GB1166046A - Improvements in or relating to Starting Up Forced-Flow Boilers - Google Patents

Abstract

1,166,046. Raising steam in forced flow boilers. BABCOCK & WILCOX CO. 4 Oct., 1966 [4 Oct., 1965], No. 44323/66. Heading F4A. In normal operation of a power plant, a pump 10 supplies feedwater from a de-aerator 37 through a high pressure feedwater heater 12 to a once through forced flow vapour generator 14 comprising an economizer 16, generating section 17 and primary and secondary superheaters 18A, 18B. From superheater 18B, vapour passes through stop valve 24 and control valve 26 to a high pressure turbine 22, and a condenser 31, the condensate being passed by pump 33 through low pressure feedwater heater 36 back to de-aerator 37. During start up, a quarter full load fluid flow is established by pump 10 from the de-aerator 37, through economizer 16 and generating section 17 to a flash tank 42 through a conduit 39 containing a valve 41 which maintains full load pressure at the outlet of generating section 17. Flow through the superheater is prevented since all valves downstream of the primary superheater 18A are kept closed. Water from flash tank 42 passes through conduit 49 back to de-aerator 37, pegging valve 51 in conduit 49 maintaining the water level in the flash tank 42 constant. Firing is started, the gas temperature at the superheaters 18 being held at 1000‹ F. The increased enthalpy of the circulating fluid causes the flash tank and de-aerator pressures to rise. At a de-aerator pressure of 20 p.s.i.a. flash tank level control is transferred from valve 51 to valve 70 in conduit 65, water from the flash tank 42 then being used to heat feedwater in high pressure feedwater heater 12 before passing to condenser 31 via conduit 71. Valve 51 then controls the pressure in de-aerator 37. Excess water from flash tank 42, passes direct to condenser 31 through valve controlled conduit 52. At a predetermined pressure, steam from the flash tank 42 is passed through valve controlled conduit 59 to seal the turbine, and at a higher pressure, e.g. 75 p.s.i.a. through conduits 54 and 57, pass de-aerator pegging valve 58 to de-aerator 37 till the de-aerator pressure reaches 25 p.s.i.a. when water pegging valve 51 in conduit 49 closes and all water from the flash tank passes through valves 70 or 53 to the feedwater heater 12 or condenser 31 respectively. When the flash tank pressure reaches 50 to 80 p.s.i.a. above that at which the de-aerator steam requirements are met, steam is also passed from flash tank 42 to heater 12 past valve 56. Once the fluid temperature at the inlet to valve 41 exceeds 300‹ F., valve 41A, in conduit 55 leading from the outlet of the primary superheater 18A to flash tank 42, is opened, thereby allowing some fluid flow through primary superheater 18A and then through conduit 55 to the flash tank 42. In subsequent operation, valve 41 is regulated to maintain full load pressure at the outlet of primary superheater 18A, whilst valve 41A is regulated to maintain a fluid temperature at its inlet, such that the fluid enthalpy is 1200 Btu/per pound. At a flash tank pressure of 300 p.s.i.g., steam is passed from the flash tank 42 through conduit 60 and secondary superheater 18B and main steam line 23 to valve controlled turbine by-pass 66, to pass to condenser 31. Once the flow rate, pressure and temperature of steam from the flash tank reach predetermined conditions (e.g. 500 p.s.i.g.) this steam flow through by-pass 66 is discontinued, and steam is admitted to warm and roll the turbine 22 under control of stop valve 25, control valve 26 being wide open. Any excess steam from flash tank 42 is passed direct to condenser 31 through conduit 62. At 8% loading of the turbine (flash tank pressure 1000 p.s.i.g.), turbine control is transferred from valve 25 which is closed to valves 26, valve 24 now being opened wide. Further loading up to 30% is then effected by increasing the steam pressure in secondary superheater 18B and steam line 23 by gradually opening valve 20A between the primary and secondary superheaters 18A, 18B to allow direct flow between the superheater sections. When the pressure at secondary superheater 18B is greater than that of the flash tank, valve 50A in conduit 60 will close preventing vapour flow from the flash tank 42 through the secondary superheater. The flow to the flash tank 42 now comprises the difference between the start up flow and that passing through valve 20A to the secondary superheater 18B, since valve 41A tends to close as valve 20A opens to maintain the primary superheater outflow temperature constant. By 10% full load flow through valve 20A, valve 41A is completely closed. By 30% full load flow through the secondary superheater 18B, which has by then achieved full load pressure, valve 20A is fully opened and valve 41 maintaining full load pressure at the outlet of the generating sections 17 is completely closed. All flows to the flash tank 42 cease and the flash tank pressure decays. Main stop valve 20 is then opened and valve 20A closed for normal operation to begin.