What should firefighters do to avoid water hammer?

To avoid water hammer, firefighters should open and close hydrants and nozzles slowly. Water hammer occurs when there is a sudden change in the flow of water, which results in a pressure surge that can damage equipment and create hazardous conditions.

When hydrants and nozzles are operated slowly, the flow of water is gradually increased or decreased, which prevents the sudden changes in pressure that lead to water hammer. This technique allows the water to move smoothly through the system, maintaining a steady pressure and reducing the risk of damaging vibrations and shock waves.

The other options suggest actions that would create abrupt changes in water flow, increasing the likelihood of water hammer. For example, sharply opening or closing hydrants or nozzles can cause a quick change in pressure, which is exactly what leads to the phenomenon of water hammer. Thus, the safest practice is to manipulate hydrants and nozzles in a gradual manner to ensure a stable flow and minimize risk.