Waterhammer

Waterhammer, also called hydraulic shock, is a pressure surge in a fluid-filled pipe caused by a sudden change in flow velocity. It commonly occurs when a valve closes rapidly, a pump starts or stops abruptly, or a flow is otherwise interrupted, producing a transient pressure wave that travels through the system.

Mechanism: The moving fluid has inertia and resists rapid deceleration or acceleration. When flow is abruptly reduced, the liquid column compresses and the pressure rises. The resulting wave propagates at roughly the speed of sound in the liquid and reflects at boundaries, potentially producing pressures well above the static operating pressure.

Causes and scenarios: quick valve closures, pump trips, sudden throttling, or abrupt stoppage of flow can trigger waterhammer. It is a concern in municipal water mains, industrial process lines, boiler feeds, and cooling circuits, especially where large-diameter pipes or long lines amplify the effects.

Effects: transient overpressures can rupture pipes, burst joints, loosen supports, and damage fittings or equipment. In severe cases, water hammer can cause leaks, structural damage, or service disruption.

Calculation: the Joukowsky equation relates pressure rise to flow changes: ΔP = ρ c Δv, where ρ is fluid density, c is the wave speed in the liquid, and Δv is the change in fluid velocity. For water (ρ ≈ 1000 kg/m3, c ≈ 1400 m/s), a Δv of 1 m/s yields about 1.4 MPa of pressure rise.

Mitigation: design and operation measures include slow-closing valves, surge tanks or air chambers (though these can become waterlogged), pressure-relief valves, dampers, and proper pipe sizing. Controlled valve actuation and gradual start/stop procedures help prevent waterhammer.