Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 METHOD FOR DETECTING DEFECT IN LNG TANK
BACKGROUND OF THE INVENTION
:, :
The present invention relates to a method for -:
detecting a defect in an LNG tank, and particularly : - .
relates to a method for detecting a defect in an LNG
~,
tank in which liquid-tight defect portions of a
secondary barrier in an LNG m~mbrane tank build in a
membrane system LNG carrier.
As has been known well, the IMO (International
Marine Organization gas carrier code requires provision
of a perfect secondary barrier in a membrane system
LNG carrier. That is, a membrane system LNG carrier
(having an LNG membrane tank as a main structure) has a
~,, .
structure for maintaining liquid-ti~htness to keeping
LNG for a certain period b~ a secondary barrier called
triplex so that an inner hull (carrier body3 should
not ~be at a dangerous temperature to make brittle
~fracture if an ultra low temperature cargo (-162C),
that i5, LNG leaks because of generation of a crack in a
memhrane (called "primary barrier") directly contacting
with LNG. Such a tank system is mainly constituted
by above-mentioned primary and secondary barriers and a
2~ thermal insulation layer of P-PUF glass-fiber
reinforced polyurethane foam). It is also necessary
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1 that satisfactory liquid~tightness of the secondary
barrier can be estimated and guaranteed surely if LNG
leaks out of the primary barrier.
As such a conventional LNG tank of a low boil-off
type, there is a system called TGZ Mark III Containment
System developed by Techniga~ (TGZ), for example, as
disclosed in Nippon Kokan K.K. Technical Report, No.104
(1984), p. 63-69.
As a method for confirming effectiveness of such a
secondary barrier, there is a vacuum test. Fig. 8 is a
model sectional view illustrating a vacuum test
equipment provided to perform a vacuum test for an LNG
tank according to the same standard as that of the above-
mentioned TGZ Mark III Containment system. Fig. 8A
shows the~configuration of an LNG tank and the vacuum
test equipment, and Fig. 8B is a detail view at the A
portion shown in Fig. 8A. In this case, the vacuum
test estimates the eflectiveness of a secondary
barrier from its air-tightness.
2~ In Figs. 8A and 8B, the reference numeral
represents an inner hull of a carrier body, 2 represènts
a secondary barrier called triple~ provided inside the
inner hull 1, and 3 represents a primary barrier called
membrane provided inside thé secondary barrier 2 and
constituting an inner wall of an LNG tank and a thermal
insulation layer 7 filled with a thermal insulation
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.1 panel such as glass-fiber reinforced polyurethane foam
(R-PUF) is provided between a front plywood 4 to which
the primary barrier 3 is attached and a back plywood 6
fixed therewith through mastics 5 on the side of the
inner hull 1. Generally the portion of the thermal
insulation layer 7 is called a thermal insulation
section sectlon (IS), and the space between the primary
barrier 3 and the front plywood 4 is an area called an
inter barrier section(IBS) 8.
The above-mentioned members are those mainly
constituting a membrane tank. In the primary barrier 3,
stainless (304L) corrugated membranes formed in a
special shape are welded to and supported by an anchor
strip 9 built in the front plywood 4, and the membranes
attached thus are overlaid and welded to each other.
The standard pitch of corrugations is 340mm, and the
corrugations are perpendicular to each other all over
the tank wall, so that the corrugations are transformed
by the expansion/contraction caused by thermal change
and carrier body transformation at the time of carrying
LNG so that exceed stress should not be Produced in the
membranes. Since the load is transmitted to the carrier
body through the thermal insulation layer 7 upon
application of the liquid pressure to LNG (cargo) 10,
all that the membranes should perform is only to
maintain the liquid tightness.
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2~2~
.1 Ne~t the secondary barrier 2 is formed by
contacting glass cloth with both sides of an aluminum
sheet, built i~side the thermal insulation layer 7 so
that the injury of the primary barrier 3 or the inner
hull 1 should not give a direct effect to the secondary
barrier 2, and having a liquid-tightness maintaining
structure as a backup system so that the carrier body
should not be brought not a low temperature even if a
crack is produced in the primary barrier 3.
The thermal insulation panel constituting the
thermal insulation layer 7 has a sandwich struc~ure in
which the panel is bonded to the front plywood 4 and
the back plywood 6, the thermal insulation thickness
thereof can be changed correspondingly to the required
boil-off-ratio(BOR). The edge portion of the back
plywood 6 is pressed by a not-shown backing plywood,
and the backing polywood is penetrated by a stud bolt
welded to the inn~r hull 1 so that the thermal
insulation panel attached to the back plywood 6 is
:
fixed to the inner hull 1 by fastening the stud bolt
with a nut. In addition, slits 11 having the same :;
pitch as the corrugations of the membrane 3 are
provided in the front polywood 4 of the thermal
insulation layer 7 and the R-PUF between the primary
Z5 barrier 3 and the secondary barrier 2, so as to prevent
an ex.cess stress from flowing in.
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In the vacuum test for detecting a defect in an LNG -
tank, air in a thermal insulation section (IS) formed
by the thermal insulation layer 7 is exhausted by use
of a vacuum pump 12 and a valve 13 so as to establish a
certain vacuum level in the thermal insulation section
(IS) so that the air-tightness of the secondary barrier ~ ~-
2 is estimated on the basis of a pressure increase
curve obtained by measuring the pressure increase
thereafter by using a mercury manometer 14. Fig. 9 is
~ a diagram illustrating an example of a pressure
"'
increase curve having been obtained as the result of :, ;
this vacuum test. In Fig. 9, the abscissa indicates the
time, and the ordinate indicates the vacuum level. As
understood from Fig. 9, when a defect of leakage is
.
produced and air-tightness is deteriorated, the time
~to return to the atmospheric pressure is short as
shown in the curve ~. On the contrary, when there is
less defect as shown in the curves ~ and ~, the time
to return to the atmospheric pressure is long. A valve
13a is that used in the case of exhausting air of
the area of the inter barrier section (IBS) 8.
If it is proved that there is a defect In the LNG
tank as a result of the vacuum test, the portion of the
defect is detected by the use of an infra-red imaging
method. Fig. 10 is a ;nodel diagram illustrating a
method for detecting defect by use of an infra-red
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imaging method. First, hot air is fed into an LNG tank
1 15 by a not-shown means so as to raise the temperature
of a membrane sheet of the primary barrier 3 uniformly,
and, at the same time, a nitrogen gas (cool air~ of
about 0C is fed into the thermal insulation sec~ion
(IS) 7 through a blower 16. At the time of the presence
of enough temperature difference by this manner, the air
is exhausted by the valve 13a, and the pressure in the
inter barrier section (IBS) 8 is raduces into a certain
vacuum level, so that the cool air rushes out of ~he
portion of a defect and membrane sheet therein is cooled
partially. Since this cooled portion indicates a
leakage portion of the secondary barrier Z, the portion
is detected by an infra-red camera 17 brought into the
LNG tank 15, and a portion or the whole of the membrane
sheet is displayed on the screen of an image
controller 18, so that by picking up the distribution of
the partially cooled portion, it is possible to detect
the defect portion.
As has been described above, a conventional method
for detecting a defect in an LNG tank has been
performed by the use of b~th a vacuum test method and an
infra~red imaging method. By such a conventional method
for detecting a defect, however, there has been a
problem that it is impossible to specify a portion of
leakage, while the vacuum test can judge the air-
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tightness of a secondary barrier as a whole. ;
Also in the infra-red imaging method, the work o~ -
image-picking up the whole in a large tank so as to
find out a defect portion therein needs much labor and
time, and the work is therefore not effective. In
addition, since material such as a membrane sheet having
intensive reflectivity provides many noise images, there
has been a problem, that sometimes it is difficult to
judge whether an image is caused by a defect or caused
by ~ noise.
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SUMMARY OF THE INVENTION
It is therefore an object of the present invention
to solve the foregoing problems in the prior art.
It is another object of the present invention to
provide a simpler method for detecting a defect portlon ~ -~
by the use of means for detecting gas density by a
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tracer gas.
,
In order to attain the above objects, accordlng to
an aspect ~ of the present invention, in the method for a ~
`
detecting a defect in an LNG tank: a lattice sampling
pipe arrangement is provided in a thermal insulation
section of a containment system of a membrane system
LNG tank constituted by the thermal insulation section
and an inter barrier section with a secondary barrier,
which is an object to be detected, disposed as a
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boundary between the thermal insulation section and the
inter barrier section, the lattice sampling pipe
arrangement having holes formed a predetermined
intervals and disposed at lattice points thereof;
sampling pipes of the lattice sampling pipe arrangement
are connected to a single gas densimeter individually
through respective valves: at tracer gas is led into
the inter barrier section so that the density of the
traGer gas leaking through the object to be detected is
measured by the gas densimeter with respective to each
of the sampling pipes: total data is prepared by using
a processing mechanism on the basis of the result of
measurement of density obtained from each of the
sampling pipes; and a leakage portion in the object to
be detected is identified and detected by estimating
the gas density of at each of the lattice points on the
basis of the total data. :-
According to the present invention a tracer gas is
flown into one of the two sections which sandwich the -~ -
object to be detected, that is, the secondary barrier.
The tracer gas leaking through the object to be detected
is taken out through the lattice sampling pipes located
in the other section and 10d into the gas densimeter so :~
that defective leakage is detected. In this case, the
respective sampling pipes have sampling holes at the
lattice points, and each sampling pipe has a plurality
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.1 Of sampling holes so as to form a plurality of lattice
points. If valves provided at the respective sampling
holes are opened, a gas in the vicinity of the plurality
of the sampling holes is sucked through the sampling
holes. Therefore, if the tracer gas leaks through the
object to be detected and exists in the vicinity of the
sampling pipe, the density of this gas can be measured
by the gas densimeter. In this case, the gas density
;~ measured through one sampling pipe is an average value
of the gas density sampled through a plurality of
sampling holes. However, if the gas density is measured
in a manner as described above through all the sampling
pipes arranged in a lattice, the gas density at each
lattice point is measured twice through two
15~ ~ongitudinally and transversally provided different
sampling pipes. That is, the gas density at a certain
lattice point is the gas density obtained through two
sampliDg pipes ~crossing each other at the lattice
point, and it is possib1e to estimate that a defect
exists in the vicinity of a lattice point when the gas
density obtained at the lattice point is large. More
specifically, if the lattice sampling pipes and the
lattice points formed by the sampling pipes are
numbered, it is possible to identify the position and
quantity of leakage.
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1 BRIEF DRSCRIBED OF THE DRAWINGS
Figs. lA and lB are model sectional views
illustrating a leakage test equipment by a tracer gas
used for the me-thod for the deect detection according
to the present invention;
Figs. 2A through 2C are views illustrating a
lattice sampling pipe arrangement according to the
present invention;
Fig. 3 i5 a model diagram for explaining a
method for measuring gas density through a sampling pipe
according to the present invention;
Fig. 4 is a model diagram illustrating a C02 laser
,
light absorbing gas densimeter with sulfur he~afluoride ~ -
for detecting a tracer has according to the present ~ ~
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invention ;
Fig. 5 is a diagram illustratin~ a measured example ~- ~
lndlcating the gas density distribution by the use of ,- -
the lattice sampling pipe arrangement according to the :~
present invention; `~
Fig. 6 is a diagram illustrating the gas density
distribution in the test surface estimated from the
test result of Fig. 5;
Fig. 7 is a diagram illustrating a three- -
dimensional image display of a measured examples of the ;
tracer gas density and distribution through real-time
measurement;
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1 Figs. 8A and 8B are model sectional views
illustrating a conventional vacuum test equipment for an
LNG tank;
Fig. 9 is a diagram illustrating pressure increase
curves obtained as a re~1lt of a conventional vacuum
test; and
Fig. 10 is a model diagram illustrating a
conventional test equipment by an infra-red image pickup
method.
'
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Figs. lA and lB are model sectional views
illustrating a leakage test equipment by a tracer gas
used for the method for the defect detection according
to the present invention. Fig. lA shows an LNG tank and
a leakage test equipment associated therewith, and
Fig. lB is a detailed diagram at the portion B shown in
Fig. lA. In Figs. lA and lB, parts the same as or
equivalent to those in the conventional example of
Fig. 6 are referenced correspondingly and the
description thereof will be omitted here.
First, in Fig. lA, a tracer gas 20 is led into an
inter barrier section (IBS) 8 through a valve 13a. In
the portion B of Fig. lA, as shown in detail in Fig.
lB, the space for forming mastics 5 is enlarged, a
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.. .. . . . .
1 lattice sampling pipe arrangement constituted by
orthogonal sampling pipes 21 and 21a is provided in the
area of the space, that is, a part of a thermal
insulation layer (IS) 7. As shown in Fig. lB,
although the lattice sampling pipes 21 and 21a are
arranged in a section between a secondary barrier 2 and
an inner hull 1, that is, on the inner hull side, they
may be buried in a polyurethane foam (R-PUF) 22. Then,
a tracer gas is led into the IBS 8 through the valve - ~
10 13a, and the thermal insulation section of the IS 7 is ~ -
connected with a vacuum pump 12 through a valve 13 so
that the air therein can be exhausted by the vacuum
pump 12. One-side ends of the sampling pipes 21 and 21a
are connected with a gas densimeter 23 through - F
respective valves 13b (see Fig. 2).
Figs. 2A through 2C are views illustrating a
lattice sampling pipe arrangement. Fig. 2A is a model
diagram illustrating the connection of the lattice
. .. . . .
sampling pipes with a gas densimeter, Fig. 2B is a
partial view showing, in detail, a portion in the
vicinity of a lattice point C shown in Fig. 2A, and Fig. ~ -
2C is a partial view showing, in detail, in a portion
at the other end D of a sampling pipe. As illustrated,
the lattice sampling pipes 21 and 21a are grouped for
every test range on each surface (side surfaces and
bottom surfaces) of an LNG tank shown in Fig. lA. As
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- 13 -
1 described above, the sampling pipes 21 and 21a are
sucked by an exhaust system provided on the gas
densimeter 23 through the respective valves 13b so that
the gas density measurement is performed (the
measurement will be described later). At the portion of
the lattice point C, sampling holes 24 which serve as
gas entrances are formed in the sampling pipes 21 and
21a so that each hole perpendicularly diametrically
penetrates each sampling pipe as shown in Fig. 22B.
In the portion D, a blind seal 25 is formed at the
other end of each pipe as shown in Fig. 2C. Then, the
sampling pipes 21 and 21a are arranged so that the
distance of the lattice is made narrow, that is, the
distribution of the lattice points is made dense, at
portions where the defect generation rate is high. In
addition, as shown in Fig. 2A, the valves 13b are
numbered into Vi=1, ~ Vi=n~ Vj=1, ~ j=n~
the sampling pipes 21 and 2la corresponding thereto are
numbered into Si=1~ ~ Si=n' S~ Sj=n'
respect~ively.
Next, the measurement procedure of defect
detection will be described. First, the valve 13 is
opened so that the portion of the IS 7 is exhaust to
a vacuum by means of the vacuum pump 12, and the valve
13a is opened to lead a tracer gas (sulfur
hexafluoride) into the IBS 8. Consequently, a pressure
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- 14 -
.1 difference is produced between the sections on the
opposite sides of the secondary barrier 2, so that if
there is a defect of leakage in the secondary barrier
2, the tracer gas leak is through thus defect into the
sampling pipes 21 and 21a. By sequentially swi~ching
the valves 13b which are provided in the respective
lattice samplin0 pipes, the leaking tracer gas is led
into the respective sampling pipes 21 and 21a, and the
gas density thereof is measured by the gas
densimeterer 23. In a p~actical measuring procedure,
first, only the valve Vi=1 of the valves 13b is opened
to perform suction and measure gas density. After the
gas density measurement through the valve Vi=1 is
finished, the valve Vl=1 is closed and the next valve
5 Vi 2 lS opened to measure the gas density. The valves
13b are repeatedly opened and closed successively one
after one from the~valve Vi=1 to valve Yi=n and from te
valve Vj 1 to the valve Vj=n, so as to measure the gas
density with respect to each of all the sampling pipes
21 and 21a successively from the pipe Si=1 to the pipe
Si=n and from the pipe Sja1 to the pipe Sj=n. Further,
the measurement by such an operation is repeatedly
performed a plurality of times. As shown in Fig. 3, a
tracer gas is sucked by a sampling pipe 21 through
sampling holes 24 near an area E at which the tracer
has leaks through the secondary barrier 2, and the
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-- 15 --
1 tracer gas is led to the gas densimeter 23 so that the
density thereof is measured in the following manner.
As the tracer gas, a sulfur hexafluoride gas, a
helium gas, a halogen group gas, etc. may be used, and
the type of the gas densimeter is selected depending
on the kind of the gas to be used. Fig. 4 is a model
explanation view illustrating a method by using
sulfur hexafluoride as an example of the tracer gas, in
which this tracer gas is irradiated with laser light so
that the gas density is measured on the basis of the
degree of absorption of this laser light. In Fig. 4,
a sample suction pipe 31 is combined with the sampling
pipes 21 and 21a shown in Figs. 1 to 3, and this sample
. .
suction pipe 31 is connected to an air entrance of a
lS suction cell 32. The opposite ends of the suction cell
32 are formed of a material which is transparent with
respect to light having a wave length in an infra-red
band. The suction cell 32 is shaped into a closed
vessel having a predetermined cell length (for example,
30 cm), and ha~ing an entrance and an exit for air. In
this embodiment, the air entrance of the suction cell
32 is connected to the sample suction pipe 31, and the
air exit of the same is connected to a suction exhaust
system 34. If the sampling pipes 21 and 2la are
arranged to pass through a leakage portion, SF6-mixed
air is led into the pipes. In order to detect the
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.
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.1 SF6-mixed air, the suction cell 32 is irradiated with,
for example, laser light having a wave length of 10.6 ~m
(hereinafter referred to as "P(16)-ray laser light") of
a carbon dioxide laser (hereinafter referred to as
''C2 laser"). The reference numeral 35 represents the
C2 laser which generates P(16)-ray laser light. The
reference numeral 36 represents a helium-neon laser
(hereinafter referred to as "He-Ne laser"). ~enerating
red light, the He-Ne laser 36 is used as a pilot laser
for checking an optical path of the C02 laser. The
reference numeral 37 represents a spectrum analyzer
which is a measurement instrument for measuring the
wave length of the light generated from the CO2 laser 35
and the He-Ne laser 36. The reference numeral 38
represents a photo-detector which detects light near the
wave length of 106 ~m, converts the detected light into
an electric single, and outputs the thus converted
electric signal. The reference numeral 39 represents an
amplified which amplifies an input signal supplied from
the photo-detector 38, and supplies the amplified signal
to a display 4d and a leakage discriminator 41. The
display 40 displays the output of the amplifier 39, and
the leakage discriminator 41 discriminates leakage on
the basis of the change of an output signal of the
amplified 39. The reference numeral 42 represents a
mirror for reflecting light, 43 represents a half
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- 17 -
.1 mirror for reflecting incident light partially and
transmitting it partially, 33 represents a pipe
connecting the exit of the suction cell 32 with the
suction exhaust system 34.
In the gas densimeter shown in Fig. 4, if a
tracer gas SF6 is led into the suction cell 32 through
the sample suction pipe 31, the quantity of the
transmitted P(16)-ray laser light is reduced by ~he
absorption of the SF6 gas, so that the SF6 gas density
can be measured on the basis of the quantity of the
reduction. As ~or a similar gas densimeter, there are a
halôgen leak detector using a halogen-group gas, and a
helium leak detector of a mass analysis type each of
which may be used as a well-known gas densimeter and is
suitabIe for use in the detection method of the present
invention, but the description thereof will be omitted
here.
Fig. 5 is a diagram illustrating a measured
example showing the distribution of gas density by the
use of a lattice sampling pipe arrangement. Lattice
points formed by the sampling pipes Si=1 to Si n and
Sj=1 to Sj=n are numbered into P11 to Pnn The gas
density obtained from a certain sampling pipe (for
example, Si=1) is an average gas density of the values
sampled through sampling holes provided at a plurality
of lattice points, so that the gas density at a lattice
.
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.1 point can be estimated from the gas density values
obtained from two crossing sampling pipes. For example,
the gas density at the point P11 is estimated by
multiplying the gas density Di=1 obtained from the
sampling pipe Si=1 by the gas density Dj=1 obtained
from the samplin0 pipe Sj=1 Estimating the gas density
all over the area from the data of Fig. 5 in the same
manner, for example, a gas density distribution diagram
can be obtained as shown in Fig. 6. In Fig. 6, the
abscissa indicates the position of a lattice point in
~he i direction, and the ordinate indicates the
position of the lattice point in the J direction. In
such a manner, a leakage defect portion of the secondary
barrier 2 (a left upper corner portion of the test
surface) can be identified and detected.
As for a method of display the display 40, besides
the display example illustrated in Figs. 5 and 6, there
:
is a display method by higher data processing. Fig. 7 ~
is a diagram illustrating a measured example in which
the density and distribution of a tracer gas is made
into a three-dimensional image by a computer. The i-j :
surface indicates a test surface, and the degree of
,
leakage is displayed in the Z direction, so that a
three-dimensional display is realized. In this
2S detection method, a measured value oE gas density
supplied into a not-shown computer in real time. The
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.1 surface subjected to test is divided into unit test
sections each including a lattice point. The degree of
leakage is estimated by the gas density values obtained
from two crossing sampling pipes. For example, the
degree of leakage in the unit test section of i=1 and
j=1 in Fig. 7 is estimated by multiplying the gas
density Di=1 obtained from the sampling pipe of i=1 by
the gas density Dj=1 obtained from the sampling pipe of
j=1. This is repeated for every unit test section
sequentially, and the degree of leakage of the whole
the test surface is displa~ed. From this result, it
is possible to detect the leakage portion, that is, the
i-j coordinates position having the highest pole in the
Z direction in each test surface.
As has been described above, according to the
present invention, sampling pipes a:re arranged in the
form of a lattice, and sampling holes are provided at
lattice polnts of the lattice arransement of the
sampling pipes to take out a tracer gas, so that it is ~ -
20~ possible to make the number of sampling pipes smaller
than that in a method for sampling gas through separated
sampling pipes each of which has a hole at a single
position. For example, in order to measure gas density
at 100 points, while the method using a separated pipe
arrangement needs 100 sampling pipes, the method using a
lattice pipe arrangement according to the present
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.1 invention needs 20 sampling pipes, so that it is
possible to reduce the test cost.
As has been described above, according to the
present invention, lattice sampling pipes having
sampling holes at lat~ice points in a one-side
section of a secondary barrier of an LNG tank upon
which a defect of leakage is tested are arranged to
detect tracer gas leaking from the other section by a
gas densimeter connected with the sampling pipes, so
that it is possible to judge a leakage portion of the
secondary barrier properly. Consequently, the portion
of the secondary barrier to be mended is made clear so
. .
as to minimize the cut area of a primary barrier
(membrane), so that a great contribution to reducing
mending processes can be obtained.
Further, since sampling pipes are provided in a
lattice arrangement, it is possiblé to make the number
of sampling pipes smaller than that in a method of
detection by separated pipes, so that there is an
effect to reduce the test cost.
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