Application of variable diameter rectifier in vortex flowmeter

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Application of variable diameter rectifier in vortex flowmeter [Copy link]

First of all, it should be pointed out that the traditional reducer can be reduced in diameter and equipped with a smaller caliber flowmeter to achieve the purpose of measuring small flow, but this method cannot expand the range ratio of the instrument because it does not change the velocity distribution state of the pipeline. We know that the theory and derivation of the vortex flowmeter are based on the infinite uniform flow field, but in the actual closed circular tube, it is a non-uniform flow field, and the velocity distribution of the cross section is a parabola of rotation. Although a reasonable column type is selected to make the velocity distribution of the arcuate surfaces on both sides of the column uniform, in fact, the influence of the velocity distribution of the parabola of rotation on the process pipeline is objective. Experiments show that at relatively large flow rates, this effect is small, or this effect is within the allowable range; but as the flow rate decreases, this effect becomes larger and larger. From a large amount of calibration data, the instrument constant always increases with the decrease of flow rate. This shows that the difference between the velocity at the sampling point and the average velocity is getting bigger and bigger. After the use of the variable diameter rectifier, the flow velocity of the shrinking section is gradually increasing, and the increase in flow velocity at each point on the section is different. The flow velocity increases less near the center, while the flow velocity increases more near the edge of the throat. The traditional fluid rectifier has matured after long-term research and practice. It generally uses a barrier to separate the flow channel to adjust the velocity distribution in the pipeline to achieve the purpose of rectification; this type of rectifier is mainly used in laboratories and flow calibration systems. However, this method is prone to cause dirt blockage and increase resistance loss, so it is rarely used in industrial pipelines. Due to its unique performance, the vortex flowmeter has always been valued by people and has been widely used, but there are still the following two problems that bother people: First, one of the main features of the vortex flowmeter is the wide range, generally around 10:1. It should be said that such a wide measurement range should be a relatively good performance, but in actual industrial applications, the maximum flow is far lower than the upper limit of the instrument, and the minimum flow is often lower than the lower limit of the instrument. Some instruments often work near the lower limit flow, resulting in a decrease in the metering accuracy of the instrument. At this time, the signal is weak and the anti-interference ability of the instrument is also reduced. Second, due to the interference of the upstream pipe flow blocker of the instrument, the flow field is distorted, affecting the normal separation of the vortex. In order to overcome the flow field disturbance, a longer straight pipe (generally 15 to 40 times the inner diameter of the process pipe) needs to be installed in front of the instrument, which is difficult to meet in the actual site. In order to measure small flow rates, people often use traditional reducers with a round inner cavity shape to increase the flow velocity at the measurement point by reducing the diameter. The vortex flowmeter works within the normal flow rate range, but this type of diameter reduction method has a large structural size (generally 3 to 5 times the inner diameter of the process pipe). At the same time, as the fluid flows through the reducer, a large number of rotating flow groups are generated at the diameter reduction point, which increases the local resistance loss and also distorts the flow field. Therefore, a straight pipe with a length greater than 15 times the inner diameter of the process pipe must be installed between the reducer and the instrument for rectification, and the resistance loss along the way is increased. This method increases the construction cost and also brings inconvenience to processing and installation.

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In order to measure small flow, people often use traditional reducers with a round inner cavity shape to increase the flow velocity at the measuring point by reducing the diameter. This allows the vortex flowmeter to work within the normal flow velocity range, but this type of reducer has a large structural size (generally 3 to 5 times the inner diameter of the process pipe). At the same time, as the fluid flows through the reducer, a large number of rotating flow groups are generated at the reducer, increasing local resistance losses and causing flow field distortion. Therefore, a straight pipe with a length greater than 15 times the inner diameter of the process pipe must be installed between the reducer and the instrument for rectification, and the resistance loss along the way is increased. This method increases construction costs and also brings inconvenience to processing and installation.