Valve Bonnet and Stem Types: Rising Stem, OS&Y and Bolted vs Pressure-Seal
Quick Answer
The bonnet closes the body and carries the stem and packing. Bolted bonnets suit most services to about Class 600-900; pressure-seal bonnets become preferable at higher pressures because line pressure energizes the seal. The stem type - rising (OS&Y) or non-rising - decides whether the open/closed position is visible from the stem.
Rising Stem (OS&Y)
Outside-screw-and-yoke (OS&Y) rising-stem valves move the stem upward as they open, so stem height shows position. They are required where visual indication matters: fire-protection systems, steam mains, and any valve an operator must confirm at a glance.
Non-Rising Stem
Non-rising-stem valves keep the stem threaded inside the valve; only the handwheel turns. They suit tight spaces and buried or underground service where stem travel is undesirable, but give no visual position cue.
Bolted vs Pressure-Seal Bonnet
| Bonnet | Typical use |
|---|---|
| Bolted | General service, up to high classes (often Class 900) |
| Pressure-seal | Very high pressure; line pressure tightens the seal |
| Union/screwed | Small, low-pressure valves |
Pressure-seal bonnets avoid the bolt-stress limits of large flanged joints, so they dominate at Class 1500 and above.
Maintenance Link
A sound bonnet and stem design also supports packing service - see blowout-proof stem design and stem packing graphite vs PTFE. Hub: gate valves.
Frequently Asked Questions
What does OS&Y mean on a valve?
Outside-screw-and-yoke: a rising-stem design where the stem moves up as the valve opens, so stem height visibly indicates open or closed position.
When is a pressure-seal bonnet used instead of bolted?
At very high pressures (commonly Class 1500 and above), because line pressure energizes the seal and avoids the bolt-stress limits of flanged bolted joints.
Why use a non-rising-stem valve?
In tight spaces or buried/underground service where external stem travel is undesirable; the trade-off is no visual position indication.