What should I know when choosing a flow meter? (1) Posted by : admin / Posted on : Mar 06， 2020

Flowmeter performance requirements
The performance aspects of the flow meter mainly include: measurement flow (instant flow) or total volume (cumulative flow); accuracy requirements; repeatability; linearity; flow range and range; pressure loss; output signal characteristics and flowmeter response time Wait.
(1) Measurement flow or total
Flow measurement includes two types, namely instantaneous flow and cumulative flow. For example, the crude oil belonging to the sub-transmission station pipeline is a custody transfer or a petrochemical pipeline for continuous ratio production or process control of production processes. Observation. In some workplaces, instantaneous flow measurement is required to control the flow. Therefore, it is necessary to choose according to the needs of on-site measurement. Some flowmeters, such as volumetric flowmeters, turbine flowmeters, etc., have a measuring principle that directly obtains the total amount by mechanical counting or pulse frequency output. Its accuracy is high and it is suitable for measuring total amounts, such as equipped with a corresponding signaling device. It can also output flow. The electromagnetic flowmeter and ultrasonic flowmeter are used to derive the flow rate by measuring the flow velocity of the fluid, and the response is fast, which is suitable for process control. If it is equipped with the accumulation function, the total amount can also be obtained.
(2) Accuracy
The accuracy level of the flow meter is specified in a certain flow range. If it is used under a specific condition or a narrow flow range, for example, it only changes within a small range, the measurement accuracy will be Higher than the specified accuracy level. If a turbine flow meter is used to measure the barrel distribution of oil products, and the valve is used with the valve fully open, 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, and material balance. If high measurement accuracy is required, the durability of accuracy measurement should be considered. Generally, it is used for flow meters in the above cases, and the accuracy level is required to be 0.2. In such a workplace, measurement standard equipment (such as a volume tube) is usually provided on-site to perform online detection of the flowmeter used. In recent years, due to the increasing tension of crude oil and the high requirements of various units for the measurement of crude oil, a coefficient transfer has been proposed for crude oil measurement. Calibrate the flowmeter monthly to determine the flow coefficient, and transfer the data calculated according to the data measured by the flowmeter and the flow coefficient of the flowmeter every day to improve the accuracy of the flowmeter, also known as zero error handover.
The accuracy level is generally determined based on the maximum allowable error of the flow meter. The flowmeter instructions provided by each manufacturer will be given. 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, which is usually expressed by "% R". The citation error refers to the measurement upper limit value or the percentage of the range, usually "% FS". Many manufacturers' instructions do not indicate this. For example, float flow meters generally use quoted errors, and some models of electromagnetic flow meters also use quoted errors.
If the flow meter is not simply used to measure the total amount, but is used in a flow control system, the accuracy of the detection flow meter must be determined under the control accuracy requirements of the entire system. Because the entire system not only has errors in flow detection, it also includes errors in signal transmission, control adjustment, operation execution and other factors and various influencing factors. For example, there is a return difference of about 2% in the operating system. It is uneconomical and unreasonable to determine the accuracy of the measurement instrument used (over 0.5 level). As far as the meter is concerned, the accuracy between the sensor and the secondary meter should also be properly matched. For example, the error of the averaging tube designed without actual calibration is between ± 2.5% to ± 4%, with 0.2%. ~ 0.5% high accuracy differential pressure gauge is not significant.
    Another problem is that the accuracy level specified for the flowmeter in the verification procedures or manufacturer's instructions refers to the maximum allowable error of the flowmeter. However, because the flowmeter is affected by changes in environmental conditions, fluid flow conditions, and dynamic conditions during field use, some additional errors will occur. Therefore, the flowmeter used in the field should be a combination of the maximum allowable error and the additional error of the meter itself. This problem must be fully considered. Sometimes the error in the field environment may exceed the maximum allowable error of the flowmeter.