Abstract:
A foreign object impact sensor for use with combustion turbines utilizes a passive acoustic waveguide within the turbine to receive acoustic signals from foreign object impact. The magnitude of these signals is displayed and or stored using a lighted display or other suitable indicating device. The information is also recorded and stored, so that a spectrum signature can be obtained from Fourier analysis.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to on-line monitoring of combustion turbines for defects. More specifically, the invention is an apparatus and method for monitoring acoustic signals within a combustion turbine to monitor the passage of foreign particles through the turbine on-line.  
           [0003]    2. Description of the Related Art  
           [0004]    Combustion turbines typically operate at extremely high temperatures, for example, 2500° F. to 2900° F. (1371° C. to 1593° C.). Such high temperatures will cause failure of various components unless they are protected from the heat. These components include the rotating blades of the turbine, and the vanes for directing gas flow within the turbine. A typical combustion turbine will have three to four rows each of blades and vanes, with approximately 50 to 100 blades or vanes per row, and will typically have approximately 500 total blades and vanes to protect. A commonly used material for vanes and blades is nickel-cobalt. These components are usually insulated by a thermal barrier coating to enable their use within high temperature environments. A typical thermal barrier coating is yttria-zirconia.  
           [0005]    Currently, it is necessary to periodically stop the turbine and inspect the components for deterioration of the thermal barrier coating, defects in other coatings, or other defects, for example, formation of cracks. It would be desirable to monitor the condition of these components while the turbine is in use. Avoiding the need to periodically stop the turbine for inspection reduces downtime, increasing the turbine&#39;s efficiency. Similarly, early detection of defects reduces repair costs and outage time, again increasing turbine efficiency. A need exists for monitoring conditions within the turbine that can cause defects, or are a symptom of a defect, such as foreign objects passing through the combustion turbine.  
           [0006]    One proposed system for detecting and locating defects within turbine components involves using a probe on the housing of the turbine to measure the acoustic spectrum of the turbine. This acoustic spectrum is then compared with a reference spectrum, with deviations from this reference spectrum indicating a damaged turbine blade or component. Preferred embodiments of this method include generating an acoustic signal to increase the intensity of the acoustic spectrum within the turbine.  
           [0007]    Another proposed system is for monitoring synchronous blade vibration. The system includes at least three sensors circumferentially arranged around a row of blades. The sensors may be of the eddy current type, microwave, or optical. The sensors detect the arrival time of each blade at the sensor, using the difference between the expected arrival time of the blade and the actual arrival time of the blade to determine the amplitude of the blade vibration. This information has been analyzed using Fourier transforms to determine the vibratory stress on each blade.  
           [0008]    Yet another proposed system for monitoring the condition of a turbine includes mounting an acoustic emission sensor on a surface of an engine component. The sensor will detect the resulting acoustic emissions when particles of debris strike the surface to which it is mounted or other surfaces.  
           [0009]    One proposed method utilizes ultrasonic inspection of rotating machinery while the machinery is in operation. The method uses an ultrasonic transducer to radiate pulses of ultrasonic energy at a frequency substantially equal to a subharmonic of the frequency of the turbine rotation. The transducer will sense reflections of the ultrasonic pulses from the blade, and convert the reflections into an electrical signal. Changes in the reflected signal can indicate a damaged blade.  
           [0010]    Another system for monitoring the intake of foreign objects into an engine includes sensors positioned near the engine intake and exhaust duct for detecting electrostatic charges induced in the sensors by passing foreign bodies. Each sensor includes a plurality of sensor elements, with each sensor element having an insulating layer of epoxy resin and the charge-collecting layer of silver-loaded epoxy resin.  
           [0011]    Accordingly, there is a need for an apparatus and method for detecting the passage of foreign debris through a combustion turbine, measuring the frequency and intensity of the resulting acoustic signals and displaying such information on line, and recording this information, thereby providing an indication of when a turbine needs to be shut down for maintenance.  
         SUMMARY OF THE INVENTION  
         [0012]    The invention is a system for monitoring the frequency and severity of impacts from foreign debris upon various components within a combustion turbine during operation of the turbine. The system relies on the detection of acoustic signals generated by the impact of foreign debris upon various components of the turbine.  
           [0013]    An acoustic sensor, such as an acoustic waveguide, is bonded to a suitable location on the combustion turbine. This location may be either inside or outside of the gas turbine, with one example location being adjacent to one of the vanes within the turbine. The other end of the waveguide is bonded to a transducer, for example, a bender transducer, for converting the acoustic signal into an electric signal.  
           [0014]    The electrical signal may then be transmitted to a radio frequency step-up transformer for ensuring that even the smallest voltages received are increased to at least approximately 1.5 volts. Lastly, the electrical signal is transmitted to a capacitive voltage divider, transmitting the voltage to one or more light-emitting diodes (LED), depending upon the voltage received. Smaller voltages will light only one LED, whereas larger voltages will light an increasing number of LEDs. For example, a voltage between 1.5 and 2.5 volts may light a green LED, a voltage between 2.5 and 4.5 volts may light a green plus a yellow LED, and a voltage exceeding 4.5 volts may light a green, yellow and red LED.  
           [0015]    The received signal may also be recorded for further study using a data logger or a storage oscilliscope. For example, a video camera may record the activation of the LEDs, recording the information on a videocassette, and displaying it on a video screen.  
           [0016]    Alternatively, instead of a narrow band [˜3 kHz] bender transducer, a wide band [˜0.1 to 1 Mhz] transducer may be bonded to the AWG and the acoustic pulse from FOD measured and stored. Once the information is stored, the spectrum signature may be obtained from Fourier analysis. The spectrum signatures from the impact of various objects, such as ceramic chips and metal chips, will be different, allowing for their identification. Certain spectrum signatures, for example, those indicating a chip from a turbine blade, will indicate that it is desirable to shut down the combustion turbine for maintenance.  
           [0017]    A sensing system of the present invention can collect and record information about the number and magnitude of foreign object impact within the combustion turbine, and transmit this information over long distances, without power supplies or batteries.  
           [0018]    It is therefore an aspect of the present invention to provide a foreign object impact sensor capable of detecting the acoustic signal resulting from a wide variety of foreign objects impacting components within a combustion turbine.  
           [0019]    It is another aspect of the present invention to provide a foreign object impact sensor capable of displaying and recording the frequency and severity of foreign object impacts within the combustion turbine.  
           [0020]    It is a further aspect of the present invention to provide a foreign object impact sensor providing a means to record and store signals from foreign object impacts, so that a spectrum signature can be obtained from Fourier analysis.  
           [0021]    It is another aspect of the present invention to provide a foreign object impact sensor capable of being set up and utilized at minimal cost.  
           [0022]    These and other aspects of the invention will become apparent through the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a cross-sectional view of a combustion turbine for which the present invention will be used.  
         [0024]    [0024]FIG. 2 is a cross-sectional view of the turbine portion of a combustion turbine.  
         [0025]    [0025]FIG. 3 is a schematic diagram illustrating a foreign object impact sensor of the present invention utilized in conjunction with the turbine portion of a combustion turbine.  
         [0026]    [0026]FIG. 4 is a schematic diagram illustrating the electrical circuitry for providing visual indication of the severity of foreign debris impact within the turbine.  
         [0027]    [0027]FIG. 5 is a graph illustrating the expected acoustic signals resulting from various debris impacting various surfaces.  
         [0028]    [0028]FIG. 6 is an example of a spectrum signature produced by a plurality of acoustic waves at various frequencies. 
     
    
       [0029]    Like reference numbers denote like elements throughout the drawings.  
       DETAILED DESCRIPTION  
       [0030]    The preferred embodiments of the invention is a system for on-line monitoring of foreign object impacts within a combustion turbine. The significance and functioning of the present invention are best understood through a description of the environment within a combustion turbine.  
         [0031]    [0031]FIGS. 1 and 2 illustrates a combustion turbine  10 . The combustion turbine  10  includes a compressor  12 , at least one combustor  14 , and a turbine  16 . The turbine  16  includes a plurality of rotating blades  18 , secured to a rotatable central shaft  20 . A plurality of stationary vanes  22  are positioned between the blades  18 , with the vanes  22  being dimensioned and configured to guide air over the blades  18 . The blades  18  and vanes will typically be made from nickel-cobalt, and will typically be coated with a thermal barrier coating, for example yttria-zirconia.  
         [0032]    In use, air is drawn in through the compressor  12 , where it is compressed and driven towards the combustor  14 . The combustor  14  mixes the air with fuel and ignites it, thereby forming a working gas. This working gas will typically be approximately 2500° F. to 2900° F. (1371° C. to 1593° C.). This gas expands through the turbine  16 , being guided across the blades  18  by the vanes  22 . As the gas passes through the turbine  16 , it rotates the blades  18  and shaft  20 , thereby transmitting usable mechanical work through the shaft  20 . The combustion turbine  10  also includes a cooling system  24 , dimensioned and configured to supply a coolant, for example steam or compressed air, to the blades  18  and vanes  22 .  
         [0033]    From the above description, it becomes apparent that the environment within the combustion turbine  10  is particularly harsh, and is therefore likely to cause various forms of component deterioration, such as deterioriation of thermal barrier coatings and coatings for corrosion prevention, or formation of cracks within varying components. Any such deterioration has the potential for causing fragments from various coatings or underlying components to be broken off, striking other surfaces within the combustion turbine as they are blown through the turbine. Additionally, there may also be some potential for foreign objects to be sucked into the compressor portion of the turbine, traveling throughout the turbine and striking various surfaces along the way. Monitoring the acoustic signals generated by these foreign object impacts, thereby determining the frequency and severity of the impacts, permits a determination of when the combustion turbine must be stopped and serviced.  
         [0034]    Referring to FIGS.  2 - 3 , means for receiving acoustic signals within the combustion turbine, which is preferably an acoustic waveguide  28 , is illustrated. The acoustic waveguide  28  may be bonded to any surface inside or outside the combustion turbine  10 , and in the present example is illustrated bonded to a vane  22 . Preferred materials for such an acoustic waveguide are nichrome, platinum, and/or tungsten and suitable alloy of such materials, with the specific material chosen for it&#39;s acoustic as well as temperature resistant properties. A special feature of acoustic waveguides is that they can be bonded to various surfaces by either a point or direct contact or by bonding a few inches of acoustic waveguide length to the surface edges, or other flat surfaces to a vane. Such bonding can be achieved by welding or brazing.  
         [0035]    Referring to FIG. 3, the acoustic waveguide  28  is attached to an acoustic receiver  30 , which is preferably a piezoceramic crystal. One example of such an acoustic receiver is a bender transducer or bender acoustic sensor. Such acoustic receivers convert acoustic signals into electrical signals. Depending on the sensitivity required, the acoustic receiver  30  may include only a single piezoceramic crystal, or alternatively, may include a plurality of piezoceramic crystals each being bonded sequentially to a plurality of acoustic waveguides  28 . If multiple piezoceramic crystals are utilized, each crystal will preferably have a different resonant frequency. For example, crystals with frequencies of 10 kHz, 30 kHz, 60 kHz, 90 kHz, 120 kHz, and 240 kHz. If only a single crystal is used, an example of a suitable resonant frequency for the crystal is 3 kHz. The use of single 3 kHz acoustic bender sensor has the advantage of producing a large terminal voltage from impact pressure signals, whereas the use of multiple bender sensors has the advantage of permitting the identification of spectrum signatures across a wide range of frequencies for each type of particle impact. The acoustic receiver  30  is connected by an electrical lead  32  to a signal display apparatus  34 .  
         [0036]    Referring to FIGS.  3 - 4 , the signal display apparatus  34  and its various components are illustrated. Because it is anticipated that most electrical signals coming from the acoustic receiver  30  will be in the 100 to 300 mV range, displaying this signal may be facilitated by using a step-up radio frequency transformer  36  to increase the signal&#39;s voltage. The step-up transformer  36  may preferably increase this electrical signal to a value exceeding 1.5 volts, which is sufficient to momentarily light a light emitting diode. This electrical signal may then be transmitted through a capacitive voltage dividing network  38  to a lighted display  40 . The lighted display  40  includes a plurality of light-emitting diodes for displaying the frequency and severity of foreign object impacts, with the present example having three light-emitting diodes: a green LED  42 , a yellow LED  44 , and a red LED  46 . The capacitive voltage divider  38  likewise includes a plurality of capacitors connected in series, dimensioned and configured to light an increasing number of the LEDs  42 ,  44 ,  46  with increasing voltage signals. In the present example, a signal greater than 1.5 volts will light the green LED  42 , a signal exceeding 2.5 volts will light the green LED  42  and yellow LED  44 , and a signal exceeding 4.5 volts will light the green LED  42 , yellow LED  44 , and red LED  46 . Referring specifically to FIG. 4, this is accomplished by electrically connecting the green LED  42  with the transformer  36  so that all three capacitors  48 ,  50 ,  52 , are by-passed. Expressed differently, the green LED  42  and capacitive voltage divider  38  are electrically connected in parallel. The yellow LED  44  is electrically connected in series with the capacitor  48 , and in parallel with the capacitors  50  and  52 . Likewise, the red LED  46  is electrically connected in series with the capacitors  48  and  50 , and in parallel with the capacitor  52 .  
         [0037]    Referring back to FIG. 3, the magnitude and severity of the foreign object impacts may additionally be displayed and recorded at a remote display apparatus  54 . A plurality of optical fiber lightguides  56 ,  58 ,  60 , with each optical fiber lightguide  56 ,  58 ,  60  corresponding to one LED  42 ,  44 ,  46 , may transmit the light generated by the LEDs  42 ,  44 ,  46  to be displayed. The remote display apparatus  54  may also include a TV camera  64  for recording the lighted display  62 , a video cassette recorder  66  for maintaining a historical record of the frequency and severity of foreign object impacts, and a video monitor  68  for viewing the frequency and severity of past or present foreign object impacts.  
         [0038]    The acoustic monitoring system of the present invention provides several advantages. Because the initial source of energy for the system is the acoustic wave generated by the foreign object impact within the combustion turbine  10 , the magnitude and severity of these foreign object impacts may be converted to a lighted display without power supplies or batteries. Either the electrical lead  32  or the optical fiber lightguides  56 ,  58 ,  60  may transmit the appropriate signals over long distances, thereby permitting remote monitoring of the combustion turbine  10 . Additionally, the frequency and severity of foreign object impacts may be recorded for further study.  
         [0039]    [0039]FIG. 5 illustrates the resulting electrical signal from utilizing an acoustic waveguide to detect the impact of various particles on various surfaces and then converting the acoustical signal into an electrical signal. One surface utilized in developing the information displayed in FIG. 5 was a graphite-epoxy composite. Various masses of metal spheres and salt grains were dropped onto this surface, and the resulting acoustic waves were detected using an epoxy-fiberglass acoustic waveguide. In this experiment, the signal received from the impact of salt grains having a mass less than 10 −4  grams was in the range 300-1,200 μV range and the signal received from the impact of a 1 gram metal sphere exceeded 100 mV. Switching to nichrome acoustic waveguide bonded to a gas turbine blade, and dropping salt grains on this blade, resulted in electrical signals in the range of 500 μV. Although this experiment differs from the environment within a combustion turbine in that a combustion turbine will produce background noise corresponding to some of the resonant frequencies of some of the piezoceramic crystals utilized as acoustic receivers, it is expected that acoustic signals from foreign object impacting on various surfaces will produce momentary signals higher than the background noise.  
         [0040]    In addition to recording and studying the visual display resulting from the foreign object impact, the frequency and severity of these impacts may be studied using Fourier analysis to obtain the spectrum signature for a plurality of impacts. Use of Fourier analysis requires the use of either a single, wide spectrum piezoceramic crystal, or a plurality of piezoceramic crystals having different resonant frequencies, as the acoustic receiver  30 . Fourier analysis is well known in the art of signal processing. Fourier analysis permits the determination and display of the proportion to which each different frequency within the entire spectrum generated by the foreign object impact contributes to the overall spectrum signature. FIG. 6 is one example of such a spectrum signature for a plurality of waves. In the example of FIG. 6, a low proportion of the overall spectrum signature is produced by high frequency waves, and a high proportion of the overall spectrum signature is produced by low frequency waves. A foreign object impact against a surface within the combustion turbine  10  will produce acoustic waves over a range of frequencies, with the proportional contribution of each frequency to the overall spectrum signature varying based on the type of foreign object and type of surface impacted. Therefore, the specific nature of the foreign object impact can be determined by comparing the spectrum signatures generated with the known spectrum signatures of various foreign object impacts. For example, a foreign object impact generating the signature spectrum of a piece of a turbine blade  18  striking a surface within the combustion turbine  10  may serve as an indication that it is necessary to shut down the combustion turbine  10  for maintenance.  
         [0041]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.