On a rising-stem valve the stem travels up out of the body as the valve opens, showing position directly; on a non-rising stem design the stem only rotates, with the wedge climbing an internal thread, saving overhead space. Rising stems dominate industrial service; non-rising stems suit buried and space-limited installations.
The distinction sounds cosmetic and is anything but. A rising stem is threaded to a fixed nut (in OS&Y valves, the yoke bushing outside the pressure boundary): turning the handwheel drives the stem — and the wedge attached to it — vertically. The stem's height above the handwheel is a truthful, unpowered position indicator, and the thread never contacts the medium. The cost is headroom: a 24" gate valve needs most of a metre of clearance for the stem to rise into.
A non-rising stem (NRS) inverts the thread: the stem is held axially in the bonnet and only rotates, while the wedge carries the nut and climbs the stem inside the body. No headroom is needed — the reason NRS valves rule buried water mains, marine engine rooms and pits — but the operating thread now works submerged in the process fluid, and position must be read from a factory-fitted indicator or counted turns.
Selection is straightforward: industrial process and steam service take rising-stem OS&Y valves for thread protection and visible position; waterworks and space-constrained installations take NRS with resilient seats. VTXFLOW supplies both patterns in gate valves from 1/2" to 60".