Reference Manual

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Figure 1-28. Characterized Cages for Globe-Style Valve Bodies
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QUICK OPENING LINEAR EQUAL PERCENTAGE
Figure 1-29. Inherent Flow
Characteristics Curves
direction through the cage openings. Therefore, it
could be necessary to reverse the valve body in
the pipeline to obtain proper flow direction.
Characterized Valve Plugs
The valve plug, the movable part of a globe-style
control valve assembly, provides a variable
restriction to fluid flow. Valve plug styles are each
designed to:
D Provide a specific flow characteristic.
D Permit a specified manner of guiding or
alignment with the seat ring.
D Have a particular shutoff or
damage-resistance capability.
Valve plugs are designed for either two-position or
throttling control. In two-position applications, the
valve plug is positioned by the actuator at either of
two points within the travel range of the assembly.
In throttling control, the valve plug can be
positioned at any point within the travel range as
dictated by the process requirements.
The contour of the valve plug surface next to the
seat ring is instrumental in determining the
inherent flow characteristic of a conventional
globe-style control valve. As the actuator moves
the valve plug through its travel range, the
unobstructed flow area changes in size and shape
depending upon the contour of the valve plug.
When a constant pressure differential is
maintained across the valve, the changing
relationship between percentage of maximum flow
capacity and percentage of total travel range can
be portrayed (figure 1-29), and is designated as
the inherent flow characteristic of the valve.
Commonly specified inherent flow characteristics
include:
Linear Flow
D A valve with an ideal linear inherent flow
characteristic produces a flow rate directly
proportional to the amount of valve plug travel
throughout the travel range. For instance, at 50%
of rated travel, flow rate is 50% of maximum flow;
at 80% of rated travel, flow rate is 80% of
maximum; etc. Change of flow rate is constant
with respect to valve plug travel. Valves with a
linear characteristic are often specified for liquid
level control and for flow control applications
requiring constant gain.
Equal-Percentage Flow
D Ideally, for equal increments of valve plug
travel, the change in flow rate regarding travel may
be expressed as a constant percent of the flow