Standard Specification for Differential Pressure Flow Meters

In the standard specifications of differential pressure flowmeters, the laying of pressure pipes and the installation of instruments are generally only briefly mentioned, that is, the differential pressure signal should not have transmission distortion. In fact, it is not easy to transmit the differential pressure signal without distortion. There are facts to prove it: when the device has just stopped, or when conditions permit to close the valve on the pipeline equipped with a differential pressure flowmeter primary device, and confirm that the flow through the flowmeter has dropped to zero, judging from the specific flowmeter reading , there are not many real indications of zero. The main reason for the zero drift is mostly the distortion in the process of differential pressure transmission. The transmission distortion of the differential pressure signal makes the differential pressure signal received by the differential pressure transmitter not equal to the differential pressure signal generated by the throttling device, thus causing additional errors. Differential pressure signal transmission distortion includes steady-state value distortion and dynamic distortion. Under steady flow conditions only steady state value distortion exists, under pulsating flow conditions both steady state value distortion and dynamic distortion may exist. This section mainly discusses the steady-state value distortion. Steady-state value distortion may be caused by the unreasonable direction of the pressure guide tube, improper setting of the cut-off valve, unequal condenser height, unequal liquid level of the isolation liquid, unreasonable slope of positive and negative pressure guide tubes, and unreasonable medium in the tube. Density is not equal, three-valve group effusion, etc. (Work) Errors caused by unreasonable guidance of the pressure guide tube and improper setting of the shut-off valve. The errors caused by unreasonable guidance of the pressure guide tube and improper setting of the shut-off valve in the differential pressure steam flowmeter have been discussed in Section 4. When the equalizing tube differential pressure flowmeter is used to measure the flow rate of low-pressure gas that may precipitate condensate, the horizontal needle cut-off valve may cause condensate to accumulate in the valve and block the gas path. analyzed. The error caused by the unequal height of the condensers The structure of the two condensers on the positive and negative pressure pipes should be symmetrical, and the installation heights should be equal, so that it is possible to make the liquid levels in the two condensers equal, because the difference in the liquid levels will cause people 10?3 differential pressure distortion. But in the field of use, not only the installation heights of the two condensers are not equal, it is common to see, and some even turn the condenser upside down, which does not play the role of the condenser at all. One of the positive and negative pressure ports of the elbow flowmeter is opened on the outer ring of the elbow, and the other is opened on the inner ring of the elbow. When the pipe diameter is large, it is easy to cause height inequality. The pressure signal is inherently small, so the differential pressure signal transmission distortion is not too large in terms of the absolute value of the differential pressure, but the relative error of the flow measurement caused by it is considerable. Shown in Fig. 9, 16 is another example from the field of use. The measured fluid is steam, and the throttling device is a linear orifice plate. The positive and negative pressure guide tubes are drawn from two horizontal directions of the circumference. Although no condensation tank is used, the part below the vertical pressure guide tube in the figure is filled with condensed water. When the construction team installs, if there is a lack of inspection methods, it is easy to have a height difference between the eighth and eighth points, and even if there is a height difference, it is difficult to distinguish with the naked eye. This height difference is converted into a liquid column difference, causing distortion in the transmission of the differential pressure signal. (^) The error caused by the unequal height of the spacer liquid level. The isolator in the pressure pipeline is to use the spacer liquid to isolate the corrosive medium from the differential pressure transmitter, as shown in Figure 9 and 17. When the spacer fluid is just filled, the level of the spacer fluid in the two isolation containers can be equalized through the balance valve of the three-valve group. The liquid levels are not equal, thus introducing additional differential pressure. Relevant knowledge: Flowmeters are classified according to the situation with compensation, and the differential pressure value corresponding to the common flow rate

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