JP2002139358A - Ultrasonic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enlarge difference in propagation time and to enable high-accuracy measurement by increasing the effective length of a sound ray, without changing the length of a measuring pipe line. SOLUTION: A main pipeline 1a having a flow and an auxiliary pipeline 1b having no flow are provided as a pipeline 1, an ultrasonic wave from a vibrator 2 (vibrator 3) passes through the main pipeline 1a having a flow, then passes through the auxiliary pipeline 1b having no flow, and again passes through the main pipe line 1a having a flow to reach the opposite vibrator 3 (vibrator 2), whereby the substantial length of the sound ray is made longer than the conventional length, so that the difference in propagation time is substantially increased to improve the measurement accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an ultrasonic flowmeter for measuring a flow rate or a flow rate of a fluid flowing in a pipeline.

[0002]

2. Description of the Related Art FIG. 4 is for explaining a conventional example.
FIG. 2 shows a cross-sectional view of a sonic flowmeter pipe section.

That is, a pair of ultrasonic transducers 2 and 3 for transmitting and receiving ultrasonic waves by utilizing the electric / mechanical energy conversion effect of a piezoelectric element PZT such as a piezoelectric ceramic are provided upstream and downstream of a pipeline 1. Are installed such that sound rays (lines connecting the centers of the vibrating surfaces of the ultrasonic vibrators 2 and 3) are oblique to the direction of flow, and transmit ultrasonic waves on the one hand and receive ultrasonic waves on the other hand. The operation is performed alternately, the respective propagation times are measured, the flow velocity is obtained from the time difference, and the flow rate is converted from the known pipe cross-sectional area.

[0004] The following is a description using mathematical expressions.

An ultrasonic wave is transmitted from the ultrasonic oscillator 2 on the upstream side, a propagation time until reception by the ultrasonic oscillator 3 on the downstream side is τ ₁ , and an ultrasonic wave is transmitted from the ultrasonic oscillator 3 on the downstream side. And
The propagation time until reception by the ultrasonic transducer 2 on the upstream side is τ
Assuming that ₂ , τ ₁ is the sound speed c + vcos of the ultrasonic wave obtained by increasing the sound ray length L (see FIG. 4) through which the ultrasonic wave propagates in the flow pipe 1 by the sound ray direction component vcos θ of the flow velocity v.
Divided by θ, it is calculated as the following equation (1), and similarly, τ
₂ is calculated by the following equation (2) by dividing the sound ray length L by the sound velocity c-vcos θ of the ultrasonic wave reduced by the sound ray direction component vcos θ of the flow velocity v. Here, as shown in FIG. 4, θ is the angle between the flow direction and the sound ray (ultrasonic driving angle).
Is shown.

Τ ₁ = L / (c + vcos θ) (1) τ ₂ = L / (c−vcos θ) (2) Here, when τ ₂ −τ ₁ is obtained, τ ₂ −τ ₁ = L / ( c−vcos θ) −L / (c + vcos θ) = 2Lvcos θ / (c ² −v ² cos ² θ) ≒ 2Lvcos θ / c ² (3), and the flow velocity v can be obtained from the following equation (4).

V = c ² (τ ₂ −τ ₁ ) / 2Lcos θ (4)

[0008]

As described above, in the ultrasonic flowmeter, since the flow velocity is converted from the measured values of the times τ ₁ and τ _{2 in which} the ultrasonic waves propagate between the ultrasonic transducers, the time measurement of the circuit is performed. Time measurement errors due to the effects of resolution, ambient noise, and the like cause deterioration in measurement accuracy. When there are errors of Δτ ₁ and Δτ _{2 in} the measurement of the propagation times of τ ₁ and τ ₂ , respectively, the flow velocity v is expressed by the following equation (5). v = c ² {(τ ₂ + Δτ ₂ ) − (τ ₁ + Δτ ₁ )} / 2L cos θ = c ² (τ ₂ −τ ₁ ) / 2L cos θ + c ² (Δτ ₂ −Δτ ₁ ) / 2L cos θ (5) The first term of the expression 5) is an expression representing a true value of the flow velocity,
The second term is an expression representing an error. As is clear from this, the error component (Δτ ₂ −Δτ ₁ ) is the flow velocity (or flow rate)
When the flow rate does not change, the lower the flow velocity (low flow rate), the smaller the true value of the flow velocity (τ ₂ −τ ₁ ), and the greater the influence of the error, the lower the measurement accuracy.

One of the means for reducing the error component is (Δ
There is a method of increasing the propagation time difference (τ ₂ −τ ₁ ) compared to (τ ₂ −Δτ ₁ ). In the case where the magnitude of (Δτ ₂ −Δτ ₁ ) is constant, if the length Lcosθ of the sound ray flow direction component is increased from the above equation (3), the propagation time difference (τ ₂ −
Since the ratio of the error is smaller than τ ₁ ), the measurement accuracy can be improved. However, if the length Lcosθ of the sound ray flow direction component is increased, the following problem occurs.

[0010] The pipe becomes longer in the flow direction, and the flow meter detector becomes larger.

The distance over which the ultrasonic wave propagates is long, and the error component (Δτ ₂ −Δτ ₁ ) due to the decrease in S / N due to signal attenuation.
Increase.

Therefore, an object of the present invention is to provide a high-precision ultrasonic flowmeter by suppressing the reduction of S / N without changing the length of the conduit and without increasing the size of the flowmeter detector. It is to enable measurement.

[0013]

In order to solve such a problem, according to the first aspect of the present invention, an ultrasonic flowmeter for measuring a flow velocity or a flow rate of a fluid flowing in a pipe is provided with a main pipe having a flow and a flow path. The ultrasonic wave transmitted from one of at least one pair of ultrasonic vibrators arranged at the upper side and the downstream side of the pipeline, respectively, passes through the main pipeline having the flow. Thereafter, the upper and downstream ultrasonic vibrators are caused to repeatedly perform the operation of passing through the sub flow path without flow, passing through the main flow path with flow again, and reaching the other ultrasonic vibrator. And performing the measurement. In the first aspect of the present invention, the flow path without flow can be formed by partitioning with a sheet-like member which has a high fluid resistance and does not allow fluid to pass therethrough but allows ultrasonic waves to pass therethrough (the second aspect of the invention).

According to the second aspect of the present invention, the sheet-like member is a mesh-like sheet-like member (the invention of the third aspect), or the sheet-like member is provided between the main conduit and the sub conduit. The partition portion may be composed of a blade arranged in parallel with the propagation direction of the ultrasonic wave (the invention of claim 4). In any one of the first to fourth aspects of the present invention, a reflecting portion that guides the ultrasonic waves from a flowing line to a flowing line, and a reflecting portion that guides the ultrasonic wave from a flowing line to a flowing line. (Invention of claim 5).

According to the fifth aspect of the present invention, the reflecting portion has a flat reflecting surface shape, and a pair of ultrasonic vibrators and the sound ray of the ultrasonic wave transmitted from one side are reflected a plurality of times. The reflecting portion may have a reflecting surface inclined with respect to the sound ray so as to reach the center of the other ultrasonic transducer later (the invention of claim 6), or the reflecting portion has a concave reflecting surface shape. In addition, the cross-sectional shape can be an elliptical arc whose focal point is the center of vibration of the ultrasonic vibrator and the center point between the reflection surfaces in the sub-channel where there is no flow (the invention of claim 7).

[0016]

FIG. 1 shows a first embodiment of the present invention, and is a cross-sectional view of an ultrasonic flowmeter pipe section as in FIG.

Here, the pipeline 1 is a main pipeline 1 through which a fluid flows.
a and a sub-channel 1b partitioned by sheets 5a and 5b having high fluid resistance that transmits ultrasonic waves but does not allow fluid to pass through. The ultrasonic waves are reflected by the sub-channel 1b to change its direction. Ultrasonic reflecting portions 6a and 6b are provided.

The flow velocity and the flow rate are measured in the same manner as in the prior art. The propagation time τ ₁ until the ultrasonic wave transmitted from the upstream ultrasonic vibrator 2 is received by the downstream ultrasonic vibrator 3, and the downstream ultrasonic wave By measuring the propagation time τ ₂ until the ultrasonic wave transmitted from the ultrasonic transducer 3 is received by the ultrasonic transducer 2 on the upstream side,
Find the flow velocity and flow rate. In FIG. 1, the ultrasonic wave transmitted from the ultrasonic transducer 2 on the upstream side passes through the main pipeline 1a, and faces the ultrasonic transducer 2 on the upstream side, and the ultrasonic transmission on the downstream side installed in parallel with the flow direction. The sheet passes through the sheet 5b. The ultrasonic wave that has passed through the ultrasonic wave transmitting sheet 5b on the downstream side is
b, and is reflected by the downstream reflecting portion 6b installed in the sub conduit 1b, and the ultrasonic wave is returned in the upstream direction. The ultrasonic wave returning from the downstream side to the upstream side in the sub-line 1b is reflected by the reflecting portion 6a, and the main line 1a installed on the upstream side of the line.
And enters the ultrasonic transmission sheet 5a that separates the sub-channel 1b from the sub-channel 1b, and is received by the ultrasonic transducer 3 installed on the sound ray. In order to enable high-precision measurement while keeping the pipe length unchanged, it is necessary to use the ultrasonic vibrators 2 and 3 to transmit the ultrasonic transmission sheets 5a and 5a.
The sound ray length L up to b is equal to the conventional sound ray length L, and the ultrasonic wave incidence angle θ is also equal to the conventional one.

For the ultrasonic transmission sheets 5a and 5b, if the fluid passing through the flow meter is air, for example, and the frequency is 100 to 1 MHz, the wire diameter is 18 to 30 μm.
A wire mesh having a pitch of 32 to 54 μm and a porosity of about 42%, or a sheet formed by arranging thin metal blades 7 like aluminum foil at a pitch of about 0.5 to 1.5 mm as shown in FIG. Can be considered. FIG. 2 shows a specific example of an ultrasonic transmission sheet in which thin metal blades 7 are arranged, and FIG.
Is a cross-sectional view along the pipeline, and (b) is a front view as viewed from the main pipeline direction. Further, the reflecting portions 6b and 6a are connected to the main pipeline 1a.
Centers 2a, 3a of ultrasonic transducers 2, 3 passing through the inside
Sound lines 4a and 4c respectively connecting the reflection parts 6b and 6a,
A plane perpendicular to a line bisecting the angle formed by the sound ray 4b connecting the points 6b1 and 6a1 where the sound rays 4a and 4c intersect with the reflecting portions 6b and 6a, respectively.

In this way, the time τ ₁ from when the ultrasonic wave is transmitted from the ultrasonic transducer 2 on the upstream side to when it is received by the ultrasonic transducer 3 on the downstream side is τ _1a which passes through the sound ray 4 a. ,
This is the sum of the time τ _1b passing through the sound ray 4b and the time τ _1c passing through the sound ray 4c.

Τ ₁ = τ _1a + τ _1b + τ _1c (6) The time τ _1a and τ _1c passing through the sound rays 4a and 4c is because the sound velocity of the ultrasonic wave is faster by the flow velocity component vcos θ in the sound ray direction. , And the following equations (7) and (8).

Τ _1a = L / (c + vcos θ) τ _1c = L / (c + vcos θ) (7) Further, the sub conduit 1b is partitioned by the ultrasonic transmission sheets 5a and 5b, and the flow velocity is almost zero. The time τ _1b passing through the sound ray 4b is not affected by the sound speed of the sound wave,
Assuming that the length of b is L ′, the following equation (8) is obtained.

Τ _1b = L ′ / c (8) From equations (6) to (8), equation (9) is obtained.

Τ ₁ = 2L / (c + vcos θ) + L ′ / c (9) Similarly, an ultrasonic wave is transmitted from the ultrasonic transducer 3 on the downstream side,
The time τ ₂ until reception by the ultrasonic transducer 2 on the upstream side is
It is expressed as the sum of the time τ _2a passing through the sound ray 4a, the time τ _2b passing through the sound ray 4b, and the time τ _2c passing through the sound ray 4c.

Τ ₂ = τ _2a + τ _2b + τ _2c (10) Times τ _2a and τ _2c passing through the sound rays 4a and 4c, and the sound ray 4b
Since the time tau _2b passing through the following equation (11) the time tau ₂ is the following formula (12).

Τ _2a = L / (c−vcos θ) τ _2c = L / (c−vcos θ) τ _2b = L ′ / c (11) τ ₂ = 2L / (c−vcos θ) + L ′ / c ( 12) From the above, the generated time difference is as shown in the following expression (13) from the expressions (9) and (12). The propagation time difference is doubled with the same pipe length as the conventional one, and the influence of the error is reduced. Measurement with high accuracy.

[0027] _{τ 2 -τ 1 = {2L /} (c + vcosθ) + L '/ c} - {2L / (c-vcosθ) + L' / c} = 4Lvcosθ / (c 2 -v 2 cos 2 θ) ≒ 4Lvcosθ / c ² (13) Further, in the measurement of the propagation time difference, in order to reduce the influence of the change in sound speed due to the temperature, the correction is performed by calculating the sum of the propagation times τ ₁ and τ ₂ . The conversion formula is as follows.

Τ ₂ + τ ₁ = {2L / (c + vcos θ) + L ′ / c} + {2L / (c−vcos θ) + L ′ / c} = 2Lc / (c ² −v ² cos ² θ) + 2L ′ / c ≒ 2 (L + L ′) / c Therefore, the following equation (14) is obtained.

C = 2 (L + L ′) / (τ ₂ + τ ₁ ) (14) FIG. 3 shows a second embodiment of the present invention, and is a cross-sectional view of an ultrasonic flowmeter pipe section as in FIG. It is.

In the first example, since the distance over which the ultrasonic wave propagates is long, the influence of signal attenuation due to the propagation causes
The error component Δτ ₂ −Δτ ₁ may increase. Therefore, in order to reduce such signal attenuation, the reflecting portions 6a and 6b are concave surfaces as shown in the figure, and the cross-sectional shape thereof is an elliptic arc shape that converges ultrasonic waves and reduces attenuation due to diffusion. The focal points of the elliptical arcs 6a and 6b are the vibration centers 2a and 3a of the ultrasonic transducer and the center point 1b of the pipeline in the sub pipeline.
Let it be 1. When the cross section is a concave surface having an elliptical arc shape as compared with the reflection on a plane, the reception sensitivity increases by about 1.3 to 1.5 times with almost one reflection depending on the directivity of the ultrasonic transducer. Therefore, it is expected that the signal will increase due to reflection, thereby reducing the influence of signal reduction due to distance attenuation.

[0031]

According to the present invention, the following effects can be expected.

1) Since the effective sound ray length can be increased without changing the length of the pipeline of the ultrasonic flow meter, the generated propagation time difference increases, and the effect of the time difference error is small and high accuracy is achieved. Measurement of flow velocity and flow rate becomes possible.

2) By providing a concave reflecting portion having an elliptical arc cross section, it is possible to converge ultrasonic waves and reduce diffusion attenuation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a specific example of an ultrasonic transmission sheet.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a configuration sectional view showing a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... pipeline, 1a ... main pipeline, 1b ... sub pipeline, 2 ... upstream ultrasonic oscillator, 3 ... downstream ultrasonic oscillator, 4, 4a, 4b
... Sound rays, 5a, 5b ... Ultrasonic transmission sheets, 6a, 6b
... reflecting part, 7 ... feather.

Claims (7)

[Claims] 1. An ultrasonic flowmeter for measuring a flow velocity or a flow rate of a fluid flowing in a pipe, comprising: a main pipe having a flow and a sub-channel having no flow, which are respectively disposed above and downstream of the pipe. The ultrasonic wave transmitted from one of the at least one pair of ultrasonic vibrators passed through the main line with the flow, then passed through the sub-line without the flow, and again the main line with the flow. An ultrasonic flowmeter characterized in that the upper and lower ultrasonic vibrators repeatedly perform the operation of passing through the second ultrasonic vibrator and reaching the other ultrasonic vibrator, thereby performing measurement. 2. The ultrasonic flowmeter according to claim 1, wherein the non-flowing pipe is formed by a sheet-like member having a large fluid resistance and impervious to fluid but transmitting ultrasonic waves. . 3. The ultrasonic flowmeter according to claim 2, wherein the sheet-like member is a mesh-like sheet-like member. 4. The ultra-thin sheet according to claim 2, wherein the sheet-like member is composed of vanes arranged at a partition between the main pipe and the sub-pipe in parallel with the propagation direction of the ultrasonic wave. Sound flow meter. 5. A reflector which guides the ultrasonic wave from a pipe having a flow to a pipe having no flow, and a reflector which guides the ultrasonic wave from a pipe having no flow to a pipe having a flow. The ultrasonic flowmeter according to claim 1. 6. The reflecting section has a flat reflecting surface shape, and a pair of ultrasonic transducers and a sound ray of an ultrasonic wave transmitted from one of the ultrasonic transducers are reflected by the other ultrasonic transducer after a plurality of reflections. The ultrasonic flowmeter according to claim 5, further comprising a reflecting surface inclined with respect to the sound ray so as to reach a center of the ultrasonic flowmeter. 7. The reflecting section has a concave reflecting surface shape, and its cross-sectional shape is an elliptic arc whose focal point is the vibration center of the ultrasonic vibrator and the center point between the reflecting surfaces in the sub-flow passage without flow. The ultrasonic flowmeter according to claim 5, wherein the ultrasonic flowmeter has a shape.