The structure of the positive displacement flowmeter
in order to meet the requirements of different media and different working conditions for flow measurement in production, various types of positive displacement flowmeter have been produced. Among them, the more common types are gear type, scraper type and rotating piston type, which are introduced as follows.
1. Gear type positive displacement flowmeter
the shell of this flowmeter is equipped with two rotors, Directly or indirectly mesh with each other to produce rotation under the pressure difference between the inlet and outlet of the flowmeter. Through the rotation of the gear, the fluid filled in the "metering space" between the gear and the housing is continuously discharged. By measuring the number of gear rotation, The fluid volume passing through the flowmeter can be obtained.
Figure 1 shows the schematic diagram of elliptical gear type volumetric flowmeter (also known as obar volumetric flowmeter)
Figure 1 working schematic diagram of elliptical gear flowmeter
it can be seen from the figure that the flowmeter works by two elliptical gears meshing with each other, and its working process is briefly described as follows: P1 in the figure represents the fluid pressure at the inlet of the flowmeter; It indicates the outlet fluid pressure. Obviously, the pressure P1 is greater than P2. In Figure 1 (a), although the lower rotor is affected by the differential pressure of the fluid, it does not produce rotating torque, while the upper gear rotates due to the rotating torque generated by the differential pressure of the two cases. Because the two gears mesh with each other, they rotate in the direction of the arrow with O1 and O2 as the axes, and at the same time, the gear O1 discharges the fluid in the half moon metering space to the outlet. In the state of Figure 1 (a), The upper gear is the driving gear and the lower gear is the driven gear. At the position shown in Figure 1 (b), both gears generate rotating torque under the action of fluid differential pressure, and rotate in the direction of the arrow under the action of this torque, and change to the position shown in Figure 1 (c). At this time, the gear position is opposite to that shown in Figure 1 (a). The lower gear is the driving gear and the upper gear is the driven gear. The lower gear rotates under the action of inlet and outlet fluid differential pressure, Once again, the fluid in the half moon "metering space" between it and the shell is discharged. In this continuous movement, the elliptical gear discharges four volumes of fluid in the "metering space" every revolution. Therefore, as long as the number of revolutions of the gear is read, the amount of liquid discharged can be calculated. Referring to figure 2-2, the total amount of fluid discharged can be calculated as v = 4nv
, where n is the number of revolutions of the gear
a, b-- long half sleeve and short half sleeve of elliptical gear
δ-- Thickness of elliptical gear.
another gear type volumetric flowmeter is a one needle cardiotonic injection flowmeter suitable for finished product inspection and single material index test of industrial and mining enterprises. The waist wheel volume is for the renewable plastic market, also known as roots type volumetric flowmeter. The work of this flowmeter was originally discovered by Geim of the University of Manchester, UK, in 2004. The principle and working process of this flowmeter are basically the same as that of elliptical gear type, which also depends on progress, The outlet fluid pressure difference produces movement, and four volumes of fluid in the "metering space" are discharged every revolution. The difference is that there are no teeth on the waist wheel, which are not directly meshed and rotated, but are driven by the transmission gear set installed outside the shell.
the above two rotor types of volumetric flow meters can be used for the measurement of various liquid flows, especially for the accurate measurement of oil flow. Under high pressure In large flow gas flow measurement, this kind of flowmeter is also used. Because the elliptical gear volumetric flowmeter directly relies on the meshing of the measuring wheel, it has high requirements for the cleaning of the medium, and solid particles are not allowed to pass through the flowmeter.
2. Scraper type volumetric flowmeter
scraper type flowmeter is also a common volumetric flowmeter. The rotor of this flowmeter is equipped with two pairs of scrapers that can slide radially inside and outside, The rotor rotates under the effect of the differential pressure between the inlet and outlet of the flowmeter, and four volumes of fluid in the "metering space" are discharged every revolution. The same as the previous type of flowmeter, as long as the number of revolutions is measured, the volume of discharged fluid can be calculated.
the structure of the more common cam scraper flowmeter is shown in Figure 2-4. The inner cavity of the shell in the figure is a circular empty cylinder, and the rotor is also a hollow cylindrical object with a certain radial width, Four slots are opened radially at 90 ° positions, and the scraper can slide freely in the slot. The four scrapers are connected by two connecting rods, which are perpendicular to each other and intersect in space. A small ball is installed at one end of each scraper, and the four balls roll on a fixed cam to make the scraper shrink from time to time. When the adjacent two scrapers extend to the inner wall of the shell, a standard volume of the metering space is formed. When the scraper moves in the metering section, It only rotates with the rotor without sliding, so as to ensure that its standard volume is constant. When it leaves the metering section, the scraper retracts into the groove, and the fluid is discharged from the outlet. At the same time, the latter scraper forms a second "metering space" with another adjacent scraper, which acts the same way. The rotor moves for a week, discharging four volumes of fluid in the "metering space".
in the scraper type volumetric flowmeter, there is also the so-called rotary valve scraper flowmeter, Its working principle is similar to that of the cam type, but its structure is different, so it will not be described in detail here.
rotary piston volumetric flowmeter
the structure and working principle of the rotary piston (also known as the swing piston) volumetric flowmeter can be seen in Figure 2-5.
as shown in the figure, the rotary piston is located between the inner and outer rings 3 and 4 of the fixed core technology that focuses on the future, and the shaft 6 of the piston rolls against the guide roller 5, The middle diaphragm 1 divides the metering space into two parts. The upper notch of the piston 2 engages with the diaphragm 1. When the piston moves in the direction of the arrow, it moves in a straight line with the diaphragm 1. Under the action of the pressure difference between the inlet and outlet fluid, the piston always rotates in close contact with the inner and outer barrel walls, alternately and continuously discharging the fluid between the piston and the inner and outer cylinders, The fluid flow can be obtained by calculating the number of piston rotations.
the rotary piston volumetric flowmeter has the advantage of large flow capacity. Its disadvantage is that there will be some leakage in the working process, so the accuracy is low
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