The working principle of differential pressure flowmeter

The basic principle is that the fluid in the pipeline is filled with fluid, when it flows through the throttle in the pipeline, as shown in Figure 4.1, the flow velocity will form a local contraction at the throttle, so the flow velocity increases and the static pressure decreases, so it will be before and after the throttle A pressure difference is created. The greater the fluid flow rate, the greater the pressure difference produced. This way, the basic principle of measuring the flow rate based on the pressure difference is filled with fluid in the pipeline. When it flows through the throttle in the pipeline, as shown in Figure 4.1, the flow rate will be at the junction. A local contraction is formed at the flow member, so the flow velocity increases and the static pressure decreases, so a pressure difference is generated before and after the throttle member. The greater the fluid flow rate, the greater the pressure difference generated, so that the flow rate can be measured according to the pressure difference. This measurement method is based on the flow continuity equation (the law of conservation of mass) and the Bernoulli equation (the law of conservation of energy). The size of the pressure difference is not only related to the flow rate, but also related to many other factors. For example, when the form of the throttling device or the physical properties (density, viscosity) of the fluid in the pipeline are different, the pressure difference generated under the same flow rate is also a different orifice plate. In the flow equation of nearby flow velocity and pressure distribution, qm--mass flow, kg/s; 　　 qv--volume flow, m3/s; 　　 C--outflow coefficient; 　　ε--expansibility coefficient; 　　 β--diameter ratio, β u003dd/D; 　　 d--the aperture of the throttle under working conditions, m; 　　 D--the inner diameter of the upstream pipeline under working conditions, m; 　　 △P--differential pressure, Pa; 　　 ρl--upstream fluid density, kg/m3 . It can be seen from the above formula that the flow rate is a function of 6 parameters C, ε, d, ρ, △P, β(D), these 6 parameters can be divided into actual measurement [d, ρ, △P, β(D)] And statistics (C, ε) two types. (1) Actual measurement 1) d, D In formula (4.1), d has a square relationship with the flow rate. Its accuracy has a greater impact on the total flow accuracy. The error value should generally be controlled at about ±0.05%. Take into account the influence of working temperature on the thermal expansion of the material. The standard stipulates that the inner diameter D of the pipeline must be measured on the spot, and multiple measurements must be taken on several sections of the upstream pipe section to find the average value, and the error should not be greater than ±0.3%. In addition to the higher requirements for numerical measurement accuracy, the serious influence of the abnormal throttling phenomenon caused by the inner diameter deviation on the upstream passage of the throttling element should also be considered. Therefore, when the throttling device is not supplied as a complete set, full attention should be paid to this problem when piping on site.　　2) ρ ρ is at the same position as △P in the flow equation. That is to say, when pursuing the high-precision level of differential pressure transmitters, never forget that the measurement accuracy of ρ should also match it. Otherwise, the increase in ΔP will be offset by the decrease in ρ.　　3) The accurate measurement of △P differential pressure △P should not be limited to the selection of a high-precision differential pressure transmitter. In fact, whether the differential pressure transmitter can accept the true differential pressure value is also determined by a series of factors. Among them, the manufacture, installation and use of the correct pressure tapping hole and pressure pipe is the key to ensure the true differential pressure value. Many influencing factors are difficult to determine quantitatively or qualitatively. Only by strengthening the standardization of manufacturing and installation can the goal be achieved. (2) Statistics 1) C Statistics C is a quantity that cannot be measured (referring to the design, manufacture and installation according to the standard, and use without calibration). The most complicated situation when used in the field is the actual C value and the C value determined by the standard. Does not match. Their deviation is caused by a series of factors in design, manufacturing, installation and use. It should be clear that all the above-mentioned links strictly follow the regulations of the standard, and the actual value will conform to the value determined by the standard. It is difficult for the site to fully meet this requirement.　　It should be pointed out that some deviations from standard conditions can be estimated quantitatively (correction can be made), and some can only be estimated qualitatively (magnitude and direction of uncertainty). But in reality, sometimes it is not just a conditional deviation, which brings about very complicated situations, because the general information only introduces the error caused by a certain conditional deviation. If many conditions deviate at the same time, there is a lack of relevant information to check. 2) ε The expansibility coefficient ε is a correction to the change of the outflow coefficient caused by the density change when the fluid passes through the throttle. Its error consists of two parts: one is the error of ε under the usual flow rate, which is the standard determined value The second is the error caused by the fluctuation of the flow rate ε value. Generally, in the case of low static pressure and high differential pressure, the ε value has a non-negligible error. When △P/P≤0.04, the value of ε