The ball valve occupies several niches along the valve spectrum, and includes inexpesive types aswell as very sophisticated high performance designs. They all work in much the same way, just like a plug valve in that there is a section of waterway that rotates 90 degrees when the valve is actuated, and that blocks off fluid flow. The differences in ball valves, as in plug valves, generally relate more to how the valve is put together than how it operates. But as with plug valves, there are some very profound differences between types of ball valves in that some of them function in opposite ways from others. Like the plug valve, a standard ball valve is capable of limited throttling or control, but with some modification it can serve very well as a throttling or modulating valve. It is classified here primarily as an on-off valve because a greater share of its use is as an on-off than as a modulating valve. The design differences in ball valves relate to how the ball comes out-through the end, through the top, or not at all. These differences are profound from the maintenance and safety stand points. Another major classification of ball valves is whether the ball is floating or nonfloating, that is, whether the ball moves into the seat when it shuts off or the seat moves into the ball. The floating-ball type is adequate from most services and is the most common. The type where the ball does not float is commonly known as a trunnion type.
Most ball valves are end-entry types, where either one entire end of the body unscrews to remove the internals or a tail piece comes out from the end, leaving part of that end intact. In smaller sizes the end or the tailpiece is threaded into the body. Sometimes each end is attached to a center section by bolting or threading. In larger sizes the tailpiece threads into the body until it is flush with the rest of the end flange, or the tailpiece is retained by cap screws. The other common type is the split-body, where the entire body is split slightly off-center of the stem with a bolted joint similar to the joint between body and bonnet in a gate valve. Here end-entry and split-body types will be treated together, since most of their other design features are much the same, in contrast to those of top-entry designs.
The main caution that applies to true end-entry flanged valves that does not apply to split-body types is that, as in the high-performance butterfly, the tailpiece often forms part of the gasket sealing surface. The radial split between the tailpiece and the main part of the body is determined by the outside diameter of the ball, but it can fall near the middle of the gasket sealing surface. This is not as severe a problem as it is in the butterfly, but the tailpiece must end up exactly flush with the remainder of the end surface for effective gasket sealing. With spiralwound gaskets, which seal more toward the inside diameter of the sealing surface, the problem is actually no more severe than with flat gaskets, but it is a potential problem nevertheless. Many tailpiece designs now are intended to bottom out against a shoulder in the body, which both ensures correct seat loading without a lot of trial and error, and also places the gasket seating surface correctly. Some designs have an enlarged end on the tailpiece that comprises the entire raised face. This is more common on small ball valves. A standard floating-ball valve, whether end-entry or split-body, is assembled by installing one of the seats into the larger half of the body, then slipping the stem from the inside into the packing gland, then setting the ball onto the first seat and the stem, followed by the second seat and the tailpiece or the smaller body half. During installation the stem and ball are turned as they would be in the closed position, so the ball can be lined up properly with the stem. The two seats are parallel to each other, and perpendicular to and concentric with the flow stream. The packing is installed from the outside, held down with a regular packing gland. Many ball valves have elastomer O-rings for a stem seal, whic are usually installed from the inside along with the stem.