Answer Posted / mdhasi

Inherent Flow Characteristics of Control Valves.




>>Valve Coefficient or Valve sizing Coefficient (Cv)
The relationship between valve flow output and
valve travel is known as the flow characteristic of the
valve. When the flow characteristics are obtained with
constant pressure drop across the valve (generally
referenced to the standard condition of a constant pressure
drop of 6.9kPa or 1lbf/sq.in.), they are called inherent
flow characteristics. Three types of them can be seen in
the applet when it is initialized. The sensitivity of the
rate of change of flow to the valve step position (dQ/dL)
at a constant pressure drop is called the valve
coefficient, Cv.

The quick-opening : it provides large changes in flow (Q)
for very small changes in lift (L). 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. In this case, Cv
will decrease as Q increases.
Q = Cv*SQRT(L*DP/gc) (where DP is pressure drop across the
valve.)

The majority of control applications are valves with
linear, equal-percentage, or modified-flow characteristics.
Linear : flow capacity increases linearly with valve
travel. For linear valves, the Cv is constant through the
whole range.
Q = Cv*L*SQRT(DP/gc)

Equal percentage : flow capacity increases exponentially
with valve trim travel; equal increments of valve travel
produce equal percentage changes in the existing Cv. For
equal-percentage valves, the Cv will increase as Q
increases.
Q = Cv*RL-1*SQRT(DP/gc) (where R is rangeability.)

A modified parabolic characteristic - approximately midway
between linear and equal-percentage characteristics. It
provides fine throttling at low flow capacity and
approximately linear characteristics at higher flow
capacity



>>Rangability (R)
R = (maximum controllable flow)/(minimum controllable flow)
The "rangeability" factor defines a valve's capability to
control small flows. The higher the number, the better. On
typical small globe valves (1/2") the rangeability is 5:1,
and on a large valve (2") it is 75:1.


>>Turn Down (T)
T = (normal maximum flow)/(minimum controllable flow)

Genrally, valves are sized so that T is around 0.7R where R
lies, depending on the type of valve, typically between 20
and 50. For example, if the design of a tank-level control
system calls for 25:1 change in inlet flow rate to maintain
the level because of a chaging outlet flow rate, then the
indicated control valve turndown ratio is at least 25 and
the rangeability ratio at least 35.

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