Patent Publication Number: US-3874170-A

Title: Fuel control apparatus for gas turbine engine reheat systems

Description:
United States Patent 1 [111 3,874,170 Lewis Apr. 1, 1975 FUEL CONTROL APPARATUS FOR GAS 3.591.968 7/1971 Arnett .1 60/243 TURBINE ENGINE REHEAT SYSTEMS 3.630.029 l2/l97l Smith. 60/243 75] Inventor Geoffre Arthur Lewis 80mm&#34; 3.714.784 2/l973 Glaze 60/24l I Englang FOREIGN PATENTS OR APPLICATIONS 905.754 919 2 U d .46 241 [73] Assignee: Lucas Aerospace Limited, 6 med King om 0/ B&#39;rmmgham&#39; England Primary ExaminerClarence R. Gordon [22] Filed: Apr. 6, 1973 Attorney, Agent, or Firm-Holman 8L Stern l I A fuel control apparatus for a gas turbine engine rel l Foreign Appllcalloll Prlorlty Data heat system has a variable metering arrangement re- June 21,1972 United Kingdom 28998172 sponsive to engine operating parameters to control fuel flow to the reheat burners, and a distributor valve [52] US. Cl. 60/241, 60/243 arrangement between the metering arrangement and [51] Int. Cl. F0214 3/10 the burners. The distributor arrangement is operable [58] Field 01&#39; Search 60/241, 243, 261, 233 in one mode to connect the metering arrangement to the burners, and in another mode to shut off fuel from [56] References Cited the metering arrangement and to connect the burners UMTED STATES PATENTS to a low pressure dram. 3.197.958 8/1965 Davies 60/243 7 Claims, 10 Drawing Figures P571 i ll i .1 I55 156] I 154 J O 0Q 19s 445 3 PF3 152 fi I47 /|AO w 1116 131 [j PFZ I i5| |33 j PFI PATEHTIW H975 3.874.170  
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 PATENTEU APR 1 75 SHEET 3 OF 8 FATENTEDAPR usvs 3 874,170  
 snmsnra HATENTEBAPR 11% 3,874,170 sum 7 or a 2 4 zea 260 E66 212 as FIG. IO.  
 FEEL CONTROL APPARATUS FOR GAS TURBINE ENGINE REHEAT SYSTEMS This invention relates to a fuel control apparatus for a gas turbine engine reheat system.  
  According to the invention a fuel control apparatus for a gas turbine engine reheat system includes a variable metering arrangement for controlling fuel flow to the engine reheat burners a low pressure drain and a valve arrangement operable in one position thereof to connect said metering arrangement to said burners in another position to connect said burners to said low pressure drain, said valve arrangement comprising first, second and third ports respectively communicating with said metering arrangement, said burners and said drain. valve seats between said first and second ports and between said second and third ports. a closure member movable to selectively engage said valve seats. means for biasing said closure member towards engagement with one of the seats. and means responsive to a control signal to move said closure member against said biasing means into engagement with the other of said seats.  
  An example of a fuel control apparatus according to the invention will now be described with reference to the accompanying drawings:  
 FIG. I is a block diagram ot&#39;the apparatus as a whole,  
  FIG. 2 shows diagrammatically a variable metering assembly forming part of the apparatus.  
  FIG. 3 shows diagrammatically a servo pressure control arrangement forming part of the apparatus.  
  FIG. 4 shows a selector valve arrangement forming part of the apparatus,  
  FIG. 5 shows diagrannnatically a distributor arrangement forming part of the apparatus.  
  FIG. 6 shows diagrammatically a relay valve arrangement forming part of the apparatus,  
  FIG. 7 shows diagrammatically a priming arrangement forming part of the apparatus,  
 FIG. 8 shows an alternative form of priming arrangement,  
  FIG. 9 shows a further form of priming arrangement. and  
 FIG. 10 shows yet another form of printing arrangement.  
  Referring firstly to FIG. I the apparatus has a centrifugal vapour core fuel pump l0 driven by the engine. Pump [0 is of the kind which includes a throttle II responsive to a servo pressure signal PS1 applied via a passage I2 to increase pump delivery. This servo pressure signal PSI is derived in a manner later to be described. Throttle II is also responsive to a further servo pressure signal PS2 in a passage 13 which opposes the pressure in passage 12.  
  A fuel metering assembly 14 includes an electrical control arrangement I411. The hydraulic parts ofassembly I4 are shown in greater detail in FIG. 2. Assembly I4 has an inlet 15 connected to the outlet of pump I0. Inlet l5 communicates via a non-return valve [6 with three variable metering devices 17. I8. I9 arranged in parallel to regulate fuel tlow to a primary burner 20. a gutter 21 and a collandcr 22 respectively ot&#39;the engine reheat system (FIG. I).  
  The metering assembly 14 includes a first control means in the form of a three-dimensional cam 23 which is fixedly secured to the rod 24 of a piston 25. Cam 23 l is formed with a series of profiles which correspond to functions of compressor intake temperature T, for a (ill number of values of compressor delivery pressure P- Piston 25 is subjected on opposite sides to servo pressures derived from the pressure at the inlet 15. One side of piston 25 communicates via a valve 26 with a chamber 27 and also via a restrictor 28 with the downstream side of valve I6. Restrictor 28 and valve 26 thus provide a variable fluid pressure potentiometer arrangement.  
  The other side of piston 25 communicates via a valve 29 with chamber 27 and also via a resistor 30 with the downstream side of valve 16. valve 29 and restrictor 30 providing a further variable fluid pressure potentiometer. Closure members for valves 26. 29 are provided by a single lever 31 movable in opposite directions by bellows 32. 33 respectively.  
  Bellows 32 is responsive to an air pressure intermediate a pair of restrictors 34. and derived from the intake pressure P. of the engine compressor. Bellows 33 is responsive to an air pressure intermediate restrictor 35 and a further restrictor 36. and derived from the engine compressor delivery pressure P A spring 37 is engaged between lever 31 and an adjacent end of piston stem 24. A further spring 38 is engaged between lever 3] and an adjustable stop 39 on assmcbly I4. Chamber 27 communicates via an axial passage 40 in stem 24 with a further chamber 4] in which cam 23 is located. In use. for a given difference of pressure P; and P there will be a corresponding equilibrium position 25. and hence a corresponding axial position of cam 23.  
  Cam 23 is mounted on a pinion 42 so as to be axially slidable but non-rotatable relative to pinion 42. Pinion 42 meshes with a rack 43 which forms part of a piston 44. Piston 44 is responsive to the operation of an electrically operated servo-control valve arrangement 45 having a pair of interconnected inlet ports 46. 47. and a pair of outlet ports 48. 49. and an exhaust port 50. Valve arrangement 45 includes electrical linear transducer 44a which is responsive to the position of piston 44.  
  A valve spool SI is axially movable in response to pressure signals at its opposite ends. these signals being derived from the pressure at port 47 by a nozzle 52 pivotally movable by a torque motor 53 so as to be alignable with either of a pair of passages 54. 55 through which pressure signals are applied to the respective ends of valve spool 51.  
  The electrical signals from arrangement l4u which are applied to torque motor 53 are derived from the engine compressor intake temperature T and the output from transducer 441: The angular position of cam 23 is thus dependent on temperature T Variable metering devices l7. l8. 19 are angularly spaced about the axis of cam 23. In order that this arrangement may be portrayed diagrammatically in FIG. 2, a part of cam 23 is shown repeated adjacent the device 17.  
  Device 17 has a sleeve with a port 6] which communicates with inlet 15. A piston control element 62 is slidable in sleeve 6&#34; and includes a port 63 which cooperates with port 6l to provide a variable metering orifice. Control element 62 defines within sleeve 60 a chamber 64 which communicates with inlet 15 through a restrictor 65. Element 62 includes a stem 66 having a bore 67 which communicates with chamber 64. A ball closure member 68 is captive on stem 66 and is operable to shut off bore 67.  
  A cam follower 69 is pivotally mounted on sleeve 60 and engages cam 23. Control element 62 is urged to the right. as seen in FIG. 2. by the pressure downstream of ports 61. 63. This rightward movement is resisted by the pressure in chamber 64. If ball 68 is allowed to open by cam follower 69, the pressure in chamber 64 falls and element 62 moves to reduce fuel flow through device 17. With ball 68 shut the pressure in chamber 64 exceeds that downstream of ports 6|. 63 and arrests element 62. Control element 62 thus acts as a follow-up servo piston responsive to the position of cam follower 69. which is in turn responsive to pressure P. and temperature T,.  
  Devices [8 and 19 are generally similar to device 17 and include respective piston control elements 70. 71. Elements 70. 7] are, however, also rotatable within their associated sleeves. An edge cam 72 is secured to a shaft 73 which extends into chamber 4]. Cam follow ers 7211 are keyed to elements 70. 7] and are biased into engagement with cam 72. Shaft 73 is rotatable by a rack and pinion arrangement, 74 controlled by an electrically operated servo-valve arrangement 75. identical with valve arrangement 45 described above. Electrical signals applied to servo-valve 75 are dependent on the position H of a power demand control for the engine. Control elements 70, 7] are thus responsive to pressure P temperature T, and control position 0.  
  The fuel pressure DP immediately downstream of device 17 communicates via a passage 8&#34; with a servo pressure control arrangement 8| (FIG. 1) shown in detail in FIG. 3. The downstream side of device 17 also communicates with a throttle valve 82 which has a spring-loaded piston control element 83 responsive to pressure DP and to a servo pressure S1 to regulate fuel flow from device l7 to an outlet passage 84. Servo pressure Si is obtained in a manner later to be described and is applied to element 83 via a passage 85.  
  Downstream of device 18 is a throttle valve 76 having an outlet 76:: and a piston control element biased against the pressure DP2 downstream of device 18 by a spring and by a servo pressure signal S2. derived in a manner to be described and applied via a passage 77. Downstream of device I9 is a throttle valve 78 identical with valve 77 and having an outlet 78a. The piston control element of valve 78 is responsive to the pressure DP3 downstream of device 19 and also to a servo pressure S3 derived in a manner to be described and applied via a passage 79.  
  Arrangement 8! has a valve spool 86 urged in one direction by the delivery pressure P, of pump and in the opposite direction by the pressure DH and by a governor mechanism 87 responsive to the engine speed. Valve spool 86 is coupled by a torsionally stiff spring 88 to an auxiliary piston 89 whose side remote from spool 86 communicates via a sequence valve arrangement 90 (later to be described) and via passage l3 with the pressure PS2 on pump throttle ll. Auxiliary piston 89 and spool 86 slide in a sleeve 91 provided with ports 92, 93 which respectively communicate with pressure PS2 and with pump delivery pressure P,,. The ports 92. 93 are axially spaced by less than the length of piston 89 so that at the rightward end ofits travel (as seen in FIG. 3) piston 89 is subjected on both sides to pressure PS2. and the leftward end ofits travel to pressure P,, on both sides. The force due to the difference between pressures P,, and PS2, and applied via spring 88 to spool 86. is thus limited.  
 (ill  
  Sleeve 91 also has a port 94 communicating with pressure PS1 via passage 12. a port 95 communicating with a servo pressure PS3. derived via valve arrangement 90. a port 96 communicating with a low pressure LP. which is conveniently the output pressure of a boost pump, not shown. in a fuel tank associated with the engine. Port 94 communicates with the bore of sleeve 91 via a pair of restricted passages. The arrangement is such that in a mid-position of spool 86 both port 95 and port 96 communicate with port 94 via the respective restricted passage, which thus act as a fluid potentiometer.  
  An increase in pump delivery pressure P,, urges spool 86 to the left, permitting increased flow between ports 94. 96 and decreased flow between ports 94, 95. If servo pressure PS3 is high, servo pressure PSI falls and throttle ll in pump [0 moves to reduce fuel flow. This movement causes a fall in pressure PS2 which is transmitted via valve arrangement 90 to urge piston 89 to the right to oppose movement of spool 86. Piston 89 thus provides a position feedback until pump delivery pressure P,, has risen sufficiently to restore an equilibrium condition. An increase in engine speed N or in pressure DPl similarly causes pump delivery pressure P,, to be increased. Pump delivery pressure P, at inlet 15 of metering assembly 14 is thus varied to maintain the difference between the pressures DPl and P,, substantially constant for a given engine speed, this pressure difference being variable with engine speed. The weights 87a of the governor mechanism 87 have a relativcly low specific gravity, whereby, when a denser fuel is used. the effect of engine speed on spool 86 is reduced.  
  Sleeve 91 includes a further port 97 which is normally in communication with port 96, but which is shut off by the valve spool 86 when the latter has moved sufficiently to the right to shut off port 96 from port 94.  
  Arrangement 81 also includes a spool valve 100 having a spool 10] responsive to pressure DH, and to pressure DP2 which is obtained, as above described, from metering assembly 14 and opposes pressure DP]. Surrounding spool 10! is a sleeve 102 having a port 103 communicating with pump delivery pressure P,,, a port 104 communicating via passage 77 with throttle valve 76 in metering assembly [4. to apply servo pressure S2 thereto, and a port 105 communicating with port 97 and hence normally with low pressure.  
  A piston element 106 is coupled to spool 10] by a torsionally stiff spring 107. By virtue of leakage around piston element 106 the latter acts as a dashpot to slow movement of spool 10] A further spool valve H0 is substantially identical with valve 100 and has a spool I ll movable in response to pressure DP] and to a pressure DP3 obtained, as above, described from the downstream side of metering device 19 in arrangement 14, to control the value of servo pressure S3 at a port 112. Port 112 communicates via passage 79 with throttle valve 78 in arrangement l4.  
  Spools 86, 101, Ill and governor 87 are rotated via a gear train by a shaft I13 driven by the engine.  
  Associated with metering assembly 14 is a shuttle valve arrangement 120, shown diagrammatically in detail in FIG. 4. Arrangement comprises a pair of shuttle valves 12], 122 having respective ball control members 123, 124. Valve 12] is connected by passages [25. 126 to pressures PF2, PF3 on the downstream sides of the respective throttle valves 76. 78 in assembly 14. Valve I23 has been an outlet I27 which forms one inlet of valve I22. The other inlet of valve I22 communicates via a passage I28 with the pressure PR in chamber 4I in assembly I4. The outlet of valve I22 communicates via a passage I29 with throttle valve 82 in assembly I4. The pressure in passage I29 provides servo pressure SI. Pressure SI is thus equal to which ever of pressures PR. PF2 or PF3 is the largest.  
  When the engine reheat system is in use. fuel is supplied by the pump to the assembly I4 and passes via a device I7 and throttle valve 82 to passage 84. and thence via a distributor arrangement I30. later to be described in detail with reference to FIG. 5. to the pri mary burners of the reheat system.  
  The output pressure P of pump I0 is controlled as above described to maintain pressure DPI substantially constant for a given engine speed and fuel density. A rise in pressure DPI urges spool IOI. in arrangement 8]. to the right. causing the value of pressure S2 to rise towards pump delivery pressure P,,. Throttle valve 76 progressively closes until pressure DP2 has risen to equal DPI. restoring equilibrium conditions. Similarly. a rise in pressure DP2 moves spool IOI to the left and throttle valve 76 opens until equilibrium is restored. Spool valve IIO in arrangement 8I operates similarly to maintain pressure DP3 equal to pressure DPI.  
  Valve 82 in metering assembly 14 acts as a pressurising valve. The springs in valves 76. 78 provide only light preloads to ensure that these valves are shut when the system is closed down. The spring in valve 82 is such that. in the absence of servo pressure SI. pressure DPI is just sufficient to operate the spools 86. &#34;II. I l l in the servo pressure control arrangement 8|. At low flows. therefore. the system pressure is low and pumping power loss is a minimum.  
  At higher rates of reheat fuel llow. pressure PF2. PF3. increase. and the highest of these pressures is applied via the shuttle valve arrangement I to provide servo pressure SI. Servo pressure SI assists the spring in valve 82 to raise the system pressure. The consequent rise in pressure DPI increases the values of servo pressures S2 and S3 to enable throttle valves 76. 78 to maintain the increased system pressure.  
  At a low value of reheat fuel flow. i.e. at low values of compressor delivery pressure P2 and of angle 0 of the power demand control the pressure in chamber 64 of valve I7 may become equal to the pressure P in chamber 41 and. if in excess of pressure DPI will urge valve I7 fully shut. Pressures PFZ and PF3 are also low. In these circumstances servo pressure SI becomes equal to pressure PR in chamber 41 of arrangement 14. The system pressure is thereby maintained high enough to prevent pressure PR from urging control elements 62. 70. 7I in the respective metering devices I7. I8. I9 to positions in which reheat fuel flow is shut off.  
  The distributor arrangement I30. shown in FIG. 5 has a housing I3] with inlet ports I32. I33. I34 respectively connected with the outlet passages 84. 76a and 7811 respectively of throttle valves 82. 76. 78. Associated with the respective inlet ports I32. I33. I34 are outlet passages I35. I36. I37 which respectively com municate with the primary burner 20. gutter 21 and collander 22 of the engine reheat system.  
  Between the inlet ports and their associated outlet passages are respective valve arrangements I38. I39. I which are operable to connect outlet passages I35.  
 (all  
 I36. I37 either to their associated inlet ports or to a gallery 141 which communicates via a fourth valve arrangement I42 with a lovv pressure drain outlet I43.  
  Valve arrangement 140 comprises a pair of frustoconical seats I44. 145 in the housing I3I. A dumb-bellshaped control member 146 has part spherical end portions I47. I48 respectively engageable with seats I44. I45. Member I46 is biased by a spring I49 in a direc tion to engage portion I47 with seat I44 and thereby to isolate inlet port I34 from outlet passage I37. In this position of control member I46 outlet passage I37 communicates with gallery MI.  
  Member I46 is movable against spring I49 by an axially aligned piston I50 responsive to servo pressure PS3 in a further gallery 15] within the housing I3]. Supply of pressure PS3 to gallery I5] is controlled. in a manner to be described. by sequence valve arrangement 90. and by a relay valve I (FIG. I) to be de scribed with reference to FIG. 6. Pressure PS3 is of sufficient magnitude that control element I46 is readily movable against spring 149 and the pressure in inlet port I34. Valve arrangement I40 thus has a bangbang&#34; action.  
  Portion I47 is slid-able with clearance in a sleeve I52 forming part of housing I3]. Portion I48 is radially movable with respect to piston I50. Control element I46 is thus self aligned with the valve seats I44. I45.  
  Valve arrangements I38. I39 are substantially identical to valve arrangement I40. differing only in their di mensions. Arrangements I38. I39 are also responsive to servo pressure PS3 in gallery 15].  
  Valve arrangement I42 has a frusto-conical seat I53 with which a part-spherical closure member I54 is engageable. Valve arrangement I42 is biased shut by a spring I55 and is movable against spring I55 by a piston I56 in response to a servo pressure PS4. also derived via sequence valve arrangement 90.  
  Sequence valve 90 is shown diagramnuitically in FIG. I and comprises a sleeve having a plurality of ports. Within the sleeve is a control element rotatable by the shaft 73 (FIGS. I and 2) to selectively interconnect the ports. As shown in FIG. I the control element has a number of metering edges which co-operate with the associated ports so as to interconnect only those ports shown associated with a particular metering location on the control element. Rotation ofthe control element is best considered as being in live anticlockwise steps. B to F. from an initial position A as seen in FIG. I. Four metering locations I70. I7I. I72. I73 are shown.  
  A high pressure source. conveniently the delivery pressure of the engine main fuel pump. is connected to ports I74. I75 at metering locations I70. I7I respectively. Ports I76. I77 at location I are respectively connected to passage I3 of pump I0 and to the PS2 connection of pressure control arrangement 8]. Thus in positions A and B ports I74. I are interconnected and in positions C to F ports I76. I77 are interconnected.  
  At metering location I7I port I75 is connected to a port I78 in position A onlyfand to a port I79 in positions C to F. The pressure at port I79 provides servo pressure PS3. and that at port I78 provides servo pressure PS4. The low pressure LP referred to above is connected to ports I80. I8] at metering locations I72. I73 respectively. A further port I82. at location 172. is connected to port I78. the arrangement being such that pressure PS4 is at high pressure in position A and at low pressure in positions I3 to F. A port I84 at location I72 is connected to port I79 at location I70. so that pressure PS3 is a low pressure in positions A and B. A port I83 at location I73 is connected to the outlet of the pump I0. so that the pump outlet is connected to low pressure in position A only of the control element. The chamber 4I in metering arrangement I4 is permanently connected to a low pressure via a passage 185 in the sequence valve 90. The reference pressure PR is thus substantially equal to this low pressure.  
  The positions A to F of the shaft 73. and hence of the sequence valve control element. correspond to positions of a pilots control lever (not shown) for the engine. Positions A and B correspond to an inoperative condition of the reheat system. movement between positions B and C corresponds to light-up or shut down of the reheat system. depending on the direction of movement. and positions D to F corresponds to different lev els of reheat flow as controlled by cam 72 (FIG. 2).  
  Relay valve I60 is shown in FIG. 6 and comprises a housing 190 within which are a pair of axially aligned nozzles I9I. I92. whose respective bores communicate via passages I93. I94 with pressures PS3 and with the gallery MI in distributor arrangement I30. A lever I95 is pivoted in housing I90 and carries a selt&#39;aligning closure I96. Lever I95 is biased by a spring I97 so that closure I96 shuts off nozzle I9I. Lever I95 is movable against spring I97 by the pressure PS3 to open nozzle I9I and shut nozzle I92. A passage I98 interconnects the inside of housing 190 with gallery I51 in arrangement I30. When pressure PS3 is sufficiently high to overcome spring 197. passages I93. 198 are interconnected and the valves in the distributor arrangement I30 are operated to supply fuel to the engine reheat system. If pressure PS3 is low gallery II is connected via passage I98. I94 to gallery I4]. and valve arrangements I38. I39. I40 operate to shut off fuel supply to the reheat burners. It pressure PS4 falls to a low value. valve I42 opens to dump l&#39;uel within those burners via passage I43 to a drain I99.  
  The fuel control apparatus also includes a priming valve arrangement 200. shown in detail in FIG. 7. Arrangement 200 is connected in parallel with metering assembly I4. between the outlet of pump 10 and the inlet ports of the distributor arrangement I30. Valve arrangement 200 includes a piston-like control member slidable in a cylinder 202 and biased by a spring 203 towards a stop 204. The bore of cylinder 202 communicates via a passage 205 with the inlet of pump I0.  
  Piston 201 has an enlarged head 206 which is subjected. via a filter 207 and a flow restrictor 208 to the pump delivery pressure P,, in passage 205 to urge piston 20I against spring 203. Surrounding piston 20! within cylinder 202 are annular passages 209. 210. 2l 1. which communicate. via respective nonreturn valves 2I2. 2 l3. 2 l4. and passages 215. 216. 2I7 with the respective inlet ports I32. I33. I34 of distributor arrangement I30. Passage 2I I communicates with a side of the piston head 206 which is remote from that subjected to pressure P,,.  
  Piston 20] has an internal blind bore which communicates with ports 2I8. 2I9. 220 in the wall of piston 20]. In the extreme rightward position of piston 20] as seen in FIG. 7. ports 2l8. 219. 220 are clear of the respective annular passages 209. 210. 21 l. The permitted travel oi piston 20] is such that ports 218. 2I9. 220  
 communicate with respective passages 209. 210. 2] I to supply fuel to the distributor arrangement I30. In the extreme leftward position of piston 20! the ports 2I8. 219. 220 are again clear of the respective annular passages. In this leftward position pump delivery pressure P,, communicates with a passage 22] connected to a &#34;hot-shot ignition device 222 (FIG. I) of a known typev The rate at which piston 201 moves leftward. and hence the quantity of fuel delivered to each reheat burner is dependent on the size of restrictor 208. The piston 20I is maintained in its leftward position by pump delivery pressure P,, so long as the reheat system remains operative. When the reheat system is shut down pressure P,, becomes effectively equal to the pressure at the pump inlet. and the piston 20] is returned to its initial position by spring 203.  
  Device 222 is of a known type and is responsive to pressure P,, in passage 221. when servo pressure PS3 in passage I93 is high. to supply a measured quantity of fuel to nozzles 223. 224. which are respectively in the engine combustion chamber and downstream of the engine turbine. thereby applying a flame jet to the reheat burners 20. 21. 22 to ignite the reheat system.  
  In use, with sequence valve in position A high pressure will be applied to passage I3 of pump I0 and throttle II will be urged fully shut. Pressure PS4 will be high. opening gallery in distributor I30 to the drain I99. PS3 will also be low. allowing fluid in passages I35. I36. I37 to be dumped to drain 199. via valves I38. I39. I40 in distributor 130. The low value of PS3 ensures. via pressure control arrangement 81. that servo pressure PS1 is also low, and movement of throttle II to reduce pump delivery is not therefore opposed. The pump outlet is connected to low pressure via sequence valve 90. The reheat system is thus shut down.  
  When sequence valve is moved to position B, pump passage I3 remains at high pressure and PS3 remains at low pressure. PS4 however, falls to a low pressure, shutting off the dump valve I42. in distributor 130. The pump outlet is also isolated from low pressure. Position. B of valve 90 is thus a position prior to lighting the reheat system. Valves I38. I39. I40 and I42 are shut. and this condition may be maintained as long as required in readiness for operation of the reheat system.  
  When valve 90 is moved to position C. pump passage [3 is isolated from high pressure and connected to pressure control arrangement 81 to provide servo pressure PS2. Pressure PS3 becomes high. enabling pressure PSI to rise to increase pump output pressure. Relay valve I60 is energised to apply PS3 to distributor I30 and open valves I38. I39. I40 to permit fuel flow to the reheat system. Fuel from pump 10 is delivered via passage 205 to priming device 200. from which measured quantities of fuel are discharged via passages 215. 2I6. 217 and distributor I30 to the respective burners 20. 21. 22.  
  When the priming valve arrangement 200 completes its priming operation. hot-shot device 222 is enabled. by virtue ofpressure PS3 to ignite the reheat system via nozzles 223. 224. Priming arrangement 200 completes its operation during the time in which metering assembly I4 starts to deliver fuel to the reheat system.  
  Subsequent rotation of shaft 73 to position F by rack and pinion 74 (FIG. 2) in response the position 60f the engine power demand control. does not affect the operation of valve )0. but causes metering arrangement 14 to increase fuel llow to the engine.  
  FIG. 8 shows a form of priming valve arrangement, which is an alternative to that shown in FIG. 7. This alternative form has a piston control member 236 which is movable within a bore 231 against a spring 232 by pump delivery pressure P,,. applied via passage 205. Annular passages 233. 234. 235 surround piston 2311 within bore 231 and respectively communicate. via non-return valves 212. 213. 214. with the passages 215. 216. 217. Valves 212. 213. 214 are identical to the correspondingly numbered valves described with reference to FIG. 7. Ports 236. 237. 238 extend through the wall of piston 230. as before. A chamber 241 on the side of piston 230 remote from passage 205 communicates via a llow restrictor 239 and a non-return valve 240 with low pressure. Chamber 241 also communicates with passage 233 via a restrictor 242.  
  In use piston 230 is moved. as before. against spring 232 by pressure P,, so that ports 236. 237. 238 move past passages 233. 234. 235 to delivery measured quantities of fuel to the reheat system. At the rightward extent of its travel piston 230 uncovers passage 221 which communicates. as before, with the &#34;hot-shot&#34; device 222. The rate at which piston 230 moves. and hence the quantity of fuel delivered to each burner. is dependent on the size of restrictor 242. Piston 230 is maintained in its rightward position, while pressure P,, is applied to passage 205. by the low pressure in chamber 24] resulting from leakage to low pressure via nonreturn valve 240 and restrictor 239.  
  The further form of priming arrangement shown in FIG. 9 has a piston 250 movable against spring 251 by pressure P,,. The rate of movement of piston 250 is dependent on the rate of flow through a restrictor 252. An annular passage 253 communciates with passage 205 and piston 250 has a single port 254 which cooperates with passage 253 to deliver a measured amount of priming fuel to the reheat burners. as before. Delivery of fuel to the burners is via the non-return valves 212. 213. 214 and respective passages 215. 216. 217 described above.  
  A fuel control apparatus which includes the device of FIG. 9 does not include a hot-shot ignition device 221. Instead. piston 250 includes a pair of passages 255. 256 which are respectively operable, aftcr piston 250 has moved to complete its priming function. to connect the high pressure source to nozzle 223 in the engine. and to connect passage 205 to nozzle 224 in the engine. Connections to nozzles 223 and 224 are made via respective spring-loaded valves 257. 258. Valves 257. 258 are also biased shut by the low pressure LP.  
  In use after piston 250 has moved sufficiently to the right to complete the priming operation, passage 255 permits high pressure flow to nozzle 223. Piston 250 continues to move to the right. shutting off the above high pressure How after approximately 0.25 seconds. Shortly after the high pressure flow has begun. passage 256 permits fuel at pump delivery pressure P,, to flow to nozzle 224. At the extreme rightward position of piston 250 both the passages 255. 256 are clear of their associated ports and flow to nozzles 223. 224 ceases. Piston 250 is maintained in its rightward position as a result of fuel via a restrictor 258.  
  The alternative form of priming valve arrangement shown in FIG. 10 includes a piston-like control member 260 slidable in a cylinder 261 and biased by an extension spring 262 towards a stop 263. Pump outlet 15 communicates with cylinder 261 via passage 205 and a restrictor 264 so that the delivery pressure of pump 10 opposes spring 262. Outlet 15 also communicates with an annular passage 265 communicating with the bore of cylinder 26] and closed by member 260 when the latter is against stop 263. Member 260 has ports 266 which can communicate with passage 265 as member 2611 is moved against the bias of spring 262. The permitted travel of member 261] is such that in its extreme rightward position. as seen in the drawing. ports 266 are again clear of passage 265. In this position member 260 uncovers the passage 221 which in this case interconnects cylinder 261 and the ignition device 222.  
  Downstream of control member 260 cylinder 261 communicates via non-return valves 212. 213. 214 with passages 215. 216. 217. The upstream side of valves 212. 213. 214 communicate. via a restrictor 267 and a further non-return valve 268. with the inlet of the pump 10. Valve 268 is such that it lifts off at a relatively low pressure.  
  In use. piston member 261) is moved. as before. against spring 262. Ports 266 first move into alignment. and subsequently out of alignment. with passage 265. As ports 266 move past passage 265 fuel flows via nonrcturn valves 212. 213. 214 and respective passages 215. 216. 217 to the distributor 130. As this fuel flow increases the consequent reduction in pressure at pump oulet 15 causes throttle l l to be moved to maintain the output ilow. As ports 266 move past their mid-position relative to passage 29 the pump output pressure tends to increase and is again corrected. A small amount of fuel flows back to the pump inlet via restrictor 267 and relief valve 268.  
  With member 260 in its rightward position. flow through passages 215. 216. 217 ceases and passage 221 is uncovered to cause device 222 to ignite the reheat system. Since there is now no flow through ports 266. the pressure immediately upstream of valves 212. 213. 214 falls to a low value. Control member 260 is thus maintained in its rightward position so long as the delivery pressure of pump 10 is sufficient to overcome spring 262. Pump delivery pressure is. in fact, maintained by the servo pressure signal in passage 12 in response to engine operating parameters as above described.  
  When the reheat system is shut down. the pump delivery pressure becomes effectively equal to pump inlet pressure and the member 260 is returned to its initial position by spring 262.  
  The arrangement shown in FIG. 10 enables a priming fuel charge to be supplied to the reheat burners substantially immediately reheat is selected. the time taken being typically less than one second. this time being ef fectively dependent on the size of restrictor 264.  
 I claim:  
  1. A fuel control apparatus for a gas turbine engine reheat system. including a variable metering arrangement for controlling fuel flow to the engine reheat burners. a low pressure drain and a valve arrangement operable in one position thereof to connect said metering arrangement to said burners and in another position to connect said burners to said low pressure drain. said valve arrangement comprising first. second and third ports respectively communicating with said metering arrangement. said burners and said drain, valve seats between said first and second ports and between said second and third ports a closure member movable to selectively engage said valve seats. means for biasing said closure member towards engagement with one ot the seats. means responsive to a control pressure signal to move said closure member against said biasing means into engagement with the other of said seats a dump valve between said third port and said low pressure drain, and means responsive to a further control pressure signal to open said dump valve.  
  2. An apparatus as claimed in claim I in which said closure member is responsive to said control signal to connect said metering arrangement to said burners, and which includes means, responsive to a requirement to light the reheat system. tor providing said control signal.  
  3. An apparatus as claimed in claim 2 in which the means for providing said signal comprises a selector valve responsive to the position of an engine power demand control.  
  4. An apparatus as claimed in claim 1 in which said selector valve is operative to remove said further controi pressure signaL&#39;and thereby to close said dump valve before providing the first-mentioned control signal to connect said metering arrangement to said burncrs.  
  S. An apparatus as claimed in claim 4 which includes a relay valve operable to apply the first-mentioned pressure signal to said third port. whereby when said dump valve is moved by said biasing means to connect said burners to said low pressure drain 6. An apparatus as claimed in claim I in which said metering arrangement includes first second and third metering orifices responsive to engine operating parameters to control t&#39;uel tlow to respective ones of the engine reheat burners.  
  7. An apparatus as claimed in claim 6 in which said valve arrangement comprises a plurality of first ports respectively communicating with said first, second and third metering orifices. a plurality of second ports re spectively associated with said first ports and communicating with respective ones of said reheat burners. a plurality of third ports associated with respective groups of first and second ports and commonly connected to said low pressure drain and a plurality ofclo-