In the realm of industrial processes, boilers play a crucial role in many sectors, ranging from power generation to manufacturing. For companies, the cost of operating boiler systems, especially with the recent steep increases in energy costs, can be significant; this is particularly true for larger systems generating more than 8 tonnes of live steam per hour.
Water is a fundamental component in the operation of boilers, serving as a medium for heat transfer and the quality of water used directly impacts the efficiency and reliability of boilers.
The difference between boiler water and feedwater
Before we look at water’s role in boiler efficiency, it’s important to understand the difference between boiler water, or boiler make-up water and boiler feedwater. Whilst interconnected in the steam generation process, they serve distinct roles and require specific attention.
Boiler makeup water serves the purpose of replenishing the water lost during boiler operation, whereas boiler feed water undergoes rigorous treatment and is supplied to the boiler for steam generation. The treatment of boiler feedwater involves more comprehensive processes to adhere to stringent quality standards, as it directly impacts the performance, efficiency, and longevity of the boiler system. In contrast, boiler makeup water is generally subjected to less extensive treatment and is primarily employed to sustain the water level in the boiler system.
Boiler feedwater is typically drawn either from a standard mains supply, or from a licensed source such as a borehole. In each case, the raw water will contain varying degrees of organic and inorganic matter, plus dissolved gasses, minerals and salts. If these are not removed, then as pressurised steam is produced the dissolved gasses and solids will become concentrated, corroding metal parts and forming deposits or scale on internal boiler, condenser and pipework surfaces. In turn, this will affect heat transfer and liquid flow, reduce boiler efficiency and increase energy consumption, while also leading to higher maintenance costs and a greater risk of boiler failure.
These problems are proportionately greater in modern boilers, with high steam pressures and heat transfer rates, in applications with large volumes of live steam. They are also exacerbated in parts of the country where hard water or raised levels of total dissolved solids naturally occur.
Chemistry of feedwater
To ensure the integrity and optimal performance of a boiler and to deliver steam with the appropriate purity for turbine or industrial processes, it is imperative to purify and chemically condition the boiler feedwater. How to treat feedwater impurities depends on several factors including boiler design, steam purity needs, boiler operating pressure, the method of internal treatment for boiler water, blowdown rate and whether the feedwater is employed for steam attemperation.
Chemistry parameters that require control include:
Water hardness: Hardness in water is primarily caused by the presence of calcium and magnesium ions. These ions can lead to the formation of scale on heat exchange surfaces, reducing the efficiency of the boiler.
Total Dissolved Solids (TDS): TDS is a measure of all inorganic and organic substances dissolved in water. Excessive TDS can contribute to scale formation and foaming in the boiler, adversely affecting heat transfer efficiency.
pH level: The pH of feedwater significantly influences corrosion rates in the boiler. High or low pH levels can accelerate corrosion of metal surfaces. Maintaining the proper pH range (typically between 8.5 and 9.5) through the addition of alkalinity builders or pH adjusters is crucial for corrosion prevention.
Oxygen content: Oxygen is a major contributor to corrosion in boiler systems. It reacts with metal surfaces, causing pitting and deterioration.
Silica concentration: Silica can cause severe scaling in boilers, particularly in high-pressure systems. Monitoring and controlling silica levels are crucial to prevent the formation of hard, glass-like deposits on heat transfer surfaces.
Conductivity: This is a measure of the ability of water to conduct an electric current, primarily influenced by dissolved ions. Monitoring conductivity helps assess the overall quality of feedwater. Sudden changes in conductivity may indicate contamination, and maintaining consistent levels is essential for efficient boiler operation.
Alkalinity: Alkalinity in feedwater helps buffer against changes in pH, providing stability to the water chemistry. It is important to control alkalinity to prevent swings in pH that could lead to corrosion. Alkalinity builders can be added to maintain the desired alkalinity levels.
Organic Contaminants: These include oils and organics which adversely impact boiler efficiency and lead to foaming and carryover.
Boiler feedwater treatments
Purifying and chemically conditioning boiler feedwater is a multifaceted process that requires careful consideration of the specific impurities present. Common treatments include:
- Filtration and ultrafiltration
- Ion exchange (softening)
- Reverse osmosis (RO)
- Chemical treatment
Adhering to specific technical specifications and implementing effective water treatment strategies are paramount to ensuring optimal performance and prolonging the lifespan of boiler systems. By understanding the importance of water quality in boiler operations, industries can mitigate the risks associated with scale formation, corrosion, and other water-related issues, ultimately contributing to increased efficiency and reduced operational costs.