JP2005180988A - Ultrasonic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly-accurate ultrasonic flowmeter capable making ultrasonic waves propagate approximately in parallel with the pipe axis, and suppressing unwanted waves propagating along the pipe axis. <P>SOLUTION: In this ultrasonic flowmeter, an inflow part 2 and an outflow part 3 are provided on a flow velocity measuring pipe 1, and ultrasonic transducers 4, 5 are mounted respectively on the inflow and outflow parts, so as to be located opposite so that each normal thereof approximately coincides with the axis of the flow velocity measuring pipe, and an internal pipe 12, having the inner diameter which, is smaller than the diameter of the wave transmission/reception face of each ultrasonic transducer and approximately constant in the pipe axis direction is formed coaxially in the flow velocity measuring pipe 12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to an ultrasonic flowmeter that propagates ultrasonic waves into a fluid to be measured and measures the flow velocity and flow rate of the fluid based on the difference in propagation time between the upstream and downstream directions of the flow path of the ultrasonic waves. In particular, the present invention relates to an ultrasonic flowmeter suitable for measuring the flow rate of a gas having a low density such as hydrogen gas.

  Conventionally, an ultrasonic wave is propagated in the fluid in the flow channel to be measured, and the flow velocity (flow rate) of the fluid is utilized by utilizing the difference in propagation time when the ultrasonic wave propagates in the upstream and downstream directions of the flow channel. Ultrasonic flowmeters that measure the pressure have been widely used.

  FIG. 3 is a cross-sectional view showing the configuration of the flow path portion of the conventional typical ultrasonic flowmeter. A fluid inflow portion 22 and an outflow portion 23 are attached to both ends of the linear flow velocity measuring pipe 21, and ultrasonic transducers 24 and 25 are attached to the outer sides of the inflow portion 22 and the outflow portion 23, respectively. The flow velocity of the fluid flowing in the upstream and downstream directions in the flow velocity measurement pipe 21 is measured from the time required for propagation of the ultrasonic waves propagating between the ultrasonic transducers 24 and 25.

  In the conventional ultrasonic flowmeter, since the ultrasonic transducer has a diameter smaller than that of the flow velocity measuring pipe, the ultrasonic wavefront expands while propagating. That is, a component in which the propagation path intersects the central axis of the pipe is generated, and multiple reflection occurs on the inner wall surface. As a result, a large number of propagation paths (multipaths) having different lengths are formed, the rising portion of the received waveform is distorted, the current output time of the received signal becomes unclear, and a detection error occurs. Conversely, if the ultrasonic beam is too narrowed, the flow velocity at a specific location such as the central portion of the channel cross section is detected. In this case, the flow velocity at a specific location must be converted to the average flow velocity in the cross section, which causes an error.

  In JP-A-10-274551 (Patent Document 1) and JP-A-2002-71409 (Patent Document 2), by smoothly changing the inner diameter of the flow velocity measuring pipe in the pipe axis direction, A configuration in which the inner diameter of the part is smaller than the diameter of the transmission / reception surface of the ultrasonic transducer is disclosed.

  Further, in the conventional ultrasonic flowmeter shown in FIG. 3, a part of the ultrasonic waves generated by one of the ultrasonic transducers 24, 25 are in the pipe wall of the inflow part 22, the outflow part 23, and the flow velocity measurement pipe 21. Or, it propagates along the tube wall as a longitudinal wave, a transverse wave, a surface wave, etc., and is received by the other of the ultrasonic transducers 24 and 25. This received signal component becomes an unnecessary interference wave with respect to a desired signal component that is propagated through the fluid in the flow velocity measuring pipe 21 and received, and there is a problem that the accuracy of measurement is lowered.

  If the flow velocity measuring pipe 21, the inflow portion 22, and the outflow portion 23 are made of resin such as epoxy resin instead of metal, the attenuation amount of the ultrasonic signal propagating on the surface and inside thereof increases, and interference by unnecessary waves. Is considerably reduced. However, when the fluid to be measured is a gas having a small atomic number such as hydrogen, if the pipe is made of resin, the fluid leaks, causing a safety and hygiene problem. In order to prevent the leakage of the fluid, there is a restriction that the flow velocity measurement pipe and the like must be made of a special material such as a metal or silica having a dense structure and a small propagation attenuation of sound waves.

  Japanese Laid-Open Patent Publication No. 2002-236042 (Patent Document 3) discloses a configuration in which ultrasonic waves propagating through a tube wall are attenuated by forming vibration damping means on the tube wall between ultrasonic transducers.

JP-A-10-274551 (summary) JP 2002-71409 A (summary) Japanese Patent Laid-Open No. 2002-236042 (FIGS. 1 and 4)

  In the configurations disclosed in Patent Documents 1 and 2, since the inner diameter of the pipe line varies greatly between the both end portions and the central portion, the flow velocity of the fluid varies significantly along the flow direction, As a result, there is a problem that calculation of the flow velocity becomes complicated and causes measurement errors. Accordingly, one of the problems to be solved by the present invention is that an ultrasonic flowmeter with improved accuracy by propagating ultrasonic waves substantially parallel to the tube axis while avoiding a significant change in flow velocity in the flow direction. Is to provide.

  In addition, in the method for attenuating an unnecessary ultrasonic signal propagating inside or on the surface of a tube wall disclosed in Patent Document 3, there is a problem that an attenuation mechanism formed on the conduit is complicated and the manufacturing cost is increased. Accordingly, one of the other problems to be solved by the present invention is to provide an ultrasonic flowmeter equipped with a simple and inexpensive attenuation mechanism capable of suppressing unnecessary ultrasonic signals propagating inside or on the surface of a tube wall. There is to do.

  The ultrasonic flowmeter of the present invention that solves the above-mentioned problems of the prior art has a fluid inflow portion and an outflow portion disposed at both ends of a linear flow velocity measurement pipe, and a wave transmitting / receiving surface at each of the inflow portion and the outflow portion. An ultrasonic transducer is attached so that the normal line of the flow velocity is substantially coincident with the axial line of the flow velocity measuring pipe, and the difference in the propagation time of ultrasonic waves propagating in the upstream direction and the downstream direction in the flow velocity measuring line The flow velocity and the flow rate of the fluid flowing in the flow velocity measuring pipe are measured based on the above. The ultrasonic flowmeter includes an internal pipe that is coaxially formed in the flow velocity measuring pipe and that has an inner diameter that is smaller than the diameter of the transmission / reception surface of the ultrasonic transducer and substantially the same in the pipe axis direction. ing.

  The inner pipe formed coaxially in the flow velocity measurement pipe has an inner diameter smaller than the diameter of the transmission / reception surface of the ultrasonic transducer, so that the inner pipe is ultrasonically distributed in a substantially parallel and uniform energy distribution. Is propagated. For this reason, the average flow velocity in the cross section is detected. Also, since the internal pipe line has almost the same inner diameter in the pipe axis direction, the flow velocity does not change significantly along the flow path, and the measured value itself is the average of the flow direction in the flow path and the cross section. It becomes a flow velocity.

  According to the preferred embodiment of the present invention, the inner wall surface of the inner pipe is formed with a taper whose inner diameter gradually increases toward the end surface, thereby preventing the spread of the emitted ultrasonic waves. It is configured as follows.

  According to another preferred embodiment of the present invention, the ultrasonic transducer comprises a plurality of materials having different acoustic impedances such as a gasket, an O-ring, and the like, with respect to a flange formed at an end of the flow velocity measuring pipe. The ultrasonic wave generated by the transducer is reflected at the boundary surface of the spacer, and the ultrasonic wave is prevented from propagating to the tube wall.

  According to still another preferred embodiment of the present invention, by using a synthetic resin gasket for the bolt for attaching the ultrasonic transducer to the flange, the ultrasonic wave generated in the transducer and propagated through the inside or the surface of the bolt. A sound wave is reflected at a gasket boundary surface made of synthetic resin having different acoustic impedance, and is more effectively prevented from propagating to the tube wall as an unnecessary interference component.

  According to still another preferred embodiment of the present invention, the reception-dedicated ultrasonic sensors installed at two points spaced apart along the flow path of the internal conduit and the reception-dedicated ultrasonic sensors receive signals. A detection unit that detects the time required for propagation by detecting a time difference between the received ultrasonic signals based on the correlation of the waveforms of the received signals, and improves the measurement accuracy by more accurately detecting the propagation time. It is configured as follows.

  According to still another preferred embodiment of the present invention, an acoustic matching layer is formed on each transmission / reception surface of the ultrasonic transducer so that the efficiency of transmission and reception of ultrasonic waves is further improved. Has been.

  FIG. 1 is a diagram showing an overall configuration of an ultrasonic flowmeter according to an embodiment of the present invention. For convenience of illustration, a pipe portion is a cross-sectional view, and a circuit portion is a functional block diagram. In the figure, 1 is a linear flow rate measuring line, 2 is an inflow section, 3 is an outflow section, 4 and 5 are ultrasonic transducers, 6 is a transmission circuit, 7 is a reception circuit, 8 is a control circuit, and 9 is signal processing. Circuits 10 and 11 are reception sensors dedicated to reception. Furthermore, 12 is an internal pipe line, and 13 is a holding part for holding the internal pipe line.

  An inflow portion 2 and an outflow portion 3 for a fluid (hydrogen gas in this embodiment) are formed at both ends of the linear flow velocity measuring pipe 1. Ultrasonic transducers 4 and 5 are attached to the inflow portion 2 and the outflow portion 3, respectively. The ultrasonic transducers 4 and 5 are mounted so as to face each other from the front so that the normal line of each transmission / reception surface substantially coincides with the axis of the flow velocity measuring pipe 1.

  The ultrasonic transducers 4 and 5 are not for transmission and reception but for transmission only. In order to receive the ultrasonic signals emitted from these ultrasonic transducers 4 and 5 in the downstream direction or the upstream direction, the reception sensors 10 and 11 dedicated to reception are kept at an appropriate distance in the flow velocity measuring circuit 1. is set up.

  The internal conduit 12 is formed coaxially in the flow velocity measuring conduit 1. The inner conduit 12 has an inner diameter slightly smaller than the diameter of the transmitting / receiving surfaces of the ultrasonic transducers 4 and 5, so that the ultrasonic waves radiated from the transmitting / receiving surfaces are substantially parallel along the center line of the inner conduit. And with a uniform energy density in the cross section of the internal conduit. The inner wall surface of the internal conduit 12 is formed with a taper whose inner diameter gradually increases toward the upstream and downstream end faces. Since the taper has a gentle slope and the length of the taper portion is small, the inner diameter of the inner pipe 12 and the direction of the flow path are maintained at substantially the same value.

  The closer the tip surface of the internal conduit 12 is to the transmission / reception surfaces of the ultrasonic transducers 4 and 5, the greater the amount of ultrasonic energy radiated from the transmission / reception surfaces can be propagated into the internal conduit. become. However, if this is done, the pressure loss at this approaching location will be excessive, making it unsuitable as a measuring instrument in that it will adversely affect the system under measurement. Therefore, the distal end surface of the internal conduit 12 is not excessively brought close to the transmission / reception surfaces of the ultrasonic transducers 4 and 5, and most of the radiated ultrasonic energy is prevented from spreading and propagated in the internal conduit. A gentle taper is formed for the purpose.

  More specifically, the propagation speed of the ultrasonic waves transmitted from the ultrasonic transducers 4 and 5 and propagating through the hydrogen gas is about 1200 m / sec, which is about 3.5 times the speed of sound in the air. For this reason, the wavelength of an ultrasonic wave also becomes long only by the same magnification. As a result, the directivity angle of the ultrasonic wave becomes large, and it becomes easy to spread in the lateral direction. In order to suppress the lateral expansion, the inner pipe 12 has an inner diameter slightly smaller than the diameter of the transmission / reception surfaces of the ultrasonic transducers 3 and 4, and a taper that gradually increases the inner diameter toward the end face. Is formed. The flow rate of hydrogen gas can be approximated to a substantially constant value in the flow path direction by setting the amount of change in the flow path direction of the inner diameter of the internal conduit 12 to a small value (preferably a value of several percent or less). Therefore, the taper is set to a very gentle value.

  The detailed structure of the ultrasonic transducer 4 is shown in FIG. In FIG. 2, only a portion necessary for the description of the structure is shown in a sectional view. The case 41 made of stainless steel has a structure in which a cylindrical side portion and a disc-shaped tip surface that closes the tip surface of the side portion are integrally formed. A disc-shaped piezoelectric element (PZT) 42 is fixed inside the front end surface of the case 41.

  The front end surface and side portions of the case 41 are covered with a coating 43 of ethylene tetrafluoride (TFE) having a thickness of about 5 mm. A portion of the covering 43 attached to the front end surface of the case 41 functions as an acoustic matching layer. That is, TFE which comprises the coating | cover 43 formed in the front end surface of case 41 is the geometric mean value of the acoustic impedance of the stainless steel which comprises the front end surface of case 41, and the acoustic impedance of the hydrogen gas which is the fluid of measurement object It has an acoustic impedance close to. The thickness of the coating 43 is set to a value of a quarter wavelength of the ultrasonic signal propagating inside the coating. Assuming that the center frequency of the transmitted ultrasonic wave is 100 KHz and the speed of sound during TFE is 2000 m / sec, λ / 4 is approximately 5 mm as described above.

  The side portion of the case 41 where the covering 43 is formed and the flange 14 formed on the pipe line 1 are sealed with a packing 44. Between the flange 14 of the pipe line and the flange 44 formed on the case 41, an acoustic reflection structure is formed in which the front and rear of the gasket 47 made of aluminum alloy are sandwiched between gaskets 45 and 46 made of TFE. Has been.

  That is, the ultrasonic wave generated by the piezoelectric element 42 is transmitted to the flange 44 of the ultrasonic transducer 4 through the front end surface and the side portion of the case 41, and the pipe 44 passes through the gasket 44, the gasket 47, and the gasket 46 from this flange 44. 1 propagates to the flange 14, and further propagates to the pipe wall of the pipe 1. Since the ultrasonic wave propagating through the pipe wall of the pipe line 1 becomes an unnecessary interference wave, it is necessary to improve the measurement accuracy how to suppress the amplitude.

  On the propagation path of the unnecessary ultrasonic signal propagating from the flange 44 of the ultrasonic transducer 4 to the flange 14 of the pipe line 1, gaskets 45 and 46 made of TFE having a significantly different acoustic impedance and an aluminum alloy are used. By alternately disposing the gaskets 47, the ultrasonic waves are reflected at the mutual boundary surfaces, thereby preventing unnecessary components of the ultrasonic waves from being propagated to the pipe wall of the pipe line 1. In order to further prevent the propagation of this ultrasonic wave due to reflection of unnecessary components, a TFE bushing is used for the bolt 48 made of stainless steel, and the interface between the bolt and the bushing with greatly different acoustic impedance. Thus, the ultrasonic waves are reflected, and the amplitude of unnecessary waves received by the reception sensors 10 and 11 through the inside and the surface of the pipeline is suppressed.

  The portion of the covering 43 that covers the side surface of the case 41 is interposed between the flanges 14 and 44 made of stainless steel as well as the side surface of the case 44 made of stainless steel. The reflection of the unnecessary wave component of the ultrasonic wave which is going to propagate from the side surface of the lens to the flanges 14 and 44 is promoted and the propagation is prevented. A portion of the covering 43 covering the side surface of the case 41 simultaneously functions as a vibration absorbing material that replaces vibration energy with heat.

  When receiving the transmission trigger pulse supplied from the control circuit 8, the ultrasonic transducers 3 and 4 dedicated for transmission transmit ultrasonic waves in the flow path. The ultrasonic wave transmitted from the ultrasonic transducer 4 propagates downstream in the hydrogen gas, and the ultrasonic wave transmitted from the ultrasonic transducer 5 propagates upstream in the hydrogen gas.

When receiving a transmission trigger from the control circuit, the receiving circuit 7 receives, filters, amplifies, and supplies the received ultrasonic signals supplied from the receiving sensors 10 and 11 dedicated for reception to the signal processing circuit 9. The signal processing circuit 9 obtains a correlation with the subsequent reception ultrasonic signal while giving a constant delay amount τ to the previous reception ultrasonic signal. The signal processing circuit 9 detects the delay amount τ ₀ given to obtain the maximum correlation value as the time required for propagation of the ultrasonic waves between the receiving sensors 10 and 11 in the downstream direction and the upstream direction, and this propagation requirement is detected. Calculate the flow rate and flow rate of hydrogen gas based on time.

  In this way, when two ultrasonic sensors dedicated to reception are arranged along the flow path and the method of detecting the required propagation time by correlating the received signals of both is used, the measurement accuracy due to the reception waveform breakage etc. Can be effectively prevented, and highly accurate detection can be performed. For further details of such a high-accuracy measurement method, refer to Japanese Patent No. 3368305 filed by the present applicant.

  The ultrasonic flowmeter of the present invention has been described above by way of example when measuring the flow rate of hydrogen gas. However, the measurement target is not limited to hydrogen gas, and may be other appropriate gas or fluid such as liquid.

  In addition, the TFE gasket and the aluminum alloy gasket are interposed as spacers between the flange of the duct and the ultrasonic transducer, and the ultrasonic waves are reflected by the mutual interface to prevent the propagation of the ultrasonic waves. The configuration is illustrated. However, as long as there are a plurality of spacers made of materials having different acoustic impedances, the present invention is not limited to this combination, and a rubber O-ring or the like can be used in combination with a metal gasket.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the whole structure of the ultrasonic flowmeter of one Example of this invention, A sectional view is shown about a pipe line part, and a functional block diagram is shown about a circuit part. It is a fragmentary sectional view which shows the detail of the ultrasonic transducer 4 of FIG. 1, and its attachment part. It is sectional drawing which shows the structure of the pipe line part of the conventional ultrasonic flowmeter.

Explanation of symbols

1 Flow measurement line
2 Inflow section
3 Outflow part
4,5 Ultrasonic transducer
6 Transmitter circuit
7 Receiver circuit
8 Control circuit
9 Signal processing circuit
10,11 Receive sensor
12 Internal pipeline
13 Holding part
41 cases
42 Piezoelectric element (PZT)
43 Coating
44 Flange
45,46 TFE gasket
47 Gasket made of aluminum alloy
48 Tightening bolt with bush
21 Flow velocity measurement line
22 Inlet
23 Outflow part
24,25 Transducer

Claims (7)

An inflow portion and an outflow portion of the fluid are formed at both ends of the linear flow velocity measuring pipe, and the normal line of the transmission / reception surface at each of the inflow portion and the outflow portion substantially coincides with the axis of the flow velocity measuring pipe. The ultrasonic transducers are attached to face each other, and the flow velocity and flow rate of the fluid flowing in the flow velocity measurement pipe are determined based on the difference in the propagation time of the ultrasonic waves propagating in the upstream direction and the downstream direction in the flow velocity measurement line. In the ultrasonic flowmeter to measure,
An ultrasonic flowmeter comprising an inner pipe formed coaxially in the flow velocity measuring pipe and having a diameter that is smaller than the diameter of the wave transmitting / receiving surface of the ultrasonic transducer and substantially the same in the pipe axis direction. . In claim 1,
2. An ultrasonic flowmeter according to claim 1, wherein inner walls of both end portions of the inner pipe are formed with a taper whose inner diameter gradually increases toward each end surface. In each of claims 1 and 2,
The ultrasonic transducer is attached to a flange formed at an end portion of the flow velocity measuring pipe while interposing a plurality of spacers made of materials having different acoustic impedances such as gaskets, O-rings, and the like. Ultrasonic flow meter. In claim 3,
The ultrasonic flowmeter is characterized in that the ultrasonic transducer uses a gasket made of synthetic resin for a bolt attached to the flange. In each of claims 1 to 4,
A reception-only ultrasonic sensor installed at two points spaced along the flow path of the internal conduit;
An ultrasonic wave comprising: a detecting unit that detects the time required for propagation by detecting a time difference between ultrasonic signals received by these dedicated ultrasonic sensors based on a correlation between waveforms of the received signals. Flowmeter. In each of claims 1 to 5,
An ultrasonic flowmeter characterized in that an acoustic matching layer is formed on each transmitting / receiving surface of the ultrasonic transducer. In each of claims 1 to 6,
2. The ultrasonic flowmeter according to claim 1, wherein the fluid is hydrogen, and the material for the flow velocity measuring pipe, the inflow portion, and the outflow portion is metal.

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