Meters measuring fluid flow are collectively referred to as flow meters or flow meters. Flowmeter is one of the most important instruments in industrial measurement. With the development of industrial production, the requirements for the accuracy and range of flow measurement are getting higher and higher, and the flow measurement technology is changing with each passing day. In order to adapt to various uses, various types of flow meters have come out one after another. At present, more than 100 flow meters have been put into use. From different perspectives, flowmeters have different classification methods. There are two commonly used classification methods. First, they are classified according to the measuring principle adopted by the flowmeter. Second, they are classified according to the structure principle of the flowmeter.
First, classified by measuring principle
(1) Mechanics: The instruments belonging to this type of principle include differential pressure type and rotor type using Bernoulli's theorem, impulse type and movable tube type using momentum theorem, direct mass method using Newton's second law, and use of fluids. Targets of momentum principle; turbines using angular momentum theorem; vortex-type, vortex-type using fluid oscillation principle; pitot-tube type with total static pressure difference, and positive displacement and enthalpy, trough and so on.
(2) Electrical principle: The instruments used for this kind of principle include electromagnetic type, differential capacitive type, inductive type, strain resistance type and so on.
(3) Acoustics principle: Ultrasonic waves are used to measure the flow rate using acoustic principles. Acoustic type (shock wave type) etc.
(4) Thermal principle: The use of thermal principles to measure the flow of heat, direct calorimetry, indirect calorimetry and so on.
(5) Optical principle: Laser type, photoelectric type, etc. are instruments belonging to this type of principle.
(6) Original in physics principle: nuclear magnetic resonance type, nuclear radiation type, etc. are instruments belonging to this type of principle.
(7) Other principles: Marking principle (tracer principle, nuclear magnetic resonance principle), related principles, etc.
Second, according to the structure of the flow meter structure classification
According to the actual situation of current flow meter products, according to the structure principle of the flow meter, it can be roughly classified into the following types:
1. Volumetric flowmeter
The volumetric flow meter is equivalent to a standard volume container, which continuously measures the flowing medium. The larger the flow, the more times the metrics are measured and the higher the output frequency. The principle of volumetric flow meter is relatively simple, suitable for measuring fluids with high viscosity and low Reynolds number. According to the shape of the rotating body, currently produced products are divided into: Oval gear flow meters, waist wheel flow meters (Roots flow meters), rotary pistons and scraper flow meters for measuring the flow of liquids; Volume flow meter, film type and simple flow meter.
2. Impeller flowmeter
The working principle of the impeller flowmeter is to place the impeller in the fluid to be measured, and it is rotated by the impact of the fluid flow, and the flow speed is reflected by the speed of the impeller rotation. A typical impeller flow meter is a water meter and a turbine flow meter, and its structure can be a mechanical transmission output type or an electric pulse output type. The mechanical meter output of the general mechanical transmission is low in accuracy and the error is about ±2%. However, the structure is simple and the construction cost is low. It has been mass-produced domestically and standardized, generalized and serialized. The accuracy of the turbine flowmeter output by the electric pulse signal is high, and the general error is ±0.2% to 0.5%.
3. Differential pressure flowmeter (variable pressure drop flowmeter)
Differential pressure flowmeter consists of primary and secondary devices. A primary device is called a flow measuring element, which is installed in the pipeline of the fluid to be measured, and generates a pressure difference proportional to the flow rate (flow rate) for the secondary device to display the flow rate. The secondary device is called a display instrument. It receives the differential pressure signal generated by the measuring element and converts it into the corresponding flow for display. The primary device of a differential pressure flowmeter is usually a throttling device or a dynamic pressure measuring device (Pitot, constant velocity tube, etc.). The secondary device is a variety of mechanical, electronic, combined differential pressure gauge with flow display instrument. The differential pressure sensitive elements of the differential pressure gauge are mostly elastic elements. Due to the square root relationship between differential pressure and flow, flow display instruments are equipped with a squarer device to linearize the flow rate scale. Most meters also have a flow totalizer to show the cumulative flow for economic accounting. This method of measuring flow using differential pressure has a long history and is relatively mature. Countries in the world are generally used in more important occasions, accounting for about 70% of various flow measurement methods. Such meters are used for flow measurement of main steam, feed water, condensate, etc. in power plants.
4. Variable area flowmeter (pressure drop type flowmeter)
The float placed in the upper and lower tapered flow channels is moved by the force of the fluid flowing from bottom to top. When this force is balanced with the "display weight" of the float (the weight of the float itself minus the buoyancy of the fluid it receives), the captive is stationary. The height at which the float is stationary can be used as a measure of the size of the flow. Because the flowmeter cross-sectional area of ​​the flowmeter varies with the height of the float, and the pressure difference between the upper and lower parts is equal when the float is stable, this type of flowmeter is called a variable area type flowmeter or a constant pressure drop type flowmeter. The typical instrument for this type of flowmeter is a rotor (float) flowmeter.
5. Momentum flowmeter
Measurement of fluid momentum to reflect the size of the flow meter flow meter. Since the momentum P of the flowing fluid is proportional to the density of the fluid and the square of the flow velocity v, ie, pv2, when the flow cross section is determined, v is proportional to the volumetric flow rate Q, so pQ2. Set the proportional coefficient to A, then Q=A Therefore, measured P, can reflect the flow Q. In this type of flowmeter, the detection element is mostly used to convert momentum into pressure, displacement or force, and then measure the flow rate. The typical instrument for this type of flowmeter is the target and rotary vane type flowmeter.
6. Impulse flowmeter
Flowmeters that use the impulse theorem to measure flow are called impulse flowmeters. They are used to measure the flow of granular solid media. They are also used to measure the flow of mud, crystalline liquids, and abrasives. Flow measurement ranges from a few kilograms per hour to nearly ten thousand tons. The typical instrument is a horizontal force-type impulse flowmeter. The measurement principle is that when the measured medium freely drops from a certain height h to a detection plate with a tilt angle, an impulse is generated, and the horizontal force component of the impulse is directly proportional to the mass flow rate of the horse. Measuring this horizontal component can reflect the size of the mass flow. According to the detection method of signal (9), this type of flowmeter is divided into displacement detection type and direct force measurement type.
7. Electromagnetic Flowmeter
Electromagnetic flowmeter is the application of conductive body in the magnetic field to generate the induced electromotive force, and the induced electromotive force and flow rate is proportional to the principle of measuring the electromotive force to reflect the flow of the pipeline and made. Its measurement accuracy and sensitivity are high. Industrially used to measure the flow of water, pulp and other media. Measurable maximum pipe diameter of 2m, and minimal pressure loss. However, the medium with low conductivity, such as gas and steam, cannot be used.
Electromagnetic flowmeters are expensive, and signals are susceptible to external magnetic fields, affecting widespread use in industrial pipe flow measurements. To this end, the product is constantly improving and updating, and the development of the computer.
8. Ultrasonic flowmeter
Ultrasonic flowmeters are designed based on the geometrical principle of the speed at which ultrasonic waves travel in a flowing medium equal to the average flow velocity of the measured medium and the velocity of the sound waves themselves. It is also measured flow rate to reflect the size of the flow. Although ultrasonic flowmeters only appeared in the 70's, they are very popular because they can be made into non-contact types and can be used to measure open flow in conjunction with ultrasonic water level gauges. They do not generate disturbances and resistance to fluids. There are promising flowmeters.
Ultrasonic Doppler flowmeters manufactured using the Doppler effect have received widespread attention in recent years and are considered as ideal instruments for non-contact measurement of dual-phase flow.
9. Fluid oscillating flowmeter
The fluid oscillating flowmeter is designed to use the fluid to oscillate when it flows under specific flow conditions, and the frequency of oscillation is proportional to the flow rate. When the flow cross section is constant, the flow rate is proportional to the flow volume of the lead volume. Therefore, measuring the oscillation frequency can measure the flow. This flow meter was developed and developed in the 70's. Because it has the advantages of no rotating parts and pulsed digital output, it has a promising future. At present, typical products include vortex flowmeters and precession vortex flowmeters.
10. Mass flowmeter
Since the volume of the fluid is affected by parameters such as temperature and pressure, the volumetric flow rate is used to indicate the flow rate of the medium. In the case of continuous changes in the media parameters, it is often difficult to achieve this requirement, resulting in distortion of the instrument display value. Therefore, mass flow meters have been widely applied and valued. Mass flow meter is divided into direct and indirect two. The direct mass flow meter uses the principle directly related to the mass flow to measure. At present, there are mass flow meters such as calorimeter, angular momentum, vibratory gyro, Magnus effect, and Coriolis force type. An indirect mass flow meter uses a density meter and a volumetric flow rate to obtain a mass flow.
In modern industrial production, the operating parameters such as temperature and pressure of the working fluid are continuously increasing. In the case of high temperature and high pressure, due to the reasons of material and structure, the application of the direct mass flowmeter has encountered difficulties and indirect quality. Flowmeters are often not practical because they are limited by the range of application of humidity and pressure. Therefore, temperature and pressure compensated mass flow meters are widely used in industrial production. It can be regarded as an indirect mass flowmeter instead of using a density meter. Instead, it uses the relationship between temperature, pressure, and density. It uses the temperature and pressure signals to calculate the density signal through a function and multiplies it with the volumetric flow rate to obtain Mass Flow. At present, temperature and pressure compensation mass flowmeters have been put into practical use. However, when the measured medium parameters vary widely or rapidly, correct compensation will be difficult or impossible. Therefore, further study of the applicable mass flow in actual production is made. Meters and densitometers are still a topic.
Chen's flow meter with the above-mentioned structural principles is much more versatile than flowmeters of various structures, such as various helium flow meters and trough flow meters for open channel flow measurement; plug-in type flow meters for large-diameter flow measurement; and measurement layers. Laminar flow meter for flow rate; related flow meter for two-phase flow measurement; and laser, nuclear magnetic resonance flow meter and various tracer methods, dilute method, etc. With the development of technology and practical application needs, new types of flow meters will continue to emerge. The types of flow meters will be more complete.
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