GB768201A - Improvements relating to forced flow once through tubulous vapour generating and superheating units and to the starting of turbines arranged to be supplied with vapour from such units - Google Patents

Abstract

768,201. Forced-flow steam boilers. BABCOCK & WILCOX, Ltd. March 24, 1955 [March 26, 1954], Nos. 8562/55. Class 123(2). A forced-flow, once-through steam boiler comprises serially-connected tube sections viz., a preheater 7, an evaporating section 8, a transition section 9, a primary superheater 10 and a secondary superheater 12, each section being composed of several parallel tubes, only one of which is shown in the Figure. The preheater inlet header 16 is supplied with water through a feed regulating valve 6 and pipes 4, 5 by a pump 3. A separator 11 is arranged in each parallel tube between superheaters 10, 12 to remove water from steam when individual tubes of the transition zone 9 and super heater 10 are being flushed, each separator being connected by pipes 22, 23 to a common collector vessel 21. The final outlet header 20 is connected by a pipe 13 to a turbine 15, variations in the steam pressure in the pipe 13 acting through control means 47, 48 to regulate the feed valve 6. When the turbine is being started, initially only about 5 per cent of the full load steam output is passed therethrough so as to limit heat stresses in the turbine: Under this condition, to ensure adequate flow velocities in the heated tubes of the boiler without the use of an auxiliary starting condenser, a temporary return flow circulation is established through the heated tubes, a circulating pump 25 drawing water from the vessel 21 and delivering it through line 26 and non-return valve 27 to the tube system. A constantlevel is maintained in the vessel 21 by passing cold feed water through a heat exchange coil 34 in the vessel, the flow of feed water, which is controlled according to the level by a valve 35 actuated by a float 36, determining the amount of steam which is condensed in the vessel and hence the level therein. The head exerted by pump 25 is such that as soon as the boiler load increases to that at which adequate flow velocity occurs in sections 7 .. 10, the valve 27 closes and an interlocked valve 28 opens. Pump 25 then delivers water through pipe 29 and individual pipes 31 fitted with non-return valves 32 to respective tubes of superheater 10 in such quantity that slight superheating occurs at the superheater outlet. With further increase in load the valves 32 close and the boiler then operates normally.