I hope I understood your query and that you probably refer to the rotary "valves" at the end of the filter drums.
From what I remember about lube solvent dewaxing vacuum rotary filters, these valves are composed of two circular touching plates or faces; one fixed and attached to external piping (for vacuum and for blow back) and the other attached to, and rotating with, the filter drum.
The fixed plate has circular grooves (and blocked bridges) to enable connecting the various filter compartments either with sucking vacuum or with blowback pressure to discharge the cake built on the drums' surfaces covered with suitable filter cloths.
The flat rotating face shows the ends (holes) of the piping connected with the various drum filtering compartments located exactly in front of the fixed plates' grooves.
On dewaxing filters, drums have about 30 longitudinal compartments and two concentrically circularly arranged rows of piping for easier draining of the rising and descending sections with the drums' rotating cycle.
You can see a sketch on Perry's Chem. Engineers' Handbook, Ed. VIII, Fig. 18-133.