Burkert 8792 Operating Instructions Manual page 203

Type 8792, 8793 Type 8792, 8793
Selection criteria for continuous valves
=
k
v s
1
k
Vges
The k value of the continuous valve should have at least the value which is calculated according to
VS
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 behavior of continuous valves!
The upper limit for the k
∆
( p )
ψ =
∆
( p )
(∆p) Pressure drop over the fully opened valve
V0
(∆p) Pressure drop over the entire equipment
0
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 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 cor-
rection characteristic is used.
1
(2)
1
−
² k ²
V a
value of the continuous valve can be specified in practice via the so-called valve authority Ψ:
VS
k ²
V 0 = V a
(3)
+
k ² k ²
0 V a V S
english
205