Introduction to Water Chemistry

Water chemistry involves the study of chemical characteristics and reactions of water in both natural and industrial contexts. In steam generation systems, feedwater quality plays a critical role in determining boiler efficiency, safety, and longevity. Even trace impurities can cause significant problems under high temperature and pressure conditions.

1. Sludge and Scale Formation

Scale: Hard, adherent crystalline deposits on heat-transfer surfaces from precipitation of sparingly soluble salts.
Sludge: Soft, loose, and non-adherent deposits of impurities, settling in low-flow areas.

Causes

• Scale: Thermal decomposition of bicarbonates, reduced solubility of CaSO₄, precipitation of Mg(OH)₂ and SiO₂.
• Sludge: Suspended solids, corrosion products, oil, and precipitated salts that do not adhere strongly.

Chemical Examples

Ca(HCO3)2 → CaCO3 ↓ + CO2 ↑ + H2O
Ca2+ + SO42- → CaSO4 ↓

Effects

• Reduced heat transfer efficiency.
• Local overheating and tube failure.
• Restricted water circulation.

Prevention

• External treatment: ion exchange, lime-soda softening.
• Phosphate conditioning to produce soft sludge.
• Regular blowdown to control solids concentration.

2. Priming and Foaming

Priming: Carryover of boiler water with steam due to violent boiling.
Foaming: Stable foam formation on water surface entraining water droplets.

Causes

• High dissolved solids (TDS) in boiler water.
• Oil/grease contamination acting as surfactants.
• Sudden steam demand changes.
• Improper water level control.

Consequences

• Reduced steam quality and thermal efficiency.
• Turbine blade erosion.
• Contamination of steam and condensate systems.

Prevention

• Control TDS via blowdown.
• Remove oil/organic contamination from feedwater.
• Use anti-foaming agents when necessary.
• Ensure proper drum design and separators.

3. Boiler Corrosion

Corrosion is the chemical or electrochemical attack on boiler metals, leading to thinning, pitting, and structural failure.

Types and Causes

• Oxygen corrosion: Dissolved O₂ forms rust: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ ↓.
• Carbonic acid corrosion: CO₂ + H₂O → H₂CO₃.
• Acidic corrosion: From low-pH feedwater or condensate.
• Pitting: Localized attack often under deposits.

Prevention

• Mechanical deaeration of feedwater.
• Oxygen scavengers: Na₂SO₃, hydrazine.
• Maintain alkaline pH (8.5–9.5).
• Condensate treatment with volatile amines.

4. Caustic Embrittlement

Caustic embrittlement is intergranular cracking of steel due to high local concentrations of NaOH in stressed areas.

Mechanism

Concentrated OH^- ions penetrate grain boundaries in steel under tensile stress, causing intergranular failure at high temperature.

Causes

• Excessive alkalinity from NaOH dosing.
• Concentration under deposits or in crevices.
• High temperature and tensile stress.

Prevention

• Use coordinated phosphate treatment instead of free NaOH.
• Maintain alkalinity within safe limits.
• Good blowdown and cleaning practices.
• Design to avoid crevice-prone joints.

Integrated Prevention Approach

• External treatment: softening, demineralization, reverse osmosis, deaeration.
• Internal treatment: phosphate conditioning, dispersants, scavengers.
• Operational controls: blowdown, monitoring pH, TDS, dissolved oxygen.
• Maintenance: inspection, cleaning, stress-relieving where necessary.