The gas turbines/generators in a combined cycle power plant are often responsible for producing two-thirds of the total power generation. As such, their reliable operation may be considered as more critical than that of the steam turbine. Water may be injected into a gas turbine for NOx control and/or to increase its generating capacity. This water must be of sufficient purity to ensure that significant deposits will not accumulate and corrosion will not occur in the combustion path. Deposits can critically affect heat transfer across the blades. Corrosion of the refractory coatings on the blade and the blade itself can occur if significant amounts of fluxing chemicals such as sodium and potassium are added to the gas. Potassium levels in makeup water are typically not an issue and therefore sodium is the critical parameter monitored. Table 12 shows an example of suggested limits for gas turbine NOx control injection water.


Gas turbines often operate during the summer peak power generation period. One constraint on the amount of power that can be generated is the density (temperature) of the air at the turbine intake. To generate more power during hot summer temperatures, the inlet air to gas turbines is often cooled with evaporative cooling systems. Water may be sprayed directly into the inlet air stream (fogging) or cascaded across loose media perpendicular to the air flow (evaporative) to reduce the air temperature. As with the gas turbine injection water, any corrosive or deposit-forming species in the water are a concern. An example of the suggested limits for direct injection water used for inlet air cooling is provided in Table 13. In addition, demineralized water may leach chemicals from fibrous cooling media in an evaporative system resulting in material breakdown, carryover, or loss of media strength. Some units establish the proper amount of dissolved solids in the water to prevent problems with the media, and then add demineralized water to replace water lost to evaporation. Table 14 shows an example of evaporative cooling water limits.

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