Abstract:
An improved temperature sensor assembly ( 15 ) comprising a temperature sensing probe ( 16 ) having a temperature sensing portion ( 18 ), a terminal portion ( 21 ), and an intermediate portion ( 20 ) between the temperature sensing portion and the terminal portion; a mounting element ( 28 ) in sliding engagement along a first axis (x-x) with the intermediate portion of the temperature sensing probe such that the temperature sensing probe is movable linearly relative to the mounting element in an axial direction; the mounting element configured to attach to an open tip thermowell ( 100 ) such that the temperature sensing portion of the probe is exposed to a process environment ( 104 ); the temperature sensing probe comprising a stop ( 19 ) configured to bear against a seat ( 108 ) in the thermowell; and a spring element ( 24 ) arranged between the stop and the mounting element and configured to bias the stop and the mounting element linearly away from each other in the axial direction.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of gas turbine sensors, and more particularly to an improved exhaust gas temperature sensor assembly. 
     BACKGROUND ART 
     Large frame power generation gas turbines that generate power from combustible fuels are often computer controlled through a series of complex algorithms and inputs from various types of sensors, including temperature sensors. Such sensor inputs play an important role in the efficiency and emissions performance of gas turbines. 
     Conventional large frame ground-based gas turbines use numerous exhaust gas thermocouples for control purposes. The number of sensors can range from sixteen on smaller output engines to as many as thirty on larger engines. Such thermocouples are typically installed in a radiation shield welded to the exhaust plenum of the turbine. The radiation shield design is intended to allow for easy replacement of the protected temperature sensor during maintenance or as a result of premature failure. 
     To avoid the problem of the temperature sensor seizing inside the radiation shield as a result of high temperature operating conditions, typically the sheath of the sensor and the radiation shield are made from different alloys. 
     SUMMARY OF THE INVENTION 
     With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for the purposes of illustration and not by way of limitation, the present invention provides an improved temperature sensor assembly ( 15 ) comprising a temperature sensing probe ( 16 ) having a temperature sensing portion ( 18 ), a terminal portion ( 21 ), and an intermediate portion ( 20 ) between the temperature sensing portion and the terminal portion; a mounting element ( 28 ) in sliding engagement along a first axis (x-x) with the intermediate portion of the temperature sensing probe such that the temperature sensing probe is movable linearly relative to the mounting element in an axial direction; the mounting element configured to attach to an open tip thermowell ( 100 ) such that the temperature sensing portion of the probe is exposed to a process environment ( 104 ); the temperature sensing probe comprising a stop ( 19 ) configured to bear against a seat ( 108 ) in the thermowell; and a spring element ( 24 ) arranged between the stop and the mounting element and configured to bias the stop and the mounting element linearly away from each other in the axial direction. The temperature sensing probe may comprise a thermocouple or a resistance temperature detector. The open tip thermowell may comprise an exhaust gas turbine radiation shield. The mounting element may comprise a generally cylindrical fitting orientated about the first axis and having outwardly-facing threads ( 84 ); the radiation shield may comprise a generally cylindrical insertion opening ( 107 ) having inwardly-facing threads ( 109 ) corresponding to the outwardly-facing threads of the fitting; and the fitting of the mounting element may be configured to rotationally attach to the radiation shield at the insertion opening. The intermediate portion of the probe may comprise a generally cylindrical outer surface orientated about the first axis and having an intermediate outer diameter ( 50 ). The spring element may comprise a helical or coil compression spring orientated about the first axis around the intermediate portion of the probe and having a coil inner diameter greater than the intermediate outer diameter. The stop may comprise a generally cylindrical collar orientated about the first axis and having a collar outer diameter ( 58 ) greater than the spring inner diameter. The assembly may further comprise a spacer tube ( 23 ) orientated about the first axis around the intermediate portion of the probe and having a spacer inner diameter ( 51 ) greater than the intermediate outer diameter and a spacer outer diameter ( 52 ) greater than the coil inner diameter. The spacer tube may be positioned between the collar and the coil spring in the axial direction. The assembly may further comprise a second spacer tube ( 25 ) orientated about the first axis around the intermediate portion of the probe and positioned between the coil spring and the mounting element in the axial direction. The assembly may further comprise a split bushing ( 26 ) orientated about the first axis around the intermediate portion of the probe and positioned between the second spacer tube and the mounting element. The bushing may be removable to unload the spring. The seat may comprise an inwardly-facing frusto-conical surface ( 108 ) orientated about the first axis and the collar may comprise an outwardly-facing frusto-conical surface ( 96 ) orientated about the first axis and configured to bear against the inwardly-facing frusto-conical surface of the seat. The collar may comprise a second outwardly-facing frusto-conical surface ( 94 ) orientated about the first axis and the spacer tube may comprise an inwardly-facing frusto-conical surface ( 72 ) orientated about the first axis and configured to bear against the outwardly-facing frusto-conical surface of the collar. The fitting may comprise a counter bore ( 88 ,  89 ) configured to receiving an end portion ( 78 ,  79 ) of the split bushing. The split bushing may comprise an outwardly-facing frusto-conical surface ( 82 ) orientated about the first axis and the second spacer tube may comprise an inwardly-facing frusto-conical surface ( 74 ) orientated about the first axis and configured to bear against the outwardly-facing frusto-conical surface of the split bushing. 
     The temperature sensing probe may be movable linearly relative to the mounting element in an axial direction between a first assembled position ( FIGS. 3-5 ) and a second installed position ( FIGS. 6-10 ). The intermediate portion of the probe may comprise a support tube ( 22 ) orientated about the first axis and fixed to the terminal portion and the mounting element may be in sliding engagement along the first axis with the support tube of the intermediate portion of the temperature sensing probe. The assembly may further comprise a spacer tube ( 23 ) orientated about the first axis around the intermediate portion of the probe and positioned between the collar and the coil spring in the axial direction; a second spacer tube ( 25 ) orientated about the first axis around the intermediate portion of the probe and positioned between the coil spring and the mounting element in the axial direction; a split bushing ( 26 ) orientated about the first axis around the intermediate portion of the probe and positioned between the second spacer tube and the mounting element; and a distance ( 40 ) between an end face ( 92 ) of the support tube and a corresponding end face ( 78 ) of the bushing may vary in length in the axial direction with movement of the temperature sensing probe between the first assembled position and the second installed position. 
     The stop may be between the temperature sensing portion and the terminal portion. The stop may comprise a tip of the temperature sensing portion. The assembly may further comprise a turbine connected to the open tip thermowell. 
     In another aspect the invention provides a method of measuring the temperature of an exhaust gas comprising the steps of providing an open tip thermowell having a seat; providing a temperature sensor assembly comprising: a temperature sensing probe having a temperature sensing portion, a terminal portion, and an intermediate portion between the temperature sensing portion and the terminal portion; a mounting element in sliding engagement along a first axis with the intermediate portion of the temperature sensing probe such that the temperature sensing probe is movable linearly relative to the mounting element in an axial direction; the mounting element configured to attach to the open tip thermowell such that the temperature sensing portion of the probe is exposed to a process environment; the temperature sensing probe comprising a stop configured to bear against the seat in the open tip thermowell; and a spring element arranged between the stop and the mounting element and configured to bias the stop and the mounting element linearly away from each other in the axial direction; installing the sensor assembly in the open tip thermowell such that the spring element is compressed and the stop bears against the seat in the open tip thermowell; and sensing temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the improved sensor assembly and a conventional radiation shield into which it is installed when in use on a gas turbine. 
         FIG. 2  is an exploded view of the sensor assembly shown in  FIG. 1 . 
         FIG. 3  is a partial cross-sectional side view of the sensor assembly shown in  FIG. 1 . 
         FIG. 4  is an enlarged detailed view of the sensing end of the sensor assembly shown in  FIG. 3 , taken generally within the indicated circle A of  FIG. 3 . 
         FIG. 5  is an enlarged detailed view of the mounting portion of the sensor assembly shown in  FIG. 3 , taken generally within the indicated circle B of  FIG. 3 . 
         FIG. 6  is a side view of the sensor assembly shown in  FIG. 1  installed in the radiation shield shown in  FIG. 1 . 
         FIG. 7  is a partial vertical cross-sectional view of the sensor assembly and radiation shield shown in  FIG. 6 , taken generally on line  7 - 7  of  FIG. 6 . 
         FIG. 8  is an enlarged detailed view of the mounting end of the sensor assembly and radiation shield shown in  FIG. 7 , taken generally within the indicated circle C of  FIG. 7 . 
         FIG. 9  is a horizontal cross-sectional view of the sensor assembly and radiation shield shown in  FIG. 6 , taken generally on line  7 - 7  of  FIG. 6 . 
         FIG. 10  is an enlarged detailed view of the sensing end of the sensor assembly and radiation shield shown in  FIG. 9 , taken generally within the indicated circle D of  FIG. 7 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, debris, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof, (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or of rotation, as appropriate. 
     Referring now to the drawings, and more particularly to  FIG. 1  thereof, this invention provides an improved exhaust gas temperature sensor assembly, the presently preferred embodiment of which is generally indicated at  15 . As shown, sensor assembly  15  is configured for use with conventional radiation shield  100 . Other types of radiation shields may be used as alternatives. 
     As shown, radiation shield  100  includes generally hollow conical body  101 , transversely extending cylindrical exhaust sampling tube  103  and annular flange  102 , by which the radiation shield is welded to an existing large scale gas turbine such that exhaust flow of the turbine passes through cylindrical passage  104  in cylindrical exhaust sampling tube  103 , the temperature of which is monitored by probe  16 . As shown in  FIGS. 1 and 7-10 , radiation shield  100  has a longitudinally extending inner cylindrical bore  105  into which sensor assembly  15  is inserted. At one end inner cylindrical bore  105  has tip opening  106  through which tip  29  of probe  16  projects into exhaust passage  104  of sampling tube  103  and at the other end inner cylindrical bore  105  has insertion opening  107  from which terminal head  21  of probe  16  projects. 
     As shown, sensor assembly  15  generally includes temperature sensing probe  16  and a dampening assembly, generally indicated at  17 . In this embodiment temperature sensing probe  16  comprises a generally cylindrical thermocouple. As shown, thermocouple  16  is orientated about axis x-x and includes sensing portion  18 , having tip  29 , intermediate portion  20 , with sealing collar  19  at one end and support tube  22  at the other end, and terminal head  21 . 
     As shown, terminal head  21  generally includes a ceramic insulated junction box having terminals adapted to connect to cabling by compression fitting. Terminal head  21  has two different sized studs for proper installation and enables a convenient, stress-free orientation of the thermocouple junction box as it relates to the mating cable. Other types of terminal heads may be used as alternatives. 
     Intermediate portion  20  extends from terminal end  21  to sealing collar  19  and generally supports dampening assembly  17 . As shown, intermediate portion  20  is a generally elongated cylindrical member having a generally constant outside diameter along its central length. 
     However, adjacent terminal head  21 , intermediate portion  20  of probe  16  includes a widened support tube  22  having a diameter greater than most of intermediate portion  20 . Support tube  22  is provided to better support the weight of terminal head  21  when cantilevered out the end of mounting fixture  28  and radiation shield  100  when in use. With reference to  FIGS. 5 and 8 , support tube  22  is generally defined by outwardly-facing horizontal cylindrical surface  93  and leftwardly-facing vertical annular surface  92 . Support tube  22  is welded or fixed at its right end face to terminal head  21  of probe  16 . 
     Adjacent sensing portion  18 , intermediate portion  20  includes a widened sealing collar having a diameter greater than most of intermediate portion  20  and chamfered outside edges. With reference to  FIGS. 4 and 10 , collar  19  is generally defined by rightwardly and outwardly-facing frusto-conical surface  94 , outwardly-facing horizontal cylindrical surface  95 , and leftwardly and outwardly-facing frusto-conical surface  96 . Outside diameter  58  of cylindrical surface  95  is about the same as outside diameter  52  of spacer tube  23 . In this embodiment collar  19  is welded to thermocouple  16 . 
     Sensing portion  18  extends beyond sealing collar  19  on the opposite end from terminal head  21 . As shown, sensing portion  18  narrows to tip  29 . Sensing portion  18  is configured to extend into the environment from which temperature readings are desired. 
     As shown in  FIG. 2 , dampening assembly  17  is generally concentric with probe  16  and includes spacer tube  23 , coil spring  24 , spacer tube  25 , split bushing  26 , spring style lock washer  27  and threaded fitting  28 . 
     As shown in  FIGS. 3-5 , spacer tube  23  is generally a specially-configured hollow cylindrical member or sleeve oriented along axis x-x and bounded by rightwardly-facing annual vertical surface  70 , outwardly-facing horizontal cylindrical surface  71 , leftwardly and inwardly-facing frusto-conical surface  72 , and inwardly-facing horizontal cylindrical surface  73 , joined at its right marginal end to the inner marginal end to surface  70 . Inner diameter  51  of spacer  23  is slightly greater than outer diameter  50  of intermediate portion  20  of thermocouple  16 . Thus, spacer  23  is in sliding engagement with the intermediate portion  20  of thermocouple  16  along axis x-x. 
     Coil spring  24  is a high temperature compression spring that is compressed between fitting  28  and sealing collar  19  when assembled. Spring  24  has an inner diameter that is generally the same as inner diameter  51  of spacer  23  and has an outer diameter that is also generally the same as outer diameter  52  of spacer  23 . Thus, like spacer  23 , spring  24  is in sliding engagement with intermediate portion  20  of thermocouple  16 . 
     Spacer tube  25  is generally a specially-configured hollow cylindrical member or sleeve elongated along axis x-x, and bounded by rightwardly and inwardly-facing frusto-conical surface  74 , outwardly-facing horizontal cylindrical surface  75 , leftwardly-facing vertical annular surface  76 , and inwardly-facing horizontal cylindrical surface  77 , joined at its right marginal end to the inner marginal end of surface  74 . Inner diameter  55  of spacer tube  25  is about the same as inner diameter  51  of spacer tube  23  and outer diameter  56  of spacer tube  25  is about the same as outer diameter  52  of spacer tube  23 . Thus, like spacer  23 , spacer tube  25  is in sliding engagement along intermediate portion  20  of thermocouple  16  along axis x-x. 
     As shown in  FIG. 5 , bushing  26  is generally a specially configured hollow cylindrical member or sleeve elongated along axis x-x and split in the longitudinal direction in two halves that may be separated from each other during assembly or disassembly to unload spring  24 . As shown, bushing  26  is bounded by rightwardly-facing vertical annular surface  78 , outwardly-facing horizontal cylindrical surface  79 , outwardly and leftwardly-facing frusto-conical surface  80 , outwardly-facing horizontal cylindrical surface  81 , leftwardly and outwardly-facing frusto-conical surface  82 , and inwardly-facing horizontal cylindrical surface  83 , joined at its right marginal end to the inner marginal end of surface  78 . 
     Fitting  28  is a generally a specially-configured hollow cylindrical member elongated along axis x-x. As shown, fitting  28  includes outwardly-facing horizontal threaded cylindrical surface  84 , leftwardly-facing vertical annular surface  85 , outwardly-facing horizontal cylindrical surface  86 , leftwardly-facing vertical annular surface  87 , inwardly-facing horizontal cylindrical surface  88 , leftwardly-facing vertical annular surface  89 , and inwardly-facing horizontal cylindrical surface  90 . 
     As shown, intermediate portion  20 , including support tube  22 , of thermocouple  16  extends through the inner hollow cylindrical bore in fitting  28  defined by inner cylindrical surface  90 . The opening defined by surface  90  in fitting  28  and the outer cylindrical surface  93  of mounting tube  22  are dimensioned such that intermediate portion  20  of thermocouple  16  is in sliding engagement along axis x-x with fitting  28 . However, outer cylindrical surface  79  of bushing  26  has a diameter that it is too large to fit through the opening defined by surface  90  in fitting  28 . Similarly, terminal head  21  on the opposite side of fitting  28  from bushing  26  is too large to fit through the opening defined by surface  90  in fitting  28 . However, terminal head  21 , and thus probe  16 , can move linearly along axis x-x a given distance  40  relative to fitting  28  between a first assembled position, shown in  FIG. 5 , and a second installed position, shown in  FIG. 8 . Surface  92  of support tube  22  and terminal head  21  act as a hard stop against movement of bushing  26  and fitting  28  away from sealing collar  19  beyond a certain distance. 
     As shown, the right portion of fitting  28  includes a hexagonal shaped nut portion  30  adapted to be engaged with a wrench or other suitable tightening tool. Nut portion  30  is also sized so that it is too large to fit through insertion opening  107  in radiation shield  100 . 
     Spring  24  is operatively compressed between fitting  28  and sealing collar  19 . In particular, the left edge of spring  24  bears against right annular surface  70  of spacer tube  23 , and the left frusto-conical surface  72  of spacer tube  23  in turn bears against the right frusto-conical surface  94  of sealing collar  19  of probe  16 . The right edge of spring  24  bears against left annular surface  76  of spacer tube  25 , and the right frusto-conical surface  74  of spacer tube  25  in turn bears against left frusto-conical surface  82  of split bushing  26 , and a portion of the right annular surface  78  of bushing  26  in turn bears against left annular surface  89  of fitting  28 . Thus, spring  24  ultimately acts between fitting  28  and sealing collar  19 , biasing fitting  28  away from sealing collar  19  in the axial direction. 
     When assembly  15  is assembled, but not yet installed within radiation shield  100 , as shown in  FIG. 5 , spring  24  is slightly compressed due to left annular surface  92  of support tube  22  and the left face of terminal  21  acting as a hard stop to bushing  26  and fixture  28  sliding further along probe  16  away from collar  19 . Thus, the relative lengths of the elements of assembly  15  and support tube  22  are such that no gap is provided between left annular surface  92  of support tube  22  and right annular surface  78  of bushing  26  when assembled but not installed. Because surface  92  acts as a hard stop to movement of probe  16  relative to fixture  28  beyond this assembled position, spring  24  is held in compression. One advantage of this arrangement is that a more constant force may be applied over the operating range of linear movement of probe  16  relative to fixture  28  between the assembled position shown in  FIG. 5  and the installed position shown in  FIG. 8 . However, it is contemplated that the spring force to displacement characteristics of spring  24  and/or the relative lengths of the elements could be varied so that spring  24  is not compressed at all in the assembled position or so that the amount of such compression is adjusted as desired. 
     As shown in  FIGS. 7 and 8 , insertion opening  107  of radiation shield  100  includes inwardly-facing threaded cylindrical surface  109 . The threads on surface  109  are threaded to correspond to the outwardly-facing threads of surface  84  of fitting  28 . Thus, sensor assembly  15  can be inserted into bore  105  of shield  100  and nut portion  30  of fitting  28  then rotated about axis x-x relative to shield  100  until nut portion  30  of fitting  28  abuts the right end face of radiation shield  100 . This tightening action attaches fitting  28  to the top end of shield  100  so it does not move linearly relative to shield  100  and further compresses spring  24 . 
     With fitting  28  properly attached to shield  100 , as shown in  FIGS. 9-10 , thermocouple  16  extends into bore  105  such that collar  19  abuts seat  108  of shield  100 . As shown, seat  108  is a rightwardly and inwardly-facing frusto-conical surface. Left frusto-conical surface  96  of collar  19  is thereby configured to abut against surface  108  of shield  100  in sealing engagement. 
     When first installed and without increased operating temperatures, spring  24  is in its most compressed operational state and probe  16  is in an installed linear position relative to fitting  28  and radiation shield  100 , as shown in  FIG. 8 . When temperatures increases, and shield  100  expands in an amount greater than probe  16  due to differences in their respective coefficients of thermal expansion, spring  24  provides a spring force that biases collar  19  against and in sealing engagement with seat  108  of shield  100 . With reference to  FIG. 9 , spring  24  provides a spring force leftward so as to close any gap formed between collar  19  and seat  108  and maintain constant contact between collar  19  and seat  108  during operating conditions. Thus, spring  23  will tend to expand as necessary to maintain constant contact between sealing collar  19  and radiation shield  100 . Dampening assembly  17  thereby maintains a constant union of contact between parts to protect from vibration that would otherwise occur from a variation in the relative length of shield  100  and thermocouple  16  caused by high temperatures and different relative coefficients of thermal expansion. 
     As shown in  FIG. 8 , the relative lengths of the elements of assembly  15  are such that gap  40  is provided between left annular surface  92  of support tube  22  and right annular surface  78  of bushing  26  when assembly  15  is installed within shield  100 . Because terminal head  21  and surface  92  act as a hard stop to movement of probe  16  relative to fixture  28  beyond the assembled position, in this embodiment gap  40  is greater than the effective difference in thermal expansion of probe  16  and radiation shield  100  under intended operating conditions. Distance  40  correlates to the spring travel range or displacement range of spring  24  between the assembled position shown in  FIGS. 3-5  and the installed position shown in  FIGS. 6-10 . Assembly  15  is configured to have enough spring travel to compensate for any relative differences in expansion of shield  100  and probe  16 . 
     Thus, high temperature compression spring  24  is built into the sensor sheath so that spring  24  dampens vibration and keeps temperature sensor tip  29  stable within radiation shield  100  by ensuring a continuity of contact between stop  19  on the probe and seat  108  of radiation shield  100 . 
     A number of benefits result from the improved assembly. First, because radiation shield  100  and sensor sheath  16  may be formed of different materials and have different coefficients of thermal expansion, at operating temperatures bore  105  of radiation shield  100  may expand in length more than sensor sheath  16 , thereby creating a gap between seat  108  of radiation shield  100  and sealing collar  19  on sensor sheath  16 . This gap can leave enough room for the sensor to vibrate due to turbine gas flows and input vibrations from the engine. And such vibration may break the tip of the sensor, causing the sensor to prematurely fail. With the improved assembly, spring  24  is configured to keep sealing collar  19  of temperature sensor  16  properly seated against radiation shield  100 , even at high temperatures, thereby reducing any gap between seat  108  of radiation shield  100  and collar  19  on sensor sheath  16 . This has been found to dampen the vibration of the sensor and to keep temperature sensor tip  29  stable within radiation shield  100 . 
     The present invention contemplates that many changes and modifications may be made. Therefore, while the presently-preferred form of a spring loaded exhaust gas temperature sensor assembly has been shown and described, and several modifications and alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims.