Impulse vs reaction — where does steam expand? In an impulse steam turbine stage, where does the pressure (and enthalpy) drop of the steam primarily occur?

Introduction / Context:
Steam turbines are classified by how the enthalpy drop is apportioned between fixed and moving elements. Recognizing where the pressure drop occurs distinguishes impulse stages from reaction stages and sets expectations for velocity triangles, blade shapes, and sealing requirements.

Given Data / Assumptions:

• Single impulse stage comprising fixed nozzles followed by moving blades.
• Idealized behavior in the rotor (no pressure drop) for the impulse model.
• Negligible leakage and secondary flows for the conceptual explanation.

Concept / Approach:
In an impulse stage, all (or nearly all) of the stage pressure drop occurs in the nozzles. The nozzles convert pressure energy into high kinetic energy. The rotor blades then turn the flow, extracting work primarily by changing momentum at almost constant pressure. This contrasts with reaction stages, where pressure drops in both stator and rotor.

Step-by-Step Solution:
Identify the function of fixed nozzles: produce high jet velocity via pressure drop.Recognize rotor action: deflect jets to change angular momentum, doing work with minimal pressure change.Conclude: expansion (pressure and enthalpy drop) is in the nozzles; rotor ideally sees constant pressure.

Verification / Alternative check:
Measured static pressure across impulse rotors shows near-constant values compared with pronounced drops across stators; reaction blading shows pressure reduction in both rows.

Why Other Options Are Wrong:
Option B describes reaction staging. Option C is incorrect: moving blades alone do not perform the expansion in an impulse stage. Option D neglects the fundamental conversion of enthalpy to kinetic energy.

Common Pitfalls:
Assuming that any work extraction requires pressure drop in the rotor; in impulse stages, work results from momentum change at nearly constant pressure.

Final Answer:
The steam is expanded in the nozzles only, experiencing a pressure drop and heat drop; across moving blades pressure ideally remains constant