Reaction staging concept in steam turbines In an impulse–reaction (reaction-type) steam turbine stage, the static pressure of the steam decreases gradually and continuously over which blade rows?

Introduction / Context:
Steam turbines are broadly categorized as impulse or reaction depending on where the pressure (enthalpy) drop occurs. Identifying whether the pressure falls in fixed blades, moving blades, or both is fundamental for drawing velocity triangles, estimating stage work, and understanding efficiency and blade loading.

Given Data / Assumptions:

• The stage in question is a reaction-type (impulse–reaction) turbine stage.
• Flow is steady and the stage operates close to its design point.
• Heat transfer across the short stage is negligible; changes are mainly due to flow acceleration and expansion.

Concept / Approach:

In a pure impulse stage, almost all the pressure drop occurs in the fixed nozzles; moving blades ideally see no pressure change and only turn the jet (with losses due to friction). In a reaction stage, the total pressure drop is shared between stator (fixed) and rotor (moving) passages. The moving blades are shaped as expanding passages (nozzle-like), so static pressure decreases and relative velocity increases within the rotor, while the stator also accelerates the flow with its own pressure drop.

Step-by-Step Solution:

Verification / Alternative check:

Parsons reaction stages (degree of reaction near 0.5) show matched velocity triangles with acceleration through both stator and rotor. Measured static pressure taps confirm a distributed pressure drop across both rows in such stages.

Why Other Options Are Wrong:

Fixed blades only: describes impulse behavior, not reaction.Moving blades only: ignores stator expansion, which is incorrect.None of these / nozzle throat only: contradicts reaction-stage fundamentals.

Common Pitfalls:

Confusing absolute velocity (stator frame) with relative velocity (rotor frame). In a reaction rotor, relative velocity tends to increase due to internal pressure drop even if absolute exit velocity is moderated by blade speed and turning.

Final Answer:

both fixed and moving blades