Which component closes against the moving disc of the valve?

• A. O-ring
• B. Gasket
• C. Packing
• D. Seat ring

Answer: (D)

The moving disc closes against the seating surface provided by the seat ring. This ring creates the smooth, hardened contact surface that mates with the disc to form a tight shutoff when the valve is closed. O-rings and gaskets are used to seal joints but aren’t the surface the disc seals against, and packing seals around the stem rather than providing the closing surface for the disc.