Abstract:
A fluid flow control system includes a valve, in which a control element regulates flow through a metering orifice. A further control element regulates flow through an orifice downstream of the orifice and is responsive to the pressure downstream of the orifice and to an opposing control pressure on a line. The element is positioned in accordance with the pressure in a chamber controlled by an electrical valve. The pressure in line is set to provide a required pressure drop across the orifice.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a fluid flow control system, and in particular to a system for controlling fuel flow to reheat burners of a gas turbine engine. 
     2. Discussion of Prior Art 
     A gas turbine engine reheat system commonly has three groups of burners which are supplied with fuel through respective manifolds, the fuel flows to the manifolds being controlled by respective metering devices. Typically two of the burner groups will require very high fuel flows and the metering devices and manifolds for those groups must be correspondingly large. It is also known to provide shut-off valves for arresting flow from the manifolds to the respective burner groups. Since a reheat system requires to be primed rapidly it is beneficial to locate the shut-off valves as close to the burner groups as possible. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fluid flow control system which can advantageously be applied to a gas turbine engine reheat fuel supply by virtue of arranging that the metering and shut-off valves are combined in a single unit which is sufficiently small and light as to be located close to the burners. 
     According to the invention there is provided a fluid flow control system comprising a valve having a metering orifice, a first control element movable to regulate the flow area of said metering orifice, a second orifice in series with said metering orifice, and a second control element responsive to an increase in a fluid pressure downstream of said metering orifice and to an increase in a control pressure for respectively increasing and decreasing the flow area of said second orifice, said system also including a device for generating said control pressure to correspond to a desired pressure difference across said metering orifice. 
     Preferably said first control element is operable to shut off said metering orifice. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:— 
         FIG. 1  shows diagrammatically a system for supplying fuel to a group of reheat burners in a gas turbine engine, 
         FIG. 2  is a diagram of a combined flow control and shut-off device forming part of  FIG. 1 , and 
         FIG. 3  is a diagram of a valve for providing a pressure signal corresponding to a desired metering pressure difference in the device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The gas turbine engine  10  shown in  FIG. 1  includes core reheat burners arranged in five groups of three spray bars, the groups being supplied by respective fuel lines  11 . Fuel flow through the lines  11  is controlled by respective ones of combined metering, shut-off and pressure drop control devices  12 , each of which is responsive to an electrical control signal from a circuit  13 . Fuel flows to the devices  12  through a manifold  14  which is supplied from a centrifugal pump  15  having a shut-off valve  16  at its inlet, the valve  16  being responsive to signals from the circuit  13 . Each of the valve devices  12  is connected to the manifold  14  by a line  17  whose dimensions need only be sufficient to provide a required fuel flow through a corresponding one of the lines  11 . 
     As shown in  FIG. 2  each of the devices  12  comprises a sleeve  20  having ports  21 ,  22  which communicate with the lines  17 ,  11  respectively. A control element  23  is slidable within the sleeve  20  and has ports  24  which co-operate with the sleeve  20  to define a variable metering orifice. The element  23  carries an elastomeric seal  25  which is engageable with a shoulder  26  of the sleeve  20  to arrest fuel flow. A shuttle control element  27  is slidable within the sleeve  20  and co-operates with the port  22 . The element  27  is urged towards a fully open position by the pressure downstream of the ports  24  and is urged against that downstream pressure by the pressure in a line  28 , this pressure corresponding to a required pressure downstream of the ports  24  and being derived in a manner later to be described. 
     The pressure in line  17  is applied through an annular chamber  30  and a flow restrictor  31  to an orifice  32 . Flow through the orifice  32  is controlled by an element  33  which is pivotally movable by a torque motor  34  which is responsive to current signals from the circuit  13  ( FIG. 1 ). A spring  35  biases the element  33  towards a shut position of the orifice  32 . The pressure intermediate the restrictor  31  and orifice  32  is applied to one end of the element  23  by way of a chamber  36 , this pressure being assisted by a spring  37 . A capacitative position transducer  38  supplies the control circuit  13  with signals corresponding to the position of the control element  23 . 
     As shown in  FIG. 1  the pressure in line  28  is provided by a valve  40  which is supplied on a line  41  with pressure from the outlet of the pump  15 . The valve  40  is shown in more detail in  FIG. 3  and comprises a fixed orifice flow restrictor  42  and a variable orifice  43  in series between the line  41  and a return line  44  to a low pressure zone upstream of the pump  15 . Flow through the orifice  43  is controlled by a plunger  45  which is responsive to the pressure in line  41  and is biased against this pressure by a spring  46 . The line  28  communicates with a zone between the restrictor  42  and orifice  43 . The loading of the spring  46  and the area of the plunger  45  which is subjected to the pressure in line  41  are selected so that the pressure in line  28  is below that in line  41  by an amount which is equal to the desired metering pressure difference through the ports  24  of the control element  23 . A stack of bimetal spring discs  47  is responsive to fuel temperature to modify the loading of the spring  46 . 
     In use the required reheat fuel flow to the group of burners supplied by the lines  11  is signalled on line  47  from the circuit  13  in response to operating conditions of the engine  10  and a reheat selector device  50  ( FIG. 1 ). The control element  23  is urged in an opening direction by the pressure downstream of the ports  24  and by the pressure in line  17 , and in a closing direction by the spring  37  and pressure in chamber  36 , this latter pressure being dependent on the magnitude of the current in line  47 , this current being responsive to the feedback signal from the transducer  38  and to a flow demand signal from the selector device  50 . Flow through the ports  24  is thus effectively dependent on the current in line  47 . If the metering pressure difference increases, the pressure downstream of ports  24  falls below that in the line  28  and the control element  27  moves to reduce flow through the ports  22 , reducing the metering pressure difference to its required level. The element  27  moves correspondingly to increase flow through the ports  22  if the metering pressure drop decreases. 
     When the reheat system is shut down the pump inlet shut-off valve  16  ( FIG. 1 ) is shut and the pressure in line  17  and downstream thereof falls. The spring  37  urges the element  23  so that the elastomeric seal  25  engages the shoulder  26 , preventing further flow to the line  11 . In the event of failure of the electrical supply on line  47  the orifice  32  is shut under the influence of the spring  35  and the pressure in chamber  36  rises to equal that in line  17 . The device  12  thus fails to a shut-off condition in the event of electrical failure. 
     It will be understood that the remaining reheat burners, that is the primary and bypass burners also comprise a plurality of spray bars arranged in one or more groups which are supplied from the manifold  14 , there being a valve  12  for each group, as before. The provision of a valve  12  for each group enables the groups to be selectively supplied with fuel, so that metering efficiency at low reheat flows is improved. 
     The arrangement reduces the weight of pipe runs by requiring only a single manifold  14 , instead of the three manifolds hitherto used.