Valve stem connects the actuator and disk for positioning the disk in accordance with requirement. Stems are typically forged and connected to the disk by threaded or welded joints. For valve designs requiring stem packing or sealing to prevent leakage, a fine surface finish of the stem in the area of the seal is necessary. Typically, a stem is not considered a pressure boundary part.
Two types of valve stems are rising stems and non-rising stems. These two types of stems are easily distinguished by observation.
For a rising stem valve, the stem will rise above the actuator as the valve is opened.
This occurs because the stem is threaded and mated with the bushing threads of a yoke that is an integral part of, or is mounted to, the bonnet.
There is no upward stem movement from outside the valve for a non-rising stem design.
For the non-rising stem design, the valve disk is threaded internally and mates with the stem threads.