JPS6315539B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for detecting leakage of low-temperature fluids such as liquefied natural gas, and in particular to a method of using an optical fiber as a sensor, the amount of transmitted light of which is significantly attenuated when exposed to low temperatures. Conventionally, the main method used to detect leaks from low-temperature storage liquids such as liquefied natural gas has been to use gas detectors, but since detection is performed after the low-temperature liquid has vaporized and gasified, there is a time delay before detection. In addition, there are restrictions on the installation location due to the shape of the detector, and it can only be detected as a point.
Moreover, since it is affected by humidity and other factors, it has drawbacks such as poor reliability. There is also a known method of detecting leakage by detecting a temperature drop using thermocouples, but this can only be detected as a point, so many thermocouples need to be installed at the base of the tank or around it. However, there were problems with cost or installation. The present invention utilizes the fact that when an optical fiber such as a silica glass core coated with a silicon clad is exposed to low temperatures, the amount of transmitted light is significantly attenuated.
It is aimed at solving the above-mentioned drawbacks and problems of the conventional methods by detecting the leakage of cryogenic fluid. For this reason, the configuration of the present invention uses an optical fiber as a sensor, whose amount of transmitted light is significantly attenuated when exposed to low temperatures, and installs the optical fiber sensor via a heat insulating material near the location where leakage of low-temperature fluid is to be detected. The leakage of the fluid is detected based on the fact that the amount of light transmitted through the sensor decreases due to a decrease in temperature near the sensor due to the leakage of the low-temperature fluid. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of an apparatus implementing the present invention. In FIG. 1, reference numeral 1 denotes an optical fiber as a sensor, which consists of a quartz glass core 3 covered with a silicon cladding 2. Further, the curve a in FIG. 2 is an example of the temperature characteristic curve of the fiber 1. That is, in FIG. 2, the horizontal axis shows temperature (°C) and the vertical axis shows the transmitted light amount ratio (%). As shown by curve a, when the optical fiber 1 is exposed to low temperatures, it becomes significantly The amount of transmitted light is attenuated by approximately 40% at -60℃ and almost zero at temperatures below -65℃. Therefore, the optical fiber 1 is installed as a sensor around a storage facility for low-temperature storage liquid such as a liquefied natural gas tank, and as shown in FIG. The element 5 receives the light, the amplifier 6 amplifies it, and the comparator 7
The alarm 8 is compared with a preset value, and if it is less than the set value, the alarm device 8 issues an alarm. That is, at normal temperatures, the optical fiber 1 is sufficiently transparent to light, but if cryogenic liquid leaks from the storage facility, the leaked cryogenic liquid will come into contact with the optical fiber 1 or vaporize. lowering the temperature of the optical fiber 1, possibly by lowering the ambient temperature of the installation point;
Since the amount of light transmitted through the optical fiber 1 is significantly attenuated, the alarm device 8 issues an alarm. FIG. 3 is a plan view showing an example in which the optical fibers 1 described above are installed concentrically under the tank bottom plate 9 of a liquefied natural gas storage tank as shown by symbols S ₁ to S ₂₂ as sensors. Further, _T1 to _T8 are detector boxes in which the light emitting elements 4 to amplifier 6 described in FIG. 1 are installed. Note that the sensor is installed through a heat insulating material so that it does not come into direct contact with the tank body. In other words, in the case of underground tanks, in order to prevent the ground from freezing, heaters are generally installed on the outside of the side walls and under the bottom plate of the tank, so the heaters are used as insulation. do. In addition, in the case of an above-ground tank, the tank bottom plate generally floats about 80 centimeters above the ground, and there is air between the tank bottom plate and the ground to provide insulation, so this air is used as insulation. . Therefore, the cryogenic liquid is the sensor S ₁ ~
Leakage of cryogenic liquid can be detected by the drop in temperature when it adheres to or comes close to S ₂₂ . In this case, three-dimensional monitoring becomes possible by burying the sensors in several depths at the base of the tank. The numbers of light emitting channels and light receiving channels of the detector boxes _T1 to _T8 are as follows.

[Table] Figure 4 is a longitudinal cross-sectional elevation showing an example in which five layers of ring-shaped optical fiber sensors S are buried in the base under the tank bottom plate 9 of a liquefied natural gas storage tank, similar to Figure 3 above. It is a diagram. Although some illustrations are omitted, in correspondence with these sensors S,
A large number of detector boxes T are installed on the ground. FIG. 5 is an elevational view showing an example in which a sensor S made of an optical fiber similar to that described above is installed around the outer periphery near the top of the outer wall bottom plate of the above-ground tank 10 using air as a heat insulator, and FIG. 6 is a plan view thereof. It is. Next, examples of installing sensors in piping lines are shown in FIGS. 7 to 9. In Figure 7, a cold insulating material 12 of a low-temperature liquid transport pipe 11 is used as a heat insulator, a sensor S made of optical fiber similar to that described above is attached along this material, and a signal cable 13 is attached to a detector box T. As an example, FIG. 8 shows an example in which a sensor fixing mat 14 is attached to the cold insulation material 12 of the flange, and a sensor S made of optical fiber similar to that described above is wrapped around the mat 14 and fixed with a fixing metal fitting 15. The figure shows an example in which a sensor S made of optical fiber similar to that described above is attached to the cold insulating material 12 of the bulb and covered with a cover 16. In either case, the low-temperature liquid that has leaked close to the sensor S is detected. As described above, the present invention uses an optical fiber as a sensor, whose amount of transmitted light is significantly attenuated when exposed to low temperature, and installs it via a heat insulating material near the point where leakage of low-temperature fluid is to be detected, and detects the leakage of low-temperature fluid. This is a method of detecting leakage of the fluid based on the fact that the amount of light transmitted through the sensor decreases due to a decrease in the temperature near the sensor, so it is targeted because it detects based on temperature rather than an effect such as a chemical reaction. The type of low-temperature fluid does not matter, and since light is used as the detection principle, unlike electricity etc., it can be used safely even in locations where explosion-proof considerations are required.Moreover, the sensor is an optical fiber. Therefore, it is possible to use a flexible linear sensor for line or planar monitoring, and the location and shape of the sensor can be freely selected.

[Brief explanation of the drawing]

The drawings show embodiments of the present invention.
Figure 2 is a schematic diagram of a device implementing the present invention, Figure 2 is a light transmission characteristic curve diagram of the optical fiber in Figure 1, Figure 3 is a plan view showing an example of a sensor installed in a liquefied natural gas storage tank, Figure 4 is a vertical elevation view showing another example, and Figure 5 is another example.
An elevation view showing two examples; Figure 6 is a plan view of Figure 5;
FIG. 7 is an explanatory diagram of the application to a pipe, FIG. 8 is an explanatory diagram of the application to a flange, and FIG. 9 is an explanatory diagram of the application to a valve. 1... Optical fiber, 2... Silicon clad, 3... Quartz glass core, 4... Light emitting element, 5
...Photodetector, 6...Amplifier, 7...Comparator, 8
... Alarm, 9 ... Tank bottom plate, 10 ... Above ground tank, 11 ... Pipe, 12 ... Cold insulation material, 13 ...
...Signal cable, 14...Sensor fixing mat, 15...Fixing metal fittings, 16...Cover, a...
Characteristic curve, S, S ₁ ~ S ₂₂ ... Sensor, T, T ₁ ~
T ₈ ...Detector box.

Claims (1)

[Claims] 1. Using an optical fiber as a sensor, the amount of transmitted light is significantly attenuated when exposed to low temperatures, and installing the optical fiber sensor via a heat insulator near the location where leakage of low-temperature fluid is to be detected, and detecting leakage of low-temperature fluid. A method for detecting leakage of a low-temperature fluid, characterized in that leakage of the fluid is detected based on a decrease in the amount of light transmitted through the sensor due to a decrease in temperature near the sensor.