All ball valves seal with a rotating ball, but how that ball is supported changes everything about pressure rating, operating torque and price. The two architectures are floating and trunnion-mounted.
In a floating ball valve the ball hangs on the stem and is free to shift slightly downstream. Line pressure pushes the ball against the downstream seat — the higher the pressure, the tighter the seal. This elegant simplicity keeps cost low, but the same force must be overcome to rotate the ball, so torque rises quickly with size and pressure.
A trunnion-mounted ball is fixed on upper and lower bearings. Instead of the ball moving to the seat, spring-loaded seats move to the ball. Sealing force stays nearly constant regardless of pressure, so torque stays low even at 40 inches and Class 2500 — and features like double block & bleed and seat injection become possible.
A trunnion valve costs more up front, but on large or high-pressure service it allows a smaller, cheaper actuator and dramatically longer seat life — total cost of ownership often favors it well before the technical limit of floating designs is reached.