Feedwater Treatment and Water Chemistry
Why water chemistry matters
Understand the three main problems: scale, corrosion and carryover. Scale forms from hardness salts (calcium and magnesium) and insulates the metal, causing overheating and tube failure. Corrosion arises from dissolved oxygen, carbon dioxide and low pH. Carryover arises from high dissolved solids, alkalinity or foaming, and contaminates steam and superheaters.
Treatment methods
Know external treatment (softeners, demineralisers, reverse osmosis) and internal treatment (phosphate, sulphite, hydrazine, amines, chelants). Understand the purpose of deaeration and why oxygen scavengers are added after it. Know how to control blowdown using conductivity or total dissolved solids, and why pH is kept slightly alkaline. Understand the meaning of alkalinity, hardness, silica and chloride tests.
Traps
Questions often confuse internal and external treatment, or ask which chemical removes oxygen versus which controls scale. Another trap is pH: too low causes corrosion, too high can cause caustic embrittlement and foaming. Do not assume that clear water is pure water; dissolved solids are invisible.
Worked example
If conductivity is rising and steam is wet, reason that dissolved solids are high and carryover is likely. Increase blowdown and check the chemical dosing, rather than simply raising the water level.
Rule of thumb
Keep the water soft, oxygen-free and slightly alkaline, and control solids with blowdown.