27.2.3Disk valves

Disk valves (often referred to as eccentric disk valves, or as high-performance butterfly valves) are a variation on the butterfly design intended to improve seat shut-o . The disk’s center is o set from the shaft centerline, causing it to approach the seat with a “cam” action that results in high seating pressure. Thus, tight shut-o of flow is possible even when using metal seats and disks.

The following photograph shows the body of a Fisher E-plug control valve, with the disk in a partially-open position: