Which element of hydroelectric power plant prevents the penstock from water hammer phenomenon?

Question

Examoogle · Accepted Answer

Surge Tank

Explanation: A surge tank is a small reservoir or tank connected to the penstock of a hydroelectric power plant. It acts as a pressure buffer, preventing the dangerous pressure rise known as 'water hammer' caused by sudden fluctuations in water velocity when load changes.

Working Principle: When a turbine load is suddenly rejected, the governor closes the gates, leading to a sudden stoppage of water flow. Without a surge tank, the momentum of the moving water column would create a massive pressure surge (water hammer) in the penstock, potentially causing it to rupture. The surge tank provides an open surface where the water can rise, absorbing the kinetic energy and converting it into potential energy (head), thereby mitigating the pressure spike.

Key Formulas: $P = \rho \cdot v \cdot a \cdot c$ — where $P$ is the pressure rise, $\rho$ is water density, $v$ is velocity change, and $c$ is wave celerity; $\Delta H = \frac{L}{g} \cdot \frac{dv}{dt}$ — head rise due to flow retardation

Key Points: Water hammer occurs due to sudden change in velocity (Inertia of water); Surge tanks effectively shorten the length of the penstock subjected to high-pressure waves; Located as close to the powerhouse as possible; Provides immediate supply of water during load acceptance

Advantages: Protects the penstock from rupture; Stabilizes the turbine speed during sudden load fluctuations

Disadvantages: Adds significant construction cost; Requires high topographical elevation for installation

Applications: High and medium head hydroelectric power plants; Pumped storage power plants

Additional Info: Draft tubes are used to recover kinetic energy from water leaving the turbine runner.. Spillways are used to discharge excess flood water from the reservoir.. The phenomenon is governed by the relation between the closing time of the valve and the wave propagation time $t = \frac{2L}{c}$.

Answer: D is correct — The surge tank acts as a shock absorber to neutralize pressure surges in the penstock during sudden changes in turbine load.

Exam Tip: Remember that surge tanks are specifically meant for load rejection, whereas the draft tube is strictly a component of the turbine assembly used for efficiency recovery.

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