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8
APPENDIX
8.1
Selection criteria for continuous valves
For optional control behaviour and the reaching of the desired maximum flowrate the following criteria must be observed:
•
The correct selection of the k
•
The correct adjustment of the valve size to the pressure conditions, taking into account the pressure differences of the
installation.
Sizing guidelines can be given based upon the k
and fluid.
value is the flowrate in m³/h though a piping port under a pressure difference of ∆p = 1 bar and T = 20°C.
The k
v
For continuous valves the k
In relation with the given data, we distinguish the two following cases for the sizing of the valves:
a) The pressures p
and p
1
calculated:
The needed k
value is calculated according to:
VS
=
k
VS
Where:
k
:
maximal flowrate of fully opened continuous valve [m³/h]
VS
Q
:
maximal volumetric flowrate [m³/h]
max
∆p
:
= 1 bar; pressure drops of the valve according to the k
0
ρ
:
= 1000 kg/m³; density of water (according to the k
0
∆p:
pressure drop of the valve [bar]
ρ:
density of fluid [kg/m³]
b) The pressures p
and p
1
calculated:
1.Step:
Calculation of the k
2. Step:
Determination of the flowrate through the installation without a continuous valve (possible by by-passing the pipe
where the valve is installed).
3. Step:
Calculation of the k
4. Step:
Calculation of the needed k
=
k
VS
Important rule:
The k
value of the valve should reach at least the value calculated by the formula (1) or (2) depending on the application. It
VS
should also not greatly exceed this value.
The often used rule for on/off valves 'a little bit higher cannot be wrong' can strongly influence the control behaviour of continuous
valves!
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value, which is in fact defined by the valve size;
V
value. The k
V
value is used in addition. It corresponds to the k
VS
must be known before and after the valve before the desired maximum flowrate Q
2
∆
ρ
p
⋅
⋅
0
Q
∆
ρ
max
p
0
at the inlet and outlet of the complete installation for the desired maximum flowrate Qmax can be
2
value for the complete installation according to rule (1).
Vges
value of the installation without the continuous valve according to rule (1).
Va
value of the continuous valve according to rule (2):
VS
1
1
1
−
2
2
k
k
Vges
Va
value relative to the normal conditions for pressure, temperature
V
value of a continuous valve, which is fully open.
V
(1)
value defined
V
value definition)
V
(2)
can be
max
127
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