Burkert Type 8635 - Manual (Page 144)

142 - 8635
G
ENERAL
R
ULES
(A
PPENDIX
)
The following criteria are of decisive importance for optimal control behaviour and attainment of the
maximum flow rate through the valve:
• Correct choice of flow coefficient, which is defined essential by the size of the valve;
• Good matching of the valve size to the pressure conditions, taking into consideration the other flow
resistances in the system.
Dimensioning guidelines can be given on the basis of the flow coefficient (k
V
).
The k
V
value refers to the standardized conditions with respect to pressure, temperature and media
properties.
The k
v
value is defined as the flow rate in m³/h of water through a component at a pressure difference
∆p of 1 bar and a temperature of 20 °C.
With continuous valves the ”k
VS
value” is additionally used. This specifies the k
V
value when the
continuous vlve is fully open.
Depending on the specified data, the following cases must be distinguished on selection of a valve:
a) The pressures before and after the valve p
1
and p
2
are known at which the desired maximum flow
rate Q
max
is to be attained:
The required k
VS
value is obtained from:
where:
k
VS
is the flow coefficient of the continuous valve when fully open [m³/h]
Q
max
is the maximum volumetric flow rate [m³/h]
∆p
0
is 1 bar; the pressure drop over the valve as in the definition of k
V
r
0
is 1000 kg/m³; the density of water as in the definition of k
V
∆p is the pressure drop over the valve [bar] and
r is the density of the medium [kg/m³]
b) The pressures at the inlet and outlet of the overall system (p
1
and p
2
) are known at which the desired
maximum flow rate Q
max
is to be attained:
Step 1: Calculate the flow coefficient of the overall system k
Vges
from equation (1).
Step 2: Measure the flow rate through the system without the continuous valve
(e.g. by short-circuiting the piping where the valve is installed).
Step 3: Calculate the flow coefficient of the system without the continuous valve (k
Vs
) from
equation (1).
Step 4: Calculate the required k
VS
value of the continuous valve (k
vs
) from equation (2):
(2)
Selection criteria for continuous valves
k Q
p
p VS
˜ ˜
max
'
'
0
0
U
U
k
k k
VS
Vges Va

1
1 1
2 2
(1)