Page 298 - Boiler plant and distribution system optimization manual

Boiler Water Treatment 283

will reduce the problem of misting to some ex- tivity of 20 to 30 micromho indicates there is a
tent. A heavy foam layer is another source of liq- small chance that carryover is significant.
uid carryover into the steam. The level of foaming
can normally be controlled to a reasonable level A High Conductivity Measurement
by maintaining the total alkalinity at a level less Indicates Carryover
than 20 percent of the TDS and maintaining the A high conductivity measurement in the
total suspended solids at a level less than 8 per- steam condensate means there is either carryover
cent of the TDS. Antifoam agents are added to the or leakage into the steam system. The hardness
sodium lignosulfonate sludge dispersant to help must also be checked in this case. If any hardness
control foaming. is found, then the contamination of the conden-
sate indicated by the high conductivity is due to
PRIMING CARRYOVER leakage into the condensate system rather than
Priming carryover is caused by liquid surg- carryover. This is because the carryover of boiler
es into the steam drum that throw water into the water should be at a very low hardness due to
steam space where it is carried into the steam chemical treatment or the very low makeup water
header. Priming is always caused by a mechanical requirement.
problem or mechanical properties such as over-
sensitive feedwater controls or incorrect blow- TWO KEY OPERATING CONTROLS FOR
down procedures. There is no chemical control DEAREATORS
method available. There are two key operating controls for
deareators that must be watched. First, the deare-
SILICA CARRYOVER ator vent must be checked to see that a plume of
The silica in the boiler water can evaporate steam is always flowing. Second, the pressure of
and enter the steam, independent of water car- the deareator and temperature of the outlet water
ryover. The silica can form a deposit on turbine must be controlled. At a given pressure the tem-
blades and other equipment when the steam con- perature should be within 2°F of the temperatures
denses. This problem is controlled by maintain- shown in Table 15.3, based on the elevation of the
ing a low silica level in the boiler water. The sug- boiler site. If there is low or no steam flow or a
gested limits are shown in Table 15.2. low water temperature, the deareator is not oper-
ating properly.
Determining the Amount of Carryover A mixture of oxygen and water is a very cor-
The best indication of carryover is a mea- rosive combination. This corrosivity doubles with
surement of steam conductivity. A steam conduc- every 18°F increase in temperature.
Oxygen corrosion can be recognized by pits,
Table 15.2—Silica limits in boiler water. typically found in the top of the steam drum or at
the waterline. Oxygen can be removed from the
feedwater by mechanical or chemical deareation;
a combination of these methods is commonly
used.

CHEMICAL REMOVAL OF OxYGEN FROM
BOILER FEEDWATER
A mechanical deareator can reduce the oxy-
gen content of feedwater to a fraction of a ppm.
Complete removal requires additional chemical
treatment. One process used is catalyzed sodium
sulfite.