A standard butterfly valve is composed of four main primary components (body, disk, stem, and seat), alongside several smaller internal parts that ensure it seals tightly and operates smoothly.

An exploded view of how these pieces fit together inside a typical resilient-seated (concentric) butterfly valve highlights their arrangement:

1. Valve Body

The body is the outer ring that fits directly between your pipe flanges. It protects and holds the internal parts of the valve in place. As shown in the diagram, it features a heavy outer casing with an integrated top neck where the actuator or handle mounts.

2. Disk (or Disc)

The disc is the closure element that sits directly in the path of the fluid flow. It acts as the gatekeeper. When you turn the valve handle, the disc rotates either parallel to the flow (allowing the fluid to pass) or perpendicular to it (blocking the flow completely).

3. Stem (Shaft)

The stem is the rod that connects the external handle or actuator to the internal disc. When you turn the handle, the stem transmits that rotational force to turn the disc.

One-Piece Shaft: A single long rod  that passes completely through the disc.

Split Shaft (Two-Piece): Consists of an upper shaft and a lower shaft that fit into the top and bottom of the disc. This design allows for better fluid flow through the center of the valve because a thick stem isn't blocking the middle.

4. Seat (Liner)

The liner or seat s an elastomeric (rubber) or metallic ring that lines the entire inside of the valve body. When the valve is closed, the edge of the disc presses firmly into this soft liner to create a bubble-tight seal, preventing any fluid from leaking past. In many standard valves, the liner also wraps around the faces of the body to act as a gasket against the pipe flanges.

5. Shaft Seals & Bushings

To keep fluid from creeping up the stem and leaking out into the environment, the valve utilizes shaft seals or packing rings (labeled at the top neck). Additionally, small bushings or bearings support the shaft's alignment, preventing it from bending or wearing down under heavy fluid pressure.

6. Bottom Plug / Lower Cover

On larger or high-pressure valves, a lower cover or bottom plug  seals the opening at the very bottom of the valve body, holding the lower portion of the shaft or its supporting bearing securely in place.