text: The seat ring provides a stable, uniform and replaceable shut off surface. Seat are usually screwed in or torqued . This pushes the cage down on the lip of the seat and holds it firmly to the body of the valve. Seat may also be threaded and screwed into a thread cut in the same area of the body. However this method makes removal of the seat ring during maintenance difficult if not impossible. Seat rings are also typically beveled at the seating surface to allow for some guiding during the final stages of closing the valve.
  Economical globe valves or stop valves with a similar mechanism used in plumbing often have a rubber washer at the bottom of the disc for the seating surface, so that rubber can be compressed against the seat to form a leak-tight seal when shut.