GB890021A - Improvements in emergency speed governing system for a steam turbine - Google Patents

Abstract

890,021. Turbines; engine stop-gear. GENERAL ELECTRIC CO. March 24, 1958 [April 26, 1957], No. 9333/58. Classes 110 (3) and 122 (4). [Also in Group XXIX] An emergency speed governing system for a steam turbine comprising a main overspeed governor and a pre-emergency governor responsive to turbine speed for controlling the turbine over its normal operative range includes fluidpressure servomotor means operable by the pre-emergency governor or another governor responsive to turbine speed to shut down the turbine on overspeeding in the event of the main over-speed governor failing to do so. As shown in Fig. 1, a high pressure turbine 1 receives steam through a main control valve 7 under the control of a governor 8, and a stop valve 6 from the primary coils 5a of a steam generator 5 and a low pressure turbine 2 receives steam through a control valve 10 and a stop valve 9 from the reheat coils 5b of the steam generator. Over the normal operative speed range of the turbine a pre-emergency governor 14 through a relay 10a controls the valve 10 to adjust the speed according to load. A conventional over-speed governor 23 having a flyweight 23a on, e.g., the turbine reaching a speed 110% of its normal rated speed, trips a lever 20 to operate an emergency trip valve 18 to cut communication between lines 25, 11 through which pressure fluid is normally supplied to hold the valves 6 and 9 and the relay 10a in open position. With the operation of the trip valve 18 the valves 6 and 9 and the relay 10a close to shut down the turbines. As shown, the pre-emergency governor 14, it could be the governor 8 or another governor responsive to turbine speed, is connected to a piston valve 15 slidable in a fixed but adjustable sleeve 38 connected by lines 36, 39 to a supply of pressure fluid and a piston chamber 35 slidably mounting a single acting piston 33c connected by a rod to a piston control valve 32. On, e.g., the turbine reaching a speed 112% of its rated speed, the piston valve 15 moves upwardly against its spring to direct pressure fluid into the chamber 35 to move the piston 33c and the control valve 32 to cut communication between -the line 25 and a pressure fluid supply line 31 and open communication between the line 25 and an exhaust line so that the valves 6, 9 and relay 10a as before close to shut down the turbines. The trip-valve 18 may be reset manually through linkage 29, 30 and a knob 28 and an indicator 27 indicates that the main overspeed governor has been actuated. The speed at which the piston valve 15 operates to shut down the turbines is adjusted by a screwthreaded stop 41. In order to adjust the stop for a speed of, say, 112% rated speed without allowing the turbines to reach that speed, the sleeve 38 is raised to take up a clearance 41a corresponding to 3% of the speed range. Thus the piston valve 15 now has a less distance to travel before directing pressure fluid into the chamber 35 and the turbine will shut down at a lower speed, say, 109% rated speed, which is below the normal rating of the main overspeed governor and hence there is no necessity to lock the main overspeed governor out of action. Having adjusted the stop 41 to correspond to 109% rated speed, the sleeve 38 is released and the piston valve is then set to operate at 112% rated speed. In a modification, a " lock-out " valve 44, Fig. 2, is fitted between the trip valve 18 and the control valve 32. Normally, pressure fluid acting on unequal area lands 45a, 45b urges its valve piston 45 to the position shown, so that pressure fluid is supplied to open the valves 6, 9 and the relay 10a. On the pulling of a knob 47 the valve piston 45 is moved leftwardly to by-pass the trip valve 18 and connect the lines 25, 11 through passage 44f, chamber 44h and port 44e to still hold the valves 6, 9 and the relay 10a in open position. The main overspeed governor 50 comprising an unbalanced ring 50a operable by centrifugal force against a spring 50b to trip the lever 20 at 110% rated speed may then be tested without cutting off the steam supply to the turbines. The governor 50 is fitted with a nozzle 53a through which oil is injected under pressure into pockets 50c having restricted drain ports 50d on the manual operation of a valve 51. The injection of the oil unbalances the ring 50a at a lower speed, e.g. normal speed, to test whether the lever 20 will operate the trip valve 18 to shut down the turbines. On release of the valve 51, the governor automatically resets itself by the drainage of oil out of the ports 50d.