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Vortex street flowmeter for pipe diameter changes in the process of measuring the application analysis of rectifier

by:KAIDI     2020-08-28
A, before and after an overview of the general flow meter is in order to adapt to variable diameter flowmeter, in order to meet the measuring range of flow meter, flowmeter measurement accuracy reaches good. Such as medium disaffection tube, flow rate on the high side or low, can by reducing or expanding to meet the requirements of flow meter. But variable diameter before and after must pay attention to keep enough straight pipe, otherwise the accuracy of flow meter not ascend, but would fall. Traditional fluid rectifier through long-term research and practice have been mature, it usually adopts block body separation flow to adjust the velocity distribution of pipe, in order to achieve the purpose of the rectifier. This type of rectifier and flow calibration system is mainly used in laboratory. But this method is easy to cause contamination jams and increase the resistance loss, so rarely used in industrial pipeline. Because of its unique performance, the vortex street flowmeter has been much attention, and in a wide range of applications, but there are still two problems bothering people, one is because of interference instrument upstream piping of choke, the distortion flow field, affect the normal dial from vortex. In order to overcome the flow field disturbance, the instrument need to be equipped with a long straight pipe before ( Usually is 15 ~ 40 times the length of the pipe inner process) And it is hard to meet at the scene of the actual. Second, one of the main characteristics of vortex flowmeter is a wide range, generally around 10:1, it should be said that such a wide measuring range should be more excellent performance, but in the practical industrial application, the upper limit of large flow rate is far lower than the instrument, small flow and tend to be lower than the lower limit value, some instrument often work near the lower limit of flow, resulting in decreased instrument measuring accuracy, the signal is weak at this moment, the anti-interference ability of the instrument are also falling. In order to measure small flow rate, people tend to use inner cavity shape for garden of traditional variable diameter tube, after reducing increase of flow velocity measurement. Vortex street flowmeter work within the scope of the normal velocity, but the way of variable diameter and structure size large ( General length for the technology of 3 ~ 5 times the pipe diameter , at the same time, due to the fluid flows through the variable diameter tube, and generate a lot of rotational flow in the variable diameter, increasing the local resistance loss, also make the distortion flow field. So must be between variable diameter pipe and instrument equipped with more than 15 times the rectification process pipe inner length of straight pipe, and increases the frictional resistance loss ( As shown in figure 1) , this kind of method to increase the construction cost, processing, installation is inconvenient. Longitudinal and transverse line of variable diameter rectifier with special shape ( Has declared national) With rectifier, and improve the flow velocity and change the velocity distribution of the multiple function, its structure size, length is only 1/3 of the process pipe diameter, can be directly in the ends of the instrument, not only does not need additional straight pipe, but also can reduce the instrument requirement for straight pipe upstream. Experiment shows that the instrument upstream resistance to a plane of two 90 & deg; Elbow in the general case, vortex street flowmeter upstream side should be equipped with more than 20 times the pipe diameter to length of straight pipe, the vortex street flowmeter with a variable diameter rectifier greatly reduces the length of straight pipe upstream test requirements, its resistance is far less than the traditional variable diameter tube. Of, can make the minimum flow downhill for a third of the above range than to 15:1. * it should be pointed out first, second, the principle and analysis of the traditional variable diameter tube can after reducing, and relatively small diameter flowmeter to achieve the purpose of measuring small flow rate, but this approach may expand the range than, don't because it and change the velocity distribution of the pipeline at the end of the state. As we know, vortex flowmeter is based on the theory and in the infinite uniform flow field, and in the actual, in a closed circular tube, but is a uniform flow cross sectional velocity distribution is a paraboloid, while selecting rational column type, make the cylinder on both sides of the bow surface velocity distribution uniformity, but in fact, process piping paraboloid of the influence of velocity distribution on is objective existence. Experiments show that the bigger of the traffic, the less affected, or say the influence in allowed range; However, as the decline in traffic, the influence is more and more big, from a large number of calibration data, always with the decrease of the flow meter constant increases. This suggests that the sampling point of velocity and the average flow velocity difference is bigger and bigger. Using the variable diameter after the rectifier ( See figure 2) , as a result of shrinkage in a gradual increase in the cross section of flow velocity, at various points on the section of the increase of flow velocity is different, near the center of the velocity increase is small, and large velocity increases near the throat diameter edge. A rectifier inlet pressure P1, average flow velocity for V1, the speed of a point is not uniformity for U1, pressure at the exit of P2, average flow velocity V2, the same at some point through the inlet flow, degree of the asymmetry of velocity at the exit of U2, along the line, from the Bernoulli equation: by type ( 6) Visible, the influence of shrinkage ratio on the exit velocity uniformity, namely for certain entrance velocity uniformity, the exit velocity is not uniformity will narrow n2 times. Tend to be more homogeneous, so the exit velocity is closer to the condition of uniform flow field of the theory of vortex flowmeter, not only make the vortex stabilized, and improves the measurement range of instrument. In addition, this kind of variable diameter rectifier, in the process of fluid kinetic energy conversion effectively suppress the interference. Three, experimental test is case 1: a diameter of 40 mm of vortex flow meter installed in & phi; 40, process piping on the calibration to meet 1% of accuracy range ratio of 8:1, when installed in & phi; 50 process piping, and instrumentation installation on both sides of bianjing rectifier, within the scope of the 15:1 accuracy of 1. 0%. Example 2: two caliber 40 mm to 50 mm and vortex flowmeter equipped with rectifier, respectively installed on the process pipe diameter is 80 mm, with the water.
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