UFC 3-240-13FN
25 May 2005
turbine blades. Maintaining a low silica level in the boiler water can control silica
carryover. The acceptable level is dependent upon the boiler operating pressure; Table
3-7 shows suggested limits.
Table 3-7. Silica Levels Allowed in Boiler Water
Allowable
Boiler Pressure
Silica
KPa (psig)
ppm (as SiO2)
0-103 (0-15)
150
110-1027 (16-149)
150
1034-2062 (150-299)
150
2068-3096 (300-449)
90
3103-4130 (450-599)
40
4137-5164 (600-749)
30
5171 (750)
20
3-2.8.5
Determining Water Carryover in Steam. The best way to determine if
any water carryover is occurring is to measure the conductivity of the steam or of the
steam condensate. If you detect less than 25 micromhos conductivity in the condensate,
then carryover is probably insignificant. If conductivity is greater than 25 micromhos,
you should investigate to identify the cause.
3-2.8.5.1
Conductivity Measurement Implications. A conductivity measurement
above 25 micromhos in the steam condensate indicates that there is either carryover or
leakage into the steam or condensate system. Accordingly, a test for hardness in the
condensate must also be performed. If you find hardness, contamination of the
condensate is likely due to raw water or leakage into the condensate system rather than
boiler water carryover. If boiler water is at a very low hardness, then raw water may be
the cause.
3-2.8.5.2
Deposit Analysis. Any deposits found in the steam or condensate
systems should be analyzed to determine the source. Deposits that are mostly magnetic
iron oxide are due to corrosion of mild steel in the condensate system. Using
neutralizing amines (as described in paragraphs 3-2.7.3 and 3-2.7.4) can control these
deposits. If the deposits are mostly silica, increasing the blowdown or removing silica
from the makeup water can reduce them. If the deposits are mostly sodium salts
(sodium hydroxide, sodium carbonate, sodium chloride, sodium sulfate, and sodium
phosphate), the cause is probably boiler water carryover from misting, priming, or
foaming.
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