Abstract:
A gas turbine fuel nozzle comprises a body and a sheath adapted to surround the body. A snap-on device is provided for releasably retaining the sheath on the body. The snap-on device is displaceable between a first position for allowing the sheath to be fitted over the body and a second position for retaining the sheath in place about the body.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to fuel nozzles for gas turbine engines and, more particularly, to a protective sheath assembly for such fuel nozzles.  
           [0003]    2. Description of the Prior Art  
           [0004]    Fuel nozzles for gas turbine engines are well known in the prior art. Such conventional fuel nozzles are used to supply fuel to a combustion chamber which is provided for igniting the fuel mixture, thereby producing the energy which is used to power the engine. Generally, the combustion chamber includes a plurality of fuel nozzles to thus ensure a proper distribution of the fuel mixture within the combustion chamber.  
           [0005]    Conventional fuel nozzles include an inlet fitting, which is coupled to a fuel manifold, and a stem defining a number of fuel passages for directing fuel from the inlet fitting to a tip assembly adapted to atomize the fuel delivered to the combustion chamber. A particular problem with gas turbine fuel nozzles is that the nozzles are located in a hot area of the engine. This heat can cause the fuel passing through the nozzle stem to rise in temperature sufficiently that the fuel can carbonize or coke. Such coking can clog the nozzle and prevent the nozzle from spraying properly. Accordingly, fuel nozzles are typically provided with a protective sheath or heat shield which surrounds the nozzle stem to form an annular air gap thereabout. The sheath and the air gap provide thermal insulation to the fuel nozzle stem in order to prevent the fuel flowing therethrough from coking.  
           [0006]    Various methods have been developed to physically attach the protective sheath to the fuel nozzle. For instance, it has been proposed to permanently secure the sheath to the fuel nozzle by brazing or welding the open upper end of the sheath to an enlarged neck provided on the nozzle stem. It has also been proposed to clamp the sheath to the nozzle stem. According to this sheath attachment method, the clamp surrounds the upper end of the sheath to clamp the sheath against the enlarged neck of the nozzle stem. It has also been proposed to secure the sheath to the nozzle stem by means of radial pins extending through the sheath and pressure fitted into the nozzle stem.  
           [0007]    The above-described sheath attaching methods are generally of a permanent nature and require the use of tools to install the sheath on the fuel nozzle. It would be highly beneficial to have a non-permanent sheath attaching method and arrangement by which the sheath could be readily installed and removed without requiring any tools.  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore an aim of the present invention to provide a new gas turbine fuel nozzle heat shield assembly, wherein the heat shield is properly attached to the fuel nozzle yet allowing the heat shield to be easily removed without damaging the fuel nozzle.  
           [0009]    It is also an aim of the present invention to provide a new fuel nozzle protective sheath assembly which can be readily installed onto a fuel nozzle without the use of tools.  
           [0010]    Therefore, in accordance with the present invention, there is provided a gas turbine fuel nozzle comprising a body, a sheath adapted to surround said body, and a snap-on device for releasably retaining said sheath on said body, said snap-on device being displaceable between a first position for allowing said sheath to be fitted over said body and a second position for retaining said sheath in place about said body.  
           [0011]    In accordance with a further general aspect of the present invention, there is provided a sheath assembly for a gas turbine engine fuel nozzle, the sheath assembly comprising a tubular sheath adapted to surround at least a portion of the fuel nozzle, and a retaining device adapted to releasably hold said sheath in place on the fuel nozzle, said retaining device being displaceable between a first position for allowing said sheath to be fitted over said fuel nozzle and a second position for retaining said sheath in position on said fuel nozzle.  
           [0012]    In accordance with a still further general aspect of the present invention, there is provided a gas turbine engine fuel nozzle comprising a tubular sheath removably mounted to a fuel nozzle stem by a snap ring retained in grooves formed in the nozzle stem and the tubular sheath.  
           [0013]    In accordance with a still further general aspect of the present invention, there is provided a gas turbine engine fuel nozzle comprising a nozzle body, a detachable protective sheath, and a deflectable sheath retainer adapted to releasably engage a catch, said deflectable sheath retainer being disposed on one of said body and said sheath, and said catch being disposed on another one of said body and said sheath.  
           [0014]    In accordance with a still further general aspect of the present invention, there is provided a method for removably mounting a sheath to a gas turbine fuel nozzle, the method comprising the steps of: a) providing a snap-on retainer on the gas turbine fuel nozzle, and b) sliding the sheath over a stem portion of said fuel nozzle until the sheath snap into engagement with said snap-on retainer.  
           [0015]    In accordance with a further general aspect of the present invention, the snap-on retainer includes a spring-loaded ring and step a) comprises the steps of: machining a peripheral groove in a said stem portion of said fuel nozzle, and placing said spring-loaded ring in said peripheral groove prior to sliding said sheath over said stem portion.  
           [0016]    In accordance with a still further aspect of the present invention, the method further comprises the step of: machining a groove in an inner surface of said sheath for receiving said spring-loaded ring. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which:  
         [0018]    [0018]FIG. 1 is a side view, partly broken away, of a gas turbine engine to which an embodiment of the present invention is applied;  
         [0019]    [0019]FIG. 2 is an exploded perspective view of a fuel nozzle and heat shield assembly in accordance with a preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 3 is a perspective view of the fuel and heat shield assembly once assembled;  
         [0021]    [0021]FIG. 4 is an enlarged side view, partly in section, illustrating how the heat shield is retained in place on the fuel nozzle; and  
         [0022]    [0022]FIG. 5 is an enlarged cross-sectional side view illustrating a nozzle stem having a rounded stem neck in accordance with a further general aspect of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    [0023]FIG. 1 illustrates a gas turbine engine  10  generally comprising in serial flow communication a fan  12  (not provided with all types of engine) through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine  18  for extracting energy from the combustion gases. Although a turbofan engine has been shown, it is noted that the present invention could used in other types of gas turbine engine, such as turboprops, turboshafts, Auxiliary power units and industrial gas turbine engines.  
         [0024]    The combustor  16  typically comprises a combustion chamber  20  and a plurality of fuel nozzles (only one being shown at  22 ), which are typically equally spaced about the circumference of the combustion chamber  20  in order to permit a substantially uniform temperature distribution in the combustion chamber  20  to be maintained. In use, the fuel provided by a fuel manifold (not shown) is atomized by the fuel nozzles into the combustion chamber  20  for ignition therein, and the expanding gases caused by the fuel ignition drives the turbine  18  in a manner well known in the art.  
         [0025]    As shown in FIG. 2, each fuel nozzle  22  is protected against heat by a heat shield or protective sheath assembly  24 . The fuel nozzle  22  is generally of conventional design and comprises an inlet fitting  26  adapted to be connected to an engine manifold (not shown), a tip assembly or atomizing nozzle  28  for spraying or atomizing the fuel into the combustion chamber  20 , and a nozzle stem  30  extending between and fluidly interconnecting the inlet fitting  26  and the atomizing nozzle  28 . A flange  32  extends laterally outwardly from the upstream end of the stem  30 . Holes  34  are defined in the flange  32  to enable the fuel nozzle to be securely mounted to the case of the combustion chamber  20 .  
         [0026]    The stem  30  has an enlarged neck portion  36  directly underneath the flange  32 . A circumferentially extending groove  38  is machined in the outer surface of the neck portion  36  for receiving a snap ring  39  forming part of the protective sheath assembly  24 . As best seen in FIG. 4, the portion of the neck  36  below the groove  38  has a frustoconical profile defining a ramp  40  for facilitating the installation of the snap ring  39  in the groove  38  by sliding the ring  39  over the stem until the ring  39  captively falls into the groove  38 . The snap ring  39  is made of a springy metallic material and is design to be received with a loose fit in the stem groove  38 .  
         [0027]    As shown in FIG. 2, the protective sheath assembly  24  further comprises an open ended tubular shield or sheath  42  adapted to be removably mounted to the fuel nozzle  22  so as to define an annular air gap about the stem  30 . The sheath  42  and the annular air gap  44  (see FIGS. 3 and 4) provide thermal insulation to the stem  30  in order to prevent the fuel flowing therethrough from coking.  
         [0028]    The sheath  42  is preferably of unitary construction and is cylindrical in shape. The sheath  42  has an inner circumferential wall  46  extending from a lower end  48  to an upper end  50 . As shown in FIG. 3, the lower end  48  is machined to define a round shaped opening  52  for accommodating the angled tip atomizing assembly  28  of the fuel nozzle  22 . The upper end  50  has a circumferential shoulder  54  extending about a circular opening  56 . A circumferential shallow groove  58  is defined in the inner surface of the sheath  42  at the level of the shoulder  54  for snap engagement with the snap ring  39  in order to releasably axially retain the sheath  42  on the fuel nozzle stem  30 . The inner surface  46  of the sheath  42  at the upper end  50  thereof is machined so as to define a chamfer  60  (FIG. 4) for allowing the snap ring  39  to be initially contracted radially inwardly when pushed by the sheath  42  while the same is being slid over the nozzle stem  30  towards its final position.  
         [0029]    The sheath  42  is installed on the fuel nozzle  22  by first placing the snap ring  39  into the stem groove  38 . This is done by sliding the ring  39  axially along the nozzle stem  30  to the groove  38 . The ring  39  is gradually expanded while moving along the ramp  40  before returning back to its rest or unsolicited position upon reaching the groove  38 . Once in the groove  38 , the ring  39  loosely surrounds the stem  30  so as to provide enough play for the ring  39  to be radially contracted towards the central axis of the nozzle stem  30 .  
         [0030]    The sheath  42  is then slid onto the nozzle stem  30  (in the direction indicated by arrow  61  in FIG. 1) until the chamfer  60  engages the snap ring  39 . As the sheath  42  continues to move along the stem  30 , the ring  39  is circumferentially compressed inward (ring gap is closed) in the stem groove  38 . When the sheath  42  reaches its final position, the snap ring  39  expands back into the sheath groove  58 , retaining the sheath  42  in position for installation of the nozzle assembly on the engine. Note that a guide pin or the like (not shown) can be used to ensure proper alignment of the sheath  42  with the tip assembly  28 , as know in the art.  
         [0031]    As best seen in FIG. 4, the snap-ring  39  has a round cross-section, which matches the outline of the sheath groove  58 . The rounded cross-sectional shape of the snap-ring  39  advantageously provides for easy removal of the sheath  42  by simply pulling it off the assembly. The sheath can be reinstalled back onto the fuel nozzle assembly or replaced by a similar one if need be.  
         [0032]    As shown in FIG. 5, the rounded cross-sectional shape of the snap-ring  39  introduces the novel concept of a rounded stem neck  41  which allows assembly of one piece sheaths, such as sheath  42 , onto gas turbine nozzles having a large nozzle tip angle A. In some nozzle configurations, the nozzle tip axis A is so large that in order to install the one piece sheath  42  onto the nozzle assembly, the sheath  4  has to be slid at an angle B from the nozzle stem axis. The rounded neck  41  allows assembly of greater combination of angles A and B.  
         [0033]    The above-described non-permanent sheath attachment method provides for a tool-free installation/removal of the sheath  42 , which constitutes another major advantage over know techniques.  
         [0034]    The utilisation of a snap-ring, which is retained captive between the sheath  42  and the nozzle stem  30 , for removably holding the sheath  42  on the fuel nozzle  22  is also advantageous in that it provides a very compact sheath retaining arrangement.  
         [0035]    However, it is understood that the present invention is not limited to the utilization of a snap-ring and that other types of deflectable or spring-loaded sheath engaging member or retainer could be used for providing releasable attachment of the sheath on the fuel nozzle assembly. Also, various types of catches could be provided on the protective sheath or on the fuel nozzle assembly for releasable engagement with a corresponding sheath retainer.  
         [0036]    The present invention is also advantageous in that mis-assembly of the sheath  42  can be easily detected by the sheath  42  not being properly retained/attached to the fuel nozzle  22  upon removal of the nozzle  22  from the combustion case  20 . The sheath  42  can be easily removed for overhaul and maintenance purposes. Furthermore, the sheath  42  and the snap ring  39  are simple and inexpensive to manufacture and assemble.