GB1489089A - Fuel systems for gas turbine engines - Google Patents

Abstract

1489089 Gas turbine engine fuel systems ROLLS-ROYCE Ltd 27 Jan 1975 [25 Jan 1974] 03505/74 Heading F1G A gas turbine engine fuel system comprises two substantially similar constant-displacement fuel pumps connected in parallel upstream of a fuel control unit, a spill valve associated with each pump being controlled in accordance with the pressure drop across the control unit so that it spills substantially the entire flow of its respective pump when the pressure drop exceeds a predetermined value, the spill valves operating at different predetermined values whereby substantially the entire flow through one pump spills at a lower pressure drop than that at which substantially the entire flow through the other pump spills. The constant displacement pumps 16, 18 and their associated non-return valves 76, 78 are connected in parallel between a backing pump 12 and the fuel control unit 28, respective spill valves 44, 46 being provided to spill fuel via a spill duct 48 to the lowpressure side of the pumps 16, 18. The pressure drop HP-P across the control unit 28 is applied via ducts 62 and 66, 64, 68 to respective pistons 50, 52 to tend to open the respective spill valves 44, 46 against the action of respective springs 58, 60. Spring 60 is stronger than spring 58 so that the spill valve 44 opens before the spill valve 46 opens. When the engine is started, both pumps 16, 18 supply fuel via the control unit 28 to burners 32. Spill valve 44 begins to open when the pressure drop (HP-P) has risen to 200 psi and is fully open at 250 psi. Substantially the whole flow to the burners is now supplied by pump 18 alone. Spill valve 46 begins to open at 260 psi and is fully open at 310 psi, so that during normal operation, in which HP-P has between these two values, valve 46 is partially open to permit a degree of spill from the pump 18. If the pump 16 fails during normal operation, there is no effect on the fuel supply. If the pump 18 fails instead, the resultant decrease in HP-P reduces the opening of the spill valve 46. If there is total failure of the pump 18, spill valve 46 closes fully and the spill valve 44 begins to close to provide an output from the pump 16, HP-P then stabilizing at a value between 200 and 250 psi. Pressure switches 80, 82 indicate which of the two pumps is supplying fuel to the engine. To test the system in a preflight check, with HP-P at its normal operating value a solenoid 86 is operated to cause a valve 70 to disconnect duct 68 from duct 66 and connect it instead to a duct 74 leading to the low-pressure side of the pumps, thereby simulating a failure of the pump 18. The resultant decrease in HP-P causes the pump 16 to resume operation as before, this being indicated by pressure switch 80.