GB1052417A - - Google Patents

Abstract

1,052,417. Steam generators. COMBUSTION ENGINEERING Inc. March 20, 1964 [March 25, 1963], No. 12021/64. Heading F4A. A method of starting a forced flow once through vapour generator, having vapour generating and superheating sections connected by valve means in parallel, the first valve means constructed for operation with critical pressure drop, the second of substantially larger flow area unsuited for such operation, circulation means forcing flow through the sections in series, and an adjustable restriction controlling flow from the superheater section to a load having a minimum initial flow requirement, comprises forcing substantially the same through flow of liquid through the sections and restriction while maintaining a preselected pressure in the generating section, establishing said minimum initial flow with the restriction adjusted to provide a superheater pressure substantially less than that producing a critical pressure drop across the valve means, opening first valve means to raise the superheater pressure to a higher pressure to eliminate the critical pressure drop, then opening second valve means while maintaining the generating section at said preselected pressure, to vary superheater pressure between said higher pressure and generating section pressure. In the super-critical steam generator shown, feedwater is forced by pumps 10, 12, Fig. 1 (not shown), through feedwater valves FW, FWB in parallel to economizer 16, generating section comprising centre and outer furnace walls 26 and steam cooled walls of the rear gas pass 28, a system of pressure actuated boiler throttle valves BT, BTB and superheaters 30, 32 from which steam passes through turbine control valves 36 to turbine sections 38, 42, 46 between which are reheaters 40, 44 and finally to a condenser 48. Condensate passes through a demineralizer 50, de-aerator 54 and feedwater heaters 52, 55, 56. Feedwater fed by pumps 10, 12 is recirculated through the generating section and conduit 64, connected to a mixing vessel 60, by pump 18. A flash tank or separator 60 is connected in parallel across the throttle valves by conduits 58, 62 containing valves BE, SA, respectively. The method of starting comprises closing valves BT, BTB, setting valve BE to automatic pressure control at a preselected pressure and with the feed pumps and valve FWB establishing a minimum flow through the generator section which returns to the condenser through a water drain valve WD. Recirculating pump 18 is started, the generator is fired and as the temperature rises fluid flashes to steam in the separator 60 part of which passes through valve SA to the superheater section returning to the condenser via valve SD. The control of valve BE is reset at a pressure higher than the highest saturation pressure of the fluid in the walls. If boiler water clean up is required the fluid temperature and recirculation flow are maintained constant until the solids concentration reaches acceptable limits, during this time L.P. steam may be used to heat and roll the turbine. When clean up is completed the firing rate is increased, producing in the separator increasing steam flow which is used to speed the turbine and establish a desired turbine chest pressure, excess steam returning to the condenser through valves SP. Valve BTB is gradually opened and valves BE, SP, SA closed, maintaining a preselected pressure in the furnace walls, and when the superheater pressure reaches a value above that producing a critical throttling pressure drop valve BT is opened. Valve BT is controlled by pressures in the furnace walls and in the superheater as indicated at 83, 85 and valves BT, BTB are prevented from opening below a preselected temperature in the furnace walls by a temperature actuated switch or valve 67. A hot restart is carried out by closing the BT, BTB valves, reducing the superheater pressure to the separator operating level and then following the normal starting sequence.