VALVE FUNCTIONS AND BASIC PARTSDOE-HDBK-1018/2-93ValvesStemThe stem, which connects the actuator and disk, is responsible for positioning the disk.Stems are typically forged and connected to the disk by threaded or welded joints. Forvalve designs requiring stem packing or sealing to prevent leakage, a fine surface finishof the stem in the area of the seal is necessary. Typically, a stem is not considered apressure boundary part.Connection of the disk to the stem can allow some rocking or rotation to ease thepositioning of the disk on the seat. Alternately, the stem may be flexible enough to letthe disk position itself against the seat. However, constant fluttering or rotation of aflexible or loosely connected disk can destroy the disk or its connection to the stem.Two types of valve stems are rising stems and nonrising stems. Illustrated in Figures 2and 3, these two types of stems are easily distinguished by observation. For a rising stemvalve, the stem will rise above the actuator as the valve is opened. This occurs becausethe stem is threaded and mated with the bushing threads of a yoke that is an integral partof, or is mounted to, the bonnet.Figure 2 Rising StemsME-04Rev. 0Page 4
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business