One, an overview of the classes in pipe flow measurement process, the uneven liquid velocity distribution in pipe and spiral of the existence of mixed flow, are some of the flowmeter
Mainly speed meter)
Measurement precision, the operation of the main causes of poor stability, especially the vortex flowmeter, orifice flow meter
, differential pressure flowmeter and vortex precession, this factor for flowmeter measurement effect is serious, so now the precession of vortex flowmeter are built-in rectifier, caused by the uneven liquid velocity and vortex flow in pipeline, is due to the upstream pipe flow meter, valve, pump, such as pipeline structure joint different heart or welding burrs, gasket to highlight the line, and other obstacles.
In order to overcome the existing in the pipeline flow velocity distribution, and eliminate vortex flow in the upper part of the pipe into a bunch of catheter (
Or other components)
This is to install the cause of the rectifier.
Rectifier is flow metering system in a major accessory equipment.
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.
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