Abstract:
A fuel control system for reheat burners of a gas turbine engine has a plurality of metering valves for respective burners, and throttle valves in series with the respective metering valves. A pressure regulating valve is provided for introducing pressurized fuel into fuel manifolds downstream of the respective throttle valves when the metering and throttle valves are shut. The pressure regulating valve also acts to relieve pressure in low pressure parts of the system when the latter is closed down and to prevent excessive temperature rise at the inlet of a fuel supply pump. The metering valves include pressure return ports which communicate with the metering valve outlets when those valves are shut.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to valves for regulating fluid flows and fluid pressures, and to gas turbine engine fuel control systems incorporating such valves. 
     2. Discussion of Prior Art 
     It is known to provide metering valves for fluids in which a control element is moved in response to variations in a servo-pressure, the servo-pressure being regulated by a valve which is energised by an electric force motor. It is a disadvantage of such known valves that failure of the force motor or its current supply may result in rapid change in the servo-pressure and consequent rapid movement of the valve control element away from its previously selected position. The present invention describes a metering valve arrangement in which this disadvantage is overcome. 
     SUMMARY OF THE INVENTION 
     Reheat systems for gas turbine engines are not normally operated for the whole time that the engine is running. Additionally, high engine operating temperature during reheat operation may cause the fuel in the reheat system to boil and to empty the system when reheat is shut off. If the system is allowed to empty, or partly empty, it will not respond sufficiently quickly when reheat is selected. It is an object of a further aspect of the invention to provide a reheat system in which the fuel supply manifolds are maintained primed with fuel at a pressure which is sufficient to prevent boiling. 
     According to one aspect of the invention a fuel control system for the reheat burners of a gas turbine engine comprises a source of pressurized fuel, a plurality of metering valves for regulating fuel flow from said source to respective ones of a plurality of burner manifolds, and means operable when one of said metering valves is shut, for introducing fuel at a predetermined reference pressure into the manifold associated with said one valve. 
     When a gas turbine engine main fuel system is shut down, heat flows from the engine or a rise in ambient temperature may expand fuel which is isolated between shut-off valves in a low pressure part of the fuel supply system, causing damage to that part of the system. 
     It is a still further object of the invention to provide a control valve for priming the engine reheat manifolds which will also act to relieve pressure in low pressure parts of the system when the engine is shut down. 
     According to a further aspect of the invention there is provided a valve for regulating pressure of a fluid, comprising a housing having first and second ports for connection to a fluid pressure source and a low pressure respectively, an outlet port communicating with said first port, a control element for regulating flow between said first and second ports, and a device for urging said control element to increase said flow in response to an increase in pressure at said first and outlet ports. 
     When a fluid flow control valve is used as a metering valve in a gas turbine engine fuel control system there is commonly provided a separate valve which is opened as the metering valve is shut, to return high pressure fuel to a drain line, thereby to reduce system pressure. It is desirable to reduce the amount of fuel discharged to the drain line, to minimise either the size of a drain tank, or the quantity of fuel discharged overboard. It is a further object of the invention to provide a fuel metering valve which incorporates means for reducing system pressure when the valve is shut. 
     According to a still further aspect of the invention a metering valve arrangement for regulating fuel flow from a pressure source to a gas turbine engine comprises a body having an inlet, an outlet and a return pressure port, and a control element movable within said body to regulate flow between said inlet and said outlet, said control element having a portion which uncovers said port to connect the latter to said outlet in a closed condition only of said control element in which flow between said inlet and said outlets prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment illustrative inventions will now be described by way of example only and with reference to the accompanying drawings in which:— 
         FIG. 1  is a block diagram of a control system for a gas turbine engine reheat fuel supply, 
         FIGS. 2 ,  3 , and  4  show details of metering arrangements for respective groups of reheat burners of the engine, and forming parts of  FIG. 1 , 
         FIG. 5  is a detailed view, to an enlarged scale, of parts of the devices of  FIGS. 2 and 3 , 
         FIG. 6  shows a pressure control valve forming part of  FIG. 1 , and 
         FIG. 7  shows a device for supplying a measured quantity of fuel for igniting the reheat burners. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The gas turbine engine  10 , shown in  FIG. 1 , has three groups of reheat burners, designated primary, bypass and core burners respectively. Fuel for these burners is supplied from a tank  11  by way of metering arrangements  12 ,  13  and  14  respectively. Outlet passages  15 ,  16 ,  17  for the arrangements  12 ,  13 ,  14  respectively pass to the engine  10  by way of an arrangement of spring-loaded shut-off valves  18 . These valves  18  lift open at a predetermined pressure difference and act, in a manner to be described, as pressure relief valves during shut down of the reheat system. The arrangements  12 ,  13  and  14  are shown in more detail in  FIGS. 2 to 4  respectively and are responsive to signals from an electrical control circuit  20 . A device  21  is also responsive to control signals from the circuit  20  to deliver a measured quantity of fuel on a line  22  to the engine  10  to transfer flame from the engine combustion chamber to the reheat burners when the reheat system is switched on. 
     The metering arrangements  12 ,  13 ,  14  receive fuel on a common supply passage  23  from a centrifugal pump  24  by way of a valve  25  which is biased to connect the outlet of the pump  24  to a drain line  19 , and is responsive to a predetermined level of delivery pressure from the pump  24  to connect the pump outlet to the passage  23  and to shut off the drain line  19 . A low pressure pump  26  supplies fuel from the tank  11  to the inlet of the pump  24  by way of a line  34  and an electrically operated valve  27 . A low pressure return line  28  communicates with the inlet of the pump  24  by way of a non-return valve  29 . As shown in  FIG. 4  the arrangement  14  includes a filter  30  from which high pressure fuel is supplied from the passage  23  to a line  31 . A manually operable valve  32  in a line  33  between the tank  11  and the inlet of the pump  26  is shut when the engine  10  is shut down. The pump  26  also supplies fuel on the line  34  to a main metering system  35  which includes a shut-off valve arrangement  36 , the system  35 , including the arrangement  36 , being responsive to signals from the circuit  20 . 
     As shown in  FIG. 2  the arrangement  12  includes a metering valve  40  having an inlet communicating with the passage  23  and a control element  41  for regulating flow between the passages  23  and  15 . The element  41  is axially movable in response to a difference between servo-pressures in chambers  42 ,  43 , these pressures being derived at ports  66 ,  67  by a flapper-controlled pilot valve  44  from the high and return pressures in lines  31  and  28  respectively. The flapper  68  of the valve  44  is biased to a null position by springs  45 ,  46 . 
     The valve  44  communicates with the chamber  42  by way of a line  39  and parallel pressure relief valves  47 ,  48  which permit flow in respective opposite directions. The valves  47 ,  48  are set to lift at a low pressure difference, for example 138 kPa, which is nevertheless above the servo-pressure difference in the null condition of the valve  44 . The springs  45 ,  46  are adjusted so that the difference between the pressure in line  39  and the pressures in chambers  42 ,  43  is very small when the valve  44  is in its null condition. The difference between the pressures in chambers  42 ,  43  is also very small when the valve  44  is operating, and in the steady-state condition of the valve  40  those pressures are substantially equal. The valve  44  is movable in either direction from its null position by a torque motor  50  which is controlled by the circuit  20  ( FIG. 1 ). Thus, if the valve  44  adopts a null position as a result of failure of the motor  50  or its electrical supply, the servo-pressure difference across the valve  47 ,  48  will be insufficient to lift either of them and the element  41  will move from its position only as a result of a very slow leakage to or from the chamber  42 , this leakage being through the gap in a piston ring  38 . A stem on the control element  41  is coupled to a displacement transducer  52  which signals the position of the element  41  to the circuit  20 . An adjustable restricted orifice  51  is located between the high pressure line  31  and the outlet port  53  of the valve  40 , to provide compensation for manufacturing tolerances in the profile of the metering orifice  69  of the valve  40 . 
     The outlet  53  of the valve  40  communicates with the passage  15  by way of a pressure drop regulating valve  54  whose control element  55  is biased open by a spring  37  and is responsive to pressure in a chamber  56  between a variable orifice  57  in the element  55  and a fixed restrictor  58 . Flow through the orifice  57  is controlled by a piston  59  which is responsive to difference between the pressures in the outlet  53  and in the high pressure line  31 , and is spring-loaded to shut the orifice  57 . The valve  54  is dimensioned to provide a predetermined metering pressure drop across the valve  40 , and regulates flow to the passage  15  to maintain that pressure drop constant. The pressure in the outlet  53  is applied to the metering arrangements  13 ,  14  through lines  60 ,  61  respectively. The valve  40  includes a port  49  through which fuel can flow to the return line  28  from the outlet  53  when the valve  40  is shut. The valve  29  ( FIG. 1 ) allows this fuel to flow to the line  34  only when the pressure in line  28  is higher than that in line  34 . 
     The arrangements  13 ,  14  are shown in  FIGS. 3 and 4  and are generally similar to the arrangement  12 , except in the sizes of their respective metering valves  62 ,  63 , and will not be described in detail. The principal difference in the arrangements  13 ,  14  resides in their pressure drop control valves  64 ,  65  respectively, which are responsive to the pressure downstream of the valve  40 , applied through the lines  60 ,  61 . The valves  64 ,  65  are also responsive to the pressures downstream of the metering valves  62 ,  63 . respectively. The arrangement is such that the pressures downstream of the valves  62 ,  63  are maintained equal to that downstream of the metering valve  40 . Since the metering valves  40 ,  62 ,  63  have a common supply passage  23 , the pressure differences across all of the metering valves are maintained equal to each other. 
     The valves  64 ,  65  are shown in more detail in  FIG. 5 . The valve  64  includes a sleeve  70  having ports  71  which communicate with the outlet passage  16 . A control element  72  is slidable in the sleeve  70  and is urged towards a shut position by a spring  73 . The sleeve  70  has a further port  74  which communicates by way of an annular passage  75  and a line  76  (see also  FIGS. 1 ,  3  and  4 ) with a valve  77  ( FIG. 1 ) which provides a constant reference pressure in the line  76 . The sleeve  70  has an annular groove  78  which communicates with one of the ports  71 . When the valve  64  is shut a further annular groove  79  in the element  72  interconnects the port  74  and the groove  78 . In the shut condition of the valve  64  fuel can flow from the valve  77  ( FIG. 1 ) through the line  76 , passage  75 , port  74  and grooves  78 ,  79  to apply the reference pressure to the outlet passage  16 . 
     In its shut condition the valve  65  can supply the reference pressure in line  76  to the passage  17 , and corresponds to the valve  64  except that the control element  80  of the valve  65  has an additional port  81  through which leakage flow from the groove  79 , between the control element  80  and its surrounding sleeve, can enter the bore of the valve  65  and pass to the line  61 . As described above the line  61  communicates with the line  60  and with the outlet  53  of the valve  40  ( FIG. 2 ). Since the valve  54  is biased open, when the reheat system is in its shut down condition fuel from the line  61  can enter the outlet passage  15 . With the reheat system shut down the valve  27  is shut and valve  25  ( FIG. 1 ) is connected to the drain line  19 , though the pumps  24 ,  26  continue running. The reference pressure in line  76 , and applied to passages  15 ,  16 ,  17  is less than that at which the valves  18  ( FIG. 1 ) will lift, so that when the reheat system is shut down no fuel reaches the reheat burners. When the reheat system is operated to shut the valve  62  ( FIG. 3 ) and supply fuel to the primary and core burners only, the valve  64  also shuts and the reference pressure is applied to passage  16 . 
     When the main fuel system and reheat system ( FIG. 1 ) are shut down, the shut-off valve arrangement  36  and the valves  27 ,  32  will be shut. Fuel is therefore trapped in the lines  33 ,  34 ,  85 ,  86  and pump  26 . Heat flow from the engine  10 , or an increase in ambient temperature may expand the trapped fuel and result in damage to those parts of the system. The valve  77 , in addition to providing the reference pressure in line  76 , also serves to prevent unacceptable pressure rise in the low pressure zones of the fuel system. 
     The valve is shown in detail in  FIG. 6  and comprises a housing  82  which has first and second ports  83 ,  84  which communicate by way of respective lines  85 ,  86  with the outlet and inlet respectively of the pump  26 . The line  85  includes a flow restrictor  87  ( FIG. 1 ). A ported sleeve  88  is located within the housing  82  between one end thereof and a fixed ported stem  89 . Ports  90  in the stem  89  communicate with the port  84 . A valve sleeve  91  is sealingly slidable on the stem  89  to control flow through the ports  90 . The interior of the sleeve  88  defines a chamber  92  which communicates with the line  76  by way of a third port  93  in the housing  82 . An evacuated bellows device  94  in the chamber  92  urges the valve sleeve  91  to shut the ports  90  in the stem  89 . The bias applied by the bellows device  94  is variable by means of a stem  95  whose position is axially adjustable by means of shim washers  96 . 
     In use, fuel initially flows from the line  85  through the restrictor  87 , port  83 , chamber  92  and port  93  to the line  76 . At a predetermined reference pressure in the line  76 , set by the stem  95 , the sleeve  91  is lifted to open the ports  90  and allow a part of the fuel in line  85  to return to the inlet of the pump  26 . The valve  77  thereby maintains a regulated reference pressure in the line  76  for priming the supply passages  15 ,  16   17  when the reheat system is shut down. After priming the line  76  and elements downstream thereof are full of fuel. Since the reference pressure in line  76  is less than that at which the valves  18  ( FIG. 1 ) will open, no fuel flows to the reheat burners. Fuel upstream of the valves  18  is maintained at a pressure sufficient to prevent boiling. 
     In a shut down condition of the engine  10 , heat may cause fuel in the lines  33 ,  34 ,  85 ,  86  and in the pump  26  to expand. A rise in pressure in the line  85  is relieved by way of the line  76  and the valves  18 . During partial operation of the reheat system the valves  62 ,  64  may be shut. Hot fuel from the outlet passage  16  may pass through the grooves  78 ,  79  and enter the reference pressure line  76 . Even if the ports  90  are open, the hot fuel from line  76  is mixed with cool fuel from the line  85  before returning by way of the line  86  to the inlet of the pump  26 . The fuel temperature at the pump inlet thus never rises sufficiently to boil, and will not become blocked by vapour. 
     The device  21  shown in  FIG. 7  comprises a spool valve  100  and a change-over valve  101  operated by a solenoid  102 . High pressure in line  31  is applied to a port  103  of the valve  100 . A further port  104  and a chamber  105  at one end of the valve spool are connected to the low pressure return line  28 . A chamber  106  at the other end of the spool is connected by the valve  101  to the return line  28  when the solenoid  102  is de-energised or to the high pressure line  31  when the solenoid  102  is energised on selection of reheat. The spool is biased by a spring  107  to connect the port  103  to a chamber  108  in which a piston  109  is slidable. The piston  109  is biased by a spring  110  to provide a maximum volume in the chamber  108  and the spring bias is opposed by the pressure in a chamber  111  which, in the de-energised condition of the solenoid  102  is connected to the low pressure line  28  through the port  104  and a port  112 . In this condition the line  22  to the engine  10  is shut-off and the chamber  108  is charged with fuel from the line  31  through port  103  and a further port  113 . The piston  109  is maintained in its charged position by the spring  110 , in the absence of pressure in line  31 , when the reheat system is shut down. 
     At reheat start-up after fuel flow to selected ones of the reheat burners has been established, the solenoid  102  is energised to apply the high pressure in line  31  to the chamber  106 , moving the spool against the spring  107  to the full extent of its travel, as determined by an abutment  114 , in which position the chamber  108  is isolated from line  31  and connected to line  22 . At the same time the high pressure line  31  is connected to the chamber  111  through ports  103 ,  112 , and the piston  109  moves to expel fuel in the chamber  108  to the line  22 , through the ports  113 ,  115  by way of a non-return valve  116 . The fuel so expelled passes to the engine combustion chamber and a location downstream thereof, resulting in a streak of flame between the combustion chamber and the selected burners, to ignite fuel at the latter. 
     When the reheat system is shut down, the valves  40 ,  62 ,  63  ( FIGS. 2 ,  3  and  4 ) are shut. The pump inlet valve  27  ( FIG. 1 ) is also shut, but the valve  25  continues to pass fuel to the passage  23  while the pump  24  empties, maintaining servo operating pressure on the line  31 . In the fully shut conditions of valves  62 ,  63  the pressures upstream of valves  64 ,  65  respectively ( FIG. 5 ) are lower than that in lines  60 ,  61 , so that valves  64 ,  65  shut, stopping flow to the passages  16 ,  17  and establishing the reference pressure in those passages, as described above. In the shut condition of valve  40  the low pressure return port  49  therein is open and fuel flows from the line  31  to the return line  28  through the restrictor orifice  51 , line  60  and port  49 . The pressure drop between passage  23  and outlet  53  is greater than the predetermined metering pressure drop of valve  54 , and that valve moves to reduce the pressure drop, shutting off flow to the passage  15 . As the pump  24  empties via the orifice  51  and port  49  ( FIG. 2 ), and their equivalents in the arrangements  13 ,  14 , the pressure in line  31  falls to that in the return line  28 , at which pressure in the line  31  the valve  25  connects the outlet of the pump  24  to the drain line  19 . The low pressure in line  31  results in absence of servo pressure for the valve  54 , which opens under influence of its spring. Reference pressure in line  76 , which has already been established in passages  16 ,  17  is established in passage  15 , as described above.