Viessmann VITOMAX 200-HS Operating And Service Instructions page 27

Water quality for steam boiler systems
General information regarding conditioning
■ Conductivity
– Saline
This means feedwater with electrical
conductivity of >30 µS/cm (e.g.
downstream of softening plants).
– Low salt content
This means feedwater with electrical
conductivity of 0.2-30 µS/cm (e.g.
downstream of desalination plants).
– Free of salt
This means feedwater with electrical
conductivity of <0.2 µS/cm and a sili-
cic acid concentration of <0.02 mg/l,
as well as condensate with electrical
conductivity <5 µS/cm (e.g. down-
stream of full desalination plants
with mixed bed quality).
■ Acid capacity K
S 8.2
A higher acid capacity in feedwater
would indicate an excess of bound car-
bon dioxide. This results in a higher
putrefaction by alkalis of the boiler
water, which leads to an increased risk
of corrosion on the steam boiler, and
also on the steam network down-
stream of the boiler through separation
of vapour-volatile carbon dioxide.
■ Adjusting the alkalinity
The selection of the alkalising agent
depends, amongst other things, on the
use of the steam, the steam pressure
and the type of water treatment. There
are solid and volatile alkalising agents.
A balanced conditioning with phosphate
or its derivatives can also be beneficial
through regulating the pH value of the
boiler water. However, organic condi-
tioning agents have also been in use for
years.
(cont.)
If organic conditioning agents are used,
the amounts and processes to be used
as well as the required analyses must be
specified by the supplier of these chem-
ical products.
■ Oxygen and carbon dioxide or oxy-
gen binders
Carbon dioxide and oxygen are expel-
led from the feedwater through full
thermal deaeration. The use of oxygen
binders is required if it proves impos-
sible in practical use, e.g. through
repeated idle periods, to keep the oxy-
gen content of the feedwater below the
permissible values. This "correcting
chemical" is admixed with the feedwa-
ter through the dosing facility.
Please note:
Amines that form a film are not oxygen
binders.
Note
In case of damage, evaporation can lead
to a concentration of the solute non-vol-
atile boiler water constituents (salts,
solid conditioning agents). Such condi-
tions can lead to local alkali concentra-
tions that result in stress fracture corro-
sion. As a result, sodium hydroxide as
alkalization agent is only permissible in
slightly buffered feedwater with conduc-
tivity of <30 µS/cm, if the recommended
pH range cannot be achieved with
sodium phosphate alone. With this
method, sudden fluctuations in pH value
result in low sodium hydroxide content.
Water quality
27