Page 197 - Industrial Ventilation Design Guidebook

4.4 WATER PROPERTIES AND TREATMENT i 51

or soda ash is used in controlled amounts. For some boiler water, rreatment-
chelating agents are used to full advantage.
For a high-pressure boiler plant with a high evaporation rate, demineral-
ized feedwater is classified as having an electrical conductivity of less than
1
0,2 jxS cm" ; for less critical plant conditions an electrical conductivity
-1
greater than 0.2 |xS cm may be acceptable.
The water chemistry relating to power plants operating at high temper-
atures and pressures is a complex issue. To determine if water is corrosive
or scale forming, use is made of the Langelier or Ryznar index. For further
information, refer to the VGB guidelines for plants operating at pressures
above 68 bar (VGB-450L) and the Scandinavian recommendations
(DENA).

Sludge
Either straightforward drainage or blowdown can readily remove
sludge from the plant. It is, however, necessary in some cases to ensure that
the residual solids are free-flowing; this is achieved by the use of tannin,
lignin, seaweed derivatives, and starch organics.
Foam
Water level control and the use of organic antifoam chemicals are essen-
tial in steam plants in order to break down the bubbles at the water surface in
steam systems, which cause foaming.

Condensate
Condensate formed in steam systems may require treatment to remove
the carbon dioxide in suspension or free-flowing in the condensate. Due to
the nature of the water source, carbon dioxide and oxygen as dissolved
gases are always present to some degree in water supplies, and in some in-
stances hydrogen sulfide (H 2S) and ammonia (NH 3 ) may be present, pro-
ducing a weak carbonic acid gas and causing elevated-temperature
corrosion of metals. Treatment in this case is achieved by the addition of
chemicals, pH control, oxygen removal, etc.
When dealing with a large steam plant, the chemistry and the methods of
water treatment required are complex.
The efficiency of water separation varies considerably from boiler to
boiler. The purity of the steam supplied to a steam turbine should be checked.
On the basis of the results, the maximum allowable salt concentration in the
boiler water can be determined. This concentration may be much lower than
the values given in the table.
2
When the heat load even locally exceeds 230 kW m~ the target values
for drum pressure, 160 bar (except for SiO 2), should be used for all boiler
pressures. For feedwater, the recommended values for >67 bar should be
used.
1. The maximum p-value is independent of feedwater treatment.
2. The gauge pressure when using phosphates to reduce the residual
hardness and when using a coordinated phosphate method for pH