Pipeline transients: What is the primary cause of water hammer in pipelines during rapid operational changes (e.g., sudden valve closures or pump trips)?

Introduction / Context:
Water hammer (hydraulic shock) is a transient phenomenon causing pressure surges in piping systems. Understanding its cause is essential for designing surge protection, valve sequencing, and safe operating procedures in water and process systems.

Given Data / Assumptions:

• Incompressible liquid approximation with finite wave speed.
• Events such as sudden valve closure, pump start/stop, or rapid load change.
• Elastic pipe walls and fluid compressibility allow pressure waves to propagate.

Concept / Approach:
Water hammer arises when the flow rate changes quickly, imparting a momentum change to the fluid column. The resulting pressure wave travels at the acoustic wave speed, reflecting at boundaries and superimposing to create high transient pressures. The fundamental cause is the rapid change in flow (velocity), not the structural failure itself.

Step-by-Step Solution:

Verification / Alternative check:
Joukowsky equation: Δp = ρaΔV confirms proportionality of surge to velocity change and wave speed, matching field observations.

Why Other Options Are Wrong:

• Option (a): Describes a consequence (bursting) rather than the cause.
• Option (c): Too specific; while valve closure is a cause, the general principle is rapid change in flow rate in any form.
• None of these: Incorrect because (b) correctly states the primary cause.

Common Pitfalls:
Confusing steady-state overpressure with transient surge; water hammer is dynamic and depends on rate of change, pipe length, and wave reflections.

Final Answer:
rapid pressure change due to a rapid change in the rate of flow