The sudden closing of a valve leads to which undesirable phenomenon?

• A. Cavitation
• B. Water hammer
• C. Flow separation
• D. Thermal fatigue

Answer: (B)

When a valve closes quickly, the moving liquid cannot stop instantly, so a rapid pressure rise develops in the pipeline. This surge propagates as a compression wave along the pipe—a hydraulic transient known as water hammer. The magnitude of the surge can be estimated with the Joukowsky relation, ΔP = ρ c ΔV, where ρ is the fluid density, c is the wave speed in the pipe (determined by the fluid’s compressibility and the pipe’s flexibility), and ΔV is the change in fluid velocity (often the initial velocity dropping toward zero). This sudden pressure spike can cause loud banging, vibration, and potential damage if the system isn’t designed to absorb or dampen it. Cavitation involves vapor formation from very low pressure, which is a different phenomenon driven by local pressure drops below the vapor pressure. Flow separation and thermal fatigue arise from adverse pressure gradients or temperature cycling, respectively, and are not the immediate consequence of a sudden valve closure.