The lowest cost metal ball valves, which can be available in brass, steel, and even stainless steel, are the two-piece threaded-body types in which the entire valve body is split with a threaded joint. They can also be built as a three-way pattern. The disadvantage of this design is that when someone attempts to apply torque to the piping or directly to the valve - for instance, to remove a plug or other piping attached to one end of the valve - it is possible for the tailpiece to unscrew. When this happens, there are two consequences. One is that, since the valve itself has come apart, it is not possible to shut off the fluid flow there. The second is that, since the ball is no longer retained, it can shoot out under line pressure. Both of these occurrences are undesirable. This inherent problem can be remedied at a slightly higher cost by tack-welding the two pieces together (in steel or stainless) or by assembling the valves with a thread adhesive.
An alternative design, which is generally no more expensive, has a one-piece body with a tailpiece that threads into the body through one end, such that both end connections are machined into the same part. Often the tailpiece has straight threads that are the same dia1neter as the bottom of the taper pipe threads. There is a shoulder just above the seat area so that the tail piece will bottom out. The tail piece has a slot in its outer end like a screwdriver slot for installation. Pipe threaded into the end of the valve will not affect the tightness of the seat loading, because of the body shoulder. With this design, no amount of wrenching can cause the valve to separate since the tailpiece is inaccessible from the outside. This design is known as the unibody.
There are a couple of other slightly more expensive designs in common use. One is the two-piece bolted body, in which the valve is split slightly to one side of the centerline and held together with four cap screws threaded into the larger half of the body. This design is somewhat larger than the one-piece body. Another type is the threepiece bolted body, in which there are two identical end pieces bolted into or around a center piece that contains the ball and seats. This design is even larger than the two-piece bolted body, but it has a couple of advantages. One is that, if the end pieces must be of special metallurgy for welding compatibility with, the piping, the center section need not be since it is not being welded to. Another advantage is that, since socket-welding and seal-welding in small ball valves are notorious for melting the seats and destroying the valve before it even goes into service, a three-piece ball valve can easily be unbolted and the center section moved out of harm's way while the end pieces are being welded. Seats and seals are also relatively easy to replace in-line since the complete valve does not have to come out of line. This style is easy to recognize since it is often almost cube shaped. There are two cliff erent types of three-piece bolted-body designs, depending on whether the center section can be swung out or not while the end pieces stay held together with a single bolt. The types that can swing out are easier to change seats in or to gain access to the interior of the valve or pipe, but the types that do not swing out, in which the body bolting is completely enclosed by the center section of the body, have more reliable sealing of the end pieces and are good for higher temperatures and pressures. These types always have four bolts for each end, while the swingout types sometimes have three bolts. The three-piece design can also be built as a three-way.