Apparatus and method for attenuating acoustic waves propagating within a pipe wall

A method and apparatus for damping an ultrasonic signal propagating in the wall of a pipe, the apparatus including at least one damping structure for securing at least one sensor to the wall of the pipe, wherein the at least one sensor includes a transmitter component and a receiver component for transmitting and receiving an ultrasonic signal, wherein the at least one damping structure is associated with the outer wall of the pipe for damping the ultrasonic signal propagating within the wall of the pipe and a processor that defines a convective ridge in the k-ω plane in response to the ultrasonic signals, and determines the slope of at least a portion of the convective ridge to determine the flow velocity of the fluid.

TECHNICAL FIELD

This invention relates to a method and apparatus for attenuating acoustic waves (or ring around acoustics) propagating through the walls of a pipe for a clamp-on ultrasonic flow meter.

BACKGROUND

Most ultrasonic flow measurements seek to leverage information contained in fluid borne disturbances of a specific temporal frequency. The specific frequency often results from natural frequencies of the drive electronics, the transducer, or the resonant transmission characteristic of the pipe wall.

Referring toFIG. 8, one of the primary challenges associated with clamp-on ultrasonic flow metering is the interference between the structural borne ultrasonic signal component100and the desired fluid borne ultrasonic signal component102. The structural borne component100of the ultrasonic signal is often of the same or similar frequency and essentially masks the fluid borne component102of the ultrasonic signal.

Standard pipes are fairly effective waveguides for structural borne acoustics components100. The ultrasonic pulse propagates along the wall of a pipe104with very little damping and rings around the circumference numerous times until the inherent damping in the pipe and the propagation of energy axially away from the initial excitation eventually dissipates the structural borne ultrasonic waves.

SUMMARY OF THE INVENTION

An apparatus for damping an ultrasonic signal propagating in the wall of a pipe is provided, wherein the apparatus includes a structural housing for securing at least one sensor having a transmitter component and a receiver component for transmitting and receiving an ultrasonic signal, wherein the housing is coupled to the outer wall of the pipe for damping the ultrasonic signal propagating within the wall of the pipe.

An apparatus for damping an ultrasonic signal propagating in the wall of a pipe is provided, wherein the apparatus includes at least one damping structure for securing at least one sensor to the wall of the pipe, wherein the at least one sensor includes a transmitter component and a receiver component for transmitting and receiving an ultrasonic signal, wherein the at least one damping structure is associated with the outer wall of the pipe for damping the ultrasonic signal propagating within the wall of the pipe. A processor is also provided, wherein the processor defines a convective ridge in the k-ω plane in response to the ultrasonic signals, and determines a slope of at least a portion of the convective ridge to determine a flow velocity of a fluid flowing within the pipe.

A method for damping an ultrasonic signal propagating within the wall of a pipe is provided, wherein the method includes introducing an ultrasonic signal into a pipe having a fluid flowing within, modifying the damping characteristics of the pipe wall by providing multiple impedance changes in the pipe wall and by providing alternate energy dissipation paths for the ultrasonic signals and processing a transit time of the received ultrasonic signals to determine a flow velocity of the fluid.

DETAILED DESCRIPTION

The present invention discloses apparatus' and methods for reducing the impact of structural borne noise, an unintended by-product of launching the fluid born ultrasonic interrogation pulse, on the operation of clamp-on flow ultrasonic flow meters, as described in U.S. patent application Ser. No. 10/756,977, filed Jan. 13, 2004, which is incorporated herein by reference.

FIGS. 1 and 2illustrate an ultrasonic clamp-on flow meter110, as described in U.S. patent application Ser. No. 10/756,977, wherein the ultrasonic flow meter110includes an array of ultrasonic sensors112having a plurality of ultrasonic sensors114-120disposed axially along the length of the pipe104. Each ultrasonic sensor114-120comprises a transmitter122and a receiver124. The transmitter122provides an ultrasonic signal to the corresponding receiver124, wherein the ultrasonic signal is orthogonal to the direction of the flow of a fluid126. While this embodiment of the present clamp-on ultrasonic meter110is described, one will appreciate that the present invention is applicable to the other embodiments, such as that described and taught in U.S. patent application Ser. No. 10/756,977, including embodiments in non-orthogonal ultrasonic signals, pitch and catch configurations, pulse echo configurations, and combined transmitter/receiver ultrasonic sensors, as shown inFIGS. 3-6.

For example, while each of the ultrasonic sensor units114-120comprises a pair of ultrasonic sensors (transmitter and receiver)122,124which are diametrically-opposed to provide through transmission, the present invention contemplates that one of the ultrasonic sensors122,124of each sensor unit114-120may be offset axially such that the ultrasonic signal from the transmitter sensor has an axial component in its propagation direction, as shown inFIG. 3.

As shown inFIG. 4, the present invention also contemplates that the sensor units114-120of the sensing device112may be configured in a pulse/echo configuration. In this embodiment, each sensing unit114-120comprises one ultrasonic sensor that transmits an ultrasonic signal through the pipe wall and fluid substantially orthogonal to the direction of flow and receives a reflection of the ultrasonic signal reflected back from the wall of the pipe to the ultrasonic sensor.

Referring toFIG. 5, the sensing device112may be configured to function in a pitch and catch configuration. In this embodiment, each sensor unit114-120comprises a pair of ultrasonic sensors (transmitter, receiver)122,124disposed axially along the pipe104disposed on the same side of the pipe104at a predetermined distance apart. Each transmitter sensor122provides an ultrasonic signal at a predetermined angle into the flow126. The ultrasonic signal propagates through the fluid126and reflects off of the inner surface of the pipe104and reflects the ultrasonic signal back through the fluid126to the respective receiver sensor124.

FIG. 6shows another pitch and catch configuration for the sensing device112contemplated by the present invention. This configuration is similar to that shown inFIG. 5except that the sensors disposed between the end sensors function as both a transmitter and a receiver. This pitch and catch configuration reduces the number of sensors needed to operate.

Referring back toFIG. 1, the signals S1(t)-SN(t) received from each ultrasonic sensor114-120are processed by an ultrasonic signal processor128and a signal processor130(having an array processor131) for determining the velocity of the fluid flow and/or volumetric flow rate. The signal processor130includes at least one of array processing logic, as will be described in greater detail hereinafter (SeeFIGS. 13 and 14); and cross-correlation processing logic, as also will be described in greater detail hereinafter (FIG. 15).

One should appreciate that the present invention is applicable to at least all the configurations of an ultrasonic flow meter considered herein (as well as others not described herein), and will be described in greater detail hereinafter.

Specifically, the present invention teaches complimentary approaches to attenuating or eliminating the structural borne component100of the ultrasonic signal. For example, one embodiment comprises a structurally significant housing, and a second embodiment including piezoelectric films applied to the outer surface of the pipe104to damp out the structural borne ultrasonic vibrations.

The first embodiment, as shown inFIG. 7, involves the use of a structurally significant housing132to clamp-on to the outside of the process piping104. The housing132is structurally significant in terms of mass and stiffness as compared to the pipe104itself and once the clamp-on ultrasonic meter110(seeFIG. 1) (including the housing132) is mounted to the pipe104, the housing132and pipe104wall essentially form a single structural body at the ultrasonic excitation frequencies of interest. The idea is to clamp the structurally significant housing132to the pipe104with sufficient force, possibly with the addition of epoxy, to effectively modify the ultrasonic vibrational characteristics of the pipe104.

More specifically, the structurally significant housing132essentially modifies the structural properties of the entire structural path (or substantially the entire path) between the transmitting and the receiving ultrasonic transducers122,124. The structurally significant housing132contacts and reinforces all areas of the pipe104except for the immediate area of the transmitting and receiving transducers122,124. Given that the flexural stiffness of a plate scales with the cube of the thickness of the plate, doubling the effective wall thickness increases the effective flexural stiffness by a factor of 8. Thus, as one rule of thumb, this invention considers doubling the flexural stiffness by at least 2× as being “significant” and thus a structural housing132of the same material as the pipe104need only result in a ˜25% increase in effective pipe104wall thickness to be considered significant. Thus, the present invention enhances the relative ability of the transmitting and receiving sensors122,124to communicate through the fluid126with respect to the structurally borne fluid path.

In addition to impeding the propagation of the structural wave component100from the transmitting sensor122to the receiving sensor124, the design of the structurally significant housing132can be optimized to increase the transmission of fluid borne ultrasonic wave component102. Referring toFIG. 9, with the structurally significant housing132in place, the unreinforced section103of the pipe104wall effectively appears as a clamped diaphragm.

Blevins, Formulas for Natural Frequency and Mode Shapes, (which is incorporated herein by reference) provides formulas for the natural frequency of a clamp-on diaphragm. For example, for a clamp-on diaphragm having a diameter, a, and thickness, h, for a material of modulus, E, Poisson ratio, v, and a mass per unit area, g, the natural frequency may be given by,

This formulation neglects the real world stiffening effect of the curvature of the pipe104wall in the unreinforced area and thus will likely under predict the natural frequency for a given geometry. However, recognizing this limitation, initial calculations show that for a pipe104wall of ˜0.3 inches, and unreinforced sensor areas of roughly 0.75 inches in diameters, a flat plate circular disk has resonant frequencies on the order of 10,000 Hz to 500,000 Hz, which is within the range of ultrasonic transducers. Thus, tuning the natural frequency of the diaphragm system that is formed using a structurally significant housing132with the primary transmission frequency of the ultrasonic sensors114-120—created by either driving the transducer at a specific frequency, or pulsing the transducer, is both practical and feasible with commonly available ultrasonic transducers and the design proposed herein.

The standard, unreinforced pipe does demonstrate frequency selectivity with respect to normal incidence ultrasonic waves. The transmission of normal incident ultrasonic waves102is maximized at frequencies that correspond to the wavelength of compression waves in the pipe104wall being an integral number of halfwave lengths,

The effect of the structurally significant housing132would be maximized if the resonant frequency of the diaphragm system designed above coincided with one of the frequency of maximum transmission.

The design task of aligning the two resonant frequencies becomes one of selecting the diameter of the “diaphragm” such that the natural frequency of the “diaphragm” lines up with the frequency of maximum transmission. Inspection of the above equations shows that this condition is essentially met for “diaphragms” with radii equal to the thickness of the pipe104wall.

Thus, under the simplified, but still realistic assumptions discussed herein, one optimal “diaphragm” diameter may be equal to 2 times the thickness of the pipe104. These values are tabulated in Table 1, shown inFIG. 9B.

Note that as the pipe104wall gets thicker, the optimal “diaphragm” diameter increases. Given the size of conventional transducers, this effect may be better leveraged for thick wall pipes, such as those used in high-pressure oil and gas wells.

Referring toFIG. 10, an additional embodiment of a structurally significant housing200is shown, wherein the presence of the structurally significant housing200provides multiple impedance changes, alternate energy dissipation paths, and augmented damping to reduce the level of structural borne noise present to interfere with the fluid borne signal required to make a flow measurement. Specifically, the structurally significant housing200includes viscoelastic damping material202introduced into slots204in the housing200. For structural waves100propagating through the housing, the design of the slots204provide for shearing of the viscoelastic material202, effectively augmenting the damping of the structural wave100.

Referring toFIG. 11, another embodiment of a structurally significant housing300is shown with viscoelastic damping material202attached between the housing300and structurally significant plates302. The structurally significant housing300and the structurally significant plates302serve to constrain the viscoelastic material202when deflected, effectively augmenting the damping of the structural wave100.

While the present invention of a structurally significant housing132,200,300attenuates the structural borne ultrasonic signals100propagating circumferentially around the pipe104, one should appreciate that the housing132,200,300will also attenuate or eliminate axially propagating structural borne ultrasonic signals100. Further, while the housing132,200,300is shown as a single housing comprised of two halves bolted together to retain the ultrasonic sensors114-120of the array of sensors112, one should appreciate that the present invention contemplates that the ultrasonic meter may comprise a plurality of discreet independent structurally significant housings, wherein each sensor114-120of the array112may be mounted to the pipe104by a respective structurally significant housing132,200,300. It is further contemplated that a housing132,200,300may also include any number of ultrasonic sensors114-120less than the total number of the array112.

Referring toFIGS. 12A,12B,12C and12D, an additional approach of attenuating or damping the structural borne ultrasonic signal or vibration100includes the use of piezo films304applied to the outer surface of the pipe104. Piezo devices304bonded to a vibrating structure and electrically shunted to dissipate charge generated by deformation are well known to serve as effective dampening devices for structural vibration. (e.g. Piezo damping of Fan blades). By tuning the electrical properties of the piezo RLC circuit306, the circuit306can be optimized to preferentially damp structural vibration of a specific frequency.

One objective of the current invention is to bond piezoelectric materials (e.g. PVDF film)304to the pipe104wall along the region of the wall in which the interfering structural borne ultrasonic vibration100(seeFIG. 8) would travel. The circuitry306could be broadband in nature or tuned to optimize attenuation of vibrations at specific frequencies.

Alternatively to the passive electronic system described above, the pvdf film304could also be used in an active circuit to preferentially damp out specific structural vibration. One piezoelectric film304is similar to that shown in U.S. patent application Ser. No. 10/712,833, filed on Nov. 12, 2003, which is incorporated herein by reference.

In one configuration envisioned, the pvdf system is applied to the pipe104as a separate sub system of the existing ultrasonic flow metering system. Typical, piezo transducers are used to launch and detect ultrasonic signals. The proposed use of piezo dampers constitute a separate system designed to reduce or eliminate the structure borne component100of the ultrasonic signal, unintentionally generated as a by-product of generating the fluid borne component200, arriving at the ultrasonic detector124ideally intended to respond to only fluid borne ultrasonic devices. An illustration of one embodiment of this concept is shown inFIGS. 12A,12B,12C and12D.

The compressional wavelength in steel at 1 MHz is approximately 0.2 inches. Ideally, the spatial extent of the PVDF patches should target an odd integral number of half wavelengths, namely ˜0.1, 0.3, 0.5 inches etc.

Referring back toFIG. 1, the flow logic in the processor130may determine the velocity of each sensor in the array of sensors114-120using one or both of the following techniques to determine the convection velocity of vortical disturbances within the process flow126or other characteristics of the process flow126that moves/convects with the process flow126by 1) Characterizing the convective ridge of the vortical disturbances or other characteristics using array processing techniques that use an array112of ultrasonic sensors114-120and/or 2) cross-correlating unsteady variations in the ultrasonic signals using ultrasonic sensors114-120. It should be appreciated that while the sensors114-120have been shown and described, the present invention is not limited in this regard and the number of sensors can vary. For example, any number of sensors may be used, such as 2 to 16 sensors, without departing from the scope of the invention.

Referring toFIG. 13, a block diagram illustrating the flow logic308in the processor130ofFIG. 1is shown and is used to characterize the convective ridge of the unsteady variations of the ultrasonic signals and determine the flow rates. As shown inFIG. 13, the flow logic308includes a data acquisition unit310(e.g., A/D converter) that converts the analog signals T1(t) . . . TN(t) to respective digital signals and provides the digital signals T1(t) . . . TN(t) to FFT logic312. The FFT logic312calculates the Fourier transform of the digitized time-based input signals T1(t) . . . TN(t) and provides complex frequency domain (or frequency based) signals T1(ω), T2(ω), T3(ω), . . . TN(ω) indicative of the frequency content of the input signals. It should be appreciated that instead of FFTs, any other technique for obtaining the frequency domain characteristics of the signals T1(t)-TN(t), may be used. For example, the cross-spectral density and the power spectral density may be used to form one or more frequency domain transfer functions (or frequency responses or ratios) discussed hereinafter.

One technique of determining the convection velocity of the coherent structures (e.g., turbulent eddies)314within the flow126is by characterizing a convective ridge of the resulting unsteady variations using an array112of sensors114-120or other beam forming techniques, similar to that described in U.S. patent application Ser. No. 09/729,994, filed Dec. 4, 2000, now U.S. Pat. No. 6,609,069, which is incorporated herein by reference in its entirety.

A data accumulator316accumulates the frequency signals T1(ω)-TN(ω) over a sampling interval, and provides the data to an array processor318, which performs a spatial-temporal (two-dimensional) transform of the sensor data, from the x-t domain to the k-ω domain, and then calculates the power in the k-ω plane, as represented by a k-ω plot.

The array processor318may use standard so-called beam forming, array processing, or adaptive array-processing algorithms, i.e. algorithms for processing the sensor signals using various delays and weighing 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 array112into 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=2πv.

It should be appreciated that 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, or combined use, of other adaptive array processing algorithms, such as MUSIC algorithm. The present invention also recognizes that such techniques can be used to determine flow rate, i.e. that the signals caused by a stochastic parameter convecting with a flow126are time stationary and may have a coherence length long enough so that it is practical to locate sensors114-120apart from each other and yet still be within the coherence length.

Convective characteristics or parameters have a dispersion relationship that can be approximated by the straight-line equation,
k=ω/u,
where u is the convection velocity (flow velocity). Referring toFIG. 14, a k-ω plot is a plot of k-ω pairs obtained from a spectral analysis of sensor samples associated with convective parameters that are portrayed so that the energy of the disturbance spectrally corresponds to pairings that might be described as a substantially straight ridge, wherein the ridge, in turbulent boundary layer theory, is called a convective ridge.

To calculate the power in the k-ω plane, as represented by a k-ω plot (seeFIG. 14) of either of the signals, the array processor318determines the wavelength and so the (spatial) wavenumber k, and also the (temporal) frequency and so the angular frequency ω, of various of the spectral components of the stochastic parameter. There are numerous algorithms available in the public domain to perform the spatial/temporal decomposition of arrays of sensors114-120.

The present embodiment may use temporal and spatial filtering to precondition the signals to effectively filter out the common mode characteristics and other long wavelength (compared to the sensor spacing) characteristics in the pipe104by differencing adjacent sensors114-120and retaining a substantial portion of the stochastic parameter associated with the flow field and any other short wavelength (compared to the sensor spacing) low frequency stochastic parameters.

In the case of suitable coherent structures314being present, the power in the k-ω plane shown in the k-ω plot ofFIG. 14shows a convective ridge320. The convective ridge320represents the concentration of a stochastic parameter that convects with the flow126and 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 line320with some slope, wherein the slope indicates the flow velocity.

Once the power in the k-ω plane is determined, a convective ridge identifier322uses one or another feature extraction method to determine the location and orientation (slope) of any convective ridge320present in the k-ω plane. In one embodiment, a so-called slant stacking method is used, a method in which the accumulated frequency of k-ω pairs in the k-ω plot along different rays emanating from the origin are compared, each different ray being associated with a different trial convection velocity (in that the slope of a ray is assumed to be the flow velocity or correlated to the flow velocity in a known way). The convective ridge identifier322provides information about the different trial convection velocities, information referred to generally as convective ridge information.

An analyzer324examines the convective ridge information including the convective ridge orientation (slope). Assuming the straight-line dispersion relation given by k=ω/u, the analyzer324determines the flow velocity and/or volumetric flow, which are output as parameters326. The volumetric flow is determined by multiplying the cross-sectional area of the inside of the pipe104with the velocity of the process flow126.

As previously noted, for turbulent Newtonian fluids, there is typically not a significant amount of dispersion over a wide range of wavelength-to-diameter ratios. As a result, the convective ridge320in the k-ω plot is substantially straight over a wide frequency range and, accordingly, there is a wide frequency range for which the straight-line dispersion relation given by k=ω/u provides accurate flow velocity measurements.

For stratified flows, however, some degree of dispersion exists such that coherent structures314convect at velocities which depend on their size. As a result of increasing levels of dispersion, the convective ridge320in the k-ω plot becomes increasingly non-linear.

2) Cross-Correlating Unsteady Pressure Variations Using an Array of Unsteady Pressure Sensors.

Referring toFIG. 15, a processor400is provided which uses cross-correlation of unsteady variations of the ultrasonic signals to determine the flow rates. The processing unit400ofFIG. 15determines the convection velocity of the vortical disturbances within the flow126by cross correlating unsteady ultrasonic variations using an array of ultrasonic sensors114-120, similar to that shown in U.S. Pat. No. 6,889,562, filed Nov. 8, 2001, which is incorporated herein by reference.

Referring toFIG. 15, the processing unit400has two measurement regions located a distance ΔX apart along the pipe104. Each pair of sensors114,116and118,120of each region act as spatial filters to remove certain acoustic signals from the unsteady pressure signals, and the distances X1, X2are determined by the desired filtering characteristic for each spatial filter, as discussed hereinafter.

In particular, in the processing unit400, the ultrasonic signal T1(t) is provided to a positive input of a summer402and the ultrasonic signal T2(t) is provided to a negative input of the summer402. The output of the summer402is provided to line404indicative of the difference between the two ultrasonic signals T1, T2(e.g., T1−T2=Tas1).

The line404is fed to a bandpass filter406, which passes a predetermined passband of frequencies and attenuates frequencies outside the passband. In accordance with the present invention, the passband of the filter406may be set to filter out (or attenuate) the dc portion and the high frequency portion of the input signals and to pass the frequencies therebetween. Other passbands may be used in other embodiments, if desired. Bandpass filter406provides a filtered signal Tasf1on a line408to Cross-Correlation Logic410, described hereinafter.

The ultrasonic signal T3(t) is provided to a positive input of a summer412and the ultrasonic signal T4(t) is provided to a negative input of the summer412. The output of the summer412is provided on a line414indicative of the difference between the two ultrasonic signals T3, T4(e.g., T3−T4=Tas2). The line414is fed to a bandpass filter416, similar to the bandpass filter406discussed hereinbefore, which passes frequencies within the passband and attenuates frequencies outside the passband. The filter416provides a filtered signal Tasf2on a line418to the Cross-Correlation Logic410. The signs on the summers402,412may be swapped if desired, provided the signs of both summers are swapped together. In addition, the ultrasonic signals T1, T2, T3, T4may be scaled prior to presentation to the summers402,412.

The Cross-Correlation Logic410calculates a known time domain cross-correlation between the signals Tasf1and Tasf2on the lines408,418, respectively, and provides an output signal on a line420indicative of the time delay τ it takes for an vortical flow field314(or vortex, stochastic, or vortical structure, field, disturbance or perturbation within the flow) to propagate from one sensing region to the other sensing region. Such vortical flow disturbances, as is known, are coherent dynamic conditions that can occur in the flow which substantially decay (by a predetermined amount) over a predetermined distance (or coherence length) and convect (or flow) at or near the average velocity of the fluid flow. As described above, the vortical flow field314also has a stochastic or vortical pressure disturbance associated with it. In general, the vortical flow disturbances314are distributed throughout the flow, particularly in high shear regions, such as boundary layers (e.g., along the inner wall of the tube104) and are shown herein as discrete vortical flow fields314. Because the vortical flow fields (and the associated pressure disturbance) convect at or near the mean flow velocity, the propagation time delay τ is related to the velocity of the flow by the distance ΔX between the measurement regions, as discussed hereinafter.

Referring toFIG. 15, a spacing signal ΔX on a line422indicative of the distance ΔX between the sensing regions is divided by the time delay signal τ on the line420by a divider424which provides an output signal on the line426indicative of the convection velocity Uc(t) of the saturated vapor/liquid mixture flowing in the pipe104, which is related to (or proportional to or approximately equal to) the average (or mean) flow velocity Uf(t) of the flow126, as defined below:
Uc(t)=ΔX/τ∝Uf(t)

The present invention uses temporal and spatial filtering to precondition the ultrasonic signals to effectively filter out the acoustic disturbances Pacousticand other long wavelength (compared to the sensor spacing) disturbances in the pipe104at the two sensing regions and retain a substantial portion of the ultrasonic signal Tvorticalassociated with the vortical flow field314and any other short wavelength (compared to the sensor spacing) low frequency pressure disturbances Tother. In accordance with the present invention, if the low frequency pressure disturbances Totherare small, they will not substantially impair the measurement accuracy of Tvortical.

While the cross-correlation was shown using four sensors, whereby two sensors were summed together to form a sensing region, the invention contemplates that each sensing region may only be comprised of one (or more) sensors disposed at an axial location along the pipe104.

As mentioned hereinbefore, the present invention contemplates that the housing and blocks for attenuating the structural ultrasonic signals may be used with any configuration of ultrasonic sensors114-120. Specifically any of the three classes of flow meters that utilize ultrasonic transducers, which include transit time ultrasonic flow meters (TTUF), doppler ultrasonic flow meters (DUF), and cross correlation ultrasonic flow meters (CCUF).

CCUF's measure the time required for ultrasonic beams to transit across a flow path at two, axially displaced locations along a pipe104. Within this measurement principle, variations in transit time are assumed to correlate with properties that convect with the flow126, such as vortical structure, inhomogenities in flow composition, temperature variations to name a few.

CCUF's utilize high frequency acoustic signals, i.e. ultrasonics, to measure much lower frequencies, time varying properties of structures in the flow126. Like all other cross correlation based flow meters, the physical disturbances which cause the transit time variations should retain some level of coherence over the distance between the two sensors.

Cross correlation ultrasonic flow meters have been around since the early 1960's. CCUF's are typically much more robust to variations in fluid composition than the other ultrasonic-based flow measurement approaches such as transit time and Doppler based methods.

Although CCFU's are operationally more robust than other ultrasonic interpretation techniques, they suffer from drawbacks attributed to most cross correlation flow meters, i.e., they have slow update rates and are relatively inaccurate.

Transit time, defined as the time required for an ultrasonic beam to propagate a given distance, can be measured using a radially aligned ultrasonic transmitter and receiver. For a homogenous fluid with a no transverse velocity components flowing in an infinitely rigid tube, the transit time may be given by the following relation:
t=D/Amix
where t is the transit time, D is the diameter of the pipe104, and Amixis the speed of sound propagating through the fluid126.

In such a flow, variation in transit time is analogous to a variation in sound speed of the fluid. In real fluids however, there are many mechanisms, which could cause small variations in transit time which remain spatially coherent for several pipe diameters. For single phase flows, variations in the transverse velocity component will cause variations in transit time. Variations in the thermophysical properties of a fluid such as temperature or composition will also cause variations. Many of these effects convect with the flow. Thus, influence of transverse velocity of the fluid associated with coherent vortical structures314on the transit time enables transit time based measurements to be suitable for cross correlation flow measurement for flows with uniform composition properties. The combination of sensitivity to velocity field perturbation and to composition changes make transit time measurement well suited for both single and multiphase applications.

Despite CCUF's functioning over a wide range of flow composition, standard transit time ultrasonic flow meters (TTUF) are more widely used. TTUF's tend to require relatively well behaved fluids (i.e. single phase fluids) and well-defined coupling between the transducer and the fluid itself. TTUF's rely on transmitting and receive ultrasonic signals that have some component of their propagation in line with the flow. While this requirement does not pose a significant issue for in-line, wetted transducer TTUF's, it does pose a challenge for clamp-on devices by introducing the ratio of sound speed in the pipe to the fluid as an important operating parameter. The influence of this parameter leads to reliability and accuracy problems with clamp-on TTUF's.

CCFU's, utilize ultrasonic transducers to launch and detect ultrasonic waves propagating normal to the flow path. Refraction of ultrasonic waves at the pipe/fluid interface is not an issue and the ratio between sound speed of pipe and the fluid does not directly effect operability.

In still another embodiment, each pair of transducers114-120comprise a single transmitter122to emit an ultrasonic signal through the flow126and a receiver,124which receives the respective signal for processing. The time it takes for the signal to arrive at the receiver transducer124for each pair is calculated and fed to the SONAR algorithms (in the array processor131) where the flow rate is calculated. One embodiment uses a very simplistic signal detection algorithm that looks for a peak in the reading obtained from the receiver124. This algorithm works well when a good signal-to-noise ratio is observed at the receiver124, however when bubbles intersect the signal path between the transmitter122and receiver124a significant attenuation can occur, which will severely degrade the received signal quality. The amount of attenuation will vary depending on the bubble characteristics such as size and density.

Referring toFIG. 17, the transmitting ultrasonic transducer array122is periodically pulsed to create the ultrasonic signal that transmits through the pipe104and fluid. Each transducer will have a fundamental oscillation frequency, which when pulsed will emit a short ultrasonic burst signal.FIG. 16shows the signal created by a 1 MHz ultrasonic transducer when pulsed with a 10 nS width pulse created in the flow meter110. In typical applications the receiving ultrasonic transducer124, located on the opposite side of a pipe104, will receive this signal once it has bisected the pipe104however in addition to this primary through-transmitted signal other unwanted secondary signals will also be detected. These secondary signals include portions of the original signal that have been refracted or reflected along a different path through the pipe104than the preferred direct transmission. Often these secondary signals possess sufficient strength to still reach the receiver transducer124and will interfere with the desired signal. Examples of these secondary signals include the ring-around signals600that travel within the pipe wall104, reflected signals that may bounce off multiple interfaces such as the transducer-pipe interface or the pipe-liquid interface, or as in the case here where an array of transducers are used, from an adjacent transducer, as shown inFIG. 17.

The dominant secondary signal is the ‘ring-around’ signal600. This is the portion of the ultrasonic signal that travels around through the wall of the pipe104and can still be detected by the receiving transducer124.FIG. 18shows a diagram of this signal as compared to the through-transmitted signal. As shown inFIG. 19, ultrasonic transmitting and receiving transmitters122,124, respectively, are shown attached to the outer surface of a pipe104. They are arranged such that the generated ultrasonic signal will be normal to the direction of the fluid flow and travel through the center602of the liquid within the pipe104. As discussed above, as the ultrasonic signal travels through the pipe104, bubbles604and other matter within the pipe104will scatter and attenuate the signal before it fully traverses the pipe104and is detected by the receiving transducer124. Also depicted is the ‘ring-around’ signal600. This signal is created through reflection and diffraction between the transmitting ultrasonic transducer122, the pipe wall104and the material present inside the pipe104due to the large impendence mismatch between the various materials. As an example, the impedance of steel is 45 MRayls in contrast to fluid which has an impedance of 1.5 MRayls. In this case, only a small percentage of the ultrasonic signal is actually injected into the fluid while the rest is reflected throughout the overall system. The majority of this excess energy is present in the pipe104wall in the form of shear and compressional ultrasonic waves600. These waves will travel throughout the pipe104and will be seen by the receiving transducer124along with any desired signals. Coupled with the fact that the through-transmitted signal can be significantly attenuated as it travels through the fluid126in the pipe104, it can be very difficult to distinguish the wanted signal from all the secondary signals.FIG. 19shows an example of a received ultrasonic signal602along with an unwanted ‘ring-around’ signal600. The arrow indicates the location of the through-transmitted pulse in relation to the large ‘ring-around’ signal. Contrast this to the clean ultrasonic signal seen inFIG. 16.

To increase the system robustness of the ultrasonic flow meter110, the amount of the noise signal may be decreased by mechanically reducing the strength of the secondary ring-around ultrasonic signals that were able to reach the detectors.

Signal to Noise

It should be appreciated that the quality of any flow measurement, independent of the technology, is typically dependent upon the signal to noise ratio (S/N). Noise, in this case, is defined as any portion of the measured signal that contains no flow information. It is desirable to maximize the S/N to obtain optimum performance. As mentioned, the dominant noise source for the ultrasonic flow meter110was determined to be ring-around noise. Ring-around noise is defined as the signal seen by the receiving transducer124that has not passed through the fluid126, but instead traveled via the pipe104wall. This signal contains no flow information and, in certain cases, can corrupt the measurement.FIG. 19shows both the signal path and ring-around path.

The ultrasonic flow meter110measures the modulation of the time-of-flight (TOF) measurement orthogonal to the flow direction. The TOF modulation is due to the vortical disturbances in the beam path and the flow velocity is determined by correlating these coherent modulations over the length of the sensor array.

Under ideal conditions, the ratio of the signal passing through the fluid126to the ring-around noise is high, and/or the differential TOF between the signals is large, and a flow measurement can be made. In situations where the straight through signal is attenuated due to properties of the fluid124(air bubbles, particulates, etc.) the S/N ratio can be substantially reduced and the flow measurement compromised. In cases where the signal and noise temporally overlap, and/or in situations where the ring-around signal is greater than the straight through signal, advanced signal processing algorithms need to be employed to detect the signal. In order to reduce the burden placed on the detection algorithm to detect small signals in the presence of a large ring-around signal, methods of reducing the amplitude of the ring-around noise were investigated.

The properties of the ring-around energy differ depending upon the wall thickness of the pipe104, transducer frequency, pipe surface quality, and transducer size. Generally speaking, higher levels of ring-around are seen at smaller pipe diameters (i.e. 2 inch) for a given transducer excitation frequency due to the tighter curvature of the wall. Ring-around signals can be generated when energy from the transducer is either directly coupled into the pipe wall and/or be a result of reflected energy from the inner pipe/liquid interface. This energy can propagate as a variety of different waves, such as shear, longitudinal and surface waves.FIG. 20shows the phase velocity of supported circumferential modes within the wall of a schedule 40, 2 inch steel pipe. It can be seen that at low excitation frequencies, such as 1 MHz, four modes can be supported in the pipe wall, wherein the number of modes capable of being supported increases with increased frequency. The phase velocity of the lower order modes converges to approximately 3000 meters/sec.

One approach to eliminate ring-around involves coupling the energy into a mechanical structure attached to the pipe104. Referring toFIG. 21, two steel blocks500were machined with a curvature slightly larger than the radius of a 2 inch pipe104. Acoustic coupling gel was applied between the pipe104and the curved face of the blocks500. The blocks500were then coupled to the pipe104which was then filled with water and the ring-around noise was measured and compared to the straight through signal. This was accomplished by first measuring and recording the received signal containing both the ring-around noise and the straight through signal, followed by a measurement with the straight through beam blocked. The difference between the measurements represents the contribution of the ring-around noise. The results of these tests showed the blocks had little impact on the attenuation of the acoustic energy propagating in the pipe104wall.

A second test was conducted where the blocks500were epoxied to the pipe104wall. Comparison of these measurements showed substantial attenuation of the ring-around energy.FIG. 22shows the received signal with and without epoxied ring-around blocks500. The first arrival signal without ring-around blocks occurs at approximately 31 usecs. This is consistence with the calculated transit time through steel. The straight through signal containing the flow information has a transit time of 41 usec. Ring-around blocks attenuate the ring-around noise resulting in an improved signal to noise at the receiver124. It should be appreciated that improvements in S/N of up to 20 dB were realized with ring-around blocks.

If should also be appreciated that while the present invention contemplates using a block of material500(e.g., steel) attached or engaged to the pipe104to attenuate acoustic waves propagating through the pipe104wall, the invention further contemplates that the blocks500may be comprised of a sheet of material (e.g., steel, tin and lead) that is epoxied or otherwise engaged or attached to the pipe104wall. The sheet material may cover a substantial portion of the circumference and length of the array of sensors114-120. The attenuation design may comprise of a plurality of respective sheets for each ultrasonic sensor pair and disposed on both sides of the pipe104between the sensor pair.

As discussed above and as seen inFIG. 23andFIG. 24, for various measurements made on pipes104the transit time of an ultrasonic wave is determined and a related pipe parameter is derived (e.g. flow velocity). Often the ultrasonic energy is coupled through a pipe104wall and then into the confined fluid126. The signal of interest is the signal that passes thru the fluid126(or other material contained in the pipe104). Sometimes this signal is difficult to see because some of the ultrasonic energy is unavoidably coupled into the pipe104wall and travels around the circumference of the pipe104wall and ends up on top of the desired signal. This unwanted signal is sometimes referred to as ring-round.

By attaching blocks500with similar impedance to the pipe104to the pipe wall the ring-round can be reduced. The blocks500reduce the ring-round by basically two methods. First, for a wave traveling in the pipe wall, the block500because of its thickness, creates a different impedance and the energy is reflected. Second, the energy that is not reflected travels out into the block500and does not continue around the pipe104. Note that the blocks500should be attached to the pipe with a solid material because a gel or liquid may not couple out the shear wave.