Patent GB-781764-A (GB)
Title: Improvements in and relating to control gear for steam turbines
Inventor: PETERS ROBERT WILLIAM
IPC: F01D17/26
Filed: 1954-05-07
Abstract: 781,764. Steam turbines. ENGLISH ELECTRIC CO., Ltd. May 6, 1955 [May 7, 1954; June 18, 1954; Nov. 5, 1954], Nos. 13384/54, 17955/54 and 32090/54. Class 110 (3). [Also in Group XXIX] In control gear for steam turbines having load limiting devices responsive to boiler pressure and condenser vacuum, a normally open switch in the circuit of the speed-setting motor is closed to cause operation of the main steam throttle valve to its no-load position when the boiler pressure or the vacuum or both drop below predetermined values and is opened when the boiler pressure or the vacuum is restored whilst the main throttle valve remains in its no-load position until re-set. As shown in the drawing accompanying the first Provisional Specification a centrifugal governor 1 acts through a linkage 3, 206, 205, 8 on a primary leak-off valve 15 to control the pressure acting on a device 21 which in turn operates the pilot valve 30 of a servomotor piston 25 operating the main throttle valve 28 in a manner similar to that described in Specification 677,097, [Group XXIX]. Speed adjustment is effected as in Specification 677,097 by rotation of the screwed spindle 8 in a nut 9 through bevel gearing 14 by a hand wheel 13 or through worm gearing 12 by the motor 11. The motor 11 is in circuit with the normally open switch 207, shown as a tilting mercury switch, and a fourposition throw-over switch 212 operated by a push-pull rod 214. The switch 207 is connected to an intermediate point of a floating beam 215, the ends of which are articulated to an extension 10 of the spindle 8 and to a spring-loaded piston 69. The piston 69 is connected to a secondary fluid system 154, 254 including leak-off valves - 152, 250 and isolating valves 153, 253. The valve 152 is opened in response to a fall in boiler pressure through a fluid-pressure servo mechanism as described in Specification 750,343, [Group XXIX]. The leak-off valve 250 is opened by a drop in vacuum and is connected to a piston 251 and a spring- loaded diaphragm 249 connected on its under side to the condenser. The opening of either leak-off valve causes the piston 69 to descend and the switch 207 to close. The motor 11 then raises the valve 15 to vent the hydraulic fluid system 52, 53 and cause the throttle valve 28 to move to the no-load position. At the same time, the beam 215 is tilted clockwise to re-open the switch 207 and the motor 11 stops. The switch 212 is shown in the "Auto" position. The valve 28 can only be restored to the open position by moving the switch 212 to the " Speed up " position in which the motor 11 is connected directly across the battery 213. The switch 212 also has a " Speed Down " position and an " Off " position. The Specification also describes with reference to the drawing accompany - ing the second Provisional Specification a further control gear similar to that described above but in which the floating beam 215 is dispensed with and the normally open switch is constituted as a micro-switch closed as before by the fall of pressure in the secondary fluid system which in this case acts on a spring-loaded bellows. This bellows is opposed by a further bellows open to the primary fluid system 52, 53, the drop in pressure in which following the operation of one or other of the load-limiting devices causes the micro-switch to be re-opened. A normally closed switch in series with the micro-switch opens when the pressure in the primary system drops to a value corresponding to the minimum load intended to be left on the turbine. The micro-switch may have an additional microswitch in parallel therewith, the first switch presetting the motor circuit and the second switch making the circuit. Fig. 1 of the third Provisional Specification shows a further control gear in which two micro-switches 307, 308 controlling the operation of the speed-setting motor 11 are operated through a floating beam 415 by a piston 69 connected to a secondary fluid system 154, 254 including leak-off valves 152, 250 responsive to a fall of boiler pressure and condenser vacuum respectively. The motor 11 operates through gearing 12, 410, 409 to cause the gear 409 to rise on a threaded stem 408 and operate through a floating beam 405 a pilot valve 30 controlling the servo piston 25 of the main throttle 28. The descent of the latter causes the beam 415 to be re-set and the switches 307, 308 to be re-opened. Fig. 2 of the third Provisional Specification shows a circuit diagram in which the field and armature windings 11a, 11b of the motor 11 are connected in series for load increasing and load decreasing purposes by contactors RC, LC respectively only one of which may be energized at once. The micro-switches 307, 308 are connected to a twoposition switch S1 which in the position shown is set for automatic operation and is in circuit with an indicator lamp L1, with an alarm A through contacts LCe, and with the energizing coil through a pressure-operated switch 63 and contacts RCc. In the alternative or manual position the contacts S1b are closed, bringing an indicator lamp L2 and a three-position switch S2 into circuit. Both sets of contacts S2a, S2b may be open or contacts S2a may be closed for load increasing or the contacts S2b may be closed for load decreasing. In a further circuit, the switch S2 is replaced by a contactor energized by closure of the micro-switches 307, 308 and with this arrangement the load is removed automatically following a fall in vacuum irrespective of the position of the switch S2. The first Provisional Specification describes also an electromagnetically-operated pilot valve 226 which over-rides the valve 30 when the steam pressure detected by a diaphragm 222 is above 50 per cent of full load and simultaneously the generator load detected by a watt-meter 224, is below 30 per cent of full load.