Non--reacttiive Sillica in boiler water

Non--reactive Silica
.1 Silica in water is present mostly as reactive or dissolved silica. In surface waters,
a small quantity of non-reactive silica (in colloidal dimensions) may also be
present during parts of the year especially during the monsoon. A DM plant
removes reactive silica almost completely, to less than 0.005 mg/l. However,
non-reactive silica is not removed and finds its way into the boiler drum where it
gets converted into reactive silica under the operating conditions of high pressure
and temperature. The station chemists usually overcome this problem by having
increased blow-downs during these periods.
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.2 Increased blow-down however means loss of valuable condensate and when one
considers the cost of condensate at Rs. 100/m3, even a small increase in blow-down
of say, 1 m3/h for four months in a year, will mean a loss of Rs. 2.7 lacs
annually. In addition, removal of silica scales is very difficult and needs shutdown
of the power station disrupting power supply. It is therefore important that nonreactive
silica also is removed from boiler feed water.
.3 Estimation of non-reactive silica in water is often difficult as it is present in very
small quantities (generally less than 1.0 mg/l) and calls for a high degree of
expertise on the part of the chemist. Besides, it requires handling of hazardous
chemicals like hydrofluoric acid.
.4 Presence of non-reactive silica is therefore best ascertained by continuous
monitoring of silica in boiler drum water. Ingress of non-reactive silica will be
seen as an abnormal increase in boiler drum silica values during some parts of
the year.
.4 Removal off Non reactive silica
There are atleast two methods for removing non-reactive silica from boiler feed
water :
- Pretreatment of water ( coagulation and clarification)
- Ultra filtration