OS&Y vs NRS Gate Valve
OS&Y stands for Outside Screw and Yoke. It describes a gate valve with a rising stem where the threaded portion of the stem is outside the valve body and exposed to the atmosphere. NRS stands for Non-Rising Stem, where the stem threads are inside the body and the stem does not move vertically when the valve is operated.
When to Use Each Type
OS&Y (rising stem) valves are the default choice for above-ground process piping in refineries, chemical plants, and power stations. The exposed stem thread provides clear visual indication of valve position: stem up means open, stem down means closed. This is critical for safety and operational verification.
NRS (non-rising stem) valves suit applications where vertical clearance is limited: underground pipelines, buried water mains, and confined spaces. The stem rotates in place without extending upward, so the valve requires no additional headroom above the handwheel.
Specifications and Comparison
| Parameter | OS&Y (Rising Stem) | NRS (Non-Rising Stem) |
|---|---|---|
| Stem movement | Rises vertically when opening | Rotates in place, no vertical travel |
| Thread location | Outside body (exposed) | Inside body (submerged in fluid) |
| Position indication | Visual (stem height) | Requires external indicator |
| Vertical space | Needs clearance equal to stem travel | Compact, no extra height needed |
| Thread exposure | Protected from fluid, exposed to atmosphere | Exposed to fluid, protected from atmosphere |
| Lubrication | External threads easily greased | Internal threads harder to maintain |
| Standards | API 600 (cast), API 602 (forged) | API 600, API 602 |
| Typical application | Above-ground process piping | Underground, buried, or confined-space piping |
| Fire safety | Preferred (stem packing visible for inspection) | Acceptable with external position indicator |
| Cost | Slightly higher (longer stem, yoke) | Slightly lower |
Design Details
In an OS&Y valve, the yoke (a bracket attached to the bonnet) supports the stem nut. Turning the handwheel rotates the nut, which drives the stem upward or downward. The stem threads never contact the process fluid, reducing corrosion and making thread inspection straightforward.
In an NRS valve, the stem threads engage directly with the gate. The stem rotates but does not translate vertically. This means the threads are immersed in the process fluid, which can cause corrosion, erosion, or fouling depending on the service. NRS valves are harder to inspect because the internal thread condition cannot be visually assessed without disassembly.
Position Indication
OS&Y valves have a built-in position indicator: the exposed stem. NRS valves require an add-on position indicator or limit switch, especially when used with actuators. For safety-critical isolation valves, ASME B16.34-rated OS&Y designs with clear visual indication are strongly preferred.
Both OS&Y and NRS designs are available in all pressure classes (150 through 2500) and in both cast and forged body materials.
Leave a Comment
Have a question or feedback? Send us a message.