Reference Manual

1−19
Flow Characteristic
The next selection criterion, “inherent flow
characteristic”, refers to the pattern in which the
flow at constant pressure drop changes according
to valve position. Typical characteristics are
quick-opening, linear, and equal-percentage. The
choice of characteristic may have a strong
influence upon the stability or controllability of the
process (see table 1-3), as it represents the
change of valve gain relative to travel.
Most control valves are carefully “characterized”
by means of contours on a plug, cage, or ball
element. Some valves are available in a variety of
characteristics to suit the application, while others
offer little or no choice. To quantitatively determine
the best flow characteristic for a given application,
a dynamic analysis of the control loop can be
performed. In most cases, however, this is
unnecessary; reference to established rules of
thumb will suffice.
The accompanying drawing illustrates typical flow
characteristic curves (figure 1-29). The quick
opening flow characteristic provides for maximum
change in flow rate at low valve travels with a fairly
linear relationship. Additional increases in valve
travel give sharply reduced changes in flow rate,
and when the valve plug nears the wide open
position, the change in flow rate approaches zero.
In a control valve, the quick opening valve plug is
used primarily for on-off service; but it is also
suitable for many applications where a linear valve
plug would normally be specified.
Rangeability
Another aspect of a valve’s flow characteristic is its
rangeability, which is the ratio of its maximum and
minimum controllable flow rates. Exceptionally
wide rangeability may be required for certain
applications to handle wide load swings or a
combination of start-up, normal and maximum
working conditions. Generally speaking, rotary
valves—especially partial ball valves—have
greater rangeability than sliding-stem varieties.
Use of Positioners
A positioner is an instrument that helps improve
control by accurately positioning a control valve
actuator in response to a control signal. They are
useful in many applications and are required with
certain actuator styles in order to match actuator
and instrument pressure signals, or to provide
operating stability. To a certain extent, a valve with
one inherent flow characteristic can also be made
to perform as though it had a different
characteristic by utilizing a nonlinear (i.e.,
characterized) positioner-actuator combination.
The limitation of this approach lies in the
positioner’s frequency response and phase lag
compared to the characteristic frequency of the
process. Although it is common practice to utilize a
positioner on every valve application, each
application should be reviewed carefully. There
are certain examples of high gain processes
where a positioner can hinder valve performance.
Pressure Drop
The maximum pressure drop a valve can tolerate
at shutoff, or when partially or fully open, is an
important selection criteria. Sliding-stem valves
are generally superior in both regards because of
the rugged nature of their moving parts. Many
rotary valves are limited to pressure drops well
below the body pressure rating, especially under
flowing conditions, due to dynamic stresses that
high velocity flow imposes on the disk or ball
segment.
Noise and Cavitation
Noise and cavitation are two considerations that
often are grouped together because both result
from high pressure drops and large flow rates.
They are treated by special modifications to
standard valves. Chapter four discusses the
cavitation phenomenon and its impact and
treatment, while chapter six discusses noise
generation and abatement.
End Connections
The three common methods of installing control
valves in pipelines are by means of screwed pipe
threads, bolted flanges, and welded end
connections. At some point in the selection
process, the valve’s end connections must be
considered with the question simply being whether
the desired connection style is available in the
valve being considered.
In some situations, this matter can limit the
selection rather narrowly. For instance, if a piping
specification calls for welded connections only, the
choice usually is limited to sliding-stem valves.
Screwed end connections, popular in small control
valves, offer more economy than flanged ends.