working conditions: normal pressure, high pressure , vacuum, room temperature, high temperature, low temperature, etc.; pipe diameter: from a few mm to several m; flow conditions: subsonic, sonic, pulsating flow. Its consumption in various industrial sectors accounts for about 1/4 to 1/3 of the total flow meter usage.
3.2.Floater flow meter The float flow meter, also known as rotameter, is a type of variable area flow meter. In a vertical conical tube that expands from bottom to top, the gravity of a float with a circular cross section is subject to liquid power. , so that the float can rise and fall freely in the cone.

The float flow meter is the second most widely used type of flow meter after the differential pressure flow meter, and plays a decisive role in the small and micro flow.

In the mid-1980s, sales in Japan, Western Europe, and the United States accounted for 15% to 20% of the flow meters. China's production in 1990 was estimated at 12 to 140,000 units, of which more than 95% were glass cone floater flowmeters.

Features:

(1) The glass cone floater flowmeter has a simple structure and is easy to use. The disadvantage is that the pressure resistance is low and there is a large risk that the glass tube is fragile.

(2) Suitable for small diameter and low flow rate;

(3) Low pressure loss.
3.3 Volumetric Flowmeters Positive displacement flowmeters, also known as fixed displacement flowmeters, referred to as PD flowmeters, are among the most accurate in flowmeters. It uses a mechanical measuring element to continuously divide the fluid into a single known volume fraction, measuring the total volume of fluid according to the number of times that the measurement chamber fills and discharges the volume fraction of fluid one after another.

Volumetric flowmeters are classified according to their measuring components and can be divided into oval gear flowmeters, scraper flowmeters, dual-rotor flowmeters, rotary piston flowmeters, reciprocating piston flowmeters, disk flowmeters, liquid-sealing tumbler flowmeters. Wet gas meter and film gas meter.

advantage:

(1) High measurement accuracy;

(2) Installation pipeline conditions have no effect on the measurement accuracy;

(3) can be used for the measurement of high viscosity liquids;

(4) Wide range;

(5) Direct-reading meters can be directly accumulated, total, clear, and easy to operate without external energy.

Disadvantages:

(1) The result is complex and bulky;

(2) The types of medium to be measured, the caliber, and the working status of the medium are limited;

(3) Not suitable for high and low temperature applications;

(4) Most meters are only suitable for clean single-phase fluids;

(5) Generate noise and vibration.

Application Overview:
Volumetric flowmeters and differential pressure flowmeters and float flowmeters are listed as the three most used flowmeters and are often used for the measurement of the total volume of expensive media (oil products, natural gas, etc.).

In industrialized countries, the sales flow of PD flowmeters (excluding domestic gas meters and household water meters) accounted for 13%~23% of the flow meters in recent years; China accounted for about 20%, and the 1990 production (excluding domestic gas meters) was estimated at 340,000. Taiwan, where the elliptical gear type and waist wheel type account for about 70% and 20%, respectively.
3.4 Turbine Flowmeters Turbine flowmeters, which are the main types of velocity flowmeters, use a multi-blade rotor (turbine) to sense the average flow rate of the fluid and to derive the flow or total meter.
It is generally composed of two parts: a sensor and a display. It can also be made into an integral type.

Turbine flowmeters and positive displacement flowmeters, Coriolis mass flowmeters are known as three types of repetitive, high-accuracy products in flowmeters. As one of the ten types of flowmeters, their products have been developed into many varieties and series. Mass production scale.

advantage:

(1) High precision, among all flow meters, the most accurate flow meter;

(2) good repeatability;

(3) zero-zero drift, good anti-interference ability;

(4) Wide range;

(5) Compact structure.

Disadvantages:

(1) The calibration feature cannot be maintained for a long time;

(2) The fluid properties have a great influence on the flow characteristics.

Application Overview:

Turbine flowmeters are widely used in several measurement objects: petroleum, organic liquids, inorganic liquids, liquefied gas, natural gas, and cryogenic fluids. In Europe and the United States, turbine flow meters are the second only to the natural measurement of orifice flow meters. Instruments, only the Netherlands in the natural gas pipeline on the use of more than 2,600 various sizes, pressure from 0.8 ~ 6.5MPa gas turbine flow meter, they have become an excellent natural gas meter.
3.5 Electromagnetic flowmeter Electromagnetic flowmeter is an instrument for measuring conductive liquids made according to Faraday’s electromagnetic induction law.
Electromagnetic flowmeter has a series of excellent characteristics, which can solve the problems that other flowmeters are not easy to apply, such as the measurement of dirty flow and corrosion flow.

In the 70s and 80s, there was a major technological breakthrough in electromagnetic flow, which made it a widely used type of flowmeter. The percentage of its use in flow meters has been rising.

advantage:

(1) The measurement channel is a smooth, straight section of tubing that will not block and is suitable for the measurement of liquid-solid two-phase fluids containing solid particles, such as pulp, mud, sewage, etc.;

(2) No pressure loss caused by flow detection, and good energy saving effect;

(3) The measured volume flow is virtually unaffected by changes in fluid density, viscosity, temperature, pressure, and conductivity;

(4) Large flow range and wide caliber range;

(5) Corrosive fluids may be applied.

Disadvantages:

(1) Liquids with very low conductivity, such as petroleum products, cannot be measured;

(2) Gas, steam, and liquids containing large bubbles cannot be measured;

(3) Cannot be used for higher temperatures.

Application Overview:

Electromagnetic flow meters have a wide range of applications. Large-diameter meters are used in water supply and drainage projects; medium and small diameters are often used in demanding or unpredictable applications such as the control of cooling water in the blast furnace tuyere in the iron and steel industry, pulp and black liquor in the paper industry, and the chemical industry. Strong corrosive fluid, non-ferrous metallurgical industry pulp; small diameter, small diameter is often used in the pharmaceutical industry, food industry, biochemical and other health requirements.
3.6 Vortex Flowmeter Vortex flowmeter is to place a non-streamlined vortex generator in the fluid. The fluid is alternately separated on both sides of the generator to release two series of regularly staggered vortex instruments.

Vortex flowmeter according to the frequency detection method can be divided into: stress type, strain type, capacitive type, thermal type, vibration type, photoelectric type and ultrasonic type.

Vortex flowmeter is the youngest type of flowmeter, but its rapid development has become a common type of flowmeter.

advantage:

(1) The structure is simple and firm;

(2) There are many kinds of applicable fluids;

3) high precision;

(4) Wide range;

(5) Small pressure loss.
Disadvantages:

(1) Not applicable to low Reynolds number measurements;

(2) Longer straight sections;

(3) Low meter factor (compared with 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 sensing the effect of fluid flow on an ultrasonic beam (or ultrasonic pulse).

According to the principle of signal detection, ultrasonic flowmeters can be classified into differential propagation velocity methods (direct time difference method, time difference method, phase difference method, and frequency difference method), beam offset method, Doppler method, cross correlation method, and spatial filter method. And noise law.
Ultrasonic flowmeters, like electromagnetic flowmeters, are flowmeters without obstructions, and are unobstructed flowmeters. They are a class of flowmeters that are suitable for solving difficult problems in flow measurement, especially in large-diameter flow measurement. In recent years it has been one of the fastest growing flow meters.

advantage:

(1) Non-contact measurement can be made;

(2) No flow obstruction measurement, no pressure loss;

(3) The non-conductive liquid can be measured, which is a supplement to the electromagnetic flowmeter without obstructing the measurement.

Disadvantages:

(1) The transit 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 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 effluents,: strange liquids, liquefied natural gas, etc.;

(2) Good experience has been used in the field of high-pressure natural gas for gas applications;

(3) The Doppler method is applicable to biphasic fluids with relatively low out-of-phase contents, such as untreated sewage, factory discharge fluids, and dirty process fluids; it is generally not suitable for very clean fluids.
3.8 Coriolis Mass Flowmeters Coriolis Mass Flowmeters (hereafter abbreviated as CMF) are a type of direct method that uses the Coriolis force principle that is proportional to the mass flow rate when fluids flow through a vibrating tube. Mass flow meter.

The application of China's CMF started relatively late. In recent years, several manufacturing plants (such as Taihang Instrument Factory) have developed their own supply markets; several factories have also established joint ventures or used foreign technology to produce serial meters.

Thermal gas mass flowmeter
The thermal flowmeter sensor contains two sensing elements, a speed sensor and a temperature sensor. They automatically compensate and correct for gas temperature changes. The electric heating part of the meter heats the speed sensor to a certain value higher than the working temperature, so that a constant temperature difference is formed between the speed sensor and the sensor that measures the working condition temperature. When the temperature difference is kept constant, the energy consumed by electric heating can also be said to be proportional to the mass flow rate of the gas flowing through the heat dissipation value.

The hot gas mass flow meter is the Mass Flow Meter (abbreviated as MFM), which is a new type of instrument in gas flow metering. Different from other gas flow meters, it does not require pressure and temperature correction, and directly measures the mass flow of gas. A sensor can Do range from very low to high range. It is suitable for the measurement of single gases and fixed proportions of multicomponent gases.

Thermal gas mass flow meters are new meters for measuring and controlling gas mass flow. It can be used in the monitoring of air, hydrocarbon gases, combustible gases, and flue gas in industrial sectors such as petroleum, chemical, steel, metallurgy, electric power, light industry, medicine, and environmental protection.

Features
High reliability and good repeatability
No moving parts range faster than wide response without temperature compensation
Different from the previous ones, it is a flow meter that measures the natural flow of a free surface in a non-full tubular open channel.

Water channels that are not full-pipelined are called open channels, and the flow of water in open channels is called an open channel flowmeter.

In addition to circular flow meters, there are U-shaped, trapezoidal, rectangular and other shapes.

The open channel flowmeter applies all the urban water diversion canals; the power plant diversion and drainage, the sewage treatment inflow and discharge canals; the industrial and mining enterprises water discharge and irrigation works and irrigation channels. Some people estimated that 1995, accounting for about 1.6% of the overall flow instrument, but there is no estimated data for domestic applications.
4, the new working principle of flow meter research and development

4.1 Electrostatic flowmeter
(electrostatic flowmeter)

The Tokyo Institute of Technology develops an electrostatic flowmeter for low conductivity liquid flow measurement in oil transmission pipelines.

The metal measuring tube of the electrostatic flowmeter is connected to the pipe system in an insulated manner. The measurement of the charge in the measuring pipe can be made by measuring the electrostatic charge on the capacitor. They conducted solid-flow tests on metal and plastic measuring tube instruments with internal diameters of 4 to 8 mm, such as copper and stainless steel. The tests showed that the flow and charge were close to linear.
4.2 Composite Effect Flow Meter
(combined effects meter)

The working principle of the instrument is based on the fluid's momentum and pressure acting on the instrument body's cavity deformation, measuring the composite effect of the deformation of the flow. The instrument was developed by the American GMI School of Engineering and Management and has applied for two patents.
4.3 Tachometer Flow Sensor
(tachmetric flowrate sensor)

It was developed by the Russian Scientific and Engineering Center Industrial Instrumentation Corporation and was developed based on the suspension effect theory. The instrument has been successfully applied in several fields (for example, more than 2,000 sets of hot water flow are measured in nuclear power plants and used continuously for 8 years), and it is still being improved to expand the application field.
5, several flow meter applications and development trends
5.1 Coriolis Mass Flow Meter (CMF)

The foreign CMF has developed more than 30 series. The development of each series is focused on the technical aspects: the design and innovation of the structure of the flow detection and measurement tube; the improvement of the stability and accuracy of the instrument zero point; the increase of the deflection of the measuring tube and the improvement of the sensitivity; Distribution, reduce fatigue damage, strengthen anti-vibration interference capabilities.

5.2 Electromagnetic Flowmeter (EMF)
Since EMF entered the industrial application in the early 1950s, the use of the field has been expanding. Since the late 1980s, it has accounted for 16% to 20% of the sales amount of flow meters in various countries.

China's rapid development in recent years, 1994 sales estimated at 6500 to 7500 units. China has produced ENF with a maximum diameter of 2~6m, and has a real-life calibration capability of 3m.
5.3 Vortex Flowmeter (USF)
USF entered industrial applications in the late 1960s, and it accounted for 4% to 6% of the sales amount of flow meters in various countries since the late 1980s. In 1992, the estimated worldwide sales volume was 3.548 million units, and domestic products were estimated to be 8,000-9,000 units during the same period.

5.4 PowerBar Flowmeter
The Veolia Flowmeter uses a fully aerodynamically engineered structural design and is a sensing element that achieves unsurpassed levels of precision, efficiency, and reliability.
6. Conclusions From the above, it can be seen that although the flowmeter has grown to maturity today, its variety is still extremely varied, and there is no flowmeter yet suitable for any occasion.

Each flow meter has its scope of application and also has limitations. This requires us to:

(1) When selecting the instrument, be sure to be familiar with the condition of the instrument and the measured object, and consider other factors, so that the measurement will be accurate;

(2) Efforts are made to develop new instruments to make them more perfect on the existing basis.

Differential pressure flowmeter
A differential pressure flowmeter (hereinafter abbreviated as DPF or flowmeter) is a meter that measures a flow rate based on a differential pressure generated by a flow detecting member installed in a pipe, a known fluid condition, and a geometric dimension of a detecting member and a pipe. The DPF consists of a primary device (test piece) and a secondary device (differential pressure conversion and flow display instrument). The DPF is usually classified by the type of test piece, such as hole pull flow meter, venturi flow meter, and constant flow tube flow meter. The secondary device is a variety of mechanical, electronic, and electromechanical integrated differential pressure gauges, differential pressure transmitters, and flow display and calculation instruments. It has been developed into a category specification with high degree of tri-ization (serialization, generalization, and standardization). A large class of instruments. Differential pressure gauges can be used to measure flow parameters as well as other parameters (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 detection parts. Among them, throttling type and dynamic pressure head type application The most extensive.

Throttle DPF test pieces are divided into standard and non-standard types according to their degree of standardization. The so-called standard throttling device is designed, manufactured, installed and used according to standard documents. It is not necessary to carry out real flow calibration to determine its flow value and estimate flow measurement error. Non-standard throttling devices are less mature and have not been included in the standard. Detection items in the file.

The development of standard throttling DPF has been a long process. As early as the 1920s, the United States and Europe began to conduct large-scale throttling device test studies. The most commonly used throttling devices - orifices and nozzles - begin standardization. One type of ISA l932 nozzle of the standard nozzle is now standardized in the 30's, and the standard orifice plate was also known as the ISA l932 orifice plate. The standardization of the throttling device's structural form is of far-reaching significance, because only the throttling device's structural form is standardized, it is possible to bring together many international research results. It promotes the depth and breadth of the theory and practice of the test piece. This is what other flowmeters do not. In 1980, the ISO (International Organization for Standardization) formally passed the international standard ISO 5167. At this point, the first international standard for flow metering throttling devices was born. ISO 5167 summarizes the results of decades of theoretical and experimental research on a limited number of throttling devices (orifices, nozzles, and venturis) internationally, reflecting the current state-of-the-art science and production technology of such test pieces. . However, from the date of the formal promulgation of ISO 5167, it has exposed many problems that need to be resolved. These problems mainly include the following aspects.

1) The obsoleteness of ISO 5167 test data Most of the data used in ISO 5167 are the test results of the 30s. Today, regardless of the throttling device manufacturing technology, flow test equipment and experimental techniques have made tremendous progress, and systematic tests have been conducted again. Higher accuracy and more reliable data are necessary. In the 1980s, both the United States and Europe conducted large-scale experiments to lay the foundation for the revision of ISO 5167.

2) When ISO voting passed ISO 5167 in the ISO 5167 issue regarding the length of straight pipe, the United States voted against it. The main reason was that there were different opinions on the length of straight pipe, and this issue should be the main problem in the revision of ISO 5167. one.

3) The scientific issues of ISO 5167 affect the flow coefficient of the throttling device particularly, mainly including the ratio β of the diameter of the throttling device, the pressure-recovery device, the Reynolds number, the installation eccentricity of the throttling device, and the front and rear choke. The type and length of the straight pipe, the sharpness of the inlet edge of the orifice, the roughness of the pipe wall, the turbulence of the fluid flow, etc., are influenced by numerous factors, and some parameters are difficult to measure directly. Therefore, some provisions of the standard are not determined scientifically, but Consensus had to be artificially determined. Well-known traffic expert E.A. Spencer proposed a series of questions that should be reviewed, such as orifice flatness, concentricity, right-angled edge sharpness, pipe roughness, upstream flow velocity distribution, and the role of flow regulators.

4) Issues regarding the improvement of the accuracy of throttling DPF measurement Given that throttling DPF plays an important role in the flow meter, it is of great significance to improve its measurement accuracy. Previous international academic conferences believed that it was necessary to enable flow measurement workers, fluid mechanics, and computer technology workers to work together in close cooperation to solve this problem.

In the 1980s, the United States and Europe began to conduct large-scale orifice plate flow test studies. 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 studies using modern state-of-the-art test equipment and statistical processing techniques for test data to lay the technical foundation for the revision of ISO 5167. In 1999, the ISO issued a revised version of ISO 5167 (ISO/CD 5167-1-4), which is a draft of the committee. It has changed a lot in terms of technical content and editing, and is a completely new standard. Originally scheduled to be reviewed at the ISO/TC30/SC2 conference held in Denver, USA, in July 1999 as a DIS (draft standard), but the meeting felt that there were still details that should be negotiated and failed. It is not yet known when the new ISO 5167 standard was formally promulgated. The new ISO 5167 standard has substantial changes in both core content of the standard. One is the formula for the outflow coefficient of the orifice plate, the Reader-Harris/Gallagher formula (RG type) is used in place of the Stolz formula, and the other is the upstream of the throttling device. The length of the side straight pipe and the use of flow regulators.

We usually refer to the throttling devices listed in ISO 5167 (GB/T2624) as standard throttling devices. The others are called non-standard throttling devices. It should be pointed out that non-standard throttling devices are not only those throttling device structures and If the standard throttling device is different, if the standard throttling device works under standard deviation, it should also be called a non-standard throttling device. For example, the standard orifice plate works in the miscible flow or the standard Venturi nozzle under critical flow. .

At present, non-standard throttling devices generally have the following types:

1) Low Reynolds number 1/4 round hole plate, conical inlet orifice plate, double orifice plate, double inclined orifice plate, semicircular orifice plate, etc.;

2) For dirty media, use circular orifice plate, eccentric orifice plate, annular orifice plate, wedge-shaped orifice plate, elbow throttling piece, etc.

3) Low pressure loss with Luo Luosi, Daoer, Daoer orifice, double venturi nozzle, generic venturi, Vasy, etc.

4) The overall diameter (contained) of the orifice is used for the small diameter;

5) Tip orifice plate, tip nozzle, Borda tube, etc.

6) Wide range of throttling device elastic loading variable area variable pressure head flowmeter (linear orifice plate);

7) Capillary throttling laminar flow meter;

8) Pulsating flow throttling device;

9) critical flow throttling device sonic venturi nozzle;

10) Miscible flow throttling device.

The continuous development of throttling DPF field applications will inevitably require the development of non-standard throttling devices. For more than a decade, ISO has also continuously developed technical documents concerning non-standard throttling devices and published them as technical reports before they can become official standards. . It can be foreseen that in the future it is possible that some of the more mature non-standard throttling devices will be promoted to standard ones.

In the mid-to-late 1990s, various types of DPF sales in the world accounted for 50% to 60% of the total number of flow meters (about 1 million units per year), and the amount accounted for about 30%. China's sales volume accounts for about 35%-42% of the total flow meter volume (excluding domestic gas meters, household water meters, and glass tube float meters) (60,000-70,000 units per year).

2 How it works
2.1 Basic principles The fluid filled with 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 before and after the throttle It created a pressure difference. The greater the flow of fluid, the greater the pressure difference that is produced, so that the magnitude of the flow can be measured on the basis of 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 throttling device type or the physical properties (density, viscosity) of the fluid in the pipe are different, the pressure difference generated at the same size flow rate is also different.

2.2 Flow equation
Where qm - mass flow, kg / s;

Qv--volume flow, m3/s;

C--outflow coefficient;

Ε--expansibility coefficient;

Β--diameter ratio, β=d/D;

d - the aperture of the throttling element under working conditions, m;

D--the inner diameter of the upstream pipeline under working conditions, m;

â–³ 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, ρ, △P, and β(D). The six parameters can be divided into actual measurements [d, ρ, ΔP, β(D)]. And statistics (C, ε) two categories.

(1) Real measurement
1) In d, D (4.1), d is related to the square of the flow rate, and its accuracy has a greater influence on the total flow accuracy. The error value should generally be controlled at ±0.05%, and the operating temperature should also be taken into account. Material thermal expansion effect. The standard stipulates that the inner diameter D of the pipe must be measured. Multiple measurements must be made on several sections of the upstream pipe section to find the average value. The error should not exceed ±0.3%. In addition to the higher requirements on the accuracy of numerical measurements, it is also necessary to consider the serious influence of the deviation of the inner diameter on the abnormal throttling phenomenon of the upstream channel of the throttle. Therefore, when the throttling device is not supplied as a complete set, piping should pay attention to this problem in the field.

2) ρ ρ is in the same position as ΔP in the flow equation, that is, when pursuing high-precision differential pressure transmitter, never forget that the measurement accuracy of ρ should also match. Otherwise, the increase in ΔP will be offset by the decrease in ρ.

3) Accurate measurement of ΔP differential pressure ΔP should not be limited to the selection of a high precision differential pressure transmitter. In fact, whether the differential pressure transmitter can receive the true differential pressure value is also determined by a series of factors. Among them, the correct manufacturing and installation and use of the pressure-reducing hole and pressure-reducing pipeline are the key to ensure the true differential pressure value. Many influencing factors are difficult to quantify or qualitatively determine. Only by strengthening the standardization of manufacturing and installation can the purpose be achieved.

(2) Statistics
1) The C statistic C is the amount that cannot be measured (refers to the standard design, manufacture and installation, and is not used by calibration). The most complicated situation when used on the site is that the actual C value does not conform to the C value determined by the standard. Their deviation is caused by the design, manufacture, installation and use of a series of factors. It should be clarified that all the above-mentioned links strictly follow the provisions of the standard, and their actual values ​​will be in conformity with the value determined by the standard. It is difficult to fully satisfy this requirement on site.

It should be noted that some deviations from the standard conditions can be quantitatively estimated (can be corrected), and some can only be qualitatively estimated (the magnitude and direction of the uncertainty). But in reality, sometimes it is not only a conditional deviation, which brings about a very complicated situation, because the general data only describes the error caused by a certain conditional deviation. If many conditions deviate at the same time, relevant data are missing.

2) ε The expansibility coefficient ε is the correction of the change of the outflow coefficient caused by the change of density when the fluid passes through the throttling device. Its error is composed of two parts: one is the error of ε at the common flow rate, that is, the standard determination value. The second one is the error caused by the fluctuation of the ε value due to the flow rate change. Generally, in the case of low static pressure and high differential pressure, the ε value has a non-negligible error. When ΔP/P ≤ 0.04, the error of ε is negligible.
Category 3
Classification principle Classification type
According to the principle of differential pressure, 1) throttle type; 2) dynamic pressure head type; 3) hydraulic resistance type; 4) centrifugal type; 5) dynamic pressure gain type; 6) jet flow type
1) Standard orifice plate; 2) Standard nozzle; 3) Classic Venturi tube; 4) Venturi nozzle; 5) Cone inlet orifice plate; 6) Quarter hole plate; 7) Round hole Plate; 8) eccentric orifice; 9) wedge orifice; 10) integral (contained) orifice plate; 11) linear orifice plate; 12) annular orifice plate; 13) Douer tube; 14) Luo Luosi tube; Bending pipe; 16) Reversible orifice throttling device; 17) Critical flow throttling device, classification by application 1) Standard throttling device; 2) Low Reynolds number throttling device; 3) Dirty flow throttling device; 4 ) Low pressure loss throttling device; 5) Small diameter throttling device; 6) Wide range throttling device; 7) Critical flow throttling device;

3.1 Classification according to the principle of differential pressure
1) The throttling type works on the principle that the fluid converts part of the pressure energy into kinetic energy to generate differential pressure through the throttling element. The detection element is called 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 to static pressure, such as an equal flow tube flowmeter.

3) The hydraulic resistance type works on the basis of the principle of differential pressure generated by the fluid resistance. The detection element is a capillary bundle, also known as a laminar flow meter, which is generally used for small flow measurement.

4) Centrifugal type is based on the differential pressure formed by the principle of centrifugal force generated by the bending pipe or the annular pipe, such as the elbow flowmeter, the annular pipe flowmeter and so on.

5) The dynamic pressure gain type works according to the dynamic pressure amplification principle, such as Pitot-Venturi tube.

6) The jet type works on the principle of fluid jet impingement, such as jet differential pressure flowmeter.

3.2 Classified by structure
1) The standard orifice plate is also called concentric right-angled edge plate. Its axial section is shown in Figure 4.2. The orifice plate is a sheet of sharp right-angled edge processed into a circular concentric shape. The upstream edge of the orifice opening should be a sharp right angle. The standard orifice plate has three methods of pressure extraction: fillet, flange, and DD/2 pressure; as shown in Figure 4.3. To measure the flow from either direction, a symmetrical orifice can be used. The two edges of the orifice conform to the characteristics of the upstream edge of the right-angle edge orifice, and the thickness of the orifice does not exceed the thickness of the orifice.

2) Standard nozzles come in two configurations: ISA 1932 nozzles and long diameter nozzles.

a. ISA 1932 Nozzle (Fig. 4.4) The upstream nozzle consists of a converging section defined by two arcs of a plane perpendicular to the axis and having a circular contour, a cylindrical throat and a groove. The pressure of the ISA 1932 nozzle is only available at angle connection pressure.

b. Long-Sized Nozzles (Figure 4.5) The upstream nozzle consists of a plane perpendicular to the axis, a 1/4 elliptical constriction, a cylindrical throat, and possibly grooves or bevels. Long-diameter nozzle pressure mode is only DD/2 pressure.

3) The classic Venturi consists of an inlet cylinder section A, a cone constriction section B, a cylindrical throat C, and a cone diffusion section E, as shown in Figure 4.6. According to different processing methods, there are the following structural forms: 1 has a coarse casting shrinkage segment; 2 has a mechanically processed shrinkage segment; 3 has an iron plate welded contraction segment. The relationship between L1, L2, R1, R2 and D, d in different structural forms is shown in Table 4.2.

4) The Venturi nozzle consists of an inlet nozzle, a cylindrical throat, and a diffusion section, as shown in Figure 4.7.

5) Tapered inlet orifice The tapered inlet orifice is similar to a standard orifice plate and is equivalent to a standard inverted orifice plate. Its structure is shown in Fig. 4.8 and the pressure is taken as a corner joint pressure.

Table 4.2 Relation between L1, L2, R1, R2 and D, d
Injection rough-welded iron plate inlet for rough cast inlet machining
1 ±0.25D (100mm<D<150mm)

L1=0.5D±0.05D L1=0.5D±0.05D

2 L2=1D or 0.25D+250mm The smaller of the two quantities L2≥D (inlet diameter) L2≥D (inlet diameter)

3 R1=1.375D+20% R1<0.25D R1=0, except for welds
4 R2=3.625d to 3.8d R2<0.25D R2=0, except for welds
1一环隙；2-夹持环；3一上游端面A；4-下游端面B；
5-轴线；6-流向；7-取压口；8-孔板；
X-带环隙的夹持环；Y-单独取压口
超声波流量计的基本原理及类型
超声波在流动的流体中传播时就载上流体流速的信息。因此通过接收到的超声波就可以检测出流体的流速，从而换算成流量。根据检测的方式，可分为传播速度差法、多普勒法、波束偏移法、噪声法及相关法等不同类型的超声波流量计。起声波流量计是近十几年来随着集成电路技术迅速发展才开始应用的一种
非接触式仪表，适于测量不易接触和观察的流体以及大管径流量。它与水位计联动可进行敞开水流的流量测量。使用超声波流量比不用在流体中安装测量元件故不会改变流体的流动状态，不产生附加阻力，仪表的安装及检修均可不影响生产管线运行因而是一种理想的节能型流量计。

众所周知，目前的工业流量测量普遍存在着大管径、大流量测量困难的问题，这是因为一般流量计随着测量管径的增大会带来制造和运输上的困难，造价提高、能损加大、安装不仅这些缺点，超声波流量计均可避免。因为各类超声波流量计均可管外安装、非接触测流，仪表造价基本上与被测管道口径大小无关，而其它类型的流量计随着口径增加，造价大幅度增加，故口径越大超声波流量计比相同功能其它类型流量计的功能价格比越优越。被认为是较好的大管径流量测量仪表，多普勒法超声波流量计可测双相介质的流量，故可用于下水道及排污水等脏污流的测量。在发电厂中，用便携式超声波流量计测量水轮机进水量、汽轮机循环水量等大管径流量，比过去的皮脱管流速计方便得多。超声被流量汁也可用于气体测量。管径的适用范围从2cm到5m，从几米宽的明渠、暗渠到500m宽的河流都可适用。

另外，超声测量仪表的流量测量准确度几乎不受被测流体温度、压力、粘度、密度等参数的影响，又可制成非接触及便携式测量仪表，故可解决其它类型仪表所难以测量的强腐蚀性、非导电性、放射性及易燃易爆介质的流量测量问题。另外，鉴于非接触测量特点，再配以合理的电子线路，一台仪表可适应多种管径测量和多种流量范围测量。超声波流量计的适应能力也是其它仪表不可比拟的。超声波流量计具有上述一些优点因此它越来越受到重视并且向产品系列化、通用化发展，现已制成不同声道的标准型、高温型、防爆型、湿式型仪表以适应不同介质，不同场合和不同管道条件的流量测量。

超声波流量计目前所存在的缺点主要是可测流体的温度范围受超声波换能铝及换能器与管道之间的耦合材料耐温程度的限制，以及高温下被测流体传声速度的原始数据不全。目前我国只能用于测量200℃以下的流体。另外，超声波流量计的测量线路比一般流量计复杂。这是因为，一般工业计量中液体的流速常常是每秒几米，而声波在液体中的传播速度约为1500m／s左右，被测流体流速(流量)变化带给声速的变化量最大也是10－3数量级．若要求测量流速的准确度为1％，则对声速的测量准确度需为10-5～10-6数量级，因此必须有完善的测量线路才能实现，这也正是超声波流量计只有在集成电路技术迅速发展的前题下才能得到实际应用的原因。

超声波流量计由超声波换能器、电子线路及流量显示和累积系统三部分组成。超声波发射换能器将电能转换为超声波能量，并将其发射到被测流体中，接收器接收到的超声波信号，经电子线路放大并转换为代表流量的电信号供给显示和积算仪表进行显示和积算。这样就实现了流量的检测和显示。

超声波流量计常用压电换能器。它利用压电材料的压电效应，采用适出的发射电路把电能加到发射换能器的压电元件上，使其产生超声波振劝。超声波以某一角度射入流体中传播，然后由接收换能器接收，并经压电元件变为电能，以便检测。发射换能器利用压电元件的逆压电效应，而接收换能器则是利用压电效应。

超声波流量计换能器的压电元件常做成圆形薄片，沿厚度振动。薄片直径超过厚度的10倍，以保证振动的方向性。压电元件材料多采用锆钛酸铅。为固定压电元件，使超声波以合适的角度射入到流体中，需把元件故人声楔中，构成换能器整体(又称探头)。声楔的材料不仅要求强度高、耐老化，而且要求超声波经声楔后能量损失小即透射系数接近1。常用的声楔材料是有机玻璃，因为它透明，可以观察到声楔中压电元件的组装情况。另外，某些橡胶、塑料及胶木也可作声楔材料。

超声波流量计的电子线路包括发射、接收、信号处理和显示电路。测得的瞬时流量和累积流量值用数字量或模拟量显示。

根据对信号检测的原理，目前超声波流量计大致可分传播速度差法(包括：直接时差法、时差法、相位差法、频差法)波束偏移法、多普勒法、相关法、空间滤波法及噪声法等类型，如图所示。其中以噪声法原理及结构最简单，便于测量和携带，价格便宜但准确度较低，适于在流量测量准确度要求不高的场合使用。由于直接时差法、时差法、频差法和相位差法的基本原理都是通过测量超声波脉冲顺流和逆流传报时速度之差来反映流体的流速的，故又统称为传播速度差法。其中频差法和时差法克服了声速随流体温度变化带来的误差，准确度较高，所以被广泛采用。按照换能器的配置方法不同，传播速度差拨又分为：Z法(透过法)、V法(反射法)、X法(交叉法)等。波束偏移法是利用超声波束在流体中的传播方向随流体流速变化而产生偏移来反映流体流速的，低流速时，灵敏度很低适用性不大．多普勒法是利用声学多普勒原理，通过测量不均匀流体中散射体散射的超声波多普
勒频移来确定流体流量的，适用于含悬浮颗粒、气泡等流体流量测量。相关法是利用相关技术测量流量，原理上，此法的测量准确度与流体中的声速无关，因而与流体温度，浓度等无关，因而测量准确度高，适用范围广。但相关器价格贵，线路比较复杂。在微处理机普及应用后，这个缺点可以克服。噪声法(听音法)是利用管道内流体流动时产生的噪声与流体的流速有关的原理，通过检测噪声表示流速或流量值。其方法简单，设备价格便宜，但准确度低。

以上几种方法各有特点，应根据被测流体性质．流速分布情况、管路安装地点以及对测量准确度的要求等因素进行选择。一般说来由于工业生产中工质的温度常不能保持恒定，故多采用频差法及时差法。只有在管径很大时才采用直接时差法。对换能器安装方法的选择原则一般是：当流体沿管轴平行流动时，选用Z法；当流动方向与管铀不平行或管路安装地点使换能器安装间隔受到限制时，采用V法或X法。当流场分布不均匀而表前直管段又较短时，也可采用多声道(例如双声道或四声道)来克服流速扰动带来的流量测量误差。多普勒法适于测量两相流，可避免常规仪表由悬浮粒或气泡造成的堵塞、磨损、附着而不能运行的弊病，因而得以迅速发展。随着工业的发展及节能工作的开展，煤油混合(COM)、煤水泥合(CWM)燃料的输送和应用以及燃料油加水助燃等节能方法的发展，都为多普勒超声波流量计应用开辟广阔前景。

流量计的种类很多，一般市场上用得比较广泛的有：电磁流量计、涡街流量计、涡轮流量计、孔板流量计、V锥流量计、金属转子流量计、玻璃转子流量计、旋进旋涡流量计、椭圆齿轮流量计、均速管流量计、超声波流量计等。它们的安装条件对直管段的要求V锥流量计是最低，而电磁、涡街、孔板等对直管段要求就较高，一般是前5D后3D,对于流量计前端有弯头、阀门等的直管段要求就更高，最高要求直管段是前50D后5D,因此在选购流量计时一定要考虑流量计现场安装的环境、位置等因素，从而选择更加适合现场工矿的流量计。

现在流量计所需要的参数：
1、被测量的介质 2、被测量介质的温度 3、被测量介质的压力 4、被测量介质的流量 5、要求的测量精度 6、现场工矿情

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