The trunnion-mounted ball types are assembled similarly, except that the trunnions provide rigid support for the ball so the body joint has to be located fairly close to the center line. In a true trunnion design, the trunnions are integral to the ball such that the trunnion supports must be split or accessible from one end.
Often the bearings are assembled around the trunnions before the assembly is installed. The other difference is that, since the trunnion-mounted ball does not move, the seats must be spring-loaded with disc or wave springs or sets of tiny coil springs. These assemblies must be put together, usually piece by piece, as the valve is assembled.
Some trunnion valves have lower stems that are inserted from the bottom, with a bolted cover that retains the trunnion. These are not the same as true blowout-proof designs but since the lower trunnion. is totally enclosed, it has the same effect.
The second type of floating-ball design is the top-entry valve. This design has a body in one piece with a cap, like a bonnet, that holds the stem and the packing.
The seats in a top-entry valve are normally at an angle to one another, like the seats in a gate valve. This valve is assembled by dropping the ball and seats simultaneously into the body, then placing the cap and stem onto the body. The angle serves to aid in assembly, and to some extent allows wedging of the ball tighter into the seats.
The top-entry design has several advantages over the end-entry designs, one being that there are fewer leak paths to atmosphere, another being that since the body is one piece, the valve is more rigid than some split-body types.
The primary advantages of the type are that the wedging action allows for mechanical loading from the top via a spring pushing on the ball to make it seat tighter and that the valve is repairable in-line that is, the entire valve does not have to be physically removed from the pipe in order to get inside it for maintenance.
There are several methods of making a top-entry valve seat tighter than the corresponding end-entry type. One is by spring-loading the ball from the top with a simple coil spring, to force the ball more firmly into the seats. This force is additive to the pressure force of the fluid flow, such that the valve still seats more tightly on the downstream side.
Another method is to machine the ball with tapers around the port openings, such that in the open position the seats are not loaded or only lightly loaded, but in the closed position the ball forces itself tightly between the two seats. This helps to avoid the phenomenon known as cold flow, where the fluoroplastic material commonly used for seat material deforms under constant load and loses its shape.
Still another type of ball valve is the true one-piece body, in which the body is literally built around the ball in one piece, by welding the components together, such that the ball cannot be removed except by cutting the body in half.
This means the valve has better integrity in terms of leak paths and strength, and can reliably be direct buried or installed undersea or used in remote locations where no maintenance would be performed anyway, but it also means that no seat renewal can be done. The inability to maintain the seats can be designed around, for instance, by providing a mechanism to rotate the seats every time the valve is actuated so that a different portion of the seat takes the greatest seating load every time.
These valves are also provided with sealant injection devices so that if the valve is leaking past the seat into the body cavity, the cavity can be filled with grease or sealant like a plug valve to help obstruct the flow of line fluid around the ball. Pipeline service ball valves designed to API 6A or API 6D have this feature, whether one-piece or top-entry valves.
The largest market for these valves is pipeline service, but they have also been extensively used in refinery service. All of these types of valves can be built as both floating-ball or trunnion-mounted designs. The end-entry is normally built as a floating-ball design, and the one-piece is almost always built as a trunnion-mounted ball design.
