Page 123 - Valve Selection Handbook

110 Valve Selection Handbook

seats and the ball by an amount that ensures a correct prestress at the
cryogenic operating temperature. However, the sealing capacity of these
valves may not be satisfactory at low fluid pressures if these valves also
have to operate at ambient temperatures.
Other means of combating the effect of differential thermal contraction
between the seats and the ball include supporting the seats on flexible
metal diaphragms; choosing a seat-ring material that has a considerably
lower coefficient of contraction than virgin PTFE, such as graphite or
carbon filled PTFE; or making the seat rings of stainless steel with PTFE
inserts in which the PTFE contents are kept to a minimum.
Because plastic seat-ring materials become rigid at cryogenic tempera-
tures, the surface finish of the seatings and the sphericity of the ball must
be of a high standard to ensure a high degree of seat tightness. Also, as
with other types of valves for cryogenic service, the extended bonnet
should be positioned no more than 45° from the upright to ensure an
effective stem seal.

Variations of Body Construction

Access to the ball valve internals can be provided in various ways.
This has led to the development of a number of variations in the body
construction; Figure 3-68 shows the most common variations.
The one-piece body has the fewest number of body joints subject to
pipeline stresses. This type of body therefore is often chosen for haz-
ardous fluids. If the valve is to be buried, the sealed-body variety is fre-
quently used. The one-piece body with top entry and the various split
body constructions offer easy entry to the valve internals. In the case of
welded-in-line valves, those with top entry may also be serviced in situ.
A selection from these types is often a matter of personal preference.

Face-to-Face Dimensions

The original practice of U.S. manufacturers was to make the face-to-
face dimensions of flanged ball valves to the nearest valve standard,
which gave minimum material content. This happened to be the gate
valve standard, but the face-to-face dimensions of class 150 and of sizes
DN 2OO (NPS 8) through DN 300 (NPS 12) of class 300 permitted only
reduced-bore construction.
In 1961, when UK manufacturers also introduced the flanged ball
valve, there was an additional demand for full-bore ball valves. When it
was impossible to accommodate the full-bore ball valve in the confines