Burkert Type 8635 - Manual (Page 145)

8635 - 143
G
ENERAL
R
ULES
(A
PPENDIX
)
Determination in practice of the upper limit to the k
VS
value of the continuous valve is possible by means
of the so-called "valve authority"
Ψ:
(3)
(
∆p)
V0
is the pressure drop over the fully opened valve and
( ∆p)
0
is the pressure drop over the entire system
<
'
' 
p
p
k
k k
V Va
Va VS
0
0
2
2 2
NOTE With a valve authority Ψ Ψ Ψ Ψ Ψ < 0.3 the continuous valve is overdimensioned.
With the valve fully open, in this case the flow resistance is significantly smaller than that of
the other fluidic components in the system. This means that only in the lower opening range
is the valve position dominant in the operating characteristic. For this reason, the operating
characteristic is strongly deformed.
By selection of a progressive (equiprocentile) characteristic between position setpoint and
valve stroke, this can be partially compensated and the operating characteristic linearized
within certain limits.
However, the valve authority Ψ Ψ Ψ Ψ Ψ should be > 0.1, even when using a
corrected characteristic.
The control behaviour (control performance, settling time) when using a corrected
characteristic is strongly dependent on the operating point.
NOTE The k
VS
of the continuous valve should have at least the value calculated from the equation
(1 or 2) relevant to the application, but under no circumstances be very much greater.
The rule of thumb often used with switching valves, "somewhat larger never hurts", can be
strongly detrimental to the control behaviour of continuous valves!