Sliding-stem valves include globe valves, angle valves, and axial flow valves. Of all the control valve types, the sliding-stem, globe style valve is the most common. As shown in Figure 3, the major components of a sliding-stem, globe style valve include the valve body, bonnet, trim, gaskets, and packing.
Valve Body – The valve body is the main fluid boundary and pressure containing component. The valve body includes provisions for securing internal parts, end connections that allow installation in the pipeline, and a means for attaching the bonnet.
Bonnet – The bonnet is also a major pressure containing component and fluid boundary. In common configurations, the bonnet is bolted or threaded onto the valve body. The bonnet locates and guides the valve stem, and it includes a bore for the packing that is referred to as the packing box. The bonnet also includes a yoke boss or some other means of mounting an actuator. When the valve design includes a yoke boss, an actuator yoke locknut secures the actuator to the bonnet.
Most globe and angle valves are top-entry designs. This means that removal of the bonnet allows access to all internal trim components for maintenance or replacement. The top-entry design allows in-line valve maintenance, provided that the valve is isolated from system pressure prior to bonnet removal.
Trim – Trim refers to all internal, process wetted components. Trim includes the valve plug, the valve plug stem, the cage, and the seat ring.
Gaskets – In a typical valve construction, a bonnet gasket provides a seal between the body and bonnet mating surfaces; a cage gasket provides a seal between the bonnet and the cage mating surfaces; and a seat ring gasket provides a seal between the seat ring and body mating surfaces.
Packing – Packing prevents leakage along the valve plug stem. Packing is compressed to form a tight seal between the packing box wall and the valve plug stem by tightening the packing flange nuts. As the nuts are tightened, the packing flange transfers the compressive load to the packing.