GB1410181A - Method of controlling the feed of a forced circulation steam generator - Google Patents

Abstract

1410181 Feed water control in forced circulation boilers SULZER BROS Ltd 18 Sept 1972 [24 Sept 1971] 43099/72 Heading F4A A method of feed water control in a forced circulation steam generator comprises the application to a feed controller of a signal formed from the combination of a predetermined base signal with one of two control signals derived from two of the operating parameters of the generator; at least under steady conditions the base signal and a first control signal being combined for feed control over a first load range and the base signal and a second control signal being combined over a second load range. In the arrangement shown a generator 9 receives feed water from feed pump 1 in accordance with command signals delivered along a line 47 to a feed controller 48 acting on a feed valve 3. A device 25 measures the water level in a separator 10 and the output of this device, after being compared at 27 with a constant set value 28, is limited to positive values at limiter 33 to provide a first control signal. Similarly a temperature measuring device 26 detecting between superheaters 11, 12 feeds its output to be compared at 37 with a load dependant set value 38 and thereafter to a positive value limiter 43 to provide a second control signal. A predetermined base signal is provided by 46 and this is summed with the inverted output of limiter 33 and with the uninverted output of limiter 43 at 45 to provide the command signal. In a first, lower, load range the signal at point 39 from device 26 is arranged to be negative, the limiter 43 will therefore produce no output. At the same time the signal at point 29 is arranged to be positive and limiter 33 will produce a signal which is subtracted from a constant base signal at 45 to produce a command signal depressed below the base level. As the load increases, the water level in separator 10 falls and this causes the output of limiter 33 to become less and allows the command signal to rise toward the base level. The feed rate increases until the end of the first load range is reached and the signal at point 29 becomes negative, limiter 33 then will produce no output and the command signal will comprise the constant base signal. As the load rises further under the constant feed conditions the temperature detected by 26 will increase until the second, upper load range is reached; at which point the signal at 39 becomes positive and limiter 43 produces an output which is added to the base signal. The command signal increases and the feed rate rises. The base signal is arranged to be constant up to the start of the second load range but then increases with increasing load, however in an alternative arrangement the base signal is constant throughout. When the load is reduced the above sequence is performed in the opposite sense. In one application of the invention the generator operates with sliding pressure and the first load range delivers sub-critical pressure steam and the second load range delivers supercritical pressure steam. Function generators 30, 40 having integral action are used in the circuit which delivers signals to the limiters 33, 43 and analog-digital converters are employed to interrupt the input to these generators when the signal becomes negative to avoid a run-away increase in output signal. In a modification (Fig. 3, not shown) it can be arranged that when both signals are positive generator 40 is blocked and when both signals are negative generator 30 is blocked.