Boiler water problems generally fall into two classes: deposit-related and corrosion-related. Because the two often interact, it is very common to find a boiler experiencing both simultaneously. There are many instances where deposits cause corrosion and corrosion causes deposits. It is important to avoid both problems.
Deposit-Related Problems
Boiler Scale
One of the most common deposit problems is boiler scale. This happens when calcium, magnesium and silica, common in most water supplies, react with tube metal found in boilers to form a hard scale on the interior of the boiler tubes, reducing heat transfer and lowering the boiler’s efficiency. If allowed to accumulate, boiler scale can eventually cause the tubes to overheat and rupture. Scaling is one of the leading causes of boiler tube failures. Scale is equivalent to having a thin film of insulation between the furnace
gases and boiler water. It can drop a boiler’s efficiency by as much as 10-12%. Scale forms as the solubilities of the scaleforming salts in water decreases and the temperature and concentrations of the salts increases. When feedwater is elevated to boiler water temperature, the solubility of the scaleforming salts is decreased, and solid scale begins to form on the boiler systems. See Table 2-1. Removing calcium and magnesium or other deposit-forming materials from the feedwater before they enter the boiler system is the best way to prevent scaling. Small amounts of hardness (calcium plus magnesium) can be effectively treated in the boiler and related system components by using boiler water treatment products such as chelates, polymers, and/or phosphates. Scale formation also occurs in economizers, feedwater pumps and related service lines. It also forms in low-pressure boilers where no pre-treatment or poorly maintained treatment chemicals, boiler water treatment products or pretreatment chemicals such as sodium zeolite are used. It is not normally found in boiler systems where demineralization is used or in high pressure, high purity systems.
Large amounts of hardness that cannot be successfully treated using boiler water treatment products must be treated by some other process.
