Some ball valves are built with the ball and stem machined in one piece, and there are a nunmber of trunnion-mounted designs in which the ball has shafts at both top and bottom. Many, but not all, of these "shafts" are really just bearing surfaces or true trunnions, wherein the length is actually less than its diameter. In both these cases the ball can no longer move into the seat at all or can rnove only in a very restricted manner. This means the seats must move into the ball. This is normally accomplished by placing springs behind each seat. The springs are either a series of small coil springs, or more usually a Belleville or disc spring that pushes on the seat frorn behind. The reasons for building a trunnion mounted valve are that the size of the valve makes the ball too heavy to float adequately, or that the pressure is high enough that the seat load caused by line pressure is so high that it would cause crushing of the seat. Placing the ball on trunnions and springing the seats removes the seat loading from any relationship with line pressure and places it under the control of the valve designer. It is often true that trunnion mounted valves will seal on both upstream and downstream seats, although the downstream seat is still somewhat tighter and the line fluid will still always get behind the ball when the valve is opened and closed. It is possible to obtain very tight sealing with seat springing.
One additional variation on the end-entry design is a valve that has a two-piece tailpiece, one threaded into the body like a small unibody valve, and a second tailpiece that threads or bolts in to provide the flange surface. The inner tailpiece can be used to adjust the seat sealing force by threading it in and out, without affecting the position of the outer tailpiece. This design still requires removal from the line for adjustment.
High-performance ball valves almost always are built with springloaded seats and can be expected to deliver bubble-tight shutoff, even under conditions that tend to cause seat damage. This is often accomplished by providing a hard overlay on the seats and a still harder coating on the ball, with a spring force great enough to hold the two in intimate contact. The torque required to operate a valve like this is, naturally, often considerably greater than that required to operate a standard ball valve. Some designs have a spring-loaded seat on only the upstream side, to add to the fluid force on the ball and to provide son1e sealing in the "wrong" direction, but in the "right" direction the seat is machined directly into the inside of the body so that the ball is pushing against a solid metal seat.
At the opposite end of the spectrum are the low-cost ball valves, which are generally the lowest priced valves of any type because they have the smallest metal mass and the least amount of machining required. A brass valve with elastomer seats will shut off adequately under domestic water pressure and is normally available from stock. Even cheaper valves can be injection molded from PVC or other resins, usually with an acetal plastic ball, sometimes even without elastomer seats. Such valves are even suitable for industrial use in water service and will perfonn adequately, but they should be considered throwaway. Their value makes it uneconomical to expend any labor repairing them.