In an impulse turbine used in hydraulic machinery, the nozzle converts the entire available head into kinetic energy before the jet strikes the runner. State whether the following assertion is correct: “The water pressure at the entry to the runner vanes and at the exit from the runner vanes is (approximately) atmospheric.”

Difficulty: Easy

Correct Answer: Agree

Explanation:


Introduction:
Impulse turbines (e.g., Pelton, Turgo) extract energy almost entirely from the jet's kinetic head. Understanding the pressure conditions at runner inlet and outlet is fundamental to distinguishing impulse from reaction turbines and explains why draft tubes are unnecessary for impulse machines.


Given Data / Assumptions:

  • Nozzle expands to atmosphere and delivers a free jet.
  • Jet strikes runner buckets in an open casing (not submerged).
  • Negligible pressure rise within the runner passage; losses limited to friction and splash.


Concept / Approach:
In an impulse turbine, the entire pressure drop occurs in the nozzle. The runner operates in air. Consequently, static pressure at the runner entry (just as the jet leaves the nozzle) is atmospheric, and after the jet is deflected by the buckets and discharged, it again mixes with the surrounding atmosphere, remaining at atmospheric pressure.


Step-by-Step Solution:
1) Convert head to velocity in the nozzle: V ≈ (2 * g * H)^(1/2).2) At nozzle exit, static pressure = atmospheric (free jet condition).3) Runner changes only the jet momentum (direction and slightly magnitude); static pressure stays ≈ atmospheric.4) Discharged water leaves to atmosphere; no draft tube or submergence is used.


Verification / Alternative check:
Pressure taps placed just upstream of the nozzle show high pressure; immediately in the free jet and around the runner, static pressure readings are essentially atmospheric.


Why Other Options Are Wrong:
Disagree/part-load/small jets: the impulse principle holds over the operating range; pressure in the runner region remains ≈ atmospheric.

Draft tube condition: draft tubes belong to reaction turbines, not impulse wheels.


Common Pitfalls:
Confusing impulse with reaction operation and assuming a pressure drop across the runner, which does not occur for ideal impulse machines.


Final Answer:
Agree

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