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Basic knowledge of various flow meters
May 14, 2018

1. Rotary flowmeter

A float flowmeter, also known as a rotameter, is a type of variable area flowmeter. In a vertical conical tube that expands from below to above, the gravity of a float of circular cross-section is subjected to liquid power. The float can be The cone rises and falls freely inside. 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 through the magnetic coupling to the dial to indicate the flow. Generally divided into glass and metal rotor flowmeter. The metal rotor flowmeter is the most commonly used in the industry. For small diameter corrosive media, glass is usually used. Due to the inherent fragility of the glass material, the key control point also uses a rotameter made of precious metals such as full titanium. . Due to its high accuracy and reproducibility, the rotameter has been widely used in the flow detection of small diameter (≤ 200MM).


2. Volumetric flowmeter

Volumetric flowmeters measure the volumetric flow of a fluid by measuring the volumetric volume formed between the housing and the rotor. According to the structure of the rotor, the volumetric flowmeter has a waist wheel type, a scraper type, an oval gear type flowmeter and the like. Volumetric flowmeters are characterized by high measurement accuracy, some up to 0.2%; simple and reliable structure; wide applicability; high temperature and high pressure; Widely used in the measurement of crude oil and other oil products. However, due to the gear transmission, the blockage of the pipeline is the greatest hidden danger. It is necessary to install a filter in front of the equipment, which has a limited life and often requires maintenance.

3, differential pressure flowmeter

Differential pressure flow meter is a kind of flow meter that shows the flow rate by measuring the static pressure difference produced by the fluid flowing through the throttling device. It is composed of throttling device, differential pressure signal pipeline and differential pressure gauge. The most commonly used throttling devices in the industry are standard orifice plates, nozzles, venturi nozzles, and venturi tubes. Now, the throttling device is integrated with the direction of integration of high-precision differential pressure transmitters and temperature compensation with the nozzles to improve accuracy. With the Pitot technology, the throttling device can be calibrated online. Industrial measurement also uses some non-standard throttling devices, such as double orifice plates, circular orifice plates, annular orifice plates, etc. These instruments generally require real flow calibration. The standard throttling device has a relatively simple structure, but due to its relatively high dimensional tolerance, shape and position tolerance requirements, the technical difficulty of processing is high. Taking the standard orifice plate as an example, it is an ultra-thin plate-shaped part, which is easily deformed by processing. Larger orifice plates are also susceptible to deformation during use and affect accuracy. The pressure-reducing orifice of a throttling device is generally not opened too much, and deformation may occur during use to affect the measurement accuracy. Due to the friction of fluid in the standard orifice during use, it also causes wear to the measurement-related structural elements (such as acute angles) and reduces the measurement accuracy.

Although differential pressure flowmeters were developed earlier, along with the continuous improvement and development of various other types of flow meters, with the constant increase in the flow measurement requirements of industrial development, the position of differential pressure flowmeters in industrial measurement has been partially Replaced by advanced, high-precision, and convenient flow meters.

4、Magnetic flowmeter

Electromagnetic flowmeter Based on Faraday's electromagnetic induction principle, a meter for measuring the volumetric flow of conductive liquid is developed. According to Faraday's law of electromagnetic induction, an induced voltage is generated in a conductor when a conductor is moved in a magnetic field to cut magnetic lines of force. The magnitude of the electromotive force and the conductor In the magnetic field, the speed perpendicular to the magnetic field is proportional to the speed, which is then converted into flow according to the diameter and medium.

Electromagnetic flowmeter and selection principle: 1), the liquid to be measured must be conductive liquid or slurry; 2), caliber and range, it is best that the normal range exceeds half of full scale, the flow rate is between 2 - 4 meters; 3 ), the use of pressure must be less than the flowmeter pressure; 4) different temperatures and corrosive media use different liner materials and electrode materials.

The measurement accuracy of the electromagnetic flowmeter is established in the case where the liquid is filled with pipes, and the measurement of air in the pipe has not been well resolved at present.

The electromagnetic flowmeter converter is its core component, which directly affects the accuracy of measurement. At present, domestic-made converters cannot meet the requirements due to various reasons.

Electromagnetic flowmeter advantages: no throttling parts, so the pressure loss is small, reducing energy consumption, only related to the average velocity of the measured fluid, a wide range of measurement; just calibrated by water can be measured after other media without correction, the most Suitable for use as a metering device for settlement. Due to continuous improvement of technology and process materials, continuous improvement in stability, linearity, accuracy, and life, and continuous expansion of the diameter, the use of replaceable electrodes and scraper electrodes for the measurement of solid-liquid two-phase media has been resolved. High-pressure (32MPA), corrosion-resistant (anti-acid, alkali lining) media measurement problems, and the continuous expansion of the diameter (up to 3200MM diameter), life expectancy (usually more than 10 years), the electromagnetic flowmeter is getting more and more Wide range of applications, the cost has also been reduced, but the overall price, especially the large diameter is still high, so it has an important position in the purchase of flow meters.

5, ultrasonic flowmeter

As long as the sound can be transmitted through the ultrasonic flow meter can be measured; Ultrasonic flow meter can measure the flow of high-viscosity liquid, non-conductive liquid or gas flow, the principle of its measurement flow is: the speed of ultrasonic waves in the fluid will be measured with Fluid flow rate changes.

Ultrasonic flowmeters are generally not used for settlement meters. They cannot be shut down for on-site measurement point failures, but they also require inspection parameters to guide production. Ultrasonic flowmeters are often used. The biggest advantage of the ultrasonic flowmeter is that it can be used for large-diameter flow measurement (diameter greater than 2 meters), which can save costs and reduce maintenance.

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