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Apparatus and method for providing a flow measurement compensated for entrained gas Number:7,367,240 from the United States Patent and Trademark Office (PTO) owispatent

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Title: Apparatus and method for providing a flow measurement compensated for entrained gas

Abstract: A apparatus 10, 110 is provided that measures the speed of sound and/or vortical disturbances propagating in a fluid or mixture having entrained gas/air to determine the gas volume fraction of the flow 12 propagating through a pipes and compensating or correcting the volumetric flow measurement for entrained air. The GVF meter includes and array of sensor disposed axially along the length of the pipe. The GVF measures the speed of sound propagating through the pipe and fluid to determine the gas volume fraction of the mixture using array processing. The GVF meter can be used with an electromagnetic meter and a consistency meter to compensate for volumetric flow rate and consistency measurement respective, to correct for errors due to entrained gas/air.

Patent Number: 7,367,240 Issued on 05/06/2008 to Gysling,   et al.

Inventors: Gysling; Daniel L. (Glastonbury, CT), Loose; Douglas H. (Southington, CT)
Assignee: CiDRA Corporation (Wallingford, CT)
Appl. No.: 11/656,848
Filed: January 22, 2007

Related U.S. Patent Documents
Application NumberFiling DatePatent NumberIssue Date
10766440Jan., 20047165464
10715197Nov., 2003
60426723Nov., 2002
60441395Jan., 2003
60441652Jan., 2003
60442968Jan., 2003
60503349Sep., 2003
60518171Nov., 2003

Current U.S. Class: 73/861.42 ; 73/61.49
Current International Class: G01F 1/34 (20060101); G01N 29/00 (20060101)
Field of Search: 73/861.42,61.49

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Sonar-Based Volumetric Flow Meter for Pulp and Paper Applications--By: Daiel L. Gysling & Douglas H. Loose--Dec. 3, 2002. cited by other .
Sonar-Based Volumetric Flow Meter for Chemical and Petrochemical Applications--By: Daniel L. Gysling & Douglas H. Loose--Feb. 14, 2003. cited by other .
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Primary Examiner: Patel; Harshad
Parent Case Text


CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 10/766,440, filed on Jan. 27, 2004, now U.S. Pat. No. 7,165,464 which is a continuation in part of U.S. patent application Ser. No. 10/715,197, filed on Nov. 17, 2003, now abandoned which claimed the benefit of U.S. Provisional Application No. 60/426,723, filed Nov. 15, 2002; U.S. Provisional Application No. 60/441,395, filed Jan. 21, 2003, U.S. Provisional Application No. 60/441,652, filed Jan. 22, 2003; U.S. Provisional Application No. 60/442,968, filed Jan. 27, 2003, U.S. Provisional Application No. 60/503,349, filed Sep. 16, 2003; and U.S. Provisional Application No. 60/518,171, filed Nov. 7, 2003, all of which are incorporated herein by reference in their entirety.
Claims


What is claimed is:

1. An apparatus for measuring a parameter of a process flow having entrained gas flowing within a pipe, the apparatus comprising: a first meter including a sensor that provides a measurement signal indicative of a parameter of the flow propagating through the pipe; a second meter including a sensor that provides a phase fraction signal indicative of a gas phase fraction of the process flow; and a processor that determines a compensated measurement signal indicative of the measurement signal compensated for entrained gas in the process flow, in response to the measurement signal and the phase fraction signal.

2. The apparatus of claim 1, wherein the second meter includes at least two strain sensors disposed at different axial locations along the pipe, each of the strain sensors providing a respective strain signal indicative of acoustic pressure disturbances within the pipe at a corresponding axial position, wherein the second meter, responsive to said pressure signals, provides the phase fraction signal.

3. The apparatus of claim 2, wherein the second meter determines the slope of an acoustic ridge in the k-.omega. plane to determine the phase fraction signal.

4. The apparatus of claim 1, wherein the first meter includes at least two strain sensors at different axial locations along the pipe, each of the strain sensors providing a respective strain signal indicative of unsteady pressure disturbances within the pipe at a corresponding axial position, wherein the first meter, responsive to said strain signals, provides a signal indicative of a parameter of the process flow flowing within the pipe.

5. The apparatus of claim 4, wherein the parameter of the process flow is one of velocity of the process flow and/or the volumetric flow of the process flow.

6. The apparatus of claim 4, wherein the first meter determines the slope of a convective ridge in the k-.omega. plane to determine the velocity of the process flow flowing in the pipe.

7. The apparatus of claim 1, wherein the first meter is a volumetric flow meter and the measurement signal is indicative of the volumetric flow rate of the process flow.

8. The apparatus of claim 7, wherein the volumetric flow meter is an electromagnetic flow meter.

9. The apparatus of claim 1, wherein the first meter is a consistency flow meter and the measurement signal is indicative of the consistency of the process flow.

10. The apparatus of claim 9, wherein the consistency meter is a microwave consistency meter.

11. The apparatus of claim 1, wherein the compensated measurement signal is indicative of the volumetric flow rate of the non-aerated portion of the process flow.

12. The apparatus of claim 11, wherein the compensated measurement signal is determine by Q.sub.comp=Q.sub.meas(1-.phi.), where Q.sub.comp is the compensated measurement signal, Q.sub.meas is the measurement signal, and .phi. is the gas phase fraction of the process flow.

13. The apparatus of claim 1, wherein the measurement signal is indicative of the consistency of the process flow flowing in the pipe.

14. The apparatus of claim 1, wherein the compensated measurement signal is indicative of the consistency of the non-aerated portion of the process flow.

15. The apparatus of claim 14, wherein the compensated measurement signal is determine by Q.sub.comp=Q.sub.meas(1-R.phi.), where Q.sub.comp is the compensated measurement signal, Q.sub.meas is the measurement signal, R is a compensation factor, and .phi. is the gas volume fraction of the process flow.

16. The apparatus of claim 15, wherein the compensation factor is approximately 1.4.

17. The apparatus of claim 1, wherein the process flow is one of a liquid having entrained gas, a mixture having entrained gas, a liquid-liquid mixture having entrained gas, a liquid-solid mixture having entrained gas, and a slurry having entrained gas.

18. The apparatus of claim 1, wherein the first meter and second meter have at least one common sensor.

19. The apparatus of claim 1, wherein the second meter measures the speed of an one dimensional acoustic wave propagating through the process flow.

20. The apparatus of claim 1, wherein the second meter includes 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 sensors disposed at different axial locations along the pipe, each of the strain sensors providing a respective strain signal indicative of acoustic pressure disturbances within the pipe at a corresponding axial position, wherein the second meter, responsive to said pressure signals, provides the phase fraction signal.

21. The apparatus of claim 20, wherein the apparatus further includes at least one of a pressure sensor and temperature sensor to respective determine the pressure and temperature of the process flow.

22. A method for measuring a parameter of a process flow having entrained gas flowing within a pipe, the method comprising: receiving a measurement signal indicative of a parameter of the process flow propagating through the pipe; receiving a phase fraction measurement signal indicative of the gas phase fraction of the process flow; and determining a compensated measurement signal indicative of the measurement signal compensated for entrained gas in the process flow, in response to the measurement signal and the phase fraction signal.

23. An apparatus for measuring a parameter of a process flow having entrained gas flowing within a pipe, the apparatus comprising: a first means for providing a measurement signal indicative of a parameter of the flow propagating through the pipe; a second means for providing a phase fraction signal indicative of the gas phase fraction of the process flow; and a third means for determining a compensated measurement signal indicative of the measurement signal compensated for entrained gas in the process flow, in response to the measurement signal and the phase fraction signal.
Description


TECHNICAL FIELD

This invention relates to an apparatus for measuring a flow having entrained gas therein, and more particularly to an apparatus that measures the speed of sound propagating through the flow to determine the gas volume fraction of the gas in the process flow and compensating the output measurement of a flow meter (e.g., a volumetric flow meter and a consistency meter) for entrained gas.

BACKGROUND ART

The present invention provides an apparatus and method of measuring volumetric flow rate and gas volume fraction in slurries used in the paper and pulp industries and in other industries. Slurries commonly used in the paper and pulp industry are mostly water and typically contain between 1% and 10% pulp content by mass. Monitoring the flow rate and consistency of the slurry can lead to improved quality and efficiency of the paper production process.

Processes run in the paper and pulp industry can often, either intentionally or unintentionally, entrain gas/air. Typically, this entrained air results in measurement errors in process monitoring equipment such as volumetric flow measurements and consistency meters.

Industry estimates indicate that entrained air levels of 2-4% are common. Since most process flow monitors are unable to distinguish between air and liquid, interpreting their output as liquid flow rates would result in a overestimate of the liquid by the volumetric flow rate of the air present at the measurement location. Similarly, for the void fraction of the air within the pipe can cause errors in consistency measurements.

Thus, providing a method and apparatus for measuring entrained air in paper and pulp slurries would provide several benefits. Firstly, it would provide a means to screen the output of process instrumentation. Secondly, in addition to screening the measurements, an accurate measurement of the entrained air would provide a means to correct the output of volumetric flow meters and consistency meters. Thirdly, monitoring variations in the amount of entrained air in a given process could be indicative of process anomalies, such a worn bushing or cavitating pumps and/or valves.

Multiphase process flow rate is a critical process control parameter for the paper and pulp industry. Knowing the amounts of liquid, solids and entrained gases flowing in process lines is key to optimizing the overall the papermaking process (Matula, 2000). Unfortunately, significant challenges remain in the achieving accurate, reliable, and economical monitoring of multiphase flow rates of paper and pulp slurries. Reliability challenges arise due the corrosive and erosive properties of the slurry. Accuracy challenges stem from the multiphase nature of the slurries. Economical challenges arise from the need to reduce total life time cost of flow measurement, considering installation and maintenance costs in addition to the initial cost of the equipment.

Currently, there is an unmet need for multiphase flow measurement in the paper and pulp industry. Real time flow measurement is typical restricted to monitoring the total volumetric flow rate in a process line without providing information on the composition of the process mixture. For example, electromagnetic flow meters are the most widely used flow meters in the paper and pulp industry, however they provide no indication of presence of entrained air, with its presence resulting in an over prediction of the volumetric flow of process fluid by the amount of air entrained. Consistency meter provide a measurement of the percentage of solids within the process, however this technology remains more of an art than a science. Furthermore, although entrained air is known to have a large, often deleterious, impact on the paper making process, instrumentation is currently not available to provide this measurement on a real time basis.

The present invention an accurate, reliable multiphase flow measurement in the paper and pulp industry.

In one embodiment of the present invention, the apparatus and method improves the determination of consistency of paper and pulp slurries. Consistency refers to the mass fraction of pulp contained in water and pulp slurries used in the paper making process. Consistency measurements are critical in the optimization of the paper making process. Currently, many companies produce consistency meters employing various technology to serve the paper and pulp industry. Unfortunately, accurate and reliable measurement of consistency remains an elusive objective. Typically, interpreting the output of a consistency meter in terms of actual consistency is more of an art than a science.

Of the various types of consistency meters on the market, microwave based meters may represent the best the solution for many applications. One such microwave-based consistency meter is manufactured by Toshiba. Microwave consistency meters essentially measure speed or velocity the microwave signal propagates through the medium being measured. For example, the speed of the microwave signal through water is approximately 0.1 time the speed of light in a vacuum (c), through air is approximately 1.0 times the speed of light in a vacuum, and through fiber (or pulp) is approximately 0.6 times the speed of light in a vacuum.

The velocity of the microwave signal propagating through the paper pulp slurry is measure by the conductive effects of the slurry, in accordance with the following equation: V=c*sqrt(E)

Where V is the velocity of the microwave signal propagating through the slurry, c is the speed of light in a vacuum, and E is the relative conductivity of the material. Typical values of relative conductivity for material comprising a paper/pulp slurry, for example, are: Water relative conductivity=80; Air relative conductivity=1; and Fiber relative conductivity=3.

These meters typically work well in the absence of entrained air. With entrained air present, the air displaces water and looks like additional pulp fiber to the microwave meter. Thus, uncertainity in the amount of entrained air translates directly into uncertainty in consistency.

SUMMARY OF THE INVENTION

Objects of the present invention include an apparatus having sensor for determining the speed of sound propagating within a pipe for determining the gas volume fraction of a process flow to correct the output of a meter for entrained gas, such as a volumetric flow meter and a consistency meter.

According to the present invention, an apparatus for measuring a parameter of a process flow flowing within a pipe includes a first meter portion and a second meter portion. The first meter portion provides a meter measurement signal indicative of a parameter of the flow propagating through the pipe. The second meter portion includes a sensor for providing sound measurement signal indicative of the speed of sound propagating within the pipe. A processor provides a compensated meter measurement signal indicative of a measurement parameter corrected for entrained gas in the flow propagating through the pipe, in response to meter measurement signal and the sound measurement signal.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus having an array of sensors onto a pipe for measuring the volumetric flow and gas volume fraction of the mixture flowing in the pipe having entrained gas/air therein, in accordance with the present invention.

FIG. 2 is a block diagram of an embodiment of the apparatus of FIG. 1, in accordance with the present invention.

FIG. 3 is a functional flow diagram of an apparatus embodying the present invention that compensates the volumetric flow measurement of a volumetric flow meter, in accordance with the present invention.

FIG. 4 is a block diagram of an apparatus for measuring the speed of sound propagating through a process flow flowing within a pipe, in accordance with the present invention.

FIG. 5 is a plot of Mixture Sound Speed as a function of gas volume fraction for a 5% consistency slurry over a range of process pressures, in accordance with the present invention.

FIG. 6 is a plot of Mixture Sound Speed a function of gas volume fraction for pure water and a 5% consistency slurry at 4 atm process pressure, in accordance with the present invention.

FIG. 7 is a plot of Mixture Sound Speed as a function of gas volume fraction for different consistency slurry over a range of process pressures, in accordance with the present invention.

FIG. 8 is a plot of Mixture Sound Speed a function of entrained air volume fraction for slurry at a process pressure, in accordance with the present invention.

FIG. 9 is a K-w plot for acoustic field within 3 inch pipe containing .about.2% air by volume entrained in water flowing 240 gpm, in accordance with the present invention.

FIG. 10 is a cross-sectional view of a pipe having a turbulent pipe flowing having coherent structures therein, in accordance with the present invention.

FIG. 11 is a block diagram of an apparatus for measuring the vortical field of a process flow within a pipe, in accordance with the present invention.

FIG. 12 a k.omega. plot of data processed from an apparatus embodying the present invention that illustrates slope of the convective ridge, and a plot of the optimization function of the convective ridge, in accordance with the present invention.

FIG. 13 is a block diagram of an apparatus for measuring the vortical field and acoustic field of a process flow within a pipe, in accordance with the present invention.

FIG. 14 is a block diagram of another apparatus for measuring the vortical field of a process flow within a pipe, in accordance with the present invention.

FIG. 15 is a functional flow diagram of an apparatus embodying the present invention that compensates the volumetric flow measurement of an electromagnetic flow meter, in accordance with the present invention.

FIG. 16 is a functional flow diagram of an apparatus embodying the present invention that compensates the consistency measurement of a consistency meter, in accordance with the present invention.

FIGS. 17-19 are configurations for an apparatus in accordance with the present invention.

FIGS. 20-22 are plots of the output of an apparatus embodying the present invention for compensating a microwave consistency meter, in accordance with the present invention.

FIG. 23 is a block diagram of a closed loop system having a microwave consistency meter compensated for entrained gas, in accordance with the present invention.

FIG. 24 is a block diagram of a closed loop system having an electromagnetic flow meter compensated for entrained gas, in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, an apparatus, generally shown as 10, is provided to measure volumetric flow rate and gas volume fraction in liquids and mixtures (e.g. paper and pulp slurries or other solid liquid mixtures) having entrained gas therein (including air). The apparatus 10 in accordance with the present invention determines the speed at which sound propagates within a pipe 14 to measure entrained gas in liquids and/or mixtures 12. To simplify the explanation of the present invention the flow propagating through the pipe will be referred to as a mixture or slurry with the understanding that the flow may be a liquid or any other mixture having entrained gas therein.

The following approach may be used with any technique that measures the sound speed of a fluid. However, it is particularly synergistic with sonar based volumetric flow meters such as described in U.S. Pat. No. 6,889,562 and U.S. Pat. No. 6,609,069, which are incorporated herein by reference, in that the sound speed measurement, and thus gas volume fraction measurement, can be accomplished using the same hardware as that required for the volumetric flow measurement. It should be noted, however, that the gas volume fraction measurement could be performed independently of a volumetric flow measurement, and would have utility as an important process measurement in isolation or in conjunction with other process measurements, which will be described in greater detail hereinafter.

FIG. 2 is a block diagram 1 of the apparatus 10 of FIG. 1 that includes a device 2 for measuring the speed of sound (SOS) propagating within a pipe 14 and a device 3 for measuring the velocity of the mixture 12 within the pipe 14. A pressure sensor 4 and/or temperature sensor 13 measures the pressure and/or temperature of the mixture flowing through the pipe. Alternatively, the pressure and/or temperature may be estimated rather than actually measured. In response to the speed of sound signal 5, the velocity 7 of the flow 12 and characteristics 6 of the flow (e.g., pressure and temperature), a processor 24 determines the gas volume fraction (GVF) of the flow 12, the uncompensated volumetric flow 9 of the mixture, and the volumetric flow 11 of the flow compensated for the entrained air therein.

A flow chart 13 shown in FIG. 3 illustrates the function of the processor 24. As shown in FIG. 2, the inputs to the processor includes the speed of sound (SOS) within the pipe 14, the velocity 7 of the mixture 12, and the pressure and temperature 6 of the mixture. The fluid properties of the mixture (e.g., SOS and density) are determined knowing the pressure and temperature of the mixture. The gas volume fraction of the mixture (GVF) is determined using the SOS measurement and fluid properties, which will be described in greater detail hereinafter. The volumetric flow rate of the mixture (including the entrained gas) is determined using the velocity and knowing the cross-sectional area of the inner diameter of the pipe. The processor 24 provides a compensated volumetric flow measurement of the mixture by correcting the uncompensated volumetric flow rate using the void fraction of the air. For example, correction for void fraction of gas may be as follows, for a no slip, homogeneous flow model: Qair+Qliquid=Qmix Qair=GVFair*Qmix Qliquid=(1-GVFair)Qmix

Other models and corrections may be used to correct for gas volume fraction.

Other information relating to the gas volume fraction in a fluid and the speed of sound (or sonic velocity) in the fluid, is described in "Fluid Mechanics and Measurements in two-phase flow Systems", Institution of mechanical engineers, proceedings 1969-1970 Vol. 184 part 3C, Sep. 24-25 1969, Birdcage Walk, Westminster, London S.W. 1, England, which is incorporated herein by reference.

FIG. 1 illustrates a schematic drawing of an embodiment of the present invention. The apparatus 10 includes a sensing device 16 comprising an array of pressure sensors (or transducers) 18-21 spaced axially along the outer surface 22 of a pipe 14, having a process flow propagating therein. The pressure sensors measure the unsteady pressures produced by acoustical and vortical disturbances within the pipe, which are indicative of the SOS propagating through the pipe and the velocity of the mixture 12. The output signals (P.sub.1-P.sub.N) of the pressure sensors 18-21 are provided to the processor 24, which processes the pressure measurement data and determines gas volume fraction (GVF), the uncompensated volumetric flow rate and the compensated volumetric flow rate, as described hereinbefore.

In an embodiment of the present invention shown in FIG. 1, the apparatus 10 has at least four pressure sensors 18-21 disposed axially along the pipe 14 for measuring the unsteady pressure P.sub.1-P.sub.N of the mixture 12 flowing therethrough. Both measurements are derive by interpreting the unsteady pressure field within the process piping using multiple transducers displaced axially over .about.2 diameters in length. The flow measurements can be performed using ported pressure transducers or clamp-on, strain-based sensors.

The apparatus 10 has the ability to measure the gas volume fraction and volumetric flow rate using one or both of the following techniques described herein below: 1) Determining the speed of sound of acoustical disturbances or sound waves propagating through the flow 12 using the array of pressure sensors 18-21, and/or 2) Determining the velocity of vortical disturbances or "eddies" propagating through the flow 12 using the array of pressure sensors 18-21.

Generally, the first technique measures unsteady pressures created by acoustical disturbances propagating through the flow 12 to determine the speed of sound (SOS) propagating through the flow. Knowing the pressure and/or temperature of the flow and the speed of sound of the acoustical disturbances, the processing unit 24 can determine the gas volume fraction of the mixture, as described and shown in FIG. 3.

The apparatus in FIG. 1 also contemplates providing one or more acoustic sources 27 to enable the measurement of the speed of sound propagating through the flow for instances of acoustically quiet flow. The acoustic sources may be a device that taps on and/or vibrates the wall of the pipe, for example. The acoustic sources may be disposed at the input end of output end of the array of sensors 18-21, or at both ends as shown. One should appreciate that in most instances the acoustics sources are not necessary and the apparatus passively detects the acoustic ridge provided in the flow 12. The passive noise includes noise generated by pumps, valves, motors, and the turbulent mixture itself.

The second technique measures the velocities associated with unsteady flow fields and/or pressure disturbances created by vortical disturbances or "eddies" 118 to determine the velocity of the flow 12. The pressure sensors 18-21 measure the unsteady pressures P.sub.1-P.sub.N created by the vortical disturbances as these disturbances convect within the flow 12 through the pipe 14 in a known manner, as shown in FIG. 10. Therefore, the velocity of these vortical disturbances is related to the velocity of the mixture and hence the volumetric flow rate may be determined, as will be described in greater detail hereinafter.

In one embodiment of the present invention as shown in FIG. 1, each of the pressure sensors 18-21 may include a piezoelectric film sensor to measure the unsteady pressures of the mixture 12 using either technique described hereinbefore.

The piezoelectric film sensors include a piezoelectric material or film to generate an electrical signal proportional to the degree that the material is mechanically deformed or stressed. The piezoelectric sensing element is typically conformed to allow complete or nearly complete circumferential measurement of induced strain to provide a circumferential-averaged pressure signal. The sensors can be formed from PVDF films, co-polymer films, or flexible PZT sensors, similar to that described in "Piezo Film Sensors Technical Manual" provided by Measurement Specialties, Inc., which is incorporated herein by reference. A piezoelectric film sensor that may be used for the present invention is part number 1-1002405-0, LDT4-028K, manufactured by Measurement Specialties, Inc.

Piezoelectric film ("piezofilm"), like piezoelectric material, is a dynamic material that develops an electrical charge proportional to a change in mechanical stress. Consequently, the piezoelectric material measures the strain induced within the pipe 14 due to unsteady pressure variations (e.g., vortical and/or acoustical) within the process mixture 12. Strain within the pipe is transduced to an output voltage or current by the attached piezoelectric sensor. The piezoelectrical material or film may be formed of a polymer, such as polarized fluoropolymer, polyvinylidene fluoride (PVDF). The piezoelectric film sensors are similar to that described in U.S. patent application Ser. No. 10/712,833, publication number 04-0168523, now abandoned, which is incorporated herein by reference.

The apparatus 10 of the present invention may be configured and programmed to measure and process the detected unsteady pressures P.sub.1(t)-P.sub.N(t) created by acoustic waves and/or vortical disturbances, respectively, propagating through the mixture to determine the SOS within the pipe 14 and the velocity of the mixture 12. One such apparatus 110 is shown in FIG. 4 that measures the speed of sound (SOS) of one-dimensional sound waves propagating through the mixture to determine the gas volume fraction of the mixture. It is known that sound propagates through various mediums at various speeds in such fields as SONAR and RADAR fields. The speed of sound propagating through the pipe and mixture 12 may be determined using a number of known techniques, such as those set forth in U.S. patent application Ser. No. 09/344,094, entitled "Fluid Parameter Measurement in Pipes Using Acoustic Pressures", filed Jun. 25, 1999, now U.S. Pat. No. 6,354,147; U.S. patent application Ser. No. 09/729,994, filed Dec. 4, 2002, now U.S. Pat. No. 6,609,069; U.S. patent application Ser. No. 09/997,221, filed Nov. 28, 2001, now U.S. Pat. No. 6,587,798; and U.S. patent application Ser. No. 10/007,749, entitled "Fluid Parameter Measurement in Pipes Using Acoustic Pressures", filed Nov. 7, 2001, each of which are incorporated herein by reference.

In accordance with the present invention, the speed of sound propagating through the mixture 12 is measured by passively listening to the flow with an array of unsteady pressure sensors to determine the speed at which one-dimensional compression waves propagate through the mixture 12 contained within the pipe 14.

As shown in FIG. 4, an apparatus 110 measuring the speed of sound in the mixture 12 has an array of at least two acoustic pressure sensors 115,116, located at two locations x.sub.1,x.sub.2 axially along the pipe 14. One will appreciate that the sensor array may include more than two pressure sensors as depicted by pressure sensors 117,118 at location ,x.sub.3, X.sub.N. The pressure generated by the acoustic waves may be measured through pressure sensors 115-118. The pressure sensors 115-118 provide pressure time-varying signals P.sub.1(t),P.sub.2(t),P.sub.3(t),P.sub.N(t) on lines 120,121,122,123 to a signal processing unit 130 to known Fast Fourier Transform (FFT) logics 126,127,128,129, respectively. The FFT logics 126-129 calculate the Fourier transform of the time-based input signals P.sub.1(t)-P.sub.N(t) and provide complex frequency domain (or frequency based) signals P.sub.1(.omega.),P.sub.2(.omega.),P.sub.3(.omega.),P.sub.N(.omega.) on lines 132,133,134,135 indicative of the frequency content of the input signals. Instead of FFT's, any other technique for obtaining the frequency domain characteristics of the signals P1(t)-PN(t), may be used. For example, the cross-spectral density and the power spectral density may be used to form a frequency domain transfer functions (or frequency response or ratios) discussed hereinafter.

The frequency signals P.sub.1(.omega.)-P.sub.N(.omega.) are fed to array processing unit 138 which provides a signal to line 140 indicative of the speed of sound of the mixture a.sub.mix (discussed more hereinafter). The a.sub.mix signal is provided to map (or equation) logic 142, which converts a.sub.mix to a percent composition of a mixture and provides a % Comp signal to line 44-144 indicative thereof (as discussed hereinafter).

More specifically, for planar one-dimensional acoustic waves in a homogenous mixture, it is known that the acoustic pressure field P(x,t) at a location x along the pipe 14, where the wavelength .lamda. of the acoustic waves to be measured is long compared to the diameter d of the pipe 14 (i.e., .lamda./d>>1), may be expressed as a superposition of a right traveling wave and a left traveling wave, as follows: P(x,t)=(Ae.sup.-ik.sup.r.sup.x+Be.sup.+ik.sup.l.sup.x)e.sup.i.omega.t Eq. 1 where A, B are the frequency-based complex amplitudes of the right and left traveling waves, respectively, x is the pressure measurement location along a pipe 14, .omega. is frequency (in rad/sec, where .omega.=2.pi.f), and k.sub.r, k.sub.l are wave numbers for the right and left traveling waves, respectively, which are defined as:

.ident..omega..times..times..times..times..times..ident..omega..times..tim- es. ##EQU00001## where a.sub.mix is the speed of sound of the mixture in the pipe, .omega. is frequency (in rad/sec), and M.sub.x is the axial Mach number of the flow of the mixture within the pipe, where:

.ident..times. ##EQU00002## where Vmix is the axial velocity of the mixture. For non-homogenous mixtures, the axial Mach number represents the average velocity of the mixture and the low frequency acoustic field description remains substantially unaltered.

The data from the array of sensors 115-118 may be processed in any domain, including the frequency/spatial domain, the temporal/spatial domain, the temporal/wave-number domain or the wave-number/frequency (k-.omega.) domain. As such, any known array processing technique in any of these or other related domains may be used if desired, similar to the techniques used in the fields of SONAR and RADAR.

One such technique of determining the speed of sound propagating through the flow 12 is using array processing techniques to define an acoustic ridge in the k-.omega. plane as shown in FIG. 9. The slope of the acoustic ridge is indicative of the speed of sound propagating through the flow 12. This technique is similar to that described in U.S. Pat. No. 6,587,798 filed Nov. 28, 2001, titled "Method and System for Determining The Speed of Sound in a Fluid Within a Conduit", which is incorporated herein by reference. The speed of sound (SOS) is determined by applying sonar arraying processing techniques to determine the speed at which the one dimensional acoustic waves propagate past the axial array of unsteady pressure measurements distributed along the pipe 14.

The signal processor 24 performs a Fast Fourier Transform (FFT) of the time-based pressure signals P.sub.1(t)-P.sub.N(t) to convert the pressure signal into the frequency domain. The power of the frequency-domain pressure signals are then determined and defined in the k-.omega. plane by using array processing algorithms (such as Capon and Music algorithms). The acoustic ridge in the k-.omega. plane, as shown in the k-.omega. plot of FIG. 9, is then determined. The speed of sound (SOS) is determined by measuring slope of the acoustic ridge. The gas volume fraction is then calculated or otherwise determined, as described hereinafter.

The flow meter of the present invention uses known array processing techniques, in particular the Minimum Variance, Distortionless Response (MVDR, or Capon technique), to identify pressure fluctuations, which convect with the materials flowing in a conduit and accurately ascertain the velocity, and thus the flow rate, of said material. These processing techniques utilize the covariance between multiple sensors 18-21 at a plurality of frequencies to identify signals that behave according to a given assumed model; in the case of the apparatus 10, a model, which represents pressure variations 20 convecting at a constant speed across the pressure sensors comprising the flow meter monitoring head 12.

To calculate the power in the k-.omega. plane, as represent by a k-.omega. plot (see FIG. 9) of eith


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