Leak detecting method for vessels

Abstract: A leak detecting method for vessels in which a gas of a given pressure is mixed with sulfur hexafluoride of a given partial pressure and the mixture is introduced into a vessel, and a gas in the vicinity of an outer peripheral surface of the vessel is sucked by a gas sucking mechanism. According to one form of the invention, the sucked air is introduced into an absorption cell (5) and a P(16)-line laser light of a carbon dioxide gas laser is introduced into the absorption cell (5). Then, the presence or absence of absorption of the P(16)-line laser light within the absorption cell is detected in accordance with a change in the transmitted light intensity of the P(16)-line laser light and the presence or absence of a leak in the pressure vessel (1) is discriminated in accordance with a result of the detection. In accordance with another form of the invention the sucked air is introduced into a multiple reflection absorption cell and a P(16)-line laser light of a carbon dioxide gas laser and a reference light slightly different in wavelength from the P(16)-line laser light are introduced into the multiple reflection absorption cell. Then, the presence or absence of absorption of the P(16)-line laser light within the multiple reflection absorption cell is detected in accordance with a change in a light intensity ratio between the reflected lights of the P(16)-line laser light and the reference light, and the presence or absence of a leak in the pressure vessel is discriminated in accordance with a result of the detection.

Description: The invention refers to a leak detection method according to the preamble of claim 1. This leak detecting method is known from DE-A-3 204 327. According to this document the suction port is located on the surface of the vessel. There is no provision for mechanically displacing the suction port and to run the leak detecting method automatically. In the known method the light originates from a metal wire, which is electrically heated. US-A-4,772,789 refers to an automatically operating leak imaging method which is capable of detecting the location and magnitude of any and all leakage flows emanating from the pressure vessel. The exterior of the vessel is illuminated with a light source at a wavelength which is capable of being absorbed by the air mixed with sulfur hexafluoride. A videocamera is used to image the vessel and the image is electronically stored. Then the vessel is illuminated with a light source at a wavelength which is not capable of being absorbed by air mixed with sulfur hexafluoride. The resulting second image is also electronically stored. The stored images are then electronically compared and analyzed for detecting, locating and measuring any leaking air mixed with sulfur hexafluoride from the vessel. The absorption rate can be raised at the P(16) 10.55 micron line of a CO 2 Laser. This known method is difficult to perform in vessels which have a complicated structure. Surface portions of the vessel, which are hidden to the videocamera, cannot be inspected. Starting from the known method of DE-A- 3 204 327 it is the object of the invention to improve the known leak detecting method, to render it automatic and easier to handle and to improve the light system. This object is solved by the method of claim 1. Fig. 1 is a block diagram showing the construction of a leak detecting apparatus for pressure vessels according to an embodiment of the present invention. Fig. 2(a), 2(b) and 2(c) are diagrams showing examples of the measurement of a P(16)-line laser light by the absorption cell of Fig. 1. Fig. 3 is a schematic block diagram showing the construction of a leak detecting apparatus for pressure vessels according to another embodiment of the present invention. Fig. 1 is a schematic block diagram showing the construction of a leak detecting apparatus for pressure vessels according to an embodiment of the present invention. In the Figure, numeral 1 designates a pressure vessel and the interior of which is pressurized by an externally supplied air mixed with sulfur hexafluoride (hereinafter referred to as SF 6 ). Numeral 2 designates the welds of the pressure vessel 1 where the tendency to cause leakage is highest. Numeral 3 designates an air suction port which is formed for example into a funnel shape and including a lower part having a flexible skirt so as to be convenient for the sucking of air while being moved and an upper part connected to one end of a flexible pipe 4 for air exhausting purposes. The position of the air suction port 3 is subjected to profiling control by a profile robot 17 which is preliminarily taught about the positions of the welds 2. The other end of the flexible pipe 4 is connected to the air inlet of an absorption cell 5. The absorption cell 5 is a closed container having ends made of a substance transparent to light of wavelengths in the infrared region and a given cell length (e.g., 30 cm) and including an air inlet and an air outlet. In this embodiment, the air inlet of the absorption cell 5 is connected to the suction port 3 through the flexible pipe 4 and its air outlet is connected to a suction and exhaust unit 6 through a pipe 15. As a result, air is continuously flowing at a constant flow rate within the absorption cell 5 so that if the air suction port 3 is positioned at the location of a leak, the SF 6 -containing air is introduced into the absorption cell 5. In order to detect the SF 6 -containing air, a P(16)-line laser light is introduced from a carbon dioxide gas laser into the absorption cell 5. Numeral 7 designates the carbon dioxide gas (hereinafter referred to as CO 2 ) laser for emitting a laser light having a wavelength of 10.6 µm (hereinafter referred to as a P(16)-line laser light). Numeral 8 designates a helium neon (hereinafter referred to as He-Ne) laser which emits a red light and is used as an identification laser for checking the optical path of the CO 2 laser. Numeral 9 designates a spectral analyzer which is a measuring device for measuring the wavelength of the light emitted by the CO 2 laser 7. Numeral 10 designates a light sensor for detecting light near a wavelength of 10.6 µm to convert and generate it as an electric signal. Numeral 11 designates an amplifier for amplifying the input signal from the light sensor 10 and applying it to an indicator 12 and a leak discriminator 16. Numeral 12 designates the indicator for indicating the output of the amplifier 11, 13 a mirror for light reflecting purposes, 14 a half-mirror for reflecting part of the incident light and transmitting part of the incident light, 15 the pipe connecting the air outlet of the absorption cell and the suction and exhaust unit, 16 the leak discriminator for discriminating a leak in accordance with a change in the output signal of the amplifier 11. Figs. 2(A), 2(B) and 2(C) are diagrams showing examples of the measurement of the P(16)-line laser light in the absorption cell of Fig. 1 and each of the examples shows the measured values of the transmitted light intensity of the incident P(16)-line laser light into the absorption cell in cases where a vacuum is produced within the absorption cell and where a mixed gas of SF 6 having a concentration of 1 ppm, 5 ppm or 10 ppm and nitrogen (N 2 ) is sealed, respectively. The operation of the embodiment of Fig. 1 will now be described with reference to Figs. 2(A), 2(B), and 2(C). In order to detect leakage from the pressure vessel 1, the interior of pressure vessel 1 is pressurized by air mixed with SF 6 having a concentraion of about 5 ppm to 10 ppm. Since the welds 2 and the terminal mounting portion of the vessel generally represent those portions having the highest tendency to cause a leak, the profile robot 17 is preliminarily taught about the shape of the pressure vessel 1 and the positions of the welds 2. The profile robot 17 controls the position of the suction port 3 connected to the flexible pipe 4 in such a manner that the suction port 3 is profile controlled along the outer peripheral portion of the pressure vessel 1 so as to scan all of the areas to be inspected such as the welds 2, thereby sucking the air in the vicinity of these areas successively. In this case, the suction port 3 is not brought into close contact with the outer peripheral surface of the pressure vessel 1 but a small gap is maintained therebetween so that the air near the suction port 3 is also sucked and thus the air of a small flow rate (e.g., several ACC/sec) flows continuously. In order that this air of the constant flow rate flows at all times, the suction and exhaust unit 6 continuously effects the suction and exhaustion of the air through the suction port 3, the flexible pipe 4, the absorption cell 5 and the pipe 15. In this way, the air of the constant flow rate is continuously introduced into the absorption cell 5 from near the suction port 3 so that if the suction port 3 is positioned on the location of a leak such as the weld 2 or the terminal mounting portion, the SF 6 -containing air is introduced into the absorption cell 5 and the air remains temporarily within the cell. The CO 2 laser 7 is preliminarily tuned by use of the spectral analyzer 9 and it outputs the P(16)-line laser light (i.e., the laser light having a wavelength of 10.6 µm). Also, the output optical path of the CO 2 laser light is preliminarily adjusted by use of the output light (red light) of the identifying He-Ne laser 8 so as to pass along the central axis of the absorption cell 5. The ends of the absorption cell 5 are made of a substance which is transparent to light of wavelengths in the infrared region so that the CO 2 laser light having a wavelength of 10.6 µm is introduced into one end of the absorption cell 5 and its transmitted light is taken out from the other end. Then, depending on whether the SF 6 is present within the absorption cell 5 when the P(16)-line laser light from the CO 2 laser passes through the optical path of absorption cell 5, there is caused a variation in the amount of the transmitted laser light. The laser light transmitted through the absorption cell 5 is first converted to an electric signal by the light sensor 10 and it is then amplified by the amplifier 11, thereby generating an output. The output of the amplifier 11 is indicated on the indicator 12 and it is also supplied to the leak discriminator 16 which in turn discriminates the presence of leakage from the pressure vessel 1. The output of the leak discriminator 16 is fed back to the robot 17 so that while referring to the results of the leak discriminating signals supplied from the leak discriminator 16, the robot 17 controls the position of the suction port 3 in a manner that the positions of leaks and the shape of leakage on the pressure vessel 1 are measured properly. Each of Figs. 2(A), 2(B), and 2(C) shows variations in the amount of transmitted light due to the absorption of the incident CO 2 laser light of 10.6 µm in wavelength in cases where a vacuum is produced within the absorption cell 5 and where N 2 is mixed with a trace amount(1 ppm, 5 ppm or 10 ppm) of SF 6 . Thus, the leak discriminator 16 can discriminate the presence of leakage of the pressure vessel 1 by discriminating the amount of such variation. Referring now to Fig. 3, there is illustrated a schematic block diagram showing the construction of a leak detecting apparatus for pressure vessels according to another embodiment of the present invention. In the Figure, numerals 1, 2, 3, 4, 6, 14, 15 and 17 designate the same component parts as shown in Fig. 1. Numeral 21 designates a multiple reflection absorption cell, and the cell 21 is a closed container having an incident light entrance and a reflected light outlet which are made of a substance transparent to light of wavelengths in the infrared region and it also includes an air inlet and an air outlet. The incident light introduced through the incident light entrance of the multiple reflection absorption cell 21 is subjected to multiple reflection (that is, the optical path length is extended in an amount corresponding to the multiple reflection) within the cell by oppositely arranged infrared reflection concave mirrors within the cell and the reflected light is delivered to the outside through the reflected light outlet. In this embodiment, the multiple reflection absorption cell 21 is a sensor so that if SF 6 is mixed with the air sucked into the cell 21, only the incident laser light of 10.6 µm in wavelength is absorbed in the course of the multiple reflection within the cell and thus its output is delivered as an attenuated light intensity through the reflected light outlet. Numeral 22 designates a CO 2 laser which outputs two laser beam of different wavelengths, i.e., a P(16)-line laser light of a wavelength λ 1 which is 10.6µm and a reference light of a wavelength λ 2 which is slightly different from the wavelength λ 1 . It is to be noted that the reference light of the wavelength λ 2 may comprise for example a P(18)-line laser light of a wavelength of λ 2 which is 10.57 µm. Numeral 23 designates a calibrator and monitor for calibrating and monitoring the CO 2 laser 22 so that two measuring lightwaves slightly different in wavelength (or frequency) from each other are generated from the CO 2 laser 22, and this measuring device is used for effecting the initial calibration of the two light waves of the wavelengths λ 1 and λ 2 which are generated from the CO 2 laser 22 and monitored during its operation. Numeral 24 designates an optical filter for the lights output from the multiple reflection absorption cell 21 and it is composed a filter ℓ 1 for transmitting only the light of the wavelength λ 1 and a filter ℓ 2 for transmitting only the light of the wavelength λ 2 . Numeral 25 designates a light sensor comprising light detectors m 1 and m 2 for detecting the lights of the wavelengths λ 1 and λ 2 transmitted by the filters ℓ 1 and ℓ 2 , respectively. Numerals 26-1 and 26-2 designates #1 amplifier and #2 amplifier for respectively amplifying and outputting the input signals from the light detectors m 1 and m 2 respectively. Numeral 27 designates a divider for dividing the output value of the #1 amplifier 26-1 by the output value of the #2 amplifier 26-2 in order to calculate the ratio of light intensities of the wavelengths λ 1 and λ 2 as the quotient of the division. Numeral 28 designates a microprocessor (hereinafter referred to as a CPU) incorporating memories,i.e., ROM and RAM for storing programs and measured data, etc. Numeral 29 designates an indicator for indicating the measurement results. The operation of the embodiment of Fig. 3 will be described. The operation of pressurizing the interior of pressure vessel 1 with the air mixed with a trace amount of SF 6 , controlling the position of the air suction port 3 connected to the flexible pipe 4 by the profile robot 17 and sucking the nearby air while profile controlling the suction port 3 along the outer peripheral surface (mainly the welds 2, etc.,) of the pressure vessel 1 for the purpose of detecting leakage from the pressure vessel 1 is all the same as in the case of Fig. 1. In the case of Fig. 3, however, the other end of the flexible pipe 4 is connected to the air inlet of the multiple reflection absorption cell 21 and one end of the pipe 15 is connected to the air outlet of the multiple reflection absorption cell 21. As a result, the suction and exhaust unit performs the continuous suction and exhaustion through the suction port 3, the flexible pipe 4, the multiple reflection absorption cell 21 and the pipe 15. As in the case of Fig. 1, if the suction port 3 sucks the SF 6 -containing air at the location of a leak, this SF 6 -containing air is introduced into the multiple reflection absorption cell 21 and it remains temporarily within the cell. After its two output wavelengths have been preliminarily calibrated by the calibrator and monitor 23, the CO 2 laser 22 introduces both the P(16)-line laser light of the wavelength λ 1 (=10.6µm) and the reference light of the wavelength λ 2 which is slightly different from the former into the multiple reflection absorption cell 21 through its light entrance. When the SF 6 -containing air is present within the multiple reflection absorption cell 21, the P(16)-line laser light of the wavelength λ 1 of the incident laser beams of the different wavelengths is absorbed in the course of multiple reflection and the reference light of the wavelength λ 2 is practically not absorbed and they are delivered as such from the reflected light outlet. Therefore, while there is caused a variation in the light intensity of the P(16)-line laser light of the wavelength λ 1 delivered from the reflected light outlet depending on whether SF 6 is present within the multiple reflection absorption cell 21, practically there is no change in the light intensity of the reference light of the wavelength λ 2 . The P(16)-line laser light of the wavelength λ 1 , which is delivered from the reflected light output of the multiple reflection absorption cell 21, is supplied to one input of the divider 27 through the filter ℓ 1 of the filter 24, the light detector m 1 of the light sensor 25 and the #1 amplifier 26-1 and similarly the reference light of the wavelength λ 2 is supplied to the other input of the divider 27 through the filter ℓ 2 of the filter 24, the light detector m 2 of the light sensor 25 and the #2 amplifier 26-2. The divider 27 divides the output value of the #1 amplifier 26-1 by the output value of the #2 amplifier 26-2 to calculate the ratio between the two light intensities. The thus calculated light intensity ratio data of the P(16)-line laser light of the wavelength λ 1 to the reference light of the wavelength λ2 is supplied to the CPU 28. In response to the input data, the CPU 28 detects the presence or absence of the absorption of the P(16)-line laser light so that the result of the detection is indicated on the indicator 29 and also the presence or absence of leakage of the pressure vessel 1 is discriminated in accordance with the detection result, thereby feeding the result of the discrimination back to the robot 17. Referring to the feed back signal, the robot 17 controls the position of the suction port 3 in such a manner that the positions of leaks and the shape of leakage on the pressure vessel 1 are measured properly. While, in above-described preferred embodiments, the present invention has been shown as applied to the detection of leaks in pressure vessels, the present invention is applicable not only to the pressure testing of pressure vessels but also to the gas-tight testing of hermetically sealed vessels. In accordance with the present invention, air, inert gases, nitrogen gas, etc., are conceivable as the suitable gases for introduction into such vessels.

Claims: A leak detecting method for pressure vessels (1) comprising the steps of: pressurizing an interior of a pressure vessel (1) with air mixed with a light absorbing fluid of a given partial pressure, sucking air in the vicinity of an outer peripheral surface of said pressure vessel by an air sucking mechanism having a suction port (3) to introduce the same into an absorption cell (5), introducing light into said absorption cell (5), detecting the presence or absence of absorption of light within said absorption cell (5) in accordance with a change in a transmitted light quantity of said light, and discriminating the presence or absence of a leak in said pressure vessel (1) in accordance with a result of said detection; characterized in that a P(16)-line laser light of a carbon dioxide gas laser is the light used, that a profile robot (17) is provided, which is preliminarily taught about the shape of said pressure vessel (1) and the position of welds (2), which profile robot (17) controls the position of the air suction port (3) of said air sucking mechanism in such a manner that said suction port (3) is profile controlled so as to scan all of the areas to be inspected including said welds (2) thereby successively sucking the air in the vicinity of the outer peripheral surface of said pressure vessel (1) and in the case there is a leak in said pressure vessel (1) the location of the leak is able to be specified. The leak detecting method according to claim 1, characterized in that a multiple reflection absorption cell (21) is used as absorption cell (5). The leak detecting method according to claim 1 or 2, characterized in that together with the P(16)-line laser light a reference light slightly different in wavelength from said P(16)-line laser light is introduced into the absorption cell (5,21) and that the presence or absence of absorption of said P(16)-line laser light within said absorption cell (5, 21) in accordance with a change in a light intensity ratio between reflected lights of said P(16)-line laser light and said reference light is detected.

Inventors: ASAI KAZAHIRO, ISHIHARA KOJI C O NKK CORPORAT, MORIMOTO SHIGERU C O KANAOKA F, OMORI HIROSHI C O KANAOKA FACT, ASAI, KAZAHIRO, ISHIHARA, KOJI, C/O NKK CORPORATION, MORIMOTO, SHIGERU, C/O KANAOKA FACTORY IN SAKAI, OMORI, HIROSHI, C/O KANAOKA FACTORY IN SAKAI
Applicants: ASAI KAZUHIRO, DAIKIN IND LTD, NIPPON KOKAN KK, ASAI, KAZUHIRO, DAIKIN INDUSTRIES, LIMITED, NKK CORPORATION
Application year: 1991
Main classification: G01M3
IPC codes: G01M3