Valve Stem Packing: Graphite vs PTFE and How to Control Leakage
Quick Answer
Valve stem packing fills the gland around the stem to block fluid escape where the stem passes through the bonnet. PTFE packing is clean and chemical-resistant for moderate temperature; graphite packing handles high temperature and forms the basis of low-emission sets. Live-loading maintains gland load as packing compresses.
How Packing Works
The gland presses a stack of packing rings against the stem and bonnet. Compression forces the rings to seal both the stem and the bore. As packing wears, gland load drops and leakage rises unless the gland is re-tightened or live-loaded.
Graphite vs PTFE
| Factor | PTFE packing | Graphite packing |
|---|---|---|
| Temperature | Up to about 260 degrees C | Well above 400 degrees C |
| Chemical use | Broad | Broad, including steam |
| Emissions | Standard | Low-emission capable |
| Cost | Lower | Higher |
For high-temperature duty, pair packing choice with the guidance in our high-temperature valve selection guide.
Live-Loading
A live-loaded gland uses a spring stack to hold constant load as packing relaxes. This slows leakage growth and extends service between adjustments. It is common on valves in emissions-controlled service.
Related Standards
Low-emission packing development references API 622 (packing qualification) and ISO 15848-1 (valve fugitive emissions). Our fugitive emissions guide expands on this.
References
Packing selection follows API 622, ISO 15848-1, and manufacturer recommendations. Confirm temperature limits from the packing data sheet.
Frequently Asked Questions
What does valve stem packing do?
It seals the gap where the stem exits the bonnet, preventing fluid leakage along the stem.
Is graphite or PTFE packing better?
PTFE is cleaner and good to about 260 degrees C; graphite handles much higher temperature and supports low-emission sets.
What is live-loading?
A spring device that keeps constant gland load on the packing as it wears, slowing leakage.
When should packing be replaced?
When stem leakage exceeds the acceptable limit or during scheduled maintenance of the valve.
Can packing reduce fugitive emissions?
Yes. Low-emission graphite sets qualified to API 622 are used to meet ISO 15848-1 targets.