Abstract:
A method of determining when a component exceeds a predetermined stress level that is less than a fatigue failure level for the component comprising providing a braze joint in a high stress area of a target component, the braze joint designed to accommodate stress/strain up to the predetermined threshold level; measuring strain at the braze joint; observing a shift in strain indicative of a failed brazed joint; utilizing information obtained to repair, replace or set a remaining service life for the target component.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to turbomachinery instrumentation and, specifically, to a technique for early detection of stress/strain in the instrumentation over a predetermined threshold level. 
       BACKGROUND OF THE INVENTION 
       [0002]    Sophisticated instrumentation used to monitor operating conditions in the harsh environment of a turbomachine is complex and expensive. Undetected fatigue failure of such instrumentation can not only involve costly shutdowns for repair and/or replacement of the instrumentation, but it can also result in damage to the machine itself. For example, inlet rakes are used to monitor temperature, pressure, velocity and/or other parameters of air entering the turbine inlet and flowing across the rake. They are typically mounted to the inlet sidewall in cantilevered fashion, with the sensor beam or tube extending into the flowpath. Conditions within the flowpath can cause vibrations that may excite the natural or resonance frequency of the beam, causing catastrophic failure of the beam, which may then also result in severe damage to the downstream turbine blades or other flowpath components. 
         [0003]    There is a need, therefore, to devise a way to monitor and detect a stress/strain level in a component (a turbine inlet rake in the exemplary embodiment) that exceeds a predetermined stress/strain level below that at which the component will fail in fatigue, thus providing the user or operator with a clear indication or early warning that the component is at risk. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In accordance with a first exemplary but nonlimiting embodiment, there is provided a method of determining when a component exceeds a predetermined stress level that is less than a fatigue failure level for the component comprising: providing a braze joint in a high stress area of a target component, the braze joint designed to accommodate stress/strain up to the predetermined threshold level; measuring strain at the braze joint; observing a shift in strain indicative of a failed brazed joint; and utilizing information obtained in steps (b) and (c) to repair, replace or set a remaining service life for the target component. 
         [0005]    In another exemplary but nonlimiting aspect, there is provided a method of determining when an instrumentation beam cantilevered from a casing wall at a structural joint, and subject to vibratory stress, exceeds a predetermined stress level that is less than a fatigue failure level for the instrumentation beam comprising: providing a braze joint in a high stress area of the instrumentation beam proximate the casing wall but forward of the structural joint, the braze joint designed to accommodate stress/strain up to the predetermined threshold level; measuring strain at the braze joint; observing a shift in strain indicative of a failed brazed joint; and utilizing information obtained in steps (b) and (c) to repair, replace or set a remaining service life for the instrumentation beam. 
         [0006]    In still another exemplary but nonlimiting aspect, there is provided an inlet rake for a turbine comprising a hollow beam fitted with plural sensors, and a flange body adapted to secure the hollow beam to a turbine inlet duct; a weld joint between said hollow beam and one end of the flange body located proximate the turbine inlet duct; and a braze joint between the hollow beam and an opposite end of the flange body, forward of said weld joint. 
         [0007]    The invention will now be described in greater detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a front elevation view of a turbine inlet rake; 
           [0009]      FIG. 2  is a side elevation of the rake shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a cross section of the flange body removed from  FIGS. 1 and 2  and taken along the line  3 - 3  of  FIG. 4 ; 
           [0011]      FIG. 4  is a top plan view of the flange body; 
           [0012]      FIGS. 5 and 6  are partial elevations of the rake, showing the locations of the braze joint and strain gauges; and 
           [0013]      FIG. 7  is a plot of microstrains vs. time, showing a strain shift upon failure of the braze joint. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In the exemplary but nonlimiting embodiment shown in  FIGS. 1-6 , there is illustrated an inlet rake  10  of the type typically used to monitor various flow characteristics of air entering, for example, a turbine inlet. The inlet rake is formed as an elongated, tapered hollow body or beam  12  of generally rounded, rectangular cross-sectional shape ( FIG. 4 ), fitted with a plurality of sensors  14  for measuring, air pressure, temperature, velocity and/or other flow parameters of air entering the turbine inlet and flowing across the rake. The inlet rake  10  mounts to a duct or casing wall (indicated at  24  in  FIG. 1 ) at a flange body  16  that includes a cylindrical portion  18  and a radial flange portion  20 . Tubes and/or sensor leads, generally indicated at  22 , are fixed to the back side of the flange portion  20  which, upon installation, lies on the other side of the duct or casing wall, outside the flow path. 
         [0015]    In use, the hollow body or beam  12  is essentially cantilevered from the casing wall and projecting into the air inlet flowpath, and is thus subject to high vibratory stresses, with peak stresses occurring at the flange portion  20 , where the latter is bolted to the casing wall, and where the hollow body or beam  12  is welded to the flange portion (indicated at W in  FIGS. 3 and 4 . If the natural frequency of the hollow beam  12  is excited into resonance, the inlet rake  10  can fail at the welded joint and a separated beam  12  can also cause damage to flow path components downstream of the turbine inlet. 
         [0016]    In an exemplary but nonlimiting embodiment, this invention provides a technique or methodology for detecting stress/strain levels in the inlet rake  10  that exceed a predetermined threshold stress level that is nevertheless below the Goodman limit of the inlet rake. The Goodman limit is material specific, and refers to the level of alternating strain the particular material can withstand for 1 million cycles before high cycle fatigue failure occurs. 
         [0017]    In order to implement the technique described herein, a braze joint is located at  26  (see  FIGS. 1-3 ,  5  and  6 ) in an area of high stress, sized for a particular material limit. In the example embodiment, the braze joint is added where the hollow beam  12  is joined with the flange body  16 , rearward of the main structural joint where the beam  12  is welded to the flange portion  20  that, in turn, is joined to the casing or duct wall  24  by bolts or other means. Since the length of the rake determines its natural frequency, the braze joint  26  effectively, albeit artificially, sets a new length for the rake in terms of establishing the natural frequency of the rake. In the exemplary embodiment, the braze joint  26  is located approximately 2.6 inches ahead of the nearest edge of the flange portion  20 , as best appreciated from  FIG. 3 . It will be understood, however, that the exact distance is not critical and may be varied. It is important that the braze joint  26  be located closer to the free edge of the beam  12  than the weld joint. In order to monitor the strain at the braze joint, at strain gauges  28 ,  30  are located along the braze joint  26  with approximately a 90° spacing therebetween as best seen in  FIGS. 5 and 6 . 
         [0018]    The braze joint  26  is sized to handle up to a desired predetermined stress level, preferably below the Goodman limit for the particular braze material. In the event higher than expected stress/strain levels are experienced by the braze joint, the braze joint begins to fail, causing a small crack that can be visually noted. In addition, a shift in the strain resulting from the crack can be monitored and plotted. For example,  FIG. 7  shows a strain curve where strain in microstrains is plotted against time in seconds. The plot represents a beam at resonance frequency with the braze joint implemented. The curve shows a significant downward shift in strain at about 6-7 thousand seconds, indicating that the braze joint has cracked, the beam has dropped out of resonance, and that the predetermined threshold stress/strain level has been exceeded. 
         [0019]    The failure of the braze joint  26  resets the length and thus the natural frequency of the beam  12 . By bringing the beam  12  (and rake  10 ) out of resonance and by resetting the natural frequency, the rake is returned to an at least temporarily safe condition. The rake will then be inspected and depending on where the threshold stress/strain level has been set, repaired or replaced. It may also be possible to permit the rake to remain in use, noting the remaining cycles before the Goodman material limit of the beam is reached. Thus, early failure of the braze joint enables the operator replace, repair or set a remaining service life for the component. In other words, the disclosed detection method allows the operator to monitor the “health” of the inlet rake in a safe and reliable manner, utilizing the braze joint in the manner of a “fusible link”. It should be noted however, that once the braze joint has failed, the rake is subject to resonant frequency failure if higher than expected strain is experienced. 
         [0020]    While the invention has been described in connection with a turbine inlet rake, it will be appreciated that there are many other applications for the described stress/strain detection method including other instrumentation components, or even turbine components that are subject to severe vibration. 
         [0021]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.