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Control
Valve Flow Characteristics
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| Trim design will affect how the valve capacity
changes as the valve moves through its complete travel. Because of the
variation in trim design, many valves are not linear in nature. THE RELATIONSHIP
BETWEEN VALVE CAPACITY AND VALVE TRAVEL IS KNOWN AS THE FLOW CHARACTERISTIC
OF THE VALVE. Valve trims are specially designed, or characterized, in
order to meet the large variety of control application needs. This is necessary
because most control loops have some inherent nonlinearities, which you
can compensate for when selecting control valve trim. |
| Charts similar to Figure 1 (see below) are
used to illustrate various control valve flow characteristics. The percent
of full flow through the valve is plotted against valve stem position.
The curves shown are typical of those available from valve manufacturers.
These curves are based on CONSTANT PRESSURE DROP across the valve and are
called INHERENT FLOW CHARACTERISTICS. |
| The quick-opening characteristic provides large changes in flow for
very small changes in lift. It usually has too high a valve gain for use
in modulating control. So it is limited to on-off service, such as sequential
operation in either batch or semi-continuous processes. |
| The majority of control applications are valves with linear, equal-percentage,
or modified-flow characteristics. |
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Linear - flow capacity increases linearly with valve travel.
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Equal percentage - flow capacity increases exponentially with valve
trim travel; equal increments of valve travel produce equal percentage
changes in the existing Cv.
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A modified parabolic characteristic is approximately midway between
linear and equal-percentage characteristics. It provides fine throttling
at low flow capacity and approximately linear characteristics at higher
flow capacity.
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| When valves are installed with a pump, pipes,
fittings, and other process equipment, the pressure drop across the valve
will vary as the plug moves through its travel. When the actual flow in
a system is plotted against valve opening, the curve is called the INSTALLED
FLOW CHARACTERISTIC. |
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Figure 1
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Inherrent
Flow Characteristics For Common Valve Trim Designs
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