Boiler Water Chemistry
Proper water chemistry is the single most important factor in boiler longevity. Learn about TDS, pH, dissolved oxygen, hardness, and the chemical treatments that keep your boiler healthy.
Why Water Chemistry Matters
Boiler water chemistry determines whether your boiler lasts 5 years or 30 years. Poor water quality causes scale formation, corrosion, foaming, and carryover, all of which reduce efficiency, damage equipment, and increase operating costs.
Steam is generated from water. Whatever is in the water ends up either in the boiler (concentrated) or in the steam (carried over). The goal of water treatment is to keep dissolved solids and dissolved gases under control while preventing scale and corrosion.
Key Parameters
| Parameter | Target | Why it matters |
|---|---|---|
| TDS (Total Dissolved Solids) | Below 3,500 mg/L (varies by pressure) | High TDS causes foaming and carryover. Control by surface blowdown. |
| pH | 10.5 to 11.5 (boiler water) | Low pH causes corrosion. High pH causes caustic embrittlement. Controlled by chemical dosing. |
| Dissolved oxygen | Below 0.007 mg/L in feedwater | Oxygen causes pitting corrosion. Removed by deaeration and oxygen scavengers. |
| Hardness (Ca, Mg) | Below 1 mg/L in feedwater | Calcium and magnesium form scale. Removed by softening or RO. |
| Sulphite (O2 scavenger) | 30 to 60 mg/L residual | Consumes dissolved oxygen to prevent corrosion. Dosed into feedwater. |
| Phosphate | 20 to 40 mg/L | Conditions hardness salts to form soft sludge instead of hard scale. |
| Alkalinity | 200 to 400 mg/L as CaCO3 | Maintains pH and prevents acid corrosion. |
Common Water Treatment Chemicals
- Oxygen scavengers (sulphite, hydrazine): React with dissolved oxygen to prevent pitting corrosion. Sulphite is most common for low to medium pressure boilers. Hydrazine is used for high pressure but is toxic and being phased out.
- Phosphate compounds: React with calcium hardness to form a soft, non-adherent sludge that is removed by bottom blowdown, rather than hard scale on tube surfaces.
- Amines (neutralising, filming): Protect condensate return piping from CO2 and O2 corrosion. Neutralising amines raise pH, filming amines create a protective barrier.
- Antifoam agents: Reduce foaming caused by high TDS or suspended solids. Usually polymeric. A supplement to proper blowdown, not a replacement.
- Chelants and polymers: Bind dissolved hardness ions, keeping them in solution rather than precipitating as scale. Used as alternatives to phosphate treatment.
Testing Frequency
| Test | Frequency | By |
|---|---|---|
| TDS (boiler water) | Every shift | Operator (conductivity meter) |
| pH (boiler and feedwater) | Every shift | Operator (pH meter or strips) |
| Hardness (feedwater) | Daily | Operator or water treatment company |
| Dissolved oxygen (feedwater) | Weekly | Water treatment company |
| Sulphite residual | Daily | Operator (drop test) |
| Phosphate residual | Daily | Operator (drop test) |
| Iron content (condensate) | Weekly | Water treatment company |
| Full water analysis | Monthly | Water treatment company |
Keep a water chemistry logbook
Record every test result. Trends are more important than single readings. A gradual increase in TDS, decrease in pH, or change in chemical consumption indicates a developing problem. Use the PlantLogica maintenance logging system to track and trend water chemistry data automatically.