Causes and hazards of corrosion of circulating water system

Causes and hazards of corrosion of circulating water system

Refrigeration equipment can be divided into compression refrigeration and absorption refrigeration according to different refrigeration methods. As the refrigeration equipment is used, the refrigeration efficiency will gradually decrease. A careful analysis is caused by the following reasons:

　　 1. The causes of various scale formation in the circulating water system:

　　 Various salts are dissolved in natural water, such as bicarbonate, sulfate, chloride, silicate, etc. Therefore, if water containing more bicarbonate is used as cooling water, it will be thermally decomposed when it passes through the heat transfer surface of the heat exchanger. The passage of cooling water through the cooling tower is equivalent to an aeration process. The CO2 dissolved in the water will escape. Therefore, the PH value of the water will increase. At this time, the bicarbonate will also react under alkaline conditions. Therefore, on the heat transfer surface of the heat exchanger, these slightly soluble salts easily reach a supersaturated state and crystallize out of the water. When the water flow speed is relatively or the heat transfer surface is relatively rough, these crystalline deposits are easily deposited on the heat transfer surface. In addition, calcium sulfate, calcium silicate, magnesium silicate, etc. dissolved in water, when the product of the anion and cation concentration exceeds its own solubility product, will also generate precipitation and deposit on the heat transfer surface. This type of deposit is often referred to as scale. Because these scales are composed of inorganic salts, they are also called inorganic scales; because these scales have dense crystals and are relatively hard, they are called hard scales. They are usually firmly attached to the heat exchange surface and are not easily washed off by water.

　　 Hazard:

　　①Reduce the heat exchange efficiency of the equipment by 3%-10%

　　②It is easy to block the steel pipe in the condenser, and at a small scale, it reduces the flow rate and affects the cooling effect. In severe cases, the steel pipe in the condenser is even blocked and even scrapped.

　　③ Provide a place for some anaerobic bacteria to multiply.

　　 2. Causes of metal corrosion in circulating water system:

　　During the normal operation of the cooling water system and the sound of chemical cleaning, different forms of corrosion often occur.

According to the knowledge of metal corrosion theory, by carefully observing the metal corrosion form of the corrosion sample or the damaged equipment, in conjunction with some other methods, people can often find out the cause of corrosion and the measures to solve the corrosion problem, so research in the cooling water system The corrosion form of metals is a very useful method.

　　1, uniform corrosion

　　 Uniform corrosion is also called general corrosion or general corrosion. Its general feature is that the short-range corrosion is uniformly carried out on all exposed surfaces of the metal. During the corrosion process, the metal gradually becomes thinner and finally destroyed.

　　 For the carbon class, uniform corrosion mainly occurs in acidic solutions with low pH. If too much acid is added, when the pH of the cooling water drops to a very low level, the carbon steel equipment will also undergo obvious uniform corrosion.

　　2, galvanic corrosion. Galvanic corrosion is also called bimetallic corrosion or contact corrosion.

　　 When two different metals penetrate into a conductive aqueous solution, there is usually a potential difference between the two metals. One example of galvanic corrosion in the cooling water system is the galvanic corrosion that occurs in the cooling water between the brass heat exchange tube in the heat exchanger and the carbon steel pipe plate or the steel water chamber. In the corrosion process, it is the thick steel tube sheet or water chamber that is accelerated to corrode, not the thin copper tube. Because the steel tube sheet or water chamber has a thicker wall, it can still be used for a long time.

　　3, crevice corrosion

　　 Metal surface immersed in corrosive medium, when it is in a crevice or other hidden area, strong local corrosion often occurs.

　　 Hazard:

　　①Accelerate the corrosion of equipment, which is easy to cause accidents.

　　②Affect normal production and shorten the service life of equipment.

　　③Improved operation and equipment maintenance costs.

　　 Three, the formation of slime (soft scale):

　　 The microorganisms in cooling water generally refer to bacteria and algae. In fresh water, there are generally fewer bacteria and algae, but in the cycle, due to the concentration of nutrients, the rise of water temperature and sunlight, the conditions for rapid reproduction of bacteria and algae are created. The mucus secreted by a large number of bacteria is like an adhesive, which can make the dust, impurities and chemical deposits floating in the water stick together, forming a slimy deposit that adheres to the heat transfer surface of the heat exchanger: this deposit Some people call it biological slime, while others call it soft dirt.

Slime accumulates on the tube wall of the heat exchanger, in addition to causing corrosion, it will also reduce the flow of cooling water, thereby reducing the cooling efficiency of the heat exchanger rudder: in severe cases, these biological sludge will all block the tube, forcing Discontinued for cleaning. For example, a large number of bacteria and algae multiplied in the heat exchanger of a factory in Beijing, which reduced the heat load to 50% within half a month, and had to stop production frequently. Reduce output.

　　 slime is generally composed of fine-grained silt, dust, insoluble salt mud, colloidal hydroxide, debris, corrosion products, oil, especially the dead bodies of bacteria and algae and their viscous secretions. Improper control of water treatment, excessively high turbidity of the supplementary water, introduction of fine silt and colloidal substances into the cooling water system, or failure to kill bacteria and algae in time, or serious corrosion, many corrosion products, and careless operation, oil pollution, process products, etc. Leakage into the cooling water will aggravate the formation of dirt. When this kind of water flows through the surface of the heat exchanger, it is easy to form fouling deposits, especially when the water travels through the shell, where the flow rate is slower, the fouling deposits are more. Because this kind of dirt has a large volume and a loose texture, it is also called soft dirt. They are the main cause of corrosion under fouling and the breeding ground for certain bacteria such as anaerobic bacteria.

　　 Hazard:

　　① The slime adheres to the metal surface of the heat exchange (cooling) part, reducing the cooling effect of the cooling water.

②A large amount of slime will block the cooling water passage in the heat exchanger (water cooler), thereby making the cooling water unable to work. A small amount of slime will reduce the cross-sectional area of the cooling water passage, thereby reducing the flow and cooling effect of the cooling water Increase pump pressure.

　　③The slime accumulates on the surface or between the fillers of the cooling tower, blocking the passage of cooling water and reducing the cooling effect of the cooling tower.

④The slime covers the metal surface in the heat exchanger to prevent the corrosion and scale inhibitors from reaching the metal surface to exert their corrosion and scale inhibition effects, and prevent the biocides from killing the microorganisms in and under the slime, reducing these The power of the refrigerant.

　　⑤The slime covers the metal surface, forming differential corrosion cells, causing corrosion of these metal devices.