In the course of operation or when the system is being shut, pressure might go below the atmospheric pressure creating a vacuum inside the system or the process equipment. Vacuum formation can damage equipment or the system and hence vacuum breakers should be installed.
Vacuum breakers remain in closed position during the normal operation. When the vacuum forms, the external (atmospheric) pressure displaces the closing element such that the air from outside can flow inside and prevent the vacuum formation.
Vacuum breakers find multiple applications in steam systems such as small heat exchangers where they can avoid stalling.