Data sheet
VFS 2
4 | VD.HC.L3.02
© Danfoss | 2016.11
Examples:
1 For fluids with specific gravity of 1
(e.g. water)
Design data:
Flow rate: 6 m
3
/h
System pressure drop: 55 kPa
Locate the horizontal line representing a flow
rate of 6 m
3
/h (line A-A). The valve authority is
given by the equation:
2
p
1
p
1
p
a
authority,
Valve
∆
+
∆
∆
=
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
5
,
0
p
2
p
a
1
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
VS
6.3 would give
a pressure drop of 90.7 kPa (point C):
62
.
0
55
7
.
90
7
.
90
authority
valve
hence
=
+
=
The largest valve, with k
VS
10, would give a
pressure drop of 36 kPa (point D):
395
.
0
55
36
36
authority
valve
hence
=
+
=
Generally, the smaller valve would be selected
(resulting in a valve authority higher than 0.5
and therefore improved control). However, this
will increase the total pressure and should be
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. )
different from 1
Design data:
Flow rate: 6 m
3
/h of fluid, S.G. 0.9
System pressure drop: 10 kPa
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
3
/h is located
(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
VS
16 diagonal. The intersection of G-G
with k
VS
16 gives a valve pressure drop of 12.7 kPa
(point H).
Flow liquid with specific
gravity of 1
Flow liquid with specific
gravity different to 1
FLOW Pressure drop kPa (100 kPa = 1 bar = ~10 m H
2
O)
Specific gravity
Control valve sizing diagram
for fluids
