Boiler feed water treatment corrosion control

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. 

Sulfite, S032-, is a bivalent polyanion. It occurs in boiler feed water that is treated with sulfite for controlling dissolved oxygen. It also occurs in waters subjected to S02 treatment for dechlorination purpose. Sulfite forms sulfurus acid, H2S03, which gradually oxidizes to sulfuric acid. Excess sulfite in boiler waters can cause corrosion. Sulfite is toxic to aquatic life. 

A boiler requires blowdown to remove concentrated dissolved solids and control the water quality. The lack of blowdown could result in a higher pH of boiler feed water (BFW) in the boiler, which could potentially lead to corrosion. Insufficient blowdown could also cause impurities to carryover to steam. On the other hand, excessive blowdown wastes energy, water, and chemicals. The optimum blowdown rate is determined by various factors including the boiler type and capacity, operating pressure, water treatment, and makeup water quality. Blowdown rate is 2—4% for relatively large boilers and 4—8% for small boilers. It can be up to 10% if makeup water contains high concentrations of solids. Industrial standards for blowdown are available and can be referenced that indicate the amount of blowdown depending on the type and pressure of the boiler. 

---