What caution should be observed when shifting main feed lube oil coolers?

When shifting main feed lube oil coolers, keeping the fill valve open is crucial to ensure that there is a continuous flow of oil during the transition. This action helps maintain adequate lubrication of the system and prevents air from entering the lube oil system, which could lead to cavitation or damage due to insufficient lubrication. The fill valve's open status allows for a proper balance and pressure to be maintained, ensuring that the changeover does not disrupt the system’s operation or cause any potential for damage.

In contrast, shutting all valves or closing the bypass valve can disrupt oil flow and pressure, leading to possible system failures. Keeping the steam line pressurized may not directly relate to the oil cooler shift and could introduce unnecessary complications during the transition.