Several methods for measuring the air volume of a large rectangular pipe are compared with their respective advantages and disadvantages, and the application is recommended. The article focuses on the wing flowmeter.
Flow measurement in large pipelines has generally attracted people's attention in recent years, and it has been seriously resolved. In order to save space and facilitate installation, many enterprises often use rectangular pipes to be wall-mounted when the fluid pressure in the pipes is not large. For example, the inlet pipes of thermal power plants and large ventilation systems use rectangular pipes with a length of 2 ~ 3m. It is not uncommon. There are some special problems with the flow measurement in this kind of pipeline, and the measurement methods and instruments in the round pipe cannot be copied, which is rarely introduced. In the past two years, the author explored to solve this problem in a project, designed and manufactured two wing flowmeters, and commissioned and put into operation successfully.
1 Fluid flow characteristics in rectangular pipes As early as 1926, Nikuradse tested the flow of fluids in rectangular pipes, and its isokinetic lines are shown. It shows different concentric lines in concentric circles and finds a secondary flow.
Essentially, the secondary flow is a normal flow perpendicular to the axial direction on the cross section of the pipe under turbulent flow conditions. It superimposes the axial flow to form a secondary flow, which can also be called a vortex. Since there is no normal flow in laminar flow, it is impossible for secondary flow to occur under laminar flow conditions.
(Since Chengdu Science and Technology Service Center, Sioooo), in 1964, Brundrett and Baines again tested the flow in rectangular smooth and rough pipes. They believed that the secondary flow in the rectangular pipe was caused by Renault in cross section. Caused by force, the main role is the normal stress intensity, which is a non-isentropic flow, and turbulent motion is the basis for the formation of secondary flow. In 1973, Landei and Ying used an analytical method to describe this complex Flow. They demonstrated that the secondary flow has a tendency to homogenize the wall stress, and pointed out that the secondary flow will change the size of the friction factor, and its amplitude is about ± 10%. For a long time, people have focused on the research of this flow. The influence of resistance, which rarely involves flow measurement, has only been considered in recent years, see the discussion.
There have also been attempts to measure the axial pressure gradient dp / d of rectangular pipes to solve the problem of flow measurement. The formula is 1/2 height; the hydrodynamic viscosity coefficient; dPl / ck is the dimensionless axial pressure gradient.
This method has not yet been heard of being applied to actual engineering. The question is whether this calculation formula accurately describes the actual flow and whether the boundary conditions can be accurately determined. These have yet to be proven in practice.
2 Common methods The following describes three common methods for measuring the flow of rectangular large pipes. Although each has its own advantages, they are not satisfactory.
2.1 Velocity area method This is a classic measurement method, which has been followed by international standards (IS03966, IS07194). The advantage of this method is that it is not limited by the size of the pipeline, and it is also accurate and reliable; the disadvantage is that it is necessary to measure the flow rate of more than 20 points for a flow value. The measurement is very cumbersome, so it is almost impossible to apply it to industrial sites. , But it can be used as a most basic and practical verification method.
It should be noted here that in international standards, the recommended velocity meter for this method is the NPL pitot tube. Although it is accurate but sensitive to the flow direction, when the flow direction deviates from the axial direction by more than 10, it will cause more than 1% error. It is not a problem to apply multiple bit streams in round pipes, but it should be treated differently for rectangular pipes. Since there is inevitably a secondary flow in the rectangular pipe, the deviation of the flow direction beyond ± 10 is common, and the NPL type pitot tube is still used, which is difficult to guarantee the necessary accuracy. For this reason, the author recommends a special pitot tube with diversion inlet, which can still maintain 1% accuracy when the flow direction is within ± 40.
2.2 Insertion method 2.2.1 Point velocity method The point velocity method is a method of estimating the flow rate by measuring the flow velocity at a certain point in the pipeline. This method is simple and easy, but the accuracy is very low. It is mostly used for monitoring in industrial sites and rarely used for measurement.
In principle, the flow rate can be measured by a general velocimeter using this method.
At present, there are plug-in turbine flowmeters, plug-in vortex flowmeters, Pito-Venturi tubes, etc. The author especially recommends the Pito-Venturi tube, which has a simple structure and reliable operation, can output a differential pressure signal that is several times larger than a pitot tube, and is resistant to high temperatures and can work in a more severe site than the previous two. Thermal power plants and metallurgy and iron and steel industries have been popular for a while. It should be noted that the repeatability of its speed-differential pressure characteristics is poor, and each one needs to be verified in the wind tunnel before it can be used.
2.2.2 Linear velocity method The linear velocity method refers to the measurement of the comprehensive value of the multi-point flow velocity along a straight line to determine the flow rate at a time. It is more accurate than the point velocity method, and the installation conditions are also better than the point velocity method. . It is simple in structure, reliable in operation and easy to install. It is more suitable for measuring the air volume of rectangular pipes in various situations. Its installation position is as shown.
When the uniform velocity tube is used in a rectangular pipe, its flow calculation formula is similar to that of a round pipe. The difference is that when calculating the cross-sectional area and facilitating the selection of the flow coefficient K, the equivalent diameter and the rectangular cross-section width 6 and height / 1 are used. The relationship is only expedient for this treatment, especially when the width 6 of the rectangular cross-section differs greatly from the height h, there will be a larger error.
2.3 Elbow flow meter (see this method has been applied for decades, is a more mature measurement method, but the output differential pressure is too small, the measurement accuracy is not high. When using rectangular pipes, due to the needs of process layout It is common to have rectangular elbows. If it is used according to the situation, it is also a method that can be considered.
Otherwise, deliberately adding a 90-elbow pipe flowmeter to the straight pipe is not necessary.
3 wing flowmeter 3.1 principle measures the flow in a rectangular pipe, and a rectangular venturi tube was used in the early days. In order to prevent the airflow from separating in the expansion section and increasing the pressure loss, the rear expansion angle should generally not exceed 1p ~ 12, so the venturi tube is very long, which causes inconvenience to installation and transportation. If one or several wings are installed in a rectangular channel (see), it is possible to control each separation angle at 10 ~ 12 ° and shorten the entire length, making the structure compact and easy to install and transport.
From the principle point of view, the wing flowmeter still uses throttling, which accelerates and decompresses the airflow. The differential pressure is used to determine the size of the flow. Therefore, it is still a throttling device. However, the inlet part of the wing flow has a diversion function. During the process of gas acceleration, it can ease the lateral flow of the airflow and reduce the size of the vortex. The flow device is so demanding that it only needs about 3 times the length of the equivalent diameter, which is particularly practical for large rectangular pipes.
3.2 Calculation formula Generally, the velocity of the airflow flowing through the wing flowmeter will not exceed 30m / s, so it can be treated as an incompressible flow. The density p of the fluid can be regarded as a constant. (The derivation is omitted), the calculation formula is the output differential pressure, pa; yv is a constant, depending on the unit of each parameter of formula (2), here is 5.09x103; K is the flow coefficient, generally between 0.95 ~ 0.99; a is Blocking ratio, defined as m = 42 / Yu; seven is the minimum channel area of ​​the throat of the flowmeter, m2; the mountain is the cross-sectional area of ​​the inlet of the flowmeter, m2. When designing a wing flowmeter, it is often known that the flow rate 9 and the inlet cross-sectional area are Therefore, the two parameters of differential pressure Ap and blocking ratio a are selected. When calculating, one of the parameters must be given to find another parameter. The author recommends that when the blocking ratio a is selected, it should be between 0.3 and 0.5; if the value a is too small, the pressure loss will be too large, which will affect the normal flow in the pipeline; if it is too large, the throttling effect is not obvious. The output differential pressure is too small, it is difficult to choose a differential pressure transmitter. The differential pressure Ap value is recommended not to be less than 400Pa, and the upper limit is unlimited.
3.3 Structure The structure of the wing flowmeter is shown below.
The curve of the wing profile is composed of three segments: the leading edge is an arc with a radius of curvature of 25 (or a circular tube directly using a shirt 0); the middle curve is the quadratic equation y2 = c (c is a constant ) As described; the trailing edge is a straight line for easy processing, and the three sections are smoothly connected, and no inflection point is allowed.
The position of the total and static pressure holes is a group of total pressure holes directly opposite to the flow direction of each wing leading edge, and the distance of each hole is 0.2 ~ 0.3m. The measured total pressure is collected in the total pressure manifold located at the leading edge. Connect the average value of the total pressure in each manifold to the total pressure header on the shell of the wing flowmeter, and connect it to the pressure end of the differential pressure transmitter through the instrument valve.
At the highest point of the wing, that is, the surface of the wing where the channel is the narrowest, there is a set of static pressure holes (for example, considering the inertia of the fluid, the narrowest channel should be about 1 cm downstream of the narrowest structure for the fluid) . The spacing of each static pressure hole is 0.1 ~ 02m, and the measured static pressure is collected in a square groove static pressure manifold with a cross section of 50mmx60mm (see). The static pressure collected by each static pressure manifold is then connected to the static pressure header on the flowmeter housing, and connected to the low pressure end of the differential pressure transmitter through the instrument valve.
Total and static pressure aperture The total pressure aperture is 2 ~ 3mm; the static pressure aperture is 1 ~ 2mm. Generally speaking, the static pressure aperture should be 1/2 of the thickness of the wing plate. The static pressure aperture is not allowed to have chamfers and the edges are smooth No burrs or welding slag. The leading edge of the total pressure hole can be allowed to have an internal chamfer of 90> ~ 120P. Even to strengthen the diversion function, a diversion sleeve is installed.
Manholes To facilitate maintenance and verification, a manhole of 0.6mx0.6m should be placed about 1 in front of the inlet of the wing flowmeter.
There are two issues to be explained here. One is why a square groove static pressure manifold is used. In principle, as long as a baffle is installed in the static pressure vent hole in the wing, it is also feasible to separate the total and static pressure. But this requires that all welding seams should be absolutely sealed when processing the wings, and no air leakage is allowed, otherwise the true static pressure cannot be measured. Secondly, due to the large volume of the back cavity of the wing, and the static pressure hole is only 1 ~ 2mm, it takes a long time to achieve a stable static pressure value, the flow; 1 time constant is too large. Therefore, in order to improve the processing conditions and improve the performance of the measurement and control system, it is necessary to use a static pressure manifold with a small volume and easy processing. The second is that if the wing flowmeter is at the inlet of the air inlet pipe, a bell mouth with a diversion function should be considered at the inlet. The bell mouth is curved and the angle is preferably 45 ° ~ 60. In this case, because The flowmeter is close to the inlet, and the pressure loss can be ignored. The atmospheric pressure can be regarded as the total pressure. The high-pressure cavity of the differential pressure transmitter directly communicates with the atmosphere, omitting the total pressure manifold and gas collector, and simplifying the structure.
The gas in the sewage industrial site will inevitably contain some dust. The wing flowmeter is generally not easy to block due to the large number of pressure measurement holes. After using for a long time, the gas collecting pipe or the pressure measuring manifold may collect more dust because of the smaller cavity. If a blowdown ball valve (or plug) is installed at the other end of the manifold or gas collection pipe to the atmosphere, and the valve (or plug) is opened regularly, (turn down to No. 16) 3 Sensor calibration result After the design, installation and commissioning of the sensor is completed, Calibration was carried out in the metrology department. In order to compare with the electromagnetic turbine flow sensor with a caliber of (425mm), this paper gives the calibration results of the optical fiber speed turbine flow sensor with the same diameter and the same bearing and turbine rotor material. The calibration procedure is strict According to the flow calibration procedure, the calibration medium is water, and a total of three round trips are performed. See the calibration results, where a is the TV-TV relationship curve and b is the Office-A: relationship curve.
It can be known from the calibration results: the measurable range of the optical fiber speed turbine flow sensor is 0.29-12.60m3 / h, and the range ratio is 43: 1. During the calibration, due to insufficient pressure of the calibration device, the flow rate can only be 12.60m3 / h. It is impossible to increase, otherwise, the upper limit of flow measurement may be increased. The measurement range of the electromagnetic turbine flow sensor with the same caliber is 1.19 ~ 12.24m3 / h, and the range ratio is only 10: 1. It can be seen that the measurement dead zone of the fiber speed sensor is greatly reduced.
The analysis and calculation of the calibration results shows that the linearity error of the sensor in the full range is 0.83%, which is a little too large, but its repeatability error is only 0.25%. Therefore, the method of nonlinear correction can be easily used To reduce linearity errors. For example, using the three-segment linear interpolation method, the linearity error of the sensor can be reduced to 0.30%. 4 Conclusion Fiber optic speed turbine flow sensor has the unique advantages of good measurement repeatability, large range ratio, strong anti-electromagnetic interference ability, safety and reliability, etc. , Especially the separated optical fiber speed turbine flow sensor, the test site is not charged, and it is a safe and reliable flow detection instrument in the measurement of low viscosity fuel and flammable gas flow.
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