Boiler house
Level controls
Local regulations may restrict the use of this product to below the conditions quoted.
In the interests of development and improvement of the product, we reserve the right to change the specification without notice. © Copyright 2015
Safety
Your attention is drawn to
Safety Information Leaflet IM-GCM-10
2.3.37
PN9123E
11.0
(8)
11.0
(8)
11.0
(8)
11.0
(8)
TI-S75-12
AB Issue 4
Selection of
Modulating Feedwater Valves
Pneumatically Actuated
Valve Kv selection graph
Pressure drop across the fully open valve psi
1.45
2
345
10
20305
0
100145
1. Selection of the valve body size
The standard valve for modulating boiler water level control is
DN40 (11⁄2") nominal pipe size (40 mm). A range of seat sizes
to suit this body is available to suit most sizes of boilers.
However, for smaller or larger boilers, alternative sized valves
can be selected from the Spirax Sarco range - See overleaf.
Valve stem seals are available in normal (PTFE) or high
temperature (graphite) material. We recommend the use of the
high temperature seal to decrease the possibility of leakage
over long term use. Valves with high temperature stem seals
are suffixed 'H'.
2. Selection of the valve body material
and pressure rating
The valve body must be suitable for the maximum pressure and
temperature in the feedwater line.
Standard valve types are as follows:
100 000
90 000
80 000
70 000
60 000
50 000
40 000
30 000
20 000
15 000
10 000
9 000
8 000
7 000
6 000
5 000
4 000
3 000
2 000
1 000
200 000
100 000
90 000
80 000
70 000
60 000
50 000
40 000
30 000
20 000
10 000
9 000
8 000
7 000
6 000
5 000
4 000
3 000
SG iron body
Cast steel body
KE71 Screwed
KE73 Flanged
KE43 Flanged
PN25 rating
(Pmax 25 bar g at 120oC)
PN40 rating
(Pmax 40 bar g at 120oC)
3. Selection of the valve Kv
The DN40 valve body size is available with various seat sizes
giving a choice of Kv values.
Use the graph to select a suitable Kv as follows:
a) The feedwater flowrate is the actual maximum steam
generation rate of the boiler plus any blowdown rate where
this is significant. In practice the use of the 'from and at'
boiler rating will give a small safety margin. In the example
this is 15 000 kg/h.
b) The pressure drop across the valve is the feedpump
pressure at the maximum flowrate, minus the boiler
pressure, minus any valve and pipework losses. In the
example the available pressure drop is 1.5 bar.
c) Select the larger Kv value, 16 in this example. If right on the
line, or if in doubt, select a larger Kv.
4. Selection of the actuator + valve adaptor
The actuator has to be capable of shutting off against the
maximum feedpump pressure to Class IV when the boiler is
not under pressure.
Select the actuator + valve adaptor from the table below:
Actuator type
Valve
size
DN40
PN9223E
Maximum feedpump
pressure bar g
0.1
0.2 0.3 0.5
Pressure drop across the fully open valve bar
Kv
value
25.0
16.0
10.0
6.3
1 1.5 2 3 4 5
10
Figures in brackets denotes the differential
valves fitted with high temperature graphite
These valves have a suffix 'H'.
Valve stroke is 20 mm.
5. Electropneumatic positioner
Specify EP5 positioner (4 - 20 mA input).
40
(40)
40
(40)
40
(40)
40
(40)
pressures for
stem sealing.
B
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B
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k
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K
v
1
0
K
v
6
.
3
K
v
2
5
K
v
1
6
