page_title: Valve
section_title: Seat
hierarchical_section_title: Valve / Components / Seat
caption_attribution_description: Balls for ball valves.
context_page_description: A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
  The simplest, and very ancient, valve is simply a freely hinged flap which swings down to obstruct fluid flow in one direction, but is pushed up by the flow itself when the flow is moving in the opposite direction. This is called a check valve, as it prevents or "checks" the flow in one direction. Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.
  Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home. Even aerosol spray cans have a tiny valve built in. Valves are also used in the military and transport sectors.
context_section_description: The seat is the interior surface of the body which contacts the disc to form a leak-tight seal. In discs that move linearly or swing on a hinge or trunnion, the disc comes into contact with the seat only when the valve is shut.  In disks that rotate, the seat is always in contact with the disk, but the area of contact changes as the disc is turned. The seat always remains stationary relative to the body.
  Seats are classified by whether they are cut directly into the body, or if they are made of a different material:
  Hard seats are integral to the valve body. Nearly all hard seated metal valves have a small amount of leakage.
  Soft seats are fitted to the valve body and made of softer materials such as PTFE or various elastomers such as NBR, EPDM, or FKM depending on the maximum operating temperature.
  A closed soft seated valve is much less liable to leak when shut while hard seated valves are more durable. Gate, globe, and check valves are usually hard seated while butterfly, ball, plug, and diaphragm valves are usually soft seated.
caption: Ball valve
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