Patent GB-839781-A (GB)
Title: Improvements in or relating to protective systems for torque-producing gas turbine engines for aircraft
Inventor: MCLEAN ARTHUR FREDERICK
IPC: F02C9/28
Filed: 1957-06-17
Abstract: 839,781. Gas turbine plant stop-gear. BRISTOL SIDDELEY ENGINES Ltd. June 5, 1958 [June 17, 1957], No. 19089/57. Classes 110 (3) and 122 (4). In an aircraft power plant including a gas turbine engine having a rotary compressor system and a turbine for supplying torque to a power consuming device, a protective system is provided and comprises means operable to reduce the supply of heat energy to the torquesupplying turbine thereby to reduce the torque developed by the turbine, the means being actuated by a triggering device which is responsive to three factors, namely the torque absorbed by the power consuming device, the pressure of compressed air in a given stage of, or at the discharge from, the compressor system, and the ambient air pressure, the arrangement being such that in operation, when the torque absorbed by the power-consuming device falls by more than a predetermined amount (which may vary with operating condition) below a value related to the values of the other two factors, the triggering device initiates operation of the means to reduce the supply of heat energy to the torque-supplying turbine. In the embodiment shown, the triggering device comprises a casing 12 in which is disposed a lever 10 pivoted at 11 and having at its other end a contact 13 disposed adjacent a fixed contact 14. The lever is acted on by a pressure PT produced in a fluid in response to torque being absorbed, the pressure being applied to diaphragm 18a and freely mounted piston 18, the piston rod 19 acting on the lever so as to tend to open the contact 13. The interior of the casing 12 is open to atmosphere through port 21 and so with increase in atmospheric pressure P1, the evacuated capsule 22 allows the lever 10 under the action of spring 16 to move contact 13 away from contact 14 and vice versa. The lever 10 is also acted on in a direction tending to close the contacts 13, 14, by a bellows 20, the interior of which is subjected to pressure P2 of air tapped from the engine compressor. Movement of the lever 10 is limited by an adjustable stop 17. The lever 10 is also acted on by the compressor pressure P2 in a direction tending to separate the contacts 13,14, the pressure being communicated to the chamber 23 by connection 24 and acting on diaphragm 25a which bears against a freely moving piston 25, the rod 26 of which bears on the lever 10. The moment of the diaphragm 25a on the lever is less than that of the bellows 20. The connection 24 incorporates a plate valve 28 having a small orifice 27 therein so that in the event of sudden fall in torque pressure PT accompanied by sudden fall of compressor pressure P2, pressure in the chamber 23 drops more slowly than pressure in the bellows 20 and so the diaphragm 25a acts to ensure that the contacts 13, 14 do not close under such transient operating conditions. In the event of sudden increase in compressor pressure, the plate valve 28 moves upwardly thereby opening by-pass passages so that the orifice 27 does not act to restrict build-up of pressure in the chamber 23. The circuit diagram of Fig. 3 shows the switch 50 of a pressure-operated triggering device such as that of Fig. 2 connected in parallel with a conventional centrifugally operated overspeed switch 51 to control the operation of a reversible motor actuator 52 for a main fuel shut-off valve for an engine unit. The actuator comprises an armature 53 and a split field winding including an " opening " coil O and a " shutting " coil S, the coils being supplied through switches Lo and Ls respectively which are interconnected for operation by the armature so that after the armature has acted to close the valve, the " shutting " circuit S is broken and the " opening " circuit O is closed ready for the next operation. Current is supplied from the positive line 65, through the fire emergency switch 68 and the fuel control switch 71 (which is in the opposite position to that shown when re-starting) to the pair of normally closed contacts 73, 74 of the electromagnetic relay 75, thence to " opening " coil O, armature 53 to negative line 78. When the fuel valve has moved to the open position, the switches Lo, Ls are moved to their alternative positions and current then flows through line 62 and indicator coil 63 to the negative line 78 so that the " open " position is shown on the indicator 60. When the plant is shut down, the fuel control switch 71 is moved to the position shown, whereupon current flows to the coil S so that the armature 53 acts to close the fuel valve. The fuel valve may alternatively be closed by switching the fire emergency switch 68 to the position opposite to that shown. During normal running of the plant the movable contacts of the triggering switch 50 and of the centrifugal overspeed switch 51 are connected through an arming switch 87 to the positive line 65. In the event of either switch 50 or 51 closing, current flows through coil 91 of relay 75 to the negative line and the relay is thus operated, the pair of contacts 93, 94 being thereby closed. A holding circuit is thus established through the coil 91 irrespective of whether the triggering switch 50 or the overspeed switch 51 remains closed or open, and can only be released by opening the arming switch 87. Operation of the relay breaks the circuit through the normally closed contacts 73, 74 and establishes a circuit through contacts 97, 98 and so through the shutting coil S whereby the fuel valve is closed. The relay also carries a normally closed pair of contacts 99, 100 which closes a circuit through coil 102 which indicates that the arming switch 87 is closed; also a normally open pair of contacts 104, 105 which close when the relay is operated and allow current to flow through coil 107 which indicates that the protective system has operated. The invention is applicable to aircraft gas turbine engines in which the power consuming device is an airscrew, a ducted fan or a bladed sustaining rotor and in which the engine has a single turbine driving both the compressor and the useful load, or a separate turbine driving the useful load, or has two turbines one of which drives only a compressor and the other of which drives a compressor and the useful load. The means to reduce the supply of heat energy to the torque supplying turbine may be either a fuel shut-off valve controlling supply of fuel to the gas turbine engine as a whole or only to a stream of working fluid passing through the torque-supplying turbine. Alternatively it may be a valve capable of diverting some or all of the working fluid from the torquesupplying turbine, or in the case of an engine receiving a major part of its heat through a heat exchanger, the reducing means may be a valve diverting the heating fluid from the heat exchanger.