Page 114 - Valve Selection Handbook

Manual Valves 101

BALL VALVES

Ball valves are a species of plug valves having a ball-shaped closure
member. The seat matching the ball is circular so that the seating stress is
circumferentially uniform. Most ball valves are also equipped with soft
seats that conform readily to the surface of the ball. Thus, from the point
of sealing, the concept of the ball valve is excellent. The valves shown in
Figure 3-60 through Figure 3-66 are typical of the ball valves available.
The flow-control characteristic that arises from a round port moving
across a circular seat and from the double pressure drop across the two
seats is very good. However, if the valve is left partially open for an
extended period under conditions of a high pressure drop across the ball,
the soft seat will tend to flow around the edge of the ball orifice and pos-
sibly lock the ball in that position. Ball valves for manual control are
therefore best suited for stopping and starting flow and moderate throt-
tling. If flow control is automatic, the ball is continuously on the move,
thus keeping this failure from normally occurring.
Because the ball moves across the seats with a wiping motion, ball valves
will handle fluids with solids in suspension. However, abrasive solids will
damage the seats and the ball surface. Long, tough fibrous material may
also present a problem, as the fibers tend to wrap around the ball.
To economize in the valve construction, most ball valves have a
reduced bore with a venturi-shaped flow passage of about three-quarters
the nominal valve size. The pressure drop across the reduced-bore ball
valve is thereby so small that the cost of a full-bore ball valve is not nor-
mally justified. However, there are applications when a full-bore ball
valve is required, as for example, when the pipeline has to be scraped.

Seat Materials for Ball Valves

The most important seat material for ball valves is PTFE, which is
inert to almost all chemicals. This property is combined with a low coef-
ficient of friction, a wide range of temperature application, and excellent
sealing properties. However, the physical properties of PTFE include
also a high coefficient of expansion, susceptibility to cold flow, and poor
heat transfer. The seat must therefore be designed around these proper-
ties. Plastic materials for ball valve seats also include filled PTFE, nylon,
and many others. However, as the seating material becomes harder, the
sealing reliability tends to suffer, particularly at low-pressure differen-
tials. Elastomers such as buna-N are also used for the seats, but they