A Cryogenic Line Leak Detector
D. S. A L L A N and D. S. S C H I F F
Arthur D. Little, Inc., Consultant
On-site detection speeds response to flammable cryogenic gas leaks.
N O V E L method of detecting leaks from cryogenic systems has been
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developed to provide a more rapid means of sensing and locating the
leak. This leak detection system can be particularly important if the cryogenic fluid is flammable when vaporized, as with liquefied natural gas
(LNG). A well-proven, basic principle of fire protection for flammable gas
systems, including flammable cryogens, is to turn off the source of escaping
liquid o r gas as soon as possible, since extinguishment, at best, is only
marginally effective. The cryogenic line leak detector offers an opportunity to greatly improve on the ability to automatically and quickly
sense and locate the leak, thus providing the operator of t h e cryogenic
system with a higher level of information needed to accomplish a rapid
shutdown of the source of escaping fluid.
BACKGROUND
The detection of leaks from flammable gas systems, such as natural gas
transmission and distribution lines, cannot usually be accomplished with
existing equipment in a way that will quickly pinpoint the location of the
leak.
Commonly employed detectors, such as ionization probes and
catalytic cells, are, in effect, point sensors that must be strategically placed
with respect to the location of potential leaks if they are to detect the
leak at all. Because practical systems usually have several potential leak
sites and since there are economic limitations on the n u m b e r of detectors
that can be employed, detectors usually must be placed at some distance
from the leak. This will result in relatively prolonged times between the
occurrence of the leak and its detection.
With liquefied gases where the liquid state is maintained b y reduced
temperatures (as for example with LNG, refrigerated propane, and refrigerated ammonia), other detection schemes are often used. Thermocouples are used to sense temperature changes caused by the escape of the
NOTE: This paper was presented at the 79th Annual Meeting of the National Fire
Protection Association in Chicago, Illinois, May 13, 1975.
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Leak Detector
271
cold fluid, and visual detection can often be made by observing either the
condensed water vapor cloud caused b y the cool gas lowering the temperature of the surrounding air or by frosting of the liquid line or container
caused by condensation of water vapor on metal surfaces cooled by the
escaping cotd gas or liquid. Thermocouples are point sensors and have the
previously mentioned limitations. Visual inspection is not possible for enclosed systems and requires an alert observer wherever it is used.
A potential improvement in the detection of cold liquefied gases, particularly cryogenic materials, exists in the line leak detector. This system
consists of a long (almost any length) liquid-containing flexible (plastic)
tube of tess than 0.5 inch in diameter, which can be run along and around
any and all equipment containing the cold liquid. A leak anywhere along
and within the vicinity of the tubing will be sensed in a very short period
of time (perhaps less than 1 second). This d e t ~ t o r , in effect, senses the
temperature difference between the escaping cold liquid or gas and the
ambient temperature.
OPERATING
PRINCIPLE
The cryogenic line leak detector contains an electrically conducting
solution, which markedly changes its electrical resistance when the liquid
is frozen by exposure to the cold temperature of the escaping cryogen. The
increased electrical resistance of the system causes a decrease in current
flow, which in turn can be used to activate alarms a n d / o r other emergency
controls.
The detector shown schematically in Figure 1 consists of a flexible tube
of electrically nonconductive material containing the conducting liquid,
typically a salt solution. An alternating current is passed through the
solution by impressing an appropriate electrical potential on the electrodes
at each end of the tube. Changes in current m a y be detected and used in an
electrical system to initiate automatic alarms or other responses.
The location of the leak (or frozen area of the solution with respect to
one end of the tubing) can also be determined by electrical means. B y
surrounding the tubing with a metal coating or a metal mesh, as is used in
Filled with Salt Solution
I/4" O.D.
/
El~trodes
Plastic Tube
Figure 1. Physical schematic of line detector,
Electrical
Circuit
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Fire Technology
coaxial cables, two parallel conductors are created. The change in electrical
impedance between these two conductors (the outer sheath and the salt
solution) caused b y the freezing of the liquid at any point along t h e length
of the tube is utilized to establish the distance at which the frozen area
occurs with respect to one end of the detector. This arrangement is shown
schematically in Figure 2.
R~ncreaseat
s Leak
/
Termination
putation
[Co7'~p~
....Circuit
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--..
ControtBoardLineDetector
Figure 2. Electrical representation of line detector.
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APPLICATIONS
The line leak detector can be installed as one continuous section or as
several separate sections along and around all cryogenic systems where the
detection of potential leaks is desired. It can be made almost a n y length
from a few inches to several hundred feet. It should be located outside of
the cryogenic insulation at close to ambient temperatures in order to
sense the large temperature difference caused by the leak. The detector can
be strung along cryogenic piping, around valves and fittings, a n d along
critical areas in process equipment, including cold boxes, compressors,
and expanders. It has special potential for rapid detection of leaks from
primary storage containers where extra precautionary measures m a y be
required. A conceptual illustration of the installation of a detector at a
cryogenic storage facility is presented in Figure 3.
A partial listing of equipment on which the line detector m a y be placed
is as follows:
• The periphery of cryogenic storage containers;
• T a n k penetrations;
• Major cryogenic transfer piping, valves, and fittings;
• Cold boxes;
• Vaporizer output lines;
• Compressors and expanders where there is either cold gas or liquid;
• Sumps and impounding areas to contain spills of cryogenic liquid;
• T a n k truck containers and transfer equipment;
• T a n k vessels and barges; and
• Transfer stations and dockside transfer facilities.
Leak D e t e c t o r
273
F i g u r e 3. Installation of a detector at a cryogenic storage facility.
The leak detector electrical circuitry can be combined with plant control
systems. It can be used as a basis for indicating sites of leakage on a control display panel and m a y be connected to the control logic system to
provide the most effective response to leaks.
CONCLUSION
The cryogenic line leak detector provides a means of detecting leaks
more quickly and positively than existing systems. It provides relatively
complete coverage of cryogenic components and can activate automatic
response systems. It is simple in concept and is fail-safe in that damage to
the detector or loss of power can be used to alert the operator. In effect,
the line leak detector has the potential for significantly improving upon
the safety of cryogenic storage, process, transfer, and transport systems.