The problem of decreased pH value in closed-loop cooling water
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After the major system overhaul, it was found that the pH value of the closed-loop water was low. System inspections revealed no leaks in the heat exchangers. After discussing this with the water quality management staff, it was suggested that bacterial growth can also cause a decrease in the water quality pH value. Are there any similar cases, and how were they resolved? Thank you!
Personally, I don’t think this reason holds up.
After being in use for a period of time, the pH of circulating water decreases. I believe there are two factors behind this: 1) Water-soluble alkaline substances in the raw water, such as calcium and magnesium, continuously deposit on the surface of the heat exchange surfaces, forming scale which lowers the pH; 2) During circulation, the water absorbs and dissolves acidic gases like carbon dioxide from the air, which also contributes to a decrease in pH. This is the same principle that explains why solid sodium hydroxide, when left exposed to the air, gradually turns into sodium carbonate.
What was it before the PH major overhaul, and what is it now?
After being in use for a period of time, the pH level of circulating cooling water decreases. This phenomenon should be analyzed from two aspects. To prevent corrosion of equipment caused by low pH levels in the circulating cooling water, appropriate measures must be taken: 1. During the operation of the circulating water, calcium and magnesium ions present in the raw water react with carbon dioxide absorbed from the air, resulting in the formation of precipitates. It also produces hydrogen ions, thereby lowering the pH value. CO2 + 2H2O + Ca2+ → CaCO3↓ + 2H+. Preventive measures include regularly adding corrosion and scale inhibitors to the circulating cooling water to prevent scaling in the system, which could affect heat exchange efficiency. Note that adding such inhibitors lowers the pH value of the circulating cooling water; therefore, after some time of operation, an appropriate amount of caustic soda should be added to restore the pH value to normal levels. 2. During the operation of circulating water, these microbial organisms such as bacteria and algae continue to multiply; they secrete acidic substances, which causes the pH level of the circulating water to drop. Preventive measures include regularly adding bactericidal and algaecidal agents to the circulating cooling water to prevent the growth of bacterial and algal microorganisms in the system. Note: After adding bactericidal and algaecidal agents to the circulating cooling water, its pH value will drop further; after it has been in use for a while, an appropriate amount of caustic soda should be added promptly to adjust the pH value of the circulating cooling water back to normal.
The original calcium bicarbonate and magnesium bicarbonate in the circulating water transform into calcium carbonate and magnesium carbonate, which then precipitate