The three-piece ball valve lends itself well to sanitary services, where the valve must be disassembled for cleaning. These valves can be equipped with a cavity-filler seat, designed to leave essentially no cavities around the ball so as not to allow a buildup of contaminated material in the valve. The three-piece ball valve can also easily be used for high purity gas service, where bleeds and purges are installed in the valve. Both services often use welded-end bodies, which can be easily built because of the three-piece design. Another style used in oilpatch work is the true-union type. This is actually a three-piece design also, but with union nuts on each end piece. The center section is still removable from the line after the end pieces are welded or threaded to the adjacent piping. In addition to the steel variety, this design is used often in plastic body valves where it is probably the most common type. Some plastic body valves are the single-union type, in which one end of the body is cemented or solvent-welded solidly to the pipe and the other end has a union for breakout.
Still another design often used in oil production services, but also used in process work, is a three-piece threaded body. This design has two ends that thread into a center section, each including the seat. These valves are generally somewhat sturdier than the two-piece threaded designs discussed above, and are sometimes available with flanged ends. To help in the prevention of accidental disassembly, the ends are often assembled with a thread adhesive.
All the designs described above are for small valves, from about ¼". pipe size (DN 8) to 2". or 3". pipe size (DN 50 or DN 80). In larger sizes, and in many cases starting from 1/2". pipe size (DN 15), ball valves are generally flanged like any other valves. Even small valves are often built with flanged ends for corrosive or high-pressure services, since flanged joints hold pressure better than do threaded joints and socketweld end ball valves are so difficult to install reliably. In flanged-end construction, the end-entry is the cheapest. For cross-country transportation piping or for services where leaks are unusually intolerable, welded-end ball valves are common and are generally of the one-piece welded-body type or the top-entry type.
For higher pressures and larger body sizes, the split-body and the one-piece valves have become more common. The split-body design can also be plastic-lined. Split-body valves in larger sizes are often made ,vith two body joints, with two identical end pieces and a center section, somewhat like the smaller three-piece bolted types. This design lends itself to an interesting variation known as the tank-bottom valve, where one of the end pieces is designed to be integrally welded to a tank or vessel. A quite different type of ball valve is known as the single-seat type, as well as by proprietary names. This valve has a rising stem, like a gate valve or a lift plug valve. Like a lift plug, it has a slot in the stem that cams the stem 90 degrees around as the hand wheel turns and lifts the stem. This valve has a ball, also referred to as a core since it really is not a complete ball, that is loosely attached to the·stem with a T-head similar to that of a gate valve, so that the ball can be lifted. There is only one seat, on the downstream side. As with any floating-ball type, in the closed position line pressure holds the ball against this seat. The difference is that the stern can exert force on the ball to seat it more tightly. The single-seat ball valve is capable of very tight shutoff. It will seal in the opposite direction, but generally not as well. Its application like that of the lift plug, is for absolute separation of two products or for better shutoff of light fluids. The single-seat type can even be machined with grooves and a bleed to atmosphere for a double block and bleed. Because there is only one seat, a special body vent is not required to prevent overpressuring since the body cavity is always open to the piping on one side.