(70) Therefore, the risk of sulphuric acid dew point attack ... - DTI Home

4.3 Improvements to Cycle and Other Plant Components
4.3.1 Steam Cooled Turbine Blades
For the gas turbine, the maximum allowable inlet temperature is governed by
both the materials available for the turbine blading and the cooling technique
employed.
Previous generations of gas turbines utilised air to cool turbine components in
an open loop system. The cooling air was supplied from a bleed in the gas
turbine compressor, and then ducted to the internal chambers of the turbine
blades and discharged through small holes in the blade walls. This air
provided a thin, cool insulating blanket along the external surface of the
turbine blade. As a result, there is a significant exhaust gas temperature drop
across the first stage nozzle and significant flow of air required to cool down
the relevant turbine stages. An integrated closed loop steam cooling system
significantly reduces this temperature drop in addition to eliminating the
requirement for air bleed for the turbine cooling. This technology is envisaged
as contributing around 2% points in thermal efficiency.
The thermodynamic advantage of utilising steam in cooling circuits was
recognised in the early 90’s [6] (Figure 32). This has been developed
accordingly over the last decade to allow integration of the HRSG steam flow
with the gas turbine cooling loop to further enhance cycle performance. The
implications of this on steam purity are significant if corrosion and fouling of
the cooling passages is to be prevented. This is discussed further in Section
3.5.9.
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