Which condition can cause water hammer?

• A. Subcooled water with condensing steam in a vertical pipe (water cannon)
• B. Air entrainment in the supply line
• C. Slow, uniform flow with no changes
• D. Continuous steady heating

Answer: (A)

Water hammer comes from a sudden change in the momentum of a moving liquid, often amplified when a vapor phase is involved. In a vertical pipe where subcooled water is in contact with steam, steam pockets can exist along the line. If conditions cause rapid condensation of that steam or collapse of a steam pocket, the vapor space vanishes quickly and the liquid must fill the space abruptly. That rapid, transient interaction between collapsing steam and the moving water generates a sharp pressure surge that propagates as a pressure wave through the pipe—the water hammer, sometimes seen as a powerful “water cannon.”

The other scenarios don’t produce that kind of sudden momentum transfer or phase-change-driven impulse: air entrainment can disturb flow but tends to dampen rather than create a sharp surge; slow, uniform flow with no changes lacks any abrupt acceleration or deceleration; and steady heating changes temperature and pressure slowly without a transient shock.