What should you do to prevent a vacuum from forming on a boiler that is coming offline?

To prevent a vacuum from forming on a boiler that is coming offline, it is essential to ensure that the steam space within the boiler can equalize with atmospheric pressure. Opening the boiler vent allows air to enter the boiler, preventing the creation of a vacuum as steam condenses or as the system cools down.

When a boiler cools, the steam that was previously in the system can condense back into water, leading to a reduction in pressure. If there is no way for air to enter the boiler to fill that space, a vacuum can form. This vacuum can create a risk of damage to the boiler structure and may hinder safe operations. By opening the vent, you allow atmospheric pressure to balance the internal pressure, thus maintaining the integrity of the boiler system.

In contrast, increasing boiler pressure without proper controls might lead to other operational issues and wouldn't directly address the issue of vacuum formation. Closing the main feedwater valve could lead to improper water levels, and shutting down the fuel supply may not prevent vacuum formation as it doesn't directly address airflow within the boiler. Therefore, opening the boiler vent is the most effective action to prevent vacuum conditions during the offline state.