Reference Manual

1−20
The threads usually specified are tapered female
NPT on the valve body. They form a
metal-to-metal seal by wedging over the mating
male threads on the pipeline ends. This
connection style is usually limited to valves not
larger than NPS 2, and is not recommended for
elevated temperature service.
Valve maintenance might be complicated by
screwed end connections if it is necessary to take
the body out of the pipeline. Screwed connections
require breaking a flanged joint or union
connection to permit unscrewing the valve body
from the pipeline.
Flanged end valves are easily removed from the
piping and are suitable for use through the range
of working pressures that most control valves are
manufactured (figure 1-13).
Flanged end connections can be utilized in a
temperature range from absolute zero (−273°F) to
approximately 1500°F (815°C). They are utilized
on all valve sizes. The most common flanged end
connections include flat face, raised face, and ring
type joint.
Welded ends on control valves are leak-tight at all
pressures and temperatures and are economical
in initial cost (figure 1-14). Welded end valves are
more difficult to remove from the line and are
limited to weldable materials. Welded ends come
in two styles, socket weld and buttweld.
Shutoff Capability
Some consideration must be given to a valve’s
shutoff capability, which is usually rated in terms of
classes specified in ANSI/FCI70-2 (table 1-4). In
service, shutoff leakage depends upon many
factors, including but not limited to, pressure drop,
temperature, and the condition of the sealing
surfaces. Because shutoff ratings are based upon
standard test conditions that can be different from
service conditions, service leakage cannot be
predicted accurately. However, the shutoff class
provides a good basis for comparison among
valves of similar configuration. It is not uncommon
for valve users to overestimate the shutoff class
required.
Because tight shutoff valves generally cost more
both in initial cost, as well as in later maintenance
expense, serious consideration is warranted. Tight
shutoff is particularly critical in high pressure
valves, considering that leakage in these
applications can lead to the ultimate destruction of
the trim. Special precautions in seat material
selection, seat preparation and seat load are
necessary to ensure success.
Flow Capacity
Finally, the criterion of capacity or size can be an
overriding constraint on selection. For extremely
large lines, sliding-stem valves are more
expensive than rotary types. On the other hand,
for extremely small flows, a suitable rotary valve
may not be available. If future plans call for
significantly larger flow, then a sliding-stem valve
with replaceable restricted trim may be the
answer. The trim can be changed to full size trim
to accommodate higher flow rates at less cost than
replacing the entire valve body assembly.
Rotary style products generally have much higher
maximum capacity than sliding-stem valves for a
given body size. This fact makes rotary products
attractive in applications where the pressure drop
available is rather small. However, it is of little or
no advantage in high pressure drop applications
such as pressure regulation or letdown.
Conclusion
For most general applications, it makes sense
both economically, as well as technically, to use
sliding-stem valves for lower flow ranges, ball
valves for intermediate capacities, and high
performance butterfly valves for the very largest
required flows. However, there are numerous
other factors in selecting control valves, and
general selection principles are not always the
best choice.
Selecting a control valve is more of and art than a
science. Process conditions, physical fluid
phenomena, customer preference, customer
experience, supplier experience, among numerous
other criteria must be considered in order to obtain
the best possible solution. Many applications are
beyond that of general service, and as chapter 4
will present, there are of number of selection
criteria that must be considered when dealing with
these sometimes severe flows.
Special considerations may require out-of-the-
ordinary valve solutions; there are valve designs
and special trims available to handle high noise
applications, flashing, cavitation, high pressure,
high temperature and combinations of these
conditions.