Trim & Internals
Balanced Trim
Balanced trim equalizes the pressure acting on the two sides of the plug. Holes through the plug admit upstream (or downstream) pressure to the opposing face, and a piston ring (or flexible seal) isolates that chamber from the other side. The static unbalance force across the plug then nearly vanishes, so the actuator sizes for control forces and seat load instead of fighting full line ΔP — often one or two actuator sizes smaller.
The price: the piston ring's clearance is a leakage path, so balanced globes rarely reach the tightest metal-seat classes without a soft seat or a separate shutoff valve. The ring is also a wear item — a leaking ring raises the minimum controllable flow and quietly degrades turndown, a classic "the valve got sticky at the bottom" diagnosis.
Unbalanced trim keeps the plug solid and seals hard, at the cost of a larger actuator; the choice is a classic sizing trade-off made per service.