The most basic type of butterfly valve design is a centric or concentric butterfly valve. This means that the stem passes through the centerline of the disc, which is in the center of the pipe bore, and the seat is the inside diameter periphery of the valve body. This zero-offset valve design is also called resilient-seated because it relies on the flexibility of the seat rubber to efficiently seal the flow when closed. In this type of valve, the disc first comes into contact with the seat at around 85° during a 90° rotation. Concentric butterfly valves are commonly used for low pressure ranges. Centric butterfly valves with fixed liners Fixed liner butterfly valves can be used under vacuum conditions as there is no movement or wear of the liner. Furthermore, the fixed liner valve is suitable for high flow velocities and high operating cycles, e.g., open and close every few seconds. Fixed-liner butterfly valves are typically used above ground, but the double-flanged types are also suitable for below-ground installations. Available for water, wastewater, gas, fire protection, and industry applications Centric butterfly valves with loose liners Loose-liner butterfly valves are typically a cost-effective solution for applications above ground where replacement and maintenance are relatively easy. The liner is replaceable without a full replacement of the valve, and the design gives more options for different liner materials. available for water, wastewater, and industry applications.