How do you size a pneumatic actuator for a control valve?

Actuator sizing ensures adequate thrust to close and modulate valve against process forces. Calculate: unbalance force (differential pressure x plug area), seat load (for shutoff class), friction (packing, bearings), and dynamic forces (flow-induced). Total required thrust with safety factor determines actuator size. Check: spring range compatible with signal range (3-15 psi), available supply pressure provides adequate output, and fail-safe thrust at minimum supply pressure. Consider: double-acting piston for high thrust, booster relays for fast response, and volume boosters for large actuators. Document thrust calculations in valve datasheet.