Reference Manual

1−18
Conventional globe-style valve bodies can be fitted
with seat rings with smaller port size than normal
and valve plugs sized to fit those smaller ports.
Valves with cage-guided trim often achieve the
reduced capacity effect by utilizing valve plug,
cage, and seat ring parts from a smaller valve size
of similar construction and adapter pieces above
the cage and below the seat ring to mate those
smaller parts with the valve body (figure 1-28).
Because reduced capacity service is not unusual,
leading manufacturers provide readily available
trim part combinations to perform the required
function. Many restricted capacity trim
combinations are designed to furnish
approximately 40% of full-size trim capacity.
General Selection Criteria
Most of the considerations that guide the selection
of valve type and brand are rather basic. However,
there are some matters that may be overlooked by
users whose familiarity is mainly limited to just one
or a few valve types. Table 1-1 below provides a
checklist of important criteria; each is discussed at
length following the table.
Table 1-1. Suggested General Criteria for Selecting Type
and Brand of Control Valve
Body pressure rating
High and low temperature limits
Material compatibility and durability
Inherent flow characteristic and rangeability
Maximum pressure drop (shutoff and flowing)
Noise and cavitation
End connections
Shutoff leakage
Capacity versus cost
Nature of flowing media
Dynamic performance
Pressure Ratings
Body pressure ratings ordinarily are considered
according to ANSI pressure classes — the most
common ones for steel and stainless steel being
Classes 150, 300 and 600. (Source documents
are ASME/ANSI Standards B16.34, “Steel
Valves,” and ANSI B16.1, “Cast Iron Pipe
Flanges and Flanged Fittings.”) For a given body
material, each NSI Class corresponds to a
prescribed profile of maximum pressures that
decrease with temperature according to the
strength of the material. Each material also has a
minimum and maximum service temperature
based upon loss of ductility or loss of strength. For
most applications, the required pressure rating is
dictated by the application. However, because all
products are not available for all ANSI Classes, it
is an important consideration for selection.
Temperature Considerations
Required temperature capabilities are also a
foregone conclusion, but one that is likely to
narrow valve selection possibilities. The
considerations include the strength or ductility of
the body material, as well as relative thermal
expansion of various parts.
Temperature limits also may be imposed due to
disintegration of soft parts at high temperatures or
loss of resiliency at low temperatures. The soft
materials under consideration include various
elastomers, plastics, and PTFE. They may be
found in parts such as seat rings, seal or piston
rings, packing, rotary shaft bearings and butterfly
valve liners. Typical upper temperature limits for
elastomers are in the 200 - 350°F range, and the
general limit for PTFE is 450°F.
Temperature affects valve selection by excluding
certain valves that do not have high or low
temperature options. It also may have some affect
on the valve’s performance. For instance, going
from PTFE to metal seals for high temperatures
generally increases the shutoff leakage flow.
Similarly, high temperature metal bearing sleeves
in rotary valves impose more friction upon the
shaft than do PTFE bearings, so that the shaft
cannot withstand as high a pressure-drop load at
shutoff. Selection of the valve packing is also
based largely upon service temperature.
Material Selection
The third criterion in table 1-1, “material
compatibility and durability”, is a more complex
consideration. Variables may include corrosion by
the process fluid, erosion by abrasive material,
flashing, cavitation or pressure and temperature
requirements. The piping material usually indicates
the body material. However, because the velocity
is higher in valves, other factors must be
considered. When these variables are included,
often valve and piping materials will differ. The trim
materials, in turn, are usually a function of the
body material, temperature range and qualities of
the fluid. When a body material other than carbon,
alloy, or stainless steel is required, use of an
alternate valve type, such as lined or bar stock,
should be considered.