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1. Overview A wide variety of flow measurement methods and instruments are available, as well as 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 measuring object, there are two types of closed pipes and open channels; according to the purpose of measurement, they can be divided into total quantity measurement and flow measurement, and the instruments are called total quantity tables and flowmeters respectively.
Aggregate meters measure the flow through a pipeline over a period of time, expressed as a quotient of the total amount of flow through a pipeline for a short period of time divided by that time. Actually, a flow meter is usually equipped with a cumulative 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.
1.1 Differential pressure flowmeter Differential pressure flowmeter is based on the differential pressure generated by the flow detection device installed in the pipeline, the known fluid conditions and the geometric dimensions of the test piece and the pipeline to calculate the flow meter.
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 constant 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, 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, and the flow values ​​and estimated measurement errors can be determined without real-world calibration.
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) A wide range of applications. No flowmeters have yet been compared;
(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.).
Application Overview:
Differential pressure flowmeters have a particularly wide range of applications and have applications in flow measurement of closed pipes, 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.
1.2 Floater flow meter Floater flow meter, also known as rotameter, is a kind of variable-area flowmeter. In a vertical conical tube expanded from bottom to top, the gravity of the float of circular cross-section is subjected 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 an important 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.
1.3 Volumetric Flowmeters Volumetric flowmeters, also known as fixed displacement flowmeters, referred to as PD flowmeters, are among the most accurate in flow meters. 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 sequentially and repeatedly.
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 amount 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 flow meters in recent years; China accounted for approximately 20%; 1990 production (excluding domestic gas meters) was estimated at 340,000 Taiwan, with elliptical gears and waist wheels, accounts for about 70% and 20% respectively.
1.4 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 and volumetric flowmeters and Coriolis mass flowmeters are called three types of repeatability and best accuracy in flowmeters. As one of the ten types of flowmeters, their products have been developed into many varieties and many Series 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. Instrumentation, only the Netherlands in the 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.
1.5 Electromagnetic Flowmeters Electromagnetic flowmeters are meters that measure conductive liquids according to Faraday’s law of electromagnetic induction.
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 tube that will not block and is suitable for the measurement of liquid-solid two-phase fluids containing solid particles, such as pulp, sludge, sewage, etc.;
(2) No pressure loss caused by flow detection, and good energy saving effect;
(3) The measured volumetric 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 flowmeters have a wide range of applications. Large diameter instruments are used in water supply and drainage projects; medium and small diameters are often used in demanding or unpredictable applications, such as steel industry blast furnace tuyere cooling water control, paper industry measurement of pulp and black liquor, 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 places with health requirements.
1.6 Vortex Flowmeter 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 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) does not apply 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.
1.7 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 principles of signal detection, ultrasonic flowmeters can be classified into differential propagation speed 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 usually not suitable for very clean fluids.
1.8 Coriolis Mass Flowmeter The Coriolis mass flowmeter (hereafter abbreviated as CMF) is a type of direct method that uses the Coriolis force principle that is proportional to the mass flow rate when a fluid flows in 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.
The 1.9 open channel flowmeter is different from the previous ones in that 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 supply diversion canals; the power plant diversion and drainage channels, 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.
2. New working principle Research and development of flow meters 2.1 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.
2.2 composite effects 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.
2.3 tachometer flow sensor (tachmetricflowratesensor)
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.
3. Several Flow Meter Applications and Development Trends 3.1 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 measuring 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.
3.2 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.
3.3 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 world's estimated sales volume was 3.548 million units, and domestic products were estimated to be 8,000-9,000 units during the same period.
4. Conclusions From the above, it can be seen that although the flowmeter has grown to maturity today, its variety is still extremely numerous. 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 in two aspects, and take into account 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.
Talking about the Development and Current Situation of Flowmeter
Abstract: According to the current most popular and most widely used classification method, it is divided into: vortex flow in volumetric flowmeter, differential pressure flowmeter, floater flowmeter, turbine flowmeter, electromagnetic flowmeter and fluid oscillation flowmeter. Meters, mass flowmeters, and plug-in flowmeters explain the principles, characteristics, application overview, and developments at home and abroad for various flowmeters.