Abstract: A plurality of concentric annular plates (1) are braced against a valve guard (2) by means of radially arranged spring-loaded bridges (5). The guided height (6) of the bridges (5) is at least equal to the maximum lifting height (7) of the annular plates (1) which abut against centering elements (8) of the bridges (5) and are guided by means of the bridges (5) relative to the valve guard (2) along the entire lift. A very simple guiding of the annular plates (1), which preferably impact against the valve guard (2) only via the bridges (5) and thus show a reduced tendency to sticking, is achieved particularly by constructing the annular plates (1) and/or bridges (5) from fiber-inforced plastic material.