5 Factors That Impact the Formation of Boiler Scale

Energy efficiency of your boiler system centers around keeping your boiler clean and free of scale. Boiler scale can be a hard, tenacious deposit or a softer coating of insoluble salts, which forms on boiler tubes and walls. It’s most often caused by makeup water hardness, which refers to the naturally occurring calcium and magnesium ions found in water. These ions, when heated to the high temperatures inside a boiler, are what creates the scale and scale buildup. This buildup will not only cause a reduction in energy efficiency of the boiler, but can lead to boiler tube overheating potentially to the rupturing of the tubes.

A reduction in energy efficiency means the boiler has to burn more fuel to maintain the same output. Over time, that increased fuel demand can get very expensive. By keeping the boiler free of scale, the unit is maintained at peak efficiency, resulting in lower fuel costs. Preventing scale may involve, but not be limited to, filtration, pretreatment of the water entering the boiler and chemical treatment.

Boiler scale is the number one issue in low pressure boiler treatment programs

Water hardness is the number one contributor to boiler scale. There are other reasons for the formation of scale, but the naturally occurring ion combinations are the predominant culprits, particularly in low-pressure systems.

The essential principle is that the thicker the scale, the greater the percentage of the reduction in the energy efficiency of the system.

5 factors that can impact the formation of scale

  1. Poor Maintenance of Boiler Systems: Poor maintenance of a boiler or the pretreatment system can quickly result in scale, and create potential hazards to employees as well.
  2. System Pressure: The pressure at which a boiler operates can also contribute to the amount of scale in the boilers. Industry guidelines dictate the type of chemistry and the level of pressure at which systems can run. Boiler systems must run at the American Society of Mechanical Engineers (ASME) guidelines for both feedwater and boiler water chemistry. These guidelines dictate how water must be pretreated before it gets to the boiler. It also influences what chemistry can be put into the boiler.
  3. Boiler Design: Certain boiler designs are more susceptible to scale than others. Boiler design can contribute to the efficiency of the system. Some systems have a different heat flux rate at the tube’s surface, even though they may have the same pressure rating. Boiler design and efficiency is constantly being studied and tested to find a combination that conforms to ASME standards while also utilizing new materials and methods of boiler construction.
  4. Condensate return: Condensate is formed when steam vapor returns to its liquid state. When this happens as a result of steam used in the process application, the recovered condensate can retain about 13% of the total energy of the steam vapor. When returned to the boiler system, this condensate can help reduce make-up costs (reduced water and treatment chemical requirement) and fuel costs (associated with heating the cold makeup water). If not properly monitored or treated, contaminated condensate water can contribute to scale in the system.
  5. Pretreatment: Pretreating your boiler system water begins with an analysis of the water going into your system. A pretreatment plan will be developed, the elements of which will be dependent on the analysis of the water source and its quality. Pretreatment will likely include the removal of solids (suspended and/or dissolved) via filtration, softening or demineralization, and also dissolved oxygen removal via deaeration or a hot water tank. An inadequate or poor pretreatment plan can contribute to scale and deposition due to hardness and iron.

Preventing boiler scale

Understanding the makeup of the water going into your system and being able to pretreat it is a major factor in preventing boiler scale. Since almost all corrosion and scale can be traced to two major factors—dissolved oxygen in the water and water hardness, knowing what kind of water you’re feeding into your boiler is the best place to begin eliminating or changing the factors that impact scale formation.

To learn more about resolving boiler system problems, download our dissolved oxygen control guide today.

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