Optical steam quality measurement system and method

The United States Government has certain rights in this invention pursuant to Contract No. DE-AC07-99ID13727, and Contract No. DE-AC07-05ID14517 between the United States Department of Energy and Battelle Energy Alliance, LLC.

1. Field of the Invention

The present invention relates generally to measuring the quality, or wetness, of steam. More particularly, the present invention relates to an optical system and method for determining the quality of a flow of steam by using laser diode emitter and photo diode detector devices to measure radiant energy absorption due to liquid water in the flow of steam.

2. State of the Art

The efficiency and lifetime of components in steam systems are affected by the quality, or wetness, of the steam. Steam quality is defined as the percentage of the total fluid mass of steam that is in the vapor phase. “Dry” steam or steam of 100% quality consists solely of water in the vapor phase. Steam quality of less than 100% indicates the steam contains a portion of water in the liquid phase corresponding to the reduction in quality percentage. Such steam is referred to as “wet” steam. The portion of water in the liquid phase is often due to condensation caused by temperature drops or other energy losses at points throughout a steam system. Excessive steam washing to remove particulates or reduce concentrations of chloride or other impurities to levels that are non-damaging to system components can also introduce excess moisture. The resulting wet steam carries water droplets that can corrode system components such as turbine blades or piping, eventually resulting in failure thereof. In addition, entrained water droplets often contain solids that can deposit on turbine surfaces, adversely affecting the flow stream and turbine efficiency as well as potentially causing imbalance and necessitating cleaning operations. Wet steam also contains less usable energy than dry steam, translating into a loss in mechanical performance. Control of steam quality during operation can be used to anticipate and correct these problems. Accordingly, online or “real time” monitoring and feedback of steam quality is important for diagnostic purposes to increase operating efficiency and to reduce equipment maintenance and replacement costs.

Historically, calorimeters have been used to measure steam quality, but difficulties with sensitivity, accuracy and range limit their suitability for use in many applications. Moreover, calorimeters in most cases require slip stream installations and the insertion of multi-port probes which may disrupt steam flow and result in measurement errors due to nonrepresentative sampling. Calorimeters also exhibit slow response times, making them unsuitable for continuous, real time monitoring. Such continuous monitoring is important if quality measurement of steam is to be used for diagnostic and failure detection purposes. Other attempts to measure steam quality while avoiding the drawbacks of calorimetry have been developed using optical methods. By observing changes in light or other radiant energy passed through a flow of steam, and comparing the changes with known effects of water droplets on the various wavelengths transmitted, a determination of the percentage of liquid water contained in the steam can be made.

In one optical technique, steam quality is measured by comparing the intensity (I) of light passed though a flow of wet steam containing water droplets in suspension to the intensity (I o ) of light passed through a flow of dry steam which contains no water droplets. In the wet steam, the light is scattered by the droplets based on the ratio of droplet diameter and the wavelength of the light, thereby reducing its intensity. Under the Mie scattering theory derived from Maxwell's equations, the ratio of intensities (I/I o ) can be used to deduce droplet size and distribution, and thereby the amount of liquid water in a given flow of steam. U.S. Pat. No. 6,128,079 to McClosky et al. and U.S. Pat. No. 4,137,462 to Wyler disclose typical examples of this scattering measurement technique, wherein a probe is inserted into a passageway for transmitting a beam of light across a flow of steam and reflecting it back to a photo detector. The invasive nature of this approach raises concerns with respect to the effect of a probe on the flow of steam, which may reduce the accuracy of the scattering measurements. Moreover, scattering of light outside of the measurement area may be reflected back from other surfaces within the passageway, generating noise which must be compensated for by a signal offset.

Another optical technique that has achieved some success is to measure a drop in radiant energy intensity due to absorption by water droplets, rather than scattering. U.S. Pat. No. 4,862,001 to Dowling et al., the disclosure of which is incorporated herein by reference, discloses a steam quality monitor using this technique. Two windows are included on opposite sides of a steam pipe. A source located outside of the steam pipe passes a beam of IR energy through the windows to a detector located on the opposite side of the steam pipe. The loss of IR energy due to absorption by water droplets in the steam is then determined by comparing the energy of the IR beam passed through the steam to one sent from the source to a detector without passing through steam. Using Beer's law, which relates the amount of energy absorbed to the mass of water droplets in a given volume of steam, the steam quality can be calculated. While well known in the prior art, this technique has not been widely used due to the cost and complexity of the instruments required. Large-scale, high-temperature broadband IR sources are required to provide sufficient transmission as the IR energy rarely has the intensity and beam quality necessary for accurate measurements. Broadband IR sources are not collimated and as a result do not present a consistent beam cross section through the measured volume of steam. Since the IR beam may spread quickly, its intensity cross section during transmission across the measured volume changes as a function of the path length. Accordingly, such systems often lack the sensitivity and stability to perform measurements under operating conditions of interest.

What is needed, therefore, is an optical system for measuring steam quality that is sensitive to slight variations in steam quality during real time monitoring while being of a compact and noninvasive nature.

An optical measurement system and method is presented that offers precision on-line monitoring of steam quality which is more versatile and accurate than the above-described prior art techniques. A closed vessel having a chamber or passageway, such as a steam pipe, is provided with first and second windows aligned across the path of a flow or other volume of steam to be contained therein. In operation, at least one wavelength and, in some embodiments, multiple wavelengths of radiant energy are passed through a volume of steam from an emitter associated with the first window to a detector associated with the opposing second window. If multiple wavelengths are employed, by comparing the amount of each wavelength of radiant energy absorbed by the steam, a highly accurate measurement of the steam quality may be determined on a continuous basis in real-time.

The present invention further improves operation by utilizing optical semiconductor-based emitter and detector technology that is robust, light-weight, portable and operates at ambient (room) temperatures. In one aspect of the present invention, for instance, laser diodes are implemented as the source of radiant energy in the emitter. These devices provide high intensity and energy stability, thereby overcoming the problems encountered in prior art optical systems with respect to device size and beam quality. Likewise, in another aspect of the present invention photo diodes are used in the detector to receive the radiant energy, thus providing a compact and reliable monitoring mechanism.

In an exemplary embodiment of the present invention, the emitter comprises three laser diodes which are each coupled to a fiber-optic cable for transmitting specific wavelengths of radiant energy through the volume of steam. The first laser diode generates a radiant energy signal at a wavelength that is significantly absorbed when transmitted through water in the vapor phase. The second laser diode generates a radiant energy signal at a wavelength that is significantly absorbed when transmitted through water in the liquid phase. The third laser diode generates a radiant energy signal at a wavelength that is minimally influenced by water in either state. The detector comprises a number of photo diodes which respectively receive the radiant energy signals after passing through the volume of steam and measure energy losses due to absorption. By comparing the signal losses at the wavelengths generated with the first and second laser diodes, the relative amounts of absorption due to the presence of liquid water and water vapor may be calculated to determine steam quality. The losses at the energy wavelength generated by the third laser diode may be used as a reference to improve the absorption measurements indicated by energy losses in the first and second wavelength signals by providing an indication of signal noise that may be present due to scattering of the radiant energy.

In a further embodiment of the present invention, the radiant energy signals generated by the three laser diodes are combined for transmission into one fiber-optic cable using multiplexing. In this manner, the signals are transmitted transversely across the volume of steam as a single beam of radiant energy. By doing so, the signals each pass through the same volume of the volume of steam, providing for a more precise correlation between the losses for each wavelength. Under one combination approach, the signals are modulated with known frequency domain multiplexing techniques such that a single photo diode is used to receive the beam of radiant energy and measure energy loss for all three signal wavelengths. Under an alternative approach, the signals are combined using known wavelength division multiplexing techniques wherein the beam of radiant energy is received by three separate photo diodes, each of which measures energy loss for a specific signal wavelength.

In yet a further embodiment of the present invention, specialized optical elements are used to guide the beam of radiant energy across the passageway. The emitter includes a collimator with a graded index lens to produce a large diameter beam of radiant energy to pass through the volume of steam. The enlarged cross-sectional area of the beam of radiant energy thereby encompasses a larger sample of steam, enabling more reliable measurements and reducing signal noise due to scattering effects. The detector includes a receiver with a graded index lens for collecting the enlarged beam of radiant energy and passing it to the photo diodes. Use of an enlarged laser beam may, in many instances, remove the need for a third wavelength of radiant energy due to a reduced significance of scattering effects and, in some instances requiring less sensitivity, may enable steam quality measurement using a single wavelength.

Other and further features and advantages will be apparent from the following detailed description of the present invention taken in conjunction with the accompanying drawings. It should be understood that the following is provided for illustrative and exemplary purposes only, and that numerous combinations and modifications of the various embodiments are possible.

In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:

FIG. 1 is a schematic view of an optical measurement system according to the present invention.

FIG. 2 is a schematic view showing the components of an emitter suitable for use with the optical measurement system depicted in FIG. 1 .

FIG. 3 is a view showing the components of a detector embodiment suitable for use with the optical measurement system depicted in FIG. 1 .

FIG. 4 is a view showing the components of an alternative detector embodiment suitable for use with the optical measurement system depicted in FIG. 1 .

Referring in general to the following description and accompanying drawings, various aspects of the present invention are illustrated to show its structure and method of operation. Common elements of the illustrated embodiments are designated with like numerals. It should be understood the figures presented are not meant to be illustrative of actual views of any particular portion of the actual optical measurement system structure, but are merely idealized representations which are employed to more clearly and fully depict the present invention.

FIG. 1 is a schematic view of the optical measurement system 2 according to the present invention being situated to measure the quality of a flow of steam 4 moving through a pipe 6 . The application of the present invention to pipe 6 is for exemplary purposes only, as the components of optical measurement system 2 may be configured to monitor steam quality within any chamber or passageway of a closed vessel which holds steam. To allow passage of radiant energy transversely across the flow of steam 4 , a first window 8 and a second window 10 are provided on opposite sides of pipe 6. Windows 8 and 10 may be formed of sapphire or any other material which is suitably transmissive of radiant energy at the desired wavelengths and is capable of withstanding the temperature and pressure exhibited by steam 4 . Sapphire is currently preferred due to its resistance to degradation and stress in high pressure environments, enabling the use of relatively thin windows. However, alternative materials such as diamond, for enhanced long-term stability, may be used as well. It is also contemplated that the radiant energy may be of a type or wavelength that may be passed directly through the material of pipe 6 , in which case windows will not be required. Of course, a pipe segment may be formed of a suitably transmissive material in lieu of using windows.

In general terms, optical measurement system 2 comprises an emitter 12 associated with first window 8 and a detector 14 associated with the opposing second window 10 . Optical measurement system 2 further comprises a power source 16 for supplying power to various transmission components of emitter 12 and detector 14 , as well as a controller 18 for the purposes of managing radiant energy transmission and data logging. Controller 18 may comprise, for instance, one or more microprocessors having associated memory and programmed to direct the operation of power source 16 and receive and interpret signal data conveyed from detector 14 . As seen if FIG. 1 , when driven by power source 16 , emitter 12 produces a beam of radiant energy 20 which is directed through first window 8 in pipe 6 and passes across the flow of steam 4 . The beam of radiant energy 20 then exits second window 10 in pipe 6 , and is collected by detector 14 . Detector 14 generates electrical signals 21 in relation to the intensity of the beam of radiant energy 20 , which electrical signals 21 are conveyed to controller 18 . In this manner, controller 18 may calculate the loss of intensity of the beam of radiant energy 20 due to absorption while passing through the flow of steam 4 , and thereby determine steam quality by comparing initial radiant energy beam intensity, or power input corresponding to the initial radiant energy beam intensity (which may be simpler when efficiency of power conversion is known or determinable) to the intensity of the corresponding beam after passage through the steam. As used herein, any reference to employing initial radiant energy beam intensity encompasses using power input to the laser diode or other radiant energy source.

FIG. 2 is a schematic view of the operation and function of the components of emitter 12 . A key feature of the present invention is the inclusion of three laser diodes 22 , 24 and 26 , each of which generate an independent radiant energy signal of specific wavelength for subsequent transmission through the flow of steam 4 . The first laser diode 22 is configured to generate a radiant energy signal 23 at a wavelength sensitive to absorption by water in the vapor phase. The second laser diode 24 is configured to generate a radiant energy signal 25 at a wavelength sensitive to absorption by water in the liquid phase. The third laser diode 26 generates a radiant energy signal 27 at a wavelength that will pass through water in either the liquid or vapor phases without significant absorption. Example wavelengths exhibiting the desired absorption characteristics are in the range of 980 nm for the first laser diode 22 , 1550 nm for the second laser diode 24 , and 1060 nm for the third laser diode 26 ; however, the use of other wavelengths is possible. By using multiple wavelengths having the described varying absorptive properties, the separate effects of the liquid and vapor components of steam 4 may be measured and correlated. Employing a third wavelength also enables compensation for scattering effects, which may affect the accuracy of absorption readings. Accordingly, the present invention provides more accurate and sensitive steam quality measurements than those offered by prior art optical systems.

Power for signal generation is provided by conductive lines 28 communicating with power source 16 . Laser diodes 22 , 24 and 26 are connected to fiber-optic cables 30 , 32 and 34 , respectively, for routing of the independent radiant energy signals 23 , 25 and 27 . Using fiber optics allows laser diodes 22 , 24 and 26 to be remotely located from a monitoring site which may be inhospitable in terms of environment or convenience of access. The use of laser diodes in the present invention is therefore desirable due in part to their ability to transmit radiant energy over long pathways without significant divergence of the radiant energy signals. It is feasible, however, that other devices for generating radiant energy at multiple wavelengths may be incorporated for use with the present invention. For example, conventional light-emitting diodes or non-diode type laser or focused conventional light sources may be used in place of laser diodes 22 , 24 and 26 .

Fiber-optic cables 30 , 32 and 34 terminate at a fiber-optic combiner 36 which merges radiant energy signals 23 , 25 and 27 into one fiber-optic cable 38 to form the single beam of radiant energy 20 . Combination of the multiple wavelengths of radiant energy generated by laser diodes 22 , 24 and 26 is accomplished by multiplexing their signals. In one exemplary embodiment, the signal combination may be carried out using frequency domain multiplexing. Under this method, laser diodes 22 , 24 and 26 are separately modulated by power source 16 such that the beam of radiant energy 20 contains wavelengths of the radiant energy signals 23 , 25 and 27 at different frequencies. In another exemplary embodiment, the signal combination is carried out using wavelength division multiplexing. With wavelength division multiplexing laser diodes 22 , 24 and 26 are not separately modulated, but rather are operated concurrently, and the resulting beam of radiant energy 20 carries all three wavelengths of radiant energy signals 23 , 25 and 27 at a common frequency. The type of multiplexing determines the manner in which detector 14 collects the beam of radiant energy 20 , as described in further detail below.

Referring again to FIG. 2 , fiber-optic cable 38 terminates in a collimator 40 positioned outside of the first window 8 in pipe 6 for aligning and transmitting the beam of radiant energy 20 across the flow of steam 4 . Collimator 40 is designed to include a graded index lens 42 which expands the beam of radiant energy 20 carried by fiber-optic cable 38 into a relatively large diameter 44 . Graded index or “GRIN” lenses rely on differential refractive indexes of material rather than shape for their optical properties, and are known for use with optical communications because of this feature. By expanding the beam of radiant energy 20 , it will be passed through a larger cross-section of steam 4 as it is transmitted across pipe 6 . This provides a larger data sample and reduces the noise contribution from scattering effects, providing a more accurate measurement. The diameter 44 of the beam of radiant energy 20 might be expanded up to a range of about 6 mm to 80 mm, for example, whereas prior art optical systems typically have beam diameters of about 1 mm. The maximum allowable beam diameter will, of course, be limited by the size of the passageway or other closed vessel where the measurement is to be made. If the diameter of pipe 2 is smaller than the upper limits of the above size range, then a correspondingly smaller diameter beam would be employed in order to pass through the windows. As noted previously, the use of an enlarged diameter laser beam of radiant energy 20 may enable accurate steam quality measurement without the use of a third wavelength due to reduction in significance of scattering effects on the larger beam. Still further, an enlarged diameter laser beam may enable steam quality measurement suitable for some applications using a single wavelength.

Returning to FIG. 1 , after the beam of radiant energy 20 passes through the flow of steam 4 within pipe 2 it is collected by detector 14 . Detector 14 utilizes a receiver 46 with a graded index lens 48 ( FIGS. 3 and 4 ) positioned outside the second window 10 for collecting the beam of radiant energy 20 . The use of graded index lenses 42 and 48 for focusing and transmission of the beam of radiant energy 20 across pipe 6 and receipt thereof reduces the sensitivity to positioning and alignment exhibited by other lens types, making optical measurement system 2 easier to install and maintain. Graded index lenses of the type and size described above are commercially available from vendors such as Lightpath Technologies of Albuquerque, N. Mex.

FIG. 3 shows a view of the components of a detector 14 ′ used in an exemplary embodiment of the present invention wherein frequency domain multiplexing has been employed ( FIG. 1 ) to combine the radiant energy signals 23 , 25 and 27 of laser diodes 22 , 24 and 26 into the beam of radiant energy 20 . As described above, detector 14 ′ includes a receiver 46 with graded index lens 48 for collecting the beam of radiant energy 20 . Graded index lens 48 focuses the beam of radiant energy 20 into a fiber-optic cable 50 connected to receiver 46 for routing. Fiber-optic cable 50 terminates at a photo diode 52 that generates electrical signals 21 corresponding to radiant energy intensity, and conveys them to controller 18 ( FIG. 1 ). Based on the modulation timing of laser diodes 22 , 24 and 26 by power source 16 , controller 18 then separates electrical signals 21 to provide measurement data in the form of energy losses for the radiant energy signals 23 , 25 and 27 and each of the corresponding wavelengths.

FIG. 4 shows a view of the components of a detector 14 ″ in another exemplary embodiment of the present invention wherein a wavelength division multiplexing scheme has been used to combine the radiant energy signals 23 , 25 and 27 . Detector 14 ″ includes receiver 46 , graded index lens 48 and fiber-optic cable 50 which perform the same function as in detector 14 ′. In detector 14 ″, however, fiber-optic cable 50 terminates at a wave division demultiplexer 54 for dividing the beam of radiant energy 20 into three separate beams, each beam respectively containing a single wavelength of one of the radiant energy signals 23 , 25 and 27 . Demultiplexing devices of this type are known in the art, and are commonly constructed as fiberoptic splitters containing gratings which separate out the specific wavelengths of radiant energy for passage into separate fibers. Each wavelength of energy signals 23 , 25 and 27 is then directed through one of fiber-optic cables 56 , 58 and 60 to a first photo diode 62 , second photo diode 64 and third photo diode 66 , respectively, each photo diode 62 , 64 and 66 being sensitive to a specific wavelength relating to one of the radiant energy signals 23 , 25 and 27 . In this manner, photo diodes 62 , 64 and 66 generate individual electric signals 21 a , 21 b and 21 c corresponding to the radiant energy intensity for each of the radiant energy signals 23 , 25 and 27 transmitted by laser diodes 22 , 24 and 26 . Accordingly, controller 18 is provided with separate measurement data indicative of energy losses for each of the associated radiant energy wavelengths.

While the detector embodiments have been described in terms of photo diodes, it is possible that photo detectors such as spectrally filtered photo diodes in the form of charge coupled devices (CCD's) or other monochrometer or spectrometer devices could be used as well. It is also possible that further wavelengths of radiant energy might be added into the beam of radiant energy 20 which are sensitive to specific contamination species possibly contained in steam 4 . This would be accomplished, for example, by including additional laser diodes to emit the further wavelengths. Under frequency domain multiplexing, the additional laser diodes would be modulated at supplemental unique frequencies and the modulation timing to would be configured to further separate the frequencies into electrical signals at photo diode 52 to provide measurement data for the contamination species. Under wavelength division multiplexing, wave division demultiplexer 54 would be configured to divide the wavelengths from the additional laser diodes into further separated beams which would then be passed to a corresponding number of additional photo diodes for measurement. With the wavelength division multiplexing scheme, the wave division demultiplexer 54 might be replaced with a conventional fiber optic splitter which does not separate out the wavelengths, but instead sends a portion of beam 20 containing all of the radiant energy wavelengths to photo detector devices formed as a spectrally filtered array.

Under any of the foregoing multiplexing approaches, the measurement data supplied to controller 18 may then be used to determine the amount of energy absorption associated with each wavelength. In the illustrated embodiments, this is accomplished by comparing the intensity of each of radiant energy signals 23 , 25 and 27 (or the equivalent power input) when they are transmitted from laser diodes 22 , 24 and 26 to their intensity when received at photo diode 52 or photo diodes 62 , 64 and 66 . Once the amount of absorption is obtained, the steam quality may be determined. For instance, using known constants for the absorption effect of liquid and vapor water on given wavelengths of radiant energy, controller 18 may be programmed to calculate steam quality based on the relationship of absorption to liquid and vapor concentrations under Beer's law. Alternatively, absorption levels recorded by optical measurement system 2 could be matched to standardized values for steam quality under the same conditions using measurement techniques such as calorimetry.

Because amounts of absorption for both the liquid and vapor components of steam 4 are provided in terms of the separate wavelengths of radiant energy generated by laser diodes 22 and 24 , they may be correlated with one another to produce a more accurate steam quality measurement. The use of laser diodes further provides high intensity and energy stability for beam transmission, and enables more precise selectivity to specific wavelengths. Because laser generated beams do not substantially diverge during transmission, a consistent beam cross section across the measurement path will be obtained which better models the true nature of the steam cross section penetrated thereby. This makes the present invention suitable for monitoring high-quality steam levels of, for example, 98% to 100% steam quality and particularly 99.5% and above where prior art measurement systems may lack the necessary sensitivity. Furthermore, for lower steam quality levels, scattering of radiant energy due to increased liquid water may influence absorption measurements of conventional optical measurement systems. By using the minimally absorbed wavelength of radiant energy generated by laser diode 26 as a reference point, any scattering effects on the absorption measurements may be recognized and compensated for, thereby enhancing the dynamic response of the system of the present invention and extending the lower end of the range of accuracy thereof.

In summary, the above-described embodiments of the present invention provide an optical steam quality measurement system that offers precision on-line monitoring of steam quality by utilizing a multiple wavelength absorption method for measuring the liquid and vapor components of steam. The system is also well suited for placement within a wide range of environments due to the incorporation of compact and reliable optical semiconductor emitter and detector technology.

Although the present invention has been depicted and described with respect to the illustrated embodiments, various additions, deletions and modifications are contemplated without departing from its scope or essential characteristics. For instance, when measuring steam of high quality where there will be minimal scattering of radiant energy, or when large beam diameters are employed such that noise from scattering effects is negligible, it might be desirable to use only one or two wavelengths of radiant energy to measure absorption. This would reduce the number of required components and simplify the system structure, while still providing the benefits exhibited by laser diode radiant energy transmission. Further, while described in terms of monitoring steam quality based on the presence of liquid water, it may also be configured to monitor for different compositions. This would be beneficial in processes like geothermal energy production wherein steam supplied from subterranean sources may be contaminated with gases and vapors which are harmful to steam turbine components or other equipment. By transmitting a wavelength of radiant energy sensitive to absorption by these harmful gases and vapors, steam flow could be monitored for their occurrence to prevent damage. Another application may involve measuring the content of liquid water in gas flows used for manufacturing semiconductor devices, which processes are highly sensitive to moisture contamination and the resulting semiconductor devices and in-process substrates to moisture damage.

The scope of the invention is, therefore, indicated by the appended claims rather than the forgoing described embodiments. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.