Sliding-Stem Valves
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 , 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 lock nut 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
- A sliding stem valve with a diaphragm actuator and digital positioner should have a precision (deadband, resolution, and threshold sensitivity) of less than 0.2%. Even the best rotary valves usually have a precision that is twice as large
- A sliding stem valve has a direct connection between the actuator shaft and the internal flow element trim (e.g., plug). The motion is linear and direct. There is no backlash from shaft connections, translation from linear to rotary motion, linkages, or rotary element seals (e.g., ball and butterfly seals).
- Sliding stem valves offer a better installed characteristic than that of rotary valves. The installed characteristic for rotary valves tends to become too flat at upper valve positions (i.e., more open); valve gain becomes too low. Sliding stem valves also offer more choices for inherent trim characteristics.
Rotary-Shaft Valves
Although they are less popular than sliding-stem control valves, a wide range of rotary-shaft control valves, including ball valves, butterfly valves, and rotary-plug valves, are also available. The components of a typical rotary-shaft control valve include the valve body and the trim as shown in Figure
An important advantage of rotary control valves over sliding-stem designs such as the globe valve and diaphragm valve is a virtually obstruction less path for fluid when the valve is wide-open.
Valve Body – The valve body is the major fluid boundary and pressure-containing component. The valve body also supports and locates internal parts such as shafts, bushings, and seals. As Figure 4 indicates, many rotary-shaft valve designs do not include a separate bonnet.
Trim refers to all internal, process-wetted parts. Trim includes:
•A closure member in the form of a disk, ball, ball segment, or valve plug.
•A seal or seat ring that mates with the closure member to provide shutoff when the valve is in the closed position.
•A retainer that locates and secures the seal or seat ring in the valve body.
•A valve shaft that transmits torque to the closure member.
•Bearings and/or bushings that locate and support the valve shaft.
•Packing that prevents process fluid leakage along the valve shaft
