TYPES OF VALVES
Ball valves are available in the venturi, reduced, and full port pattern. The full port
pattern has a ball with a bore equal to the inside diameter of the pipe.
Balls are usually metallic in metallic bodies with trim (seats) produced from elastomeric
(elastic materials resembling rubber) materials. Plastic construction is also available.
The resilient seats for ball valves are made from various elastomeric material. The most
common seat materials are teflon (TFE), filled TFE, Nylon, Buna-N, Neoprene, and
combinations of these materials. Because of the elastomeric materials, these valves
cannot be used at elevated temperatures. Care must be used in the selection of the seat
material to ensure that it is compatible with the materials being handled by the valve.
Ball Valve Stem Design
The stem in a ball valve is not fastened to the ball. It normally has a rectangular portion at the
ball end which fits into a slot cut into the ball. The enlargement permits rotation of the ball as
the stem is turned.
Ball Valve Bonnet Design
A bonnet cap fastens to the body, which holds the stem assembly and ball in place. Adjustment
of the bonnet cap permits compression of the packing, which supplies the stem seal. Packing for
ball valve stems is usually in the configuration of die-formed packing rings normally of TFE,
TFE-filled, or TFE-impregnated material. Some ball valve stems are sealed by means of O-rings
rather than packing.
Ball Valve Position
Some ball valves are equipped with stops that permit only 90 rotation. Others do not have
stops and may be rotated 360. With or without stops, a 90 rotation is all that is required for
closing or opening a ball valve.
The handle indicates valve ball position. When the handle lies along the axis of the valve, the
valve is open. When the handle lies 90 across the axis of the valve, the valve is closed. Some
ball valve stems have a groove cut in the top face of the stem that shows the flowpath through
the ball. Observation of the groove position indicates the position of the port through the ball.
This feature is particularly advantageous on multiport ball valves.