# Patent

## Title
Apparatus and method of measuring gas volume fraction of a fluid flowing within a pipe

## Abstract

A clamp on apparatus 10,110 is provided that measures the speed of sound or acoustic disturbances propagating in a fluid or mixture having entrained gas/air to determine the gas volume fraction of the flow 12 propagating through a pipe 14. The apparatus includes an array of pressure sensors clamped onto the exterior of the pipe and disposed axially along the length of the pipe. The apparatus measures the speed of sound propagating through the fluid to determine the gas volume fraction of the mixture using adaptive array processing techniques to define an acoustic ridge in the k-ω plane. The slope of the acoustic ridge 61 defines the speed of sound propagating through the fluid in the pipe.

## Background
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
This application is a continuation application of U.S. patent application Ser. No. 10/762,410, filed on Jan. 21, 2004 now U.S. Pat. No. 7,062,976 that claimed the benefit of U.S. Provisional Patent Application No. 60/528,802 filed Dec. 11, 2003, U.S. Provisional Patent Application No. 60/441,652 filed Jan. 22, 2003; U.S. Provisional Patent Application No. 60/441,395 filed Jan. 21, 2003; which are all incorporated herein by reference.


TECHNICAL FIELD
This invention relates to an apparatus for measuring a flow having entrained gas therein, and more particularly to a clamp on apparatus that measures the speed of sound propagating through the flow to determine the gas volume fraction of the gas in the process.
BACKGROUND ART
The present invention provides a clamp on apparatus and method of measuring gas volume fraction in a process flow or fluid, such as 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 gas volume fraction of a 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, for example, 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. 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 lifetime 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 processing industry, such as 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.
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, uncertainty 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 a device for determining the speed of sound propagating within a pipe to determine the gas volume fraction of a process fluid or flow flowing within a pipe, which can be clamped on or attached to the exterior wall of the pipe.
According to the present invention, an apparatus for measuring the gas volume fraction process flow flowing within a pipe is provided. The apparatus includes at least one sensor for providing a sound measurement signal indicative of the speed of sound propagating within the pipe. A processor determines the gas volume fraction of the flow, in response to the sound measurement signal.
According to the present invention, a method of measuring the gas volume fraction process flow flowing within a pipe comprises measuring the speed of sound propagating within the pipe, and determining the gas volume fraction of the flow, in response to the measured speed of sound.
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  ̃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 piezoelectric film sensor in accordance with the present invention.
FIG. 11 is a top plan view of a piezoelectric film sensor in accordance with the present invention.
FIG. 12 is a cross-sectional view of a portion of the piezoelectric film sensor and clamp, in accordance with the present invention.
FIG. 13 is top plan view of a portion of the multi-band sensor assembly, in accordance with the present invention.
FIG. 14 is a prespective view of an assembled multi-band sensor assembly of FIG. 13, 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 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 through the fluid 12 within a pipe 14 to measure entrained gas in liquids and/or mixtures 12. To simplify the explanation of the present invention, the flow 12 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 flow or speed at which sound propagates through the flow 12. However, it is particularly synergistic with flow meters using sonar-based array processing, such as described in U.S. patent application, Ser. No. 10/007,736 and U.S. patent application Ser. No. 09/729,994, filed Dec. 4, 200, now U.S. Pat. No. 6,609,069, which are incorporated herein by reference. While the sonar-based flow meter using an array of sensors to measure the speed of sound of an acoustic wave propagating through the mixture is shown and described, one will appreciate that any means for measuring the speed of sound of the acoustic wave may used to determine the entrained gas volume fraction of the mixture/fluid.
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 through the flow 12 within a pipe 14. A pressure sensor and/or temperature sensor 3,4 measures the pressure and/or temperature, respective, of the mixture 12 flowing through the pipe. In response to the speed of sound signal 5 and the characteristics 6 of the flow (e.g., pressure and temperature), an entrained gas processing unit 25 determines the gas volume fraction (GVF) of the flow 12. The pressure and temperature sensors enables the apparatus 10 to compensate or determine the gas volume fraction for dynamic changes in the pressure and temperature of the flow 12. Alternatively, the pressure and/or temperature may be estimated rather than actually measured.
A flow chart 13 shown in FIG. 3 illustrates the function of the entrained gas processing unit 25. As shown in FIG. 2, the inputs to the processing unit 25 includes the speed of sound (SOS) 5 within the mixture 12 in the pipe 14, and the pressure and/or temperature 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.
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 one 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 disturbances within the pipe, which are indicative of the SOS propagating through the mixture 12. The output signals (P1-PN) of the pressure sensors 18-21 are provided to the processor 24, which processes the pressure measurement data and determines the speed of sound and gas volume fraction (GVF).
In an embodiment of the present invention shown in FIG. 1, the apparatus 10 has at least pressure sensors 18-21 disposed axially along the pipe 14 for measuring the unsteady pressure P1-PN of the mixture 12 flowing therethrough. The speed of sound propagating through the flow 12 is derived by interpreting the unsteady pressure field within the process piping 14 using multiple transducers displaced axially over  ̃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 by determining the speed of sound of acoustical disturbances or sound waves propagating through the flow 12 using the array of pressure sensors 18-21. While the apparatus of FIG. 1 shows at least four pressure sensors 18-21, the present invention contemplates an apparatus having an array of two or more pressure sensors and having as many as sixteen (16) pressure sensors.
Generally, the apparatus 10 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 source may be a device the taps or vibrates on 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 apparatus 10 of the present invention may be configured and programmed to measure and process the detected unsteady pressures P1(t)-PN(t) created by acoustic waves propagating through the mixture to determine the SOS through the flow 12 in the pipe 14. 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 one embodiment of 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 embodying the present invention has an array of at least three acoustic pressure sensors 115,116,117, located at three locations x1,x2,x3 axially along the pipe 14. One will appreciate that the sensor array may include more than three pressure sensors as depicted by pressure sensor 118 at location xN. The pressure generated by the acoustic waves may be measured through pressure sensors 115-118. The pressure sensors 15-18 provide pressure time-varying signals P1(t),P2(t),P3(t),PN(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 P1(t)-PN(t) and provide complex frequency domain (or frequency based) signals P1(ω),P2(ω),P3(ω),PN(ω) 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 P1(ω)-PN(ω)) are fed to an array processing unit 138 which provides a signal to line 40 indicative of the speed of sound of the mixture amix, discussed more hereinafter. The amix signal is provided to an entrained gas processing unit 142, similar to the processing unit 25, which converts amix to a percent composition of a mixture and provides a gas volume fraction or % Comp signal to line 44 indicative thereof (as discussed hereinafter).
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-ω) 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-ω 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 P1(t)-PN(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-ω plane by using array processing algorithms (such as Capon and Music algorithms). The acoustic ridge in the k-ω plane, as shown in the k-ω 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-ω plane, as represent by a k-ω plot (see FIG. 9) of either the pressure signals, the processor 58 determines the wavelength and so the (spatial) wavenumber k, and also the (temporal) frequency and so the angular frequency ω, of various spectral components of the acoustic waves created passively or actively within the pipe. There are numerous algorithms available in the public domain to perform the spatial/temporal decomposition of arrays of sensor units 18-21.
In the case of suitable acoustic pressures being present, the power in the k-ω plane shown in a k-ω plot of FIG. 9 so determined will exhibit a structure that is called an acoustic ridge 61 associated with sound propagating with the flow and one associated with sound propagating against the flow. The acoustic ridge represents the concentration of the disturbances that propagate with and against the flow and is a mathematical manifestation of the relationship between the spatial variations and temporal variations described above. Such a plot will indicate a tendency for k-ω pairs to appear more or less along a line with some slope, the slope indicating the speed of sound traveling in both directions, as is described in more detail below. The power in the k-ω plane so determined is then provided to a acoustic ridge identifier, which uses one or another feature extraction method to determine the location and orientation (slope) of any acoustic ridge present in the k-ω plane. Finally, information including the acoustic ridge orientation (slope) is used by an analyzer to determine the speed of sound.
The array processing unit 23 uses standard so-called beam forming, array processing, or adaptive array-processing algorithms, i.e. algorithms for processing the sensor signals using various delays and weighting to create suitable phase relationships between the signals provided by the different sensors, thereby creating phased antenna array functionality. In other words, the beam forming or array processing algorithms transform the time domain signals from the sensor array into their spatial and temporal frequency components, i.e. into a set of wave numbers given by k=2π/λ where λ is the wavelength of a spectral component, and corresponding angular frequencies given by ω=2πν.
The prior art teaches many algorithms of use in spatially and temporally decomposing a signal from a phased array of sensors, and the present invention is not restricted to any particular algorithm. One particular adaptive array processing algorithm is the Capon method/algorithm. While the Capon method is described as one method, the present invention contemplates the use of other adaptive array processing algorithms, such as MUSIC algorithm. The present invention recognizes that such techniques can be used to determine speed of sound propagating through the fluid 12.
Also, some or all of the functions within the processor 130 may be implemented in software (using a microprocessor or computer) and/or firmware, or may be implemented using analog and/or digital hardware, having sufficient memory, interfaces, and capacity to perform the functions described herein.
It is within the scope of the present invention that the pressure sensor spacing may be known or arbitrary and that as few as two sensors are required if certain information is known about the acoustic properties of the process flow 12. The pressure sensors are spaced sufficiently such that the entire length of the array (aperture) is at least a significant fraction of the measured wavelength of the acoustic waves being measured. The acoustic wavelength is a function of the type or characteristics of flow 12.
Based on the above discussion, one may use a short length scale aperture to measure the sound speed. For example, the characteristic acoustic length scale is: λ=c/f; where c is the speed of sound in a mixture, f is frequency and λ is wavelength.
If Aperture=L and if L/λ is approx. constant.
Then Lwater/λwater=Lwater*f/Cwater≈LGVF*f/cGVF 
Therefore: LGVF=Lwater (CGVF/Cwater); where GVF is gas volume fraction.
Thus for SOS of water (Cwater=5,000 ft/sec), and SOS of the Gas volume fraction (C GVF=500 ft/sec) and a length aperture of L water=5 ft (which we have shown is sufficient to accurately measure the SOS of water), the length aperture for a gas volume fraction LGVF would be about 0.5 feet.
The entrained gas processing unit 25 assumes a nearly isothermal condition for the flow 12. As such the gas volume fraction or the void fraction is related to the speed of sound by the following quadratic equation:

Ax2+Bx+C=0

wherein x is the speed of sound, A=1+rg/rl*(Keff/P−1)−Keff/P, B=Keff/P−2+rg/rl; C=1−Keff/rl*ameas^2); Rg=gas density, rl=liquid density, Keff=effective K (modulus of the liquid and pipewall), P=pressure, and ameas=measured speed of sound.
Effectively,
Gas Volume Fraction (GVF)=(−B+sqrt(B^2−4*A*C))/(2*A)
Alternatively, the sound speed of a mixture can be related to volumetric phase fraction (φi) of the components and the sound speed (a) and densities (ρ) of the component through the Wood equation.



  
    
      1
      
        
          ρ
          mix
        
        ⁢
        
          a
          
            mix
            ∞
          
          2
        
      
    
    =
    
      
        ∑
        
          i
          =
          1
        
        N
      
      ⁢
      
        
      
      ⁢
      
        
          φ
          i
        
        
          
            ρ
            i
          
          ⁢
          
            a
            i
            2
          
        
      
    
  




  where




  
    
      ρ
      mix
    
    =
    
      
        ∑
        
          i
          =
          1
        
        N
      
      ⁢
      
        
      
      ⁢
      
        
          ρ
          i
        
        ⁢
        
          φ
          i
        
      
    
  



One dimensional compression waves propagating within a mixture 12 contained within a pipe 14 exert an unsteady internal pressure loading on the pipe. The degree to which the pipe displaces as a result of the unsteady pressure loading influences the speed of propagation of the compression wave. The relationship among the infinite domain speed of sound and density of a mixture; the elastic modulus (E), thickness (t), and radius (R) of a vacuum-backed cylindrical conduit; and the effective propagation velocity (aeff) for one dimensional compression is given by the following expression:




  
    
      
        
          a
          eff
        
        =
        
          1
          
            
              
                1
                /
                
                  a
                  
                    mix
                    ∞
                  
                  2
                
              
              +
              
                
                  ρ
                  mix
                
                ⁢
                
                  
                    2
                    ⁢
                    R
                  
                  Et
                
              
            
          
        
      
    
    
      
        (
        
          eq
          ⁢
          
            
          
          ⁢
          1
        
        )
      
    
  




Note: “vacuum backed” as used herein refers to a situation in which the fluid surrounding the pipe externally has negligible acoustic impedance compared to that of the mixture internal to the pipe 14. For example, meter containing a typical water and pulp slurry immersed in air at standard atmospheric conditions satisfies this condition and can be considered “vacuum-backed”.
The mixing rule essentially states that the compressibility of a mixture (1/(□a2)) is the volumetrically-weighted average of the compressibilities of the components. For gas/liquid mixtures 12 at pressure and temperatures typical of paper and pulp industry, the compressibility of gas phase is orders of magnitudes greater than that of the liquid. Thus, the compressibility of the gas phase and the density of the liquid phase primarily determine mixture sound speed, and as such, it is necessary to have a good estimate of process pressure to interpret mixture sound speed in terms of volumetric fraction of entrained gas. The effect of process pressure on the relationship between sound speed and entrained air volume fraction is shown in FIG. 5.
Conversely, however, detailed knowledge of the liquid/slurry is not required for entrained air measurement. Variations in liquid density and compressibility with changes in consistency have a negligible effect on mixture sound speed compared to the presence of entrained air. FIG. 6 shows the mixture sound speed as a function of entrained air volume fraction for two slurries, one with 0% wood fiber and the other with 5% wood fiber by volume. As shown, the relationship between mixture sound speed and gas volume fraction is essentially indistinguishable for the two slurries. Furthermore, mixture sound speed is shown to an excellent indicator of gas volume fraction, especially for the trace to moderate amounts of entrained air, from 0 to 5% by volume, typically encountered in the paper and pulp industry.
For paper and pulp slurries, the conditions are such that for slurries with non-negligible amounts of entrained gas, say <0.01%, the compliance of standard industrial piping (Schedule 10 or 40 steel pipe) is typically negligible compared to that of the entrained air.
FIGS. 7 and 8 above show the relationship between sound speed and entrained air for slurries 12 with pulp contents representative of the range used in the paper and pulp industry. Referring to FIG. 7, two slurry consistencies are shown; representing the lower limit, a pure water mixture is considered, and representing the higher end of consistencies, a 5% pulp/95% water slurry is considered. Since the effect of entrained air on the sound speed of the mixture is highly sensitive to the compressibility of the entrained air, the effect of the entrained air is examined at two pressures, one at ambient representing the lower limit of pressure, and one at 4 atmospheres representing a typical line pressure in a paper process. As shown, the consistency of the liquid slurry 12, i.e., the pulp content, has little effect on the relationship between entrained air volume fraction and mixture sound speed. This indicates that an entrained air measurement could be accurately performed, within 0.01% or so, with little or no knowledge of the consistency of the slurry. The chart does show a strong dependence on line pressure. Physically, this effect is linked to the compressibility of the air, and thus, this indicates that reasonable estimates of line pressure and temperature would be required to accurately interpret mixture sound speed in terms of entrained air gas volume fraction.
FIG. 7 also shows that for the region of interest, from roughly 1% entrained air to roughly 5% entrained air, mixture sound speeds (amix) are quite low compare to the liquid-only sound speeds. In this example, the sound speed of the pure water and the 5% pulp slurry were calculated, based on reasonable estimates of the constituent densities and compressibilities, to be 1524 m/s and 1541 m/s, respectively. The sound speed of these mixtures with 1% to 5% entrained air at typical operating pressure (1 atm to 4 atms) are on the order of 100 m/sec. The implication of these low sound speed is that the mixture sound speed could be accurately determined with a array of sensors, ie using the methodology described in aforementioned U.S. patent application Ser. No. 10/007,749, with an aperture that is similar, or identical, to an array of sensors that would be suitable to determine the convection velocity, using the methodology described in aforementioned U.S. patent application Ser. No. 10/007,736, which is incorporated herein by reference.
For the sound speed measurement, the apparatus 110 utilizes array processing algorithms. The temporal and spatial frequency content of sound propagating within the process piping is related through a dispersion relationship.




  k
  =
  
    ω
    
      a
      mix
    
  




As before, k is the wave number, defined as k=2π/λ, ω is the temporal frequency in rad/sec, and amix is the speed at which sound propagates within the process piping. Unlike disturbances, which convect with the flow, however, sound generally propagates in both directions, with and against the mean flow. For these cases, the acoustic power is located along two acoustic ridges, one for the sound traveling with the flow at a speed of amix+Vmix and one for the sound traveling against the flow at a speed of amix−Vmix.
FIG. 9 shows a k-ω plot generated for acoustic sound field recorded from water flowing at a rate of 240 gpm containing  ̃2% entrained air by volume in a 3 in, schedule 10, stainless steel pipe. The k-ω plot was constructed using data from an array of strain-based sensors attached to the outside of the pipe. Two acoustic ridges are clearly evident. Based on the slopes of the acoustic ridges, the sound speed for this for this mixture was 330 ft/sec (100 m/s), consistent with that predicted by the Wood equation. Note that adding 2% air by volume reduces the sound speed of the bubbly mixture to less than 10% of the sound speed of single phase water.
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, which is incorporated herein by reference.
FIGS. 10 and 11 illustrate a piezoelectric film sensor (similar to the sensor 18 of FIG. 1), wherein the piezoelectric film 32 is disposed between and pair of conductive coatings 34,35, such as silver ink. The piezoelectric film 32 and conductive coatings 34,35 are coated onto a protective sheet 36 (e.g., mylar) with a protective coating 38 disposed on the opposing side of the upper conductive coating. A pair of conductors 40,42 is attached to a respective conductive coating 34,35.
The thickness of the piezoelectric film 32 may be in the range of 8 um to approximately 110 um. The thickness is dependent on the degree of sensitivity desired or needed to measure the unsteady pressures within the pipe 14. The sensitivity of the sensor 30 increases as the thickness of the piezoelectric film increases.
The advantages of this technique of clamping the PVDF sensor 30 onto the outer surface of the pipe 14 are the following:

    
    
        1. Non-intrusive flow rate measurements
        2. Low cost
        3. Measurement technique requires no excitation source. Ambient flow noise is used as a source.
        4. Flexible piezoelectric sensors can be mounted in a variety of configurations to enhance signal detection schemes. These configurations include a) co-located sensors, b) segmented sensors with opposing polarity configurations, c) wide sensors to enhance acoustic signal detection and minimize vortical noise detection, d) tailored sensor geometries to minimize sensitivity to tube modes, e) differencing of sensors to eliminate acoustic noise from vortical signals.
        5. Higher Operating Temperatures (125C) (co-polymers)
    
    


As shown in FIG. 12, the piezoelectric film sensor 30 is adhered or attached to a strap 72 which is then clamped (or strapped) onto the outer surface of the pipe 14 at each respective axial location, similar to that described in U.S. Provisional Application No. 60/425,436, filed Nov. 12, 2002; and U.S. Provisional Application No. 60/426,724, which are incorporated herein by reference.
As shown in FIG. 12, the piezoelectric film sensor 30 is attached to the outer surface 73 of the strap in relation to the pipe 14. The conductive insulator 36 is attached to the outer surface of the strap by double side tape or any other appropriate adhesive. The adhesive is preferably flexible or compliant but minimizes creep between the strap and piezoelectric film sensor during the operation of the sensor 30. The length of the strap is substantially the same as the circumference of the pipe 14. The piezoelectric film sensor may extend over the substantial length of the strap or some portion less than the strap. In the embodiment shown in FIG. 12, the piezoelectric film sensor 30 extends substantially the length of the strap 72 to provide a circumferentially averaged pressure signal to the processing unit 24.
Referring to FIG. 12, an attachment assembly 75 comprising a first attachment block 76, a second attachment block 77 and a spacer 78 disposed therebetween, which are welded together to provide slots 79 between each of the attachment blocks and the spacer. The slots receive respective ends of the strap 72 to secure the ends of the strap together. One end of the strap 72 and the pair of conductors 40,42 are threaded through the slot disposed between the first attachment block and the spacer. The strap 72 and conductors 40,42 are secured to the attachment assembly by a pair of fasteners 80. The other end of the strap is threaded through the slot disposed between the spacer and the second attachment block. The other end of the strap is pull tightly between the spacer and the second attachment to draw up and take-up the tension and securely clamp the strap to the pipe 14. A set screw within the second attachment block is tighten, which then pierces the other end of the strap to secure it to the attachment assembly. The excess portion of the other end of the strap is then cut off. The piezoelectric film sensor may then be covered with a copper sheet to provide a grounding shield for EMI or other electrical noise.
While the piezoelectric film sensor 30 was mounted to the outer surface of the straps 72, the present invention contemplates the piezoelectric film sensor may be mounted to the inner surface of the strap, thereby resulting in the piezoelectric sensor being disposed between the strap and the outer surface of the pipe 14.
The present invention also contemplates that the piezoelectric film sensors 30 of FIGS. 10 and 11 may be mounted directly onto the outer diameter of the pipe 14 by epoxy, glue or other adhesive.
FIGS. 13 and 14 show another embodiment of the sensing device 16 (or sensor head) mounted to a pipe 14 having fluid 12 flowing therethrough, similar to that described in U.S. patent application Ser. No. 10/795,111, filed on Mar. 4, 2004, which is incorporated herein by reference. The sensor head 16 includes a multi-band sensor assembly 40 wrapped and mounted to the outer surface 22 of the pipe 14. The multi-band sensor assembly 40 of the sensor head 16 includes a strip of piezoelectric film 50 attached to each band 44 of a multi-band strap 52. The multi-band strap 52 is formed of a single sheet of metallic material (e.g., stainless steel) by stamping or punching voids into the sheet material. The multi-band strap 58 includes a plurality of bands 44 that are spaced a predetermined distance apart. In the embodiment shown, the bands are equi-spaced, however, the present invention contemplates that the straps may be disposed at different spacings. In one embodiment, the spacing is approximately 40% of the diameter of the pipe 14.
The type of unsteady pressure measurement being made (SOS v. Vortical Disturbances) determines the spacing of the sensors 18-21. Measurement of unsteady vortical pressures prefers the array of sensors to be spaced such that the length of the array is less than the coherence length of the vortical disturbances which is typically on the order of a pipe diameter. Measurement of the acoustic pressures prefers the array of sensors to be space such that the length of the array of sensors 18-21 is as long as a predetermined portion of the wavelength of the measured acoustic signal (e.g., greater than 20% of the wavelength of the acoustic signal). The desired wavelength of the measured acoustic signal is dependent upon the dispersion of particles in the fluid flow, which is dependent on the particle size, such as that described in U.S. patent application Ser. No. 10/349,716, filed Jan. 23, 2003 and U.S. patent application Ser. No. 10/376,427, filed Feb. 26, 2003, which are all incorporated by reference.
The multi-band strap 52 also includes a plurality of cross members 62 spaced along the length of the bands44 to maintain the spacing between the bands over their lengths. The respective ends of the bands are also interconnected by opposing end strips 61. The cross members 62 are formed in the shape of an X, however, the invention contemplates that the cross members may be in the form of straight members extending perpendicular between the bands 44 or diagonal to the bands. These diagonal members may be angled in the same direction or different directions. The cross members 62 advantageously provide that the sensors are properly spaced apart and maintained at the proper distance during the mounting of the sensor assembly 40 to the outer surface 22 of the pipe 14. The interconnection of the bands 44 also permits all the sensors 18-21 to be mounted to the pipe 14 simultaneously and thus reduces the time of mounting the sensor assembly 16 to the pipe. The unitary multi-band strap 52 ensures the sensors 18-21 are properly space.
The present invention also contemplates the multi-band strap 52 may simply comprise a single sheet of metallic material without cut outs to define individual bands 44, however, when mounted to the pipe, the sheet may not uniformally contact the surface 22 of the pipe.
Referring to FIG. 13, each piezoelectric film 50 is mounted, which will be described in greater detail hereinafter, along the length of a respective band 44 of the sensor strap 52. The electrodes 70 of each piezoelectric film 50 are electrically connected (e.g., soldered) to a flexible circuit board 72 mounted along the length at one end strip 61 of the multi-band strap 52. The circuit board 72 is secured to the multi-band strap 52 by a plurality of tabs 76 that are welded or otherwise attached to the multi-band strap. The piezoelectric film 50 wraps from below the circuit board 72 to the top of the circuit board, where it is soldered thereto. The electrical runs on the circuit board 72 interconnect each piezoelectric film 50 to the electrical cable 66 at location 78. The electrical cable 41 interconnects with a pre-amplifier unit adjacent an access window of a cover mounted over the sensor assembly and to the pipe.
The multi-band sensor assembly 40 is wrapped around the pipe 14 and the ends are attached to each other by a pair of stiffening rails 46. The stiffening rails 46 are attached (e.g., welded) to the ends of the multi-band strap 52 of the sensor assembly 40. The rails 46 extend the length of the end strips 61 of the multi-band strap 52. As shown, the ends of the multi-band strap 52 are bend to engage the inner surface of the rails 46. The bent ends of the multi-band strap 52 are then welded to the inner surface of each respective rod 46. While the multi-band strap 52 is welded to the rails, other fastening means may be used, such as bolts and screws.
When mounting or clamping the sensor assembly 40 to the pipe 14, the ends of the sensor assembly 40 are secured together by bolts or screws 54, or other fasteners, which interconnect the stiffening rods 46. To insure proper alignment of the rails 46, one rail may include a guide pin and the other rail a hole for receiving the pin. As best shown in FIG. 9, a spring 56 may be disposed on the bolts to provide constant tension on the rails 46.
While the rails 46 are shown to be one continuous rail, the present invention contemplates that each rail may comprise a plurality of shorter rails disposed at the end of each band 44, effective providing a split rail. Similar to that described, each of the shorter rails opposing each other are bolted together to secure the sensor assembly 40 to the pipe 14. This split rail (i.e., plurality of shorter rails) configuration isolates each band 44 from the others an thus permits each band 44 to more uniformally engage the pipe 14 with out the stress and influence of the clamping of the other bands created by the single unitary rail 46.
As shown in FIG. 14, the sensor assembly 40 includes a shield 43, dispose around the outside of the multi-band strap 52 to provide a grounding shield. The grounding shield may comprise metallic sheet material, screen or web. In one embodiment, the shield 42 is attached to the sensor assembly 40 by welding one end of the shield to one end of multi-band strap 52. The shield 42 wraps around the sensor assembly 40 to the opposing end thereof. The opposing end of the shield 42 includes a pair of through holes or windows for receiving bend tabs, which are integral to the multi-band strap 52. The bend tabs temporarily support the shield in place to enable the attachment of the cable ties 48 around the shield.
In a addition to the metallic grounding shield 46, a sheet of polyimide material 86 or other suitable non-conductive material is secured to the inner surface of grounding shield, such as by rivets. The polyimide material 86 (e.g., Kapton) provides an electrically insulative barrier between the piezoelectric film and the shield 46. Further, the polyimide material provides a water barrier for the piezoelectric film 50 should any water or moisture pass through the shield 46, particularly shield in the form of a screen or web.
Alternatively, the shield 46 may be secured, such as by welding to both ends of the multi-band strap 52. This method is particularly suited for shields that are in the form of a web or screen, and therefore flexible.
Referring to FIG. 13, the sensor assembly 40 includes a plurality of stand-offs 81 disposed at the outer edges of the multi-band strap 51 and between each of the bands 44. The stand-offs extend substantially the length of the bands 44. The stand-offs are formed of a flexible foam-like material having a thickness great than the thickness of the piezoelectric film 50 to ensure the shield 46 and polyimide sheet does not contact the piezoelectric film when the sensor assembly 40 is clamped/mounted onto a pipe 14.
Advantageously, the single strap 40 having multiple bands 44, each of which having a PVDF sensor 50 mounted thereon, allows the sensor spacing to be set at the time of manufacture to thereby eliminate the positioning and measuring at the time of installation. Further, the single strap 40 allows more accurate positioning (spacing) of the sensors 18-21 than can be attained in a field installation. The single strap 52 also provides a more time efficient installation technique over installing individual bands 44.
While six sensors have been shown, one will appreciate that sensor assembly 40 may have any number of PVDF sensors 50, including as few as two sensors and more than six sensors, such as 8, 16 or more sensors.
Another embodiment of the present invention include a pressure sensor such as pipe strain sensors, accelerometers, velocity sensors or displacement sensors, discussed hereinafter, that are mounted onto a strap to enable the pressure sensor to be clamped onto the pipe. The sensors may be removable or permanently attached via known mechanical techniques such as mechanical fastener, spring loaded, clamped, clam shell arrangement, strapping or other equivalents. These certain types of pressure sensors, it may be desirable for the pipe 12 to exhibit a certain amount of pipe compliance.
Instead of single point pressure sensors 18-21, at the axial locations along the pipe 12, two or more pressure sensors may be used around the circumference of the pipe 12 at each of the axial locations. The signals from the pressure sensors around the circumference at a given axial location may be averaged to provide a cross-sectional (or circumference) averaged unsteady acoustic pressure measurement. Other numbers of acoustic pressure sensors and annular spacing may be used. Averaging multiple annular pressure sensors reduces noises from disturbances and pipe vibrations and other sources of noise not related to the one-dimensional acoustic pressure waves in the pipe 12, thereby creating a spatial array of pressure sensors to help characterize the one-dimensional sound field within the pipe 12.
The pressure sensors 18-21 of FIG. 1 described herein may be any type of pressure sensor, capable of measuring the unsteady (or ac or dynamic) pressures within a pipe 14, such as piezoelectric, optical, capacitive, resistive (e.g., Wheatstone bridge), accelerometers (or geophones), velocity measuring devices, displacement measuring devices, etc. If optical pressure sensors are used, the sensors 18-21 may be Bragg grating based pressure sensors, such as that described in U.S. patent application Ser. No. 08/925,598, entitled “High Sensitivity Fiber Optic Pressure Sensor For Use In Harsh Environments”, filed Sep. 8, 1997, now U.S. Pat. No. 6,016,702, and in U.S. patent application Ser. No. 10/224,821, entitled “Non-Intrusive Fiber Optic Pressure Sensor for Measuring Unsteady Pressures within a Pipe”, which are incorporated herein by reference. In an embodiment of the present invention that utilizes fiber optics as the pressure sensors 14 they may be connected individually or may be multiplexed along one or more optical fibers using wavelength division multiplexing (WDM), time division multiplexing (TDM), or any other optical multiplexing techniques.
In certain embodiments of the present invention, a piezo-electronic pressure transducer may be used as one or more of the pressure sensors 15-18 and it may measure the unsteady (or dynamic or ac) pressure variations inside the tube 14 by measuring the pressure levels inside of the tube. These sensors may be ported within the pipe to make direct contact with the mixture 12. In an embodiment of the present invention, the sensors 14 comprise pressure sensors manufactured by PCB Piezotronics. In one pressure sensor there are integrated circuit piezoelectric voltage mode-type sensors that feature built-in microelectronic amplifiers, and convert the high-impedance charge into a low-impedance voltage output. Specifically, a Model 106B manufactured by PCB Piezotronics is used which is a high sensitivity, acceleration compensated integrated circuit piezoelectric quartz pressure sensor suitable for measuring low pressure acoustic phenomena in hydraulic and pneumatic systems. It has the unique capability to measure small pressure changes of less than 0.001 psi under high static conditions. The 106B has a 300 mV/psi sensitivity and a resolution of 91 dB (0.0001 psi).
The pressure sensors incorporate a built-in MOSFET microelectronic amplifier to convert the high-impedance charge output into a low-impedance voltage signal. The sensor is powered from a constant-current source and can operate over long coaxial or ribbon cable without signal degradation. The low-impedance voltage signal is not affected by triboelectric cable noise or insulation resistance-degrading contaminants. Power to operate integrated circuit piezoelectric sensors generally takes the form of a low-cost, 24 to 27 VDC, 2 to 20 mA constant-current supply. A data acquisition system of the present invention may incorporate constant-current power for directly powering integrated circuit piezoelectric sensors.
Most piezoelectric pressure sensors are constructed with either compression mode quartz crystals preloaded in a rigid housing, or unconstrained tourmaline crystals. These designs give the sensors microsecond response times and resonant frequencies in the hundreds of kHz, with minimal overshoot or ringing. Small diaphragm diameters ensure spatial resolution of narrow shock waves.
The output characteristic of piezoelectric pressure sensor systems is that of an AC-coupled system, where repetitive signals decay until there is an equal area above and below the original base line. As magnitude levels of the monitored event fluctuate, the output remains stabilized around the base line with the positive and negative areas of the curve remaining equal.
It is also within the scope of the present invention that any strain sensing technique may be used to measure the variations in strain in the pipe, such as highly sensitive piezoelectric, electronic or electric, strain gages and piezo-resistive strain gages attached to the pipe 12. Other strain gages include resistive foil type gages having a race track configuration similar to that disclosed U.S. patent application Ser. No. 09/344,094, filed Jun. 25, 1999, now U.S. Pat. No. 6,354,147, which is incorporated herein by reference. The invention also contemplates strain gages being disposed about a predetermined portion of the circumference of pipe 12. The axial placement of and separation distance ΔX1, ΔX2 between the strain sensors are determined as described herein above.
It is also within the scope of the present invention that any other strain sensing technique may be used to measure the variations in strain in the tube, such as highly sensitive piezoelectric, electronic or electric, strain gages attached to or embedded in the tube 14.
While a number of sensor have been described, one will appreciate that any sensor the measures the speed of sound propagating through the fluid may be used with the present invention, including ultrasonic sensors.
It should be understood that any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incorporated with any other embodiment described herein.
Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention.

## Claims

1. An apparatus for measuring the gas volume fraction process flow flowing within a pipe, the apparatus comprising:
at least two sensors that provide a sound measurement signal indicative of sound waves propagating in the process flow flowing within the pipe, wherein the sensors clamp onto or attach to the outer wall of the pipe; and
a processor that determines the slope of an acoustic ridge in the k-ω plane, in response to the sound measurement signals, to provide a sound speed signal indicative of the speed of sound propagating through the process flow, and that determines the gas volume fraction of the flow, in response to the sound speed signal.

2. The apparatus of claim 1, wherein the at least two sensors include at least two pressure strain sensors at different axial locations along the pipe, each of the pressure strain sensors providing a respective strain signal indicative of an acoustic pressure disturbance within the pipe at a corresponding axial position.

3. The apparatus of claim 2, wherein the strain sensors are pressure sensors.

4. The apparatus of claim 2, wherein the strain sensors are pressure sensors.

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

6. The apparatus of claim 1, wherein the processor determines the gas volume fraction using at least one of the pressure and temperature of the process flow.

7. The apparatus of claim 6, 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.

8. The apparatus of claim 1, wherein the sound waves are one dimensional acoustic waves.

9. The apparatus of claim 1, wherein the processor determines the gas volume fraction using at least one of the pressure and temperature of the process flow.

10. The apparatus of claim 9, 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.

11. The apparatus of claim 1, wherein the sound waves are one dimensional acoustic waves.

12. The apparatus of claim 1, wherein the at least two sensors include 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 sensors.

13. The apparatus of claim 1, further includes an acoustic source for generating sound waves within the process flow.

14. The apparatus of claim 1, wherein the sound wave is a passive noise.

15. The apparatus of claim 1, wherein the gas volume fraction is determined using the following formula:


Gas Volume Fraction=−B+sqrt(B^2−4*A*C))/(2*A)

wherein A=1+rg/rl*(Keff/P−1)−Keff/P, B=Keff/P−2+rg/rl; C=1−Keff/rl*ameas^2; Rg=gas density, rl=liquid density, Keff=effective K (modulus of the liquid and pipewall), P=pressure, and ameas=measured speed of sound.

16. The apparatus of claim 1, wherein the each of the pressure sensors comprises a strap and a piezoelectric film sensor attached to the strap.

17. The apparatus of claim 1, wherein the piezoelectric film sensor is attached to the outer surface of the strap and/or the inner surface of the strap.

18. The apparatus of claim 1, further includes a clamping device for attaching the ends of one of the pressure sensors to clamp the pressure sensor onto the pipe.

19. The apparatus of claim 1, wherein the pressure sensors are removably clamped to the pipe.

20. The apparatus of claim 1, wherein the piezoelectric film sensor includes at least one of polyvinylchlorine fluoride (PDVF), polymer film and flexible PZT.