The classification of flow meter

The classification of flow equipment can be divided into: volumetric flowmeter, velocity flowmeter, target flowmeter, electromagnetic flowmeter, vortex flowmeter, rotameter, differential pressure flowmeter, ultrasonic flowmeter , Mass flow meter, etc.

1. Rotameter

Float flowmeter, also known as rotameter, is a kind of variable area flowmeter. In a vertical cone tube that expands from bottom to top, the gravity of the float of circular cross section is borne by the hydrodynamic force, and the float can be in The cone can rise and fall freely. It moves up and down under the action of flow velocity and buoyancy, and after balancing with the weight of the float, it is transmitted to the dial to indicate the flow rate through a magnetic coupling. Generally divided into glass and metal rotameters. Metal rotor flowmeters are the most commonly used in the industry. For corrosive media with small pipe diameters, glass is usually used. Due to the fragility of glass, the key control point is also a rotor flowmeter made of precious metals such as titanium. . There are many domestic rotor flowmeter manufacturers, mainly Chengde Kroni (using German Cologne technology), Kaifeng Instrument Factory, Chongqing Chuanyi, and Changzhou Chengfeng all produce rotameters. Due to the high accuracy and repeatability of rotameters, It is widely used in the flow detection of small pipe diameters (≤ 200MM).　　

2. Positive displacement flow meter

The positive displacement flowmeter measures the volume flow of fluid by measuring the metering volume formed between the housing and the rotor. According to the structure of the rotor, positive displacement flow meters include waist wheel type, scraper type, elliptical gear type and so on. Positive displacement flow meters are characterized by high measurement accuracy, some up to 0.2%; simple and reliable structure; wide applicability; high temperature and high pressure resistance; low installation conditions. It is widely used in the measurement of crude oil and other oil products. However, due to the gear drive, the bulk of the pipeline is the biggest hidden danger. It is necessary to install a filter in front of the equipment, which has a limited life span and often needs maintenance. The main domestic production units are: Kaifeng Instrument Factory, Anhui Instrument Factory, etc.

3. Differential pressure flow meter

The differential pressure flowmeter is a measuring device with a long history of use and complete experimental data. It is a flow meter that measures the static pressure difference generated by the fluid flowing through the throttling device to display the flow rate. The most basic configuration is composed of throttling device, differential pressure signal pipeline and differential pressure gauge. The most commonly used throttling device in the industry is the “standard throttling device” that has been standardized. For example, standard orifice, nozzle, venturi nozzle, venturi tube. Now the throttling device, especially the nozzle flow measurement, is moving towards integration, and the high-precision differential pressure transmitter and temperature compensation are integrated with the nozzle, which greatly improves the accuracy. Pitot tube technology can be used to calibrate the throttling device online. Nowadays, some non-standard throttling devices are also used in industrial measurement, such as double orifice plates, round orifice plates, annular orifice plates, etc. These meters generally require real-flow calibration. The structure of the standard throttling device is relatively simple, but due to its relatively high requirements for dimensional tolerance, shape and position tolerance, the processing technology is relatively difficult. Taking the standard orifice plate as an example, it is an ultra-thin plate-like part, which is prone to deformation during processing, and larger orifice plates are also prone to deformation during use, which affects accuracy. The pressure hole of the throttling device is generally not too large, and it will deform during use, which will affect the measurement accuracy. The standard orifice plate will wear out the structural elements related to the measurement (such as acute angles) due to the friction of the fluid against it during use, which will reduce the measurement accuracy.

Although the development of differential pressure flow meters is relatively early, with the continuous improvement and development of other forms of flow meters, and the continuous improvement of flow measurement requirements for industrial development, the position of differential pressure flow meters in industrial measurement has been partially It is replaced by advanced, high-precision and convenient flow meters.

4. Electromagnetic flowmeter

An electromagnetic flowmeter is developed based on the Faraday electromagnetic induction principle to measure the volume flow of conductive liquid. According to Faraday’s law of electromagnetic induction, when a conductor cuts the magnetic field line in a magnetic field, an induced voltage is generated in the conductor. The magnitude of the electromotive force is consistent with that of the conductor. In the magnetic field, the velocity of the movement perpendicular to the magnetic field is proportional, and then according to the diameter of the pipe and the difference of the medium, it is converted into a flow rate.

Electromagnetic flowmeter and selection principles: 1) The liquid to be measured must be conductive liquid or slurry; 2) The caliber and range, preferably the normal range is more than half of the full range, and the flow rate is between 2-4 meters; 3 ). The operating pressure must be less than the pressure resistance of the flowmeter; 4). Different lining materials and electrode materials should be used for different temperatures and corrosive media.

The measurement accuracy of the electromagnetic flowmeter is based on the situation where the liquid is full of the pipe, and the measurement problem of air in the pipe has not yet been well solved.

The advantages of electromagnetic flowmeters: There is no throttling part, so the pressure loss is small, and the energy consumption is reduced. It is only related to the average velocity of the measured fluid, and the measurement range is wide; other media can be measured only after the water calibration, without correction, the most Suitable for use as a metering device for settlement. Due to the continuous improvement of technology and process materials, the continuous improvement of stability, linearity, accuracy and life, and the continuous expansion of pipe diameters, the measurement of solid-liquid two-phase media adopts replaceable electrodes and scraper electrodes to solve the problem. High pressure (32MPA), corrosion resistance (anti-acid and alkali lining) medium measurement problems, as well as the continuous expansion of the caliber (up to 3200MM caliber), the continuous increase in life (generally greater than 10 years), electromagnetic flowmeters are getting more and more Widely used, its cost has also been reduced, but the overall price, especially the price of large pipe diameters, is still high, so it has an important position in the purchase of flow meters.

5. Ultrasonic flowmeter

Ultrasonic flowmeter is a new type of flow measurement instrument developed in modern times. As long as the fluid that can transmit sound can be measured with ultrasonic flowmeter; ultrasonic flowmeter can measure the flow of high-viscosity liquid, non-conductive liquid or gas, and its measurement The principle of flow rate is: the propagation speed of ultrasonic waves in the fluid will vary with the flow rate of the fluid being measured. At present, high-precision ultrasonic flowmeters are still the world of foreign brands, such as Japan’s Fuji, the United States’ Kanglechuang; domestic manufacturers of ultrasonic flowmeters mainly include: Tangshan Meilun, Dalian Xianchao, Wuhan Tailong and so on.

Ultrasonic flowmeters are generally not used as settlement metering instruments, and the production cannot be stopped for replacement when the on-site metering point is damaged, and it is often used in situations where testing parameters are required to guide production. The biggest advantage of ultrasonic flowmeters is that they are used for large-caliber flow measurement (pipe diameters greater than 2 meters). Even if some metering points are used for settlement, the use of high-precision ultrasonic flowmeters can save costs and reduce maintenance.

6. Mass flow meter

After years of research, the U-shaped tube mass flowmeter was first introduced by the American MICRO-MOTION company in 1977. Once this flowmeter came out, it showed its strong vitality. Its advantage is that the mass flow signal can be directly obtained, and it is not affected by the physical Parameter influence, accuracy is ± 0.4% of the measured value, and some can reach 0.2%. It can measure a wide variety of gases, liquids and slurries. It is especially suitable for measuring liquefied petroleum gas and liquefied natural gas with quality trading media, supplemented The electromagnetic flowmeter is insufficient; because it is not affected by the flow velocity distribution on the upstream side, there is no need for direct pipe sections on the front and back sides of the flowmeter. The disadvantage is that the mass flowmeter has high processing accuracy and generally has a heavy base, so it is expensive; because it is easily affected by external vibration and the accuracy is reduced, pay attention to the choice of its installation location and method.

7. Vortex flowmeter

The vortex flowmeter, also known as the vortex flowmeter, is a product that only came out in the late 1970s. It has been popular since it was put on the market and has been widely used to measure liquid, gas, steam and other media. The vortex flowmeter is a velocity flowmeter. The output signal is a pulse frequency signal or a standard current signal proportional to the flow rate, and is not affected by fluid temperature, pressure composition, viscosity and density. The structure is simple, there are no moving parts, and the detection element does not touch the fluid to be measured. It has the characteristics of high accuracy and long service life. The disadvantage is that a certain straight pipe section is required during installation, and the ordinary type does not have a good solution to vibration and high temperature. The vortex street has piezoelectric and capacitive types. The latter has advantages in temperature resistance and vibration resistance, but it is more expensive and is generally used for the measurement of superheated steam.

8. Target flow meter

Measuring principle: When the medium flows in the measuring tube, the pressure difference between its own kinetic energy and the target plate will cause a slight displacement of the target plate, and the resulting force is proportional to the flow rate. It can measure ultra-small flow, ultra-low flow rate (0 -0.08M/S), and the accuracy can reach 0.2%.