Patent GB-982418-A (GB)
Title: Forced-flow once-through vapour generators
IPC: C01G19/00
Filed: 1963-01-10
Abstract: 982,418. Starting forced flow steam boilers. SULZER FRERES S.A. Jan. 10, 1963 [Jan. 18, 1962], No. 1263/63. Heading F4A. In a method of starting a once-through steam boiler particularly from the hot state, damage by thermal shock to the final superheater which is made of material susceptible thereto, e.g. austenitic steel, is prevented by admitting the working fluid to the superheater only when its temperature corresponds substantially to the temperature of the final superheating surface or of the flue gases surrounding said surface, the temperature of the working medium being adjusted towards the superheater surface or flue gas temperature by regulating the flow of the medium in the heating surfaces upstream of the superheater or by varying the heat impinging thereon. The Figure shows a unit power plant operating at supercritical pressure. The combustion chamber 1 which is fired by burners 68, the supply of fuel to which is controlled by a valve 70 and is lined with evaporating tubes 5, contains in its upper part a reheater 36. A lateral offtake 3 contains a superheater section 11 and leads to a downpass 2 containing final and primary superheater sections 16, 10 and water preheater sections 30, 31. Live steam passes from the final superheater section 16 through a line 7 to the H.P. turbine 8 and after reheating flows through the L.P. turbine 9 to exhaust into a condenser 38 whence condensate is extracted by a pump 40 and passes through heaters 41 to a feed water tank 37. A feed pump 34 draws water from the tank and forces it through heaters 33 and the preheaters 30, 31 to the evaporating sections 5. Between the heating surfaces 5 and 10 a pipe 42 controlled by a pressure maintaining valve 6 branches off to the tank 37. Between the heating surfaces 11 and 16 a pipe 47 controlled by a valve 12 branches off to a by-pass separator 13, the latter being connected by a separated steam pipe 53 controlled by a valve 54 to the inlet of the reheater and by a separated water pipe 55 controlled by a valve 56 to the tank 37, the valve 56 being adjusted to maintain a constant level in the separator. From the live steam line 7 a by-pass pipe 19 controlled by a valve 20 leads to the separator 13 whilst the reheater outlet pipe has a branch pipe 57 controlled by a valve 58 and provided with an injection cooler 59 for by-passing steam to the condenser 38. In normal operation the valves 6, 12, 20, 54 and 58 are closed and the turbine inlet valves 21, 22 are open, the flow of feedwater being adjusted by a regulator 62 which compares an actual value t 1  of evaporator outlet temperature with a desired value t 1s  obtained from a starter unit 25 and transmits an appropriate signal to a servomotor 66 which also receives a limit value signal W min  for minimum water flow. When the plant is shut down the firing is reduced to 30% of normal load firing and the valve 21 is closed. The boiler pressure rises and a regulator 60 opens the valve 20 to by-pass steam to the separator 13. The firing is then stopped. On restarting the plant whilst still hot the valves 20, 21, 22 are closed and the valves 6, 12, 54, 58 are opened. The starter unit 25 is actuated to give the following sequence of operations. The feed pump 34 is started and delivers the flow W min,  part of the water returning from the evaporator to the feed tank 37 through the line 42 and part flowing through the superheaters 10, 11 and the line 47 to the separator 13 and thence to the feed tank. Firing is started and the temperature of the superheaters 10, 11 rises. Steam separated in the separator 13 flows through the reheater to cool the latter. The temperature of the flue gases adjacent final superheater 16 is measured by a pick-off 75 and, as soon as the temperature of the working medium leaving the superheater 11 as measured at 15 reaches the flue gas temperature, a desired value pressure signal is transmitted from the starter unit to the regulators 44, 60 for the valves 6, 20 respectively whereupon the valve 6 begins to close and the valve 20 begins to open. The working medium then begins to flow through the superheater 16 without thermal shock to the latter. Finally the valves 6, 12 are completely closed, the firing and feed water supply rate are increased until the required steam temperature is reached when the valves 20, 54, 58 are closed and the turbine inlet valves 21, 22 opened. In a modification when the plant has to be started frequently the working medium is released to the final superheater after a known time has elapsed. In a further modification the flow through the surfaces upstream of the final superheater is determined according to measures of pressure in the tube system and temperature at the outlet of the surfaces.