A unit for diversion of a gas which may be hot, dust-laden, saturated with water vapor and containing water droplets, e.g. from steelworks converters, the unit enabling at one and the same time the flow of gases which it receives tangentially and horizontally from the gas collecting network (8) to be caused to flow either into a first vertical duct (14) directed upwards or into a second vertical duct (3) directed downwards, the water droplets and consequently part of the dust to be eliminated, and simultaneously gastightness to be ensured towards that one of these two ducts which is at the moment not being fed with gas, this unit being characterized in that it comprises a fixed body (1) containing a movable bell (2) capable of forming a water seal to ensure gastightness alternatively towards each of the two vertical ducts (14) (3), and in that the fixed body (1) being of special shape cooperates with the bell (2) in order to form a cyclone which eliminates the water droplets and part of the dust contained in the gas at the inlet to the body (1).