Analysis of the influence of the orifice flowmeter on the transmission difference

1. The influence of the aperture ratio β on the transmission difference.

According to the error theory, the error of the flow measurement system composed of a standard orifice plate and a secondary instrument is composed of three parts: random error, determined system error and undetermined system error. In fact, only the undetermined systematic error caused by the instrument itself is considered. The magnitude of the undetermined systematic error can be calculated using the uncertainty of the flow measurement. According to SY/6143-1996, the uncertainty of flow measurement is equivalent to twice the standard deviation. Therefore, the standard deviation of the measurement system can be obtained by calculating the measurement uncertainty of the flow rate. The general CFD PHOENICS software is used to numerically simulate the loss phenomenon. Through numerical simulation, we can establish the relationship between uncertainty and the aperture ratio β, as shown in Figure 2. OIt can be seen from the figure that the higher the value of the aperture ratio β, the higher the uncertainty and the greater the difference in natural gas transmission. Therefore, the use of orifice plates with large aperture ratios should be avoided. In practical application, it should be tried to avoid exceeding the upper limit stipulated in the national standard 0.10≤β≤0.75 to reduce the transmission difference.

2. The influence of orifice plate deformation, passivation and dirt on the transmission difference.

There are not many incidents of orifice plate deformation. Most of the reasons for the deformation are that on the one hand, when the pipeline is opened for gas supply, the operator opens the upstream and downstream shut-off valves at too fast an opening speed, which leads to a rapid increase in differential pressure at both ends of thechoke piece, causing excessive impact on the choke piece, resulting in deformation of the orifice plate; on the other hand, the inner diameter of the orifice plate is small due to the sudden large increase in gas transmission, which easily leads to deformation of the orifice plate. Deformation of the standard orifice plate often manifests as a concave face surface and a prominent downstream surface. There are no standard or experimental data on the influence of orifice plate deformation on the flow measurement indication, but the influence is obvious. Orifice plates that have been deformed and fail the test can only be scrapped, and the surface cannot be artificially restored. This will cause a big system error. If the flow rate is found to have large deviation during on-site production and is not caused by human adjustment, iIt should be determined whether it is caused by the deformation of the orifice plate. Moreover, in use, due to the abrasive effect of the fluid, in particular for high-pressure or high-speed fluids containing particles, its inlet edge will dull more quickly and be rounded. In this way, under the same flow, the fluid shrinkage behind the orifice is weakened, and the differential pressure is continuously reduced, which will form more and more negative flow error. The small flow stream cross section at the orifice plate outlet increased after the inlet was eroded. If it can be calibrated under this condition, it will naturally be found that the flow coefficient of the orifice plate has increased, but the standard formula is still used. The smaller the coefficients, the more negative systematic errors there will be. Fouling of the orifice surface and changes in the flow cross section. EtaSince the orifice flow meter is composed of throttle parts, dirt accumulates upstream of the orifice plate during long-term use, resulting in inaccurate differential pressure signals and directly affecting measurement accuracy. Dirt and passivation of the orifice plate can cause a measurement deviation greater than 2 to 10%. In summary, deformation, passivation and dirtiness of the orifice plate have a great influence on the transmission error. Therefore, the correct use of the orifice flowmeter throttle orifice has nothing to do with irregular routine maintenance can effectively reduce the transmission difference.

3. The influence of secondary support instruments on the transmission difference.

Due to the long time of use, the zero point of the transmitter will drift. If it is a negative drift, the output current of the transmitter will be lowerur at the standard 4 mA and the flow rate will be low. The output current of the device is higher than the standard 4mA, and the flow rate is displayed on the high side. If the range setting is larger, the flow will be displayed on the low side, and if the range setting is small, the flow will be displayed on the high side. If the natural gas is not completely dehydrated, it will contain some water vapor, and it will accumulate in the pressure pipe of the secondary instrument (differential pressure, pressure transmitter) after long-term use . If it is not discharged in time, it will damage the secondary instrument. The measurement accuracy of the instrument will be affected. For winter users in the north where the outside temperature is below zero, if the secondary instrument and the pressure induction pipe are not insulated, the pressure induction pipe will freeze, which will cause inaccurate measurement, resulting in a transmission error. Thereforeuent, the correct use and maintenance of secondary instruments can effectively reduce the transmission error.

In long distance pipeline engineering practice, orifice valves have dominated for a long time. Objectively speaking, when the orifice flow meter is used for measurement, it is necessary to select and design the primary device (orifice throttling device) and the secondary instrument (different pressure, pressure, temperature, totalizer) of the orifice flow meter. Measurement and installation should be carried out in strict accordance with relevant national standards, coupled with irregular maintenance and servicing, to ensure measurement accuracy. Through a qualified metering system, the transmission difference can be effectively reduced, the transmission difference can be controlled within a reasonable range, the gas transmission cost can be controlled.health and the vital interests of the company can be guaranteed.

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