Trim & Internals
Packing
Packing is the deformable sealing system in the valve's stuffing box that prevents process leakage along the stem (or shaft) while still allowing it to move. A stack of rings — PTFE for clean, low-temperature services; flexible graphite for high temperature, fire-safe and emission-critical services — is compressed by a gland flange and studs.
The engineering trade-off is eternal: tighter packing leaks less but adds friction, which raises dead band and hysteresis and can make a control valve too sticky to throttle. Modern answers: live loading (Belleville springs keep constant gland load through wear and thermal cycling), low-friction ring geometries, and monitored packing for fugitive-emission services (ISO 15848 / API 624 type-tested).
Field practice: never over-tighten "just to stop a drip" on a throttling valve without measuring the friction consequence; when packing is replaced, re-run a ramp test and re-verify bench set. Emission standards have pushed packing from a commodity to an engineered component — treat it as one.