During the long-term operation of production units, water coolers are prone to process material leakage, which affects the normal operation of the unit and may even force the system to shut down, resulting in significant economic losses. Water coolers prone to material leakage are usually made of carbon steel, mainly due to corrosion and perforation of the heat exchange wall, causing leakage from the high-pressure area to the low-pressure area between the process side and the circulating water side. Given the significant impact of leakage on the circulating water system and production units, this article summarizes and analyzes the causes, hazards, detection methods, and post-leakage treatment measures of leakage.
Most process-side materials are weakly acidic or contain small amounts of acidic gases, which corrode carbon steel water coolers rapidly, directly causing perforation of the heat exchange wall and subsequent leakage.
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a. If circulating water is not treated with corrosion inhibitors or is improperly treated, the water body will corrode and perforate the heat exchange wall of the water cooler.
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b. For water coolers that have been in operation for more than 10 years, their service life has been reached. Although adding corrosion and scale inhibitors to circulating water can effectively control corrosion and provide good corrosion inhibition, long-term use will still lead to corrosion and perforation of the water cooler.
Poor manufacturing and maintenance quality of water coolers often cause material leakage. Issues such as substandard welding quality, inadequate thermal expansion or loose tube expansion, and problems with the material type or installation of gaskets in floating-head water coolers can all lead to leakage during operation.
When organic substances leak into circulating water, they trigger massive reproduction of microorganisms and rapid growth of algae. This causes the water to develop a fishy odor, turn yellowish-brown or dark brown, and float with a large amount of flocculent or goose feather-like suspended solids. Meanwhile, the massive growth of microorganisms severely contaminates the filter media of side filters, significantly reducing their filtration efficiency—manifested by the outlet turbidity of side filters being almost equal to or even higher than the inlet turbidity. Additionally, the amount of biological slime in the entire system increases; such slime deposits inside water coolers, ultimately reducing heat exchange efficiency.
Furthermore, most microorganisms corrode metals. Sulfate-reducing bacteria, in particular, cause significant corrosion to iron. The iron ions corroded from the metal are oxidized to form Fe(OH)₃ precipitates under the action of oxygen. Fe(OH)₃, a yellow or brown precipitate, darkens the color of circulating water and increases turbidity.
On the other hand, microorganisms and algae themselves scatter light, further increasing turbidity. During leakage, the number of microorganisms rises sharply, metal corrosion intensifies, and the total iron and turbidity in circulating water increase significantly—more than double the normal values. For example, in a coal-to-methanol project in Inner Mongolia, methanol and hydrogen sulfide gas leaked into the circulating water through a heat exchanger, leading to a substantial increase in biological slime (up to 15ml/L), obvious floating matter on the water tank surface, and total iron and turbidity exceeding control indicators.
Acidic material leakage lowers the pH of circulating water, increases total iron content, turns the water light reddish-brown, and causes a sharp rise in the system corrosion rate.
Alkaline material leakage raises the pH of circulating water, and may even cause a "white water" phenomenon. The system shows a significant scaling tendency, and the heat exchange efficiency of water coolers decreases.
System leakage can be judged using physical and chemical methods:
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Physical methods: Observe the color, odor, and surface conditions of circulating water in the tank.
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Chemical methods: Monitor changes in circulating water indicators such as pH, turbidity, total iron, COD, ammonia-nitrogen, and oil content.
When a leak is confirmed in the circulating water system, leakage detection should be carried out promptly.
The detection process follows a "main pipe first, then branch pipe" approach: measure indicators such as pH, turbidity, and COD at the inlet and outlet of circulating water to narrow down the search range, and finally identify the specific water cooler.
Leaking equipment should be immediately isolated from the system or repaired. If isolation is not feasible, the circulating return water should be discharged on-site to avoid affecting other heat exchange equipment and the entire circulating water system.
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a. Organic leakage provides nutrients for microorganisms, promoting their massive growth, making sterilization crucial. Daily sterilization of circulating water mainly relies on chlorine-based or chlorine-bromine oxidizing biocides. However, organic leakage consumes large amounts of oxidizing biocides, resulting in low or undetectable residual chlorine. In such cases, the dosage of oxidizing biocides should be increased to maintain a residual chlorine level of 0.3-0.7 ppm for more than 2 hours.
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b. While using oxidizing biocides, non-oxidizing biocides such as isothiazolinones should be used in combination. Organic leakage does not affect the efficacy of non-oxidizing biocides, so they are preferred for daily sterilization during organic leakage. To prevent microbial resistance, 2-3 types of non-oxidizing biocides should be alternated.
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c. The selection and dosage of scale and corrosion inhibitors should be optimized and adjusted based on on-site leakage conditions and measured corrosion rates, typically increasing the dosage to 2-3 times the normal operating level.
For acidic or alkaline leakage, first increase blowdown and make-up water to stabilize pH control, avoiding blind addition of acids or alkalis.
If the leaking water cooler cannot be isolated or repaired and the leakage volume is large, acid or alkali can be added appropriately based on the total alkalinity and pH of the circulating water. However, the dosage must be strictly calculated and controlled based on circulating water data to prevent over-dosing. After eliminating the leak, chemical cleaning and pre-filming of the system can be performed.
Water coolers are critical components of the circulating water system; their leakage directly deteriorates circulating water quality, endangering the safe production of enterprises.
We must attach great importance to the daily management of circulating water, as well as the manufacturing and maintenance quality of water coolers. Mastering leakage judgment and detection methods enables timely identification of leaks, rapid isolation of leaking water coolers, and improvement of water quality through biological slime stripping, chemical cleaning, and pre-filming to restore heat exchange efficiency. This lays the foundation for the safe, stable, long-term, full-load, and optimal operation of the unit, supporting bi-annual or even tri-annual maintenance cycles.
