Question 193Steam Plants - Assistant Engineer

The correct answer is D) prevent the formation of a vacuum within the boiler. When a boiler is taken off the line and begins to cool, the air cock is opened to allow air to enter the boiler. This prevents the formation of a vacuum inside the boiler, which could occur as the steam condenses and the volume decreases. Allowing air to enter through the open air cock ensures that the boiler cools evenly and without creating a dangerous vacuum condition. The other answer choices are incorrect because they do not accurately describe the purpose of opening the air cock during boiler cooldown. Purging air from the steam drum (A), guarding against gas pockets in the superheater (B), and allowing even cooling of the steam drum (C) are not the primary reasons for opening the air cock in this situation.