What causes water hammer in a piping system?

Water hammer is a phenomenon that occurs when a fluid in motion is forced to stop or change direction suddenly. This abrupt change creates a pressure wave that travels back through the piping system, leading to banging or knocking sounds, along with potential damage. The primary cause of water hammer is related to the sudden opening or closing of valves. When a valve is closed quickly, the moving water is abruptly halted, generating a shockwave that reverberates through the pipes.

In contrast, restrictions in the line, constant velocity, or constant pressure do not specifically trigger the rapid stop or change in flow direction that leads to the water hammer effect. While these factors can influence fluid dynamics, they do not inherently cause the pressure surges associated with water hammer. The key aspect that directly contributes to this phenomenon is the rapid action of opening or closing valves, making it the correct choice in identifying the cause of water hammer.