Exclusion of Oxygen in Water-Tube Boilers

Importance of Excluding Oxygen

The removal of oxygen from boiler water is extremely important because oxygen is the primary cause of pitting corrosion in boiler systems. Under high temperature and pressure conditions, even small amounts of dissolved oxygen react with the steel surfaces inside the boiler, forming localized pits. Unlike uniform corrosion, these pits penetrate deeply into the metal and can quickly lead to tube failure.

In addition, high-pressure boilers rely on a protective layer of black iron oxide (magnetite, Fe₃O₄) on internal surfaces. Dissolved oxygen destroys this protective film, exposing the base metal to further corrosion. The pits formed also act as stress concentration points, increasing the risk of cracks due to thermal and mechanical stresses during operation (corrosion fatigue).

Measures to Achieve Oxygen-Free Boiler Water

1. Mechanical Methods (De-aeration)

Mechanical removal of oxygen is mainly achieved by heating the feed water, since the solubility of gases decreases as temperature increases.

• De-aerator: Feed water is sprayed into a steam atmosphere, raising its temperature to near boiling point. This releases dissolved gases such as oxygen and carbon dioxide, which are then vented out.
• Main Condenser: In regenerative systems, the condenser is designed to maintain condensate at a temperature close to saturation under vacuum conditions, minimizing air absorption.
• Air Ejectors: These remove any air that may leak into the condenser and vacuum system, preventing it from entering the feed water.

2. Chemical Methods (Oxygen Scavenging)

Chemical treatment is used to remove any residual oxygen that mechanical methods fail to eliminate.

• Hydrazine (N₂H₄): Widely used because it reacts with oxygen to form only water and nitrogen (N₂H₄ + O₂ → 2H₂O + N₂), leaving no solid residues. It also helps maintain the protective magnetite layer.
• Sodium Sulphite (Na₂SO₃): Commonly used in low-pressure boilers. It reacts with oxygen to form sodium sulphate, but increases the total dissolved solids (TDS) in the boiler water.
• Tannins: Used in some low-pressure systems to form a protective film on metal surfaces, reducing corrosion.

Laying-Up of a Water-Tube Boiler

When the boiler is to be kept out of service for a long duration, it must be protected from moisture to prevent corrosion.

1. Drain and Dry: Drain the boiler while it is still warm so that residual heat helps in drying internal surfaces.
2. Use of Desiccants: Place moisture-absorbing substances such as silica gel or quicklime (calcium oxide) inside the boiler drums to absorb any remaining moisture.
3. Sealing the Boiler: Close all manholes, valves, and openings tightly to prevent the entry of atmospheric air and moisture.
4. Periodic Inspection: Check the desiccants at regular intervals (e.g., monthly) and replace them if they become saturated.

When the boiler is expected to be restarted soon, it is kept filled with treated water to prevent air entry.

1. Complete Filling: Fill the boiler completely until water flows out of the air vents, ensuring that no air pockets remain inside.
2. Chemical Dosing: Increase the concentration of oxygen scavengers (such as hydrazine) and maintain higher alkalinity than normal operating levels to prevent corrosion.
3. Maintain Positive Head/Pressure: Keep a slight pressure or maintain a head of water using a header tank to prevent air from entering the system through glands or valves.
4. Water Circulation: If possible, circulate the water periodically to maintain uniform chemical concentration throughout the boiler.