💧 WHAT IS WATER HAMMER? Water hammer is a pressure surge that occurs when a moving column of water in a hose, pipe, or hydrant is suddenly stopped or rapidly changed in direction. According to IFSTA Essentials of Firefighting, 8th Edition (Fire Hose Operations), water hammer occurs because water has mass and momentum. When a firefighter abruptly closes a fire nozzle or valve, the moving water has nowhere to go. The energy is instantly converted into a pressure shockwave that travels back throug