JP2984134B2 - Coriolis flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Coriolis flowmeter, and more particularly, to a straight-tube Coriolis flowmeter in which a vibrating tube through which a measurement fluid flows is a straight tube and the straight tube is driven in a high-order vibration mode.

[0002]

2. Description of the Related Art One end or both ends of a flow tube through which a fluid to be measured flows is supported, and when the flow tube is vibrated around the supporting point in a direction perpendicular to the flow direction of the flow tube, a flow tube (hereinafter referred to as a vibrating tube) is provided. 2. Description of the Related Art Mass flow meters (Coriolis flow meters) that utilize the fact that the Coriolis force acting on ()) is proportional to the mass flow rate are well known. The vibrating tube in this Coriolis flowmeter is an important part and determines the characteristics of the flowmeter. The shape as a vibrating tube is roughly classified into a curved tube and a straight tube. The curved tube type can detect mass flow rate with high sensitivity in terms of selecting a shape for effectively extracting Coriolis force, but has a disadvantage that the shape is large. In contrast, a straight pipe type Coriolis flowmeter is designed so that when the center of a straight pipe supported at both ends vibrates in a direction perpendicular to the straight pipe axis, the Coriolis flow between the straight pipe support and the center is The mass flow rate is detected as a displacement difference of the straight pipe due to the force of, i.e., a phase difference signal, and the straight pipe is vibrated in a primary vibration mode in which the supporting portions at both ends are nodes.

FIG. 7 shows a conventional straight tube type Coriolis flowmeter.
FIG. 3 is a diagram for explaining a next vibration mode, in which 30 and 31 are illustrated.
Is a support point, and 32 is a straight pipe.

As shown in the drawing, in a conventional straight tube type Coriolis flow meter, a straight tube 32 is formed by a vibrator (not shown) disposed at a center portion A with a solid line having nodes at support points 30 and 31. It is driven in the primary vibration mode indicated by the dotted line. A phase detector (not shown) is disposed at a position B between the central portion A and the support point 30 and a symmetrical point C with respect to the central portion A of B.
The phase difference based on the Coriolis force is detected by the flow of the measurement fluid to the sample. However, the above-described conventional straight tube type Coriolis flowmeter has the following problems. The Coriolis force is generated in proportion to the vector product of the mass flow rate, which is the integrated value of the flow velocity and the density, and the drive frequency, and the Coriolis force is detected as a phase difference due to the bending displacement of the straight pipe 32. In order to detect a high-sensitivity Coriolis force with a high ratio, it is necessary to reduce the thickness of the straight pipe 32 so as to be easily deformed or to increase the driving frequency.
However, reducing the wall thickness of the straight pipe 32 means lowering the natural frequency, so reducing the wall thickness and increasing the natural frequency give contradictory conditions. Further, as a means for improving the sensitivity, there is a method of increasing the flow rate. However, since the Coriolis force is proportional to the flow velocity, it is necessary to make the flow velocity twice as high in order to obtain twice the sensitivity, and as a result, the pressure loss in the straight pipe increases nearly four times. If the straight pipe 32 is a single pipe, the supporting portions 30 and 31 vibrate in the direction opposite to the vibration direction of the straight pipe 32 in order to balance the mass by vibration. The flow is divided into two parallel straight pipes and vibrated in the direction of approach and separation to achieve mass balance with the flow meter itself. The flow becomes uniform, resulting in poor balance and difficulty in production. In order to solve this problem, a method has been proposed in which a dummy pipe (a rod or a plate) is provided for one straight pipe to balance the straight pipe and the dummy pipe as a pair. In this case, when the density of the measurement fluid changes, only the straight pipe changes in the natural frequency, so that the vibrational balance is lost and the performance is greatly reduced.

[0005]

The present invention has been made in view of the above circumstances, and in a straight tube type Coriolis flowmeter, the pressure loss is increased by setting the excitation frequency to a frequency at which a higher order vibration mode of the straight tube is set. It is an object of the present invention to provide a simple and highly sensitive Coriolis flowmeter at a low cost without causing any problems.

[0006]

In order to achieve the above object, the present invention relates to a Coriolis flowmeter for measuring a flow rate of a fluid by detecting a Coriolis force generated by vibration of a straight pipe through which a measurement fluid flows.
(1) When the length between both ends of the straight pipe is L,
A straight pipe having a length L between the end supports, and L from the one support end of the straight pipe.
Driving means disposed at the position of ３ to vibrate the straight pipe;
It is disposed at a position L / 3 from the other support end of the straight pipe.
An amplitude detector and L / L from one of the support ends of the straight pipe.
Position within the section 2 and symmetric with respect to the position L / 3
And a pair of phase detectors
The driving frequency of the means becomes the secondary vibration mode of the straight pipe
Frequency and vibration of the straight pipe based on the signal of the amplitude detector.
Controlling the amplitude to be constant ; and (2) controlling the straight pipe
When the length between both ends of the support is L, a straight pipe having the length between both ends of the support L, a rotary vibrator disposed at a position of L / 2 of the straight pipe, and one support of the straight pipe Within the section of L / 2 from the end
And symmetrically disposed with respect to the position of L / 3.
A pair of phase detectors, wherein the rotary vibrator is repeatedly driven at a fixed central angle at a rotational speed of a secondary vibration mode around the L / 2 position of the straight pipe, or ,
(3) two straight pipes having the same natural frequency and having a parallel length L;
Support means for supporting both ends of the two straight tubes, the two
Disposed from one support end of the straight tube at the position of L / 3, before
Each of the two straight pipes is formed by the two straight pipes.
Driving means that vibrate with phases opposite to each other in a direction parallel to the plane
And L / 2 of the distance from one of the support ends of the two straight pipes.
Within the section and at a position symmetrical with respect to the L / 3 position
The driving means, comprising a pair of phase detectors disposed
Frequency that is driven at a frequency which is a second-order vibration mode of the straight tube of the two, or, (4) and straight tube of two identical natural frequencies of the parallel length L, a the two At both ends of the straight pipe
A supporting means for supporting, disposed at a position L / 3 from one supporting end of one of the two straight pipes;
In a direction perpendicular to the plane formed by the two straight pipes.
A driving means for vibrating in opposite phases, and any one of the two straight pipes
L / 2 section from one of the support ends, and L / 3
Pair of phase detectors located at symmetrical positions
And driving the driving means at a frequency at which the two straight pipes have a secondary vibration mode. Hereinafter, a description will be given based on an example of the present invention.

FIG. 1 is a cross-sectional view of a Coriolis flowmeter according to the present invention during use, in which 1 is a support cylinder,
Reference numerals 2 and 3 are flanges, 4 is a straight pipe, 5 is an amplitude detector, 6 is a vibrator, 7 and 8 are phase detectors, and 9 and 10 are support parts.

In FIG. 1, reference numeral 4 denotes a straight pipe (in the figure,
It is shown in a curved state for explanation of the operation), and is supported at both ends by the support portions 9 and 10 on both end surfaces of the support tube 1. The straight pipe 4 has a length L between the supports 9 and 10, and is interposed between the flanges 2 and 3 in a flow pipe (not shown), through which a measurement fluid flows. The vibrator 6 is disposed at one end of the straight pipe 4, at a position L / 3 from the support 9 in the drawing. Support part 9
The position from to L / 3 is a position where the amplitude becomes maximum when the doubly supported straight pipe 4 whose both ends are fixed by the support portions 9 and 10 is vibrated at the natural frequency of the second mode. Exciter 6
For example, a rod-shaped magnet (or core) 6a having one end fixed to the straight pipe 4 in the direction perpendicular to the straight pipe axis, and one end fixed to the support cylinder 1 and one end connected to the support member 6c. A core 6a is inserted into the coil 6b. An amplitude detector 5 having the same configuration as the vibrator 6 is fixed to the support cylinder 1 and the straight pipe 4 at a position L / 3 from the support portion 10 at the other end of the straight pipe 4. Further, on both sides of the vibrator 6, phase detectors 7 and 8 comprising magnets and coils having the same configuration principle as the amplitude detector are arranged. The disposition positions of the phase detectors 7 and 8 are sections within the same side L / 2 as the vibrator 6. Excitation 6, amplitude detection
The positions of the detector 5 and the phase detectors 7 and 8 are
It was obtained based on the analysis and research of the

The operation of the Coriolis flow meter according to the present invention having the above-described configuration will be described. The straight pipe 4-2 is attached to the coil 6b of the vibrator 6.
Oscillator (not shown) that supplies AC power at the next mode frequency
Is applied. The natural frequency fn at this time is

[0010]

(Equation 1)

## EQU1 ## Here, Cn is a constant determined by the vibration mode, and C _{1 in the case of the} primary vibration mode.
= 3.507, C ₂ = 9.815 for the _second order, E is Young's modulus of the straight pipe 4, I is the second moment, ρ is the density (including fluid), A is the cross-sectional area, and L is the two-sided This is the length between the support parts 9 and 10. Assuming that the driving conditions of the Coriolis flowmeter within the square root of the equation, such as the length L of the straight pipe 4, are constant, the natural frequency fn of the straight pipe is determined by the order of the vibration mode. That is,
In the second mode vibration, f is 2.8 times that of the first mode vibration.
₂ = the 2.8f _1. This vibration is detected by the amplitude detector 5, and the vibrator 6 is driven by an amplitude control circuit (not shown) so as to have a constant amplitude and a constant second mode vibration. However, in the vibration in the second mode, the position of the vibration exciter and the position of the L / 3 amplitude detector 5 from the support unit 10 are exactly in opposite phases, so that the vibration exciter 6 is connected to the amplitude detector. Alternatively, the amplitude detector 5 may be replaced with the position 5a of the vibrator 6 at the position 6a of FIG. As described above, when the vibration mode is changed from the first order to the second order under the same condition, a 2.8 times higher frequency can be obtained, so that a 2.8 times Coriolis force, that is, a change can be obtained under the same condition. As a result, the SN ratio is greatly improved. In addition, having a high frequency makes it less likely to be affected by a low frequency such as pipe vibration, thereby further improving the SN ratio.

FIG. 2 is a view for explaining the second mode vibration. In the figure, M is the central portion of the straight pipe 4, P and Q are the maximum amplitude positions in the second mode vibration, A, B, C, and D is a position of the phase detector, C is a symmetric position of B with respect to M, D is a symmetric position of B with respect to M, and a central portion M of the straight pipe 4 is a portion serving as a node in the second-order vibration mode. The solid line and the dotted line show the vibration state of the straight pipe 4 having a phase difference of 180 °. The straight pipe 4 undergoes an amplitude displacement based on the inertial force of −Δ on the right side of M and + Δ on the left side of M. As described above, the inertial force acting on one straight pipe is symmetrical in the vertical direction, so that the inertial force acting in the vertical direction is canceled. Although the moment due to the difference in position is generated, there is no problem because the Coriolis flowmeter is rigidly connected to the piping.

FIGS. 3A, 3B and 3C are views for explaining Coriolis force in the secondary vibration mode.
FIG. 3A illustrates the Coriolis force in the process reaching the phase indicated by the solid line in FIG. 2, and FIG. 3B illustrates the Coriolis force in the process reaching the phase indicated by the dotted line in FIG. That
FIG. 3A and FIG. 3B have a 180 ° phase difference.
Note that a vibration exciter 6 or an amplitude detector 5 is provided at the maximum amplitude positions P and Q. Here, the straight tube 4 is + △ displaced position P, at Q position - △ displacement due to the Coriolis force is represented by the curve C _L when displaced.

At the position A of the phase detector 7 in the step of FIG. 3A, the Coriolis force is a force in a direction opposite to the vibration direction, that is, a force directed downward with respect to the line 9-10 connecting the supporting positions, and At the position B of the detector 8, since the vibration of the fluid decreases, an upward Coriolis force acts, and a phase difference occurs between the positions A and B in the drawing. Similarly, a downward force is generated at the C position, an upward force is generated at the D position, and a phase point is generated at the C and D positions. In the step of the opposite phase in FIG. 3B, a phase difference in a direction completely opposite to that in FIG. Therefore, when passing through the line connecting the support positions 9-10 between the phase detectors 7 and 8, a maximum phase difference Δφ occurs as shown in FIG. 3 (c). Since this phase difference is based on the Coriolis force and is an amount proportional to the mass flow rate of the measurement fluid,
The mass flow rate can be detected based on the phase difference signal. Further, a phase point occurs between the A position and the D position and between the B position and the C position, and this phase point can be detected.

FIG. 4 is a view showing another embodiment of the Coriolis flowmeter according to the present invention. In the figure, reference numeral 11 denotes a rotary vibrator, and portions having the same functions as those in FIG. doing.

The illustrated rotary vibration exciter 11 is a driving means for reciprocating the straight pipe 4 in the directions of arrows R and F at a predetermined angle in the central portion M of the straight pipe 4, and its structure is not limited. For example, the rod can be easily driven by fixing a rod of a magnetic material orthogonal to the straight pipe 4 to the center M of the straight pipe 4 and driving the free end of the magnetic material in a circumferential direction by an electromagnet or the like. . According to this method, since the phase detectors 7 and 8 and the rotary vibration exciter 11 can be disposed separately from each other, noise to the phase detectors 7 and 8 can be reduced.

FIGS. 5A and 5B are diagrams for explaining still another embodiment of the Coriolis flow meter according to the present invention.
5A is a cross-sectional view, and FIG. 5B is a view of FIG. 5A as viewed from the flow direction.
4, 15 are straight tubes, 16 is a vibrator, 17 is an amplitude detector, 1
8, 19 are phase detectors and 20, 21, 22, 23 are support positions.

The straight pipes 14 and 15 shown are straight pipes having the same natural frequency of the length L and having both ends fixed to support members 12 and 13 so as to be parallel and flush with each other. In FIG. 13, a straight pipe 14 is supported by 20 and 21 and a straight pipe 15 is supported by 22 and 23, respectively. The vibrator 16, the amplitude detector 17, and the phase detectors 18 and 19 are the same as the vibrator 6, the amplitude detector 5, and the phase detectors 7 and 8 in FIG.
The same dimensional positional relationship is chosen for each.

In the figure, the vibrator 16 is connected to each straight pipe 1.
Driving is performed so that 4 and 15 are in opposite phases at the frequency at which the secondary vibration mode is set. Also at this time, the central portion M becomes a node, and at a certain phase, the straight tubes 14 and 15 are separated on the same surface on the vibrator 16 side as shown by the solid line in FIG.
On the other hand, the vibration becomes closer on the side of the vibrator 16 and on the side of the amplitude detector 17 in the opposite phase (not shown). As a result, the mass balance due to the vibration of the straight pipes 14 and 15 is completely achieved, and no vibration is applied to the external flow pipe, and the external flow pipe is not affected by the external vibration.

FIGS. 6A and 6B are views for explaining still another embodiment of the Coriolis flowmeter according to the present invention.
FIG. 6A is a side view, and FIG. 6B is a front view, in which 24 and 25 are support members and 26 and 27 are straight pipes.

Although the illustration of the vibrator, the amplitude detector, and the phase detector is omitted in the drawing, the solid line has the same structure as that of FIG. 5 and is arranged at the same interval. In the case shown, straight pipes 26 and 27 having the same natural frequency are fixed to support members 24 and 25 on the same surface at both ends, and are the same as those in FIG. 27 is different from the driving direction. That is, the vibration direction of the straight pipes 26 and 27 is
Each straight pipe 2 extends in a direction perpendicular to the plane formed by the straight pipes 26 and 27.
Vibrations are performed in opposite phases in the secondary modes 6 and 27, respectively.
Accordingly, the Coriolis force is detected as a phase difference between positions symmetrical with the position A of L / 3 in a section between the support member 24 and the intermediate point M. In this case, the detection sensitivity of the phase difference signal is twice as high as that of a single tube.

[0022]

As is clear from the above description, the present invention has the following effects. (1) Effects corresponding to the first to fourth aspects: In a straight pipe type Coriolis flowmeter, the drive frequency is vibrated at a frequency that is a higher vibration mode of the straight pipe, so that pressure loss is not increased. A simple and highly sensitive Coriolis flowmeter can be provided at low cost. (2) Effects corresponding to the third and fourth aspects :
Since straight pipes having the same natural frequency are arranged in parallel and driven at the frequency of the secondary vibration mode, mass balance due to vibration can be obtained, the vibration given to the outside is small, the detection sensitivity is doubled,
In addition, the influence of external vibration can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view of a Coriolis flow meter according to the present invention during use.

FIG. 2 is a diagram for explaining second-order mode vibration.

FIG. 3 is a diagram for explaining Coriolis force in a secondary vibration mode.

FIG. 4 is a view showing another embodiment of the Coriolis flow meter according to the present invention.

FIG. 5 is a view for explaining still another embodiment of the Coriolis flowmeter according to the present invention.

FIG. 6 is a view for explaining still another embodiment of the Coriolis flowmeter according to the present invention.

FIG. 7 is a view for explaining a primary vibration mode of a conventional straight tube Coriolis flowmeter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Support cylinder, 2, 3 ... Flange, 4 ... Straight pipe, 5 ... Amplitude detector, 6 ... Exciter, 7, 8 ... Phase detector, 9, 10 ... Support part.

Claims (4)

(57) [Claims] In a Coriolis flowmeter for measuring a flow rate of a fluid by detecting Coriolis force generated by vibration of a straight pipe through which a measurement fluid flows, when a length between both ends of the straight pipe is L, L and a straight pipe both ends supported between the length L, a driving means for vibrating said straight pipe is arranged from one of the support ends at the position of L / 3 of the straight tube, the other support end of said straight pipe / 3, and an amplitude detector disposed at a position of one of the straight pipes.
Within the section of L / 2 from the support end and at the position of L / 3
Consists of a pair of phase detectors disposed at symmetrical positions against, the drive frequency of the drive means and the frequency of the secondary vibration mode of the straight pipe, straight on the basis of a signal of the amplitude detector A Coriolis flowmeter characterized by controlling the vibration amplitude of a pipe to be constant. 2. The vibration of a straight pipe through which a measurement fluid flows causes
Coriolis that measure the flow rate of fluid by detecting the shearing Coriolis force
In the orifice flow meter, the length between both ends of the straight pipe is L.
When the straight tube of the both-ends-supported between length L, a of the straight tube L /
A rotary vibrator arranged at the position 2 and one of the straight pipes
Within the section of L / 2 from the support end and at the position of L / 3
A pair of phase detectors disposed symmetrically with respect to each other , wherein the rotary vibrator is centered on the L / 2 position of the straight pipe,
A Coriolis flowmeter characterized in that it is repeatedly driven at a constant central angle at a rotation speed at which a straight vibration mode of the straight pipe is set to a secondary vibration mode. 3. Two straight pipes having the same natural frequency and having a parallel length L; and support means for supporting both ends of the two straight pipes ;
The two straight pipes are disposed at a position L / 3 from one support end of each of the two straight pipes, and each of the two straight pipes is formed by the two straight pipes.
Drives in opposite directions in the direction parallel to the plane formed
Moving means and L from either one of the two straight pipes.
/ 2 and symmetric with respect to the position of L / 3
And a pair of phase detectors disposed at
A Coriolis flowmeter characterized in that the frequency of the moving means is driven at a frequency that is a secondary vibration mode of the two straight pipes. 4. Two straight pipes having the same natural frequency and having a parallel length L, and supporting means for supporting both ends of the two straight pipes ;
The two straight pipes are disposed at a position L / 3 from one support end of each of the two straight pipes, and each of the two straight pipes is formed by the two straight pipes.
Drives in opposite directions in the direction perpendicular to the plane formed
And motion means, the supporting end of any hand of the two straight tubes L
/ 2 and symmetric with respect to the position of L / 3
And a pair of phase detectors disposed at
A Coriolis flowmeter characterized in that the frequency of the moving means is driven at a frequency that is a secondary vibration mode of the two straight pipes.