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Analysis of the Influence of Plate Flowmeter on Transmission Difference

Author:    Source:    Date: 2021-06-01 14:36:47
1. The influence of aperture ratio β on the output difference.
  According to the error theory, the error of the flow measurement system composed of secondary instruments on the standard orifice plate is composed of three parts: random error, set system error and undetermined system error. In fact, only undecided system errors caused by the instrument itself are considered. The size of the undetermined system error can be calculated by the uncertainty of the flow measurement. According to SY/6143-1996, the uncertainty of flow measurement is equivalent to twice the standard deviation. Therefore, if the uncertainty of flow measurement is calculated, the standard deviation of the measurement system can be obtained. Using CFD general-purpose software PHOENICS to numerically simulate aberration phenomena, the relationship between uncertainty and aperture ratio can be established, as shown in Figure 2. As shown in the figure, the higher the hole diameter, the higher the uncertainty, and the larger the natural gas truck. Therefore, orifice plates with larger hole diameters should be avoided. In practical applications, the upper limit specified in the national standard 0.100.75 should be kept as low as possible to reduce the number of transport vehicles.

  The influence of orifice plate deformation, passivation and dirt on transportation difference.

   There are not many orifice deformation events. Most of the reason for the deformation is that when the pipeline opens the plug, when the operator opens the upstream and downstream stop valves, the valve speed is too fast, and the pressure difference between the two ends of the throttle increases rapidly, which causes a great impact on the throttle and causes the orifice to deform. On the other hand, due to the sudden and substantial increase in gas transportation, the inner diameter of the orifice plate is small, and the orifice plate is easily deformed. The deformation of the standard orifice plate often appears in the depression of the British oil surface and the kerosene surface protruding. There is no standard for the influence of the orifice plate deformation on the flow measurement, and there is no experimental data, but the orifice plate that has been deformed and has not passed the verification can be discarded, and the surface cannot be artificially restored. This may cause large system errors. If a large deviation is found in the on-site production instead of manual adjustment, it is necessary to consider whether it is caused by the deformation of the orifice plate. In addition, during use, the abrasion effect of the fluid, especially the inlet edge of the fluid with particles under high pressure or flow velocity, will passivate into a circle faster. Therefore, under the same flow, the degree of fluid shrinkage behind the hole will decrease, and the differential pressure will continue to decrease, resulting in an increased negative flow error. The minimum beam cut of the outlet of the orifice increases after the inlet is worn, in this case, the outflow coefficient of the orifice plate increases. However, the small coefficient calculated according to the standard formula is still used, so there will be an increased negative system error. The scaling of the orifice surface and the change of the circular cross-sectional area. Since the orifice flowmeter is composed of throttle components, dirt will accumulate in the upstream of the orifice during long-term use, and the differential pressure signal will be inaccurate, directly affecting the measurement accuracy. Dirt and orifice plate passivation can cause measurement deviations of 2% to 10% or more. In summary, the deformation, passivation and dirt of the orifice plate have a great influence on the transport vehicle. Therefore, the correct use and irregular routine maintenance of the orifice plate to the orifice plate flowmeter can effectively reduce the transport vehicles. Support the influence of the secondary instrument on the transmission difference.

Because of the long time of use, the transmitter has zero drift. If it is negative, the output current of the transmitter will be lower than the standard 4 mA, and the flow will be lower. If it is positive, the output current of the transmitter will be greater than the standard 4 mA. The flow rate will be high. If the distance setting is large, the flow rate is low, and the distance setting is small, and the flow rate is high. Incomplete dehydration treatment of natural gas will produce some moisture. Long-term use will accumulate liquid in the second instrument (differential pressure, pressure transmitter) guide pipe. If it is not discharged in time, it will affect the measurement accuracy of the second instrument. . For winter users in the north where the external temperature is below zero, if the secondary instrument and high-pressure pipe are not insulated, the high-pressure pipe will freeze and the measurement will be inaccurate, resulting in looseness. Therefore, the correct use and maintenance of secondary meters can effectively reduce trucks.

  In the practice of long-distance water pipeline engineering, advanced orifice valves have long occupied a dominant position. Objectively speaking, when using an orifice flowmeter to measure, the selection, design and installation of the primary device (orifice plate throttling device) and secondary instrument (differential pressure, pressure, temperature, totalizer) of the orifice flowmeter must be Strictly abide by the relevant national standards, coupled with irregular maintenance and maintenance, can ensure the measurement accuracy. Through the qualified metering system, the transportation vehicles are effectively reduced, the transportation vehicles are controlled within a reasonable range, the transportation cost is controlled, and the vital interests of the enterprise are protected.

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