Recommended Chlorine Levels for an Open Loop Water Cooling

Open loop systems are a method of using renewable energy sources for cooling power plants and manufacturing plants. An open looped water cooling system is powered by natural water sources from groundwater and lakes. The system is built with metal pumps that draw the water in and return it to another source. These elements carry concentrations of chloride that can corrode the metal structures that house the open loop water system.

Chlorine Basics

• Chlorine is important to the open loop water cooling systems. It cleanses and disinfects the system and prevents microbial organisms from growing and thriving. Central Utah Water Conservancy District explains that "The typical pre-chlorine dose is about 0.8 mg/L and the typical post-filter chlorine dose is about 0.7 mg/L with the chlorine residual in the range of 0.8 to 1.0 mg/L leaving the TWRs." However, depending on your steel materials, you can go as high as 1.5mg/L.

Stress Corrosion Cracking

• When metal gets corroded, it can crack in the region in which the corrosion had grown unattended. Stainless steel and brass are the most common metals used in open cooling systems. Chloride is the main culprit in this type of corrosion. As the temperature increases, so do the effects of stress corrosion cracking. As the concentrations of chlorides increase, so does the potential of stress corrosion cracking. A General Electric report suggests that "chloride can concentrate from 100 ppm [parts per million] in the bulk water to as high as 10,000 ppm (1%) in a crevice (<103 cfu/mL)."

Pitting

• Chlorine causes a form of corrosion called pitting on aluminum. Unlike stress corrosion cracking, chlorine needs the presence of oxygen to polarize the metal in an open loop water cooling system. Pitting occurs when the oxide layer of aluminum breaks down; high levels of chlorine cause the depletion of that layer. The chlorine levels must be checked to ensure the waters' pH level remains between 4.0 and 8.5. Aluminum open cooling systems also need to maintain concentrations of chlorides below 100 ppm in an oxidizing environment.

Temperature Control

• Cracking and pitting can lead to system failure. Temperature plays a part in corrosion. A NACE international report says "If the temperatures are limited below 140°F (60°C), the chloride stress corrosion cracking can be controlled. In most nuclear service water systems the temperature is controlled to about 125-128°F (52-53°C), which is below the threshold for chloride stress corrosion cracking." (Reference 3, pg 3-4,12)