Ultrasonic transducer device

An ultrasonic transducer assembly includes a proximal end and an opposing distal end. The transducer assembly includes an outer housing and an electroacoustic signal generating element secured within the outer housing. The signal generating element transmits an ultrasonic signal at a characteristic frequency along an ultrasonic path that is perpendicular to the face of the generating element. An isolation diaphragm is coupled to the proximal end of the outer housing. The isolation diaphragm is thin relative to a characteristic wavelength of the diaphragm material. A fluidic transmission layer is disposed between the electroacoustic signal generating element and the isolation diaphragm. In one embodiment, the isolation diaphragm is at an angle relative to the proximal face of the electroacoustic signal generating element. In another embodiment, a flow meter includes the ultrasonic transducer assembly, and the isolation diaphragm substantially matches the contour of the flow passage within the flow meter body.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to ultrasonic transducers, and more particularly to an ultrasonic transducer having improved flow rate measurement accuracy.

Ultrasonic flow meters are used to determine the mean pipe flow rate (Vm) of a variety of fluids (e.g., liquids, gases, multiphase, etc.). Knowledge of the flow rate of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the volume (Q) of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe to determine the costs for the transaction, where the volume is equal to the flow rate multiplied by the cross sectional area (A) of the pipe.

A conventional ultrasonic transducer typically includes a cylindrical housing with the ultrasonic transducer affixed within one end (usually the tip) and an electronics package mounted within the opposing end. An acoustic dampening material typically separates the two to prevent sound waves from reverberating inside the housing. The transducer crystal is fragile and therefore is not normally exposed to the medium being measured. Accordingly, the housing tip is typically hermetically sealed to prevent moisture and contaminants from entering the inner cavity where the transducer is located. Within the inner cavity, the transducer abuts the tip and propagates an ultrasonic signal through the tip material into the medium being measured. The planar face of the transducer crystal is perpendicular to (e.g., normal to) the direction of ultrasonic wave propagation. For applications in which the transducer is utilized as a flow meter, the ultrasonic transducer is then mounted through an access aperture in the fluid conduit.

One noted drawback to this approach is that the ultrasonic signal must first pass through the transducer tip material prior to entering the medium to be measured. The tip material may be quite thick to withstand the pressure of the fluid in the conduit, which in one example is several thousand pounds per square inch (psi). The thick tip material may absorb or otherwise attenuate the ultrasonic signal, causing degraded performance such as decreased signal-to-noise ratio. Furthermore, due to the tip thickness, the probe tip or face must be perpendicular to the direction of propagation. Otherwise, the thickness of the tip material will skew the wave propagation path, leading to measurement errors.

In one type of ultrasonic flow meter employing transit time flow metering, one or more pairs of ultrasonic transducers can be mounted to a pipe (or spool piece attached to a pipeline). Each pair can contain transducers located upstream and downstream from each other forming an ultrasonic path between them. Each transducer, when energized, transmits an ultrasonic signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and detected by the other transducer. The path velocity (i.e., path or chord velocity (Vp)) of the fluid averaged along an ultrasonic path can be determined as a function of the differential between (i) the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer upstream to the upstream transducer against the fluid flow direction, and (2) the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer downstream to the downstream transducer with the fluid flow direction.

One type of transit time flow meter used in industrial and commercial applications is a flare gas flow meter, which measures the flow rate in a combustible gas that is vented to atmosphere and subsequently burned. Combustible gases are common byproducts in oil refinery operations, oil drilling and exploration, and industrial processes, for example. The safest manner in which to dispose of the combustible gas is to vent it to atmosphere and ignite it. However, environmental regulations sometimes require the flare gas operator to document the amount of combustible gas burned in the atmosphere over a given period of time. The flare gas flow meter allows the operator to measure and document the gas flow in order to remain in compliance with regulations.

Ultrasonic flare gas flow measurement typically utilizes at least one pair of transducers as described above, each transducer being fitted within a probe. Since the flare gas typically flows through the pipe at a very high velocity (e.g., 150 m/s), accurate measurements may be difficult if the probes are spaced far apart, as may be the case in large diameter pipe. Therefore, in some applications each transducer probe protrudes into the flare gas pipe approximately one quarter of the pipe diameter. Each probe protruding into the pipe reduces the separation distance between the probe pair, which allows for a more accurate measurement.

Several problems with this approach arise. One noted problem is that the probes are large and present obstructions to the flow. Due to the dynamic forces in the high velocity flow, the probes may begin to shake or vibrate. The vibration may induce fatigue stress. Also, the velocity of the fluid may tend to bend the probe, either elastically or permanently. In either case, the probe may eventually fail.

Another type of transit time flow meter is a multi-phase flow meter, which measures the flow rate in pipes that contain more than one phase, such as liquid and solid. One example of a multi-phase flow may be found in oil drilling operations, where sand particles are admixed with the liquid oil flowing in the pipe. The sand particles tend to interfere with the ultrasonic waves being transmitted between sensors. One solution to this problem is to insert the probes into the pipe to minimize the distance between the transducers, similar to the flare gas application. One drawback to this approach is that the sand particles erode the probe tips and, over time, cause the probe to fail.

Another type of flow meter is a custody transfer flow meter, which necessitates a very accurate flow measurement. Custody transfer flow meters often measure expensive (and sometimes volatile) fluids such as gasoline. Safety regulations prohibit obstructions in the pipe flow path (such as probes) that could pose an ignition hazard. Therefore, the transducers are typically recess-mounted in the pipe. Due to the geometries involved (e.g., the upstream and downstream cross-mounting) and the requirement that the probe face is perpendicular to the wave propagation, the recess-mounted transducer will form recesses or cavities in the conduit wall. One drawback to this approach is that the flow meters with recessed transducers, such as those found in liquid custody transfer or multiphase flow meters, may experience erosion and blockage in the cavities formed by the recess. In some configurations, the flow velocity passing over the recess forms eddies which, if solid particles such as sand were present in the flow, erode the cavity and conduit. In other configurations, solid particles can settle into the cavities and obstruct the ultrasonic path, leading to erroneous readings.

It would be advantageous to improve flow rate measurement accuracy without inserting the transducer probe into the fluid stream, or recessing the probe from the inner wall of the fluid conduit.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect of the invention, an ultrasonic transducer is provided that improves accuracy by increasing signal-to-noise ratio. The transducer assembly includes a proximal end oriented towards a medium to be a measured, and an opposing distal end. The transducer assembly includes an outer housing and an electronics package coupled to the distal end of the outer housing. An electroacoustic signal generating element is secured within the outer housing. The generating element transmits an ultrasonic signal at a characteristic frequency along an ultrasonic path that is perpendicular to the face of the generating element. An isolation diaphragm is coupled to the proximal end of the outer housing. The isolation diaphragm is thin relative to a characteristic wavelength of the diaphragm material. A fluidic transmission layer is disposed between the electroacoustic signal generating element and the isolation diaphragm.

In another aspect of the invention, a flow meter is provided having a flow meter body with a flow passage therethrough. The flow meter body includes an aperture extending from an outer surface to the flow passage. A transducer assembly is disposed in the aperture. The transducer assembly includes an outer housing, an electroacoustic signal generating element secured within the outer housing, an isolation diaphragm coupled to a proximal end of the outer housing, and a fluidic transmission layer disposed between the electroacoustic signal generating element and the isolation diaphragm. The isolation diaphragm substantially matches the contour of the flow passage in the flow meter body, and a thickness of the isolation diaphragm is at least an order of magnitude less than a characteristic wavelength of the diaphragm material.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, which shows a schematic cross-section of a transducer assembly according to one exemplary embodiment of the invention, an ultrasonic transducer assembly2includes a proximal end4oriented towards a medium to be measured and an opposing distal end6. The transducer assembly2further includes an outer housing8which may be cylindrical in shape. The outer housing8of the transducer assembly2may be metallic, e.g., aluminum, stainless steel or titanium, or may be plastic. If plastic, it is preferably shielded electrically on the inside. An end cap10secured to the distal end6of the outer housing8includes a threaded bore12to removably secure an electronics package14to the transducer assembly2. For example, the outer housing8and the end cap10may be fabricated from stainless steel and electron beam welded to form a unitary construction.

Within the housing8, an electrical lead16conducts electrical signals from the electronics package14to an electroacoustic signal generating element18, which in the disclosed embodiment is a piezoelectric crystal. The wafer-shaped crystal18transmits and receives ultrasonic signals along an ultrasonic path20perpendicular to the planar face of the crystal18. Transmission of ultrasonic signals is responsive to a voltage applied to the crystal18, while voltage is generated at the crystal18upon reception of ultrasonic signals.

The electroacoustic signal generating element18may alternately include a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.

The transducer assembly2may further include an acoustic dampening compound22disposed on the back side of the electroacoustic signal generating element18. The acoustic dampening compound22minimizes reverberation of the ultrasound signal within the inner chamber24of the transducer assembly2, thereby increasing the performance of the transducer. The acoustic dampening compound22may be comprised of tungsten-loaded epoxy, or graphite. In some constructions, the acoustic dampening compound22may serve as a buffer to isolate delicate internal components from environmental extremes, such as high temperature applications.

The transducer assembly2further includes an isolation diaphragm26coupled to the proximal end4of the outer housing8. In one example, the coupling is implemented by welding. The diaphragm26isolates the medium being measured, such as fluid flow, from internal components of the transducer. In some fluid flow measurement applications, the fluid pressure may be several thousand pounds per square inch. Therefore, prior art ultrasonic transducers typically were fabricated from high strength metals such as stainless steel or titanium, and the diaphragm typically measured 0.080 to 0.100 inches thick to withstand the large pressure differential.

In contrast, the isolation diaphragm26of the disclosed transducer assembly2is quite thin. The minimum thickness of the isolation diaphragm26may be limited only by manufacturing practicalities. Thus, in one example, the thickness of the isolation diaphragm26is 0.02-0.50 millimeters (0.001-0.020 inches). In another example, the thickness of the isolation diaphragm26may be in the range of 0.07-0.13 millimeters (0.003-0.005 inches). Candidate materials for the isolation diaphragm26include metallic foils, such as stainless steel, titanium, or aluminum. The maximum thickness of the isolation diaphragm26is limited by the characteristic wavelength of the diaphragm material. In one embodiment, the thickness of the diaphragm26may be thin relative to the characteristic wavelength of the diaphragm material, so that the acoustic energy being transmitted and received by the electroacoustic signal generating element18does not become attenuated or reflected back into the housing or off on an angle. The characteristic wavelength λcof the isolation diaphragm material may be defined as the distance a sound waveform travels between its peaks, or
λcc/f(1)
where c is the speed of sound of the material and f is the characteristic frequency of the ultrasonic signal. In one example, the speed of sound in aluminum is 5960 meters per second and the frequency of the ultrasonic pulse is 1 MHz. The characteristic wavelength is thus 5.969 millimeters (0.235 inches). The thickness of the isolation diaphragm26may be expressed as a ratio or fraction of the characteristic wavelength. In one example, the thickness is one tenth, or one order of magnitude, less than the characteristic wavelength. In another example, the thickness is one one-hundredth, or two orders of magnitude, less than the characteristic wavelength. In any event, the thickness of the isolation diaphragm26is much less than the characteristic wavelength.

To eliminate the pressure differential across the isolation diaphragm26, a fluidic transmission layer28is disposed between the distal side of the isolation diaphragm26and the proximal face of the electroacoustic signal generating element18. The fluidic transmission layer28may be a liquid or a gel, but not a solid. For example, if the fluidic transmission layer28is a liquid, candidate liquids may include water, oil, silicon oil, or glycerol. If the fluidic transmission layer28is a gel, candidate gels may include petroleum jelly, grease, polymeric gel, polyurethane gel, or silicone gel. The fluidic transmission layer28may fill the entire inner chamber24of the transducer assembly2.

The fluidic transmission layer28serves at least two beneficial purposes. In one aspect, the layer28acts as a fluid matching layer to better couple the ultrasonic signal to the medium being measured. For example, if the transducer assembly2forms part of an ultrasonic flow meter measuring crude oil flow, the fluidic transmission layer28may comprise silicon oil. In this manner, the sound wave from the electroacoustic signal generating element18travels through the silicon oil, through the isolation diaphragm26, and through the flowing crude oil. Because the thickness of the isolation diaphragm26is much less than the characteristic wavelength, very little loss of signal integrity is encountered through the transmission layer. And, because the fluid characteristics of the transmission layer28closely match the flow being measured, very little loss of signal integrity is encountered. Thus, the disclosed transducer assembly2provides superior signal-to-noise ratio over transducers in the prior art.

In another beneficial aspect, the wetted boundary30of the transducer assembly2remains in the same location, e.g., against the fluid to be measured, but the pressure boundary32(e.g., the surfaces reacting the fluid pressure) is relocated away from wetted boundary30. In the disclosed example, the pressure boundary32is relocated behind the electroacoustic signal generating element18to a backplate34. The backplate34may be of any suitable construction and thickness to withstand the pressure forces. In one example, the backplate34is stainless steel, 2.54 millimeters (0.100 inches) thick, and welded to the outer housing8. In another example (not shown), the pressure boundary32is against the proximal face of the electroacoustic signal generating element18. In either case, the pressure drop or differential pressure across the isolation diaphragm26is eliminated because the fluidic transmission layer28transfers the reaction forces to another location within the transducer.

The relocation of the pressure boundary32allows the isolation diaphragm26to be positioned at an angle α relative to the proximal face of the electroacoustic signal generating element18without deflecting the ultrasonic signal from the ultrasonic path20. In this manner, the isolation diaphragm26may be fabricated to substantially match the inner contour of a pipe surface. For example, if the disclosed transducer assembly2forms part of an ultrasonic flow meter that is mounted through an access aperture in a fluid conduit, the diaphragm26may have a curvature matching the inner diameter of the conduit.

In some constructions, portions of the electrical lead16will be exposed to the pressures in the fluidic transmission layer28. As illustrated inFIG. 1, the pressure boundary32exerts fluid pressure on the backplate34, which may be several thousand pounds per square inch. To prevent the fluid from leaking (e.g., wicking) along a central conductor35to an unpressurized zone such as cavity37, the transducer assembly2may include a metal-to-glass seal36.

Referring toFIG. 2, wherein like numerals indicate like elements fromFIG. 1, a perspective view of a custody transfer flow meter utilizing a transducer assembly according to another exemplary embodiment of the invention is shown. A transducer assembly102according to the present disclosure is shown as part of a liquid custody transfer (LCT) ultrasonic flow meter138that is mounted in a spool piece configuration. The LCT flow meters typically meter crude oil, distillates, gasoline, diesel fuel or the like from a refinery operation. The LCT flow meters require 0.15% accuracy or better, due to the high volume flow and high retail price of the commodity.

The flow meter138includes an upstream flange140to mate with a portion of an upstream custody transfer conduit, a main body142to provide a flow passage144and ultrasonic measurement of a custody transfer fluid, a downstream flange146to mate with a downstream portion of the custody transfer conduit, and an electronics module148to route electrical wiring from the transducers. The body142is positioned at an oblique angle (e.g., at a 45° angle) relative to the custody transfer fluid flow direction150in order to provide upstream and downstream velocities for the ultrasonic transducer paths, as will be explained below. Mounted to the main body142are an upstream cover (not shown) and a downstream cover152to close off and seal the transducer assemblies102to allow the wiring to be routed through the electronics module148mounted atop the flow meter138.

Turning toFIG. 3, wherein like numerals indicate like elements fromFIG. 1, a cross-sectional schematic view of the custody transfer flow meter and transducer assembly ofFIG. 2is shown. The flow meter138is shown in cross section along the flow meter body142with the electronics module148removed for clarity. Because the flow meter138is installed obliquely to the flow axis of the spool piece, the cross-sectional view of the flow passage144appears elliptical when in fact it is round.

The flow meter138includes four pair of ultrasonic transducer assemblies102, the proximal end104of each transducer in facing relationship to each other at opposite sides of the body142. The four pair of transducers sample the flow profile in the flow meter and determine the actual flow rate by interpolating across the four sections. Each transducer assembly102includes an electroacoustic signal generating element118that emits an ultrasonic pulse across the custody transfer fluid flow path to the opposing transducer in the pair. In the illustrated embodiment, the four transducer assemblies102on the left side of the drawing emit pulses in the downstream flow direction, and the four transducer assemblies102on the right side of the drawing emit pulses in the upstream flow direction.

Each transducer assembly102includes an isolation diaphragm126that substantially matches the contour of the inner wall of the flow passageway. A fluidic transmission layer128is disposed between the distal side of the isolation diaphragm126and the proximal face of an electroacoustic signal generating element118. As can be appreciated with respect to the various installations and geometries shown inFIG. 3, the angle α of the isolation diaphragm126relative to the proximal face of the electroacoustic signal generating element118varies according to the location within the pipe. In one example, the pair of transducer assemblies102may be located such that the ultrasonic path120passes through the centerline of the flow passage. In that example, the angle α would be zero, e.g., the isolation diaphragm126would be parallel to the face of the generating element118. In another example, the pair of transducer assemblies102may be positioned to bisect approximately the quarter chord of the flow passage, as illustrated by the dashed line inFIG. 3. In that particular example, the angle α of the isolation diaphragm126relative to the face of the generating element118may be in a range of approximately 30-45 degrees. In yet another example, wherein the pair of transducer assemblies102may be positioned to bisect only a small portion of the flow passageway, the angle α of the isolation diaphragm126relative to the face of the generating element118may be approximately 60 degrees. This example is illustrated by the top and bottom pair of transducers inFIG. 3.

An advantage that may be realized in the practice of some embodiments of the described transducer assembly is that the isolation diaphragm may be configured to substantially match the contour of the inner wall of the flow passage. Thus, a transducer assembly according to an embodiment of the present invention, when installed in a flare gas flow meter, would not encounter the problems associated with transducer probe erosion and vibration.

A further advantage that may be realized when the isolation diaphragm substantially matches the contour of the inner wall of the flow passage is that, when installed in a liquid custody transfer or multiphase flow meter, the problems associated with erosion and blockage in the recess cavities are eliminated.

Another advantage that may be realized in the practice of some embodiments of the described transducer assembly is that the disclosed transducer assembly will not induce local flow perturbations in the vicinity of the access aperture. Local flow perturbations arise because a foreign object (e.g., the transducer) is introduced to the flow stream. Although not detrimental to the structural integrity of the transducer assembly, the flow perturbations necessitate software modifications to compensate for the decreased signal-to-noise ratio. The software compensation is time-consuming and is based upon trial and error.

Another advantage that may be realized in the practice of some embodiments of the described transducer assembly is that the transducer assembly does not transmit ultrasonic sound to the structural surfaces of the pipe or conduit. Thus, the cross-talk phenomenon or short-circuit noise between transducers is minimized, which improves timing accuracy and signal-to-noise ratio.

For example, the disclosed transducer assembly may be utilized in a Doppler flow meter. In the mode utilizing the Doppler effect, the flow rate is determined by measuring a rate of particle or bubble flowing with the fluid, under the assumption that the particle or bubble moves at a rate equal to that of the moving fluid. The moving rate of the particle or bubble can be determined by detecting variation of ultrasonic frequency from that of an ultrasonic wave applied to the moving particle or bubble to that of an ultrasonic wave reflected to the moving particle or bubble.

In another example, the transducer assembly may be utilized not to determine flow rate, but to detect the presence of foreign material in a flow stream, such as sand.