Heat exchanger section: [Weekly Topic] What are the common methods currently used for descaling the surfaces of heat exchange tubes? (2011.7.18-7.24)

It is understood that there are two methods: 1. Mechanical descaling, and 2. Chemical descaling. For mechanical descaling, steel chisels are commonly used for manual cleaning, as well as pressurized water.

What are the common methods currently used for descaling the surface of heat exchange tubes? 1. Mechanical descaling; 2. Chemical descaling; 3. A combination of mechanical and chemical descaling. 4. High-pressure jet cleaning. Different chemicals and methods are used to clean heat exchangers made of different materials and in various forms. Chemical Cleaning Plan for Heat Exchangers 1. Basis for Formulation This plan is formulated based on the technical data and requirements related to the chemical cleaning and pre-coating of heat exchangers. It also takes into account the following technical documents: (1) DL/T957-2005 “Guidelines for Chemical Cleaning and Pre-coating of Condensers in Thermal Power Plants”; (2) SD135-86 “Guidelines for Chemical Cleaning of Boilers”; (3) HG/T2387-92 “Quality Standards for Chemical Cleaning of Industrial Equipment”; (4) “Chemical Cleaning Plan for Heat Exchangers and Condensers at Xing’an Thermal Power Plant, Inner Mongolia Huaneng Group”. 2. Causes and Hazards of Scaling The circulating cooling water in heat exchangers contains large amounts of salts, corrosion products, and various microorganisms. Due to the lack of proper water treatment, after a period of operation, calcium and magnesium carbonate scales, as well as algae, microbial sludge, and mud, accumulate on the inner surface of the copper tubes. These deposits adhere firmly to the tube surfaces, leading to reduced heat transfer efficiency, increased circulation pressure, and lower vacuum levels in the unit, thereby affecting its operational efficiency and causing significant economic losses. 3. Cleaning principle: Calcium and magnesium carbonate scales are soluble in strong acids; a reaction takes place that releases carbon dioxide gas, resulting in the formation of substances that are soluble in water, thereby achieving the purpose of cleaning and removing scale. The dissolution reaction equations are as follows: CaCO3 + 2H+ → Ca2+ + H2O + CO2↑; Mg(OH)2 + 2H+ → Mg2+ + 2H2O. During the cleaning process, H+ ions can cause corrosion of metal surfaces and lead to hydrogen embrittlement; therefore, appropriate corrosion inhibitors must be added to the cleaning agent ； Oxidizing ions such as Fe3+ and Cu2+ generated by dissolution can cause pitting and copper plating in metal components; therefore, a masking agent must also be added to the cleaning solution. 4. Preparatory work before chemical cleaning 4.1 Disconnect other systems that are not related to the heat exchanger. 4.2 Open the high-point vent valve on the water side of the heat exchanger and the low-point drain valve on the steam side, to ensure that the large amount of gas generated during cleaning can be discharged promptly and to maintain an adequate level of cleaning fluid ； At the same time, the leakage of the copper tubes in the heat exchanger during the cleaning process is monitored through the drain valve. 4.3 To monitor the cleaning effectiveness of the system as well as the corrosion of the equipment during the cleaning process, standard corrosion test pieces corresponding to the material of the equipment and monitoring sections are suspended in the cleaning tank prior to the cleaning operation. 5. Chemical cleaning and pre-coating of heat exchangers. Chemical cleaning process: Pressure testing → Water flushing → Acid cleaning to remove scale → Water flushing → Passivation pre-coating. 5.1 Pressure testing: The purpose of pressure testing is to check for leaks in the cleaning system under simulated conditions. 5.2 Water flushing: The purpose of water flushing is to remove loose contaminants from the equipment. Water flushing is completed when no large particles of impurities can be seen in the flushing water at the outlet. 5.3 Acid cleaning for descaling: After the water flushing is complete, \"Yunqing Brand Heat Exchanger Cleaner\" is added cyclically into the cleaning tank; the concentration of the main cleaning agent is maintained at 3–10%, and cyclic cleaning is carried out within the system to remove dirt. The cleaning process lasts for 8–12 hours. Samples are taken at regular intervals to analyze the concentration of the main cleaning agent. The acid cleaning process is terminated once this concentration stabilizes within 2 hours and no gas is released from the cleaning system. Monitoring items: Concentration of cleaning main agent (%) – 1 time every half hour, value: 5.4. Water rinsing – After the cleaning process is completed, water rinsing is carried out. The system’s circulation pump is activated to replace the discharged acid waste solution with industrial water, thereby flushing out any remaining sludge and residues; at the same time, NaOH is used in the waste treatment tank to neutralize the waste liquid (pH value of 5–9). The water washing is terminated when it is visually observed that there are few impurities at the outlet and the pH value is greater than 5. Monitoring item: pH value of the neutralization treatment, 1 time every 10 minutes. Water flushing at a pH of 5.5, once every half hour; passivation treatment – after successful water flushing, 2-3% (by weight) of “Yunqing Brand Heat Exchanger Passivator” is added in a cyclic manner to carry out chemical pre-coating treatment, thereby improving the corrosion resistance of the copper tubes. After the solution concentration throughout the system is evenly mixed, stop the circulation and soak for 8–10 hours. The passivation solution is then neutralized with hydrochloric acid (pH 5–9), and the cleaning process is completed. Monitoring items: pH value during the cyclic dosing process – once every half hour; pH value after neutralization treatment – once every 10 minutes. 6. Cleaning Effect 6.1 After cleaning is completed, the monitoring tube is taken out of the cleaning tank for inspection; its inner and outer surfaces should be clean, free from any residual dirt, pitting, or zinc loss corrosion. Visually, the scale removal rate should be 100% ； 6.2 After the entire cleaning process is completed, open the access panel of the heat exchanger for inspection; the surface of the copper tubes that have been cleaned should be clean with no residue of dirt left. The cleaning efficiency of the equipment should be ≥98%, and there should be no copper plating on the tube sheets or end caps ； After chemical cleaning, there were almost no leaks in the heat exchanger’s pipelines. 6.3 After the cleaning is completed, the standard corrosion test pieces used for monitoring shall be removed from the system, and their corrosion rates must meet the requirements specified in HG/T 2387–92 \"Quality Standards for Chemical Cleaning of Industrial Equipment\". 7. Conclusion Practice has shown that this technology features high descaling efficiency and fast cleaning speed, causes minimal corrosion to the metal substrate, and provides passivation for the metal during the cleaning process; as a result, no zinc loss or over-cleaning occurs, and the passivation film remains dense and intact after cleaning. This product contains no toxic or harmful substances, making it safe to use; its waste fluid is non-polluting, and it is environmentally friendly. Its usage method is simple and easy to master. The promotion and application of this product ensure the safe and efficient operation of heat exchangers. 8. Waste liquid treatment: The waste liquids generated in the aforementioned processes must not be discharged without treatment; otherwise, it will pose safety risks as well as cause environmental pollution. After treating the aforementioned industrial waste liquid with the waste liquid treatment agent produced by our company, the content of harmful substances is reduced to a minimum, and all indicators meet environmental discharge standards. 9. Precautions during construction: (1) Construction workers must wear the required personal protective equipment when entering the site; they need to wear rubber shoes, rubber gloves, masks, and eye protection ； (2) The construction site must have good ventilation, and the operation area must have convenient and sufficient water supply. (3) When handling corrosive chemicals, special handling tools such as forklifts should be used as much as possible. It is strictly prohibited to let it get into the eyes, mouth, or skin. In case of accidental contact, rinse immediately with plenty of water; seek medical attention promptly in severe cases. (4) Construction chemicals should be stored in a cool and well-ventilated area. Prominent labels such as “Do Not Touch – Hazardous Materials” must be affixed, and they should be kept sealed to maintain their effectiveness over a long period.

Cleaning methods for scaling in plate heat exchangers: 1. Selection of cleaning agents. For the selection of cleaning agents, acid cleaning is currently used, which includes organic acids and inorganic acids. The main organic acids include oxalic acid, formic acid, etc. Inorganic acids mainly include: hydrochloric acid, nitric acid, etc. Based on the analysis of fouling in the heat exchanger, as well as the process, materials, and composition of the scale, it can be concluded that: 1) The heat exchanger has a small flow area and a complex internal structure, making it difficult to discharge the precipitates formed by the cleaning solution. 2) The heat exchanger is made of nickel-titanium alloy, and hydrochloric acid is used as the cleaning solution. This can easily cause severe corrosion to the plates, thus shortening the service life of the heat exchanger. Through repeated experiments, it was found that formic acid is the most effective choice as a cleaning solution. Adding buffers and surfactants to the formic acid cleaning solution improves the cleaning effect and reduces the corrosion of the plates caused by the cleaning solution. Chemical tests on scale samples have shown that formic acid can effectively remove scale. Through acid soaking tests, it was found that formic acid can effectively remove the scale adhering to the plates; meanwhile, its corrosive effect on the heat exchanger plates is minimal. 2. Basic principles of removing scale 1) Dissolution: Acidic solutions readily react with calcium, magnesium, and carbonate scales to form soluble compounds, thereby dissolving the scale. 2) Stripping effect: Acidic solutions can dissolve the oxides on the metal surface, breaking the bonds with scale. This causes the scale attached to the surface of the metal oxide to peel off. and fall off. 3) Gas evolution effect: When the acid solution reacts with calcium, magnesium, and carbonate scale, a large amount of carbon dioxide is produced. Carbon dioxide gas during the leakage process. It has a certain lifting force on insoluble or slowly dissolving scale layers, causing the scale to detach from the heated surfaces of the heat exchanger. 4) Disintegration: In the case of scale composed of a mixture of silicates and sulfates, as calcium, magnesium, carbonate, and iron oxides dissolve in acidic solutions, the remaining scale becomes loose and can be easily washed away by the flowing acidic solution. 3. Process requirements for cleaning scale: 1) Acid cleaning temperature: Raising the acid cleaning temperature helps improve the scale removal effect. However, if the temperature is too high, it will exacerbate the corrosion of the heat exchanger plates by the acid solution. Through repeated tests, it has been found that a temperature range of 60–E is appropriate for acid cleaning. 2) Acid pickling solution concentration: Based on repeated tests, the acid pickling solution should be prepared at a concentration of 81.0% formic acid, 17.0% water, 1.2% buffer, and 0.8% surfactant, which yields an excellent cleaning effect. 3) Pickling method and time: The pickling method should combine static immersion with dynamic circulation. The pickling time consists of 2 hours of static soaking, followed by 3–4 hours of dynamic circulation. During the pickling process, samples should be taken regularly to test the pickling concentration. When the difference between the concentrations measured in two consecutive tests is less than 0.2%, it can be considered that the pickling reaction has ended. 4) Passivation treatment: After acid cleaning, most of the scale and metal oxides on the surface of the plate heat exchanger are dissolved and removed, exposing the bare metal, which is highly susceptible to corrosion. Therefore, passivation treatment is applied to the heat exchanger plates after acid cleaning. 4. Specific steps for scaling removal
1) Flushing: Before acid cleaning, perform an open flushing of the heat exchanger to ensure that there are no impurities such as mud or scale inside it. This not only enhances the effectiveness of the acid cleaning process but also reduces the amount of acid required. 2) Pour the cleaning solution into the cleaning equipment, and then inject it into the heat exchanger. 3) Pickling: Statically soak the heat exchanger filled with acid solution for 2 hours. Then, it undergoes continuous dynamic cycling for 3–4 hours. During this period, forward and reverse flushing is performed alternately every 0.5 hours. After the pickling process is completed, if the pH value of the acid solution is greater than 2, it can be reused; otherwise, it should be diluted and neutralized before being discarded. 4) Alkaline cleaning: After acid cleaning, a solution prepared from NaOH, Na, PO, and softened water in specific proportions is used to carry out alkaline cleaning of the heat exchanger through dynamic circulation, thereby achieving acid-base neutralization and preventing further corrosion of the heat exchanger plates. 5) Washing: After the alkaline washing is complete, use clean softened water to rinse the heat exchanger repeatedly for 0.5 hours, in order to thoroughly remove any residues remaining inside it. 6) Recording: During the cleaning process, the time taken for each step should be carefully recorded in order to assess the effectiveness of the cleaning. In short, after the cleaning is completed, a pressure test must be conducted on the heat exchanger. It can be used only after passing the inspection. 5. Measures to prevent scaling in plate heat exchangers
1) During operation, strict control over water quality is essential. It is necessary to conduct rigorous water quality tests on both the water in the system and the softened water in the softening tank; only after passing these tests can the water be introduced into the pipeline network. 2) When the new system is put into operation, the heat exchanger should be separated from the system; after circulating for a period of time, the heat exchanger should then be connected to the system again. This is done to prevent impurities in the pipeline from entering the heat exchanger. 3) Throughout the entire system, in addition to periodically cleaning the strainers and filters, it is also necessary to keep the piping network clean to prevent blockages in the heat exchangers. Strictly following the cleaning methods for plate heat exchangers is an important guarantee for the normal operation of production

Methods we’ve tried: 1. Backwashing – reversing the flow of the material; this has some effect. 2. Disassemble for manual cleaning. 3. Shell-and-tube types are cleaned with a high-pressure water gun. 4 Pickling.

I. Flushing method There are two types of the flushing method. The first method is countercurrent flushing, which is typically used during operation or when the equipment is stopped for a short period. It does not require disassembling the equipment; however, a countercurrent bypass line must be pre-installed on the equipment. This method proves to be quite effective when scaling is not severe. The second method is high-pressure water gun flushing. Different rotating water nozzles are used for different heat exchangers; they can be rigid or flexible, and the pressure can be freely adjusted from 10 MPa to 200 MPa. High-pressure water is used to remove dirt, and it is applicable to both between pipes and inside pipes as well as in the housing. Flushing heat exchangers with a high-pressure water gun yields good results. Widely applied. II. Chemical descaling: Scaling in the tube side of heat exchangers is mainly caused by poor water quality, which leads to the formation of scale as well as carbonized deposits and adherences from oils. To remove this scale using chemical methods, it is first necessary to conduct laboratory analysis of the scaling substances in order to determine their properties, thereby deciding which solvent should be used for cleaning. Generally, alkaline washing (using soda ash, caustic soda, trisodium phosphate, etc.) is used for sulfate and silicate scale, while acidic washing (using hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.) is employed for carbonate scale. For cleaning oil sludge and carbon deposits, a cleaning solution can be prepared by mixing sodium hydroxide, sodium carbonate, washing powder, liquid detergent, sodium silicate, and water in certain proportions. Using chemical cleaning requires re-piping on site, which is time-consuming.

Manual descaling with steel rods, mechanical descaling, pressure water descaling, chemical descaling

I. Manual or mechanical methods: When there is slight blockage and fouling in the tube bundle, manual or mechanical methods such as scraping and wire brushing are used for cleaning, followed by blowdown using compressed air, high-pressure water, steam, etc. When the pipes are severely scaled or completely blocked, a tubular flushing drill (also known as a pipe cleaning machine) can be used for cleaning. II. Irrigation method: There are two types of irrigation methods. The first method is countercurrent flushing, which is typically used during operation or when the equipment is stopped for a short period. It does not require disassembling the equipment; however, a countercurrent bypass line must be pre-installed on the equipment. This method proves to be quite effective when scaling is not severe. The second method is high-pressure water gun flushing. Different rotating water nozzles are used for various heat exchangers; they can be rigid or rotary, with pressure adjustable freely from 10 MPa to 200 MPa. High-pressure water is used to remove dirt, and it is applicable to both between pipes and inside pipes as well as in the housing. Flushing heat exchangers with a high-pressure water gun yields good results. Widely applied. Scaling in the tube side of heat exchangers is mainly caused by poor water quality, which leads to the formation of scale as well as the deposition and adhesion of oil residues. When using chemical methods for descaling, it is first necessary to conduct laboratory analysis of the scaling substances in order to determine their properties, thereby deciding which solvent should be used for cleaning. Generally, alkaline washing (using soda ash, caustic soda, trisodium phosphate, etc.) is used for sulfate and silicate scale, while acidic washing (using hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.) is employed for carbonate scale. For cleaning oil sludge and carbon deposits, a cleaning solution can be prepared by mixing sodium hydroxide, sodium carbonate, washing powder, liquid detergent, sodium silicate, and water in certain proportions. Using chemical cleaning requires re-piping on site, which is time-consuming.

Mechanical descaling, chemical acid washing for descaling, high-pressure water descaling!

I. Manual or mechanical methods: When the tube bundle is slightly blocked or fouled, cleaning is carried out using manual or mechanical methods such as scraping and wire brushing, in combination with cleaning agents like compressed air, high-pressure water, and steam. When the pipes are severely scaled or completely blocked, a tubular flushing drill (also known as a pipe cleaning machine) can be used for cleaning. II. Irrigation method; there are two types of irrigation methods. The first method is countercurrent flushing, which is typically used during operation or when the equipment is stopped for a short period. It does not require disassembling the equipment; however, a countercurrent bypass line must be pre-installed on the equipment. This method proves to be quite effective when scaling is not severe.     The second method is the high-pressure water jet cleaning technique: different rotating water nozzles are used for various heat exchangers, which can be rigid or rotatable, with pressure adjustable freely from 10 MPa to 200 MPa. High-pressure water is used to remove dirt, and it is applicable to both between pipes and inside pipes as well as in the housing. Flushing heat exchangers with a high-pressure water gun yields good results. Widely applied. III. Chemical descaling: Scaling in the tubes of heat exchangers is mainly caused by poor water quality, which leads to the formation of scale as well as the deposition and adhesion of oil residues. To remove this scale using chemical methods, it is first necessary to conduct laboratory analysis of the scaling substances in order to determine their properties, thereby deciding which solvent should be used for cleaning. Generally, alkaline washing (using soda ash, caustic soda, trisodium phosphate, etc.) is used for sulfate and silicate scale, while acidic washing (using hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.) is employed for carbonate scale. For cleaning oil sludge and carbon deposits, a cleaning solution can be prepared by mixing sodium hydroxide, sodium carbonate, washing powder, liquid detergent, sodium silicate, and water in certain proportions. Using chemical cleaning requires re-piping on site, which is time-consuming.

I. Manual or mechanical methods: When there is slight blockage and fouling in the tube bundle, manual or mechanical methods such as scraping and wire brushing are used for cleaning, followed by blowdown using compressed air, high-pressure water, steam, etc. When the pipes are severely scaled or completely blocked, a tubular flushing drill (also known as a pipe cleaning machine) can be used for cleaning. II. Irrigation method: There are two types of irrigation methods. The first method is countercurrent flushing, which is typically used during operation or when the equipment is stopped for a short period. It does not require disassembling the equipment; however, a countercurrent bypass line must be pre-installed on the equipment. This method proves to be quite effective when scaling is not severe. The second method is high-pressure water gun flushing. Different rotating water nozzles are used for different heat exchangers; they can be rigid or flexible, and the pressure can be freely adjusted from 10 MPa to 200 MPa. High-pressure water is used to remove dirt, and it is applicable to both between pipes and inside pipes as well as in the housing. Flushing heat exchangers with a high-pressure water gun yields good results. Widely applied. III. Chemical descaling: Scaling in the tube side of heat exchangers is mainly caused by poor water quality, which leads to the formation of scale as well as carbonized deposits and adherences due to oil. To remove this scale using chemical methods, it is first necessary to conduct laboratory analysis of the scaling substances in order to determine their properties, thereby deciding which solvent should be used for cleaning. Generally, alkaline washing (using soda ash, caustic soda, trisodium phosphate, etc.) is used for sulfate and silicate scale, while acidic washing (using hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.) is employed for carbonate scale. For cleaning oil sludge and carbon deposits, a cleaning solution can be prepared by mixing sodium hydroxide, sodium carbonate, washing powder, liquid detergent, sodium silicate, and water in certain proportions. Using chemical cleaning requires re-piping on site, which is time-consuming.