Abstract:
A measuring system for measuring axial displacement of a tube relative to an axially stationary component in a rotating rotor assembly includes at least one displacement sensor adapted to be located normal to a longitudinal axis of the tube; an insulated cable system adapted for passage through the rotor assembly; a rotatable proximitor module located axially beyond the rotor assembly to which the cables are connected; and a telemetry system operatively connected to the proximitor module for sampling signals from the proximitor module and forwarding data to a ground station.

Description:
This invention was made with Government support under Contract No. DE-FC21-95MC31176 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a system for measuring axial displacement in a rotating environment, and more specifically, to the measurement of relative axial movement between adjacent components in a gas turbine rotor assembly. 
     BRIEF SUMMARY OF THE INVENTION 
     In an advanced gas turbine power plant developed by the assignee of this invention, the stage  1  and  2  gas turbine buckets are closed circuit cooled with steam or air fed by a delivery system extending through the gas turbine rotor assembly. It is important to evaluate wear resulting from relative axial movement between the axial delivery tubes and the rotating turbine wheels. The rotor for which this measurement is required is approximately 59 inches in diameter and has a maximum rotational speed of 4200 rpm. The steam delivery tubes are buried deep within the massive rotor assembly and the sensors used to measure the axial displacement of these tubes must be mounted in an area which is approximately 24 inches from the machine center line, i.e., the rotor axis, and therefore must withstand repeated exposure to a centrifugal acceleration of over 12,000 g&#39;s. In addition, the temperature of the component to be measured will range from 75° F. to 250° F. and the area through which the signal leads must pass will be nearly 1000° F. The system must survive a series of 40-minute cycles to maximum speed and temperature; a life expectancy which exceeds 5000 cycles is desirable. The unit may be either self-powered or run from an external power source through a rotating power coupling device. The displacement signal will be transferred to a stationary platform through a rotating telemetry system capsule. 
     In accordance with an exemplary embodiment of the invention, a commercially available Bentley Nevada proximity transducer system is employed to sense the axial displacement measurement of four axially oriented steam supply cooling tubes arranged at 90° intervals about the rotor axis relative to stationary components in the rotor assembly, while the tubes and stationary components are rotating about the rotor axis. 
     The measurements are taken with transducers having no moving parts to jam, wear out or degrade, and the standard transducer probe is rated to a maximum temperature of 350° F. which is slightly above the expected temperature in the immediate area of the measurement. The transducers or sensors are oriented normal to the direction of motion of each steam delivery tube, concentrating on a measurement site which, in this case, is a sloped or curved region (or fillet radius) on the tube. This allows the tube to stay within the measurement distance range of the probe. Custom cabling is connected to the circumferentially spaced sensors, the cabling including mineral insulated metallic sheathed sections which will span the hot area, and lower temperature flexible sections for connection to the probes at one end and to oscillator/demodulator modules at the other end. 
     All of the oscillator/demodulator modules rotate near the machine center line in a module support ring supported by two permanently lubricated ball bearings between the machine bearing housing and the telemetry capsule. The modules are powered via a rotating induction power system mounted to the OD of the module support ring. Rectifiers, filters, and regulators within the induction power system rotating ring provide the 24 VDC required by the modules. The 2 to 18VDC output signals from the modules connect to the telemetry capsule via rotating circular connectors and a flexible coupling arrangement. The telemetry capsule samples the signals at a rate of approximately once per second and then digitally transmits the data to the ground station across a radio frequency link. The test cell data acquisition system then receives the digital data from the ground station via a serial link. 
     Due to the shape of the fillets on the tubes used as the targets for the measurement system, the output of the system will be inherently non-linear. For this reason, a detailed calibration must be performed in situ prior to running if an accurate absolute measurement of the tube movement is desired. Following the calibration, a curve-fit may be made and the resultant coefficients inserted into the data acquisition computer for engineering unit calculation. 
     Accordingly, in its broader aspects, the present invention relates to a measuring system for measuring axial displacement of a tube relative to an axially stationary component in a rotating rotor assembly having a longitudinal axis, the measuring system comprising at least one displacement sensor adapted for placement adjacent a longitudinal axis of the tube; an insulated cable system adapted for passage through the rotor assembly; an oscillator/demodulator module assembly located axially beyond the rotor assembly to which the cables are connected; and a telemetry system operatively connected to the module assembly for sampling signals from the module assembly and forwarding data to a ground station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view, also partially sectioned, of a turbine rotor assembly incorporating the measurement system in accordance with the invention; 
     FIG. 2 is a partial section in an area of the rotor assembly which supports steam delivery tubes, and illustrates the location of a transducer mounted on a turbine wheel, with the tip of the transducer in close proximity to a fillet on an axially oriented steam delivery tube; 
     FIG. 3 is a partial side elevation of a rotating electronics package for use with the subject invention; 
     FIG. 4 is a section taken along the line  4 — 4  in FIG. 3; 
     FIG. 5 is a section taken along the line  5 — 5  in FIG. 3; 
     FIG. 6 is a section taken along the line  6 — 6  in FIG. 3; and 
     FIG. 7 is a functional block diagram of the axial displacement measurement system in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a turbine rotor assembly  10  includes the rotor  12 , four turbine wheels  14 ,  16 ,  18  and  19  separated by spacer wheels  20 ,  22  and  24 . Each wheel  14 ,  16  and  18  supports a plurality of buckets or blades (not shown) which interact with stationary nozzles (also not shown) in a well known manner. 
     Inside the rotor assembly, a plurality of axially extending steam conduits or tubes (two shown at  26 ,  28 ) supply cooling steam to the buckets in the first and second stage wheels  14 ,  16 , the cooling tubes  26 ,  28  extending from an aft wheel  30 , where the cooling circuit extends radially inwardly via a plurality of tubes  32 ,  34 , etc., before extending axially out of the rotor. 
     The location referenced by arrow A is one where the relative axial movement of the cooling tube  28  may be measured in accordance with this invention. Cables from the measurement transducers must extend through the rotor assembly to the oscillator/demodulator modules described further below. The cables first extend radially inwardly to the center region of the rotor and extend rearwardly to the electronics passage as also discussed further below. In FIG. 1, the axial portions of the cables are shown at  36 . Since the four measurement locations and sensor probe arrangements are virtually identical, only one need be described in detail. 
     Turning now to FIG. 2, an exaggerated detail view shows the axially extending steam cooling tube  28 , passing through the stage  1 - 2  spacer wheel  20  where it is engaged by a fixed sleeve coupling  38 . The free end  40  of the tube engages a fixed ring  42  in the stage  1  wheel  14  and receives a “spoolie” coupler  44 . For purposes of this disclosure, the measurement of axial movement on the tube  28  relative to the sleeve coupling  38  is taken along a curved fillet or ramp  46  of the tube immediately adjacent the fixed sleeve coupling  38 . A sensing transducer or sensor probe  48  is oriented normal to the direction of motion of the tube  28 . By concentrating on the sloped ramp or fillet  46 , the tube  28  will stay within the measurement distance range of the sensor probe. The sensor probe  48 , which may be part of a commercially available Bentley Nevada 3300 Series Proximity Transducer System, has certain advantages, e.g., it has no moving parts to jam, wear out or degrade; the sensor probe  48  is rated to a maximum temperature of 350° F. which is slightly above the expected temperature in the immediate area; and the sensor probe is relatively inexpensive. The standard 3300 series transducer may, however, be ruggedized for rough service operation. 
     A flexible cable  50  extends from the sensor probe  48  through a wire guide  52  to a connector  54 , from which the “hard” cable  56  extends to the center region of the rotor and then axially to the modules  58 ,  60 ,  62  and  64 , recalling that there are four such axial cables  36  from four respective transducers or sensor probes adjacent four steam cooling tubes, located at approximately 90° intervals about the rotor structure. The modules  58 ,  60 ,  62  and  64  are preferably Proximity® modules available from the Bentley-Nevada Corporation. 
     The tube  28  in use will be subject to axial growth due to the high temperatures experienced inside the rotor assembly  10 . Since the temperature and material properties of the tube are known, the extent of the axial movement due to thermal expansion can be predicted with some degree of accuracy. The tube  28  is also subject to radial movement, however, as a result of centrifugal forces on the tube due to rotor rotation. In this regard, note how the tube  28  engages the radially outer but not the radially inner end off the sleeve coupling  38  in FIG.  2 . The centrifugal forces may also cause the round tube to “ovalize” and thus increase the radial movement adjacent the probe. Since the measured surface is a sloped ramp (or fillet)  46 , any movement of the tube  28  will alter the distance between the sensor probe  48  and the ramp  46 . The sensor probe  48  can, however, sense both radial and axial movement, albeit the radial movement is quite small. The axial movement may be substantial (e.g., about 0.383 inch) in that along the length of the tube  28 , there are three or four axially spaced couplings like the sleeve coupling  38 . The coupling at the stage  1 - 2  spacer wheel  20  will take up the loading on the other sleeve couplings, and the couplings have low friction surfaces to allow the tube  28  to expand smoothly. Since the system can be calibrated according to the thermal expansion characteristics of the tube material, the axial movement due to other causes, e.g., centrifugal ovalization and/or slip and slide motion, can be detected with great accuracy, i.e., down to 200 mils. 
     The sensed axial movement is transmitted via the cables  36  to the four circumferentially arranged oscillator/demodulator modules  58 ,  60 ,  62  and  64  arranged on the electronics package  66  shown in partially schematic form in FIGS. 3-6, and in the functional diagram in FIG.  7 . With specific reference to FIG. 3, the package  66  is adapted to be attached to the rearward end of a bearing housing (not shown) for the turbine rotor by bolts (two shown at  68 ), guide pins (two shown at  70 ) and/or other appropriate means on the package frame  72 , so that the centerline  67  of the package is in alignment with the axis of the rotor assembly  10 . An instrument tube  74  with the cables  36  enclosed therein extends into the frame  72  and is connected via a spline  76  to the proximitor module assembly  78 . The proximitor module assembly  78  is secured via bolts  80  to the frame  72 . The modules  58 ,  60 ,  62  and  64  are enclosed within a front support plate  82 , a support ring  84  and a rear plate  86 . This subassembly is rotatable with the tube  74  by means of bearings  88 ,  90 , the latter held in place by a retaining nut  92 . A bearing load spring  87  is located axially behind the bearing. A larger diameter wire guide tube  91  (axially abutting the tube  74 ) extends centrally of the four modules  58 ,  60 ,  62  and  64  and connects to the rear support plate  86 . Forward of the proximity modules, there are stationary primary and rotating secondary power coils  93 ,  94 , respectively. Coil  93  is supported by a ring  96  and supports  98 . Coil  94  is connected to power supply  100 . This enables self-powering of the proximitor module assembly and thus the gathering of real time data during testing. Coils  93 ,  94  are connected to a linear amplifier  108  and an AC source  110  (see FIG.  7 ). 
     Three stationary circular connectors  102 ,  104  and  106  are located behind the modules  58 ,  60 ,  62  and  64 , one connector used to calibrate the package to a reference temperature and the remaining two connectors accommodating two each of the four cables  36  from the sensor probes  48 . A flexible drive coupling  112  (employing Kevlar® lacing  114 ) is located between the module rear support plate  86  and a slip ring  116  supported by a mounting plate  118 . FIG. 5 illustrates the lacing in the drive coupling from an end view. It is the flexible drive coupling and slip ring which convert the signals from the rotating to the stationary components. A functional diagram of the arrangement is shown in FIG.  7 . 
     The flexible drive  112  and slip ring  116  are part of a rotating telemetry system available from Accumetrics, Inc. The telemetry system samples the signals at a rate of approximately once per second and then digitally transmits the data to a ground station via a radial frequency link. A test cell acquisition system then receives the digital data from the ground station via a serial link. 
     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.