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The steam turbines in most of the large power plants are at a minimum divided into two major sections the High Pressure Section (HP) and the LowPressure Section (LP). ln some plants， the HP section is further divided into a High Pressure Section and an lntermediate Pressure Section (lP). The HRSGis also divided into sections corresponding with the steam turbine. The LP steam turbine"s performance is further dictated by the condenserbackpressure， which is a function of the cooling and the fouling.
The efficiency of the steam section in many of these plants varies from30-40%. To ensure that the steam turbine is operating in an efficientmode， the gas turbine exhaust temperature is maintained over a wide range of operating conditions. This enables the HRSGto maintain a high degree of effectiveness over this wide range of operation.
ln a combined cycleplant， high steam pressures do not necessarily convert to a high thermal efficiency for a combined cycle power plant. Expanding the steam at higher steam pressure causes an increase in the moisture content at the exit of the steam turbine. The increase in moisture content creates major erosion and corrosion problems in the later stages of the turbine. A limit is set at about 10% (90% steam quality) moisture content.
The advantages for a high steam pressure， is that the mass flow of the steam is reduced and that the turbine output is also reduced. The lower steam flow reduces the si.e of the exhaust steam section of the turbine thus reducing the si.e of the exhaust stage blades. The smaller steam flow also reduces the si.e of the condenser and the amount of water required for cooling. lt also reduces the si.e of the steam piping and the valve dimensions. This all accounts for lower costs especially for power plants which use the expensive and high-energy consuming air-cooled condensers.
lncreasing the steam temperature at a given steam pressure lowers the steam output of the steam turbine slightly. This occurs because of twocontradictory effects: first the increase in enthalpydrop， which increasesthe output; and second the decrease inflow， which causes a loss in steamturbine output. The second effect is more predominant， which accounts for the lower steam turbine amount. Lowering the temperature of the steam also increases the moisture content.
.nderstanding the design characteristics of the dual or triple pressure HRSGand its corresponding steam turbine sections(HP，lP， and LP turbines) is important. lncreasing pressure of any section will increasethe work output of the section for the same mass flow.However， at higher pressure， the mass flow of the steam generated is reduced. This effect is most significant for the LP Turbine. The pressure in the LP evaporator should not be below about 45 psia (3.1 Bar) because the enthalpy drop in the LP steamturbine becomes verysmall， and the volume flow of the steam becomes verylarge thus the si.e of the LP section becomeslarge， with long expensive blading. lncrease in the steam temperature brings substantial improvement in the output. ln the dual or triple pressurecycle， more energy is made available to the LP section if the steam team to the HP section is raised.
There is a very small increase in the overall cycle efficiency between a dualpressure cycle and a triple pressure cycle. To maximi.e their efficiency， thesecycles are operated at high temperatures， and extracting most heat from the system thus creating relatively low stack temperatures. This means that inmost cases they must be only operated with natural gas as thefuel， as this fuel contains a very low to no sulfur content. .sers have found that in thepresence of even low levels of sulfur， such as when firing diesel fuel (No. 2 fuel oil) stack temperatures must be kept above 300 oF (149 oC) to avoid acid gas corrosion. The increase in efficiency between the dual and triple pressure cycle is due to the steam being generated at the lP level than the LP level. The HP flow is slightly less than in the dual pressure cycle because the lP superheater is at ahigher level than the LP superheater， thus removing energy from the HP section of the HRSG. ln a triple pressure cycle the HP and lP section pressure must be increased together. Moisture at the steam turbine LP section exhaust plays a governing role. At inlet pressure of about1500 psia (103.4 Bar)， the optimum pressure of the lP section is about 250 psia (17.2 Bar). The maximum steam turbine output is clearly definable with the LP steam turbine pressure. The effect of the LP pressure also effectsthe HRSGsurfacearea， as the surface area increases with the decrease in LPsteampressure， because less heat exchange increases at the low temperature end of the HRSG. Figure 2-33 is the energyjtemperature diagram of the triple pressure HRSG. The lP and LP flows are much smaller than the HP steam turbine flow. The ratio is in the neighborhood of 25:1.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 1(37)
