PURPOSE: To prevent a rupture of a rubber expandable joint as well as abnormal vibration of turbine in a condenser for a nuclear reactor etc. by an arrangement in which when introducing the turbine bypass steam, a movable guide plate disposed inside the wall of the condenser shell is raised upright by a piston and linkage mechanism.

CONSTITUTION: The steam 1 out of a steam introducing pipe 2 tends to move upward 4 as it hits the shell wall 6. By detecting such superheated steam 4, a piston 10 is actuated by a damper valve open signal to raise a movable guide plate 8 in a position as shown by a solid line so that the superheated steam 4 may be deflected as shown by arrow 11 to be directed to pipe bundles 7, and it may be directed away from a rubber expandable joint 5. By this arrangement, the rupture of the rubber expandable joint 5 as well as abnormal vibration of turbine can be prevented.