The essential difference between the floating-ball and the trunnionmounted ball designs is that in the former, as is apparent from its name the sealing action is obtained by the ball moving, or floating, into the seat. The driving force for this is line pressure. When the valve is open, the hydrostatic forces on the ball are minimal. In fact, if the ball valve is full port, the inside of the valve appears to the flow stream to be nothing more than a continuation of the pipe. Regardless, the space between the ball and the body wall is exposed to line fluid almost as if it were a gate valve, since the seats do not contact the ball with any great force when the ball is in the open position, and as the ball rotates, line fluid can flow completely around the ball. As the ball closes, the fluid flow forces begin to push on the ball and, in the closed position, the differential pressure is such that the downstream seat takes the full pressure load exerted on the ball. This is not much different from a gate valve, except that no outside wedging or mechanical loading occurs. Sealing occurs by the def ormation of the downstream seat, which in standard ball valves is an elastomer or a semiresilient polymer such as PTFE. The seats limit the pressuretemperature rating of the valve. An all-metal seat is possible, but the springiness required is such that low-pressure metal-seated valves are difficult to build. A metal backup seat, which is geometrically different from an all-metal seat, must be provided in any valve designated as fire-safe.
The ball ordinarily is mnachined as a true sphere, which is a relatively easy shape to cut on a machine tool. However, because it is more difficult to lap sn1ooth than a flat surface would be, most balls are machined as s1nooth as possible (about 16 RMS; 0.6 µm) and then plated with chrome or, preferably, nickel to reduce the surface roughness to about 4 RMS (0.15 µm). When the ball is a corrosion-resistant material like stainless steel, the surface must be polished to this finish. The coating is not totally impermeable anyway, and is normally not permitted to be considered in evaluating the corrosion resistance of the valve. Some very corrosive services will go directly through coatings, and balls for these services must be uncoated. When metal seats are used, the ball and seats must be installed as a set because the ball and seat set must be lapped together to get good shutoff.
The ball in a floating-ball valve has a slot on top to engage the stem, wrhich has a tongue on the bottom. Early ball valves were designed so that the stem could be inserted from the outside, through the packing area, and engage the ball. This design is now prohibited in ball valves for process service, because if the stem was able to go in through the top of the valve it was also able to come out through the top of the valve, under pressure and with hydrocarbon or steam coming right behind. Almost all ball valve standards now require a blowout-proof stem, in which a section of the stem below the packing area is larger in diameter than the portion of the stein that extends out. to the outside, thus making it impossible to pass the stem cmpletely through the packing gland. This, incidentally, provides a greater area of contact between the ball slot and the stem tongue, which spreads the operating torque over a greater area of the ball. If the ball hangs up or overtorques in some way, it is possible to twist the ball and stem apart, either by twisting the stem in two or by deforming the top of the ball like the screwdriver slot in a household screw.
Many balls, especially in smaller sizes and in valves designed for fairly severe services, are machined out of a solid block or bar. In some valves, the ball is formed out of a tube section, roughly pressed into a spherical shape and then finish machined. The inside of this ball has a roughly spherical shape. Some large trunnion balls are cast with voids in areas where no metal is required, which cuts down the weight and moving mass. In some cases, a large ball may be cast in a hollow spherical shape and a piece of pipe welded inside to form the flow surface. In all cases except the very cheapest, the edges of the ball where they contact the inside flow surface are rounded and polished partway into the rounded edge.