Viessmann Vitomax HS M73 Operating And Service Instructions page 22

Water quality
Saline boiler feedwater requirements
Salt-free
■ Feedwater with an electrical conductivity of
< 0.2 S/cm and silicic acid concentration
µ
< 0.02 mg/l
■ Feedwater with an electrical conductivity of
< 5 S/cm (e.g. downstream of a full desalination
µ
system with mixed bed quality)
Acid capacity K
S 8.2
A higher acid capacity in feedwater would indicate an
excess of bound carbon dioxide.
This results in a higher putrefaction by alkalis of the
boiler water, which leads to an increased risk of corro-
sion on the steam boiler, and also on the steam net-
work downstream 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. Organic condi-
tioning agents can also be used.
The amounts, procedures and analysis regulations of
the supplier of the chemical products must be speci-
fied.
Oxygen and carbon dioxide or oxygen binders
Carbon dioxide and oxygen are expelled from the
feedwater through full thermal deaeration.
In practical operation, if the oxygen content of the
feedwater cannot be maintained below the permissible
values, e.g. by increasing the idle periods, then the
use of oxygen binders is required.
These "corrective chemicals" are mixed with the feed-
water via the dosing equipment.
!
Please note
Film forming amines may cause damage to the
boiler.
Do not use film forming amines.
Stress corrosion cracking
In case of damage, evaporation can lead to a concen-
tration of the solute non-volatile boiler water constitu-
ents (salts, solid conditioning agents). Such conditions
can lead to local alkali concentrations that result in
stress corrosion cracking.
With low buffered feedwater, conductivity < 30
*5
22
(cont.)
Preventing stress corrosion cracking
If the recommended pH range is not achieved with
sodium phosphate alone, sodium hydroxide may be
used as the alkalising agent as an exception. With this
method, sudden fluctuations in pH value result in low
sodium hydroxide content.
Deviation during conditioning
Possible causes of differing values in continuous oper-
ation:
Inadequate treatment of the top-up water
■
■ Inadequate feedwater conditioning
■ Advancing corrosion on some system components
■ Water contamination: Ingress of contaminants from
other systems, e.g. condensate tank, heat exchang-
ers
Restoring correct operation immediately:
■ Take suitable measures immediately.
■ Recycled condensate must not be allowed to affect
the feedwater quality.
■ Recycled condensate must be treated if necessary.
Check the chemical composition of the boiler water:
■
Through dosed addition of specified chemicals
–
Through continual or intermittent partial blow-down
–
of the water volume
Recommendation
Remove dissolved and suspended impurities.
Conditioning and warranty
Note
Voiding the warranty
■ If film forming amines are used
If dosing agents are used that are not listed in the
■
guidelines
S/cm
µ
*5