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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the detection of fluid leaks and to a
determination of the rate of fluid leakage from various and individual
leak sites. More particularly, the present invention relates to a method
and apparatus for detecting and determining the rate of flow of fluid
leaks in a piping system having a variety of different piping system
components.
2. Description of the Related Art
In industrial piping systems, fluid leaks are one of the main causes of
system inefficiencies, resulting in wasted energy costs, environmental
hazards, and increased manufacturing costs. Most distribution systems have
been estimated to waste about 10-20% of their capacity to leakage, and it
is not uncommon in large piping systems to have from 200-500 or more fluid
leaks occurring simultaneously. The accumulative effects of these leaks
can result in losses of hundreds of thousands of dollars per year and can
result in needless capital expenditures for new or additional equipment
installed to offset the effects of the leakage.
Past efforts to locate and quantify these leaks for repair scheduling have
not been very successful. For example, industries formerly would take
annual or bi-annual plantwide "cold shutdowns" in order to make repairs or
add new equipment. When the system was ready to be started up again, there
would be a small period of time when leaks could be detected and repaired.
This occurred because the fluid distribution systems were often one of the
first systems brought back on line after shutdown and., while the other
systems were not yet running, many leaks could be audibly located.
However, this brief time period was often not enough to perform all the
necessary repairs and generally only the large leaks were repaired if time
allowed.
More recently, industry has adapted to obviate the need for complete
shutdowns for repair and modification of equipment by taking more frequent
but smaller sectional shutdowns, called "partial outages." While this
allows the facility to continue operation and avoid start up expenses, it
hinders even more the ability to accurately locate and quantify leaks
because the leaks are more difficult to hear with part of the system
running. This, in turn, hinders the ability to strategically plan partial
shutdowns for leak repair while also making cost and environmental effect
estimates difficult.
Nevertheless, because the leakage at most fluid leak sites emits sounds
having components in the ultrasonic frequency range, an ultrasonic sound
detector, as described, for example, in U.S. Pat. No. Re. 33,977, can
provide an instrument to locate a leak site. The use of such an ultrasonic
sound detector does not require facility shutdown, thus allowing for "on
the run" leak detection.
However, one of the main problems with current ultrasonic leak detection
methods is the inability to accurately compute the size and flow rate of a
given leak once it has been detected. Some leak flow quantification
methods only categorize the leak flow rate as "small", "medium", or
"large". Others, such as that disclosed in U.S. Pat. No. 5,136,876, for
example, determine an approximate leak flow rate by comparing the measured
sound pressure level with a standard curve that plots gas leak rates
versus sound pressure level without respect to the nature of the leak
site. The inaccuracies resulting from such methods are generally due to
failure to deal with variations in the size of the leak site, the
influence of the configuration of the fluid system element at the leak
site, variations in the properties associated with the type of fluid which
is leaking, and variations in the system temperature and pressure of the
fluid. Additionally, the qualitative "small-medium-large" categorization
is wholly inadequate for determining dollar amount lost per year, possible
environmental damage, and system maintenance and repair requirements.
It is thus one object of the present invention to provide a more accurate
method for quantifying leak flow rates.
It is another object of the present invention to provide a more accurate
method for detecting leak flow rates which can accurately estimate the
yearly cost and volume loss of one individual leak or a system of leaks.
It is yet another object of the present invention to provide a method for
creating a database which provides a more accurate representation of the
rate of flow of fluid leaks.
It is another object of the present invention to provide a mobile apparatus
which can more accurately determine leak flow rates.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method for providing a database to
be used in determining the size and leak rate of a fluid leak through an
orifice in a distribution system. This is done by measuring low through
high flow rates of a simulated fluid leak through an orifice, by using a
sound detector to measure the sound pressure level corresponding to each
of the low through high flow rates and by attenuating the sound pressure
level to a desired level. Then, the equivalent orifice areas corresponding
to the low through high flow rates of the simulated fluid leak are
determined and the obtained data, including appropriate sound detector
measurements, are recorded in a database. In one embodiment of the
invention, the flow rate and attenuated sound detector measurements are
plotted on a curve and the data corresponding to the estimated
mathematical relationship between the flow rate and the attenuated sound
detector measurements as described by the curve are recorded in a
database.
Another aspect of the present invention is a method and apparatus for
determining the size and leak rate at a fluid leak site by measuring the
sound pressure level of the fluid leak, attenuating the sound pressure
level measurement to a known level, comparing the attenuation setting to a
predetermined standard for fluid leak areas, and computing the leak rate
from the obtained leak area. In one embodiment of the invention, the
attenuation setting measurement is compared directly to a predetermined
standard for leak flow rates.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall block diagram showing several steps of a method of
providing a fluid leakage rate database in accordance with the present
invention.
FIG. 2 is a fragmentary perspective view of fluid leak sites in a portion
of a piping system and an ultrasonic sound detector for measuring sound
pressure levels in accordance with the present invention.
FIG. 3a shows a front view of one form of sound pressure level meter and
attenuation dial as found on an ultrasonic sound detector showing a high
sound pressure level reading.
FIG. 3b shows the ultrasonic sound detector dial and meter of FIG. 3a with
the attenuation dial turned down so as to bring the meter level reading to
a desired level.
FIG. 3c shows a front view of the sound pressure level meter and
attenuation dial of FIG. 3a showing a low sound pressure level reading.
FIG. 3d shows the ultrasonic sound detector dial and meter of FIG. 3a with
the attenuation dial turned up so as to bring the meter level reading to
the desired level for taking a reading.
FIG. 4 is a fragmentary side elevational view, in cross section, showing a
typical screw-type pipe joint with common leak sites.
FIG. 5 is a fragmentary side elevational view, in cross section, showing a
typical flange joint with a common leak site.
FIG. 6 is a fragmentary side elevational view, in cross section, showing a
typical tube and tube end fitting joint with common leak sites.
FIG. 7 is a fragmentary side elevational view, in cross section, showing a
typical pipe union fitting with a common leak site.
FIG. 8 is a view similar to FIG. 7 showing a misaligned pipe union fitting,
with common leak sites.
FIG. 9 is a side elevational view, in cross section, showing a typical gate
valve with a common leak site.
FIG. 10 is a fragmentary perspective view of a welded pipe joint showing
common joint and bore hole leak sites.
FIG. 11 is a block diagram showing several steps of a method of determining
leak flow rates and leakage costs for a single leak site.
FIG. 12 is a fragmentary view of a portion of a piping system having
several fluid leaks that can be detected and measured using a method and
apparatus in accordance with the present invention.
FIG. 13 is a sample curve plotting air leak rates versus sound detector
sensitivity readings for a given pipe size and configuration at a given
pressure and given temperature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 represents a block diagram showing the several steps of a method of
providing actual leakage site and leakage flow rate data for a plurality
of leakage sites and leakage flow rates to provide the information to
prepare a database for more accurately measuring leakage rates in
accordance with the present invention. Starting with any potential leak
source, such as a pipe of given size and joint configuration, a source 20
of pressurized fluid, such as an air compressor, is connected with a known
leak source having a known structure, through a pressure regulator 22 to
provide compressed fluid such as air at a predetermined pressure and
temperature through a created leak 25 (see also FIG. 2) in the known leak
source. The flow rate of the air through the system or the leak site is
measured by a suitable flow meter 26. The pressure is determined by
pressure regulator 22 and the temperature by a suitable thermometer 24.
The flow meter 26 can be any conventional flow meter capable of measuring
air flow, such as, for example, Pneumatic Flow Meter Models G-32201-00,
G-32201-21, or G-03228-91, manufactured by Headland Mfg., Racine, Wis. The
flow meter 26 may, for example, use the well-known sharp edge orifice
method for measuring the flow rate of a fluid through an orifice.
Low through high leakage flow rates are simulated by changing the flow area
at the leak site 25, such as by physically enlarging the leak site area.
The object is to simulate the flow rates which still provide a detectable
sound pressure level reading on the ultrasonic sound detector. Leak flow
rates which do not provide such a reading are not relevant for the present
invention because they generally occur in the audible frequency range and
are readily detected without the need for instruments and therefore are
usually repaired immediately. The ultrasonic sound detector 28 is used in
conjunction with the flow meter 26 to set the appropriate area at the leak
site 25 to use in obtaining the low through high flow rates.
As an example, in determining the high leakage flow rate level to be
simulated in a flange joint 29 as shown in FIG. 5, the retaining bolt 35
is incrementally loosened while the flow rate and sound pressure level are
measured at each increment. When the gasket 33 no longer makes full
contact with one side of the flange 29 or when the flange connection is
disconnected altogether, the resulting leak does not provide a useful high
flow rate for the database of the present invention because such a leak
would not require ultrasonic sound detection. Rather, this type of leak,
similar to a blown gasket leak, would be readily audibly or visually
noticeable and the flange joint 29 would then be quickly repaired or
tightened. Thus, the increment tested immediately prior to this unhelpful
increment is used as the appropriate size leak for measuring the high flow
rate because it represents the highest flow rate for the simulated leak
which provides a readable ultrasonic sound pressure level measurement.
Similarly, the low leakage flow rate level for a flange joint 29, for
example, is simulated by creating as small a leak site area as possible,
such as by tightening retaining bolt 35, to provide a leakage area which
will still provide a readable sound pressure level measurement on the
ultrasonic sound detector, and then measuring the flow rate for that
leakage area.
In measuring the sound pressure level of the fluid escaping through the
leak site 25 as shown in FIG. 2, the ultrasonic sound detector 28 is
maintained at a predetermined distance "A" from the leak site at the
potential leak source, i.e., the pipe joint undergoing testing. The
distance "A" can be six inches or twelve inches, for example, and the
sound pressure level reading is taken at the maximum sound pressure value,
determined by moving detector 28 around the leak site, at the
predetermined distance, until the maximum reading is obtained.
An ultrasonic sound detector 28 suitable for use in the present invention
can be any conventional ultrasonic sound level meter, preferably one
having an attenuation dial 32 for regulating the sensitivity with which
the sound detector receives sound signals. One type of ultrasonic sound
detector 28 that has been found to be suitable is a portable, hand held
self-contained ultrasonic transducer assembly/transmission unit,
commercially available as Model 2000 from U.E. Systems, Inc., Elmsford,
N.Y. The detector 28 senses the sound pressure level or amplitude of the
sound produced by a leak, in the desired frequency range, which can be the
ultrasonic frequency range (20-100 KHz), and indicates this sound pressure
level by an indicator needle 31 that is movable across a scale 30 which
can be converted to decibels, if desired. It is important that the sound
detector 28 is maintained at substantially the same distance from each
simulated leak site 25 for uniformity purposes and for subsequent
correlation of actual detected sound level data with the previously
acquired sound level data at the simulated leak sites.
As shown in FIGS. 3a through 3d, once the sound pressure level is indicated
by the indicator needle 31 that is movable across the sound detector scale
30, the sensitivity level of the sound detector 28 is adjusted by turning
the attenuation dial 32 to bring the indicator needle 31 to a convenient
position, such as the midpoint 36 of scale 30. This position corresponds
to a value of 50.00 on the scale of the U.E. Systems sound detector
identified above. The setting of attenuation dial 32 can range from 0 to
10. Thus, the higher the intensity level of the sound signal being emitted
from the intentionally created leak 25, the greater attenuation or signal
weakening that will be required to achieve the midpoint 36 on the scale 30
of the ultrasonic sound detector 28, and thus the lower the setting of
attenuation dial 32, as shown in FIGS. 3a and 3b. A weaker sound pressure
level, such as shown in FIG. 3c, requires a correspondingly higher setting
of attenuation dial 32, as shown in FIG. 3d, for indicator needle 31 to
reach the desired position relative to scale 30. The setting 101 of
attenuation dial 32 is then recorded as a data point in the database 38,
along with its corresponding leakage flow rate 102 measured by the flow
meter 26 and the values for fluid temperature 103, fluid pressure 104,
properties associated with the fluid type (e.g. density, viscosity, gas
pressure, etc.), pipe diameter size 105 and category 106 of piping system
element. Examples of the categories of piping system elements for which
data can be obtained are flanged joints, unions, gate valves, tube fitting
joints, and the like, depending upon the piping elements that are included
in the systems to be tested for leakage rates. Examples of the fluid type
for which data can be obtained include liquids which flash into gases upon
exiting the piping system and gases such as air, nitrogen, natural gas,
and the like, depending upon the fluid types included in the systems to be
tested for leakage rates.
Thus, the attenuation dial setting is simply the level of sensitivity to
which the sound detector 28 must be set in order to receive a sound signal
from the created leak 25 at a sound pressure level at the midpoint 36 of
the sound detector scale 30. This attenuation dial setting is important
because the actual piping system survey data will have its own attenuation
dial setting and, by comparison with the simulated data, the flow rate and
area of actual survey leaks can be computed as will be explained
hereinafter.
Table 1 is an example of a compilation of fluid leakage data for one size
of one type of pipe, at a given temperature and at different fluid
pressures and leakage flow rates. For this example, as shown in Table 1,
the fluid type is air, the temperature is 78 degrees F., and the pipe is a
0.5 inch screw joint. As the fluid supply pressure is incrementally varied
by adjusting a pressure regulator 22 on the pressure source 20, values for
leak rates and sensitivity readings are obtained and recorded in tables as
shown in Tables 1a and 1b. As shown in Table 1c, corresponding values for
equivalent orifice areas are calculated in a manner to be described later
and are then recorded to complete the table. Each table is then recorded
in the database 38.
TABLE 1
__________________________________________________________________________
FLUID TYPE: AIR AT 78.degree. F.
0.5 inch Screw Joint
__________________________________________________________________________
FLOW RATE (SCFM'S)
Press
PSIG
F.LO
F1 F2 F3 F4 F5 F6 F7 F.HI
__________________________________________________________________________
20 0.016
0.02
0.18
0.25
0.40
0.55
0.70
0.86
1.01
30 0.016
0.03
0.18
0.25
0.44
0.87
1.00
1.12
1.25
40 0.016
0.03
0.20
0.30
0.44
0.77
0.88
0.99
1.10
50 0.018
0.05
0.22
0.33
0.50
0.80
0.95
1.10
1.25
60 0.016
0.08
0.25
0.33
0.53
0.77
1.03
1.25
2.00
80 0.021
0.11
0.50
0.63
1.06
1.50
2.00
3.50
4.00
100
0.016
0.15
0.45
0.61
1.16
1.25
2.00
5.00
6.00
__________________________________________________________________________
DIAL SENSITIVITY READING (at 6 inches)
Press
PSIG
S.LO
S1 S2 S3 S4 S5 S6 S7 S.HI
__________________________________________________________________________
20 8.58
7.44
6.40
6.17
6.04
5.89
5.75
5.60
5.46
30 8.30
7.14
6.38
5.83
5.68
5.56
5.45
5.33
5.22
40 8.28
7.13
6.38
5.83
5.69
5.56
5.44
5.32
5.20
50 8.19
6.95
6.10
5.47
5.23
5.41
5.60
5.78
5.97
60 8.05
6.65
6.02
5.81
5.45
5.12
4.96
5.00
4.62
80 8.10
6.64
5.50
4.73
4.56
4.38
4.28
3.25
2.78
100
8.30
6.20
6.15
5.47
5.34
4.63
5.20
3.50
2.94
__________________________________________________________________________
AREA (SQ. FT.)
Press
PSIG
A.LO
A1 A2 A3 A4 A5 A6 A7 A.HI
__________________________________________________________________________
20 .00026
.00035
.00045
.00054
.00064
.00088
.00112
.00137
.00026
30 .00019
.00035
.00052
.00069
.00086
.00100
.00115
.00129
.00144
40 .00014
.00025
.00037
.00047
.00059
.00069
.00079
.00089
.00099
50 .00013
.00022
.00031
.00040
.00049
.00059
.00070
.00081
.00092
60 .00010
.00023
.00031
.00063
.00063
.00110
.00157
.00205
.00252
80 .00010
.00019
.00024
.00051
.00051
.00072
.00097
.00169
.00194
100
.00006
.00013
.00031
.00045
.00045
.00049
.00078
.00197
.00236
__________________________________________________________________________
The low through high equivalent orifice areas corresponding to the low
through high leak flow rates are computed by the standard formula for flow
of compressible fluids through nozzles; and orifices,
##EQU1##
where q=flow rate, cubic feet per second at flowing conditions, Y=a
constant related to the net expansion factor for compressible flow through
a sharp-edged orifice,
C=a constant related to the flow coefficient for a sharp-edged orifice,
A=cross sectional area of orifice, square feet,
g=acceleration of gravity, 32.2 feet per second squared,
P=pressure differential between fluid within the pipe system and the
atmospheric pressure, psig,
.rho.=gas weight density, pounds per cubic foot, which can be represented
from the ideal gas equation as P/RT,
where P=absolute gas pressure, psia,
R=individual gas constant, foot pounds per lb per degree Rankine,
T=gas temperature, degrees Rankine=degrees F.+460.
Although Y and C are considered for practical purposes to be constant, Y
varies slightly based upon the pressure ratio across the orifice, and C
varies slightly based upon the Reynolds number. Since pressure ratios are
generally high, Y can range from about 0.75 to about 0.83; orifice
coefficients, C, can range from about 0.6 to about 1.0, depending on the
Reynolds number. Using the lower values for each of Y and C, for
simplicity of computation, yields more conservative results. The flow rate
equation is solved for the calculated leakage areas corresponding with the
test data under the various test conditions recorded in the tables of
data. These calculated areas are called equivalent orifice areas because
the flow meter 26 is designed to measure the flow rate of a fluid through
an orifice. Since the leaks 25 created to obtain the database in
accordance with the present invention emanate from leak sites of unknown
size, using the measured flow rates and related data in the equation above
enables the calculation of the equivalent orifice areas which correspond
to each of the measured leakage flow rates under the given conditions.
Thus, Table 1 is an example of data collected for a screw joint having a
pipe size of 0.50 inches, for air at a given temperature of 78 degrees F.,
and over a range of fluid supply pressures.
Once a sufficient number of data points have been established and recorded,
the fluid supply temperature is varied to different levels while keeping
all other factors the same, and the leakage areas are again calculated
based on measured leakage flow rates. These flow rate values and leakage
area values are recorded with their corresponding sensitivity readings in
another table in the database 38.
In one embodiment of the invention, fluid supply temperatures between 70
and 110 degrees F. and pressures between 20 and 100 psig were provided,
with the low through high leakage flow rates, to obtain low through high
area parameters 107 (see Table 1c and FIG. 1) for each pipe size and joint
type at the various fluid supply temperatures and pressures. The database
38 can be supplemented by calculated leakage area values 107 for selected
temperatures and pressures greater than the actual measured temperature
and pressure values. The database 38 can also be supplemented by
calculated leakage area values 107 for selected fluids based on their
unique properties. The fluids used in this invention include liquids which
flash into a gas upon exiting the piping system as well as gases such as
air, helium, argon, carbon dioxide, nitrogen, natural gas, hydrogen,
oxygen, and steam.
The same data accumulation process is continued until each individual pipe
size and joint type has been tested at the varying conditions. Once all
data points are gathered for a range of pipe sizes of the given joint
type, such as from 0.5 inch to 3 inches, the next joint type is tested
according to the same procedures just described. The joint types tested in
this invention, as shown in FIGS. 4 through 10, include: 1) screw joints
60 (FIG. 4), ranging from an inside diameter of 0.125 inch through three
inches N.P.T. (National Pipe Thread), including 90 degree elbows, 45
degree elbows, couplings, male adapters, etc., 2) flange joints 62 (FIG.
5), ranging from 0.5 inch to 3 inches inside diameter, 3) tube fittings 64
(FIG. 6), ranging from 0.125 inch to 0.5 inch in outside diameter, 4)
union fittings 66 (FIG. 7), ranging from 0.25 inch to 2.5 inches inside
diameter, 5) misaligned unions 68 (FIG. 8) with varying degrees "B" of
external force or misalignment, 6) valve packing 70 (FIG. 9), in which the
valve stem inside diameter ranges from 0.25 inch to 0.5 inch, and 7) area
or bore diameter type leaks 74 (FIG. 10) not located at the other types of
connection points that were tested. In general, larger sizes of these
configurations are not often found in industry. However, their flow rates
tend to correspond relatively closely with the flow rates of the largest
sizes tested as identified above.
For "area" 74 category leaks (see FIG. 10), the attenuation dial settings
need not be taken. This is because the cross-sectional areas of these
leaks may be obtained using the formula
.tau.d.sup.2 /4
where d is the measured or estimated bore or hole 74 diameter.
The assembled database 38 therefore includes a series of tables of data,
such as Table 1, containing a plurality of empirically determined data
points, allowing one to determine with a higher degree of accuracy than
heretofore possible the ranges of the leak area and leakage flow rate for
a given fluid, at a given temperature and given pressure, traveling
through a specified piping joint configuration of a given size. The data
constituting database 38 can be stored in a database-type computer program
for later access. In one embodiment of the invention, the data are
compiled and organized in a commercially available computer database
program such as Microsoft Access, version 1.1, available from Microsoft
Corp., Redmond, Wash.
In one embodiment of the invention, the data used to create each table is
used to plot the obtained sensitivity values against the obtained leak
rate values on a graph. A curve is then generated to fit the plotted
points based on a mathematical curve estimation method. The curve and the
formula describing the curve can then be stored in the database with
corresponding parameters of fluid type, temperature, pressure, pipe size
and pipe configuration. The curve estimation method provides a more
accurate way to determine the leak rate of a given leak based on an
obtained sensitivity reading.
For example, FIG. 13 shows a sample curve based on air leakage data
recorded for a 0.50 inch screw pipe at 60 psig and at 78 degrees F. The
actual empirically derived leak rate and sensitivity values are plotted
and then a curve is fitted to the plotted points using a mathematical
curve fitting method. In one embodiment of the invention, the method of
least squares is employed as the formula for describing the empirically
determined data points. The method of least squares assumes that the
formula is of the form:
y=C.sub.0 +C.sub.1 x+C.sub.2 x.sup.2 +C.sub.3 x.sup.3 + . . . +C.sub.n
x.sup.n
where
C.sub.n =constant coefficients
n=order of the polynomial
y=leak rate
and x=sensitivity reading
The formula may be solved using the empirically derived le | | |
