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the principle of flow meter structure
Apr 28, 2018

According to the principle of flow meter structure


    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, and servo types suitable for measuring gas flow. 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. The primary device is called a flow measuring element and it is installed in the pipeline of the fluid to be measured, generating a pressure difference proportional to the flow (flow rate) for the secondary device to display the flow. 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, i.e., pv2, when the flow cross section is determined, v is proportional to the volume flow rate Q, so p Q2. 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 kind of flow meter is the target and rotary wing type flow meter.


    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 is proportional to the flow rate, by measuring the electromotive force to reflect the principle of the pipeline flow 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 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 medium being measured and the speed of the sound waves themselves. It is also measured by the 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 type and can be used to measure open flow in conjunction with an ultrasonic water level gauge. They do not generate disturbances and resistance to fluids. There are promising flowmeters.


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