In linear valves the stem rises or turns to move a gate, disc or plug; in quarter-turn valves it rotates a ball, plug or disc. It passes through the packing, so its finish and straightness decide how well the packing seals. Ball valves use blowout-proof stems inserted from inside the body so that pressure cannot eject them, and a scored or bent stem is a common reason for packing leaks.
Stem's got a bend in it from somebody's cheater bar, that's why the packing won't stop leaking.
On butterfly and other rotary valves the stem is usually called the shaft; it does the same job.
Also heard as
- spindle
- shaft
Related terms
Packing
Compressible rings packed around a valve stem to stop process fluid leaking out to atmosphere.
OS&Y
Outside screw and yoke: a rising-stem valve design with its stem threads above the packing, out of the fluid.
Yoke
The frame on top of a valve bonnet that holds the stem nut or mounts the actuator above the packing.
Backseat
A seat on the underside of the bonnet that a stem shoulder seals against at full open, isolating the packing.