What parameters must be controlled in boiler feed water?

The key parameters are total hardness (ideally nil for high-pressure systems), dissolved oxygen (below 0.005 mg/L for high-pressure boilers), pH (8.5–9.6 depending on system metallurgy), silica (below 5 ppm for high-pressure systems), and total dissolved solids controlled through blowdown.

What is the difference between softening and demineralisation for boiler feed water?

Softening removes calcium and magnesium (hardness) through ion exchange, replacing them with sodium. It does not remove TDS. Demineralisation removes all ionic species both cations and anions producing water with very low TDS suitable for high-pressure boilers. RO achieves similar results through membrane separation.

What is a deaerator and why is it essential?

A deaerator is a pressure vessel that heats feed water to drive out dissolved oxygen and carbon dioxide before they enter the boiler. It reduces dissolved oxygen from several mg/L in cold makeup water to below 0.02 mg/L, a level at which supplemental chemical oxygen scavenging can reduce it to below 0.005 mg/L. Without de-aeration, oxygen scavenging chemical costs would be prohibitive and oxygen pitting would be inevitable.

What is boiler blowdown and why is it necessary?

Blowdown is the controlled discharge of a fraction of boiler drum water to prevent the build-up of dissolved solids above the threshold at which carryover, scale formation, or foaming occurs. Continuous blowdown, controlled by conductivity monitoring, is the standard approach for industrial boilers. Heat recovery from blowdown water through heat exchangers reduces the energy cost of blowdown.

Boiler Feed Water Treatment System