What is a steam boiler? A steam boiler is a piece of equipment that converts liquid water into steam using a heat source, typically generated by a fuel such as natural gas or wood, or by an electric heating system.

Steam is used in many residential, institutional, industrial, and commercial settings, particularly for heating, sterilization, cleaning, food processing, and certain chemical processes. Its appeal lies in the availability of water, its non-toxicity, and its ability to efficiently transfer thermal energy.

Depending on their design, boilers can take various forms. For example, fire-tube boilers circulate combustion gases through tubes surrounded by water.

Since it produces steam, a boiler is considered a pressure vessel. As such, it is specifically regulated by the Pressure Vessel Regulations (B-1.1, r. 6.1), enacted pursuant to the Quebec Building Act.

Above a certain capacity, the pressure-bearing components of industrial boilers are manufactured from carbon steels or low-alloy steels, such as carbon-manganese steels. These materials are chosen for their thermal and mechanical strength, their formability, and their compliance with the requirements applicable to pressure equipment.

These materials can offer good durability, with an equipment service life of up to 30 years or more under appropriate operating conditions. However, they remain sensitive, in particular, to water chemistry when it is not properly controlled.

Water Chemistry

In a steam boiler, water is one of the two raw materials of the system. Its quality is critical, since untreated water can cause degradation of the materials making up the boiler.

It is important to note that water suitable for drinking in a conventional plumbing system is not necessarily suitable for a steam generation system. This is due in part to the materials used and to the fact that, as the water evaporates, dissolved contaminants and mineral salts become concentrated in the residual water remaining in the shell.

The water must therefore be treated before entering the steam boiler. The treatment is designed, in particular, to control pH, alkalinity, hardness, oxygen concentration, dissolved mineral salts, and other contaminants.

In addition, boiler blowdowns and inspections are routine maintenance activities that help limit the accumulation of certain contaminants and/or deposits in the steam boiler’s water jacket.

Failures

The main forms of degradation in a steam boiler related to water chemistry are deposit formation, corrosion, and the entrainment of microdroplets in the steam.

When deposits form on heated surfaces, they reduce heat transfer between the combustion gases and the water, which can increase the system’s energy requirements as well as the temperature of the metals involved in heat transfer, and may lead to premature failure of metal components. Beneath the scale deposits, the local concentration of certain contaminants, such as chlorides or oxygen, may be higher than that measured in the circulating water, which can promote localized corrosion.

Corrosion can also be promoted by a high concentration of dissolved oxygen in the water, which creates an oxidizing environment. A pH that is too low can lead to acid corrosion, particularly in the presence of condensate contamination. Conversely, conditions that are too alkaline can promote caustic corrosion.

The entrainment of boiler water into the steam is another concern. Microdroplets laden with mineral salts or treatment chemicals can be carried along with the steam, particularly when the concentration of dissolved solids is too high. These contaminants can then affect equipment and piping further down the system.

Conclusion

During a failure analysis, documentation is often just as important as the component itself. Maintenance records, inspection results, and water analyses make it possible to reconstruct the system’s history. Combined with a materials analysis, this approach helps to understand the cause of a steam boiler failure.

This is where our experts in materials, metallurgy, and chemistry come in!

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