Combined cycle plants today operate in sliding pressure mode; if extraction steam is desired at a given pressure for process reasons, then a constant pressure may be required at the steam turbine inlet. Typically the steam pressure is allowed to float by keeping the turbine throttling valves fully open and ensuring full arc admission. The load range over which sliding operation is allowed varies from about 40% or 50% to 100%. Large variations in steam pressure affect the specific volume of steam, which in turn affects the velocity and pressure drop through superheater tubes and pipes, valves, etc. Large variations in steam pressure also affect the saturation temperature at the drum and hence thermal stresses across

Project ■ study-100% Units — British Case ■ 100 % toad Remarks — unfired

Amb temp ■ F= 60 heat loss-%= 1 gas tempto HRSG F= 1Q19 gas flow — Lb/h= 14B768 % vol C02 =3 04 H 20 = 7,33 N2 =75,5 02 = 14.13 S02 — ASM E eff=68 75 tot duty-M M Btu/h=25.9

TOC o "1-5" h z Surf gas temp waUstm duty pres flow pstm pinch apprch US module no in/out-F in/out-F MMEtfh psia Ib/h % F F BtutfiF sh 1019 924 492 750 3.37 615 21041 100 1123B 1

Evap 924 512 472 492 16 39 535. 21941 100 20 20 122275 1

Ђ00 512 365 230 472 5 68 645. 22161 0 72653 1

Gas-Steam Temperature profiles ■tots

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Figure 1.12a HRSG performance at 100% load of gas turbine.

Thick components such as drum and superheater headers, which in turn limits the rate of load changes. Sliding pressure operation increases the efficiency of the turbine at low loads due to lower throttling losses and also lowers the cost of pumping if variable-speed pumps are used.

The steam pressure at turbine inlet increases linearly as the load increases; however, the unfired HRSG steam output decreases as the steam pressure increases. By matching the steam turbine and HRSG characteristics, one can

Arrive at the operating points at various loads. Because of the large variations that occur in drum pressure during sliding pressure operation, the drum level controls should be pressure-compensated.

As an example, using the HRSG simulation program, the effect of steam pressure on a single-pressure unfired HRSG was evaluated; the results are shown in Table 1.5. Note that when multiple-pressure HRSGs are involved, the

Table 1.5 Effect of Steam Pressure on HRSG Performancea

Performance of a given module is affected by the module preceding it, so unless the configuration is known it is difficult to make generalized observations.

In the case for which data are given in Table 1.5, the HRSG was designed to generate steam at 1000 psia and 800°F and the off-design performance was evaluated at selected pressures.

The steam flow decreases as the pressure increases due to the higher saturation temperature, which limits the temperature profiles.

The exit gas temperature increases as the pressure increases, again due to the higher saturation temperature.

The steam temperature does not vary by much.

The duty or energy absorbed by steam decreases as pressure increases due to the higher exit gas temperature.