Feedwater Systems and Water Treatment
This domain covers how water reaches the boiler and how it is kept chemically safe. Know the feedwater path: condensate returns, makeup water, deaerator, feedwater heater or economizer, feed pump, feedwater regulator, and check valve before the boiler. Understand the purpose of each: a deaerator removes dissolved oxygen and other gases; an economizer recovers heat; a check valve prevents backflow; the feedwater regulator maintains level.
Water chemistry matters. Scale insulates tubes and overheats metal; corrosion thins metal; carryover contaminates steam. Learn the common treatments: softening removes hardness; dealkalization and reverse osmosis reduce dissolved solids; oxygen scavengers such as sulfite or hydrazine remove residual oxygen; amines control pH; blowdown controls dissolved solids and sludge. Know that continuous blowdown removes dissolved solids from the steam drum surface, while bottom blowdown removes sludge and sediment.
Traps: confusing continuous and bottom blowdown; assuming a deaerator only heats water (it also removes oxygen); thinking a check valve regulates flow (it only prevents reverse flow); and forgetting that excessive blowdown wastes energy and chemicals. Questions often test the relationship between dissolved solids, conductivity, and blowdown rate.
Rule of thumb: treat feedwater for oxygen and pH, treat boiler water for scale and solids, and use blowdown to hold dissolved solids within limits.