Valve Disk and Valve Seat

For a valve having a bonnet, the valve disk is the third primary principal pressure boundary. The valve disk provides the capability for permitting and prohibiting fluid flow. With the disk closed, full system pressure is applied across the disk if the outlet side is depressurized. For this reason, the disk is a pressure-retaining part.

Disks are typically forged and, in some designs, hard-surfaced to provide good wear characteristics. A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed. Most valves are named, in part, according to the design.

The seat or seal rings provide the setting surface for the disk. In some designs the body is machined to serve as the seating surface and seal rings are not used. In other designs, forged seal rings are threaded or welded to the body to provide the seating surface.

To improve the wear-resistance of the seal rings, the surface is often hard-faced by welding and then machining the contact surface of the seal rings. A fine surface finish of the seating area is necessary for good sealing when the valve is closed.


Valve disk and valve seat are components commonly found in valves used in various mechanical systems, such as engines, pumps, and industrial machinery. They work together to control the flow of fluids or gases through the valve.

Valve Disk

The valve disk, also known as the valve closure element or valve plug, is the movable part of the valve that controls the flow. It is typically a flat or curved plate that can be positioned to block or allow the passage of fluid or gas through the valve. The disk is attached to a stem or spindle, which connects it to the valve actuator or handle, allowing it to be opened or closed.

Valve Seat

The valve seat is the stationary part of the valve against which the valve disk seals when closed. It provides a sealing surface for the disk and helps control the flow rate and direction. The valve seat is usually made of a hard and wear-resistant material, such as stainless steel or other alloys, to withstand the pressure and erosive nature of the fluid or gas passing through the valve.

When the valve is closed, the disk comes into contact with the valve seat, creating a tight seal that prevents the fluid or gas from flowing through. When the valve is opened, the disk moves away from the seat, allowing the fluid or gas to pass through the valve unobstructed.

Proper alignment and condition of the valve disk and valve seat are essential for the valve to function effectively. Over time, due to wear and tear or damage, these components may require maintenance or replacement to ensure optimal valve performance and prevent leakage.