VALVE FUNCTIONS AND BASIC PARTS
Figure 3 Nonrising Stems
There is no upward stem movement from outside the valve for a nonrising stem design.
For the nonrising stem design, the valve disk is threaded internally and mates with the
The actuator operates the stem and disk assembly. An actuator may be a manually operated
handwheel, manual lever, motor operator, solenoid operator, pneumatic operator, or hydraulic
ram. In some designs, the actuator is supported by the bonnet. In other designs, a yoke
mounted to the bonnet supports the actuator.
Except for certain hydraulically controlled valves, actuators are outside of the pressure boundary.
Yokes, when used, are always outside of the pressure boundary.
Most valves use some form of packing to prevent leakage from the space between the stem and
the bonnet. Packing is commonly a fibrous material (such as flax) or another compound (such
as teflon) that forms a seal between the internal parts of a valve and the outside where the stem
extends through the body.
Valve packing must be properly compressed to prevent fluid loss and damage to the valve's
stem. If a valve's packing is too loose, the valve will leak, which is a safety hazard. If the
packing is too tight, it will impair the movement and possibly damage the stem.