Patent Publication Number: US-9410444-B2

Title: Shaft break detection

Description:
The present invention relates to a method of detecting shaft break and a shaft break detection system. It finds particular, though not exclusive, utility in detecting shaft breakage in a gas turbine engine. 
     It is an object of the present invention to provide a more accurate and more timely method and system of detecting shaft break. 
     Accordingly the present invention provides a method of detecting shaft break in a shaft system comprising a shaft coupled between two masses, the method comprising steps to: define a time-dependent rotational speed equation for the shaft in terms of system inertia for an engine transient event; discretize the rotational speed equation in terms of a discrete time constant in the discrete domain; recursively define the discretized equation to give a recursive equation; solve the recursive equation to determine the discrete time constant; define a threshold as a function of engine power; and set a shaft break signal to TRUE if the discrete time constant is greater than the threshold. 
     Advantageously, this method is robust to high frequency noise. Additionally it can be applied to any shaft system with minimal set up burden, as only the system inertia is required. 
     The rotational speed equation may be a first order linearised equation that approximates the shaft system. The rotational speed equation may be exponential in terms of an inverse time constant of speed decay. The inverse time constant of speed decay is inversely proportional to inertia of the shaft system. The inertia of the shaft system may be equal to the sum of the inertias of the masses. 
     The discrete time constant may be defined as an exponential of the sampling rate. 
     The recursive equation may be solved using a recursive least squares method. The recursive least squares method may use the last n speed samples, wherein n may be in the range 4 to 20. More preferably n may be in the range 8 to 12. 
     The steps of solving the recursive equation, defining the threshold and setting the shaft break detection signal may be performed iteratively. Thus they may be performed each time a speed sample is taken, or after a group of speed samples have been taken. 
     The method may further comprise a step of sampling the rotational speed of the shaft before the step of solving the recursive equation. This step may also be performed iteratively with the following three steps. 
     The shaft system may be a gas turbine engine shaft system, particularly an intermediate pressure shaft system. Alternatively it may be a high pressure or a low pressure shaft system. The two masses may comprise a compressor and a turbine of a gas turbine engine. 
     The engine power may be indicated by at least one engine parameter. The at least one engine parameter may be one of the group comprising altitude, compressor exit pressure, another shaft speed, lagged compressor exit pressure and corrected shaft speed of another shaft. 
     The engine transient event may comprise surge. Surge initially may similar characteristics to a shaft break event. 
     The present invention also comprises a gas turbine engine comprising a method as described above. 
     The present invention also comprises a shaft break detection system comprising: a shaft coupled between two masses; at least one sensor to sample rotational speed of the shaft; a processor to process the sampled speed to recursively solve a discretized rotational speed equation to determine a discrete time constant; a processor to determine a threshold as a function of engine power; and a comparator to set a shaft break detection signal to TRUE if the discrete time constant is greater than the threshold. 
     Advantageously, the system of the present invention sets a shaft break detection signal that is robust to high frequency noise. Additionally the set up burden is small as a shaft system is likely to already comprise a speed sensor; the remainder of the elements may be implemented in software if desired. Alternatively the elements may be implemented in hardware or a combination of hardware and software. 
     The system may comprise a sensor to sense an engine power parameter. The engine power parameter may be one of the group comprising altitude, compressor exit pressure, another shaft speed, lagged compressor exit pressure and corrected shaft speed of another shaft. 
     The system may further comprise memory to store the last n speed samples, where n may be in the range 4 to 20, more preferably 8 to 12. 
     The two masses may comprise a compressor and a turbine of a gas turbine engine. Alternatively the two masses may be a torque generator and a load. 
     The present invention also comprises a gas turbine engine comprising a system as described. 
    
    
     
       The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional side view of a gas turbine engine. 
         FIG. 2  and  FIG. 3  are a schematic illustration of a shaft system in unbroken and broken configurations. 
         FIG. 4  is a graph showing an engine transient event and its first order fitted line. 
         FIG. 5  is a flow chart of the method according to the present invention. 
         FIG. 6  is an exemplary look up graph for use in the method according to the present invention. 
     
    
    
     A gas turbine engine  10  is shown in  FIG. 1  and comprises an air intake  12  and a propulsive fan  14  that generates two airflows A and B. The gas turbine engine  10  comprises, in axial flow A, an array of inlet guide vanes  40 , an intermediate pressure compressor  16 , a high pressure compressor  18 , a combustor  20 , a high pressure turbine  22 , an intermediate pressure turbine  24 , a low pressure turbine  26  and an exhaust nozzle  28 . The fan  14  is coupled to the low pressure turbine  26  by a low pressure shaft  34 . The intermediate pressure compressor  16  is coupled to the intermediate pressure turbine  24  by an intermediate pressure shaft  36 . The high pressure compressor  18  is coupled to the high pressure turbine  22  by a high pressure shaft  38 . 
     A nacelle  30  surrounds the gas turbine engine  10  and defines, in axial flow B, a bypass duct  32 . A control system  46 , such as an electronic engine controller (EEC), is provided on the engine  10  and is configured to control aspects of the operation of the engine  10 . 
     In rare circumstances one of the shafts  34 ,  36 ,  38  may break. When this occurs the fan  14  or compressor  16 ,  18  decelerates rapidly because it is no longer driven. However, the turbine  22 ,  24 ,  26  rapidly accelerates because the load on it is substantially reduced. This in turn may cause the turbine disc to burst releasing high energy debris and resulting in catastrophic failure of the engine  10 . Where the engine  10  is used to power an aircraft the released high energy debris may not be captured and there is thus a risk of some debris impacting or piercing the fuselage of the aircraft. Therefore there is a need to identify shaft breakages and to shut down the engine  10  quickly by shutting off the fuel supply. Typically a shaft break event must be controlled in less than 1 second or the release of high energy debris cannot be reliably prevented. 
     A simplistic illustration of a shaft system  48 , for example the intermediate pressure shaft system, is shown in  FIG. 2 . The shaft system  48  comprises the intermediate pressure shaft  36  coupled between the intermediate pressure compressor  16  and the intermediate pressure turbine  24 . The shaft system  48  rotates as a whole as indicated by arrow  50 . A measuring device  52  is arranged to measure the rotational speed of the intermediate pressure shaft  34  and is coupled to a processor  54 . The measuring device  52  is preferably a speed probe located close to the intermediate pressure compressor  16 . The measuring device  52  may measure the rotational speed substantially continuously or may sample the rotational speed at defined intervals. This interval may be in the range 1 ms to 30 ms. Preferably samples are taken every 3 ms to 5 ms. Alternatively the measuring device  52  may measure the rotational speed indirectly, for example by measuring the frequency of phonic wheel teeth passing a fixed point. The processor  54  receives the measured rotational speed from the measuring device  52  and processes it as will be described below. 
     The intermediate pressure compressor  16  has inertia J c  whilst the intermediate pressure turbine  24  has inertia J t . The inertias are known properties of the shaft system  48 . 
       FIG. 3  shows the intermediate pressure shaft system  48  when the intermediate pressure shaft  36  has broken in a shaft break event. Thus the intermediate pressure shaft  36  comprises a first portion  36   a  that remains coupled to the intermediate pressure compressor  16  and a second portion  36   b  that remains coupled to the intermediate pressure turbine  24 . Although drawn approximately equal in length, it will be apparent to the skilled reader that the first portion  36   a  and second portion  36   b  of the intermediate pressure shaft  36  may be different lengths depending on where the break occurs and the cause of the break. Equally the break may not be a clean break but may leave jagged ends to the first and second portions  36   a ,  36   b.    
     In normal operation the turbine  24  drives the compressor  16  at a rotational speed resulting in the rotation  50  shown in  FIG. 2 . In the event of a shaft break the turbine  24  no longer drives the compressor  16  which therefore continues to rotate in the same direction but decelerates rapidly as indicated by arrow  56 . Meanwhile the turbine  24  accelerates as indicated by arrow  58  because it no longer experiences such a large load. 
     In normal operation the intermediate pressure shaft system  48  behaves as a third order mechanical system which can be approximated by a first order system. Such an approximation is sufficiently accurate to show relatively long term trends (&gt;50 ms) in speed reduction. 
       FIG. 4  is a graph of the speed of the intermediate pressure shaft  36 , as measured by the speed probe  52 , as a function of time. Line  60  shows an exemplary profile when the gas turbine engine  10  surges, which is an engine transient event. The first order approximation can be used to fit a curve to the line  60 , first order fit line  62 . The equation governing this line  62  is a first order differential, linearised, rotational speed equation in the form 
                 (       J   c     +     J   t       )     ⁢       ⅆ   ω       ⅆ   t         =         -   c     ⁢           ⁢   ω     +   τ           
where
 
               ω   ⁡     (   t   )       =         ω   ⁡     (   0   )       ⁢     ⅇ       -   α     ⁢           ⁢   t         +       ⅇ       -   α     ⁢           ⁢   t       ⁢       ∫   0   t     ⁢       τ       J   c     +     J   t         ⁢     ⅇ     α   ⁢           ⁢   t       ⁢           ⁢       ⅆ   t     .                   
The rotational speed measured by the speed probe  52  is ω and the total torque of the system is τ, being the sum of the torque of the intermediate pressure compressor  16  and the intermediate pressure turbine  24 . The exponential factor α is an inverse time constant of speed decay in the continuous domain and is defined as
 
             c       J   c     +     J   t             
where c is a damping factor, which is unknown.
 
       FIG. 5  is a flow chart of the method of detecting shaft break according to the present invention. Thus the first step  64  comprises defining the linearised first order rotational speed equation as described above. 
     For a shaft break event, the rotational speed ω measured by the speed probe  52  initially follows a similar profile over time but then deviates. When a shaft break event occurs there is a sudden change in system torque from τ 0  to τ 0 −Δτ, where τ 0  is the initial torque, because only the compressor  16  remains coupled to the first portion  36   a  of the shaft  36 . By defining ω 0  as the rotational speed at which a shaft break event occurs, and substituting into the equation for ω(t), the first order rotational speed equation can be written in the form ω(t)=Ae −αt +B where 
             B   =         τ   0     -     Δ   ⁢           ⁢   τ       c           
and A=ω 0 −B.
 
     The second step  66  of the method comprises discretizing the rotational speed equation. This is achieved by sampling the rotational speed ω at a rate T to give the k th  speed sample as ω(kT)=Ae −αkT +B. The discretized equation can be defined recursively, the third step  68  of the method, as ω((k+1)T)=βω(kT)+(β−1)B, where β=e −αT  is a discrete time constant, that is the time constant of speed decay in the discrete domain. 
     The fourth step  70  of the method of the present invention requires that the recursive equation be solved for the discrete time constant β. Preferably the recursive equation is solved using the recursive least squares method, an algorithm known to the skilled reader. This is an iterative method that requires the last n points to be used, where n is an integer. In a preferred embodiment n is in the range 4 to 20; more preferably 8 to 12. 
     A parallel step of the method of detecting shaft break according to the present invention requires sensing of at least one engine parameter, step  72 , that is indicative of engine power. Typical parameters include altitude, other shaft speeds, ‘raw’ or corrected, and compressor exit pressure (P 30 ), which may be lagged. However, other parameters or combinations of parameters known to the skilled reader may be substituted with equal felicity. 
     At step  74  a look up table, graph, function or other mechanism is provided to convert the at least one sensed parameter value to a threshold. An exemplary look up graph is shown in  FIG. 6  which plots the discrete time constant β against an engine parameter  80 . The threshold  82  is a line in this two-dimensional space. It will be understood that the threshold  82  may be a function of two or more engine parameters  80 , in which case the line may be visualised as a plot in three or more dimensions. For a threshold  82  that depends on multiple parameters a functional, rather than graphical, look up may be more appropriate. 
     At step  76  the discrete time constant β is compared to the threshold in a comparator, the output of which is used to set a shaft break signal at step  78 . If the discrete time constant β is greater than the determined threshold, thus the calculated β is above the threshold line  82  in  FIG. 6 , the shaft break signal is set to FALSE. Conversely, if the discrete time constant β is less than the determined threshold, thus the calculated β is below the threshold line  82  in  FIG. 6 , the shaft break signal is set to TRUE. 
     The shaft break signal can then be provided to the control system  46  of the gas turbine engine  10  which causes safe and rapid engine shutdown. For example, if the TRUE shaft break signal is received by the control system  46 , it may cause the fuel supply to the engine  10  to be cut off or a fuel metering valve to be slewed towards closed. Either of these actions will starve the engine  10  of fuel and cause it to shut down. Alternatively or additionally, variable geometry vanes in the engine  10  may be slewed to cause the engine  10  to surge and thereby accelerate dissipation of energy. 
     The present invention also comprises a shaft break detection system for a shaft system such as the intermediate pressure shaft system  48 . The shaft break detection system includes a processor, for example processor  54 , that receives the sampled rotational speed ω(kT) from the speed probe  52  and recursively solves the recursive equation to determine the discrete time constant β. The shaft break system also includes a processor, which may be the same or another processor, that determines the threshold  82  from the at least one parameter  80  indicative of engine power. This processor comprises the look up table, graph, function or other mechanism described with respect to step  74  of the method. The shaft break detection system also includes a comparator to compare the discrete time constant β to the threshold  82 . 
     The system may comprise one or more sensors to sense the one or more engine parameters  80 . There may also be memory associated with the processor or processors to store the data points for the solution of the recursive equation. 
     Although the method according to the present invention has been described as incorporating the recursive least squares method to determine the discrete time constant β, it will be apparent that other methods of solving the recursive equation may be substituted with equal felicity. For example, a Kalman filter may be used. 
     Although the method of the present invention has been described with respect to the intermediate pressure shaft system  48 , it is equally applicable to the high pressure shaft system comprising the high pressure compressor  18 , the high pressure shaft  38  and the high pressure turbine  22  or to the low pressure shaft system comprising the fan  14 , the low pressure shaft  34  and the low pressure turbine  26 . 
     The present invention has been envisaged for use in a gas turbine engine  10  for propelling an aircraft since the effects of shaft breakage are potentially catastrophic. However, the present invention also has utility for other types of gas turbine engine  10  including for marine applications and for industrial applications such as gas and oil pumping engines.