U.S. patent application number 10/331914 was filed with the patent office on 2003-07-03 for anti-parallel tag flow measurement system.
Invention is credited to Liu, Yi, Lynnworth, Lawrence C..
Application Number | 20030121335 10/331914 |
Document ID | / |
Family ID | 24819757 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030121335 |
Kind Code |
A1 |
Liu, Yi ; et al. |
July 3, 2003 |
Anti-parallel tag flow measurement system
Abstract
A tag flow measurement system wherein a first and a second
measurement path are provided across a flowing fluid, and a
receiver in each path receives signals modulated by scatterers in
the fluid. The direction of signal propagation in one path faces in
an opposite sense to, e.g., is anti-parallel to, the direction of
propagation in the other path, and the two receiver outputs are
correlated to determine a time interval representative of flow
velocity. In one embodiment each path is defined by a transmitter
on one side of the conduit and a receiver on the other side of the
conduit, and the positions or orientations of transmitter and
receiver are reversed in the second pair. Thus, the first
transmitter may lie on the same side of the conduit as the first
receiver. Diametral or chordal paths may be used. A prototype
clamp-on system detects flowing air at atmospheric pressure in a
schedule 40 one inch steel pipe over an extended range, at flow
rates as low as several meters per second. The coherent crosstalk
between one transmitter and the receiver of the other pair is
greatly reduced, and travels only through the pipe and not across
the fluid, so the cross-correlation signal to noise ratio may be
enhanced by a factor of ten or more over that of a conventional tag
correlation system. This allows effective operation in small
conduits, at small spacings, at low flows and in other difficult
measurement situations. Each transmitter may operate at a different
frequency, and the received signals may be demodulated in phase
quadrature to further enhance channel separation and received
signal power. Frequencies or frequency pairs in the range of
approximately one to four megahertz may be useful for one inch
pipe, while lower frequencies in the range of 0.1 to 0.5 megahertz
are advantageously employed for larger conduits. Spacings may be
{fraction (1/4 )} to {fraction (1/1)} of a pipe diameter, and a
common spacing, e.g., two inches may be employed for conduits over
a diameter range of one to ten inches with high accuracy.
Inventors: |
Liu, Yi; (Bolton, MA)
; Lynnworth, Lawrence C.; (Waltham, MA) |
Correspondence
Address: |
IANDIORIO & TESKA
260 BEAR HILL ROAD
WALTHAM
MA
02451-1018
US
|
Family ID: |
24819757 |
Appl. No.: |
10/331914 |
Filed: |
December 30, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10331914 |
Dec 30, 2002 |
|
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09702074 |
Oct 30, 2000 |
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Current U.S.
Class: |
73/861.18 |
Current CPC
Class: |
G01F 25/13 20220101;
G01F 1/667 20130101; G01F 1/662 20130101; G01F 1/7082 20130101 |
Class at
Publication: |
73/861.18 |
International
Class: |
G01F 001/20 |
Claims
What is claimed is:
1. An ultrasonic measurement system for determining flow of a fluid
in a conduit, such system comprising: a first transmitter receiver
pair configured to define a first ultrasonic signal chordal path
across fluid flowing in the conduit; a second transmitter receiver
pair configured to define a second ultrasonic signal chordal path
across fluid flowing in the conduit; said second transmitter
receiver pair being mounted so that the second signal chordal path
is antiparallel to the first signal chordal path and spaced a fixed
distance therefrom; and a processor operative to correlate a
tag-modulated output signal of said first pair with a tag-modulated
output signal of said second pair to determine a time interval
representative of flow.
2. The system of claim 1 wherein said first pair operates at a
different frequency than said second pair.
3. The system of claim 1 operating in a frequency range above 100
kilohertz.
4. The system of claim 3 wherein said frequency range lies above
approximately 900 kilohertz.
5. The system of claim 1 wherein said first pair operates at a
frequency different than frequency of operation of said second
pair, and received signals are demodulated at their transmission
frequency.
6. The system of claim 5 wherein said first pair operates at a
frequency within approximately ten percent of said frequency of
operation of said second pair.
7. The system of claim 5 wherein said first pair and said second
pair operate in a continuous mode.
8. The system of claim 1 wherein the fluid is a liquid.
9. The system of claim 8 wherein the liquid contains
scatterers.
10. The system of claim 8 wherein the liquid is turbulent.
11. The system of claim 1 wherein the fluid is a gas.
12. The system of claim 9 wherein the gas is a low density gas.
13. An ultrasonic measurement system for measuring flow of a fluid
in a conduit, such system comprising: first and second transmitter
receiver pairs defining first and second transit paths across a
conduit, the second transit path being anti-parallel to the first
transit path and wherein said paths are chordal paths; a signal
processor for processing signals received along said first and
second chordal paths; and a correlator for determining a time
interval between correlated tag modulated signals on said first and
second chordal paths.
14. The system of claim 13 wherein said transducers are coupled to
a steam pipe of a building heating system.
15. The system of claim 13 wherein said transducers are attached to
a process feed gas pipe of a chemical plant.
16. The system of claim 13 wherein said transducers are attached to
a conduit having a nominal diameter under about two inches.
17. The system of claim 13 wherein at least one of said transducers
is attached to a conduit by clamp-on.
18. A method of measuring flow of a fluid in a conduit, such method
comprising the steps of: providing a first transmitter/receiver
pair defining a first signal path through fluid in the pipe such
that a first receiver output is modulated by tags in the fluid;
providing a second transmitter/receiver pair defining a second
signal path through fluid in the pipe such that a second receiver
output is modulated by tags in the fluid; the second signal path
being anti-parallel to the first path, the first and second signal
paths being chordal paths; and correlating the second receiver
output with the first receiver output to determine flow rate.
19. The method of claim 18 further including the steps of operating
the first transmitter/receiver pair at a first frequency and
operating the second transmitter/receiver pair at a second
frequency different from the first frequency, wherein the first
frequency is sufficiently high to be well modulated by the tags,
and the second frequency is close to the first frequency.
20. The method of claim 19 wherein the steps of providing
transmitter/receiver pairs include providing clamp-on transducers.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 09/702,074 filed Oct. 30, 2000 entitled
"ANTI-PARALLEL TAG FLOW MEASUREMENT SYSTEM", and this continuation
application hereby claims priority to and hereby incorporates
herein by reference U.S. patent application Ser. No.
09/702,074.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to measurement of fluid flow
and to such measurements performed by receiving and processing
ultrasonic signals that are transmitted into the fluid. It
particularly relates to tag flow meters wherein inhomogencities or
turbulence in the fluid itself, or matter such as bubbles, droplets
or particles that are moving in the fluid flow, constitute "tags"
that modulate the ultrasonic signal. For operation of such tag flow
measurement systems, signals are generally launched across the
fluid along two parallel paths and received by separate receivers
after being modulated by the fluid flow.
[0003] This mode of measurement, useful for certain flows that
naturally contain, or are seeded, heated, agitated or injected to
include, discrete entrained scatterers, offers a number of
advantages. By detecting a similar pattern of modulation on the two
paths at different times, the fluid velocity may be directly
derived from the distance between paths divided by the elapsed time
between occurrence of correlated modulation patterns. Thus,
baseline calibration steps for amplitude, density, temperature or
the like, commonly required for other types of ultrasonic
measurements, are not needed. Moreover, tag measurements often are
applicable to situations in which the scatterers would introduce
too much attenuation or noise for other modes of ultrasonic
measurement to be effective. Tag measurement are also useful when
most of the interfering noise is stationary, e.g., coherent
crosstalk, but not overwhelming.
[0004] In general, an effective tag measurement requires that the
tags or modulators present in the fluid flow be displaced
coherently, i.e., as a group along the direction of fluid flow so
that they modulate the signal similarly as they cross each of the
interrogation paths. This may require that the paths be located
relatively close together with a spacing that decreases with
decreasing flow velocity. When the transducers are mounted outside
the conduit wall, or are not well isolated acoustically from the
conduit wall, as frequently occurs in clamp-on measurement
applications, this may result in high levels of noise and
crosstalk, substantially degrading the signal to noise ratio and
making effective correlation of the two received tag-modulated
signals difficult or impossible.
[0005] Recently, Chang Shen and Saul Jacobson have proposed, in
commonly owned U.S. patent application Ser. No. 09/417,946 filed
Oct. 13, 1999, that a clamp-on tag measurement system may be
effectively implemented, even for relatively low impedance fluids
such as low pressure steam traveling in a steel conduit, by
arranging the transmitter to energize a region of the wall and
positioning plural receivers to discriminate received signals along
different parallel paths of known spacing. That patent application
describes a tag measurement method using two different signal
frequencies in the two transmitter/receiver pairs. The entire
specification of that patent application is hereby incorporated
herein by reference. While the approach described therein has
extended the feasibility of clamp-on measurements to low pressure
gases in noisy conduits, the nature of such clamp-on applications
and systems involves a low signal to noise ratio, and in many
practical applications involving small pipes or low flow rates,
such as in domestic heating plants where a fluid such as wet steam
otherwise appears appropriate for tag measurement, the environment
itself provides such a level of intrinsic system noise that even
the improved transducer and processing configurations described
above may fail to yield discernible signals, or fail to provide
effective and repeatable correlations.
[0006] It is therefore desirable to provide a more effective tag
flow measurement system.
[0007] It is also desirable to provide an ultrasonic tag flow
measurement system applicable to small conduits, and/or noisy
environments.
[0008] It is further desirable to provide an ultrasonic measurement
system adapted to perform either continuous or occasional
measurements with certainty and accuracy.
[0009] It is also desirable to provide an ultrasonic tag flow
measurement system useful for measuring flow of steam, flare gas,
and industrial process gases in small conduits.
SUMMARY OF THE INVENTION
[0010] One or more of the foregoing results are obtained in
accordance with the present invention by providing a tag flow
measurement system wherein a set of preferably clamp-on transducers
define first and second measurement paths spaced apart along the
conduit and directed across a fluid flowing in the conduit such
that a receiver in each path receives signals modulated by
scatterers in the fluid. The direction of signal propagation in one
path is in an opposite sense, e.g., anti-parallel, to the direction
of propagation in the other path, and a correlation processor
operates on both received signals to determine the flow rate. In
one embodiment each path is defined by a transmitter on one side of
the conduit and a receiver on the other side of the conduit, with
the positions of transmitter and receiver being reversed in the
second pair. Thus, typically, the first transmitter lies on the
same side of the conduit as the second receiver, and the second
transmitter lies on the same side of the conduit as the first
receiver. A communicating transmitter and receiver may be
diametrically opposed, or may be positioned at opposite ends of a
chordal path.
[0011] Advantageously, the system may be applied to small conduits
having a diameter below 100 millimeters, and it has even been found
effective, for example, on a schedule 40 one inch steel pipe
carrying low pressure steam or gas. The use of anti-parallel paths
greatly reduces coherent crosstalk extending from one transmitter
to the receiver of the other pair, and signal to noise ratio may be
enhanced by a factor of ten or more, allowing effective correlation
of the two received signals. Preferably, each transmitter operates
at a different frequency, and the received signals are received and
demodulated in phase quadrature to further enhance signal quality.
Frequencies or frequency pairs in the range of approximately one to
four megahertz may be useful for one inch pipe, while lower
frequencies in the range of 0.1 to 0.5 megahertz are suitable for
larger conduits.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other features of the invention will be understood
from the description of illustrative embodiments below, taken in
together with the Figures, wherein:
[0013] FIG. 1 illustrates a prior art tag measurement system;
[0014] FIG. 2 illustrates a tag measurement system of the present
invention;
[0015] FIGS. 2A-2E illustrate chordal tag measurement systems;
[0016] FIGS. 3A and 3B illustrate signal paths and different
driving arrangements for the system of FIG. 2;
[0017] FIG. 4 illustrates a processor used in the system of FIG.
2;
[0018] FIG. 5 shows a table of tag measurements made in a prototype
system of the present invention; and
[0019] FIG. 6 illustrates range, repeatability, and comparability
of measurements made with the present invention and with the prior
art.
DETAILED DESCRIPTION
[0020] FIG. 1 illustrates a prior art transducer arrangement and
signal path in a clamp-on tag flow meter arrangement 10. As shown,
a first transducer pair T1, R1 attaches to a conduit 4 to launch
the transducer signal denoted S.sub.T1 such that the fluid-borne
portion of the signal crosses the fluid at a defined path angle
.theta..sub.3 on path P1, and a second transducer pair, T2, R2 is
positioned offset a distance L along the flow stream to define
second fluid path P2 for a transmitted signal S.sub.T2. P2 is
located parallel to, and a short but fixed or known distance along
the conduit from, path P1. Thus, for entrained scatterers in a well
behaved flow, the signal modulation produced by a given set of tags
will be substantially identical at the two different times when
that set of tags pass through the respective beams. Apart from
profile considerations, flow velocity .nu. is then given by this
time interval (.tau.) and the path spacing L, namely .nu.=KL/.tau.,
where K is a meter factor that takes in account the flow profile
and the manner in which the sound beams interact with the flow
along the paths. In most cases .theta..sub.3 is small-commonly
0.degree., or 5.degree.-7.degree. for air or steam, or
15.degree.-25.degree. for hydrocarbon liquids or water,
respectively.
[0021] As shown in FIG. 1, each receiver also receives crosstalk
from the transmitter of the other pair. The interfering signal from
transmitter T2 to receiver R1 is denoted S.sub.T21, and the
interfering signal to receiver R2 from transmitter T1 is denoted
S.sub.T12. The interfering signals are propagated in large part,
and possibly with high amplitude, through the conduit wall.
Furthermore, when a region of the wall is energized with a CW
signal, this gives rise to a periodic sound field with coherent
noise along additional paths through the fluid. As described in the
aforesaid patent application, to address the wall-borne component,
the transducers of a pair may be positioned closely to place a
receiver in a quiet shadow zone, and special wall damping may be
applied to reduce coherent or correlated noise propagating along
the pipe. However, such isolation techniques have their limits.In a
typical path spacing on a three inch schedule 40 steel pipe, the
unwanted signal or crosstalk received from the other transducer may
be approximately equal to the amplitude of the signal from the
proper transmitter. Moreover, even when configured as described in
the above-referenced patent application, only a small percentage of
the signal received from the transmitter of a transmitter/receiver
pair (e.g., of the signal S.sub.T1 received by receiving transducer
R1) consists of the desired information-bearing signal transmitted
through the fluid and modulated with the appropriate tag
information. The received signal also includes short circuit noise
carried by the wall, as well as other ringing, mode-converted
signal, echoes and system noise. As a result, for smaller conduits
such as a one inch schedule 40 pipe, where the short circuit signal
suffers little attenuation and dispersion, the signal to noise
ratio is so low that it may prevent effective tag correlation
measurements. This may be the case even when transmitting and
demodulating with different frequencies on the two paths as
described in the above-referenced patent application, and even when
applying damping material to reduce the level of crosstalk or short
circuit signal traveling within the conduit wall. In one experiment
to determine signal levels for such a configuration, for air at
atmospheric pressure, the received crosstalk signal S.sub.T12 was
1.7V, while S.sub.T2 was 0.06V and constituted too low a component
to be able to permit a meaningful measurement.
[0022] FIG. 2 illustrates a tag measurement system 20 of the
present invention. As shown, transmitter/receiver pairs define two
paths P1, P2 with the signal paths being antiparallel and separated
by a distance L. That is, not only are the paths at different
locations, but the signal of one path traverses the fluid in the
opposite sense from the signal of the other path. Thus, T1 and R2
lie on one side of the pipe and T2 and R1 lie on the other side of
the pipe. Applicant has discovered that despite this reversal of
the path direction, the signal modulation passively introduced
along the two paths by eddies or other scatterers in the flowing
fluid remains relatively similar; the signals received along the
two paths are well-correlated. Advantageously, the reversal of
orientation substantially suppresses coherent crosstalk. The two
transmitters may further be driven at similar but different
frequencies, e.g., frequencies that differ by under about fifteen
percent. The corresponding receivers are connected to circuits for
demodulating each of the received signals at its transmission
frequency. Most preferably, each signal is demodulated in phase
quadrature and its components are combined to produce an output
signal of enhanced amplitude.
[0023] In a prototype embodiment used to measure the flow of air in
a one inch schedule 40 pipe, signals of 888 kilohertz and 1
megahertz were employed in the two different paths. As illustrated
schematically in the Figure, each transducer is coupled to the
conduit via a wedge W to define a precise launch angle. The
preferred wedge is a low sound speed polymer wedge which converts
to, or couples the signal as, a shear wave into the conduit wall,
so that leaving the wall, the beam refracts along the path P1 or P2
at an appropriate angle, i.e., an angle to the conduit wall
(corresponding to .theta..sub.3 of FIG. 1), through the low density
gas in the conduit interior. The transmitters may typically operate
in a continuous wave (CW) mode. However, to inject a time reference
as would be useful for measuring c, the CW wave may be coded or
otherwise modulated. A similar polymer wedge W and mode conversion
receiving geometry at each receiving transducer produces respective
receiver output signals S.sub.R1, S.sub.R2 which are continuously
processed and sorted into multi-point signal value measurement
bins, such as successive time ordered sets of 1024 measurement
points. The two sets of received signals are then correlated. The
correlation processing may be carried out for example as described
in commonly owned U.S. Pat. No. 4,787,252 of Saul Jacobson et al.,
the text of which is hereby incorporated herein by reference in its
entirety.
[0024] FIG. 2A shows the use of antiparallel paths P1, P2 in a
chordal interrogation arrangement. The paths may, for example, be
midradius paths, and may include once- or twice-reflected paths. In
the illustrated configuration, the respective transmitter/receiver
pairs are oppositely-facing, e.g., in clockwise and
counterclockwise orientations, respectively.
[0025] Referring to FIG. 2A, two parallel measuring planes are
spaced axially a distance L. The paths may be referred to as
chordal to distinguish them from diametral paths. In this Figure
the paths for tag are shown schematically as path ABC associated
with T1 and R1, and path DEF associated with T2 and R2. As would be
best seen in an end view, the paths may be inscribed congruent
equilateral triangles, each interrogated in a sense opposite the
other, e.g., clockwise for ABC and counterclockwise for DEF. If the
interrogating wave were a pulse of continuous wave (amplitude
modulated CW), for example, one cycle of a sine wave, then such a
pulse may travel along the fluid path in a time short compared to
the time for the fluid in the first plane to flow to the second
plane.
[0026] In FIG. 2B, four parallel planes are shown by shading. The
interrogating wave now follows a helical path, e.g., in the left
region, from T1, the path is A'B'C'C", exiting to R1. The points
where this path intersects the two planes are emphasized by dots at
points A' and C". The two planes in this left region are spaced L",
whereas the important length is L between the first pair of planes
and the second pair at the right. The path at the right starting
with T2, is: D'E"F"F", exiting to R2. If viewed from the end of the
pipe, these paths would be congruent. The directions or senses of
interrogation would be opposite, however. The paths both in FIG. 2A
and those in FIG. 2B are over twice as long as diameter paths of
FIG. 2 and so allow more than twice as much time for crosstalk
pulses or waves to decay, before the desired fluid-borne pulse or
wave signal is received. In continuous wave mode, the result is
that SNR is improved, because one can think of the CW interrogation
as being made of the serial superposition of an endless stream of
individual cycles from a continuous source of sine waves.
[0027] In an application such as air in a steel pipe, the mismatch
of acoustic impedance is so great that the wave introduced by
transducer T1 primarily stays in the pipe and reradiates many times
into the fluid before the energy is totally dissipated. This is
illustrated conceptually in FIG. 2C. The wave incident from T1 at
incident angle .theta..sub.1, in a plane perpendicular to the pipe,
gives rise to a first refracted ray at refracted angle
.theta..sub.3 and that ray proceeds tangent to a construction
circle of reduced radius R.sub.cc. The broken-line path zigzagging
around the pipe radiates another ray which typically will also be
tangent to the same construction circle, or to one of similar
radius. This zigzag model is well known and may be found in patents
by Brazhnikov, or by Lynnworth and others, or elsewhere in the
technical literature. The chordal paths in FIGS. 2A, 2B thereby
interact primarily with eddies or turbulence outside the
construction circle, i.e. they exclude a core, while the diameter
paths of FIG. 2 interact with the core too. Thus the two
interrogations may complement one another and may be used to define
the proper meter factor K.
[0028] Another characteristic of the chordal paths of FIGS. 2A, 2B
is that, in end view, the transducers can all be on one side of the
pipe and are contained within a narrow arc corresponding to the
angle .theta..sub.x in FIG. 2D. the subtended angle .theta..sub.x
is less than or equal to 60.degree. and preferably is less than or
equal to 30.degree.. Conduit 200 may be a steam pipe of a building
heating system or a process feed gas pipe of a chemical plant. See
FIG. 2E.
[0029] By employing anti-parallel paths between the respective
pairs of transmitter and receiver, the crosstalk from one
transmitter reaching the receiver of the other path consists
entirely of signal passing through the wall of the conduit, and has
no contribution or a greatly reduced coherent component in the
relevant time window that has crossed the fluid path or encountered
the scatterers of interest for a tag correlation measurement. This
situation is illustrated in FIG. 3A where the cross talk signal
S.sub.T12 from transmitter T1, and the cross talk signal S.sub.T12
from transmitter T2 are each shown propagating along the pipe wall
to an adjacent receiver of the opposite path. The tag correlation
measurement depends on the presence of scatterers, and it is
immaterial whether the two pairs are upstream or downstream with
respect to the flow as seen in FIG. 3A, or vice versa as shown in
FIG. 3B. In both cases, interfering cross talk through the fluid is
eliminated resulting in received signals that dependably correlate
with a well defined first maximum corresponding to the flow
velocity of fluid in the pipe.
[0030] Advantageously, applicant has found that the anti-parallel
path configuration of the present invention allows the two opposed
paths, and the transducers that define those paths, to be spaced
quite closely to each other. Thus, for example, the distance L may
be two inches or less, even for large (e.g., ten-inch) conduits.
Transducers as shown in commonly-assigned U.S. Pat. No. 6,047,602
may advantageously be used to define even closer diametral or
chordal signal paths, and may be positioned in planes transverse to
the pipe axis or placed for reflective interrogation paths.
[0031] FIG. 4 schematically illustrates a signal detection or
demodulation section used for the measurement signal processor in
one prototype implementation. The two transmitting transducers T1,
T2 are each driven with a different frequency signal f.sub.0,
f.sub.1, and the frequency is set sufficiently high so that the
signal traveling through the flowing liquid, multiphase mixture,
gas or steam is effectively modulated by the type of
inhomogeneities or disturbances that are present in the flow. The
two frequencies are also set sufficiently close to each other,
e.g., within about ten percent, so that the modulation of both
signals by the flow will remain highly correlated. By way of
numerical examples, the transducers may be driven frequencies of
about 450-475 kHz and 500 kHz, or about 900 kHz and 1 MHz. For
smaller conduits, the higher frequency range is preferred both to
achieve adequate modulation by the fluid, and for its greater
attenuation of crosstalk. As described further below, the separate
frequencies permit the received signals to be separately
demodulated, without relying on specialized filters, or on physical
damping or isolation structures between receivers, to both reduce
crosstalk and produce two distinct fluid path signals. In other
embodiments, applicant has achieved accurate and repeatable
measurements using the same frequency for both paths.
[0032] The two receiving transducers R1, R2 produce receiver output
signals S.sub.R1(t), S.sub.R2(t), respectively, which are
separately quadrature demodulated at frequencies f.sub.0, f.sub.1,
respectively, as shown in FIG. 4. As show therein, the received
signals may be band pass filtered and amplified before quadrature
demodulation. The conditioned I, Q signals in each channel may also
be low pass filtered and provided as a further input to an A/D
converter prior to passage to the digital signal processor for
sorting into time-ordered sets of signal values (e.g.,
(I.sup.2+Q.sup.2).sup.1/2) and correlation of the two sets of
signals. The correlation processor thus operates by sampling the
demodulated output signals at a sampling frequency F, gathering the
sample values into successive bins, for example 2.sup.10 signal
value sampling points, and then correlating the two received
signals in time to determine a peak correlation interval from which
the flow rate in the measured region of pipe is readily determined
for the given path spacing.
[0033] In general, it is desirable that each receiver R1, R2 be
positioned under about one pipe diameter along the flow direction
from its corresponding transmitter T1 or T2, and further that its
beam reception width or aperture along the axial direction be less
than that of the transmitters. The receivers may use a shielded PZT
crystal transducer, and a plastic wedge at an angle designed to
launch a shear wave at about 70.degree. in the pipe wall, and
refract into the fluid at an angle such that the path P1 or P2 is
oriented fairly directly across the conduit.
Example 1
[0034] A test arrangement using atmospheric air as the test fluid
was set up to compare signal quality and measurement feasibility of
an anti-parallel tag correlation measurement of the invention with
that of the Shen-Jacobson measurement configuration described
above. Two pairs of transducers were repeatedly installed and
uninstalled five times at the same location to determine the
efficacy and repeatability of measurements using clamp-on
transducers for each of these tag correlation measurements. A
one-inch schedule 40 stainless steel (SS) conduit served as the
measuring conduit, with transducers spaced on the same side of the
conduit 2.7 inches apart. The SS pipe was connected to a 2.5 inch
PVC pipe that was used for a reference meter where dependable
measurements could be made on a non-ringing conduit wall, and air
at substantially atmospheric pressure was forced through the
conduit by a fan.
[0035] FIG. 5 is a table showing the results of the repeatability
tests for transducers arranged to perform a parallel tag
correlation measurement (denoted P), and arranged to perform an
anti-parallel tag correlation measurement (denoted AP). Motor
speeds ranging from 3,000 to 9,000 rpm were employed in each of the
five runs, corresponding to scaled reference flow velocities of 6.8
to 25.7 feet per second. The reference meter was a Panametrics'
GM868 gas meter using 500 kilohertz CPT transducers clamped on the
2.5 inch PVC pipe, with one reverse path, and using a PRE868-1-40
preamplifier. The fan was a Transicoil 14SH brushless DC motor,
with an SE10E drive and a 2.5 inch fan blade.
[0036] As shown in the table, flow velocity was unmeasurable in the
first run using the parallel tag technique until the flow velocity
in the loop was driven to a maximum flow velocity three times above
the test range, and in the fifth run it remained unmeasurable at
the lowest motor speed. Velocity measurements were comparable using
tag or anti-parallel tag measurements and were comparable to the
scaled measurement. However, significantly the SNR (signal-to-noise
ratio) was many times higher using the anti-parallel paths of the
present invention. A typical improvement on cross correlation SNR
was about five-fold, so that the range of velocities and achievable
measurements in the low signal regime and a low flow velocities was
appreciably improved.
[0037] FIGS. 6A and 6B show plots of the tag cross correlations
using parallel (FIG. 6A) and anti-parallel (FIG. 6B) signal paths
arrangements. The X-axis represents the sample number, and the
Y-axis is the cross correlation ratio. The test conditions were the
same as described in the Table of FIG. 5, and the meter factor was
set at K=1 for both tag and the reference. It should be emphasized
that the reference was not a calibrated meter but was in fact a
clamp-on transducer arrangement on a relatively small, nominally
2.5 inch diameter, pipe with one reverse path. As show in FIGS. 6A
and 6B, the anti-parallel path arrangement produced an extremely
narrow peak correlation, which is desirable, with a value
substantially higher than all surrounding neighborhoods, while the
parallel path arrangement produced a large number of points of high
correlation value that were closely distributed on both sides of
the true peak, potentially making identification difficult or
questionable. Thus, the antiparallel path system produced velocity
measurements with a very low uncertainty, whereas the parallel path
arrangement resulted in indeterminate or highly uncertain
measurements.
[0038] In other simulations and tests, applicant has explored an
air flow range between about five feet per second and about two
hundred seventy five feet per second, or between about
2.5.times.10.sup.3 and 1.5.times.10.sup.5 in terms of Reynolds
number. This is a flow turndown ratio of nearly sixty to one, with
the low end being in or very close to the laminar flow regime.
While the noted velocities are simply meter readings, and the
actual flow rates unknown, the high signal to noise ratio, the
definite and distinct correlation, and the repeatability of results
indicate that the use of an anti-parallel path for tag correlation
is a greatly improved and effective method for flow measurement
under these difficult small conduit or high noise conditions. The
system has also been found advantageous in tests on water, and on
water mixed with a small percentage of air bubbles taken as
representative of a two-phase fluid. Clamp-on tests with air have
also been conducted successfully on ten-inch PVC plastic pipe using
antiparallel paths and Reynolds numbers from about 30,000 to over
150,000, using a vortex meter for reference.
[0039] The invention being thus disclosed and illustrative
embodiments described herein, further variations and modifications
will occur to those skilled in the art, and all such variations and
modifications are considered to be within the spirit and scope of
the invention as defined herein and by the claims appended hereto
and equivalents thereof.
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