Scale in Steam Boilers

The feed water of the steam boiler must be free of hardness. Sometimes, due to a short or improper operation of the softener or the failure to regenerate the resins in time, some hardness enters the feed water of the boiler. Increasing the temperature of the water in the steam boiler reduces the solubility of water salts. The water adjacent to the hot surfaces becomes saturated and conditions are created for the deposition of poorly soluble substances. The scale formed in steam boilers is a heat insulator. If the tube wall is covered with scale, the thermal efficiency of the boiler decreases. As a result, the heat absorption from the combustion gases decreases, energy is wasted and steam production decreases. In order to compensate for the decrease in steam production, the fuel and thermal load of the boiler increase. Under these conditions, the tubes melt and lose their resistance. In order to prevent a change in the steam temperature of 260 C, it is necessary to increase the temperature of the thermal surface of the scale by about 110 C compared to the clean surface.

Effect of scale on boiler thermal surfaces

The type of scale and its composition and what type of scale will be formed in the boiler with a given water feed cannot be predicted. Even with the same type of water, the type of scale varies from place to place. The chemical composition of several types of scale is shown below. A 5 mm thick scale increases fuel consumption by 8%. Under these conditions, an additional 20,000 liters of fuel are required for a plant that consumes 250,000 liters of fuel per year.

Chemical composition of some of the deposits observed in the boiler:

• Calcium sulfate (CaSO4)
• Calcium carbonate (aragonite) (Caco3)
• Serpentine (3MgO.2SiO2.H2O)
• Xenotellite (5Caco.5SiO2.H2O)
• Analite (Na2O.Al2O3.4SiO2.2H2O)
• Magnesium phosphate (Mg5(PO4)3OH)
• Brusite (Mg(OH)2)
• Hematite (Fe2O3)
• Magnetite (Fe3O4)
• Hydroxyapatite (CCa10(OH)2(PO4)6)
• Pectolite (Na2O.4CaO.6SiO2.H2O)

Prevention of deposit formation in boilers

In order to reduce and prevent deposit formation in boilers, the following is recommended:

1. When operating the softener, take sufficient care in its regeneration and maintenance To prevent the passage of hard water.
2. The residual hardness in the boiler feed water should be controlled by adding appropriate chemicals (such as phosphates and dispersants). These compounds combine with the residual hardness in an alkaline environment and produce insoluble sludge-like materials that do not stick to the boiler walls or pipes.
3. With timely and adequate management, the concentration of salts inside the boiler should be controlled.
4. The amount of chemical anti-scale chemicals injected into the boiler feed water should be adjusted so that their residual in the boiler is at the desired level to prevent scale formation in the boiler.