Detailed explanation of the three characteristics of orifice plate flowmeters

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　　In China’s energy structure, oil and natural gas have always played a dominant role, and their transportation still relies on long-distance and gathering pipeline systems. In the process of oil and gas extraction and transportation, orifice plate flow meters, and particularly advanced orifice plate valves, hold an absolute dominant position. With the large-scale development of the domestic oil and gas industry, there has been a growing demand for measurement at high pressures and high flow rates. Due to the limitations of its structure, the orifice plate flow meter has obvious shortcomings. The following provides a detailed overview of some of the characteristics of orifice plate flow meters:
I. Requirements for using orifice plate flow meters:
The operating conditions, range of application, and requirements for the pipeline when using orifice plate flow meters (where flow rate is proportional to the square of the pressure difference):
(1) Fluid: It should be a single-phase, homogeneous Newtonian fluid that does not undergo phase changes or the precipitation of impurities when passing through the throttling device; no form of substance should adhere to or accumulate within the throttling device. 　　(2) Pipelines: Applicable only to circular pipes; there are certain restrictions on the pipe diameter. There must be long straight sections upstream and downstream, and the internal surface roughness and roundness of the straight sections 20D upstream and 10D downstream of the throttling element must strictly comply with specific regulations. 　　(3) Flow regime: The flow should be continuous and stable, without pulsations; a typical, fully developed velocity profile (turbulent velocity profile) should have already been established before being affected by the throttling element, the streamlines should be parallel to the pipe axis, and it must not be a rotational flow. 　　II. Comparison of technical performance: 2.1 Low range ratio Due to its structural characteristics, orifice plate flow meters use a throttling element for measurement; therefore, their range ratio is usually 1:10. 　　2.2 Accuracy The measurement accuracy of orifice plate flowmeters can theoretically reach 1%, but extensive practical experience has shown that due to their poor resistance to interference, the actual accuracy in the field is at most 2%; under normal circumstances it is around 3%. 　　2.3 Measuring pulsating flow Since orifice flow meters rely on the pressure difference before and after the orifice to measure flow rate, pulsating flow can cause inaccuracies in this pressure difference; therefore, orifice flow meters are not suitable for measuring pulsating flow. 　　2.4 Measurement of two-way flow The orifice plate flow meter relies on a throttling element to carry out measurements, and this throttling element has a strict directionality; therefore, it is not possible to use an orifice plate flow meter to measure two-way flow. 　　2.5 Moist gases Orifice plate flow meters are not suitable for measuring moist gases; if the gas being measured is moist, liquid can accumulate in front of the orifice plate, causing changes in the pressure difference between the upstream and downstream areas. Orifice plate flow meters rely on this pressure difference to measure flow rate, and if the pressure difference changes, it becomes impossible for such meters to accurately determine the flow rate of the gas. 　　2.6 Cleaning the metering pipeline The orifice flow meter itself contains flow-blocking elements, preventing cleaning balls from passing through; therefore, it is not possible to clean the metering pipeline while the orifice flow meter is installed in the pipeline. The pipeline can only be cleaned by removing the orifice flow meter first. 　　2.7 Effect of eddies 　　Orifice flow meters use the differential pressure method to measure gas flow, and eddies have a direct impact on the differential pressure across the orifice. As a result, orifice flow meters are very sensitive to eddies, and a long straight pipe section is required to meet the accuracy requirements. 　　2.8 Influence of flow velocity distribution Due to the structural principles underlying orifice plate flow meters, it is required that the flow velocity distribution be uniform during measurement. However, due to the complexity of the actual piping systems, the flow velocity distribution of gas in such pipelines cannot be uniform and symmetrical; as a result, orifice plate flow meters are highly sensitive to asymmetrical flow velocity distributions. 　　2.9 Repeatability For orifice plate flowmeters, as the edges of the orifice plate wear out over time, both the accuracy and repeatability of such flowmeters decrease. 　　2.10 Complexity of process pipelines For orifice plate flowmeters, the range ratio is narrow, there are numerous measurement pipelines, and the straight sections upstream and downstream are long, resulting in complex on-site process pipelines. 　　2.11 Maintenance rate Flow meter with orifice plates have flow-blocking elements, which can lead to fluid accumulation upstream; in the case of natural gas with high sulfur content, the orifice plates wear out quickly, resulting in a high maintenance rate. 　　2.12 One-time investment 　　Due to their narrow range ratio, orifice plate flow meters require more measurement pipelines for the same flow measurement requirements. Although the cost of the actual measuring instruments is low, the one-time investment in related valves, temperature transmitters, pressure transmitters, straight sections of pipe, manifolds, etc., is high. 　　3. On-site installation: (1) Length of the straight pipe section: There must be at least 20D of straight pipe section in front of the orifice flow meter; if there is a manifold upstream of the orifice flow meter, then the length of the straight pipe section upstream should be at least 10D. 　　(2) Impact of installation For orifice plate flowmeters, the installation conditions have a direct effect on their measurement accuracy, and high precision is required regarding the concentricity during on-site installation. 　　(3) Operating conditions 　　Due to the principle of orifice plate flow meters, their operating conditions in the field must match the design conditions; they have poor adaptability to changes in pressure and flow rate. 　　III. Comparison of long-term use: 　　(1) Accuracy changes 　　Due to long-term use, the wear of the inlet edge of the orifice plate and the bending deformation of the orifice plate can lead to a loss of accuracy in orifice plate flow meters. 　　(2) Effect of contamination 　　Since the orifice plate flow meter consists of a throttling element, over time contaminants accumulate upstream of the orifice plate, resulting in inaccurate differential pressure signals and directly affecting the measurement accuracy. Contamination and orifice plate passivation can cause metering errors of more than 2–10%. 　　(3) Troubleshooting 　　Since the performance characteristics of orifice plate flowmeters depend on the geometry and size of the throttling element, it is necessary to inspect this element regularly; it must be replaced whenever any changes occur in it. The lifespan of the throttling element depends on the composition of the gas, as well as the flow rate and pressure. 　　(4) Spare parts 　　Since the throttling elements of orifice plate flow meters wear and deform frequently, it is necessary to have multiple sets of such elements on hand. 　　(5) Routine maintenance 　　Orifice plate flow meters require regular maintenance, including checking parameters such as the geometric dimensions of the throttling elements. It is difficult to ensure no leakage after replacing the orifice plate online, which leads to inaccurate pressure differences and makes it hard to maintain measurement precision. 　　(6) Inspection cycle: Orifice plate flow meters should be inspected once a year, usually using the geometric calibration method. 　　As a highly precise instrument, a flow meter requires strict adherence to its specifications not only during manufacturing and use, but also demands special attention during maintenance in order to prevent it from reaching the end of its useful life prematurely.