764,072. Gas turbine plant; centrifugal pumps. HOBSON, Ltd., H. M., and GREENLAND, L. S. Feb. 28, 1955 [March 15, 1954; March 31, 1954], Nos. 7429/54 and 9429/54. Classes 110 (1) and 110 (3). [Also in Groups XI, XXIV and XXIX] A gas turbine fuel system comprises a centrifugal pump arranged to circulate fuel in a loop through spill flow burners and another centrifugal pump arranged to supply fuel to the loop at the inlet side of its pump through a metering orifice controlled by a needle valve. The fuel system, Fig. 1, consists of a main centrifugal pump 1 drawing fuel from a pressurized fuel tank and delivering it through an altitude control valve 9 and speed control valve 8 into a loop including a secondary centrifugal fuel pump 2 and spill type burners 4. A plate-type non-return valve 23 is provided in the suction of the pump 2. To maintain a minimum fuel flow to prevent flame extinction, a bypass line including a filter 20, a calibrated jet 22 and a minimum flow valve 21 is arranged around the control valves 8, 9. The speed control unit 10 which is fully des. cribed in Specification 764,071 comprises a speed-sensitive element 5 and an accelerationsensitive element 6 which jointly control a leak-off valve 67. The valves 8, 9 are attached to spring-loaded pistons 33, 38 to the upper surface of which pressure-fluid is supplied through a line 30. Orifices 29, 34 allow the pressure fluid to.pass through the pistons 33, 38. The flow of fluid from beneath the piston 38 and therefore the position of the valve 8 is controlled by the leak-off valve 67. The flow of fluid from beneath the piston 33 and therefore the position of the valve 9 is controlled by a leak-off valve 36 regulated by a ram-pressure control 7. An overspeed governor 11 and a combustion temperature limiter 12 are also provided to control the position of the valve 8. The limiter 12 has a variable datum so that in the lower part of the speed range, the temperature is controlled so as to prevent compressor stall and in the higher part of the speed range to prevent excessive temperatures. When the system is used in a compound gas turbine having independent high and low pressure turbinecompressor rotors, the speed control unit 10 controls the speed of the low pressure rotor and the governor 11 prevents overspeeding of the high pressure rotor. The pilot's control lever 13 determines the setting of the speed control unit 10 and the position of the five fuel cocks 14, 19, 24, 27 and 28. In the position shown, the fuel system is arranged for operation between flight idling rotational speed (approximately 30 per cent of maximum speed) and maximum rotational speed. When the lever 13 is moved to the " stop " position, the valves 24, 27, 28 are moved through 90 degrees and a gear pump 3, which is driven from the engine through a speed-responsive clutch, is set in operation. The pump 3 then draws fuel from the burners 4 and spill gallery 26 through the cock 27 and returns the fuel to the tank through the ground idling valve 18 and cock 19. When the lever is moved away from the " stop " to the " ground idling " position, the valves 14, 19 and 28 are moved through 90 degrees. Fuel is then fed by the gear pump 3 from the tank via the cook 14 and filter 15, an adjustable jet 17 and ground idling valve 18 to the inlet of the centrifugal pump 1. From the pump 1 the fuel flows through the control valves 8, 9 to the inlet of the centrifugal pump 2. From the burners the fuel recirculates through the cocks 27, 28 to the inlet of the main pump 1. The pressure of the supply to the burners is then the sum of the pressure rises across the pumps 1 and 2. The gear pump 3 may be used to prime the system in flight by moving the control lever 13 to the " ground idling " position. In a modification of this arrangement a non-return valve is inserted in the spill line between the cock 28 and the inlet of the pump 1. The cock 14 may be left open under " flight idling " conditions in which case it is merely necessary in order to prime the system, to reduce the engine speed to cause the clutch driving the gear pump to engage. The centrifugal pumps 1, 2 speed control and altitude control valves 8, 9, filter 20, calibrated jet 22 and flow valve 21 are housed in a common housing 100, Fig. 7. The impellor of the pump 2 is triangular and that of the pump 1 rectangular in shape, Figs. 10 and 11 (not shown). The cocks 14, 19, 24, 27, 28 are also housed in a common housing and are actuated by cams on a shaft driven by a vane type servomotor described in Specification 764,071 controlled by the pilot's control lever 13. The gear pump 3 associated valves 16, 18 and the overspeed governor 11 are also contained in a common housing. The arrangement of the pumps 1, 2, gear pump 3 and the cocks 14, 19, 24, 27 and 28 in their various housings is described in detail.