In fluid mechanics, a water hammer is a pressure surge or wave caused when a fluid in motion is forced to stop or change direction suddenly. It commonly occurs when a valve is closed abruptly at an end of a pipeline system, and this sudden closure causes a pressure wave to propagate in the pipe.

To avoid water hammer, the valve should be:

1. Left open: This option is not feasible as it contradicts the requirement to shut the valve during maintenance.
2. Closed halfway: While this might reduce the flow, it does not solve the problem entirely as the valve needs to be completely shut at some point.
3. Closed gradually: This is the correct answer. By closing the valve gradually, the change in flow is more controlled, and the pressure build-up is minimized. This gradual transition avoids the rapid pressure changes that lead to water hammer.
4. Closed immediately: This action would cause a sudden stoppage of flow, potentially leading to a water hammer effect due to the abrupt change in pressure.

Thus, the valve should be closed gradually to prevent the occurrence of water hammer.