The butterfly valve is light in weight and is relatively small and quick acting. Butterfly valves are used in freshwater, fuel, lube oil, and chilled water systems where quick action and positive flow control are required.

Butterfly valves operate in the same manner as the throttle valves and the choke valves in carburetors. A disk attached to a shaft pivots between the open and closed positions as the shaft is turned.

Figure 13-20 shows an older design of butterfly valve that is still widely used. This valve provides for positive shutoff, but it should not be used for throttling as standard practice. It consists of a body, a resilient seat, a butterfly-type disk, a stem, packing, a notched positioning plate, and a han-dle. The seat is under compression when it is in-stalled in the valve body. This design provides a seal for the disk and the upper and lower points where the stem passes through the seat. The pack-ing provides a positive seal around the stem for added protection in case the seal formed by the seat becomes damaged. To gain access to the seat for replacement, you must first remove the stem and valve disk.

Figure 13-21 shows a high-performance butterfly valve. This improved design has higher pressure capabilities and allows for a full range of throttling positions not offered in the older design of butterfly valve shown in figure 13-20. The newer design of butterfly valve has been introduced into the fleet and is gradually replacing the older design. Unlike the older style of butterfly valve, the new style has a removable retaining ring that holds the seat in place. To change the seat,

Figure 13-20.-Butterfly valve.

Figure 13-21.-Cutaway view of a high-performance butterfly valve.

Figure 13-22.-Automatically operated butterfly valve.

all you must do is remove the retaining ring, remove the old seat, place a new seat into its groove, and reinstall the retainer ring. To open or close a butterfly valve, turn the handle only one quarter of a turn to rotate the disk 90 degrees. Some larger butterfly valves may have handwheels or actuators that are driven by pneumatic, electric, or hydraulic means through a gearing arrangement. (See fig. 13-22.)

Ball Valves

Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. Most ball valves are the quick-acting type. They require only a 90-degree turn to operate the valve either completely open or closed. The ball, shown in figure 13-23, performs the same func-tion as the disk in the globe valve. When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the ball is rotated so the hole is perpen-dicular to the openings of the valve body, flow is stopped.