Trim & Internals
Seat (Seat Ring)
The seat ring is the fixed orifice ring in a globe valve against which the plug closes. Together, plug contour and seat define both the throttling behaviour near closure and the achievable leakage class.
Construction variants: one-piece screwed or welded-in seat rings (simple, but field replacement is hard), threaded rings (replaceable, can loosen under cycling), and clamped/pinned rings held by the cage (today's standard — changeable without machining the body). Seat surfaces are lapped flat or angled; metal-to-metal seats achieve Class IV-V, while inserts of PTFE or other elastomers reach Class VI (bubble-tight) at the price of temperature and erosion limits.
Damage modes: erosion by cavitation and solids (pitted seats leak at "closed"), wire-drawing at high ΔP near closure (a narrow jet that cuts a groove), and fretting from plug/seat misalignment. Remedies range from harder seat materials and Stellite facing, to trim redesign (multi-stage cages that keep ΔP per stage low), to proper actuator seat-load — an under-sized actuator is a frequent root cause of "leaky" seats.