What is the difference between wet and dry steam, and why does it matter in turbines?

• A. Wet steam contains droplets and lower energy; dry (superheated) steam carries more energy and reduces blade erosion and thermal stress.
• B. Wet steam is hotter than dry steam.
• C. Dry steam has more moisture than wet steam.
• D. Wet steam always increases turbine efficiency.

Answer: (A)

Steam quality, or dryness, is about how much liquid water is carried along with the steam. This matters in turbines because energy from the steam comes from the vapor phase; liquid water don’t contribute to shaft work the same way and can cause problems.

Wet steam has droplets of liquid water mixed with vapor. Those droplets add mass and can strike turbine blades, causing erosion and surface damage. They also don’t transfer heat and momentum as efficiently as clean vapor, so part of the steam’s energy is tied up in the liquid phase and isn’t converted into useful work. Dry steam, or superheated steam, has no liquid droplets and is hotter and contains more usable energy per kilogram, so it provides a larger enthalpy drop and a smoother, more efficient expansion through the turbine. With less moisture, there’s less blade erosion and thermal stress from condensation, which helps maintain blade life and performance.

So the best description is that wet steam contains droplets and lower energy; dry steam carries more energy and reduces blade erosion and thermal stress. Temperature alone doesn’t define dryness, and having more moisture does not improve efficiency.