Cement and Water butterfly valves typically have two flanges that “sandwich” the valve under a cement silo or in a water line.  The inside of the valve includes a neoprene seat (soft rubber) that allows the valve disk to open and close within the casting of the valve.  This seat seals the valve when in the closed position.  When installing, the valve needs to be in the OPEN position to allow the neoprene seat to center its self around the outside edge of the disk.  If installed between two flanges with the valve closed, a bulge can occur on the seat.  The bulge will impact the open/close action of the disk and cause excessive wear or chaffing to the seat.  As an option the WAM butterfly valves have the mounting flange  apart of the butterfly valve casting.  This eliminates any miss-alignment issues for cement valves.  WAM also manufactures a single flange butterfly valve that has a lip on one side for a cement boot.