Another type of ball valve, which is most commonly used as a control valve, is the eccentric ball valve. It is essentially one-half of a ball with one seat, and as such it is very directional, more so than the rising-stem ball described above. The eccentric ball is a true quarter-turn valve, like other actuated ball valves.
It is often referred to as a "rotary" valve or other names to distinguish it from a more standard ball valve. Because of its eccentric construction, the ball is rigidly mounted with a trunnion and sometimes with a top bearing where the stem does not serve to guide the ball at all, only to rotate it.
Almost all eccentric actuated ball valves and some of the high-performance or special-purpose actuated ball valves have a distinctive shape to the edge of the ball that enters the flow stream first.
These are known as characterized balls. Just like special globe disks, these contours provide some type of special flow characteristic. Some are V-shaped, to provide accurate control when the percent open is very small and to provide a constant increase in flow rate with a constant rotation of the actuator (equal-percentage opening). Some are U-shaped, to provide a somewhat faster opening (quick opening).
Some high-performance actuated ball valves, the metal-seated types for severe service, have a notch that roughly parallels the edge of the ball but is set in slightly, known as arcuate cut, which has the effect of opening the flow passage by a large percentage all at once and then gradually. This is good for erosive flows and high differential pressures, so as to minimize the tendency for high opening velocities to cut into the ball.
The arcuate cut is also used in a number of high-performance actuated ball valves intended for on-off service, because of its improved performance under high cliff erential pressures and erosive services. Another technique used in high cliff erential services where cavitation could be a problem is to provide internal baffles to break up the flow stream so that cavitation cannot occur.
The eccentric actuated ball valves used for flow modulation are almost all wafer types, installed between two flanges with a set of bolts that passes from one line flange to the other, and only centering the body and not actually bolting into it. This is not really a requirement for this type of valve, but the design is well ingrained in standard practice.
These valves are end-entry design just like many others, and as with most eccentric butterfly valves, their preferred orientation is with the flow into the disc from the tailpiece end. These valves are often designed to rotate only 60 degrees from full open to full closed since there is only one seat and these valves are often not designed for tight shutoff anyway, just like many other modulating valves.
For proper control, wafer-type valves almost always have spline connections between the ball and shaft and between the shaft and actuator linkage bracket for minimal slop or hysteresis, when the ball and stem are not machined as one piece.