Boiler water, treatment oxygen removal

In neutral or slightly alkaline, oxygen-free aqneons systems, corrosion of fairly clean steel occurs at a very low rate at elevated temperatures. This principle is the basis for most boiler water treatment to prevent corrosion, i.e., treatment is designed to provide alkalinity, to remove oxygen, and to prevent scale deposition. Other additives are also nsed to prevent foaming, but these will not be considered here. 

For some applications, notably feed-water treatment for high-pressure boilers, removal of oxygen is essential. For most industrial purposes, however, de-aeration is not applicable, since the water used is in continuous contact with air, from which it would rapidly take up more oxygen. Attention must therefore be given to creating conditions under which oxygen will stifle rather than stimulate corrosion. 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Sur-Gard [Nalco]. TM for chemicals used for treatment of boiler water to inhibit scale and corrosion and to remove oxygen from the boiler water. 

Sulphites rarely occur in natural waters. They are chiefly of artificial origin (wastewaters from the production of sulphite cellulose and thermal processing of coal). They are washed out into atmospheric waters from urban and industrial air pollutants. In waters, sulphites are slowly oxidized into sulphates, consuming dissolved oxygen. Chemical oxidation is accelerated by catalytic effects of various metals, particularly the Co(II) compounds. In water treatment, sulphites are used for dechlorination, removal of oxygen from feed waters for steam boilers, and in the technology of wastewaters for reduction of Cr(VI) to Cr(III). 

Boiler feed treatment also must remove oxygen and alkalinity from the water. Dissolved oxygen leads directly to corrosion, and it is most dangerous when corrosion takes the form of pitting. Alkalinity refers primarily to carbonates and bicarbonates. These... 

The other major separation task in boiler feed water treatment is the removal of dissolved gases, especially oxygen and carbon dioxide, which will cause corrosion in the boiler. These can be removed by mechanical de-aerators, or chemicals may be added that will scavenge these gases. 

When condensate returns to a boiler, it may be contaminated by pipeline corrosion or with process materials. Likely sources of the latter include exchanger leakage and poorly installed process tie-ins. Carbon dioxide and oxygen are the chief causes of corrosion in steam and condensate systems. Section 12.4.3.3 discusses the treatment of boiler feed water to remove these contaminants, as well as hardness. 

---