Reducing rectifiers used in flow measurement
Published on:2015-3-11 14:26:45 Hits:

First. Overview

Conventional fluid rectifier through long-term research and practice has matured, it is commonly used speed barrier separating the body to adjust the flow channel of distribution pipeline in order to achieve the purpose of rectification; this type of rectifier is mainly used for laboratory and flow calibration system . However, this method can lead to increased resistance to dirt clogging and loss, so rarely used in industrial piping.

Vortex because of its unique properties, has been people's attention, and has been widely used to, but there are two aspects of the problem plaguing the people, first, due to the interference meter upstream pipeline Baffles, the flow field distortion impact aside from the normal vortex. In order to overcome the disturbance flow field, the instrument needs to be equipped with a long straight pipe (typically 15 to 40 times the length of the process tube inner diameter), while in the actual field are difficult to meet. Second, one of the main features of the vortex flowmeter is a wide range, typically about 10: 1, it should be said that this should be a relatively wide measurement range excellent performance, but in practical industrial applications, much lower than the maximum flow meter the upper limit, the minimum flow is often lower than the lower limit of the instrument, some of the instruments often work in the vicinity of the lower flow rate, resulting in decreased accuracy of measuring instruments, when the signal is weak, the instrument of anti-jamming capability also declined. To measure low flow, people tend to use cavity shape for traditional adjustable tube Park station, after necking at increasing the flow rate measurement. So vortex flowmeter operates in the normal range of flow rates, but this way adjustable, structure size is large (typically 3 to 5 times the length of the process tube diameter), and at the same time, due to the fluid flow through the adjustable tube, adjustable office at produce large group rotating flow, increasing loss of local resistance, but also the flow field distortion. Straight pipes must be between the adjustable tube and process instrumentation installation of more than 15 times the length of the tube diameter is rectified, and increases the resistance losses along the way (see Figure 1), this method increases the cost of construction, but also to the process installation inconvenience. Reducing the use of vertical end face rectifier special shaped line (already declared national patent), a rectifier, increase the flow rate and the change in the velocity distribution of the multiple roles of the small size of its structure, the length of the tube diameter of only 1/3 of the process can be directly snapped at both ends of the instrument, not only does not require additional add-straight pipes, and can reduce meter straight pipe upstream requirements. Experimental results show that: meter upstream resistance member for two 90 ° elbows in one plane under normal circumstances, the vortex flowmeter upstream side should be installed more than 20 times the pipe diameter length of straight pipe, and the installation of a vortex flowmeter Reducing rectifier greatly reduces the measured upstream straight pipe length requirements, the resistance is much less than traditional adjustable tube. More importantly, the lower limit of the flow rate can be reduced to 1/3, turndown ratio up to 15: 1 or more. '

Second, the principle and Analysis

First it should be noted that traditional adjustable tube can be subjected to necking, and with a smaller diameter flowmeters to achieve the purpose of measuring low flow, but this method can not expand the instrument's range ratio, because it does change the end of the pipe velocity distribution state. We know that the theory and the derivation of the vortex flowmeter is based on infinite uniform get in the flow field, and in the actual closed tube, and it is non-uniform flow field, cross-sectional velocity distribution is a paraboloid of revolution, although a reasonable choice The cylindrical, so that both sides of the bow velocity distribution cylinder surface, but in fact, the impact on the process pipeline rotating parabolic velocity distribution is an objective reality. Experiments show that when a relatively large flow, this effect is small, or that the impact within the allowable range; but with the decline in traffic, the increasing influence from a large number of calibration data, always with the meter constant reducing the flow rate increases. This shows that the sampling points and the average velocity of the flow rate difference is growing.

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