Selection principle of flow meter

Abstract: The information on the selection principle of flow meter is provided by the excellent flow meter and flow meter manufacturers. The principle of choosing a flowmeter is first to have a deep understanding of the structural principles and fluid characteristics of various flowmeters, and at the same time, choose according to the specific conditions of the site and the surrounding environmental conditions. Economic factors must also be taken into account. . For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of the article on the selection principles of flowmeters. The principle of choosing a flowmeter is first to have a deep understanding of the structural principles and fluid characteristics of various flowmeters, and at the same time, choose according to the specific conditions of the site and the surrounding environmental conditions. Economic factors must also be taken into account. Under normal circumstances, the selection should be mainly from the following five aspects: ① the performance requirements of the flowmeter; ② the fluid characteristics; ③ installation requirements; ④ environmental conditions; ⑤ the price of the flow meter. 1. The performance requirements of the flowmeter The performance of the flowmeter mainly includes: measuring flow (instantaneous flow) or total amount (cumulative flow); accuracy requirements; repeatability; linearity; flow range and range; pressure loss; output signal characteristics and flowmeter response time, etc. (1) There are two types of flow measurement or total flow measurement, namely instantaneous flow and cumulative flow. For example, the crude oil in the pipeline of the sub-transportation station belongs to the custody transfer or the petrochemical pipeline for continuous proportional production or process control of the production process, etc. need to be measured The total amount, occasionally supplemented by the observation of instantaneous flow. In some workplaces, flow control requires instantaneous flow measurement. Therefore, the choice should be made according to the needs of on-site measurement. Some flowmeters such as positive displacement flowmeters, turbine flowmeters, etc., the measurement principle is to directly obtain the total amount by mechanical counting or pulse frequency output, which has high accuracy and is suitable for measuring the total amount, if equipped with a corresponding signaling device Flow can also be output. Electromagnetic flowmeters, ultrasonic flowmeters, etc. deduce the flow rate by measuring the fluid flow rate, with fast response and suitable for process control. (2) The accuracy level of the accuracy flowmeter is specified within a certain flow range. If it is used under a certain condition or within a relatively narrow flow range, for example, it only changes within a small range. At this time The measurement accuracy will be higher than the specified accuracy class. If a turbine flowmeter is used to measure oil in barrels for distribution, when the valve is fully opened, the flow rate is basically constant, and its accuracy may be improved from 0.5 to 0.25. It is used for trade accounting, storage and transportation transfer and material balance. If the measurement accuracy is required to be high, the durability of the accuracy measurement should be considered. Generally, the flowmeter is used in the above cases, and the accuracy level is required to be 0.2. In such workplaces, measurement standard equipment (such as volume tubes) is generally equipped on site to conduct online detection of the flowmeters used. In recent years, due to the increasing tension of crude oil and the high requirements of various units for crude oil measurement, the implementation of coefficient handover for crude oil measurement is proposed, that is, in addition to the periodic inspection of the flowmeter every six months, the two parties in the trade handover negotiate every 1 month or 2. The flowmeter is verified monthly to determine the flow coefficient, and the data is calculated according to the data measured by the flowmeter and the flow coefficient of the flowmeter for handover to improve the accuracy of the flowmeter, also known as zero-error handover. The accuracy class is generally determined based on the maximum allowable error of the flowmeter. It is given in the flowmeter instructions provided by each manufacturer. It is important to note whether the percentage of error refers to relative error or reference error. The relative error is the percentage of the measured value, commonly used“%R”express. The reference error refers to the upper limit of the measurement or the percentage of the range, commonly used“%FS”. Many manufacturer's instructions do not indicate this. For example, float flowmeters generally use reference errors, and some models of electromagnetic flowmeters also use reference errors. If the flowmeter is not simply measuring the total amount, but is used in the flow control system, the accuracy of the detection flowmeter should be determined under the control accuracy requirements of the entire system. Because the whole system not only has the error of flow detection, but also includes the error and various influencing factors of signal transmission, control adjustment, operation execution and so on. For example, if there is a hysteresis difference of about 2% in the operating system, it is uneconomical and unreasonable to determine an excessively high accuracy (above 0.5) for the measuring instrument used. As far as the instrument itself is concerned, the accuracy between the sensor and the secondary instrument should also be properly matched.±2.5%～±Between 4%, it is meaningless to match a differential pressure gauge with a high accuracy of 0.2% to 0.5%. Another problem is that the accuracy level specified for the flowmeter in the verification procedure or the manufacturer's manual refers to the maximum allowable error of the flowmeter. However, due to the influence of changes in environmental conditions, fluid flow conditions and dynamic conditions when the flowmeter is used in the field, some additional errors will occur. Therefore, the flowmeter used in the field should be the combination of the maximum allowable error and additional error of the instrument itself. This problem must be fully considered. Sometimes the error within the range of the use environment on site may exceed the maximum allowable error of the flowmeter.

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