Page 296 - Boiler plant and distribution system optimization manual

Boiler Water Treatment 281

tem corrosion is caused by carbon dioxide. can shut down on low water cut off.
Carbon dioxide enters the system with the What has happened? The piping and heat
boiler feed water in the form of bicarbonate and exchange equipment serviced by the distribution
carbonate alkalinity. When exposed to boiler tem- header is cold and the steam is quickly condensed
peratures, bicarbonates and carbonates break possibly forming a vacuum. This causes a violent
down to form carbon dioxide. The carbon dioxide boiling action in the boiler, it could even start a
is carried away in the steam and is condensed to series of oscillations where the water surface ris-
form carbonic acid. es or “mounds” towards the steam nozzle. The
lift of the water can not be sustained and it drops
THE PRIMING BOILER starting a sloshing effect. A wave or bouncing ef-
Priming occurs when slugs of water enter fect can start which will allow water to enter the
the steam distribution system. There are a num- steam system in slugs; a dangerous condition.
ber of causes such as: high steam demand, large This condition can also upset boiler water circula-
and sudden drops in system pressure, blowdown, tion patterns.
quick opening valves in the distribution system One rule of thumb used in raising boiler
and unsuitable steam nozzle and steam header water temperature is to go at a rate of 100 F per
size. hour. Open valves slowly, allowing the system
One way priming could happen is after an to warmup and balance out before introducing
operator has brought a boiler up to operating sudden surges on the system.
pressure he opens the distribution header quick-
ly. Then, within a few minutes, he notices the wa- Deareators
ter bouncing up and down in the sight glass. This Oxygen and carbon dioxide are very harm-
bouncing can become quite violent and the boiler ful to boiler systems. Deareators are designed to

Figure 15.1—Blowdown is reduced dramatically when Cycles of Concentration is increased.