With the development of flow measurement technology, vortex flowmeters have been used more and more. However, due to the relatively short use time of such flowmeters, many problems are encountered in actual selection. The article briefly describes vortex street. The working principle of the flow meter, and gives a specific selection of the use of recommendations. Vortex flowmeter is a new flow meter with modern advanced level. It is a kind of natural oscillation flowmeter developed in the end of the 70's and early 80's. Since its inception, it has shown that other instruments are incomparable with a new principle and structure. Its superiority, it has a simple structure, easy installation and maintenance, a variety of applicable fluids, high accuracy, wide flow range, low pressure loss, etc., in a very short period of time has been a lot of applications. However, due to the short application time, less in the application of theoretical research and practical experience, years of use proved my plant, a reasonable selection is the key to good use applications such flowmeter. 1 How the flow meter works A non-linear object placed in the flow field will alternately separate and release two rows of regular intertwined vortices behind the object. This vortex is called the Karman vortex street. Non-streamlined objects are called vortex generators. In a certain flow range, the vortex separation frequency is proportional to the average flow velocity (flow rate) in the pipeline. The flow rate of the fluid can be measured by detecting the vortex separation frequency in various forms. The average flow velocity of the media. If the width of the inflow plane of the vortex generator is d, then there is the following relation: Where f is the vortex street frequency; d is the width of the vortex body; is the incoming flow velocity; s is the Strouhal number; d is the path body diameter. s, is a dimensionless number, for a certain vortex generator, the Strouhal number s; and the Reynolds number r. When Reynolds number r. The Strouhal number s in the range of 2x104 to 7xro can be considered constant. From equation (1), it can be seen that the average flow velocity in the pipeline can be determined by measuring the separation frequency of the sensor under the known conditions of the width d of the inflow surface of the vortex generator and the Strouhal number 5; And volume flow. 2 Vortex Flowmeter Selection When choosing a vortex flowmeter, the first thing to notice is the temperature and pressure of the measuring medium. The instrument selected must be able to adapt to the actual state of the temperature and the pressure to which it is subjected. Secondly, we must choose a suitable caliber. If the caliber is chosen too large, it will not only increase the cost, but also will not give full play to the capacity of the flowmeter, and the measurement data will be inaccurate. If the caliber is too small, the pressure drop will be too large, or cavitation will occur. This will affect the accuracy of the meter. In addition, speeding can also affect the life of the instrument, resulting in mechanical damage. Before vortex flowmeters are selected, the flow of the pipe media should be accounted for to determine if the actual operating flow will exceed the measurement range of the selected flowmeter. Under normal circumstances, the maximum working flow rate (flow rate) within the tube will not exceed the upper limit of the vortex flowmeter (air 50 s, steam 70 s, water 7 m/s). Therefore, only the minimum flow of the pipeline medium is accounted for. If the vortex flowmeter is used for gas measurement, since the vortex flowmeter measures the volumetric flow in the use state, the flow value generally provided is often the flow value in the standard state. Therefore, the standard state must first be converted into Working conditions (liquids are incompressible fluids and generally do not consider this condition). The conversion formula is as follows: Where q. Working state flow, m3/h;q. For the standard state flow, expand / h; p, for the working state pressure, pa; this is the working state temperature, °C; t. The standard state temperature, °C (Note, t. Generally 20 °C). The minimum flow rate of vortex flowmeters is generally limited by the following factors: (l) Limited by the Reynolds number of the flow field, as previously stated, when the Reynolds number r of the flow field. When >2x104, the Strouhal number sr is a constant, and the generated vortex frequency is linear with the flow velocity. Therefore, the Reynolds number re>2x104 is the basic working condition of the vortex flowmeter when the Reynolds number r. At < 2x104, even if a vortex is generated and can be detected, the meter's measurement data is incorrect. (2) The limitation of vortex energy, when the flow velocity of the fluid medium is low, the vortex strength is not enough (the vortex strength is proportional to the lift sand), and the vortex rotation speed is also low and cannot generate sufficient lift force. Especially for the piezoelectric detection, especially Magnetic sensing, the energy is not enough to cause the deformation or vibration of the sensor, can not produce a signal, even if the Reynolds number r. Can meet the detection requirements can not achieve the test, the instrument still can not work. (3) Measure the liquid medium, and check whether the minimum working pressure is higher than the saturated vapor pressure at the working temperature, that is, whether cavitation occurs. If cavitation occurs, the flowmeter cannot work. Even if it works, the measured value is biased. Big. The above parameters are generally stated in the manufacturer's manual, but the minimum flow rate given by the general manufacturer is in a specific calibration state (liquid: room temperature water, '=20°C, . = 995.zkg/m,,,= 1.006/ro a 6m2/s; gas: room temperature air, t 20 °C, p = 0. impa, eve = l.zoskg/m,, Bu 2 15/10 1 "mz / s given. △p is the pressure loss of the sensor at the maximum flow rate c. 2 2 a 2.6 (resistance coefficient, each plant has instructions) . The average flow rate for the pipeline, m/s; p, is the saturated vapor pressure of the liquid at the operating temperature, pa. Compare (.fmin). And (, m,.), Select a larger flow that is the lower limit of the flow under the conditions, use this flow lower limit to select the flow meter. If both calibers can meet the measurement range, choose a small-calibre sensor as much as possible to increase the flow rate in the pipeline. At the same time, the determination of the flow range should also check whether the scope of work of the instrument, as far as possible to make the flowmeter work in the upper limit of the flow of 1/2 to 2/3 at the most ideal. Also, in some cases, such as when measuring liquids and large-diameter pipes, the frequency of vortices is very low. For excessively low frequency signals, circuit amplification filtering and shaping are difficult, and the upper limit of measurement is affected by the sensor. Frequency response and circuit response limitations. All kinds of instruments have their own characteristics, and there are bound to be limitations. In order to achieve the correct measurement, when choosing a vortex flowmeter, it should also be noted that the various sensor types of vortex flowmeters have their own characteristics. The thermal sensor has high sensitivity, large measuring range and low lower flow rate. It can show its unique superiority in gas measurement, especially in gas measurement with small density, but its corrosion resistance and stain resistance are not ideal. Piezoelectric instruments have good sensitivity, large sensor output amplitude, and good anti-fouling properties. The disadvantage is that they have poor resistance to system vibration. The biggest advantage of magnetic sensitive instruments is that they can measure high-temperature mediums, up to 427°C, which is a measure of high temperatures. The ideal instrument for steam is also good in corrosion resistance. In contrast, the sensitivity and measurement range are not as good as thermal and piezoelectric types. To grasp the advantages and disadvantages of various sensors, avoid weaknesses, and make correct choices to use the advantages and advantages of the instrument. 3 Conclusion Vortex flowmeters have many advantages that make vortex flowmeters one of the fastest-growing flow technologies in the world. However, due to some of the aforementioned limitations, the use of vortex flowmeters is limited in some demanding applications. Fortunately, in the past few years, the design of vortex flowmeters has made considerable progress, and the technical level has been greatly improved. Some limitations have been initially overcome. In the future, with the continuous development of flow technology, vortex Street flowmeters will surely be used more widely.
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