WO2008103837A1

WO2008103837A1 - Method and apparatus for monitoring gases in fluid tanks

Info

Publication number: WO2008103837A1
Application number: PCT/US2008/054600
Authority: WO
Inventors: Henrik Hofvander; Andrew D. Sappey
Original assignee: Zolo Technologies, Inc.
Priority date: 2007-02-22
Filing date: 2008-02-21
Publication date: 2008-08-28

Abstract

A method and apparatus for detecting the presence of a gas associated with fluid residing in a tank. The method includes transmitting radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas into a space associated with a tank containing gas associated with a liquid. The transmitted radiation is then detected and analyzed. The apparatus includes a laser producing radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas and plurality of pitch/catch optic pairs, each pair being configured to transmit radiation therebetween through a headspace of a tank. The laser is optically coupled to the pitch optics and a detector is optically coupled to each catch optic.

Description

METHOD AND APPARATUS FOR MONITORING GASES IN FLUID TANKS

TECHNICAL FIELD

[0001] The present invention is directed to methods and apparatus for monitoring gases in fluid tanks, and more particularly to the use of tunable diode laser absorption spectroscopy to monitor gases in fluid tanks.

BACKGROUND OF THE INVENTION

[0002] Monitoring of gases released from fluids within fluid tanks can be useful for determining the condition of the fluid within the tank and for detecting the buildup of gases which may be indicative of a failure of a process associated with the fluid in the tank or which may create danger to the tank and surrounding equipment and personnel if, for example, the gases are explosive.

[0003] As used herein, the term "tank" means any container for a fluid and includes fluid storage tanks as well as reservoirs for fluids associated with the operation of equipment such as lubricating or insulating fluids. One particular "tank" application relates to oil- insulated electrical apparatus used in the electric power industry. With regard to these applications, the electric power industry has recognized that certain electrical and thermal phenomena occur in oil-insulated electrical apparatus that can lead to the generation of "fault gases." These phenomena occur in equipment such as oil-filled transformers, load tap changers, current transformers and bushings and the like, all of which are intended to be included under the general category of "tanks."

[0004] Examples of fault gases associated with oil insulated electric equipment include H₂, O₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆. The presence of fault gases may be a measure of the condition of the associated equipment. Thus, the detection and classification of fault gases can help in the efficient maintenance of the apparatus. In addition, because many of the fault gases are highly combustible, their early detection can minimize the risk of catastrophic failures such as explosions.

[0005] The benefits associated with monitoring the build up of fault gases in fluids such as transformer oil are well known, but designing equipment suitable for monitoring fault gases has proven problematic for a variety of reasons. A common monitoring technique used by a number of electrical utilities is routine sampling of transformer oil in the field and conveying the oil samples to laboratories to run dissolved gas analysis using gas chromatographs. In some instances portable field gas chromatographs are used. However, these methods do not give real-time analysis and may result in data that is not a true measure of actual, ongoing operating conditions. Further, physical sampling cannot be done on a continuous, ongoing basis, but instead requires scheduled visits. This intermittent sampling creates the risk of missing a substantial transformer fault. Moreover, the sampling protocols are labor intensive and expensive.

[0006] Serveron, Inc. sells a True Gas Transformer Gas Analyzer which is intended to provide continuous online monitoring of transformer gases. The True Gas system in one embodiment taps into the headspace of a transformer and provides head gas for analysis in a chromatograph column. Data is provided to a microprocessor which analyzes the results of the chromatograph data to provide an alarm if dangerous conditions exist or to transmit the data via a communication system for remote monitoring. While overcoming some of the problems of the prior monitoring systems, the True Gas system is not ideal because it requires removal of a gas sample to a gas chromatograph for analysis. This is cumbersome and requires the gas chromatograph to be in close proximity to the transformer. The various pipes and connections necessary for the transporting gas to the gas chromatograph creates a risk of leakage which can effect the accuracy of collected data. The True Gas system also requires a minimum pressure of the gas in the headspace (e.g., 0.5 psig) to make the gas available for analysis. Furthermore, at sites where there are numerous transformers, a correspondingly large number of gas chromatographs and associated equipment will be required for monitoring all of the various transformers.

[0007] For conservator design transformers that do not have a headspace, the True

Gas system circulates the sample of transformer oil through a gas extractor to produce a gas sample for analysis in a gas chromatograph. While facilitating monitoring of dissolved gases in transformers without a headspace, this True Gas system shares many of the problems of the True Gas headspace analyzer described above, including requiring a large number of gas chromatographs in order to monitor electrical generation sites having numerous transformers or other electrical apparatus being monitored.

[0008] The present invention is directed toward overcoming one or more of the problems discussed above. SUMMARY

[0009] One aspect is a method of detecting the presence of a gas associated with fluid residing in a tank, the tank having a headspace in communication with the fluid. The method includes transmitting radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas into the headspace and detecting absorption of the radiation transmitted into the headspace. In one embodiment a pitch and catch optic pair is provided in optical communication with the headspace and the radiation is transmitted into the headspace between the pitch and catch optic. In one embodiment a plurality of pitch and catch optics are provided, with each pair being in optical communication with a distinct tank headspace. In such an embodiment radiation may be routed from a single source, such as a laser, to each of the pitch optics. Such an embodiment may further include a detector for detecting the absorption of the radiation transmitted into the distinct tank headspaces. The detector is in optical communication with each catch optic and radiation received by each catch optic is routed to the detector. In one embodiment, the radiation reaching the detector may be switched between various catch optics. In yet another embodiment, a multiplexed radiation beam comprising at least two wavelengths of radiation, each corresponding to a predetermined spectral absorption line of a distinct gas, is transmitted into the headspace. The multiplexed radiation is subsequently demultiplexed with the demultiplexed wavelengths of radiation being provided to a corresponding detector. The method may further include determining the temperature of the fluid, determining the pressure of the gas and determining the concentration of the gas in the headspace to enable calculating the concentration of the dissolved gas in the fluid.

[0010] Another aspect is a system for detecting the presence of gas associated with a fluid residing in a plurality of tanks, with each tank having a headspace in communication with the fluid. The system includes a laser producing radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas. A plurality of pitch and catch optic pairs are provided with each pitch optic and catch optic pair being configured to transmit radiation therebetween through the headspace of a distinct tank. Means are provided for routing the radiation from the laser to each pitch optic. A detector for detecting absorption of radiation transmitted between the pitch and catch optic pairs is in optical communication with each catch optic and means are provided for routing radiation between each catch optic and the detector. In one embodiment at least two lasers produce radiation at a distinct wavelength corresponding to a predetermined spectral absorption line of a distinct gas to be detected. A multiplexer is optically coupled to each laser for producing a multiplexed radiation beam which is conveyed to the pitch optic. A demultiplexer is optically coupled to each catch optic for demultiplexing the radiation to each wavelength and a detector for detecting the absorption of each wavelength of radiation is coupled to a respective output of the demultiplexer.

[0011] Yet another aspect is a method of detecting the presence of a gas associated with a fluid residing in a tank. The method includes separating gas dissolved in the fluid from the fluid into a space. Radiation is transmitted at a wavelength corresponding to that of a predetermined spectral absorption line of the gas into the space containing the gas separated from the fluid. Absorption of radiation transmitted into the space is detected. The method may further include transmitting a multiplexed radiation beam comprising at least two wavelengths of radiation each corresponding to a predetermined spectral absorption line of a distinct gas. Pitch and catch optics may be provided in optical communication with the space, the radiation being transmitted into the space between the pitch and catch optics. The radiation received by each catch optic is demultiplexed and a detector is provided for detecting each wavelength of the radiation comprising the multiplexed beam. A plurality of pitch/catch optic pairs each in communication with a space associated with a different tank may be provided to allow the monitoring of multiple tanks.

[0012] The method and apparatus described herein enable continuous in situ sampling of gas associated with a fluid in a tank. The method and apparatus have particularly beneficial application for the monitoring of fault gases in, for example, electrical apparatus such as transformers. The tanks may be continuously sampled to provide real time fault gas concentrations and, in those embodiments where pressure within a space containing the gas and temperature of the fluid in the tank are monitored, concentrations of dissolved fault gas can be determined. The method and apparatus have particular advantages where large numbers of transformers are being monitored. By using routers and switches only a single set of radiation sources and radiation detectors are required for a relatively large number of transformers and the radiation sources and detectors may be deployed remotely from the transformers in a controlled environment protecting the radiation sources and detectors and, if used, multiplexers, demultiplexers and routers, from potentially harsh ambient conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Fig. 1 is a schematic representation of a first embodiment of an apparatus for the detection of a gas associated with a fluid residing in a tank;

[0014] Fig. 2 is a schematic representation of an apparatus for the detection of a gas associated with a fluid residing in a plurality of tanks;

[0015] Fig. 3 is a schematic representation of an apparatus for detecting the presence of a plurality of gases associated with a fluid residing in a tank;

[0016] Fig. 4 is a schematic representation of an apparatus for detecting the presence of a plurality of gases associated with a fluid in a plurality of tanks; and

[0017] Fig. 5 is schematic representation of an apparatus for detecting the presence of gas associated with a fluid with the gas extracted from the fluid.

DETAILED DESCRIPTION

[0018] In each of the embodiments of the method and apparatus for detecting the presence of a gas associated with a fluid residing in a tank expressly described herein, the "tank" is a transformer containing a dielectric oil. However, the methods and apparatus described herein could be used with other electrical apparatus using insulating oils and indeed any tank containing a fluid where the detection of gas in a dissolved or gaseous state is desired.

[0019] A first embodiment of an apparatus for the detection of gas associated with a fluid residing in a tank 10 is illustrated in Fig. 1. This embodiment is suitable for monitoring a single gas within a headspace of a tank. In this embodiment, the tank 12 is an electric transformer having a headspace 14. A pitch optic 16 and a catch optic 18 are in optical communication in the headspace 14. A radiation source 20 is in optical communication with the pitch optic 16 by means of a fiber optic cable 22. In the embodiment of Fig. 1, the radiation source 20 is a tunable diode laser which produces radiation at a wavelength corresponding to that of a predetermined spectral absorption line of a gas to be monitored within the headspace 14. A detector 24 is optically coupled to the catch optic 18 by a fiber optic cable 26. The detector 24 in the embodiment of Fig. 1 is a photodetector, which may be a photo optic diode sensitive to the wavelength of radiation produced by the tunable diode laser 20. The detector 24 generates an electric signal based upon the nature and quantity of light transmitted to the detector at the detector wavelength. The electrical signal from each detector 24 is typically digitized and analyzed in the data processing system 28. The data processing system 28 may be a microcomputer, minicomputer, personal computer, microprocessor or any other suitable computing device programmed to process the signals from the detector 24.

[0020] As used herein, "coupled," "optically coupled" or "in optical communication with" is defined as a functional relationship between components where light can pass from a first component to a second component either through or not through intermediate components or free space.

[0021] Fig. 2 illustrates a second embodiment of an apparatus for detecting gas associated with a fluid in a plurality of tanks. In this embodiment a plurality of catch optic pairs 16, 18 are each provided in optical association with the headspace distinct transformers 12i-12_n. Like elements of Figs. 1 and 2 have the same reference numbers. In this embodiment a routing device 32 delivers radiation from the tunable diode laser 20 to each of the pitch optics 16. Suitable routing devices include optical switches which may be implemented to route the radiation beam with minimal attenuation to each pitch optic in a predetermined sequence or an optical splitter which simultaneously routes a fractional portion of the radiation beam to each pitch optic may be used. A similar routing device 34 is in optical communication with each of the catch optics 18. The routing device 34 in the second embodiment of Fig. 2 is a multimode optical switch which can sequentially provide radiation from each catch optic 18 to the detector 24. The embodiment illustrated in Fig. 2 includes only three transformers 12i, 12₂, 12_n simply for the sake of clarity. Any number of transformers n could be associated with a single tunable diode laser 20 and detector 24 limited only by the number of divisions possible by the routing device 32 and routing device 34. 1 x n optical switches with n up to 16 are currently available. Combinations of splitters and switches could be used to greatly increase the number of transformers which could be monitored using a single tunable diode laser 20 and detector 24.

[0022] Fig. 3 is a third embodiment 40 of an apparatus for detecting gas associated with a fluid residing in a tank. Like elements of Figs. 1 and 3 have the same reference numbers. The embodiment depicted in Fig. 3 includes multiple tunable diode lasers 2Oi , 2O₂, 2O₃, 2O_n, each producing radiation at a wavelength corresponding to that of a predetermined spectral absorption line of a distinct gas. Corresponding detectors 24i, 24₂, 24₃, 24_n, are provided for detecting the radiation at the select wavelengths. A multiplexer 42 is provided for coupling radiation from the tunable diode lasers 2Oi , 2O₂, 2O₃, 2O_n to the single optical fiber 22. In this manner a multiplexed beam can be transmitted through the headspace 14 for detection of up to n gases. On the catch side, a demultiplexer 44 demultiplexes the multiplexed beam to its various wavelengths which are then routed to the corresponding detector 24i, 24₂, 24₃, 24_n. In this manner a number of gases such as fault gases may be simultaneously monitored in a transformer headspace using a single pitch/catch optic pair. [0023] Fig. 4 is a fourth embodiment of an apparatus for detecting gas associated with fluid residing in a tank combining attributes of the second and third embodiments illustrated in Figs. 2 and 3, respectively. Like elements in Fig. 4 have the same reference numbers as Figs. 1, 2 and 3. The embodiment illustrated in Fig. 4 provides a multiplexed beam of radiation to the headspace of a number of transformers 12_r12_n for the purpose of detecting the presence of a plurality of gases in the headspace of the transformers. The embodiment of Fig. 4 includes an additional element of a communication interface 46 for transmitting data processed by the data processor or computer 28. The communication interface 46 may be any known device for conveying data from the computer 28 including, but not limited to, a wireless modem, a radio transmitter, a phone modem connected to a telephone line and the like.

[0024] Use of the fiber optics 22, 26 allows components such as the tunable diode lasers 20, the detectors 24, the multiplexer 42, the demultiplexer 44, the routing devices 32, 34 and the computer 28 to be located remotely from the transformers 12. For example, these components could be located in a remote control room situated one kilometer or more from the various transformers. Such a control room would typically have a controlled environment to protect these relatively delicate instruments from the elements and extreme temperature fluctuations. Fiber optics also allow optical coupling to large numbers of transformers spread about an electrical transmission facility.

[0025] Fig. 5 is another embodiment of an apparatus for detecting gas associated with fluid residing in a tank which can be utilized as an alternative to pitch/catch optic pairs associated with the headspace of a tank or in tanks without a headspace. For example, in the embodiment illustrated in Fig. 5, the tank is a conservator transformer 50. In this embodiment, a tap 52 is provided in the transformer 50 casing in communication with a pipe 54 and fluid is driven through the pipe 54 by pump 56. The fluid is conveyed to a gas extractor 58 and dissolved gas in the fluid is released through a membrane 60 to the gas detection chamber 62. A pitch/catch optic pair 16, 18 is in optical communication within the gas detection chamber 62 for delivery of a suitable radiation beam from the radiation source 20 to the detector 24. The embodiment illustrated in Fig. 5 could likewise involve multiple transformers and gas extractors 58 used with a detector apparatus similar to the embodiment illustrated in Fig. 2 or a multiplexed beam produced by an apparatus of the type depicted in Figs. 3 and 4. Thus, the embodiment illustrated in Fig. 5 differs from those depicted in Figs. 1-4 only in that instead of providing pitch/catch optic pairs in communication with a headspace, they are provided in communication with a space containing gas separated from a fluid such as the gas detection chamber 62. Fluid is recycled from the gas extractor 58 to the transformer through return conduit 64. One example of a gas extractor 58 which may be suitable for use with the embodiment of Fig. 5 is depicted and described in Walters, U.S. Patent No. 6,391,096, the content of which is expressly incorporated herein by reference in its entirety.

[0026] In certain applications it may be desirable to calculate the amount of dissolved gas in the fluid contained within the tank. For those tanks where the gas is being monitored in the headspace, the precise temperature of the fluid at the point of the gas/fluid interface must be determined. By way of example, referring to Fig. 1, a floating thermocouple or thermopile 66 may be provided in communication with the computer 28. In addition, a pressure transducer 68 may be provided in the headspace and coupled to the computer 28. The concentration of dissolved gas may then be determined by the computer 28 using Henry's law. For embodiments using the gas extractor 58, temperature and pressure transducers 66, 68 may be provided at the point of gas exchange to accurately determine the oil temperature and gas pressure.

[0027] The pitch/catch optics 16, 18 must convert light from the fiber 22 to a collimated beam, direct the beam accurately through the headspace 14 or the gas detection chamber 62, capture the beam on the far side of the headspace and couple the beam to the fiber 26. The choice of optics to accomplish this is determined by the transmission distance, any turbulence within the headspace or detection chamber, effects on the transmitted beam's quality and the core size of the fiber 26. In one embodiment, the catch optic may be a multimode fiber with a relatively large core diameter, for example, 50 microns. In one embodiment, the pitch optic may be a single mode fiber with, for example, a 10 micron core diameter. Fiber coupling on the catch optic has several advantages. In particular, only radiation in the same location as the radiation source and traveling in the same direction is focused into the fiber 70. This feature drastically reduces the amount of background light that is sent. The pitch and catch optics 16, 18 may be custom-designed and aberration- corrected for wavelengths from 660 nm to 1650 nm so that multiple radiation beams can be efficiently transmitted and received at the same time.

[0028] The various embodiments may be practiced using tunable diode laser absorption spectroscopy ("TDLAS"). As described above, TDLAS is typically performed by the transmission of laser light through a target environment, followed by the detection of the absorption of the laser light at specific wavelengths due to the presence of target gases. Spectral analysis of detected light allows identification of the type and quantity of gas along the laser path. The details of direct absorption spectroscopy are discussed in Teichert, Fernholz and Ebert, "Simultaneous in situ Measurements of CO, H₂O and Gas Temperature in a Full-Sized Coal-Fired Power Plant by Near-Inferred Diode Lasers," Applied Optics, (42(12):2043, 20 April 2003), which reference is incorporated herein in its entirety. Detection of multiple gases may require the performance of TDLAS with multiple widely spaced frequency of laser light. For example, the absorption lines of typical transformer fault gases are as follows: [0029] Table 1.

[0030] In addition to the detection of specified gases, TDLAS can be used for accurate measurement of temperature of a target gas environment. For example, this can be done by detecting the presence of water at known wavelengths. Temperature may also be monitored using other spacing at known wavelengths including O₂ and CH₄. [0031] In some instances it may be desirable to configure the system to multi-pass the beam through the headspace or detection volume, particularly where the fault gasses are at low concentrations.

[0032] The various embodiments discussed herein may be deployed using relatively inexpensive commonly available optical components designed for use in the telecommunications industry. The telecommunications components serve well to couple the pitch and catch sides of the system. Multiplexers and demultiplexers capable of handling widely dispersed wavelengths are available from ZoIo Technologies, Inc. of Boulder, Colorado. Such multiplexers and demultiplexers are described in greater detail in U.S. Patent Publication No. US-2006-0133714, which is incorporated herein in its entirety. [0033] While the invention has been particularly shown and described with reference to a number of embodiments, it would be understood by those skilled in the art that changes in the form and details may be made to the various embodiments disclosed herein without departing from the spirit and scope of the invention and that the various embodiments disclosed herein are not intended to act as limitations on the scope of the claims.

Claims

CLAIMSWhat is claimed is:

1. A method of detecting the presence of a gas dissolved in a fluid residing in a tank, the method comprising: separating a portion of gas dissolved in the fluid from the fluid into a space; transmitting radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas to be detected into the space containing the portion of gas separated from the fluid; and detecting absorption of the radiation transmitted into the space.

2. The method of claim 1 further comprising providing a pitch optic and a catch optic pair in optical communication with the space, the radiation being transmitted into the space between the pitch optic and the catch optic.

3. The method of claim 2 further comprising providing a laser emitting the radiation, the laser being optically coupled to the pitch optic.

4. The method of claim 3 further comprising providing a plurality of pitch optic and catch optic pairs, each pair being in optical communication with a distinct space containing a portion of gas separated from a fluid, and providing means for routing the radiation from the laser to each pitch optic.

5. The method of claim 4 further comprising providing a detector for detecting absorption of radiation transmitted into the distinct spaces, the detector being in optical communication with each catch optic, and routing radiation received by each catch optic to the detector.

6. The method of claim 5 further comprising switching the radiation reaching the detector between each catch optic and the detector.

7. The method of claim 1 further comprising transmitting a multiplexed radiation beam comprising at least two wavelengths of radiation corresponding to a predetermined spectral absorption line of a distinct gas.

8. The method of claim 7 further comprising: providing a pitch optic and a catch optic pair in optical communication with the space, the radiation being transmitted into the space between the pitch optic and the catch optic; demultiplexing the radiation received by the catch optic; and providing a detector for detecting each wavelength of radiation comprising the multiplexed beam.

9. The method of claim 1 wherein the space is a headspace in communication with the fluid.

10. The method of claim 4 further comprising: providing at least two lasers each emitting radiation at a distinct wavelength corresponding to that of a predetermined spectral absorption line of a distinct gas; multiplexing the at least two radiation wavelengths; optically coupling the multiplexed radiation to each pitch optic; providing a demultiplexer for demultiplexing the multiplexed radiation received by each catch optic; providing a detector for detecting each wavelength of radiation comprising the multiplexed radiation coupled to the demultiplexer; and switching the multiplexed radiation between each catch optic and the demultiplexer.

11. A method of detecting the presence of a gas associated with a fluid residing in a tank, the tank having a headspace in communication with the fluid, the method comprising: transmitting radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas into the headspace; and detecting absorption of the radiation transmitted into the headspace.

12. The method of claim 11 further comprising providing a pitch optic and a catch optic pair in optical communication with the headspace, the radiation being transmitted into the headspace between the pitch optic and the catch optic.

13. The method of claim 12 further comprising providing a laser emitting the radiation, the laser being optically coupled to the pitch optic.

14. The method of claim 13 further comprising providing a plurality of pitch optic and catch optic pairs, each pair being in optical communication with a distinct tank headspace, and providing means for routing the radiation from the laser to each pitch optic.

15. The method of claim 14 further comprising providing a detector for detecting absorption of radiation transmitted into the distinct tank headspaces, the detector being in optical communication with each catch optic, and routing radiation received by each catch optic to the detector.

16. The method of claim 15 further comprising switching the radiation reaching the detector between each catch optic and the detector.

17. The method of claim 11 further comprising transmitting a multiplexed radiation beam comprising at least two wavelengths of radiation each corresponding to a predetermined spectral absorption line of a distinct gas.

18. The method of claim 17 further comprising: providing a pitch optic and a catch optic pair in optical communication with the headspace, the radiation being transmitted into the headspace between the pitch optic and the catch optic; demultiplexing the radiation received by the catch optic; and providing a detector for detecting each wavelength of radiation comprising the multiplexed beam.

19. The method of claim 11 wherein the tank comprises a transformer.

20. The method of claim 11 further comprising determining the temperature of the fluid, determining the pressure of the gas, determining the concentration of the gas in the headspace and calculating the concentration of the dissolved gas in the fluid.

21 A system for detecting the presence of a gas associated with a fluid residing in a plurality of tanks, each tank having a headspace in communication with the fluid, the system comprising: a laser producing radiation at a wavelength corresponding to that of a predetermined spectral absorption line of the gas; a plurality of pitch optic and catch optic pairs, each pitch optic and catch optic pair being configured to transmit radiation therebetween through a headspace of a distinct tank; means for routing the radiation from the laser to each pitch optic; a detector for detecting absorption of radiation transmitted between the pitch and catch optic pairs; and means for routing radiation between each catch optic and the detector.

22. The system of claim 21 further comprising: at least two lasers producing radiation at a distinct wavelength corresponding to a predetermined spectral absorption line of a distinct gas to be detected; a multiplexer optically coupled to each laser for producing a multiplexed radiation beam; a detector for detecting the absorption of each wavelength of radiation transmitted between the pitch and catch optic pairs; and a demultiplexer optically coupled between each catch optic and each detector for demultiplexing the radiation to each wavelength.