Thread Content
I. Working principle: A vortex generator (flow obstruction element) is placed in the fluid, which alternately generates regular vortices on both sides of it; these vortices are known as Karman vortices, as shown in Figure 1. The vortex rows are arranged asymmetrically downstream of the vortex generator. Let the frequency of vortex generation be f, the average velocity of the flow of the medium being measured be U, the frontal width of the vortex generator be d, and the outer diameter of the body be D. According to the Karman vortex street principle, the following relationship holds: f = SrU1/d = SrU/md (1), where U1 is the average flow velocity on both sides of the vortex generator, in m/s ； Sr--Strouhal number ； m--the ratio of the arcuate area on each side of the vortex generator to the cross-sectional area of the pipe. Figure 1: Karman vortex street; the volumetric flow rate qv is given by qv=πD2U/4=πD2mdf/4Sr (2). K=f/qv=-1 (3), where K is the metering coefficient of the flowmeter, expressed in pulses per m3 (P/m3). In addition to being related to the geometric dimensions of the vortex generator and the pipe, K is also related to the Strouhal number. The Strouhal number is a dimensionless parameter that is related to the shape of the vortex generator and the Reynolds number. Figure 2 shows the relationship between the Strouhal number of a cylindrical vortex generator and the pipe Reynolds number. As can be seen from the graph, within the range of ReD=2×104 to 7×106, Sr can be considered constant, which is the normal operating range of the instrument. When measuring gas flow rate, the flow rate calculation formula for VSF is given by the equation in Figure 2, which shows the relationship between the Strouhal number and the Reynolds number; here, qVn and qV represent the volumetric flow rates under standard conditions (0°C or 20°C, 101.325 kPa) and under the actual operating conditions, respectively, in m3/h ； Pn and P represent the absolute pressures under standard conditions and under operating conditions, respectively, in Pa ； Tn and T represent the thermodynamic temperatures under standard conditions and under operating conditions, respectively, in K ； Zn, Z represent the gas compression coefficients under standard conditions and operating conditions, respectively. As can be seen from the above equation, the pulse frequency signal output by VSF is not affected by changes in the fluid properties and composition; that is, within a certain range of Reynolds numbers, the instrument coefficient depends only on factors such as the shape and dimensions of the vortex generator and the pipeline. However, as a flow meter used in material balance and energy measurement, it is necessary to measure the mass flow rate; in such cases, the output signal of the flow meter should simultaneously reflect both the volume flow rate and the fluid density. The physical properties and composition of the fluid also have a direct impact on flow measurement. II. Structure The VSF consists of a sensor and a converter, as shown in Figure 3. Sensors include vortex generators (flow obstructing elements), sensing elements, instrument casings, etc ； The converter includes a preamplifier, filtering and shaping circuit, D/A conversion circuit, output interface circuit, terminals, bracket, and protective cover, etc. In recent years, intelligent flowmeters have also incorporated microprocessors, display and communication modules, as well as other functional components within the converter. Figure 3 Vortex street flowmeter (1) Vortex generator. The vortex generator is the main component of the detector; it is closely related to the flow characteristics of the instrument (such as instrument coefficient, linearity, range, etc.) as well as its pressure loss characteristics. The requirements for it are as follows. 1) It can control the synchronous separation of vortices along the axis of the vortex generator ； 2) Over a wide range of Reynolds numbers, there is a stable vortex separation point, maintaining a constant Strouhal number ； 3) It can generate strong vortex streets, resulting in a high signal-to-noise ratio ； 4) Simple shape and structure, facilitating processing and standardization of geometric parameters, as well as the installation and combination of various sensing elements ； 5) The material should meet the requirements of the fluid properties, being corrosion-resistant, wear-resistant, and resistant to temperature changes ； 6) The natural frequency is outside the frequency band of the vortex street signal. A wide variety of vortex generators have been developed, which can be divided into two categories: single vortex generators and multi-vortex generators, as shown in Figure 4. The basic shapes of single vortex generators are cylinders, rectangular prisms, and triangular prisms; all other shapes are variations of these basic forms. The triangular prismatic vortex generator is the most widely used type, as shown in Figure 5. In the figure, D represents the instrument diameter. To improve the strength and stability of vortex streets, multiple vortex generators can be used, although their application is not widespread. d/D=0.2～0.3; c/D=0.1～0.2; b/d=1～1.5; θ=15°～65° ⑵ There are 5 ways in which flow meters using sensing elements can detect vortex signals. 1) Directly detect the differential pressure on both sides of the vortex generator using the detection element installed within it ； 2) Pressure guide holes are provided on the vortex generator, and detection elements are installed in these holes to measure the pressure difference on both sides of the generator ； 3) Detect the alternating circumflow around the vortex generator ； 4) Detect the alternating differential pressure on the back side of the vortex generator ； 5) Detect vortex rows in the wake. Based on these 5 detection methods, different detection techniques (thermographic, ultrasonic, stress, strain, capacitive, electromagnetic, photoelectric, fiber optic, etc.) can be used to create various types of VSF, as shown in Table 1. Table 1: List of vortex generators and detection methods. The detection element converts the vortex street signal into an electrical signal; this signal is weak and contains noise of various types. Amplification, filtering, and waveform shaping are necessary to obtain a pulse signal that is proportional to the flow rate. ⑶ Different detection methods of the converter should be equipped with preamplifiers with different characteristics, as listed in Table 2. ⑷ The gauge body can be divided into clamp type and flange type, as shown in Figure 7. Figure 7: Instrument body III. Installation notes: VSF is a flow meter that is sensitive to distortions in the pipe flow velocity distribution, rotational flows, and flow pulsations; therefore, due attention must be paid to the installation conditions of the pipes in the field, and the requirements specified in the manufacturer’s instructions must be followed. VSF can be installed indoors or outdoors. If it is installed in a well where flooding is possible, a dew-type sensor should be used. Sensors can be installed on pipes horizontally, vertically, or at an angle, but when measuring liquids and gases, care must be taken as to their installation position to avoid interference from bubbles and droplets, as shown in Figure 16. Figure 16 Installation of mixed-phase fluids (a) Installation of instruments for measuring gas flow with liquid content ； (b) When installing a flow meter for gaseous liquids with VSF, it is necessary to ensure that there are sufficient lengths of straight pipe sections upstream and downstream, as shown in Figure 17. The variations in data across different sources may be due to the fact that vortex generators have not yet been standardized, and it remains to be determined how much the differences in their shape and size affect the results ； Experimental research on the required straight pipe length for various flow obstructing elements is still insufficient; in other words, it is not yet mature. Compared to throttle-type differential pressure flowmeters, work in this area is still in its initial stages. Figure 17 Requirements for the length of straight sections upstream and downstream of a vortex street flow meter (a) a 90° elbow ； (b) Concentric expansion ； (c) Concentric contraction fully open valve ； (d) Two 90o elbows on different planes ； (e) Control valve partially open ； (f) The connections of the two 90° elbow sensors in the same plane to the pipeline are shown in Figure 18. When connecting to pipes, pay attention to the following issues. Figure 18 Connection of sensor to pipeline 1) The inner diameters D of the upstream and downstream pipes are equal to the inner diameter D` of the sensor, with the difference satisfying the following condition: 0.95D ≤ D` ≤ 1.1D. 2) The piping should be concentric with the sensor, and the coaxiality should be less than 0.05D`. 3) The gasket must not protrude into the pipeline; its inner diameter can be 1–2 mm larger than that of the sensor. 4) To perform flow interruption checks and clean the sensor, a bypass pipe should be installed as shown in Figure 19. Figure 19 Schematic diagram of the bypass pipe 5) Reducing the impact of vibrations on VSF should be addressed as a key issue in the on-site installation of VSF. First, when selecting the location for installing the sensor, try to avoid areas with vibration sources. Secondly, the use of flexible hoses for connection in small-diameter pipes can be considered. Thirdly, installing pipe supports is an effective vibration reduction method; one such pipe support method is shown in Figure 20. Figure 20 shows an example of a complete set for installing pipe supports, including straight sections at the front and back, as well as flow regulators. These elements are measures to ensure high-precision measurements; in particular, having them assembled in the manufacturing plant helps guarantee the quality of installation. Figure 21 illustrates an example of such an installation. Figure 21: For high-precision measurements in piping installation and electrical installations, it is important to use shielded or low-noise cables to connect the sensors and converters, with the distance between them not exceeding the values specified in the user manual. During wiring, it should be kept away from high-power power cables, and preferably protected with a separate metal sleeve. The \"one-point grounding\" principle should be followed, with the grounding resistance to be less than 10Ω. Both the integrated and separate types should be grounded on the sensor side, and the grounding point of the converter housing should be at the same potential as that of the sensor. IV. Fault Symptoms and Handling