Danfoss VFS 2 Fiche technique - Page 4

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Data sheet
Control valve sizing diagram
for fluids
4 | VD.HC.L3.02
VFS 2
Flow liquid with specific
gravity of 1
FLOW Pressure drop kPa (100 kPa = 1 bar = ~10 m H
Examples:
1 For fluids with specific gravity of 1
(e.g. water)
Design data:
Flow rate: 6 m
/h
3
System pressure drop: 55 kPa
Locate the horizontal line representing a flow
rate of 6 m
/h (line A-A). The valve authority is
3
given by the equation:
∆
=
Valve
authority,
a
∆
+
1 p
Where:
Δp1 = pressure drop across the fully open valve
Δp2 = pressure drop across the rest of the circuit
with a full open valve
The ideal valve would give a pressure drop equal
to the system pressure drop (i.e. an authority of
0.5):
If Δp1 = Δp2
p
1
a
0
5 ,
2
p
1
In this example an authority of 0.5 would be
given by a valve having a pressure drop of 55
kPa at that flow rate (point B). The intersection
of line A–A with a vertical line drawn from B lies
between two diagonal lines; this means that no
ideally-sized valve is available. The intersection
of line A–A with the diagonal lines gives the
pressure drops stated by real, rather than ideal,
valves. In this case, a valve with k
a pressure drop of 90.7 kPa (point C):
The largest valve, with k
pressure drop of 36 kPa (point D):
Generally, the smaller valve would be selected
(resulting in a valve authority higher than 0.5
1 p
and therefore improved control). However, this
∆
will increase the total pressure and should be
p
2
checked by the system designer for compatibility
with available pump heads, etc. The ideal
authority is 0.5 with a preferred range of
between 0.4 and 0.7.
2 For fluids with specific gravity (S.G. )
Design data:
For this example, the left hand axis of the
diagram must be ignored. Starting from the
right hand axis, the flow rate of 6 m
(point E). The intersection of the diagonal line
from point E with a vertical line from S.G. = 0.9
(point F) gives the starting point for the effective
flow rate line G-G. The process then continues as
for Example 1, so 10 kPa intersects G-G nearest
to the k
with k
6.3 would give
VS
(point H).
Flow liquid with specific
gravity different to 1
O)
Specific gravity
2
90
=
hence
valve
authority
90
7 .
10, would give a
VS
36
=
hence
valve
authority
+
36
different from 1
Flow rate: 6 m
/h of fluid, S.G. 0.9
3
System pressure drop: 10 kPa
16 diagonal. The intersection of G-G
VS
16 gives a valve pressure drop of 12.7 kPa
VS
© Danfoss | 2016.11
7 .
=
. 0
62
+
55
=
. 0
395
55
/h is located
3