Abstract:
A fuel control system according to an exemplary aspect of the present disclosure includes, among other things, a fuel delivery valve selectively moveable to a closed position to shut off a flow of fuel to a downstream location. The system further includes a windmill bypass valve, and a shutoff pressure line between the windmill bypass valve and the fuel delivery valve. The windmill bypass valve is selectively operable to direct fuel to the shutoff pressure line to assist the movement of the fuel delivery valve to the closed position. A method is also disclosed.

Description:
BACKGROUND 
       [0001]    This disclosure generally relates to a system for controlling turbomachine fuel flow. Turbomachines, such as gas turbine engines, typically include at least a compression section, a combustor section, and a turbine section. Many engines include a fuel control system configured to deliver fuel to the combustor section. These systems may include a windmill bypass valve that directs fuel away from a fuel delivery valve, such as a minimum pressure shutoff valve (MPSOV), during engine shutdown. Further, the windmill bypass valve may maintain sufficient pressure (sometimes called “muscle” pressure) to position fuel control system components as well as various engine actuators during windmilling and start. Shutdown of the engine may occur on the ground or in flight during, for example, an over-speed condition. 
         [0002]    One example fuel control system includes a windmill bypass valve selectively movable between an open position and a closed position. When in the open position, some fuel is directed away from an MPSOV, and thus the pressure of the fuel flowing to the MPSOV drops. This drop in pressure allows the MPSOV to close under the bias of a spring and shut off a flow of fuel to the engine. 
       SUMMARY 
       [0003]    A fuel control system according to an exemplary aspect of the present disclosure includes, among other things, a fuel delivery valve selectively moveable to a closed position to shut off a flow of fuel to a downstream location. The system further includes a windmill bypass valve, and a shutoff pressure line between the windmill bypass valve and the fuel delivery valve. The windmill bypass valve is selectively operable to direct fuel to the shutoff pressure line to assist the movement of the fuel delivery valve to the closed position. A method is also disclosed. 
         [0004]    The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following description and drawings, including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The drawings can be briefly described as follows: 
           [0006]      FIG. 1  schematically illustrates an example fuel control system. In  FIG. 1 , a fuel delivery valve is an open position such that fuel is allowed to flow to a downstream location. 
           [0007]      FIG. 2  schematically illustrates the system of  FIG. 1 , with the fuel delivery valve in a closed position such that fuel is prevented from flowing to the downstream location. 
           [0008]      FIG. 3  schematically illustrates an alternative fuel delivery valve arrangement. In  FIG. 3 , the fuel delivery valve is in an open position. 
           [0009]      FIG. 4  schematically illustrates the fuel delivery valve of  FIG. 3 , with the fuel delivery valve in a closed position. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  schematically illustrates an example fuel control system  20 . In this example, the system  20  is arranged to provide a flow of fuel F generally from an upstream location, at  22 , to a downstream location, at  24 . In this example, the system  20  receives a flow of fuel F from a fuel filter at the upstream location  22 . The downstream location  24  is fluidly coupled to fuel nozzles of a combustor section of a turbomachine, such as a gas turbine engine. 
         [0011]    Between the upstream location  22  and the downstream location  24 , the fuel F flows along a main fuel line  26 . In this example, immediately downstream of the upstream location  22 , there is a pump  28  configured to pressurize the fuel F. It should be understood that there may be additional pumps along the main fuel line  26 . 
         [0012]    Downstream of the pump  28 , the main fuel line  26  is fluidly coupled to a pressure regulating valve  30 . The pressure regulating valve  30  is illustrated schematically. This disclosure is not limited to any particular pressure regulating valve type. 
         [0013]    A metering valve  36  is fluidly coupled downstream of the pressure regulating valve  30 . The metering valve  36  has an inlet  38 , a main outlet  40  fluidly coupled to the main fuel line  26 . Further, like the pressure regulating valve  30 , the metering valve  36  is also illustrated schematically. This disclosure extends to all types of metering valves  36 . 
         [0014]    A high pressure source  42  supplies fuel F to a shutdown line  44 . The high pressure source  42  is illustrated schematically. The high pressure source  42  could originate from a location along the main fuel line  26  between the pump  28  and the metering valve  36 . The shutdown line  44  could be supplied by fuel F from another high pressure source in other examples. 
         [0015]    In this example, the shutdown line  44  is fluidly coupled to a spring chamber  46  of a windmill bypass valve  48 . The shutdown line  44  further includes a shutoff orifice  44 D fluidly coupled to the high pressure source  42 , which essentially allows an electromechanical valve  58  (discussed below) to control the pressure in the shutdown line  44 . The windmill bypass valve  48  includes a piston  48 P and a spring  48 S arranged in the spring chamber  46 . The position of the piston  48 P is dictated by the biasing force of the spring  48 S and the pressure of the fuel F in the shutdown line  44 . As the piston  48 P moves, the windmill bypass valve  48  moves between a closed position (shown in  FIG. 1 ) and an open position ( FIG. 2 ). 
         [0016]    In the closed position of  FIG. 1 , the piston  48 P is positioned to direct all fuel F within the main fuel line  26  downstream of the windmill bypass valve  48  and to a fuel delivery valve, which in this example is a minimum pressure shutoff valve (MPSOV)  50 . As is known in the art, an MPSOV is operable to ensure there is a minimum operating pressure in the fuel F before allowing a flow of fuel F to the downstream location  24  (e.g., the engine), and shuts off the fuel flow to downstream location  24  in certain modes of operation. The positioning of the MPSOV  50  is discussed in detail below, and is dictated by both a spring  50 S and fuel within a pressure shutoff line  52 . 
         [0017]    When in the open position ( FIG. 2 ), the windmill bypass valve  48  is fluidly coupled to a shutoff pressure line  52  via a first outlet  53 , which is in communication with a spring chamber  54  of the MPSOV  50 . The windmill bypass valve  48  is also fluidly coupled to a low pressure line  56  via a second outlet  57 , which directs fuel F to a downstream location, such as pump interstage, for example. The windmill bypass valve  48  varies the flow of fuel F in line  56  to set pressure, which, when added to the metering valve  36  pressure drop (which is typically set by the pressure regulating valve  30 ), is directed to one or more actuators via a pressure line  49  to maintain sufficient pressure (sometimes called “muscle” pressure) to position those actuators during windmilling and engine start. Example actuators include a bleed actuator of an engine or a stator vane actuator. As is known, windmilling is a condition in which the rotatable elements of the gas turbine engine rotate under the force of the passing airstream. 
         [0018]    When the windmill bypass valve  48  is closed ( FIG. 1 ), fuel F is directed to the downstream location  24  via the MPSOV  50 . The MPSOV  50  has an inlet  60  and an outlet  62 . The MPSOV further includes a piston  50 P and a spring  50 S within the spring chamber  54 . The spring chamber  54  is in communication with a damping orifice  54 D to allow the spring chamber  54  to breathe. The damping orifice  54 D is fluidly coupled to a low pressure location. When the MPSOV  50  is open, the piston  50 P is positioned to allow fuel F to flow from the inlet  60 , to the outlet  62 , and ultimately to the downstream location  24 . 
         [0019]    In one example, in order to effect movement of the piston  48 P (and, in turn, move the windmill bypass valve  48  between the open and closed positions), the pressure of the fuel F within the spring chamber  46  is adjusted. In one example, the shutdown line  44  is fluidly coupled to an electromechanical valve  58 , which may include a servo, and which is electrically coupled to a control unit C. The electromechanical valve  58  is operable to relieve the pressure in the shutdown line  44  by directing the fuel F to a low pressure location  59 . To build or maintain pressure in the shutdown line  44 , the electromechanical valve  58  does not allow any fuel F to flow to the low pressure location  59 . While the illustrated example includes a shutoff orifice  44 D supplied by a high pressure source  42  and a two-way electromechanical valve  58  coupled to a low pressure location  59 , this disclosure could employ a three-way electromechanical valve. 
         [0020]    The control unit C may be any known type of controller including memory, hardware, and software. The control unit C is configured to store instructions and to provide instructions to the various components of the fuel control system  20 , including the electromechanical valve  58 . The control unit C may be part of a main controller of an engine, or may receive instructions from such a controller. 
         [0021]    During engine shutdown, which may occur on the ground or in flight during, for example, an over-speed condition, the control unit C provides instructions to the electromechanical valve  58  to direct a portion of the fuel F within the shutdown line  44  to the low pressure location  59 . Doing so decreases the pressure of the fuel F within the spring chamber  46 . In response, the piston  48 P is moved to the open position of  FIG. 2 . An over-speed condition may be the result of some type of system failure. An example could include if a computer (or controller) of an engine failed and commanded the metering valve  36  towards a higher than required fuel flow. 
         [0022]    While moving to its the open position, the windmill bypass valve  48  is fluidly coupled to the shutoff pressure line  52  and the low pressure line  56 . As shown in  FIG. 2 , the fuel F flows within the shutoff pressure line  52  into the spring chamber  54  of the MPSOV  50 . This relatively high pressure fuel urges the piston  50 P of the MPSOV  50  toward the closed position (e.g., in the left hand direction, relative to  FIG. 2 ). When the MPSOV is closed ( FIG. 2 ), fuel F is not allowed to flow beyond the inlet  60  of the MPSOV  50 . Thus, the MPSOV  50  shuts off flow to the downstream location  24 . 
         [0023]    In order to resume engine operation, the control unit C provides instructions to the electromechanical valve  58  to cease directing fuel F to the low pressure location  59 . Pressure then builds in the spring chamber  46 , which moves the piston  48 P back to the closed position of  FIG. 1 . As the windmill bypass valve  48  closes, fuel is no longer directed to the shutoff pressure line  52 , and the piston  50 P is allowed to move to the open position of  FIG. 1 . 
         [0024]    Another feature of the windmill bypass valve  48  is the relative vertical arrangement of the outlets  53  and  57 . As illustrated, the outlet  57  is vertically spaced-apart, and in this example is above, the outlet  53 . Thus, when the windmill bypass valve  48  closes, the outlet  53  closes first, which allows the MPSOV  50  to open before cutting off pressure to the low pressure line  56 . This prevents a pressure spike (also sometimes referred to as “water hammer”) in the main fuel line  26 . Further, when opening the windmill bypass valve, the positioning of the outlets  53  and  57  essentially prioritizes a flow of fuel F to the low pressure line  56  when in an over-speed condition, for example. 
         [0025]    Providing the flow of fuel F to the MPSOV  50  via the shutoff pressure line  52  allows the MPSOV  50  to rapidly close, which increases the effectiveness of the MPSOV  50 . Further, the outlet  53  of the windmill bypass valve  48  can be sized to limit the rate at which fuel F flows to the fuel nozzles and thereby limiting the subsequent pressure spike in the fuel system. 
         [0026]      FIG. 3  illustrates a second example MPSOV  150  according to this disclosure in an open position. Corresponding structures from the first example MPSOV  50  of  FIGS. 1 and 2  are preappended with a “1” in  FIGS. 3 and 4 . 
         [0027]    In this example, the shutoff pressure line  152  is fluidly coupled to an annulus  164  circumferentially disposed about an outer housing  166  of the MPSOV  150 . Further, an metering edge  167  is provided in the outer housing  166 , and an orifice  168  is provided within a spring receipt portion  169  of the piston  150 P. When the MPSOV  150  is open, the orifice  168  is aligned relative to the metering edge  167  to allow fuel F to enter the spring chamber  154  (e.g., the orifice  168  is on the right hand side of the metering edge  167 ). During an engine shutdown, for example, the fuel F enters the spring chamber  154  and urges the piston  150 P toward the closed position substantially as described above. The closed position is illustrated in  FIG. 4 . 
         [0028]    In the closed position, the orifice  168  is not aligned with the annulus  164 . In particular, a portion of the piston  150 P (e.g., the spring receipt portion  169 ) substantially blocks annulus  164  from communicating fuel F into the spring chamber (e.g., the orifice  168  is arranged on the left hand side of the metering edge  167 ). Thus, fuel F is not allowed to enter the spring chamber  154 . Preventing flow into the spring chamber  154  when resuming engine operation allows the piston  150 P to move back to the open position without needing to overcome the pressure from the shutoff pressure line  152 , which could lead to a momentary delay in opening the MPSOV  150  and cause a pressure spike in the mail fuel line  26  upstream of the MPSOV  150 . The arrangement of  FIGS. 3-4  provides for the rapid shut off of the MPSOV  150  while also reducing the likelihood of a pressure spike during engine start in the event the windmill bypass valve  48  is in an open position. 
         [0029]    Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. 
         [0030]    One of ordinary skill in this art would understand that the above-described embodiments are exemplary and non-limiting. That is, modifications of this disclosure would come within the scope of the claims. Accordingly, the following claims should be studied to determine their true scope and content.