Feedwater, Water Treatment and Steam Purity
Feedwater, Water Treatment and Steam Purity
This domain tests how impurities cause scale, corrosion, carryover, and deposits, and how treatment, deaeration, blowdown, and sampling control them. Expect practical questions on hardness, pH, oxygen, TDS, silica, and chemical addition.
Core rules and concepts
- Hardness salts form scale on hot surfaces. Scale insulates, raises metal temperature, and can cause tube failure.
- Dissolved oxygen causes pitting, especially in economizers and feed lines. Mechanical deaeration plus chemical oxygen scavenging protects the system.
- pH control reduces corrosion. Low pH promotes acid attack; high pH can promote caustic attack in some conditions.
- TDS and silica can cause foaming, priming, and carryover, contaminating steam and downstream equipment. Blowdown controls boiler-water concentration.
- Internal treatment may use phosphate, chelant, sulfite, hydrazine substitutes, or amines, depending on pressure and system metallurgy. Chemical feed must follow the treatment program, not guesswork.
Worked example
Feedwater chloride is 20 ppm and boiler water chloride is 400 ppm. Cycles of concentration = 400 ÷ 20 = 20. Approximate blowdown as a percentage of feedwater = 100 ÷ 20 = 5%. If feedwater flow is 50,000 lb/hr, blowdown is about 2,500 lb/hr. The reasoning is that dissolved solids concentrate by evaporation; blowdown must remove the same solids entering with the feedwater. Chloride is often used because it stays dissolved and is easy to measure.
Traps that cost marks
- Using feedwater TDS instead of boiler-water TDS to calculate cycles.
- Overfeeding oxygen scavenger and assuming more chemical is always safer.
- Ignoring silica carryover at high pressure.
- Treating carryover as a mechanical problem when high TDS or foaming is the cause.