Conversion of the steam-supply system of 800-MW supercritical-pressure units to lower steam temperature upon introduction of a sliding startup process

Abstract

Results of testing a novel process of startup of 800-MW supercritical-pressure units at sliding pressure in the entire conduit of the once-through boiler with the temperature of extraneous steam in the station-service header supplying the turbine-feeding pump reduced to~250°C and intermediate transfer of its feed to its own steam through the available station-service starting-shedding device are given. It is shown that the use of this process makes it possible to fully pass to steam supply of the unit from the colder second extraction of the turbine. A variant of updating the steam feeding circuit is suggested.The steam feeding circuit is one of the main components of the starting circuit of supercritical-pressure units (SPU). In domestic SPU rated for 250 -300 MW it is constructed with feeding the station and unit service headers (SH) from the second extraction of the turbine (P = 4.0 MPa, t = 280°C) through the reducing installation of the cold reheat conduits (RI CRC) with design parameters in the AH equal to 1.3 MPa and 250 -300°C.A distinctive feature of the steam feeding circuit of 800-MW SPU is the transition of the station SH to hot steam primarily supplied by the 1.6/1.3-MPa RI from the third extraction of the turbine (P = 1.6 MPa, t = 380°C). In the modes of start and stop they are fed from the reducing-cooling installation of the hot reheat conduits (RCI HRC) with parameters of 3.7 MPa and 545°C. When the output of the generating unit is shed to no-load running and to station service, the unit SH is switched from being fed from the fourth extraction of the turbine (1.2 MPa, 340°C) to being fed from the stationservice starting-shedding high-speed cooling installation (SSHCI) with rated parameters of 24.0 MPa and 545°C. Then the design steam temperature in the station SH is increased to about 400°C.The transition to an elevated temperature level in the system of steam feeding of 800-MW generating units has primarily been connected with the absence of backup starting feed pump (BSFP) in their starting circuit. The availability of BSFP in generating units with lower unit capacity (the maximum capacity of domestic feed pumps developing a pressure of about 38.0 MPa amounts to 600 tons/h or 60% of the rated flow of feed water) makes it possible to perform full start of the generating unit and deep shedding with the help of the feed pump and to use a back-pressure turbine with removal of exhaust steam into the sixth extraction of the main turbine and to the low-pressure preheater (LPP-5) as the turbine drive.Another no less important reason behind the transition of the SH of 800-MW generating units to elevated steam parameters is the requirement that startups of the SPU be performed at the rated pressure in the evaporating conduit of the boilers, which have initially been designed for elevated pressure. Already in the initial stage of the startup it is necessary to feed a relatively high amount (about 30% of the rated value) of extraneous steam to the pipeline in order to ensure the pilot flow of the feed wate...