(carbon dioxide consumed)
Principles of Water Treatment
DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL
Sodium sulfite reacts rapidly with oxygen and is a very efficient scavenger. However, being a
solid and the source of another solid (Na SO ) that is produced during the reaction, sodium
sulfite has the potential of fouling heat transfer surfaces. An additional problem associated with
the use of sodium sulfite is corrosion of secondary system components resulting from its
decomposition products. At the temperatures present in the steam generators, sodium sulfite
can decompose as follows.
Sulfur dioxide (SO ) is a gas and is carried over to the remainder of the steam facility. With
water (in the steam or in the feed/condensate system), the SO reacts in the following manner.
This acidic condition is corrosive to all components in the secondary system.
Because of the problems associated with sodium sulfites, many facilities use volatile chemistry
control of the secondary steam system to control dissolved gases in conjunction with air
removal systems. This control utilizes hydrazine (Reaction 4-11) and morpholine (Reaction 4-
15) to eliminate oxygen and carbon dioxide, respectively.
As can be seen by Reaction (4-11), no solids are formed; thus, the tendency of fouling heat
transfer surfaces is reduced. An additional benefit results from the decomposition of hydrazine
by the following reactions.
These reactions result in an alkaline pH condition that decreases corrosion in the steam facility.
As can be seen in Reaction (4-15), the consumption of CO takes place. Two benefits result
from this reaction; 1) the inventory of dissolved gases in the steam facility is reduced, and 2)
is the reaction contributes to maintaining a higher pH by eliminating carbonic acid (H CO ),
thus reducing corrosion.