How does steam dryness fraction affect turbine performance and steam quality?

• A. It indicates the proportion of vapor in the steam; lower dryness means moisture leading to erosion, reduced efficiency, damage.
• B. It measures moisture in the condensate returned.
• C. It determines the turbine blade geometry.
• D. It indicates the water content in the feedwater.

Answer: (A)

Steam dryness fraction, or quality, is the fraction of a steam–water mixture that is vapor. When dryness is less than one, the steam carries liquid water droplets. In a turbine, this wet steam reduces performance because the liquid portion does not contribute to the work as effectively as vapor; it also causes blade erosion from droplet impacts and can lead to flow disturbances and higher losses. As moisture increases, the average enthalpy drop available for generating useful work decreases, so both efficiency and power drop. In short, drier (higher-quality) steam delivers more work and protects turbine blades, whereas moisture in the steam compromises both efficiency and blade life. The other options describe moisture in condensate, feedwater, or a design parameter, which are not what dryness fraction measures.