Water Hammer in Pipes – Cause of pressure surge Water hammer (hammer blow) in a pipeline occurs under which operating condition?

Introduction:
Water hammer is a transient hydraulic phenomenon characterized by a rapid pressure rise (or fall) and associated elastic waves in the pipe-fluid system. It is a key topic in fluid transients, affecting design of valves, pipes, supports, and surge protection devices. The question asks the practical trigger condition for hammer blow.

Given Data / Assumptions:

• Incompressible liquid approximation with finite compressibility and pipe wall elasticity for transient analysis.
• Long, rigid-supported pipeline with valves and possibly pumps.
• Sudden closure implies closure time shorter than or comparable to the pipeline wave travel time.

Concept / Approach:

When a flowing liquid column is abruptly decelerated, momentum change generates a pressure wave that travels at the acoustic wave speed of the fluid-pipe system. The classical Joukowsky relation gives the initial surge: delta_p = rho * a * delta_v, where a is the wave speed and delta_v is the sudden velocity change. Therefore, rapid closure of a downstream valve (or rapid pump trip causing sudden velocity change) produces the largest surge magnitudes.

Step-by-Step Solution:

Verification / Alternative check:

Field experience and transient modeling show highest surge when valve closure time is less than 2L/a (round trip time), confirming the risk from rapid operations.

Why Other Options Are Wrong:

Excessive leakage: Causes pressure loss, not surge.Pipe burst: It is a possible consequence, not the cause.Gradual closure / slow pump start: These mitigate transients by minimizing delta_v per unit time.

Common Pitfalls:

Confusing symptoms and causes, or assuming hammer occurs only on closure; rapid opening can also create negative pressure waves, but the strongest classic case is sudden stop of flow.

Final Answer:

the flow of fluid through the pipe is suddenly brought to rest by closing of the valve