JP4675490B2 - Ultrasonic flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic flow meter.
[0002]
[Prior art]
A pair of transducers that transmit and receive an ultrasonic beam in the flow direction and obliquely in the flow of fluid flowing in the flow path are arranged on the tube wall, and are based on the forward propagation time and backward propagation time of the ultrasonic wave. In an ultrasonic flowmeter that measures the flow velocity and flow rate of a fluid, the flow velocity and flow rate are calculated from the difference between the forward propagation time and the reverse propagation time, or the difference between the inverse of the forward propagation time and the inverse of the reverse propagation time. And ask.
[0003]
[Problems to be solved by the invention]
The conventional technique has a problem that the measurement lower limit value is determined by the measurement accuracy of the forward and reverse propagation times and the measurement resolution, and it is difficult to improve the measurement accuracy and rangeability as a flow meter.
[0004]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an ultrasonic flowmeter capable of solving such problems and improving measurement accuracy and rangeability as a flowmeter.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 is characterized in that a first transducer pair for transmitting and receiving ultrasonic waves in an oblique direction to the flow in the flow of fluid flowing through the flow channel, and an axis of the flow channel. A second transducer pair for transmitting and receiving ultrasonic waves along the
In the flow rate range above a certain level, the flow rate is measured based on the propagation time measured by the first transducer pair, and in the flow rate range below a certain level, the flow rate is measured based on the propagation time measured by the second transducer pair. This is an ultrasonic flowmeter.
[0006]
The invention according to claim 2 is the ultrasonic flowmeter according to claim 1, wherein the cross section of the flow path perpendicular to the flow is substantially rectangular, and the straight line connecting the transducers constituting the first transducer pair is a flow It is characterized in that it is inclined in the rectangular long side direction of the flow path cross section perpendicular to the flow with respect to the axis of the path.
[0007]
According to a third aspect of the present invention, in the ultrasonic flowmeter according to the first or second aspect, the transducer constituting the second transducer pair includes means for suppressing the spread of the ultrasonic beam. It is a feature.
[0008]
According to a fourth aspect of the present invention, in the ultrasonic flowmeter of the second aspect, the width near the center of a substantially rectangular section of the flow path cross section perpendicular to the flow is widened.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, preferred embodiments of the present invention will be described with reference to examples of the drawings.
[0010]
[Example 1]
As shown in FIG. 1, the first ultrasonic transducer pair 2 and 3 provided on the tube wall of the flow tube 1 having a circular channel cross section are arranged along a straight line 4 connecting the two transducers. The ultrasonic waves are transmitted and received obliquely with respect to the flow direction. The linear average flow velocity on the straight line 4 can be calculated from the forward propagation time of the ultrasonic waves from the transducers 2 to 3 and the backward propagation time of the ultrasonic waves from the transducers 3 to 2 by the time reciprocal difference method or Measurement is performed by a well-known method using a time difference method. As a calculation formula for converting the linear average flow velocity into the average flow velocity at the cross section of the flow tube, a well-known equation can be used. A well-known calculation is used to calculate the flow rate by multiplying the average flow velocity at the cross section of the flow tube thus obtained by the cross sectional area of the flow tube.
[0011]
The flow velocity distribution in the flow tube differs between the laminar flow region where the flow rate is small and the turbulent flow region where the flow rate is large. Therefore, the well-known calculation formula for converting the linear average flow velocity into the average flow velocity in the cross section of the flow tube is different. . In the laminar flow region, as shown in FIG. 2, when the coordinate in the direction of the axis XX of the flow path is x, the flow velocity V as a function of the coordinate r in the radial direction is
^{V (r) = (R 2} / 4μ) (- dp / dx) {1- (r / R) 2} ... (1)
It is represented by
[0012]
Where R: radius of flow tube 1 μ: fluid viscosity −dp / dx: friction loss due to tube wall.
[0013]
The distribution of the flow velocity V (r) is parabolic as shown in FIG. 2, has a maximum value on the axis XX, and is calculated to be twice the cross-sectional average flow velocity. Further, the linear average flow velocity on the straight line 4 is 1.5 times the cross-sectional average flow velocity. Therefore, when the flow rate measured by the first transducer pair 2 and 3 with the flow meter of FIG. 1 becomes a minute flow rate less than a certain value which is a relatively small value in the laminar flow region, the axis XX of the flow path By switching to the second transducer pair 5 and 6 arranged above, ultrasonic waves are transmitted and received on the straight line 7 connecting both transducers, and the forward propagation time and the reverse propagation time on the straight line 7 are changed. Based on this, the maximum flow velocity Vmax is measured. Since this measured value is 1.33 times that is (2 / 1.5) times the average flow velocity on the straight line 4 at the same flow rate, the measurement accuracy of the propagation time difference is improved by 30% or more. Since the flow rate measurement accuracy is improved, the measurement lower limit flow rate is lowered as compared with the case where the first transducer pair 2 and 3 are measured. Therefore, the rangeability as a flow meter is also expanded. In addition, 8 is a flow path.
[0014]
In this way, measurement is performed by the first transducer pair 2 and 3 provided obliquely with respect to the flow direction in the flow region of small and large flow rates, and on the axis of the flow path in a minute flow region of less than a certain amount. Measurement is performed by the provided second transducer pair 5 and 6. Therefore, the measurement accuracy in the minute flow rate region and the lower limit flow rate that can be measured can be improved as compared with the case where only the first transducer pair is measured.
[0015]
[Example 2]
FIGS. 3A and 3B and FIG. 4 show principle diagrams, and FIGS. 5A and 5B show specific examples. In the second embodiment, as shown in FIGS. 3 and 4, the vertical cross section of the flow path including the axis XX of the flow path 8A is a rectangle having a height H and a length L indicated by reference numeral 9, and the cross section of the flow path 8A. Is formed in a rectangular shape having a height H and a width W as shown in FIG. 1B, and constitutes a first ultrasonic transducer pair obliquely by an angle θ with respect to the axis XX of the flow path 8A. The transducers 2 and 3 are arranged so as to face each other across a straight line 4 connecting both transducers. The straight line 4 is in a plane including the rectangle 9. Further, on the axis XX, the transducers 5 and 6 constituting the second ultrasonic transducer pair are arranged to face each other. 7 is a straight line connecting the transducers 5 and 6.
[0016]
5A and 5B show a specific structure of the second embodiment. In this specific example, the straight line 4 is provided horizontally, and the axis of the flow path 8A and the straight line 7 are inclined. The fluid to be measured enters the measuring chamber 11 from the inflow part 10 and enters the flow path 8A of the rectangular flow path part 12 from the left as indicated by the arrow A. It flows along the straight line 7 in the flow path 8A to the right in the figure, exits from the right side of the flow path 8A, and flows out as indicated by the arrow B. Reference numeral 13 denotes a partition wall that partitions the inflow portion side and the outflow portion side, and the rectangular flow path portion 12 is fixed through the partition wall 13. Transceivers 2, 5, 3, and 6 are disposed opposite to the opposite left and right wall surfaces 14 and 15 of the measuring chamber 11.
[0017]
The rectangular channel portion 12 is attached so as to be inclined so that the straight line 4 on the axis XX of the flow channel 8A forms an angle θ with respect to the straight line 7 connecting the transducers 5 and 6. Also in this embodiment, the first transducer pair 2 and 3 transmit and receive ultrasonic waves to each other, thereby obtaining the line average flow velocity on the oblique straight line 4 based on the forward propagation time and the backward propagation time. . The second transducer pair 5 and 6 obtains the maximum flow velocity during laminar flow on the straight line 7 in the minute flow rate region by transmitting and receiving ultrasonic waves to each other. Switching from measurement by the first transducer to measurement by the second transducer is automatically performed when the flow rate measured by the first transducer is less than a constant flow rate in the laminar flow region. It is configured to switch to Note that the switching time may be based on the difference between the propagation times of the ultrasonic waves in the forward direction and the reverse direction, instead of depending on the magnitude of the flow rate. The transmitting and receiving ports 16, 17, 18, and 19 of the transducers 2, 3, 5, and 6 protrude from the wall surfaces 14 and 15 so that a flow velocity is generated around the entire circumference.
[0018]
The rectangular flow path of Example 2 is a flow path with a small difference in flow velocity distribution state between laminar flow and turbulent flow, and an ultrasonic transducer is placed in the rectangular flow path in the long side (H) direction of the cross section of the flow path. By installing it diagonally, the difference becomes smaller, and the measurement accuracy is improved.
[0019]
In order to effectively measure the maximum flow velocity on the axis of the flow path in the laminar flow area, the ultrasonic wave propagation area by the second transducer pair is narrowed to suppress the spread of the ultrasonic beam. For this purpose, the directivity of the ultrasonic transmitter / receiver is increased, the launch surface is reduced, or the transmitter / receiver port provided in front of the transmitter / receiver is reduced.
[0020]
Example 3
FIG. 6A is the same view as FIG. 3B in the second embodiment, showing a cross section of the flow path 8A of the second embodiment, and is a rectangular cross section having a height H and a width W. 6 (b), 6 (c) and 6 (d) are cross-sectional shapes in another embodiment compared with FIG. 6 (a), and all the heights H are the same as those in the embodiment, but the vicinity of the center of the height H The widths of the upper part and the lower part of the height H are made smaller than those of FIG. By doing so, the maximum flow velocity on the axis of the flow path in the laminar flow region with a minute flow rate can be further increased, so that the measurement accuracy in the minute flow region can be improved and the measurement lower limit flow rate can be increased.
[0021]
【The invention's effect】
Since the ultrasonic flowmeter of the present invention is configured as described above, the measurement accuracy at a minute flow rate is improved, the measurement lower limit flow rate measurement on the minute flow rate side is further reduced, and the rangeability is expanded.
[0022]
In addition, since the transducer used for the second transducer pair is specialized in the measurement of a minute flow rate, it should not adversely affect the flow of small to large flows flowing between the first transducer pair. In a minute flow rate region, the flow does not disturb the propagation of the ultrasonic wave, a stable received wave can be obtained, and even a weak ultrasonic beam with a narrowed propagation region can be reliably received.
[0023]
And in invention of Claim 2, since the transmission wave by the 1st transducer pair spreads to the whole channel by using a rectangular channel section, it will be in the propagation state correlated with the section average flow velocity of the channel, Regardless of changes in flow velocity distribution such as laminar flow and turbulent flow, it is easy to obtain the desired cross-sectional average flow velocity directly, which also contributes to improving the accuracy of the flow meter. In addition, restrictions on the height direction of the flow path have been relaxed, and a larger flow rate can be measured.
[0024]
In the invention of claim 3, since the maximum flow velocity on the flow axis in the minute flow rate region can be reliably measured and acquired, the measurement accuracy from the surface can be improved.
[0025]
In the invention of claim 4 as well, the maximum flow velocity on the flow path axis in the laminar micro flow rate region can be increased, and this also contributes to the improvement of measurement accuracy.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of Embodiment 1 of the present invention.
FIG. 2 is a diagram for explaining a flow velocity distribution in a laminar flow region in the embodiment of FIG.
FIGS. 3A and 3B are schematic diagrams for explaining a flow path shape and an arrangement relationship of ultrasonic transducers according to a second embodiment of the present invention, in which FIG. 3A is a longitudinal sectional view, and FIG. 3B is a transverse sectional view;
4 is a schematic diagram for explaining a propagation state of ultrasonic waves in the embodiment of FIG.
5A and 5B are specific examples of the second embodiment of the present invention, in which FIG. 5A is a longitudinal plan view, and FIG. 5B is a longitudinal front view.
FIGS. 6A and 6B are schematic diagrams showing various types of rectangular cross-sections of a flow path, wherein (a), (b), (c), and (d) show different cross-sectional shapes.
[Explanation of symbols]
1 Flow tube 2, 3, 5, 6 Ultrasonic transducer

Claims (4)

In the flow of the fluid flowing through the flow path, a first transducer pair that transmits and receives ultrasonic waves in a direction oblique to the flow, and a second transducer pair that transmits and receives ultrasonic waves along the axis of the flow path And
In the flow rate range above a certain level, the flow rate is measured based on the propagation time measured by the first transducer pair, and in the flow rate range below a certain level, the flow rate is measured based on the propagation time measured by the second transducer pair. An ultrasonic flowmeter characterized by: The cross section of the flow path perpendicular to the flow is substantially rectangular, and the straight line connecting the transducers constituting the first transducer pair is the rectangular length of the cross section of the flow path perpendicular to the flow with respect to the flow axis. The ultrasonic flowmeter according to claim 1, wherein the ultrasonic flowmeter is inclined in a side direction. The ultrasonic flowmeter according to claim 1 or 2, wherein the transmitter / receiver constituting the second transmitter / receiver pair includes means for suppressing the spread of the ultrasonic beam. 3. The ultrasonic flowmeter according to claim 2, wherein the width near the center of the rectangular shape of the flow path cross section perpendicular to the flow is widened.

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