Boiler Water Treatment: From Feedwater to Condensate

Boiler Water Treatment Stages

Stage	Chemical	Purpose
Feedwater deaeration	Mechanical deaerator + chemical O₂ scavenger	Remove dissolved O₂ to < 7 ppb
Internal treatment	Phosphate or all-volatile treatment	Prevent scale on boiler tubes
Condensate protection	Filming amine or neutralizing amine	Prevent CO₂/O₂ corrosion in return lines
Blowdown	Controlled continuous + intermittent	Maintain TDS/alkalinity within limits

Oxygen Scavenger Selection

Scavenger	Boiler Pressure	FDA Steam	Reaction Speed	Cost
Sodium sulfite	< 60 bar	No (adds TDS)	Fast	Lowest
Hydrazine	> 60 bar	No (carcinogen)	Slow below 150°C	Medium
Carbohydrazide	All pressures	Yes	Medium	Higher
DEHA	40–100 bar	Yes	Fast at low temp	Higher

Key rule: Always maintain measurable O₂ scavenger residual in feedwater (5–20 ppm sulfite, or 50–100 ppb hydrazine equivalent).

Internal Treatment Programs

Coordinated phosphate treatment (most common for medium-pressure):

• Trisodium phosphate: Maintain 5–15 ppm PO₄ in boiler water
• pH control: 9.5–10.5 in boiler water
• Prevents CaCO₃ and CaSO₄ scale by converting to soft hydroxyapatite sludge

All-volatile treatment (AVT) for high-pressure (> 100 bar):

• Ammonia or morpholine for pH control (pH 9.0–9.6)
• No phosphate — avoids solid deposits
• Requires ultra-pure feedwater (conductivity < 0.3 µS/cm)

Condensate Protection

CO₂ from bicarbonate decomposition dissolves in condensate → carbonic acid → corrosion.

Two approaches:

1. Neutralizing amines (morpholine, cyclohexylamine): Neutralize carbonic acid, raise pH to 8.5–9.0
2. Filming amines (octadecylamine): Form hydrophobic barrier on pipe walls

Monitoring

Parameter	Where	Target
Dissolved O₂	Deaerator outlet	< 7 ppb
O₂ scavenger residual	Feedwater	5–20 ppm (sulfite)
pH	Boiler water	9.5–10.5
Phosphate (PO₄)	Boiler water	5–15 ppm
Conductivity	Boiler water	Per OEM limits
Iron	Condensate	< 20 ppb
pH	Condensate	8.5–9.0