Speaking of differential pressure flow measurement, we all know orifice plates, venturi, flow nozzles, twisted bars, etc., but there is also a good differential pressure flow measurement method that is not widely used in China, this is the V cone to be introduced in this article形 流 流。 Shaped flow meter. V-cone flowmeter is a new type of differential pressure flowmeter. It was introduced by American Mc-Crometer company in the 1980s. Its measurement principle is basically the same as other differential pressure flowmeters. However, due to the unique design of V-cone flowmeter, It does not have some disadvantages of other differential pressure flowmeters. The characteristics and applications of this type of flowmeter are described in detail below.
Measurement principle and structural characteristics of 1 V cone flowmeter
V-cone is a differential pressure flowmeter. Like other differential pressure flowmeters, it is based on Bernoulli's law of the mutual conversion of energy in a closed pipe. That is, under the condition of a stable flow field, the flow velocity and differential pressure in the pipe The square root is proportional. Because this measurement principle is widely used, this article will not introduce it in detail.
的几何形状，它是通过在管线中心悬挂一个锥形体来节流。 The geometry of the V-cone flowmeter is throttled by hanging a cone in the center of the pipeline. Because in a pipeline, the flow velocity distribution of an ideal flow field is: the velocity near the pipe wall is close to zero, the closer to the center of the pipeline, the greater the velocity, and the maximum velocity is at the center of the pipeline (because the friction of the pipe wall reduces the flow of fluid through the pipe wall. speed). Therefore, when the fluid flows through the V-cone flowmeter, the cone directly interacts with the high-speed center portion of the fluid, forcing the high-speed center to be homogenized with the low-speed fluid near the pipe wall, thereby generating the correct differential pressure. This is even more the case when measuring at low flow rates. it works. However, other differential pressure flow meters (such as orifice plates) with central openings cannot measure fluids with too low flow rates (reynolds number is too small) because of this effect.
All differential pressure and flow measurements are based on the ideal fluid state, but due to the wide range of actual conditions on the site, this ideal state rarely exists. Any changes in the installation of the pipeline, such as elbows, valves, shrinkage, expansion, pumps, Tee, etc. will destroy the ideal flow field, so it is difficult for general flow meters to obtain accurate measurement values in the disturbed flow field. The V-cone overcomes these shortcomings. Due to its unique design, the shape and position of the cone rearrange the upstream velocity distribution and make the flow field ideal, so as to obtain accurate measurement values. Practice has shown that under extremely harsh installation conditions (such as two elbows on different planes immediately upstream of the cone), the V-shaped cone can also make the fluid velocity distribution flat and symmetrical, thereby ensuring measurement accuracy.
Advantages of 2 V cone flowmeter
The following table analyzes the advantages of V-cone flowmeters (compared to ordinary differential pressure flowmeters such as orifice plates, venturi, etc.):
Higher accuracy and repeatability; ± 0.5% of reading, repeatability better than ± 0.1%, better than straight pipe sections of traditional differential pressure flowmeters such as orifice plates, requiring lower 0 ~ 3D, lower 0 ~ 1D, far better than The first 10D (minimum requirement) of the differential pressure flowmeter and the last 5D. The same is true for complex flow paths. If it is a differential pressure flow meter such as an orifice plate or Venturi, the requirements can reach 20 to 40D before and 5D after.
Wide range ratio
Because the cone can make the fluid flow field uniform, it can measure the flow velocity lower than the general differential pressure flowmeter, that is, the lower limit of the range is lower than the general differential pressure flowmeter, so the range can be extended downwards, and the range ratio can be 15: 1. Reynolds number As low as 8000, the linearity of the signal can still be maintained.
The fluid flows through the cone without sudden fluctuations, but instead becomes a boundary layer along the cone and guides the fluid away from the rear corner of the cone. This makes the angle of the cone not subject to the abrasion of unclean fluid, and the flow coefficient β rate It can be unchanged for a long time and ensure accurate measurement for a long time. Therefore, the flowmeter can be used for a long time in dirty places, without regular maintenance and periodic verification (of course, regular maintenance and verification of the secondary meter is still necessary).
Low pressure loss The permanent pressure loss is lower than the orifice plate and close to Venturi. Signal stabilization
As the differential pressure generated by the throttling element also causes the eddy current behind the element, each type of differential pressure instrument has a signal fluctuation, even if the fluid is stable. For the standard orifice plate, the long eddy currents caused large signal fluctuations at low frequencies, which seriously interfered with the accuracy of differential pressure readings, while the eddy currents generated after the V-cone were very short, so the signal fluctuations were low-frequency. Amplitude, which makes the V-cone produce a stable signal.
Wide range of β values
The unique geometry of the V-cone allows a wide range of β values. The standard β values are 0.45, 0.55, 0.65, 0.75, 0.85, and specific β values can be used to ensure user-specific differential pressure output.
No Retention Zones The V-Cone thorough purge design avoids the retention of residues, condensates or particles in the fluid.
Wide range of applications Measurable dirty media, will not cause damage to the cone. Can measure vapor-liquid two-phase flow (liquid phase does not exceed 5%) and guarantee accuracy.
有管道式和插入式两种形式，其尺寸可从1/2”大到72”。 Wide range of pipelines V-cone flowmeters are available in both pipe and plug-in versions, with sizes ranging from 1/2 "to 72".
Practical application of 3 V cone flowmeter
A 16 "pipe in an old plant of an ethylene plant requires an additional flowmeter due to the transformation, but the pipeline is short and the installation space is limited. It is not possible to make major changes to the pipeline. Because the installation of a flowmeter cannot affect production, it can only be stopped for maintenance. During this period, construction time is also tight.
The medium of this pipe is quenched water, normal flow rate is 2220t / h, temperature is 85 ° C, pressure is 1.06MPa. In this case, a V-cone flowmeter is selected. Since the straight pipe section requires only the first 3D and then 1D, the pipeline is satisfied. Short straight pipe problems. After being put into use, the flowmeter's condition has made the user quite satisfied with both the accuracy and stability, and the transformation has been successful.
For another example, because the pipe for measuring coke oven gas in a certain plant contains impurities, the measurement elements are seriously damaged. The plant originally used standard orifice plates for measurement, which not only had low accuracy, but also had to replace one piece a year, which affected the production Great. The factory also replaced other types of measuring flowmeters, but the effect was not obvious. Later, the factory changed to V-cone flowmeters, which not only improved the measurement accuracy, but also avoided regular maintenance and periodic inspection. The V-cone flowmeter has been used for several years, and the condition is still good. The users are quite satisfied, and several other flowmeters with the same problem have been replaced. Although it seems that the investment is more than that of the orifice plate at one time, due to the extension of the service life, the actual cost of use has not increased, but it has decreased.
Recommended applications for 4 V cone flowmeters
Suitable for the following occasions:
1) small and medium diameter gas;
2) dirty media;
3) Where the straight pipe section is insufficient;
4) occasions that require high accuracy.
有着相当明显的优势，所以其使用前景相当光明。 Because the V-cone flowmeter has quite obvious advantages, its application prospect is quite bright. Of course, as a type of differential pressure flowmeter, it cannot completely replace the position of the orifice plate and venturi due to cost. Compared with vortex flowmeters and electromagnetic flowmeters, it also has disadvantages such as the installation of pressure guiding pipes. But for suitable occasions, the use of V-cone flowmeters should be an option worth considering.