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(3) Corrosion Protection
1) Maintaining the Appropriate PH
Chemicals to the boiler changes the tube water to alkali property from acid property which is the most bad condition
for corrosion. Appropriate PH is PH 11.0 - 11.8.
2) Removing the Dissolved Oxygen
The boiler is corroded when using the water including dissolved oxygen. This dissolved oxygen could be removed by
chemical injection.
3) Blowing -Off
The blowing-off should be performed so that chloride concentration in the water is not exceeded
400mg Cl/l. When using the underground water near the seaside, special attention is required. If tube water is not
maintained in appropriate condition during operation suspension, small corrosion occurred during operation could be
increased or new corrosion could be appeared.
The quality of the boiler feed water and the tube water shall be in compliance with the following standard values:
(Feed Water Quality) (Tube Water Quality)
* Sampling Method of theTube Water for Quality Test
The water sample is taken through the drain valve when the steam pressure is reduced and water has been drained a
half of the tube water after finished operation. The water sample must be taken 3 hours after operation finished. The
amount of water sample is about 500cc.
5-2 Instruction for Blowing-off of Water
The tap water or underground water used for boiler water is transparent and is seen as not included any foreign
substance, but it has a lot of organic and inorganic matter such as calcium, magnesium, silica, etc. The softener
cannot remove the other foreign substance but calcium and magnesium which is caused for scale. Therefore, for
adjusting the concentration of the boiler water to a standard value, the blowing-off should be performed through
exhausting the concentrated tube water. Blowing-off should be performed at least once a day(8 hour basis a day) for
prevention of superheating which is caused by scale and sludge deposit.
Inspection Item
Std. Value
PH (25
°
C)
6 - 9
Hardness (mgCaCO3/l)
1 and below
Oil(s) and Fat(s) (mg/l)
nearly 0
Dissolved Oxygen (mgC/l)
nearly 0
Total Iron (mgFe/l)
0.3 and below
Electrical Conductivity (
µ
s/cm)
350 and below
M Alkalinity (mgCaCo3/l)
80 and below
Chloride Ion (mgC l/l)
50 and below
Silica (mgSiO2/l)
50 and below
Inspection Item
Std. Value
PH (25
°
C)
11.0 - 11.8
M Alkalinity (mgCaCO3/l)
250 - 800
P Alkalinity (mgCaCO3/l)
150 - 600
Total Evaporation Residue (mg/l)
2800 and below
Electrical Conductivity (
µ
s/cm)
4000 and below
Chloride Ion (mgC l/l)
400 and below
Phosphate Ion (mgPO4/l)
20 - 40
Hydrazine (mgN2O4/l)
0.1 - 05
Silica (mgSiO2/l)
350 and below
Sulfite Ion (mgSO3/l)
10 - 20
