Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
1999-10-13
2001-09-25
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
C073S861060
Reexamination Certificate
active
06293156
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to flow measurement, and in particular to ultrasonic flow measurement wherein a fluid flowing in a conduit is measured by transmitting ultrasonic waves into or across the flowing stream. Such measurement systems are widely used in process control and other situations where fluid measurement is required. In general the constraints involved in setting up any such system involve generating appropriate ultrasonic signals, coupling them into the fluid, receiving the signals after they have traveled through the fluid, and processing them to determine a parameter of interest such as flow-rate, fluid density or the like. Measurement by ultrasonic signals offer several advantages, among which are the possibility of performing the measurement without intruding into the fluid or its container or causing a pressure drop in the fluid line. When the situation permits the use of a transducer clamped to the outside so that no special machining is needed, then the further advantages of simple installation, low maintenance cost and portability may be obtained.
However, there are many trade-offs in configuring an ultrasonic fluid measurement system. Generally, the conduit or vessel wall carries noise and may also constitute a significant short circuit noise path between transducers. Further, when the fluid is a gas, the gas carries relatively little signal energy compared to that in the pipe wall, and the acoustic impedance mismatch may result in passage of an extremely weak signal, possibly along an inconveniently refracted path angle which may drift with changing parameters, making detection or reception difficult or impossible. Furthermore, when the desired measurand is the flow velocity, relatively high frequencies may be needed to obtain sufficient resolution, and these in turn may be subject to relatively strong scattering fluctuations from turbulence or scatterers in the fluid or solid path.
The foregoing factors have generally prevented the design of an economic and effective ultrasonic flow measurement system for gas or steam in pipes, particularly in steel pipes. Currently, steam flows are measurable only with wetted transducers fitted in a special spool piece. To fit a measurement system to a plant therefore entails a cumbersome and expensive procedure involving shutting down the steam line, cutting the line and fitting a spool piece or custom mount. This entails enormous costs. Thus, while wetted transducers adjacent to the free stream may be implemented with special installation or custom spool pieces, it would appear that substantial improvements in attainable signal quality will be required before an effective external measurement system can be devised for flowing steam, particularly at relatively low pressures, e.g., under 35 psia (20 psig).
It would therefore be desirable to develop an improved ultrasonic system for measuring the flow of steam or gas.
It would further be desirable to develop an ultrasonic system which conveniently clamps onto a steam or gas conduit without special tapping, cutting or machining operations, and which is capable of launching and receiving signals from which a flow measurement is accurately determined.
SUMMARY OF THE INVENTION
The present invention achieves one or more of the foregoing objects by energizing a region of a steam or gas conduit with one, or in some embodiments more than one, clamp-on ultrasonic signal transmitter coupled or attached to the conduit, and providing two or more receiving transducers which clamp onto the conduit away from the transmitter region and are positioned to receive signals along paths through the flowing fluid from the energized region. The ultrasonic signals in the pipe which are transmitted across the fluid to the receivers are modulated by inhomogeneities traveling with the fluid in the conduit, and the receiving transducers are positioned such that each receiving transducer receives signal energy that has crossed a different, but spatially well-defined path, and are spaced such that the two received signals may be cross-correlated to accurately determine the flow time between the two receiving positions. Preferably the transmitter operates in a continuous wave mode, and provides a beam which refracts at a high incident angle into the pipe wall to produce multiple internal reflections along one side of the conduit. The main energy of the transmitted sound propagates along the pipe wall with the incident angle, leaving a shadow zone or quiet region of the conduit, where the receivers are installed. The transducer may be an air backed crystal effective to transfer the desired wave mode, preferably a vertically polarized shear wave signal at a frequency between about 100 kHz and 1 MHz, to the pipe wall, permitting the signal to propagate through the gas or steam without excessive attenuation. The diameter of the receiving transducers is preferably small, under about two centimeters such that they are sensitive to anomalies or discontinuities of small size in the flowing fluid.
Applicant contemplates two aspects of the invention. In the first aspect a transmitter may provide a single-frequency signal at frequency f
0
that refracts into the conduit wall producing multiple internal reflections along one side of the conduit. In this case two separate receiving transducers receive signal energy that has passed through the fluid along separate paths. In a second aspect, a system of the present invention may employ two transmitters at different frequencies, with each signal received and demodulated separately.
In the single frequency system, the spacing L, of the receiving transducers is less than the skip distance L
p
of a single path reflection of the transmitted signal in the conduit wall. When signal quality or quiet background conditions permit, the receiver spacing may be set larger than, but an integral multiple of L
p
, in which case a peak absorber structure, which may, for example, be fabricated of steel or stainless steel, is preferably coupled to the conduit wall between receiving transducers to couple out fluid-borne energy which has entered the steel pipe wall upstream. This effectively blocks the more highly correlated components of short circuit signal energy from contributing to the downstream receiver's reception signal of the energy received along a direct path through the fluid to the downstream receiver, thus effectively decoupling the signals received at each receiver and preventing the occurrence of false correlation peaks.
In a system according to the second aspect of the invention, when two transmission signals are launched from separate regions and at different frequencies f
0
, f
1
, two separate receiving transducers are spaced to receive the transmission signals at the frequencies f
0
, f
1
respectively after traveling through the fluid. The two frequencies f
0
, f
1
are selected to be comparable, e.g., to be close enough to each other that noise from the moving fluid modulates both signals similarly. Further, the spacing L between the two receiving transducers is set to be identical to the spacing between the two transmitting transducers. This distance L is further restricted by the consideration that the fluid paths defined between the respective transmitter-receiver pairs are to be sufficiently close that the noise modulation of the two received signals is highly correlated. This noise-coherence distance varies with flow velocity and conduit diameter, but L may typically be a spacing of one to three pipe diameters.
Preferably the signal from each transducer passes through a band-pass filter with center frequency at the transmission frequency, and is quadrature demodulated to produce a detection signal of enhanced signal to noise ratio; this signal is cross-correlated with the signal from the other receiver to determine the correlation peak or time delay between the two received signals. The transducers may be coupled to the conduit via a plastic wedge to mode convert the transducer energy to a shear wave signal, or may be cou
Jacobson Saul A.
Shen Chang
Fuller Benjamin R.
Nutter & McClennen & Fish LLP
Panametrics, Inc.
Patel Jagdish
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