A water hammer is caused by?

• A. Air in the pipes
• B. High water pressure
• C. The gradual acceleration of flow
• D. The sudden stop of water flow in the system

Answer: (D)

Water hammer comes from the inertia of moving water. When a valve closes suddenly or a pump stops quickly, the flowing water cannot stop instantly, so its momentum is converted into a rapid pressure rise that travels through the pipe as a shock wave. That sudden stop of water flow in the system is what creates the hammer. Air in pipes can cause loud banging and can complicate the issue, but the fundamental cause is the abrupt deceleration of the moving water. High pressure makes the resulting surge bigger, but it isn’t by itself the trigger. Gradual acceleration doesn’t generate a shock because the change in velocity happens slowly enough that no sudden pressure wave forms. To prevent water hammer, engineers use slow-closing valves, surge dampeners, or air cushions to absorb or smooth out the pressure surge.