Burkert 8626 Instructions De Service page 40

NOTE The k
calculated by the equation (1), (2) or (3) applying for the application,
but should not exceed it unnecessarily.
The rule of thumb often used with on/off valves, "somewhat larger
can do no harm", can result in serious detriment to the control
behaviour with continuously adjustable valves.
In practice, dimensioning of the upper limit for the k
may be done via the so-called valve authority
ψ ψ ψ ψ ψ= (∆ ∆ ∆ ∆ ∆ p)
where (∆p) is the pressure drop over the fully open valve and (∆p)
V0
drop over the overall installation.
The valve authority ψ should in the interest of an acceptable operating characteristic
for the installation, not lie below 0.3 ... 0.5.
If ψ is smaller than this guideline, the MFC Type 8626 or its actuator will be so
overdimensioned that when fully open, its flow resistance is considerably lower than
that of the other fluidic components in the installation. Then, even with an ideally
linear flow characteristic of the proportional valve, the operating characteristic will be
strongly deformed, since the latter is only dominated by the valve position in the
lower opening range. When the valve is wider open, the other fluid resistances
determine the flow behaviour.
A deformed operating flow characteristics could result in a slower dynamics,
especially in the upper flow range.
Operating Instructions-No. 801 147
value of the MFC Type 8626 should be at least equal to that
Vs
/(∆ ∆ ∆ ∆ ∆ p)
= (k 2 )/[(k 2 )+(k
V0 0 Va Va
value of the MFC Type 8626
Vs
2 )]
Vs
is the pressure
0
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