GB1016807A - Control system for a forced flow once-through steam generator - Google Patents

Abstract

1,016,807. Controlling steam generators. COMBUSTION ENGINEERING Inc. Dec. 13, 1963 [Dec. 28, 1962], No. 49273/63. Heading F4A. [Also in Division G3] A method of controlling a forced flow once-through steam generator to maintain the final steam temperature and pressure substantially constant under varying loads comprises sensing the superheat temperature and providing rapid correction by controlled desuperheating. In the steam generator which may be either supercritical or subcritical a feed pump 10 forces the working fluid through flow measuring device 12, control valve 14, economiser 16, primary heating surface 18, desuperheater 36 and finishing heating surface 22 through valve 24 to a prime mover, e.g. a turbine which may drive an electric generator, the steam returning from the turbine to the pump 10 in conventional manner. The generator is heated by firing means 26 having its air and fuel supplies controlled by valves 30 and 34 respectively. The desuperheater may be of the indirect heat exchanger type or as shown, the direct contact spray type in which relatively cool working fluid is introduced through a flow regulator 40. The pressure of steam leaving the generator is sensed by device 42 and its output signal compared with a set point signal at summation point 44 to provide an error signal which is transmitted to controllers 46 and 48. An error signal representing the difference between desired and actual electrical output of the generator is fed to controller 52 the output of which passes to summation points 50, 56 where it is compared with the outputs from controllers 48, 46 respectively. The resultant signal from point 50 is used to regulate valve 24 to control the outlet pressure and the signal from point 56 is conveyed to summation points 66, 68. The outlet steam temperature T 3 is primarily controlled by adjusting the ratio of the fluid flow rate and firing rate. This however has a considerable time lag and rapid control is provided by adjusting the fluid flow to the desuperheater. To permit the desuperheater to both increase and decrease the temperature T 3 the said ratio is further adjusted to maintain the operating point of the desuperheater 36, as indicated by the temperature difference across it, (T 1 - T 2 ), at a desired value. The tempera tures immediately before and after the desuperheater T 1 , T 2 and temperature T 3 are sensed by devices 82, 76, 58 respectively. The output signal of 58 is compared with a set point signal at summation point 60 and the resulting error signal passed to controller 62. The output of this and of device 76 are then compared at point 78 and the resulting signal passed to controller 80 which operates valve 40. The outputs of devices 82, 76 are compared and the resultant signal compared with a set point signal at summation point 86, the output from which is combined at summation point 88 with the error signal from point 60 to provide the input of controller 64 from which a signal passes to summation points 66, 68. The signal from point 68 controls the air and fuel valves 30, 34 and the signal from 66 compared with a signal from the flowmeter 70 passes to controller 74 which controls the flow valve 14. The control means may be hydraulic, pneumatic or preferably electric using D.C. signals.