When the steam pressure drop occurs only in the stationary steam passages, the turbine is known as:

• A. An impulse turbine
• B. A reaction turbine
• C. A Parsons turbine
• D. A single stage turbine

Answer: (A)

In an impulse turbine, the steam’s pressure is allowed to drop mainly in stationary passages before the rotor. The high-pressure steam expands in fixed nozzles, turning into a high-velocity jet. As this jet strikes the rotor blades, it transfers energy through a change in momentum (impulse) rather than a further pressure drop across the moving blades. The rotor blades themselves see only a small pressure change; the work comes from pushing the steam to change its direction and speed.

If the pressure were dropping mainly across the moving blades, it would be a reaction turbine, where energy transfer occurs through pressure changes in both stationary and moving blades. The term Parsons turbine refers to a historical design that combines features of both concepts, but the key trait described—pressure drop confined to stationary passages—points to an impulse turbine.