Water Treatment, Chemistry, and Corrosion Control
What the exam tests
This domain tests feedwater and boiler-water chemistry, scale and corrosion control, blowdown, deaeration, condensate treatment, and layup. The goal is to recognize which impurity drives which failure mode.
Core rules
- Hardness causes scale; dissolved oxygen causes pitting; low pH causes acid corrosion; high pH can cause caustic attack; silica can carry over into steam.
- Deaeration removes dissolved oxygen mechanically; chemical oxygen scavengers handle the remainder. pH and conductivity are routine control tests.
- Blowdown controls dissolved solids and sludge. Too little blowdown causes carryover and scale; too much wastes heat and water.
- Condensate return should be monitored for iron and copper, which indicate corrosion in the return system.
Worked example
Feedwater chloride is 20 ppm and boiler water chloride is 400 ppm. Cycles of concentration = 400/20 = 20. Percent blowdown = 100/20 = 5%. The reasoning: if the boiler concentrates chloride 20 times, 5% of feedwater must be blown down to hold that concentration steady. If the boiler chloride rises to 600 ppm at the same feed, cycles rise to 30 and blowdown should fall to about 3.3%, provided other limits allow.
Traps
Do not use pH alone as a complete treatment check; do not confuse scale with sludge; do not assume more blowdown is always safer; and do not ignore oxygen pitting because it is often localized and deep.