Automatic verification device of the hottest gas f

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With the development of national economy, industrial, civil and national defense departments have put forward higher requirements for the medium type, state parameters, flow range and measurement accuracy of gas flow measurement. In particular, the implementation of policies such as the west to east gas transmission, the pipelining of coal gas in major cities, and the commercialization of civil natural gas in major oil fields have made gas measurement more and more valued. However, at present, the variety of gas measuring instruments is not complete, and the accuracy is not high, especially for gas measurement with large diameter and large flow, there are still many problems. Therefore, the development of gas flow meters has become the focus of the development of flow measurement instruments in China, and the establishment of high accuracy flow standard devices is the basis for the research and manufacture of flow meters. Structure principle of sonic venturi nozzle gas flow standard device because gas flow is more obviously affected by its temperature, pressure, humidity and properties than liquid, there are many kinds of standard devices established to verify different types of gas flow meters, and the more common ones are PVTT method, standard volume tube method, bell jar method, sonic nozzle method, etc. Among them, PVTT method and standard volumetric tube method are primary standard devices, which are usually set up only by flow instrument research institutions and measurement departments to verify standard flowmeters such as sonic nozzles. Bell jar method and sonic nozzle method are standard transmission devices. Bell jar method is mainly applicable to small flow (<10m2/s), low differential pressure (<3kpa) and small pipe diameter (<) Φ 50) calibration of gas flow instruments. For oil field, petrochemical, natural gas and other industries, the sonic nozzle method is mostly used because of its biodegradable range

sonic venturi nozzle gas standard device has the following advantages: (1) as a standard flowmeter, it can calibrate or offline calibrate various gas flowmeters; (2) It can calibrate various gas flow meters at high pressure, low pressure and negative pressure; (3) high accuracy, nozzle accuracy of 0.25 grade, and the accuracy of the whole standard device is better than 0.5 grade; (4) it is superior to other standard devices in the measurement of high-pressure and large flow gas flow, such as simple structure, small volume and good reproducibility. After calibration on the primary standard device, the accuracy will not be reduced under various working conditions. This device adopts the atmospheric pressure method, as shown in Figure 2-1. from Φ 3、 Φ 4.2、 Φ 6、 Φ 8.5、 Φ 12、 Φ 17、 Φ 24、 Φ 30 and other 8 kinds of sonic Venturi nozzles with throat diameters are connected in parallel, and 3 kinds of pipe diameters are flange connected to the calibrated instrument. By selecting different nozzle combinations according to the flow size through the solenoid valve, 255 different flows can be generated, ranging from 6 to 1338m3/h (standard state), and the flow interval is 6m3/h. The accuracy of the whole device is better than ± 0.5%. Among them, the structural shape of sonic venturi nozzle is shown in Figure 2-2. Its working principle is equivalent to the function composed of a shrinking nozzle and a diffuser. When p/P0 is greater than the critical pressure ratio (0.528 for air), like other subsonic throttling devices, the flow rate of the nozzle is related to both the upstream pressure P0 and the downstream pressure P4, and the outflow coefficient is related to the throat Reynolds number and China Hengrui Mach number. At this time, it is very difficult to improve its accuracy by micro foaming with pp/2 silica composite. When p/P0 is less than or equal to the critical pressure ratio, the velocity of the gas passing through the minimum section (throat) of the nozzle reaches the local sonic speed, and the velocity remains unchanged, that is, the Mach number is equal to 1. The sonic venturi nozzle can be used as a gas measurement standard, that is, when the gas state at the nozzle inlet remains unchanged (referring to temperature and pressure), and the pressure ratio p/P0 is less than or equal to the critical pressure ratio, the mass flow of gas through the sonic venturi nozzle will be a fixed value, that is, the mass flow is only related to the upstream pressure and temperature, so it is conducive to improving its measurement accuracy. The function of the diffuser is that when the air flow passes through the throat of the sonic venturi nozzle (it has reached sonic speed), the formed sonic air flow is immediately transformed into subsonic flow by the action of shock wave. Because the shape of the diffuser is conical and its cross-sectional area is getting larger and larger, when it flows out, the pressure loss is partially restored, that is, P4/P0 ≤ 0.8 ~ 0.88 (different values are taken with different Reynolds numbers), This will minimize the pressure loss. According to the continuity principle of fluid flow, energy conservation, adiabatic process and boundary layer theory, the following flow formula can be obtained: QM - mass flow of sonic nozzle under actual conditions; A - internal sectional area of sonic nozzle throat; P0 - absolute stagnation pressure of gas in front of sonic nozzle; T0 - the (4) mixed friction in front of the sonic nozzle can be divided into semi dry friction and semi fluid friction; 

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