APPLICATION TO CORROSIVE WORKING CONDITION!
In the cast iron butterfly valve design, the stem passes through the center of the gate into the pipe bore which is also located at the center of the valve. To seal the flow, concentric valves should have a very flexible seat rubber because the disk touches the seat first when rotated in a quarter turn.
Since the disk and the seat are always in contact, the body of the valve is less prone to corrosion as the media flows. It only touches the seat that acts like a sleeve.
Low-pressure applications are the common uses for this valve. They also go by these names: centric butterfly valve, centerline butterfly valve, or the resilient-seated valve.