GB1089663A - Control systems for steam turbines and other prime movers - Google Patents

Abstract

1,089,663. Steam turbine plant; engine stop-gear. ENGLISH ELECTRIC CO. Ltd. March 16, 1965 [March 18, 1964], No. 11389/64. Headings F1K and F1T. A steam turbine 10, Fig. 1, driving an alternator 11 is controlled by a speed-responsive governor 16 driven by the turbine and acting through a hydraulic circuit 17 on a throttle valve 50 controlling the steam supply, whereby the turbine speed is governed during normal operation. The governor is also used to unload the turbine if the steam pressure at the main stop valve 13 or the vacuum in condenser 14 fall to predetermined low values. For this purpose, a speeder motor 16A can be energized by a contactor 18 to decrease the governor speed. The contactor 18 can be energized by a low-steam-pressure unloader 19 and by a low-vacuum unloader 20, separately or together. The unloaders 19, 20 comprise electrical switching means shown in detail in Fig. 2 (not shown). The unloader 19 is operated by a switch 23 sensitive to the steam pressure and by an unloading regulator 22 which compares the steam pressure with the oil pressure in the hydraulic circuit 17. The unloader 20 is operated by vacuum switches 24, 25, 26 and oil pressure switches 27, 28, 29. When the stop valve 13 is opened, a limit switch 21 is operated to activate the unloaders 19, 20 by connecting them to electric power supplies, the connection to the unloader 20 being via the vacuum switches 24, 25, 26. When the steam pressure has risen to 90% of its normal value, switch 23 closes to energize a relay which completes the activation of the unloader 19, the relay having a hold contact which retains the energization even if a subsequent drop of steam pressure below 90% opens the switch 23. If such a drop occurs, the unloading regulator 22 acts through the unloader 19 and the contactor 18 to energize the speeder motor 16A which acts on the governor 16 to reduce the opening of the throttle valve 50 whereby the turbine load is reduced from 100% to, say 10% of full load as the steam pressure at the stop valve 13 decreases, say, from 90% to 85% of the normal value. During full-load operation, if the vacuum in the condenser 14 falls to 26 inches, vacuum switch 24 closes and acts through the unloader 20, the contactor 18 and the speeder motor 16A to cause the governor 16 to reduce the opening of the throttle valve 50 until the turbine load has decreased to 75%. At this point, the oil pressure switch 27 operates to prevent further closing of the throttle valve 50. If the vacuum continues to decrease until it reaches 24 inches, vacuum switch 25 produces a further closing of the throttle valve 50, this action being terminated by the oil pressure switch 28 when the turbine load has decreased to 50%. If the vacuum now falls to 22 inches, vacuum switch 26 causes the throttle valve to close until the turbine load is zero, whereupon the oil pressure switch 29 operates a turbine trip 33 to close the stop valve 13. Consequently, the limit switch 21 de-activates the unloaders 19, 20. A further vacuum switch 34 is provided to operate the turbine trip 33 if the vacuum falls as low as 20 inches when the stop valve 13 is still open.