WO2000003206A1

WO2000003206A1 - Multichord ultrasonic flowmeter

Info

Publication number: WO2000003206A1
Application number: PCT/FR1999/001628
Authority: WO
Inventors: Dominique Certon; Cédric MONOD; Didier Pabois; Frédéric PATAT; Jean-Pierre Remenieras
Original assignee: Faure Herman
Priority date: 1998-07-10
Filing date: 1999-07-06
Publication date: 2000-01-20
Also published as: NO20010139D0; FR2781047A1; JP2002520583A; FR2781047B1; RU2226263C2; EP1097355A1; NO20010139L

Abstract

The invention concerns an ultrasonic measuring device for reducing errors in measurements and the number of transducers, comprising at least three transducers (31, 32, 33) forming at least two sets (31; 32, 33) each including at least one transducer, the sets are mutually offset in the direction of the fluid flow main axis. The invention is characterised in that the radiation diagram of at least one transducer (31) of at least one of the sets covers at least two transducers (32, 33) of the other set. The invention enables to reduce the number of transducers while improving the quality of the measurement. The invention is useful for measuring flow rates and speeds.

Description

MULTI-CORD ULTRASONIC FLOW METER

The present invention relates to a device for measuring the flow of a fluid in the liquid or gaseous phase, and more particularly a device for measuring flow using ultrasonic waves.

Flow measurements are used in metering and instant measurement applications of fluid flows in the liquid or gaseous phase and for example for transactional counting of fluids. The useful information provided by the measurement system can be the instantaneous value or instantaneous flow, the time-averaged value or average flow, or a volume counted between two time marks. Instantaneous or average flow measurements are more particularly used in production processes for which it is necessary to know, control or regulate the flow of one or more fluids involved in the process. The volume measurements, resulting from the integration of the flow measurements over a defined time interval make it possible to carry out counts in particular used for filling and / or emptying of tanks, as well as for transactions in volumes of fluids between customer and supplier. . The value of the volume thus transferred should preferably be precise enough to be used, in particular, in calculating the taxes relating to this transaction.

Furthermore, the flow measurement system is advantageously presented as an autonomous assembly requiring no other input than a source of electrical energy of the storage battery type or standard distribution in alternating voltage 50 or 60 Hz.

In the field of flow measurement in general and of metrology of fluids in particular, numerous techniques using ultrasonic devices have been proposed. Most of its systems use the so-called transit time method. Figure 1 shows a block diagram of a prior art flow meter, of the single cord type. First and second ultrasonic transducers 1 and 2 are arranged on the edge of a pipe 3 through which a fluid flows in a direction symbolized by an arrow 4. In the example of FIG. 1, the pipe is cylindrical with circular section, and the two transducers are arranged on diametrically opposite generators. In addition, the two transducers are offset along the line 2. The line joining the centers of the transducers 1 and 2 is called a cord. It makes an angle θ with the principal axis of the flow, which is the axis of revolution of the pipe in the example of figure 1. The length of the cord is noted L and the inside diameter of the pipe is denoted D. If the first transducer 1 emits an ultrasonic wave, this is detected after propagation by the second transducer 2 at the end of a time offset T ₁₂ called transit time. If it is the speed of sound in the fluid, and

V the average speed of the fluid along the chord, T ₁₂ is given by the relation:

T _{] 2} = L / (c + Vcosθ) (1) By reversing the role of each of the two transducers, the second transducer 2 becoming transmitter and the first transducer 1 receiver, a transit time T _{21 is} measured, which verifies-

T ₂₁ = L / (c - Vcosθ) (2) The combination of relations (1) and (2) makes it possible to obtain an expression of the average speed V along the longitudinal axis of the pipe, eliminating the variable vs. Since we do not necessarily know the celéπté of sound in the fluid with great precision, and as its value depends on the nature, temperature and pressure of this moving medium, we can thus avoid a source of error. The average speed V is then:

V = L. (T2i - T ₁₂ ) / (2cosθ.Ti2T ₂ ι) (3)

The expression of the flow rate, Q, is obtained by multiplying the average speed V, calculated according to (3) by the flow section πD / 4, for a circular section, and taking into account, where appropriate, certain corrective factors discussed above. after

In this formula, Ti and T ₂ are respectively the durations of propagation of the ultrasonic wave in the non-debitant part of the flow, external to the diameter D, for the paths from the first transducer to the second, and from the second transducer to the first. T] and T ₂ are equal except in the particular case where there is a movement in these non-debiting zones These times correspond in particular to the time taken by the ultrasonic wave to pass through the different layers of materials constituting the transducer and the connection zone between the transducer and the fluid vein.

K is the hydraulic coefficient of the ultrasonic flow meter. It is intended to correct the sampling during the measurement. Indeed, the transit time difference principle provides a measure of the average speed of the flow along the measuring string connecting the transducers. This rope is not necessarily representative of the total discharge area. There therefore appears an error in the calculated flow rate which depends on the actual speed profile within the flow section. The coefficient Kh is used to correct this error. This coefficient is generally fixed after a measurement in the laboratory. This type of flow meter has the following disadvantages. Since the flow meter only measures on a single cord, generally diametral as in the example in Figure 1, it cannot guarantee measurement accuracy over a large range of flow rates and Reynolds numbers. The Reynolds number, known to those skilled in the art, makes it possible to describe the degree of turbulence of the flow of a fluid. Figure 2 shows by way of example a cross-sectional view of the flowmeter of Figure 1, with the lines of axial iso-speed for an asymmetrical flow, these lines are formed of points whose axial speed is identical Figure 2 shows an example of flow for which the projection of the measuring string in a plane orthogonal to the axis of the pipe is not a axis of symmetry of the iso-speed lines, the measurement of the flow rate, for such a flow, using a monochord flowmeter, does not take into account the maximum speed of the flow, and provides a measurement value lower than the speed real. Consequently, the flow estimated from this speed measurement is also erroneous. Figure 3 shows another cross-sectional view of the flowmeter of Figure 1, with streamlines; as shown in figure 3, the flow is vortex, with a vortex on each side of the measuring string. Such vortex flows can be generated by single or multiple bends in the pipe leading to the flow meter. As in the case of FIG. 2, the speed or flow measurement supplied by a single-cord flow meter is erroneous.

It has therefore been proposed parallel multi-string systems to minimize these effects of dissymmetry of the axial profile. These systems allow finer and therefore more precise sampling. FIG. 4 shows a cross-section view of a multicord flowmeter of the prior art, using four pairs of transducers, of the type decπt in the document by F Multon, "Measurements of flow rates by ultrasonic method ", La Houille Blanche, n ° 7-1994. The flowmeter of FIG. 4 comprises four pairs of transducers 11 to 18, arranged along the line 20 The transducers are associated in pairs so that each transducer upstream corresponds to a single transducer downstream, whether in direct sight or after reflection on the internal wall of the measuring cuff. Four measuring strings 21 to 24 are thus obtained in parallel, at the pπx of a multiplication of the number of transducers, 8 in the example of FIG. 4, included in the flowmeter. In this type of system, the average flow rate is calculated as the weighted average of the velocities measured along each of the strings This flowmeter makes it possible to correct the erroneous values supplied in the flowmeter of FIG. 1, due to the asymmetry of the flow profile The flowmeter of FIG. 4 makes it possible to provide a more precise value for measuring the speed or the flow rate than those of the type of FIGS. 2 and 3 However, this type of solution implies a multiplication of the number of transducers, and is penalizing from the point of view of cost. In addition, with the number of transducers also increases the cost of the flowmeter, that of maintenance and the frequency of interventions. The number of transducers therefore quickly becomes limiting. In addition, this type of flowmeter does not necessarily make it possible to measure with certainty the flow or the speed under more complex flow conditions, for example vortex or otherwise non-uniform flow profiles.

Finally, measurement systems of the type of those of FIGS. 1 to 4 can present a problem in the case of vortex flows. In such cases, certain non-debitant components of the velocity field will be taken into account by the measurement along a single string, or along each of the parallel strings and will give a systematic error to the measurement result

The invention proposes a solution to the new problem of precise and reliable measurement of the flow rate or of the speed of the vortex flows. It also proposes a solution to the new problem of taking non-debitant components into account when measuring flow or speed. It improves the performance of state-of-the-art flowmeters, by increasing the number of test leads while limiting the number of transducers. More specifically, the invention provides a device for ultrasonic measurement of the displacement of a fluid in a pipe, comprising at least three transducers forming at least two sets of each at least one transducer, the sets being offset from each other in the direction of the main axis of the fluid flow, characterized in that the radiation pattern of at least one transducer from at least one of the assemblies covers at least two transducers from the other assembly.

In one embodiment, the assemblies include the same number of transducers.

It can also be provided that each transducer is in the radiation pattern of the transducers located in its own radiation pattern. Preferably, the transducers of each of the groups are distributed regularly over the periphery of a cross section of the pipe.

In one embodiment, the transducers of one of the groups are images of the transducers of the other group in translation parallel to the flow of the fluid.

Advantageously, the device comprises two pairs of transducers each defining a measuring string, and in that the vector sum of the projections of said strings on the cross section of the pipe is zero.

Preferably, the device comprises a system for stabilizing the flow upstream of the transducer assemblies.

The invention also relates to a method of ultrasonic measurement of the displacement of a fluid in a pipe, using at least three transducers forming at least two sets of each at least one transducer, the sets being offset by one relative to the others in the direction of the main axis of the fluid flow, characterized by the emission of an ultrasonic signal from at least one transducer and by the reception of this signal on at least two transducers of the other set. Other characteristics and advantages of the invention will appear on reading the following detailed description of embodiments of the invention, given by way of example and with reference to the accompanying drawings, which show

- Figure 1 a block diagram of a flow meter of the prior art, of the monocord type;

- Figure 2 a cross-sectional view of the flowmeter of Figure 1, with the lines of axial iso-speed for an asymmetrical flow;

- Figure 3 another cross-sectional view of the flowmeter of Figure 1, with streamlines;

- Figure 4 a cross-sectional view of a multicord flowmeter of the prior art, using four pairs of transducers; - Figure 5 an axial sectional view of the flow meter according to a first embodiment of the invention;

- Figure 6 a cross-sectional view of the flow meter of Figure 5;

- Figure 7 an axial sectional view of the flow meter according to a second embodiment of the invention;

- Figure 8 a cross-sectional view of the flowmeter of Figure 7.

- Figure 9 an axial sectional view of the flow meter according to a third embodiment of the invention;

- Figure 10 a cross-sectional view of the flowmeter of Figure 9; - Figure 11, a view in the plane of a measuring string of the device of Figure 7.

Figures 5 and 6 show a first embodiment of the invention. Figure 5 shows an axial section view of the flow meter of the invention, and Figure 6 shows a cross section view of the flow meter. The flow meter of FIG. 5 comprises three transducers 31, 32, 33, constituting two groups of transducers spaced along the pipe 30; the first group comprises the first transducer 31 and the second group comprises the second and third transducers 32 and 33.

As shown in Figure 6, in projection on the cross section of the pipe, the transducers are regularly distributed over the circumference of the pipe.

According to the invention, the radiation pattern of at least one of the transducers of one of the groups covers at least two of the transducers of the other group. This makes it possible to obtain a ratio between the number of strings and the number of transducers greater than 0.5. This limits the cost of the device. We also limit the number of transducers, without reducing the number of strings; moreover, the strings according to the invention are not parallel; this avoids the problems posed by single-rope flowmeters, without having to multiply the number of strings.

In the flow meter of Figures 5 and 6, the measurement is carried out as follows; a signal is transmitted from the first transducer 31, which is received by the second and third transducers 32 and 33; the transit times T ₁₂ and T1 ₃ are measured; signals are then transmitted successively from the second and third transducers 32 and 33, and the respective transit times T2 ₁ and T31 are measured up to the first transducer 31. Using these transit times, calculates the speed or the flow, using the formulas mentioned above.

FIG. 7 and FIG. 8 show views in axial and cross section of flow meters according to another embodiment of the invention; the flowmeter of these figures comprises six transducers 41 to 46 forming two groups of three transducers each, spaced along the line 40. The transducers of each of the groups of transducers are regularly distributed over the periphery. of driving. The transducers in one group have a radiation pattern that covers at least two of the transducers in the other group. Thus, the radiation pattern of the transducer 41 of the first group covers the transducers 45 and 46 of the second group, ie all the transducers of the second group with the exception of the transducer 44 which is located on the same generator.

The device of FIGS. 7 and 8 makes it possible to generate, with six transducers, six measurement cords. The operation of the device of Figures 7 and 8 is as follows; signals are successively transmitted from each of the transducers of the first group, then from each of the transducers of the second group. The propagation times from each of the transducers to the two other transducers of the other group which are covered by its radiation diagram are thus measured. Determining the durations T ₁₅ and

T ₂ 4 and T ₂₆ , T3 ₄ and T ₃₅ , on the one hand, T ₄ 2 and T ₄ 3, T ₅₁ and T ₅ 3, TOI and Tg ₂ provides using formulas (3) or (4 ) mentioned above, six values of speed or flow; the average of these values gives a more precise and reliable measurement of the speed or the flow rate than the known devices.

The invention makes it possible to reduce the variations of the coefficient Kh as a function of the flow rate. A variation of the coefficient Kh as a function of the flow rate which is less than 0.5% is obtained during the calibration of an apparatus according to the invention, for flow values between 60 and 500 m ^ / h. Figures 9 and 10 show views in axial and cross section of flow meters according to another embodiment of the invention; the flowmeter of these figures comprises twelve transducers 51 to 62 forming two groups of six transducers each, spaced along the pipe 50. The transducers of each of the groups of transducers are regularly distributed over the periphery of the pipe. The transducers of one of the groups have a radiation diagram which covers at least two of the transducers of the other group, and more precisely in the case of the figures, three of the transducers of the other group. Thus, the radiation pattern of the transducer 51 of the first group covers the transducers 59 to 61 of the second group, i. e. the transducers of the second group with the exception of the transducer 57 which is located on the same generator and of the two adjacent transducers. The device of FIGS. 9 and 10 makes it possible to generate, with twelve transducers, eighteen measuring cords. The operation of the device of Figures 9 and 10 is similar to that of Figures 7 and 8, so that it is not necessary to describe it in more detail.

In the case of the devices in FIGS. 7 to 10, each measuring rope corresponds to a so-called crossed rope, which lies in the same plane parallel to the axis of the pipe. FIG. 11 shows the cord between the transducers 41 and 45, in the device of FIGS. 7 and 8, and its crossed cord, between the transducers 42 and 44. The plane of FIG. 11 is the plane containing the four transducers 41, 42 , 44 and 45. The presence of crossed cords makes it possible to avoid the drawback according to known systems: in the case of vortex flows, or complex flows, such as those represented in FIG. 11, certain non-debitant components of the Velocity fields are taken into account by the measurement along a simple cord, such as the cord between the transducers 41 and 45. The measurement along the cord thus gives rise to a systematic error on the result of the measurement. The invention proposes, to remedy this drawback, to carry out the average of the measurement on two strings whose projection is the same on the cross section of the driving, but on which the measurements are taken in opposite directions; in the example of FIG. 11, the strings between the transducers 41 and 45, on the one hand, and between the transducers 42 and 44 on the other hand constitute two strings of this type, here called cross strings. In other words, the projections of these two cords on the cross section of the pipe are opposite vectors. As can be seen in FIG. 11, the errors caused by the non-discharging flows on these two cords are opposite; the averaging of the values measured on the two strings eliminates the error. In the case of FIG. 11, the transducers form a rectangle; insofar as the non-discharging flows remain stable over the measurement length, the transducers could form a trapezoid. We could also, in addition or as a substitute, systematically use a flow stabilizer upstream of the flow meter. This has the effect of reducing the average size of the vortices existing in the fluid and thus of reducing the non-debitant speed component integrated along a rope, and thus makes it possible to avoid the problems of the prior art. The invention thus allows a more precise and more reliable measurement than that of the prior art, by limiting the number of transducers.

Of course, the present invention is susceptible of numerous variations, with respect to the exemplary embodiments described with reference to the figures. Thus, the pipe can be a pipe of any type, size and material used in practice, and does not necessarily have a circular section. Likewise, the position of the transducers on the pipe can vary depending on the desired distribution of the test leads. The choice of the number and the geometry of the transducers, as well as their method of attachment to the pipe, may depend on the type of flow to be measured, that is to say on the nature of the fluid, its temperature, viscosity, speed. , pressure etc. as well as the type and geometry of the pipe, that is to say its diameter, roughness of its walls, presence of bends or any other type of irregularity. Piezoelectric transducers known per se to those skilled in the art may be used.

Finally, the invention has been described with reference to an ultrasonic flow meter; it more generally applies to other types of measurement, for example a speed measurement.

Claims

1.- Device for ultrasonic measurement of the displacement of a fluid in a pipe, comprising at least three transducers (31, 32, 33) forming at least two assemblies 31; 32, 33) of each at least one transducer, the assemblies being offset with respect to each other in the direction of the main axis of the flow of the fluid, characterized in that the radiation pattern of at least one transducer ( 31) of at least one of the assemblies covers at least two transducers (32, 33) of the other assembly.

2.- Device according to claim 1, characterized in that said sets comprise the same number of transducers.

3.- Device according to claim 1 or 2, characterized in that each transducer is in the radiation pattern of the transducers located in its own radiation pattern.

4 - Device according to claim 1, 2 or 3, characterized in that the transducers of each group are distributed regularly over the periphery of a cross section of the pipe.

5.- Device according to one of claims 1 to 4, characterized in that the transducers of one of the groups are images of the transducers of the other group in a translation parallel to the flow of the fluid.

6 - Device according to one of claims 1 to 5, characterized in that it comprises two couples (41, 45; 42, 44) of transducers each defining a measuring string, and in that the vector sum of the projections of say ropes on the cross section of the pipe is zero.

7.- Device according to any one of the preceding claims, characterized by a system of tranquilization of the flow upstream of the transducer assemblies.

8.- Method for ultrasonic measurement of the displacement of a fluid in a pipe, using at least three transducers (31, 32, 33) forming at least two sets of each at least one transducer, the sets being offset from each other in the direction of the main axis of the fluid flow, characterized by the emission of an ultrasonic signal from at least one transducer (31) and by the reception of this signal on at least two transducers (32, 33) of the other set. AMENDED CLAIMS

[received by the International Bureau on 04 November 1999 (04.1 1.99); claims 1-8 replaced by new claims 1-7

(1 page)]

1.- Device for measuring the displacement of a fluid in a pipe, by difference in the transit time of ultrasound between transducers, comprising at least three transducers (31, 32, 33) forming at least two sets (31; 32 , 33) of each at least one transducer, the assemblies being offset with respect to each other in the direction of the main axis of the fluid flow, characterized each assembly comprises six transducers, and in that the radiation diagram d 'at least one transducer (31) from at least one of the assemblies covers at least two transducers (32, 33) from the other assembly.

2.- Device according to claim 1, characterized in that each transducer is in the radiation pattern of the transducers located in its own radiation pattern.

3.- Device according to claim 1 or 2, characterized in that the transducers of each of the groups are distributed regularly over the periphery of a cross section of the pipe.

4.- Device according to claim l, 2 or 3, characterized in that the transducers of one of the groups are images of the transducers of the other group in a translation parallel to the flow of the fluid.

5 - Device according to one of claims l to 4, characterized in that it comprises two couples (41, 45; 42, 44) of transducers each defining a measuring string, and in that the vector sum of the projections of say ropes on the cross section of the pipe is zero.

6.- Device according to any one of the preceding claims, characterized by a system of tranquilization of the flow upstream of the transducer assemblies.

7.- Method for measuring the displacement of a fluid in a pipe, by difference of the transit time of ultrasound between transducers, using transducers (31, 32, 33) forming at least two sets of each six transducers, the assemblies being offset from each other in the direction of the main axis of the fluid flow, comprising the emission of an ultrasonic signal from at least one transducer (31) and by the reception of this signal on at least two transducers (32, 33) of the other set.