0 Preface
Vortex flowmeter is a speed flowmeter, which can be used to measure the flow of various media such as liquid, gas and steam. It has a wide measuring range, no moving parts, small pressure loss, reliable work, high accuracy and easy installation and maintenance. Such advantages, the application is more extensive. The following describes the application of vortex flowmeters in the flow measurement of boiler air supply in a thermal power plant. Combined with the actual situation, the problems in use and the corresponding solutions are analyzed in detail.
1 vortex flowmeter working principle
Vortex flowmeter, also known as vortex flowmeter, uses a regular vortex shedding phenomenon to measure fluid flow. As shown in Fig. 1, a non-streamlined bluff body (also called a vortex generator) is inserted perpendicular to the fluid flow direction in the pipeline. When the Reynolds number of the fluid reaches a certain value, the fluid alternates on both sides of the bluff body. Two rows of regular vortex vortex streets are separated and released. Within a certain flow velocity range, the vortex separation frequency is proportional to the flow rate.
If we set the distance between two columns of vortices to h and the distance between two adjacent vortices in the same column to L, then when h/L=0.281, the resulting vortices are periodic and stable. At this time, for the vortex generated by the cylindrical vortex generator shown in Figure 1, the unilateral frequency f can be expressed as: where f is the unilateral vortex frequency, Hz; St is the Strouhal number, dimensionless, numerical The size is related to the shape of the vortex body and the Reynolds number of the fluid; v is the average flow velocity of the fluid, m/s; d is the diameter of the cylinder, m.
The Strouhal number can be considered constant when the fluid Reynolds number is in the range of 2 x 104 to 7 x 106 (this is the normal working range of the instrument). At this time, the frequency of vortices is proportional to the average flow velocity of the fluid. Measured frequency f can be obtained by the fluid volume flow (1-2).
The vortex frequency can be measured by various methods such as heat, electricity, sound, etc., such as thermal detection method, stress detection method, capacitance detection method, ultrasonic detection method, etc. These methods utilize the pressure, velocity, or density of the vortex at the sensing element. The periodic changes of other parameters are converted into periodical electrical signals, which are then processed by amplification and shaping to obtain square wave pulses, which are finally displayed, recorded or accumulated by the secondary instrument (1).
2 vortex flowmeter in the flow measurement of boiler air supply
Air flow rate is one of the most important thermal parameters of power station boilers. Its size not only reflects the working status of the air blower, but also is an indispensable variable for building a combustion automatic control system (1).
The cross section of the air supply duct is mostly rectangular, and the situation is more complicated, and there is a certain degree of difficulty in measuring the flow rate. In this paper, the vortex flowmeter is used to measure the flow of air supply, and it has received better results.
2.1 Measurement System Structure and Principle
The vortex flow measurement system consists of a sensor, a converter, and a display/recorder. as shown in picture 2. Among them, the sensor mainly includes a vortex generating body and a vortex detector for converting the flow rate of the air to be measured into a corresponding frequency signal; the converter amplifies and reshapes the frequency signal output from the transmitter, and finally outputs 4 to 20 mADC. Standard signal; The display/recorder receives the converter's output signal and displays/records the amount of air flow to be measured.
2.2 Application Problem Analysis
The analysis of the working principle of the vortex flowmeter shows that, in practical use, the vortex flowmeter has very strict technical requirements. If the selection of the instrument and supporting equipment, installation conditions, installation methods, working environment, and use and maintenance, etc. can not meet the relevant requirements , it will cause the measurement error to increase or the indication instability, or even normal measurement. The relevant problems in practical application are summarized below.
2.2.1 Instrument Selection Problems
1) The measurable lower limit flow of the vortex flowmeter is affected by the sensitivity of the detection element, and is mainly related to the Reynolds number, which is related to the geometric characteristics of the flow field, fluid density, viscosity, and flow velocity. If you work below the minimum Reynolds number, the Strouhal number is no longer a constant and the vortex flowmeter enters the non-linear workspace, which increases the measurement error.
2) The flow rate of the fluid to be measured should be between 1/2 and 2/3 of the measuring range of the instrument to reduce the measurement error. If the instrument range is too large, it may cause the flowmeter to display no instantaneous flow. At this point, one can lower the lower cutoff frequency of the flowmeter. Although this can make the flowmeter work normally, it will cause the measurement accuracy to decrease; the second is to replace the small size vortex flowmeter (3).
3) The vortex flowmeter must be used to measure the air flow. The temperature and pressure compensation must be taken into consideration. The appropriate temperature and pressure measuring instruments and intelligent flow integrator should be selected at the same time. Moreover, these supporting instruments and vortex flowmeters must be established correctly. Flow, temperature, pressure and reference temperature, reference pressure (5), otherwise it will cause a large measurement error.
2.2.2 Installation Issues
1) When selecting the installation site, strong electric equipment, high-frequency equipment, strong switching power supply, high-temperature heat source, radiant heat source, high-humidity environment and strong corrosive atmosphere should be avoided as far as possible, and collision impact and mechanical vibration should be avoided as far as possible, especially It is the vibration in the horizontal direction perpendicular to the pipe axis. The mechanical vibration of the boiler air supply duct is relatively large, which is the biggest problem encountered with the use of a vortex flowmeter. Vortex flowmeter under the action of pipeline vibration inertial force will produce oscillation interference, when the measured medium flow rate is larger, it will make the waveform distortion, interference pulse output number (4), and then affect the measurement accuracy. In practice, measures such as installing a pipe support, installing a vibration-reducing device, or enhancing filtering may be considered. Of course, an anti-vibration vortex flowmeter may be selected, which can reduce the effect of vibration on the flow measurement accuracy to some extent.
2) The vortex flowmeter is sensitive to the effects of flow characteristics such as pulsating flow, swirling flow, and flow rate distribution distortion. It must pay attention to the distribution of upstream and downstream flow blockers (such as valves and temperature sensors) and ensure that there are sufficient upstream and downstream flows. Straight section length. For the fluctuating frequency of the measured fluid, if it is within the frequency band of the vortex flowmeter, it will have an impact on the measurement accuracy, especially when the pulsation frequency is in synchronism with the vortex frequency, which will destroy the normal operation of the instrument and even damage the instrument. If necessary, consider installing buffers or orifice plates on the upstream of the vortex flowmeter to attenuate the pulsating pressure and reduce the effect of pulsation on the measurement.
3) The pipe inner diameter DP should be the same as the flow meter inner diameter DN. If there is a difference, it should meet 0.98DN≤DP≤1.05DN, otherwise it will increase the measurement error. When installing the vortex flowmeter, the flowmeter should be kept concentric with the measuring pipe. The misalignment should be less than 0.05DN. If the eccentricity is installed, the measurement accuracy will be reduced.
2.2.3 Use and Maintenance Issues
1) The vortex flowmeters currently used are mostly smart meters. After the meters are installed, in order to ensure the flow measurement accuracy, the user must accurately set the relevant parameters according to their own actual needs, which mainly include: fluid type selection, minimum fluid density setting , meter coefficient (according to actual pipe diameter, common flow rate and pipe roughness, etc.) setting, standard operating temperature and pressure setting, cut-off frequency setting, range setting and output parameters (4～20mA or pulse) selection, etc. . The intelligent vortex flowmeters all have flow correction and self-diagnosis functions. It can automatically perform correction of relevant parameters and error segmentation correction according to different operating conditions. This can improve the accuracy of flow measurement to some extent.
2) The instrument should be calibrated regularly to ensure the accuracy of the instrument's long-term work.
3) If the flow meter measuring tube is clogged or the sensing element is contaminated by dirty media, clean it in time to avoid affecting the measurement accuracy.
3 Conclusion
The use conditions of vortex flowmeters are rather harsh, and there are also many factors that affect its measurement accuracy. Through careful analysis of its working principle and various aspects of the measurement system, and taking corresponding measures, the vortex flowmeter is guaranteed to a certain extent. Stable operation and accurate measurement.
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