GB1073581A - Improvements in and relating to start-up or low load operation of forced-flow once-through steam generating units - Google Patents

Abstract

1,073,581. Raising steam in once-through forced - flow steam generators. FOSTER WHEELER CORPORATION. May 20, 1964 [May 20, 1963], No. 20916/64. Heading F4A. [Also in Division F1] A forced-flow once-through steam generating unit comprising steam generating sections 14, 16 and superheating sections 22, 24 in series, whose output passes to a two-stage turbine 26, 30 and then to a condenser 32, to be recirculated through heaters 36, 42 and a de-aerator and storage tank 38, 40 by pumps 85, 34 back to economizer 12, has a start-up arrangement including a pressure reducing station 44 before the first superheater 22 and a turbine by-pass 48 between the superheating sections 22, 24 respectively. The pressure reducing station, Fig. 7 (not shown), comprises three reducing valves in parallel with one another and with a double-port high capacity valve which is in series with an isolation shut-off valve, all in parallel with the main flow line valves used during normal operation. The by-pass line 48 includes stop valve 52, itself by-passed by line 98, spray at temperator 54 arid flash tank 46, from whence water lines 68a, 68b, controlled by valves 72, 74 respectively lead to the condenser hot well 70 and the high pressure heaters, whilst steam line 56 with valve 58 leads to the inlet of superheater 24 (isolatable from superheater 22 by valve 50 in the main flow line) and steam line 60 with valves 66, 64, 62 and 82 respectively in its branches, leads to the high pressure heaters 42, deaerator 38, the turbine gland seal regulator and into a second turbine by-pass 76 controlled by valve 78, leading from the outlet of superheater 24 to the condenser 32. Raising steam.-A 30% full load flow through the furnace circuitry to cool it, first at 600 p.s.i., then at operating pressure (3650 p.s.i. for supercritical or 2250 p.s.i. for subcritical operation) is established, throttled at pressure reducing station 44 to 100 p.s.i., the output all passing via line 48 to flash tank 46 and thence through drain line 68 to the condenser hot well 70 and the high pressure heaters 42 to the deaerator storage tank 40. All other lines are closed. The burners are controlled to keep the final superheater gas temperature below 1200‹ F. Steam starts to flash in tank 46, controlled at a set point of 500 p.s.i. and is passed to superheater 24 via line 56 and to the gland seal regulator and the de-aerator. Steam from superheater 24 passes to the condenser hot well 70 via line 76 or any main drain line. When 5% of the full load steam flow is available from flash tank 46 part is used to warm and roll the turbine, the excess being disposed of through by-pass lines 60 and 76. Once the steam leaving superheater 24 becomes superheated valve 50 is opened to open the main flow circuit, the pressure downstream of station 44, however, still being controlled by the set point of flash tank 46. Once the turbine ports reach equilibrium temperature, by-pass line 76 is shut and the 5% load is applied to the turbine through the turbine stop-valve by-pass 90, Fig. 4 (not shown), which is increased to 10% by raising the flash tank set point pressure from 500 to 1000 p.s.i. The load is increased to 30% by control through governor valves 88 in series with the main turbine throttle stop valve 86, Fig. 4 (not shown), when the start-up circuit is shut off by closing valve 52, the load being increased up to 100% by normal combustion control in response to demand. When the throttle steam pressure reaches 3150 p.s.i. pressure reducing station 44 is also taken out of action, the normal main flow line valves being used. A hot restart of the turbine is performed in the same way as the cold start above save that the by-pass flow through line 76 is only switched to the turbine for initial rolling when the steam temperature leaving the final superheater 24 matches that of the turbine inlet ports. The start-up circuit may also be used for low-load control (below 30%). Details of the automatic control circuit (shown as dashed lines in Fig. 1) for performing the above operations are given.