What prevents a boiler from collapsing when being dumped?

• A. Manual Air Vent
• B. Safety Valve
• C. Vacuum Breaker
• D. Blowdown Valve

Answer: (A)

When a boiler is being dumped, water and steam are removed from the shell. As the water leaves and the steam space volume changes, the inside can start to pull a vacuum if air cannot enter. That vacuum tends to pull the boiler walls inward, risking collapse. The remedy is to let air into the boiler from the outside as the internal pressure drops, so the pressure inside stays near atmospheric and the shell isn’t squeezed.

A vacuum breaker serves this purpose by opening when the internal pressure falls below ambient, letting air in and equalizing the pressure. That automatic air ingress directly prevents the vacuum from forming and protects the boiler during drainage.

A manual air vent, by contrast, is designed to vent air out of the system as it fills or heats, not to admit air during dumping, so it wouldn’t prevent the collapse risk. The other devices have different roles—safety valves release excess pressure, and a blowdown valve removes water and impurities—not about preventing vacuum during draining.