In a reaction turbine, what happens to the steam as it passes through moving blades?

• A. Pressure increases
• B. Pressure decreases
• C. Velocity remains constant
• D. Velocity decreases

Answer: (B)

In a reaction turbine, steam loses energy as it passes through the moving blades because those blades extract work from the steam. This extraction of energy manifests as a drop in pressure: the steam expands and gives up pressure energy to turn the rotor. The rotor blades are part of the expansion process, so the pressure falls as the steam continues through the stage.

Elsewhere in steam turbines, no increase in pressure would occur, and the idea that velocity simply remains the same isn’t right because energy transfer changes the flow conditions. Saying the velocity decreases isn’t typical for a turbine stage, where expansion and work extraction generally drive changes in both pressure and flow speed.

So, the steam’s pressure decreases as it passes through the moving blades.