GB1411955A - Furnace circuit for variable pressure once-through generator - Google Patents

Abstract

1411955 Once-through vapour generator FOSTER WHEELER CORP 20 Dec 1972 [27 Dec 1971] 58893/72 Heading F4A In a once-through vapour generator in which tube panels 14 define a furnace enclosure and receive water from a feed pump 34, separators 50 are provided to deliver water output from the enclosure at low loads into a downcomer 42 for natural circulation, a check valve 90 in the inlet portion 70 of the downcomer being operable to close the natural circulation loop for once-through operation at higher loads, and the natural circulation being assisted by a jet pump 72 provided in the downcomer and comprising a Venturi 76 and a nuzzle 74 through which water from the feed pump 34 is introduced into the Venturi. During start up of the generator the valve 90 is open and flow through the furnace enclosure circuit and feed flow through the jet pump are respectively about 45% and at least 10% of the full load flow. As the load increases the feed pump flow is increased to maintain this furnace circulation until about 45% load is reached at which point there is little or no driving force for natural circulation and the vlave 90 then opens automatically for the generator to continue to full load on once-through operation; the generator pressure in the upper load regions being supercritical. A portion of the water from the feed pump is passed by a line 84 to a point downstream of the jet pump and a valve 86 maintains a pressure ratio of about 1 : 33 between the pressure in the nozzle 74 and the pressure in Venturi inlet 78 to ensure proper operation of the pump. To prevent flooding of the separators 50 a valve 88 operates in response to the separators water level to direct water down a by-pass line 44. The furnace enclosure is defined by panels of parallel upright tubes and cross-over means are provided above the burner zone to transfer flow in the panels in an area of high heat absorption to those in an area of low heat absorption and vice versa. An economiser 38 is provided in the upper region of the enclosure and division tube walls define a convection pass containing primary 18 and final 20 superheaters. In the embodiment shown the generator is gas or oil fired and steam is passed to turbine stages 22, 26, condensed then returned to the feed pump via low and high pressure heaters and a deaerator, being discharged through the economiser into the downcomer. In the generator a slightly different arrangement is employed (Figs. 9, 10, 11, not shown). Feed pump flow is firstly to a circuit comprising a portion of the furnace enclosure, e.g. the centre portions of the lower side walls, is then mixed with the recirculated flow from the whole enclosure, if any, and the combined flow is delivered to the remaining portions of the furnace enclosure. Superheat temperature is regulated by water spray between the superheat stages.