Bronkhorst EX-FLOW Mass Flow - Manual (Page 12)

BRONKHORST HIGH-TECH B.V.
1.6.2 Kv-value calculation
This calculation method can be used to determine the K
v
-value of the main orifice of a control valve.
Determine desired ∆p across valve.
∆ p must be at least 20% of supply pressure, or in closed loop systems, of total pressure difference in loop. If
∆ p is 20-50% of supply pressure, use formula:
K
T
p p
v
n
=
⋅
⋅
Φ
∆
vn
514
ρ
2
undercritical
If ∆P is 50-100% of supply pressure, use formula:
K
p
T
v
vn
n
=
⋅
⋅
Φ
257
1
ρ overcritical
Units:
Flow : Φ
vn
in m h
n
3 (gasflow)
Supply pressure : p
1
in bara
Downstream press. : p
2
in bara
Pressure difference : ∆ p = ( p p
1 2
− ) in bar
Temperature : T in Kelvin
Density : ρ
n n
inkg m / 3
The orifice diameter can be determined by: d= 7.6 K
v
[mm]
1.6.3 Maximum pressure drop
For (pilot) solenoid operated control valves with small orifices the maximum allowable pressure drop for
gases is according to the table.
Diameter [mm] K
v
Normally closed
∆ p max. [bard]
Normally opened
∆ p max. [bard]
0,05
0,07
0,10
0,14
0,20
0,30
0,37
0,50
0,70
1,00
1,30
1,50
1,70
2,00
4,33 x 10 -5
8,48 x 10 -5
1,73 x 10 -4
3,39 x 10 -4
6,93 x 10 -4
1,56 x 10 -3
2,37 x 10 -3
4,33 x 10 -3
8,48 x 10 -3
1,73 x 10 -2
2,93 x 10 -2
3,90 x 10 -2
5,00 x 10 -2
6,63 x 10 -2
40
30
30
30
30
30
30
30
24
12
8
6
5
3,6
30
20
20
20
20
20
20
20
15
8
5
n.a.
n.a.
n.a.
For pilot operated valves the maximum pressure drop is limited to 20 bard. If the the pressure drop during
start-up is higher, it is preferred to install a bypass valve. During start-up this valve should be opened. Also
the minimum pressure drop is limited. For exact figures consult factory or proceed according to the technical
data and/or additional instructions given by the sales office or department.
page 12 9.17.028