Long pressure pipes subject to rapid flow changes can experience severe transients called water hammer. What is the primary purpose of installing a surge tank on such a pipeline system?

Introduction / Context:
Water hammer is a transient pressure phenomenon caused by rapid changes in flow velocity (e.g., sudden valve closure or pump trip). The resulting pressure spikes can damage pipes and appurtenances. Surge tanks are a classical protective measure in hydroelectric penstocks and long transmission mains.

Given Data / Assumptions:

• Long closed conduit conveying water to or from a reservoir/turbine/pump.
• Rapid transients are possible due to operational changes.

Concept / Approach:
A surge tank provides an adjacent volume of water exposed to atmosphere (or controlled pressure) that can rapidly accept or supply water during transients. This moderates the velocity change in the main and caps the pressure rise and drop, reducing water hammer severity.

Step-by-Step Reasoning:
Valve closes or turbine load changes → momentum change in main line.Pressure would rise sharply without relief → risk of overpressure or column separation.Surge tank allows water to flow into it (during rise) or out of it (during drop), cushioning the transient.Result: reduced peak pressure and moderated subsequent oscillations.

Verification / Alternative Check:
Water hammer analysis (e.g., Joukowsky equation Δp = ρ * a * ΔV) shows peak pressures scale with velocity change. By supplying storage to limit ΔV, the surge tank limits Δp.

Why Other Options Are Wrong:
Additional static head: not the principal purpose; some designs may change steady head slightly, but surge control is primary.Overflowing the line: a surge tank is not a spillway; it manages transient flows.Removing friction losses: impossible; friction is inherent and only mitigated by design and materials.Sight-glass measurement: unrelated to surge protection.

Common Pitfalls:

• Placing surge tanks where local head losses isolate them from the main transient.
• Ignoring air admission and venting requirements for proper operation.

Final Answer:
to relieve pressure surges due to water hammer