Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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UNDERGROUND CONDUIT DEFECT LOCALIZATION
This is a divisional application of co-pending Canadian
Patent Application Serial No. 2,158,669, that was filed on
September 20, 1995.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of locating defects in
underground conduits, and in particular to locating leaks in
steam pipes buried in noisy environments, determining the rate
and direction of flow within a conduit, and locating defects
in electrical conduits.
2. Description of the Related Art
Conduits for transmission are often buried to protect
them and to save space. This is common in and around cities
for the transmission of water and steam, around industrial
plants for the transmission of chemicals or fuels, or even
across the country for the transmission of natural gas and
electric power. Leaks in any of these conduits can be costly
due to the loss of the transmitted fluid and dangerous because
of the accumulation of toxic or explosive fluids outside the
conduit.
Responsible practice therefore requires the detection of
a leak, its precise location, and its repair. The location of
the leak is most important in crowded environments due to the
disruption caused by excavation. One method to detect the leak
is to use surface sensor techniques which are hindered by
traffic, by turbulent flow within the conduit, by any
discontinuities in the conduit such as joints and traps, or by
noise generated by adjacent conduits. Another method is to
drill "bar holes" down to the conduit. This can be dangerous
to the integrity of the conduit and to repair personnel.
Common systems of leak detection include: acoustic
emission, infrared spectroscopy, tracer gas, and electrical
(Detection and Location of Leaks in District Heating
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Systems, D. S. Kupperman et al., Argonne National
Laboratories, ANL 92/5, March 1992). Half of the users of
acoustic technology feel that current acoustic methods are
not as effective as they desire (ibid.).
B. Characteristics of Steam Conduits
Acoustic methods of detection are further complicated
by the differing transmission properties of the conduit
itself, typically a metal, and the medium being
transmitted, either a gas or a fluid. Experience has shown
that the acoustic energy released by a steam leak
propagates down the metal conduit in three modes with
velocities of about 2500, 4000, and 6000 feet per second
for 16 inch or 24 inch diameter steam conduit. The
frequencies propagating in the metal are strongest below
1000 Hz and are severely attenuated at higher frequencies.
Steam conduits also contain thermal expansion joints, and
the leak noise is not discernible across these joints.
The primary medium for acoustic leak location is
therefore the steam, where propagation is unaffected by the
expansion joints. The propagation is multimode, with the
strongest amplitudes propagating with velocities about 500
and 1000 feet per second, and with least attenuation
between 1000 and 8500 Hz. The leak noise attenuates at a
rate between 0.07 and 0.15 dB per foot between these
frequencies. The movement of steam also creates a flow
noise caused by discontinuities such as joints and by
turbulence at higher flow rates'. Experiment has shown that
the flow noise amplitude is greatest below 2500 Hz.
A steam conduit is clearly not an ideal transmission
line for the propagation of information. Solving the wave
equation, which is well known in the communication art,
predicts what modes might be supported by the conduit, but
factors such as the location of the leak and the structural
support for the conduit determine which modes actually
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propagate, and this differs at every site. The propagation
velocity is therefore different at each site depending upon
which modes propagate.
C. Needs
Accordingly, there is a need to accurately locate
leaks in conduits, particularly where the cost of
excavation is high and where the danger to property, the
environment, and humans is great. Non-intrusive methods to
determine the propagation velocity of leak noise, the
medium flow rate, and its direction would enhance locating
the leak. Extending the range of detection, particularly
in low signal to noise ratio locations, would also improve
the process.
3. Summary of the Invention
The present invention relates to a method to more
accurately locate a leak in a conduit, particularly in a
noisy environment. The present invention also relates to a
method of determining the flow rate and direction of a
medium in a conduit.
In one embodiment of the invention, sensors are
attached to the conduit at three locations where they are
separated by a known distance from each other. Noise from
,..the steam leak propagating in the conduit is detected by
the sensors and an electrical signal is generated at each
location. The signals are recorded and converted into
digital form to preserve them.
Each signal is filtered to pass a frequency band from 4000
to 8500 Hz to discriminate against turbulent flow noise in
the steam, noise transmitted by the conduit, and single
frequency tones. A cross-correlation function from leak
noise data obtained from a first pair of sensors located
along the conduit is calculated to obtain a raw plot of a
first time differential. A cross-correlation function is
also calculated from leak noise data obtained from a second
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pair of sensors located along the conduit to get a raw plot
of a second time differential. These plots are smoothed to
obtain a peak time differential in each plot. The velocity
of propagation for leak noise in the conduit is then
calculated using the first peak time differential and the
known spacing between the first pair of sensors. An
uncorrected location of the leak is determined using the
velocity of propagation, the second peak time differential,
and the known spacing between the second pair of sensors.
This location may be adjusted by considering the rate and
direction of flow of a medium within the conduit to
determine the final leak location.
In another embodiment of the invention, an acoustic
method determines the flow rate and direction of steam
flowing in a conduit. Sensors are attached to the conduit
at two locations which are separated by a known distance.
A vibration is imposed upon the conduit on one side of the
sensors, and it is detected by both sensors. The process
is repeated where a second vibration is imposed on the
other side of the sensors. The signals generated by the
sensors are recorded, in either analog or digital form,
filtered to pass a frequency band from 4000 to 5500 Hz to
discriminate against turbulent flow noise in the steam,
noise transmitted by the conduit, and single frequency
tones, and then a cross-correlation function from data
obtained from the sensors from the first imposed vibration
is calculated to obtain a raw plot of a first time
differential, and the same is done for the second
vibration. Each raw plot of time differential is smoothed
to obtain a peak time differential in each plot. The
center velocity of propagation is determined using the
first peak time differential and the known spacing between
the sensors, and the process is repeated for propagation in
the other direction. The flow rate and direction of the
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medium in the conduit are then calculated from the
difference in velocities.
In a further embodiment of the invention, a defect in
an electrical conduit is located by imposing an electrical
5 pulse upon the conductor with sufficient potential to cause
an electric discharge at the defect. The acoustic energy
liberated by the electric discharge is determined with two
sensors, each mounted to the conduit and separated from the
other along a length of the conduit which does not include
the defect. Recording of the acoustic data can be
synchronized to the leading edge of the pulse. The
envelope of a cross-correlation function for each sensor
signal is calculated to determine its peak which provides
the time differential between the leading edge of the
electrical pulse received at the first sensor and that
received at the other sensor. The velocity of acoustic
energy propagation is calculated from the time differential
provided by a cross-correlation function of data from the
two sensors and the known spacing between them. Inspection
of the sign of the time differential determines which
sensor is nearest to the defect. The location of the
defect is computed knowing the velocity of propagation of
the acoustic energy, the nearest sensor location, and the
time of propagation to that sensor. The defects in this
case may include an electrical leakage path in a solid
dielectric or a leak in a fluid dielectric.
In still another embodiment of the invention, a defect
in an electrical conduit is located by imposing an
electrical pulse upon the conductor with sufficient
3o potential to cause an electric'discharge at the defect.
The acoustic energy liberated by the electric discharge is
determined by three sensors, each spaced from the others by
a known distance along the conduit. The span between one
pair of these sensors includes the location of the defect.
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Computing cross-correlation functions between data from pairs
of sensors, and smoothing the plots to get peak time
differentials provides a velocity of propagation of acoustic
energy and the location of the defect. The defects in this
case may include an electrical leakage path in a solid
dielectric or a leak in a fluid dielectric.
The previously described versions of the present
invention have many advantages including: the ability to
better locate the leak in a buried conduit, to protect the
environment and to reduce the cost and disruption of
excavation; the ability to locate leaks in noisy environments
which are typical in cities, and to reject uninteresting
sources of noise which include turbulent flow within the
conduit, discontinuities in the conduit such as joints and
traps, or noise generated by adjacent conduits; the ability to
detect a leak at greater distances from the sensors; the
ability to determine the direction and flow rate of a medium
within a conduit in a non-intrusive manner; and the ability to
locate a defect in an electrical transmission line.
In accordance with one aspect of the present invention
there is provided a method of determining the flow rate and
direction of a medium within a conduit comprising the steps
of: coupling a pair of sensors to said conduit at a first and
second location thereof; imposing a first vibration upon the
conduit at a position proximate a first sensor in said pair of
sensors, said position being outside of a region located
between said pair of sensors which are separated by a distance
and mounted to the conduit; detecting a first transmitted
vibration propagating along the conduit in response to said
imposed vibration at both sensors; repeating the steps above
wherein a second vibration is imposed at a second position
proximate a second sensor in said pair of sensors, said
position being outside of said region located between said
pair of sensors; determining the center velocity of each
transmitted vibration by computing an envelope correlation
function; and calculating the direction of fluid flow and its
velocity.
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4. Brief Description of the Drawings
The present invention taken in conjunction with the
invention described in co-pending Canadian Patent Application
Serial No. 2,158,669 which was filed on September 20, 1995,
will be described in detail hereinbelow with the aid of the
accompanying drawings, in which:
FIG. 1 shows strength versus velocity plots for single
and multimode transmission of sound in a conduit filled with a
medium, and in particular the transmission of steam leak noise
through the metal of the conduit and through the steam;
FIG. 2 shows a typical conduit supporting sensors which
detect acoustic energy, and diagrams representing the
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inputs to a cross-correlation calculation and typical
outputs therefrom;
FIG. 3 shows a typical conduit supporting sensors and
transducers which are used to determine the rate and
direction of flow of a medium in a conduit and a typical
output from smoothing raw data from a cross-correlation
calculation; and
FIG. 4 shows a power transmission line with elements
used to locate a fault therein.
5. Detailed Description of the Preferred Embodiments
The undesirable properties of steam conduit as a
transmission medium which were previously described may be
summarized in FIG. 1. FIG. lA shows an idealized plot of
correlation strength versus velocity where there is only
one mode propagating. FIG. 1B shows the correlation
strength versus propagation velocity for sound traveling in
the metal and in the steam. Three characteristic peaks for
metal propagation are above 2000 feet per second while a
series of peaks for steam propagation is below 2000 feet
per second. FIG. 1C shows a raw plot of correlation
strength versus frequency for the case of steam leak noise
propagating in a multimode case. The raw data are smoothed
by computing the envelope of the raw cross-correlation
curve to define a center velocity at the peak of the
envelope.
The cross-correlation method is well known in signal
processing, for example, see "D'igital Signal Processing"
Oppenheim & Schafer, Prentice Hall, 1975, pages 556-562.
The method is insensitive to noise because each sensor
needs to detect it. The resulting improvement in signal to
noise ratio allows the sensors to be placed three times
farther apart than in previous methods.
The correlation strength is derived from two sensor
measurements along the conduit where analog data is
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converted to digital form and cross-correlated to provide
the time delay as noise propagates between pairs of
sensors. Knowing the distance between sensors yields the
velocity. This well known method of signal processing is
used for continuous noise where there is no time equals
zero event.
In principle, the process can also be applied to an
analog signal, but it is easier in digital form. The
signals are also usually filtered to select a band of
frequencies, 4000 to 8500 Hz in the case of steam, where
the leak noise is strongest as compared to flow noise or
noise transmitted by the metal. Single frequency tones may
be removed by Smoothed Coherence Transform, SCOT, filtering
(Carter et al. Proc.IEEE (Lett). 61, 1497, 1973).
Referring now to FIG. 2, there is shown apparatus 200
in accordance with one embodiment of the invention which is
a method to locate a continuous leak in a conduit 5, which
may be a buried steam conduit. Sensors 21, 22, and 23 are
mounted to the conduit, by fasteners, by an adhesive, or
preferably by welding. Sensors 22 and 23 are in the same
manhole and constitute a first pair of sensors, separated
from each other by approximately one to ten feet. Sensor
21 is in another manhole which may be 500 feet away.
Sensors 21 and 22 constitute a second pair of sensors. In
one embodiment, the sensors were Model 4378 Accelerometers
from Bruel & Kjaer Instruments, Inc., Marlborough, MA.
Steam leak vibration data is acquired from all three
sensors, at a minimum, one pair at a time, and recorded,
typically upon digital audio tape. First receiver 61 and
second receiver 62 may provide a synchronization pulse to
recorders 31 and 32, respectively, where they are widely
separated. Alternatively, a cable could be run along the
right-of-way and all the sensor data could be recorded on
different channels of the same recorder.
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The signals are filtered to pass a frequency band from
4000 to 8500 Hz to discriminate the steam leak noise
propagating in the steam from turbulent flow and from noise
transmitted by the conduit. Filtering may also be
performed to eliminate single frequency tones.
A cross-correlation function is computed between
sensors 22 and 23. This raw data plot, containing a
multiplicity of peaks, is smoothed in an envelope
processing step to develop a curve with a single peak, as
shown in FIG. 2B, which corresponds to the time
differential the leak noise needs to traverse the known
distance between sensors 22 and 23. A center velocity for
leak noise propagation in this particular conduit is
calculated from this information.
The same steps are performed for data gathered by
sensors 21 and 22 to obtain the average time delay for
noise from the leak to reach sensor 22 as compared to
sensor 21. An uncorrected leak location, Lo, measured from
the midpoint, C-C, between sensors 21 and 22 is given by,
Lo = 0.5 v(tl - t2}
where v was calculated from the cross-correlation and
smoothing operations upon the signals from sensors 22 and
23 to give
v = d23/(t3 - t2~.
The uncorrected leak location, Lo, does not account
for the direction and velocity of the steam. This
information may be obtained by the method described in FIG.
3.
Referring now to FIG. 3, there is shown apparatus 300
which
is in accordance with another embodiment of the invention.
Where the elements of apparatus 300 are the same as
apparatus 200 the same reference numbers are shown.
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In many utilities there are no pressure gauges or flow
meters along conduits of interest. To determine the flow
in a conduit, in a non-intrusive way where there is no
leak, a method employing two sensors is used.
5 FIG. 3 shows a conduit 3, along an axis X,
transporting a fluid of unknown speed and direction.
Sensors 21 and 22 are attached to the conduit at locations
xl and x2, respectively. separated by a known distance D12.
A transducer 41, located at any position, xa, less than or
l0 equal to xl impresses a vibration upon the conduit. The
transmitted components of the vibration are sensed and
recorded at positions xl and x2 in a manner described
above. The process is repeated whereby a second transducer
42 located at any position, xb, equal to or greater than x2
impresses a vibration whose components are sensed and
recorded at xl and x2. The functions of the transducers at
xa and xb may be performed by a simple hammer blot.
The center velocities of propagation in each direction
are computed by cross-correlation and envelope smoothing,
as before. In one.measurement, the sound will propagate in
the same direction as the flow; while in the other case it
will propagate in a direction opposing the flow. The flow
rate is half of the difference between the two velocities
and the direction of flow corresponds to the faster
velocity.
Referring now to FIG. 4A, there is shown apparatus 400
in accordance with one embodiment. of the invention. Power
conduit 2 has a jacket 4 which surrounds dielectric 6 which
surrounds conductor 8. The dielectric may be a solid or a
fluid. Power conduit 2 contains a defect 10 which may be a
leak through jacket 4 for the case of a fluid dielectric,
or an electrical leakage path through the dielectric if it
is a solid. The conduit is considered to lie along an axis
X. A first sensor 21 is mounted to the conduit at location
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xl, and a second sensor is mounted to the conduit at
location x2. The sensors are separated by a distance D12.
Defect 10 is located outside the span of the sensors at a
distance L from the nearest one. A first recorder 31 is
connected to first sensor 21, and a second recorder 32 is
connected to sensor 22. A pulse generator 50 is connected
to conductor 8. One end of the pulse generator and the
jacket are electrically grounded. Commercial FM receivers
61 and 62 provide synchronization signals to the recorders
coincident with a pulse from generator 50 via transmitter
63.
In the process of locating the defect, arcing is
induced at the defect by a short, high potential pulse
propagating along the conduit from the generator. This so-
called "hi-pot" method is also used to clear shorted
capacitors. The pulse travels at much higher velocity than
sound waves generated at the defect, so it may be regarded
as instantaneous all along the line and may be used to
synchronize the data gathering process.
The transmitted acoustic energy liberated by the
electric discharge is detected with the two sensors and
recorded. An envelope of a cross-correlation function is
computed to obtain tl-t2, the time the acoustic energy
takes to propagate the distance D12. The velocity of
propagation is determined from the time and distance. The
sign of the quantity (tl-t2) indicates on which side of the
sensors the defect is located. The location of the defect
is determined from the nearest sensor knowing tl or t2 from
the electrical impulse. Therefore
L = -vtl for L < xl measured from xl; or
L = vt2 for L > x2 measured from x2.
Referring now to FIG. 4B, there is shown apparatus 401
which is similar to apparatus 400 except that the defect 10
is located between sensors 21 and 22, and sensor 23 has
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been mounted at x3. In this case, the procedure to locate
the defect is the same as that given in the discussion for
apparatus 200 in FIG. 2.
Some advantages over the use of surface detection
methods are: the ability to better locate a leak in a
buried conduit to protect the environment and to reduce the
cost and disruption of excavation; the ability to locate
leaks in a noisy environments which are typical in cities,
and to reject uninteresting sources of noise which include
turbulent flow within the conduit, discontinuities in the
conduit such as joints and traps, or noise generated by
adjacent conduits; increasing the distance over which a
leak may be found; the ability to determine the direction
and flow rate of a medium within a conduit in a non-
intrusive manner; and the ability to locate a defect in an
electrical transmission line. For the electrical line, the
number of "hi-pot" attempts needed to locate the defect is
greatly reduced, thereby minimizing the potential for
destroying good dielectric by over-stressing it.
Changes and modifications in the specifically
described embodiments can be carried out without departing
from the scope of the invention. In particular, the
apparatus and method described for determining the
direction and flow rate of a medium may be incorporated
with the embodiment for locating a leak. The spectrum of
frequencies which are used in the cross-correlation
operation may be selected for each combination of conduit
and medium to maximize leak noise data and to discriminate
against unwanted frequencies. Sensors may be connected
directly into a computer wherein processes such as, but not
limited to: filtering, digital conversion, signal
processing, smoothing, synchronization, and the
calculations of flow rate, direction, and defect location
are performed.
