Impulse turbine operation — purpose of a braking jet In an impulse turbine (e.g., Pelton), what is the function of the braking jet during shutdown or speed reduction?

Introduction:
High-inertia runners in impulse turbines can spin for long periods after the main jet is deflected off the buckets. A braking jet provides controlled deceleration to reduce stopping time and improve operational safety and responsiveness.

Given Data / Assumptions:

• Pelton-type runner with one or more nozzles and deflector/gate control.
• Shutdown procedures aiming to avoid overspeed and long coast-downs.

Concept / Approach:
The braking jet is a small jet directed against the back of the buckets or in a direction that produces a torque opposite to the current rotation. This negative torque extracts kinetic energy from the rotating runner and dissipates it in the water, shortening the stopping time without mechanical friction brakes on the shaft.

Step-by-Step Solution:

Verification / Alternative check:
Operator manuals and control sequences show coordinated jet deflection plus braking jet application during trips and load rejections to avoid excessive rotational coast-down.

Why Other Options Are Wrong:

• Breaking the jet into droplets: not the objective; atomization wastes water without controlled torque.
• Changing direction of runner: not feasible by a braking jet.
• Increasing nozzle coefficient: unrelated to shutdown behavior.
• “None”: the function is specific and well established.

Common Pitfalls:
Confusing the braking jet with deflectors; assuming mechanical brakes alone handle all stopping (they are often supplemented or avoided to reduce wear).

Final Answer: