169
Type 8692, 8693
english
General Rules - Appendix
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 calculated value.
The rule of thumb "slightly higher is never harmful" often used for switching valves may greatly impair the control behav-
iour 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
²k
p
p
S Va V
a V
0
0 V
+
=
Δ
Δ
=ψ
)(
)(
(Δp)
V0
Pressure drop over the fully opened valve
(Δp)
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 predominates 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 nominal value and valve stroke,
this can be partially compensated and the operating characteristic linearised within certain limits. However, the valve
authority Ψ should also be > 0.1 if a correction characteristic is used.
The control behaviour (control quality, transient time) depends greatly on the working point if a correction characteristic
is used.
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