Heat exchanger cleaning: Scaling in heat exchangers and treatment methods

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For the oil and chemical industries, heat exchangers are essential equipment; however, over time various problems can arise, the most prominent of which is scaling in the heat exchangers. In mild cases, this leads to reduced production efficiency, while in severe cases it can result in safety accidents such as explosions. The news of an explosion at a chemical plant in Rizhao some time ago surely left a deep impression on everyone; **recently, there has been renewed focus on safety and environmental protection issues. So what are the components that cause scaling in heat exchangers? So how should it be handled? The book \"Heat Exchanger Cleaning and Anti-corrosion\", written by Ji Yulin, General Manager of Shandong Lanxing Cleaning and Anti-corrosion Company, provides a detailed description of the different types of scale that can form on heat exchangers. One can determine the type of scale by considering the medium flowing through the heat exchanger in use: A. Scale – Whether it comes from surface water or groundwater, such water contains minerals; the main components of this scale are carbonates, sulfates, silicates of calcium and magnesium ions, as well as iron oxides. In chemical production, the circulating cooling water used in heat exchangers is mostly treated, with most of the salts removed. However, some salts have a reduced solubility as temperature increases, while other salts decompose at higher temperatures to form compounds with even lower solubility ； As circulating water is used, it continuously evaporates and concentrates; in cases where it is not treated, its concentration gradually increases, and once it exceeds its saturated concentration, crystallization and deposition occur ； During water treatment, if prior analysis and testing, the addition of chemicals, or the drainage of waste from the system are not carried out in a timely manner for other reasons, it can also lead to an excessive concentration of certain substances, which then settle on the inner walls of the system and form scale. Classification of scale: a. Carbonate scale, in which the carbonate content is over 50%. b. Sulfate scale, in which the content of sulfates is over 50%. c. Silicate scale, in which the siliconate content is over 20%. d. Mixed scale, which contains at least the above three components. B. Rust scale: Rust scale is a type of scale composed mainly of rust. Its formation is usually due to the following reasons: the surface of steel is corroded during the rolling process or under natural conditions, resulting in the formation of iron oxide scales, which are primarily FeO, Fe2O3, Fe3O4, and their hydrates ； Secondly, the equipment substrate is subject to chemical erosion by the media, which can occur in either acidic or alkaline environments; for example, substances such as FeS and FeCl2 are generated in oil refining systems ； Thirdly, the equipment substrate is corroded by microorganisms in the circulating water system, such as sulfur sulfate-reducing bacteria and iron bacteria; their corrosion products include FeSO4, FeS, etc. Classification of rust deposits: iron oxides and hydrates, iron fungi, FeS, etc. C. Oil residues: In industries such as petroleum refining, petrochemicals, coal chemistry, and organic fine chemicals, the raw materials used accumulate to varying degrees throughout the system during the production process. Their properties also change depending on the production processes, resulting in various types of oil residues such as heavy oil, residue oil, petroleum coke, coal tar, kerosene, and other substances that are difficult to remove. D. Biological sludge: Biological sludge is formed due to the very large number and variety of microorganisms that live in water. Their optimal incubation temperature is 30–38°C; therefore, the cooling water used in the petrochemical industry is highly suitable for the growth and reproduction of these microorganisms. They not only multiply to form gel-like masses but also accumulate inorganic substances such as sand, Fe2O3, Fe(OH)3, Mn3O4, Al2O3, CaSO4, SiO2, CaCO3, and other solid particles, thereby creating what is known as slime. This kind of sludge is often easily confused with scale, which is primarily composed of inorganic substances. Classification of biological sludge: It is divided into algae, iron bacteria, sulfur bacteria, fungi, sulfates, and reducing bacteria. E. Special-type scale deposits: During the circulation or flow of the medium within the system, due to friction against the vessel walls and the physical and chemical properties of the medium itself, these substances accumulate over time on the inner walls of the system, forming special-type scale deposits made up of the medium itself. Classification of fouling on special materials: Their types are not necessarily the same as those of the medium; as they come into contact with the vessel walls, their physicochemical properties change. Substances with similar compositional characteristics will also form and deposit. After discussing the issue of scaling in heat exchangers, let’s now talk about how to deal with this scaling: The methods for removing dirt from the surface of heat exchangers can be divided into two categories based on their working principle – physical cleaning and chemical cleaning ； Online cleaning and offline cleaning are distinguished based on whether the equipment is in operation. 1. Physical cleaning method: It relies on the flow of fluids or mechanical action to exert a force greater than the adhesion force of the dirt, thereby removing the dirt from the heat exchange surface. There are two types of physical cleaning methods: one is the forceful cleaning methods, such as water jet cleaning, steam jet cleaning, sandblasting cleaning, and descaling using scrapers or drills ； Another category is soft mechanical cleaning, such as wire brush cleaning and rubber ball cleaning. One of the more commonly used methods is high-pressure water jet cleaning. It utilizes the principle of liquid pressure enhancement: through a high-pressure pump, the mechanical energy from the power source (electric motor) is converted into pressure energy. The water with this high pressure then passes through a small-orifice nozzle, which acts as another conversion device, where the pressure energy is transformed into kinetic energy, thereby creating a high-speed jet (WJ). It has the following advantages: A. High cleaning quality – when cleaning pipes and the inner surfaces of heat exchangers, it removes all types of contaminants and blockages from within the pipes, allowing the metal surface to be seen; at the same time, it causes no corrosion or damage to the metal or pipes, thus achieving high-quality cleaning. The clearance rate reaches 100% and the cleanliness rate reaches 95%, significantly improving the operational efficiency of the equipment being cleaned. Moreover, it is easy to assess the construction results after completion. Since the pressure of the water jet is adjustable, therefore. The pressure and flow rate of the water jet can be easily adjusted, thus avoiding damage to the substrate being cleaned. B. Fast cleaning speed: Due to the scouring, chipping, and grinding effects of the water jet (also known as water bomb abroad), the structure can be immediately broken apart and removed. Its cleaning speed is several times to dozens of times faster than that of traditional chemical methods, sandblasting and shot blasting methods, as well as simple mechanical and manual methods. Assuming that tap water is used as the medium for the jet, its high energy allows for thorough cleaning without the need for any additives or cleaners, resulting in very low costs. C. The cleaning cost is low, and the water consumption is extremely low at 2 cubic meters per hour. It is only about 1/3 of that required for chemical cleaning; in other words, high-pressure water jet cleaning uses fine jets, and thanks to its continuous operation, it consumes less water and energy, making it an energy-efficient device. D. Wide range of applications: It is flexible and convenient to use in company inspection and maintenance tasks. Anywhere that the water jet can reach directly – whether it’s inside pipes and containers, on the surface of equipment, or in the form of hard deposits or other obstructions – can have those substances removed quickly, allowing for thorough cleaning. This cleaning method imposes no special requirements regarding the material, properties, shape of the equipment, or the type of dirt present; it only requires that the water jet be able to reach those areas directly. Therefore, it is widely used. E. It does not cause secondary pollution. After cleaning, no additional purification is required unless specified otherwise. Water jet cleaning, unlike sandblasting, shot blasting, or simple mechanical methods, does not generate large amounts of dust that could pollute the atmosphere, harm the environment, or affect human health. I also don’t want to use chemical methods, which generate large amounts of acid and alkali waste liquid, polluting cities, rivers, soil, and water quality. The water jet cleaning method enables the reduction of atmospheric dust from 80 milligrams per cubic meter with other methods to below 2 milligrams per cubic meter, in line with the specified safety standards; it eliminates the cause of silicosis and removes the harmful effects of acidic and alkaline waste fluids, representing a significant step forward in China’s environmental protection efforts. F. High safety: The water jet cleaning method is safe and produces no sparks, allowing operations to be carried out even when tanks are filled with oil. This reduces the number of cleaning steps and saves costs. High-pressure water jet cleaning boasts unparalleled advantages in terms of cleaning efficiency, power consumption, cleaning costs, and environmental friendliness; its widespread adoption is an inevitable trend. 2. Chemical cleaning method: The chemical cleaning method involves adding scale removers, acids, enzymes, etc. to the fluid in order to reduce the adhesion of dirt to the heat exchange surface, allowing it to peel off from that surface. The chemical cleaning methods currently in use include: the circulation method: the cleaning solution is forced to circulate using a pump for cleaning purposes. Impregnation method: Fill the equipment with the cleaning solution and let it stand for a certain period of time. Surge method: Fill the device with cleaning solution, then periodically remove a portion of the solution from the bottom and pour it back into the device to achieve thorough mixing and cleaning. The advantages of chemical cleaning over mechanical cleaning are as follows: it is possible to clean areas that cannot be reached by mechanical cleaning without having to disassemble the equipment. Chemical cleaning provides uniform cleaning; even the smallest gaps can be cleaned, and no residual particles remain. It prevents the formation of cores for new deposits, thus avoiding damage to the metal surface. In contrast, sharp corners present in mechanical cleaning can promote corrosion and lead to the formation of dirt there. Rust prevention and passivation treatments can be applied after cleaning to prevent rusting. Chemical cleaning can be carried out on-site, with less labor intensity compared to mechanical cleaning. To save labor and costs associated with shutdown cleaning, extend operation cycles, and reduce maintenance expenses, various online mechanical and chemical cleaning systems have been developed. Online chemical cleaning has been developed over the past few decades based on conventional chemical cleaning; its main difference from conventional chemical cleaning is that the equipment does not need to stop operating. Initially, this method was only applicable to the removal of fouling on the cooling water side in condensers and coolers; it is now also successfully used in heat exchangers for hydrocarbon processing. That’s all there is to know about fouling in heat exchangers; I hope it can help everyone.