Connecting the second exhaust-heat boiler to the operating first one under the conditions of flow circuits of combined-cycle plants with two gas-turbine units and one steam turbine

Abstract

Problems arising with connecting the second exhaust-heat boiler to the first exhaust-heat boiler under load in the case of flow circuits of combined-cycle plants of type PGU-450 are considered. Similar problems exist for two-boiler single-turbine units of type PGU-325 and for two-boiler single-turbine units of traditional steampower plants with an output of 200 -300 MW upon connection of the second furnace of a two-furnace boiler to the operating first furnace. Analysis of the temperature-induced stressed state and of the cyclic strength of thick-walled elements of the steam-water conduit of PGU-450 and of two-furnace boilers of two-boiler single-turbine units with an output of 200 -300 MW (the check valve, the inlet header of the water economizer) is used for determining the steam temperature differences in the high-pressure conduits to the steam turbine and the order of connection of the second exhaust-heat boiler to the operating fist one. Modes of enabling a two-boiler single-turbine unit of a combined-cycle plant, which are most "rigid" from the standpoint of thermal strength, are determined.The flow circuit of a combined-cycle plant of type PGU-450T is represented by a two-boiler turbine unit consisting of two gas turbines with their exhaust-heat boilers and one steam turbine. High-pressure and low-pressure steam is fed into the turbine from both exhaust-heat boilers through two pairs of high-pressure and low-pressure steam conduits.A flow circuit of such configuration, as well as that of the traditional two-boiler single-turbine unit, ensures the possibility of operation with an incomplete set of basic equipment (a single-furnace mode), when one of the two exhaust-heat boilers and the respective GTU have been stopped or have not been connected to the operating half.In this case the problem of connecting the secondenabled boiler to the already operating first boiler under load comes to the fore. This stage of startup of the two-boiler gas-steam-turbine unit should be considered with allowance for the following factors:-in accordance with the standard process, the secondenabled exhaust-heat boiler is connected to the operating first boiler after feeding steam from the first-enabled EHB into the steam turbine (when the steam turbine has gained the speed of idle running or is operating under load), when the unit is enabled in series according to the scheme 1GTU + 1EHB + ST + 2GTU + 2EHB + ST, or after the attainment of a load of 100 MW by both GTU enabled in parallel in the process of startup of the steam turbine according to the scheme 1GTU + 1EHB + 2GTU + 2EHB + ST;-when the second exhaust-heat boiler of PGU-450 is connected to the first boiler that has been operating for a long time, the half-unit 1GTU + 1EHB + ST operates for a long time at the rated load or within the regulatory range, i.e., at the rated gas temperature at the inlet to the working EHB; the turbine operates in a mode of sliding pressure with fully opened valves;-the second furnace of the two-furnace boiler of the gas-steam unit (for exampl...