Feedwater Systems and Water Treatment
This domain tests the feedwater path and the chemistry that keeps boiler metal clean and protected. Expect deaeration, feed pump and economizer function, external and internal treatment, blowdown control, and the damage caused by poor chemistry.
Feedwater system
Feedwater passes through a deaerator, feed pump, feedwater regulator, and often an economizer before entering the drum. The deaerator removes dissolved oxygen and carbon dioxide and heats the water. A feed check valve prevents backflow; the feed stop valve isolates the line.
Treatment program
- External treatment: softening, reverse osmosis, or demineralization reduces hardness and dissolved solids.
- Internal treatment: phosphate for hardness control, sulfite or other oxygen scavenger for residual oxygen, amine for condensate pH, and chelant where appropriate.
- Control tests include pH, conductivity, hardness, silica, oxygen, and total dissolved solids. Blowdown controls dissolved solids and suspended sludge.
Worked example: Feedwater TDS is 200 ppm and the maximum allowable boiler-water TDS is 2,000 ppm. Blowdown fraction equals feedwater TDS divided by (maximum boiler water TDS minus feedwater TDS): 200 / (2,000 - 200) = 200 / 1,800 = 0.111, or about 11.1% of feedwater. That blowdown rate holds boiler water at the limit if the feedwater value stays steady. Recalculate whenever feedwater quality changes.
Traps: oxygen pitting from poor deaeration; hardness scale from softener breakthrough; carryover from high TDS or silica; overfeeding chemicals; testing a sample that is not representative or properly cooled.