Trim & Internals
Plug
The plug is the movable closure member of a globe valve: the shaped element that approaches the seat ring to throttle or shut off flow. Its contour is the primary tool used to shape the inherent flow characteristic (in cageless valves), and its geometry — quick-opening flat, linear parabolic, equal-percentage contoured — is specified alongside the body.
Key variants: contoured (unbalanced) plugs take the full unbalance force but seal well; balanced plugs with pressure-equalizing holes and a piston ring work with a cage to cut actuator size at the cost of a small leakage path past the ring; vented plugs handle two-phase duties. Plug materials range from 316/17-4 stainless to chrome and tungsten carbide where cavitation erosion or solids attack.
Field relevance: plug erosion and seat damage are the leading causes of leakage-class loss; plug/seat lapping restores metal-seated Class IV-V; a scored plug on balanced trim also leaks through its ring, raising minimum flow and spoiling turndown.