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Additional technical information
The k
VS
value of the continuous valve should have at least the value which is calculated according to
equation (1) or (2) which is appropriate to the application, however it should never be far above the calcu-
lated value.
The rule of thumb "slightly higher is never harmful" often used for switching valves may greatly impair the
control behavior of continuous valves!
The upper limit for the k
VS
value of the continuous valve can be specified in practice via the so-called valve authority Ψ:
(3)
²k²k
p
S Va V
a V
0
0 V
+
=
∆
=ψ
)(
(∆p)
V0
Pressure drop over the fully opened valve
(∆p)
0
Pressure drop over the entire equipment
If the valve authority Ψ < 0.3 the continuous valve has been oversized.
When the continuous valve is fully open, the flow resistance in this case is significantly less than the flow
resistance of the remaining fluid components in the equipment. This means that the valve position pre-
dominates in the operating characteristic in the lower opening range only. For this reason the operating
characteristic is highly deformed.
By selecting a progressive (equal percentage) transfer characteristic between position set-point value and
valve stroke, this can be partially compensated and the operating characteristic linearised within certain
limits.
However, the valve authority Ψ should be > 0.1 even if a correction characteristic is used.
The control behavior (control quality, transient time) depends greatly on the working point if a correction
characteristic is used.
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