191
Typ EP 501
Additional technical information
34.
SELECTION CRITERIA FOR CONTINUOUS
VALVES
The following criteria are crucial for optimum control behavior and to ensure that the required maximum flow is reached:
• the correct selection of the flow coefficient which is defined primarily by the orifice of the valve;
• close coordination between the nominal width of the valve and the pressure conditions in consideration of the
remaining flow resistance in the equipment.
Design guidelines can be given on the basis of the flow coefficient (k
V
value). The k
V
value refers to standardised
conditions with respect to pressure, temperature and media properties.
The k
V
value describes the flow rate of water through a component in m³/h at a pressure difference of
∆
p = 1 bar
and T = 20 °C.
The “k
VS
value” is also used for continuous valves. This indicates the k
V
value when the continuous valve is fully open.
Depending on the specified data, it is necessary to differentiate between the two following cases when selecting
the valve:
a) The pressure values p1 and p2, known before and after the valve, represent the required maximum flow-rate
Q
max
which is to be reached:
The required k
VS
value is calculated as follows:
(1)
p
p
Q
k
0
0
max
s
v
ρ
ρ
⋅
∆
∆
=
⋅
Meaning of the symbols:
k
VS
flow coefficient of the continuous valve when fully open [m³/h]
Q
max
maximum volume flow rate [m³/h]
∆
p
0
= 1 bar; pressure loss on the valve according to the definition of the k
V
value
ρ
0
= 1000 kg/m³; density of water (according to the definition of the k
V
value)
∆
p pressure loss on the valve [bar]
ρ
density of the medium [kg/m³]
b) The pressure values, known at the input and output of the entire equipment (p
1
and p
2
), represent the required
maximum flow-rate Q
max
which is to be reached:
1st step: Calculate the flow coefficient of the entire equipment k
Vges
according to equation (1).
2nd step: Determine the flow-rate through the equipment without the continuous valve
(e.g. by "short-circuiting" the line at the installation location of the continuous valve).
3rd step: Calculate the flow coefficient of the equipment without the continuous valve (k
Va
)
according to equation (1).
4th step: Calculate the required k
VS
value of the continuous valve according to equation (2):
(2)
²
k
1
²
k
1
1
k
a
V
Vges
s
v
−
=
english
