GB1246452A - Natural circulation start-up system for once-through vapour generator - Google Patents

Abstract

1,246,452. Starting-up steam generators. FOSTER WHEELER CORP. 9 Oct., 1968 [11 Oct., 1967], No. 47915/68. Heading F4A. A once-through type vapour generator is started-up by inducing an unassisted natural circulation of flow in the furnace riser tubes prior to changeover to pump assisted circulation for normal operation. During the latter operation a feed pump 22 forces water through an economizer 28 to a drum 30 via line 31, then via a downcomer 32 to the bottom of radiantly heated riser tubes 16 of front and rear walls, 18a, 18b, in the furnace section 14. The flow leaving the top of walls 18a, 18b is collected in a header 38 and then recycled through downcomer 24 to the bottom of riser tubes 16 of side walls 20a, 20b for a second pass through the furnace. The flow from the top of walls 20a, 20b is collected in header 40 and then passed through the roof 48 of the generator, convection wall passes 50 and superheater 52. The generator is started-up by establishing a water level in drum 30, closing valve 26 in downcomer 24 and opening valve 36 in line 34 so that downcomer 32 supplies furnace walls 18a, 18b, 20a, 20b, in parallel. On firing of the furnace burners, an upward natural fluid circulation through the furnace walls is established, the flow passing from the top of the walls to drum 30 through the headers 38, 40 and the now open valves 42, 44. At this stage valve 56 remains closed. Steam/water separators are disposed in the drum 30, steam from the drum flowing via line 46 and valve 58 to the remaining heating sections of the generator, whereas water from the drum is returned through downcomer 32 to the furnace walls 18a &c. The downcomer 32 and other components. such as the drum 30, are of sufficiently large size to lower the resistance in the circuits to such a degree that the difference in densities between the flow in riser tubes 16 and the flow in downcomer 32 is sufficient to achieve unassisted natural circulation up to about 30% full load. When this point is reached, valves 36, 42, 44 and 58 are closed and valves 26, 56 opened to changeover to normal once-through forced flow operation using the feed pump 22, the pairs of furnace walls 18a, 18b and 20a, 20b being in series. Operating pressures are approximately 1000 p.s.i. at start-up rising to 2500 p.s.i. at changeover and 3500 for full load supercritical operation.