Impulse turbine inlet – steam velocity approaching the nozzle In a single-stage impulse turbine, the absolute velocity of steam immediately upstream of the nozzles (approaching the nozzles) is typically:

Introduction / Context:
Impulse turbines rely on nozzles to convert pressure energy to high-speed jets that strike the rotor. The flow conditions just before the nozzles help set up the velocity triangles used for stage analysis.

Given Data / Assumptions:

• Single-stage impulse arrangement with casing leading steam into stationary nozzles.
• Large pressure drop occurs in the nozzles, not in the inlet plenum.
• Negligible pre-nozzle acceleration.

Concept / Approach:
Because the pressure drop is arranged to occur across the nozzles, the steam in the casing upstream of the nozzles is nearly stagnant relative to the subsequent jet speed. Thus, inlet absolute velocity is taken as negligible when forming inlet velocity triangles.

Step-by-Step Solution:
Note design intent: maximize acceleration within the nozzle passages.Therefore, V_upstream ≈ 0 compared with V_exit from nozzle.Adopt this assumption for constructing velocity triangles used to compute blade work.

Verification / Alternative check:
Stage design literature treats inlet absolute velocity as small, ensuring that most kinetic energy appears at nozzle exit, where it is directed onto the rotor blades.

Why Other Options Are Wrong:

• Equal to or greater than nozzle exit velocity contradicts the function of the nozzle.
• “Equal to blade speed” confuses gas velocity with rotor peripheral speed.
• Supersonic upstream of the nozzle is unrealistic and undesirable.

Common Pitfalls:
Mixing up “approaching the nozzles” with “approaching the moving blades.” The very high velocity appears at the nozzle exit, not before the nozzles.

Final Answer:
negligible compared with the nozzle exit velocity