Feedwater Systems, Water Treatment, and Steam/Water Quality
Core concepts
Feedwater systems deliver water at a rate matching steam demand. Know the components: feedwater tank or deaerator, feed pumps (reciprocating or centrifugal), feedwater regulator (level control), check valve, and stop valve. The deaerator removes dissolved oxygen and other noncondensable gases by heating water to saturation and providing scrubbing; it protects downstream metal from oxygen pitting. Know the difference between makeup water (added to replace losses) and condensate return.
Water treatment addresses scale, corrosion, carryover, and foaming. External treatment (softening, demineralization, reverse osmosis) removes hardness before the boiler; internal treatment adds chemicals such as oxygen scavengers (sulfite, hydrazine substitutes), scale inhibitors, and alkalinity builders. Key control parameters: pH (typically 10.5-11.5 for high-pressure boilers), total dissolved solids, alkalinity, silica, conductivity, and phosphate. Blowdown controls dissolved solids: continuous (surface) blowdown from the steam drum removes high-solids water near the surface; bottom blowdown removes sludge and sediment. Foaming and priming cause carryover, which contaminates steam and can damage turbines; high TDS, high alkalinity, and oily contamination are common causes.
Traps
Questions test the difference between scale (deposits on tubes, causing overheating) and corrosion (metal loss), and between priming (carryover of boiler water) and foaming (bubbles on the surface). Another trap: confusing the deaerator with a softener.
Worked example
Rule of thumb: if steam is wet and the gauge glass shows high TDS, suspect carryover; reduce TDS by increasing surface blowdown and check for oil contamination before assuming a mechanical cause.