Engineering implementation method for improving measurement accuracy of curved pipe flowmeter
The effects of engineering factors such as conductivity, flow velocity distribution and straight pipe section, electrode fouling and adhesion layer, installation conditions and operating environment on the accurate measurement of the pipe flowmeter were analyzed. The engineering of improving the accuracy of electromagnetic flow measurement was proposed. Countermeasures. This ensures that the elbow flowmeter can be accurately measured in engineering applications, so that it can fully exert its metering function and provide reliable measurement data for flow detection.
Elbow flowmeter
Flow measurement is widely used in industrial and agricultural production, national defense construction, scientific testing, domestic and foreign trade, etc. However, flow measurement is a complex technology. Since flow is a dynamic quantity, the characteristics of the measured fluid are complex and the measurement conditions are Different, so the accuracy and reliability of flow measurement is difficult to guarantee. In order to accurately measure the flow rate, people have researched and developed speed flowmeter, volumetric flowmeter, momentum flowmeter, elbow flowmeter, ultrasonic open channel flowmeter, etc. There are dozens of flow meters. Among them, the elbow flowmeter is widely used in many fields because of its superior performance. However, in actual use, there are many factors affecting the accurate measurement of the flowmeter, and the flowmeter itself is often qualified. Actually, it feels the measurement.. The phenomenon is not allowed.. On the basis of ensuring the qualified measurement of the elbow flowmeter itself, this paper analyzes the factors affecting the measurement accuracy of the pipe flowmeter in the project, and proposes to improve the pipe flowmeter. Engineering implementation method for measurement accuracy.
1 Factors affecting the accurate measurement of the elbow flowmeter
Figure 1. Working principle of elbow flowmeter The elbow flowmeter works according to Faraday's law of electromagnetic induction, that is, the conductive fluid flows at an average velocity v through a magnetic field perpendicular to the flow direction, and its induced potential E is in direct contact with the fluid. The electrode (also called the sensor) is detected. E = K BvD .. , (1) where: K is the meter constant; B is the magnetic induction (T); v is the average velocity of fluid motion (m/s); D is the pipe Inner diameter (m). When K, B, D are determined, E is proportional to V. Its working principle and structure are shown in Figure 1. According to the volume flow formula of the fluid: Q = 1/ 4..D2v .. , (2) Q is a proportional function of v and is substituted into the formula (1). Then E is also a proportional function of Q. Thus, the induced voltage E is measured, that is, the volume flow rate of the medium is measured. Q. The flow rate of the elbow flowmeter The sensor and the converter are composed of two parts. The induced voltage E measured by the sensor is sent to the converter by the cable, and then intelligently processed, and then displayed by the LCD, or converted into a standard signal 4~20 mA output. According to the above measurement principle, the actual use The engineering factors affecting the accurate measurement of the elbow flowmeter are as follows: Aspects:
(1) The influence of the conductivity of the measured medium. The conductivity of the measured fluid determines the input impedance required by the converter. The fluid conductivity decreases, the output impedance of the electrode increases, and the load effect is caused by the input impedance of the converter. The error is generated. Therefore, the lower limit of the conductivity of the fluid is specified in the application of the elbow flowmeter. Theoretically, the electrode is regarded as a point electrode, ignoring its size. In fact, the electrode has a certain size, when a circular electrode of diameter d When it is in contact with a semi-infinitely broadened fluid with a conductivity of .., its spread resistance is 1/( 2..d ). Therefore, if the pipe diameter is D..d, the output impedance of the electrode is the sum of the two broadening resistances. , which is equal to 1 / ( ..d ). The lower limit of the general measured fluid conductivity is 5 ~ 10 ..s / cm, so if the electrode diameter d is 1 cm, the output impedance of the electrode is 1 / ( ..d ) = 100 ~ 200 k.. , in order to limit the influence of the output impedance to 0.1% or less, the input impedance of the converter should be 200 M.. Left and right. For the elbow flowmeter, it must be selected Consider a fluid conductivity greater than the threshold of 5 ..s/cm (ie lower limit).
(2) Velocity distribution and the influence of the straight pipe section. According to formula (1), if the flow velocity flows symmetrically with the central axis, the induced potential is independent of the velocity distribution and is only proportional to the average flow velocity. If the flow velocity is a non-central axis symmetric distribution, 2 represents the flow line distribution and velocity profile of the 90.. elbow and the expansion tube. The flow position of each flow particle is different with respect to the electrode. The induced electromotive force generated by the electrode is also different. The closer to the electrode, the larger the mass point is. The larger the induced electromotive force is, the more likely it is to cause errors. Therefore, the fluid velocity must be guaranteed to be centrally axisymmetric. In engineering, proper installation can reduce such errors. Figure 2. Flow velocity distribution of elbows and mutant tubes Despite elbow flow Manufacturers are constantly pursuing the accuracy of the flowmeter itself, but in actual engineering, the bends, valves, etc. in the process piping can cause flow distortion, secondary flow or vortex, which destroys the original sufficiently uniform flow velocity distribution. The long straight pipe section allows the fluid to recover its axisymmetric flow velocity distribution. If the actual process pipe has insufficient straight pipe sections upstream and downstream, it can be adjusted by installing a flow regulator. whole.
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