Patent GB-1200009-A (GB)
Title: Improvements in the operation of once-through steam generator systems
IPC: F22B29/12
Filed: 1968-12-23
Abstract: 1,200,009. Starting-up steam generators. COMBUSTION ENG. Inc. 23 Dec., 1968 [26 Dec., 1967], No. 60947/68. Heading F4A. A method of operating a once-through steam generator having furnace wall tubes and a superheater comprises the steps of establishing a small through-flow through the tubes at high pressure; decreasing the pressure of the small through-flow to a low pressure after traversal of the tubes; simultaneously recirculating at high pressure a portion of the through-flow through the tubes; passing the small throughflow at low pressure in heat exchange relationship with a recirculating portion at high pressure ; firing fuel in the furnace thereby increasing the temperature of the effluent from the tubes; passing the steam portion of the lowpressure through-flow to the superheater as the temperature rises to a point where steam becomes available at low pressure; and thereafter increasing the through-flow through the tubes and simultaneously decreasing a portion of throughflow recirculating through the tubes at high pressure. During normal operation of the steam generator shown, a high head feedwater pump 2 takes water from a hot well 27 and supplies it at supercritical pressure through an economizer 5 and a mixing vessel 8 to vertical furnace wall tubes 13. After passing through superheaters 19, 20, the steam drives a turbine 24 and passes to a condenser 25. During starting, a recirculating flow of water through the tubes 13, a recirculation line 32 and the mixing vessel 8 is established by a low-head centrifugal pump 12. With boiler throttle valve 17 closed, a through-flow of the order of 5% of the full load rating is then established by the feedwater pump 2. After passing through the tubes 13, the through-flow passes through a line 38 to a flash tank 39 and thence to the hot well 27. A boiler extraction valve 37 throttles the through-flow to maintain a pressure of 3500 p.s.i. in the tubes 13. Burner 34 is now lit, and eventually flashing occurs in the flash tank 39. The fluid recirculating through line 32 is cooled, via a heat exchanger 43, by the fluid passing through the line 38. Some of the fluid in line 38 is evaporated in the heat exchanger 43 before reaching the flash tank 39. Steam for cooling the superheaters 19, 20 and rolling the turbine 24 is supplied by passing steam, as required, from the flash tank 39 via lines 47 and 48 and a steam admission valve 49. To permit maximum cooling by means of the heat exchanger 43 of the recirculated flow in line 32, the pressure in the flash tank 39 is kept at a low value by operation of a spillover valve 50 to spill steam via a line 52 to the condenser 25. The turbine 24 is brought up to synchronous speed, and as the load on the turbine is increased, some of the steam flow being spilled through line 52 is transferred to line 48. Further increase in load is achieved by increasing the through-flow through the tubes 13 by opening boiler throttle valve 17. As the through-flow increases, the consequent increase in the pressure drop in the tubes 13, which is in opposition to the pressure rise produced by the recirculating pump 12, causes the flow through the recirculating line 32 to decrease. As the load is increased, the through-flow through the extraction valve 37 is reduced and stopped so that the entire through-flow passes through the throttle valve 17 to the superheaters, and the heat-exchanger 43 is out of use. As through-flow is further increased towards maximum load, the recirculation decreases until at, say, 70% load it stops, reverse flow in the line 32 being prevented by a check valve 33. In a modification, the pump 12 is replaced by a similar pump located in the line 32.