How to Use Sodium Sulfite for Boiler Oxygen Scavenging in Water Treatment

Overview

Sodium sulfite (Na₂SO₃) is the most cost-effective chemical oxygen scavenger for boilers operating below 60 bar (900 psi). It reacts with dissolved oxygen: 2Na₂SO₃ + O₂ → 2Na₂SO₄. The reaction is fast above 40°C and can be catalyzed by cobalt. Used after mechanical deaeration to remove the last 7–20 ppb of dissolved oxygen.

Preparation & Dissolution

Stock solution: Dissolve sodium sulfite powder in clean water to make 5–10% solution.

1. Add powder to water in dissolving tank with agitation
2. Dissolves quickly (10–15 minutes)
3. Solution is not stable long-term — prepare weekly maximum
4. Can add catalyzed version (contains 0.1% cobalt chloride) for faster reaction at lower temperatures

Dosing Guide

Boiler Type	Sulfite Dose	Residual Target
Low pressure (< 20 bar)	8 ppm per 1 ppm O₂	20–40 ppm in boiler
Medium pressure (20–60 bar)	8 ppm per 1 ppm O₂	10–20 ppm in boiler
After deaerator (7 ppb O₂)	~30–50 g/m³ feedwater	5–20 ppm in boiler

Stoichiometry: 7.88 ppm Na₂SO₃ reacts with 1 ppm O₂. In practice, dose at 1.5–2x stoichiometric.

Application Procedure

1. Measure feedwater dissolved oxygen at deaerator outlet
2. Calculate sulfite dose: O₂ (ppm) × 8 × safety factor (1.5–2.0)
3. Set metering pump to inject into feedwater line after deaerator
4. Monitor sulfite residual in boiler water weekly using sulfite test kit (starch-iodide method)
5. Maintain 10–40 ppm residual depending on pressure
6. Adjust dose based on residual — too low = oxygen corrosion risk, too high = wasted chemical and increased TDS

Monitoring & Control

Parameter	Frequency	Target
Dissolved O₂ (feedwater)	Daily	< 7 ppb after deaerator
Sulfite residual (boiler water)	2x weekly	10–40 ppm
Iron in feedwater	Weekly	< 50 ppb (corrosion indicator)
Boiler water TDS	Weekly	Within OEM limits

Common Mistakes

• Using in high-pressure boilers (> 60 bar): Sulfite adds TDS (Na₂SO₄ reaction product). Above 60 bar, TDS limits are tight — switch to hydrazine or carbohydrazide.
• Not using catalyzed version in cold feedwater: Below 40°C, uncatalyzed sulfite reacts slowly. Use cobalt-catalyzed grade or DEHA for low-temperature systems.
• Ignoring TDS contribution: Each ppm of O₂ removed adds ~14 ppm TDS (Na₂SO₄). Monitor boiler water conductivity.
• Forgetting the deaerator: Chemical scavenging should be the BACKUP to mechanical deaeration, not the primary defense. Fix deaerator first if O₂ > 20 ppb.

Storage & Handling

• Shelf life: 12 months (powder), keep sealed to prevent air oxidation
• Temperature: Store at room temperature, dry conditions
• Safety: Low toxicity. Generates SO₂ if mixed with acids — do not mix with HCl or H₂SO₄. Wear dust mask.