What effect does a water hammer have on water pipelines?

A water hammer is a pressure surge or wave that occurs when a fluid in motion is forced to stop or change direction suddenly. This phenomenon typically happens in pipelines when a valve closes quickly or when a pump starts or stops. The sudden change creates a rapid transient pressure that can lead to severe vibrations within the pipeline.

Thus, the correct answer highlights that water hammers cause vibrations, which can have several consequences for the integrity of the pipeline, including potential damage to joints, fittings, and the overall structure. These vibrations can lead to fatigue and even failure over time if not managed properly.

The other options may seem plausible but do not accurately encompass the primary effect of a water hammer. Increased pressure is a direct effect, but it doesn't capture the continuous implications of the phenomenon on the system. Decreased flow is not a direct result of water hammer, as flow can still be maintained even with vibrations occurring. Lastly, although a water hammer can impact system efficiency negatively, it does not lead to overall improved efficiency. Thus, understanding the vibrations caused by a water hammer is essential for maintaining the integrity and longevity of water pipelines.