Diversion device with single valve element flag damper for diverting a gaseous flow, possibly hot and/or filled with dust and/or containing at least a dangerous even toxic component, between a primary aeraulic circuit and a secondary aeraulic circuit, of the type comprising: - a section of primary duct, rectilinear and tubular of quadrangular cross-section, whose entrance and exit are intended to be connected to the primary aeraulic circuit, - a section of secondary duct, also rectilinear, tubular and of quadrangular cross-section, whose entrance opens into one of the four walls of the section of primary duct, and whose exit is intended to be connected to the secondary aeraulic circuit, - a valve element, of quadrangular shape, mounted pivotingly, substantially along one of its edges, in the section of primary duct, about a geometric axis transverse to the said wall into which the section of secondary duct opens and downstream of the latter, between two positions, a first of which is a position of opening towards the primary aeraulic circuit in which the valve element closes the entrance of the section of secondary duct, and the second of which is a position of closure of the primary aeraulic circuit, in which the valve element is moved away from the entrance of the section of secondary duct and pivoted so as to prevent the gaseous flow from discharging through the exit of the section of primary duct and that the gaseous flow entering via the entrance of the section of primary duct is diverted towards the section of secondary duct, and - a link arm assembly, connected to the valve element in order to operate it by pivoting from one to the other of the said two positions, the valve element having, on its face turned towards the section of secondary duct in its first position, a quadrangular annular bearing surface to press in sealed manner against a seat formed in the upstream end of the section of secondary duct, delimiting the entrance of the latter in the section of primary duct, the valve element being mounted pivotingly relative to the geometric pivot axis, the two edges of the valve element which are perpendicular to its pivoting edge having, with the facing walls (20, 21) of the section of primary duct, a limited clearance so as not to block the pivoting actions of the valve element between its two positions, characterized in that the quadrangular annular bearing surface and the seat are formed by machining, the valve element being mounted pivotingly with a limited radial clearance relative to the geometric pivot axis, and in that, in the second position of the valve element, its free edge opposite its pivoting edge is resting against the wall of the section of primary duct, which is opposite the wall into which the section of secondary duct opens. - a section of primary duct, rectilinear and tubular of quadrangular cross-section, whose entrance and exit are intended to be connected to the primary aeraulic circuit, - a section of secondary duct, also rectilinear, tubular and of quadrangular cross-section, whose entrance opens into one of the four walls of the section of primary duct, and whose exit is intended to be connected to the secondary aeraulic circuit, - a valve element, of quadrangular shape, mounted pivotingly, substantially along one of its edges, in the section of primary duct, about a geometric axis transverse to the said wall into which the section of secondary duct opens and downstream of the latter, between two positions, a first of which is a position of opening towards the primary aeraulic circuit in which the valve element closes the entrance of the section of secondary duct, and the second of which is a position of closure of the primary aeraulic circuit, in which the valve element is moved away from the entrance of the section of secondary duct and pivoted so as to prevent the gaseous flow from discharging through the exit of the section of primary duct and that the gaseous flow entering via the entrance of the section of primary duct is diverted towards the section of secondary duct, and - a link arm assembly, connected to the valve element in order to operate it by pivoting from one to the other of the said two positions,