The seat of a valve is the area of the body where the disc of the valve sits to form a tight seal. Usually the disc of the valves only comes in contact with the seat when the valve is closed, but this is not the case in rotating discs where the seat is in contact with the disc at all times while the area of contact changes as the disc rotates.

There are two different types of Valve Seats:

• Hard Seats which are those cut out of the valve body. This type of valve is liable to have a small amount of leakage but it is durable. Gate, globe, and check valves usually use hard seats.
• Soft Seats which are those fitted into the valve body and made from softer material which depends on the highest operating temperature. This type of valve Seat is much less likely to leak but less durable than hard Seats. Butterfly, ball, plug, and diaphragm valves usually use Soft Seats.

Valve seats are more commonly formed by initially press-fitting an almost cylindrical piece of a hardened metal alloy. The surfaces of these conical segments are designed to match with the valve. An alloy such as Stellite is commonly used and a depression is made into a cylinder head above each eventual valve stem position. Then the cast is machined to create several conical segments to form the valve seat into a shape that will match the coupling surface on the matching valve.

The seat ring provides a stable, consistent and expendable shut off surface. Seat rings are normally held in place by pressure from the fastening of the bonnet to the top of the body. This pushes the cage down on the rim of the seat ring and holds it securely to the body of the valve. Seat rings may also be threaded and screwed into a thread which has been machined in the body but this makes it difficult to remove the seat ring during maintenance. Seat rings are also classically beveled at the seating surface which allows for some positioning, during the later stages of the closing of the valve.