The ball-valve body features a straight-through style, allowing uninhibited
flow with minimal pressure drop. A number of body configurations are
available, although the most common are the split body (again refer to
Fig. 3.4), solid body with side entry (Fig. 3.5), or solid body with top
entry (Fig. 3.6). The defining factor for determining the body design is
the complexity of installing the ball inside the body. While the split body
offers the easiest disassembly and reassembly, it may present problems
with an additional joint that can be affected by piping stresses as well as
another potential leak path. Face-to-face dimensions for ball valves are
established by ANSI Standard B16.10, although with some pressure
classifications or special designs manufacturers may use the gate valve face-
to- face standard. Face-to-face dimensions are usually specified according
to a short pattern (ANSI Class 150) or long pattern for higher-pressure
classifications. The most common end connection used with manual ball
valves is the integral-flange design.
The ball itself can be either round or tapered, depending on the
internal seat design. The flow-through port is a reduced area from the
body port, approximately 75 percent of the valve’s full area. Full-area
ports are also available when minimal pressure drop is needed, such
as with on–off service, or when a pig is used to scrape the inside diameter
of the pipe and a narrow flow restriction in the line would prevent
this. Unlike the one-piece plug of plug valves where the stem is an
integral part of the plug, the ball is separate from the stem in manual
ball valves. A key slot is machined or cast into the top of the ball, into
which a key machined into the bottom portion of the stem fits.
Although a ball’s port is normally produced in a round flow passage,
with either full or reduced area, characterizable balls are also
available (Fig. 3.7) with the inlet port of the ball shaped to provide the
correct flow-to-position relationship for that flow characteristic. C-shaped
balls are also available for eliminating dead spots (Fig. 3.8).
When two round seats are fixed on the upstream and downstream
side of the ball, this is commonly called double seating. The two seats
are designed to conform with the ball’s sealing surface. With moderate
pressure drops and elastomeric seating materials, bubble-tight shutoff
is possible with double-seated ball valves. Several other seating
arrangements are utilized with ball valves. One of the most common
arrangements is the floating ball, in which the ball is not fixed to the
stem and is allowed some freedom of movement through the key slot.
With the floating ball, the upstream fluid pressure assists the seal by
pushing the ball back against the rear or downstream seat. Another
seating arrangement involves a floating seat, in which the ball is fixed
(called a trunnion-mounted ball) at two pivot points, and the process
pressure pushes the upstream seat against the ball’s sealing surface.
The seat can also be prestressed during assembly, using seats that have
a spring action. This design applies continuous pressure against a
trunnion-mounted ball after the ball is installed, while the top-works
apply a load to the entire closure element.
Most seats are made from PTFE, which provides excellent bubbletight
sealing and a temperature range that covers most general services.
Buna-N and nylon materials are also specified, but may be limited
in pressure ranges and process compatibility. For higher temperatures,
metal seats and carbon-based materials are specified, although higher
leakage rates are common.
With ball-valve design, the stem is usually sealed by packing rings,
with a packing follower and gland flange applying compression. With
split bodies and solid bodies with side entry, the stem is installed
through the body and the packing installed above the body. Because of
the keyed slot, the ball can be turned so that the key and the slot are
parallel with the flow passage, allowing the ball to enter from the side
and the stem to intersect with the stem key.
With top-entry ball valves that use trunnion-mounted balls and
spring-loaded seats, the ball has either an integral or separate lower
post that is seated in the bottom of the body. The seats are placed on
both sides of the ball and the entire assembly is placed in the body.
The top-works—consisting of a bonnet cap, packing box, gland flange,
and separate stem—are installed above the ball. When the bonnet-cap
bolting is tightened, the resulting compression energizes the seats. The
joint between the bonnet cap and the body is sealed using a gasket.
In addition to PTFE, linings can be produced from PVDF, PVDC,
polyethylene, and polypropylene. Because of the corrosive nature of
the service, lined ball valves are painted with a corrosion-resistant
coating on the exterior surface of the body. Although lined valves may
be more expensive than normal plug valves, they are considerably less
expensive than requesting corrosion-resistant metals. The one disadvantage
of lined valves is that the plastic-on-plastic seal provides a
higher breakout torque than the metal-on-elastomer seal.
To ensure quarter-turn motion without over- or understroking the
valve, a stop-collar arrangement is used. The stop collar is designed to
allow only a 90° travel of the wrench or handlever.
The ball-valve body features a straight-through style, allowing uninhibited
flow with minimal pressure drop. A number of body configurations are
available, although the most common are the split body (again refer to
Fig. 3.4), solid body with side entry (Fig. 3.5), or solid body with top
entry (Fig. 3.6). The defining factor for determining the body design is
the complexity of installing the ball inside the body. While the split body
offers the easiest disassembly and reassembly, it may present problems
with an additional joint that can be affected by piping stresses as well as
another potential leak path. Face-to-face dimensions for ball valves are
established by ANSI Standard B16.10, although with some pressure
classifications or special designs manufacturers may use the gate valve face-
to- face standard. Face-to-face dimensions are usually specified according
to a short pattern (ANSI Class 150) or long pattern for higher-pressure
classifications. The most common end connection used with manual ball
valves is the integral-flange design.
The ball itself can be either round or tapered, depending on...
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