Anti-corrosion of circulating water system
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Current status of corrosion problems As enterprises attach great importance to energy and water conservation, the reclaimed water after the water treatment device is replenished into the circulating water and demineralized water systems. In addition, in order to save energy, it is required to control the temperature difference between the inlet and outlet of the cooler's circulating water to not exceed 6°C. In order to meet the requirements, the device closed the throttling valve of the circulating water inlet and outlet, but the speed measurement work did not keep up in time. Since 2019, the frequency of leaks in circulating water coolers has increased rapidly and significantly, and the device has been forced to remove the cooler for leak plugging, which has affected stable production. The leakage situation of enterprise circulating water coolers since 2019 is shown in Table 7. As can be seen from Table 7, the circulating water cooler has leaked 23 times in the past 3 years. The main reason is: ①Excessive energy conservation. This causes the linear velocity of the circulating water to drop, and the circulating water velocity in many coolers does not reach 0.9m/s, causing corrosion under the scale. ②Excessive water conservation. The introduction of urban gray water (reclaimed water) reduces the amount of fresh water extracted. Part of the reclaimed water enters the circulating water system after treatment ； However, some trace elements in reclaimed water cannot be detected by current analysis methods. If they enter the circulating water system, they will inevitably aggravate the corrosion of the cooler. Process anti-corrosion and corrosion control measures The implementation details of Sinopec’s “Regulations on Anti-corrosion Management of Refining Processes” are as follows:: (1) The flow rate of the circulating cooling water pipe side should not be less than 1.0m/s. When the flow rate of the circulating cooling water shell side is less than 0.3m/s, anti-corrosion coating, reverse flushing and other measures should be taken. It is recommended that the outlet temperature of the circulating cooling water water cooler should not exceed 50°C. Use an ultrasonic flow meter to spot-check the flow rate on the water side of the water cooler to prevent biased flow or low flow rate. For water coolers with serious fouling tendencies, backwash facilities can be added or technologies such as ultrasonic descaling can be used. (2) The quality of circulating cooling water should comply with the control index requirements of GB50050 circulating cooling water quality. The use of recycled water as supplementary water should comply with Q/SH0628.2 "Technical Requirements for Water Management Part 2": Circulating water" requirements. (3) Corrosion and scale inhibitors should be environmentally friendly agents with high efficiency, low toxicity, good chemical stability and compounding performance based on water quality and working conditions. When zinc-containing agent formula is used, the zinc salt mass concentration in the circulating cooling water should be less than 2 mg/L (calculated as zinc ions) ； When there is copper alloy heat exchange equipment in the circulating cooling water system, the water treatment agent formula should include copper corrosion inhibitors. (4) Microbial control of circulating cooling water should be based on oxidizing fungicides and supplemented by non-oxidizing fungicides. When the oxidizing fungicide is continuously added, the mass concentration of residual chlorine should be controlled to 0.1~0.5 mg/L. When adding, it should be added 2 to 3 times a day. The mass concentration of residual chlorine in the water should be controlled to 0.5~1 mg/L each time and maintained for 2~3 hours. Non-oxidizing fungicides should be used alternately. (5) The concentration ratio of circulating cooling water should be controlled in accordance with relevant requirements. When it exceeds the standard, it can be adjusted by increasing the discharge volume. ； Control the temperature of circulating cooling water out of the heat exchanger to not exceed 50℃ ； Each device should establish a corrosion monitoring system and corrosion management system according to the actual situation to ensure the safe operation of production.
This post was last edited by Wang Wei 2 on 2023-7-23 12:54 Corrosion of the circulating water system, especially the corrosion of the water cooler, is indeed related to improper use. First, excessive energy saving； The second is excessive water conservation. Causes of transitional energy-saving hazards: Using the valve method for throttling may temporarily solve the problem. However, this method makes the circulating water side flow rate fail to meet the flow rate required by the process. However, this will cause scaling and corrosion of the water cooler, reduce the heat exchange effect of the water cooler, and accelerate corrosion of the heat exchanger tube bundle. So this method is really not advisable. Solution: First, valves cannot be used for throttling. To re-evaluate the cooling effect of the water cooler, change the pipe diameter or replace the pipe bundle to match its process. The second is to adopt the anti-corrosion method of the heat exchanger tube bundle. This method is really effective and solves the problems of corrosion and scaling of the tube bundle. Dangers of excessive water conservation: Nowadays, more petrochemical companies process sewage through membrane permeation and other treatments. The treated water is called gray water (colorless and transparent). However, reclaimed water is pumped into the circulating water field as cooling water for the water cooler. After being used for a period of time (usually about 1 month), it will cause microbial corrosion of the tube bundle, and the corrosion rate is relatively fast. About 2 months after the reclaimed water is used, leakage of the tube bundle will occur, seriously affecting the use of the equipment. Because after all, gray water comes from sewage treatment. Although most of the harmful media in the treated sewage can be cleaned, the iron ions in the sewage cannot be cleaned. We have done such a test and conducted laboratory evaluation on gray water. As a result, the reclaimed water began to corrode the carbon steel coupons in just 48 hours. The reason is that gray water contains a large amount of iron ions. Solution: Do not mix gray water into the circulating water field.
Thumbs up to the original poster. Water system corrosion is indeed a problem nowadays. The control of water quality and the selection of new corrosion-resistant materials are very important.
A few years ago, our unit also studied sewage recycling, but I won’t mention it now. It seems that it may not be a bad thing: lol
Please tell me whether the anti-corrosion process involves heat exchange.
Please tell me whether the anti-corrosion process involves heat exchange. Thank you.
Chemical dosing is the current traditional water treatment method. However, in view of the scaling and corrosion problems that have always existed in local heat exchange equipment, you can learn about the physical water treatment technology of special multi-component alloys and add micro-chemical dosing.* mi59
Reclaimed water is used as a supplement to circulating water. Because the reclaimed water itself contains high organic matter, high turbidity, and high total iron, it is extremely difficult to control corrosion using traditional chemical agents to stabilize water quality. We use EGS electrochemical EST equipment to completely control the problems of scaling, corrosion, bacteria, algae and slime with the circulating water (rechargeable water) of a steel plant in Yunnan.