Ball valves are among the least expensive but most widely used of all valve types,as well as being available in an extremely wide range of sizes.Basic geometry in volves as spherical ball located by two resilient sealing rings in asimple bodyform.The ball has a hole through one axis,connecting inletto outlet with full bore flow when aligned with the axis of the valve. Rotating the ball through 90 degree completely closes the flow passage with positive sealing via the sealing rings. Sealing is equally effective in both directions.
 Body forms and matching ball hole may provide straight through (fullbore parallel), reduced flow, or venturi flow.
The ball itself may be free floating, in which case the squared off or splined end of the stem fits into a matching recess in the op of the ball. On larger valves the ball may be trunnion-mounted. Trunnion mounting reduces operating torque to about two-thirds that of a floating ball.

   Ball valves are produce in top entry and split body forms for assembly and for renewal of the seal and ball.They are also produced in multiport configurations.

   Full operating movement is 90 degree ratation of the ball.Steps may be in corperateed to limit movement of the operating lever,or continuous rotation may be possible.In either case the lever position is in line with the axis of the valve in the open position and at right angles to it in the closed position.Lager ball valves may beoperated by handwheel through reducing gearing,or by powered actuators.Inall cases opening/closing torqueis low because the only frictions forces involved are those of the ball rotating against it seals and the friction offer by the stem gland.the latter can range from O-ring to glands fitted with die-formed packing ring.

Fire-Safe ball valves
   Extensive use is made of ball valves in gas and oil pipeline,where 'fire-safe' characteristics are necessary feature.Fire-safe requires that the valves remain operable with nil or specified minimal leakage(depending on test specification adopted) after being burnt. Burning temperatures will destroy soft seats and seals and various method may be used to overcome this effect.Once approch is to use down steam sael.This can result in the valve being very difficult to operate.Others provide secondarymetal seats to maintain contracr between  ball and a sealing surface in the event of the non-metallic seat being destroyed by heat.