Causes and Effects of Scaling in Water Systems

Scaling in water systems in power plants, heating and cooling plants, and other processing facilities can reduce the efficiency of various applications by obstructing flow and decreasing the heat transfer capabilities of piping systems. Scaling is a dense layer of inorganic material that forms when soluble contaminants in the water precipitate. These deposits on surfaces impair heat transfer and can affect both boiler water and cooling water systems, though they are less of a concern in closed cooling water systems.

What Causes Scaling?

Scaling consists of salts with limited solubility. These salts reach the deposition site and precipitate as their concentration increases through evaporation. Common contaminants that can form deposits include calcium, magnesium, iron, copper, aluminum, silica, and to a lesser extent, silt and oil. Most deposits can be classified into two types:

• Deposits that crystallize directly on pipe surfaces
• Sludge deposits that settle elsewhere and are transported to the metal surface by flowing water

You can learn more about the types of deposits expected on the membranes of different water systems.

Scaling by the Flowing Cooling Water

Scaling is typically not a significant problem for closed cooling water systems due to the minimal makeup water needs. This allows the use of otherwise costly high-quality circulating and makeup waters. However, over a long period, even small amounts of hard makeup water will cause a buildup of scaling in the system.

Some Common Scales in Cooling Water Systems Include:

• Calcium carbonate
• Calcium phosphate
• Magnesium silicate
• Silica

Factors Affecting the Development of Scaling

Four major factors determine whether water encourages scaling formation. As any of these factors change, scaling tendencies also change:

• Temperature
• Alkalinity or acidity (pH)
• Amount of scale-forming material present
• Influence of other dissolved materials, which may or may not be scale-forming

How Does Scaling Occur?

Scaling occurs when dissolved ions in the water exceed their solubility. Most salts become more soluble as the temperature increases. However, some salts, like calcium carbonate, become less soluble as temperature increases, thus often causing deposits at higher temperatures.

A change in pH can significantly affect scale formation. For example, as pH increases, calcium carbonate solubility decreases and eventually precipitates on system surfaces. Some materials, such as silica, are less soluble at lower alkalinity.

When the amount of scale-forming material dissolved in water exceeds its saturation point, scaling may result. In addition, other dissolved solids may influence scale-forming tendencies. In general, higher levels of scale-forming dissolved solids increase the chance of scaling formation.

Methods of Controlling Scaling

There are four basic means to control scaling:

• Limit the concentration of scale-forming minerals: This can be done by controlling the concentration ratio or by removing the minerals before they enter the system. “Concentration ratio” is the ratio of dissolved solids in the blowdown to dissolved solids in the makeup. In short, use high-quality makeup water.
• Feed acid to keep the common scale-forming minerals dissolved: Most minerals are more soluble at lower pH.
• Make mechanical changes in the system to reduce the chances of scale formation: Examples include increasing water flow and using heat exchangers with larger surface areas.
• Treat with chemicals designed to prevent scaling.