Rising Stem vs Non-Rising Stem
A rising stem gate valve (OS&Y; outside screw and yoke) has a stem that moves upward as the valve opens, providing a clear visual indication of the valve position. A non-rising stem (NRS) gate valve has a stem that rotates in place without moving vertically, requiring a position indicator to show whether the valve is open or closed.
Definition
Rising stem (OS&Y): the stem threads are external, located between the handwheel and the yoke. When the handwheel is turned, the stem rises out of the bonnet. The exposed thread length indicates the degree of opening. The stem thread never contacts the process fluid.
Non-rising stem (NRS): the stem threads are internal, located inside the valve body. The stem rotates in place and the gate travels up or down on the internal threads. The stem does not extend above the bonnet. The stem thread is in direct contact with the process fluid.
When Used
Rising stem (OS&Y): refineries, petrochemical plants, power plants, and all aboveground piping where visual valve position indication is required. Fire protection systems (per NFPA standards) mandate OS&Y valves for visual position confirmation. Standard for most API 600 and API 602 gate valves.
Non-rising stem: underground installations, confined spaces, pipeline burial, and applications where vertical clearance is limited. Common in waterworks (per AWWA standards) and some API 6D pipeline valves.
Specs Table
| Parameter | Rising Stem (OS&Y) | Non-Rising Stem (NRS) |
|---|---|---|
| Stem movement | Rises vertically when opening | Rotates in place (no vertical travel) |
| Position indication | Visual (exposed stem length) | Requires indicator or counter |
| Stem thread location | External (above packing) | Internal (inside body) |
| Thread exposure to fluid | No (protected by yoke) | Yes (threads in process fluid) |
| Vertical clearance needed | High (stem extends above valve) | Low (compact height) |
| Thread lubrication | Easy (accessible for greasing) | Difficult (internal) |
| Thread wear inspection | Visual (external threads) | Not visible without disassembly |
| Corrosion resistance | Better (threads not in fluid) | Threads exposed to corrosion/erosion |
| Standards | API 600, API 602 | API 6D, AWWA C500/C509 |
| Fire protection | Required by NFPA | Not permitted by NFPA |
| Common sizes | All sizes | 2” to 48”+ (pipeline) |
| Underground service | Not suitable (stem exposed) | Standard |
Comparison
The primary advantage of the rising stem design is reliable visual confirmation of valve position. An operator can see from a distance whether the valve is open, closed, or partially open by observing the stem extension. This is critical for safety, lockout/tagout procedures, and fire protection compliance.
Non-rising stem valves are compact and practical for space-constrained or underground installations. However, the internal stem threads are subject to corrosion, erosion, and fouling by the process fluid. In sour gas or corrosive services, this can lead to stem binding and failure. For this reason, OS&Y is the preferred design for most refinery and petrochemical applications.
Both designs share the same pressure-temperature ratings per ASME B16.34 and use the same body and trim materials.
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