Hazards and causes of internal leakage in heat exchangers

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Heat exchangers are common heat exchange devices in chemical plants, used for heating, cooling materials, or recovering heat. If the heat exchanger leaks, it may result in poor product quality at the least, and in severe cases, equipment damage, material leakage that can lead to fires or explosions, or poisoning of personnel. What are the hazards of internal leakage in heat exchangers? 1. Acidic substances leak into the circulating water, causing it to corrode other cooling equipment. Such leaks can be detected by monitoring parameters such as the pH value or conductivity of the circulating water; therefore, a management system for analyzing the quality of circulating water should be established, specifying analysis parameters, acceptable range values, and frequency of analysis. It is advisable to use online pH or conductivity meters for this purpose. Special attention should be paid to enamel equipment cooled by circulating water, as the acidity of the circulating water can cause hydrogen-induced corrosion in the enamel reactors, leading to cracking of the enamel. 2. Internal leakage in the heat exchanger can lead to fires and explosions. Leakage is the cause of fires and explosions, but the hidden risk of internal leaks in some heat exchangers requires extra attention. For example: when a steam heater is used to heat combustible materials, the materials (including gases) may leak into the steam condensate system ； When circulating water is used to cool combustible materials, combustible materials or combustible gases may leak into the circulating water system. During equipment maintenance and hot work at condensate and circulating water facilities, fires, explosions, and human poisoning may occur. 3. Internal leakage in the heat exchanger may cause overpressure on the low-pressure side. In operating conditions where there is a significant pressure difference across the heat exchanger, a pressure alarm and safety valve should be installed on the low-pressure side to detect leaks within the heat exchanger promptly and to provide pressure relief for protection of the equipment. If high-temperature heat transfer oil leaks into the water side, it can cause superheating and overpressure of the water. 4. If the two phases involved in heat exchange are incompatible, internal leakage can lead to chemical reactions, corrosion, or blockage of the equipment. For example: In a sulfuric acid cooler, leakage of circulating cooling water into the sulfuric acid leads to accelerated corrosion of the tubes and pipelines on the sulfuric acid side ； A leak in the heat exchanger of the isocyanic acid reaction system can cause the polymerization of isocyanic acid to block the pipelines, among other issues. Inspection measures involve strict monitoring of the pressure, temperature, and various analytical parameters of the materials in each phase. 5. Affects product quality. Mixing the material with heating or cooling media can lead to product quality issues. To meet cost reduction requirements, some devices are designed with heat exchange between materials in order to recover heat; however, internal leaks can affect product quality or cause other safety issues. What are the possible causes of internal leakage in a heat exchanger? 1. Production quality. Winning bids at low prices, or problems with the management of the manufacturers, result in defects during the production of the heat exchangers, which in turn cause internal leakage when these heat exchangers are in use. This requires selecting the right manufacturer before production, ensuring proper supervision throughout the production process, and exercising strict quality control after production is complete. 2. Vibration friction piercing. The partitions and supports of heat exchangers can experience vibration and friction, leading to perforations under conditions such as material turbulence and periodic changes in flow velocity. Factors such as partitions and material disturbance need to be taken into account during design, and construction must be carried out strictly in accordance with the drawings during installation. 3. Erosion, cavitation, etc. During heat exchange and reactions, some materials exist in a liquid-solid two-phase or even three-phase state; gas formation due to instantaneous condensation can also occur, leading to cavitation. These effects of erosion and cavitation can cause equipment leaks. During the design of the heat exchanger, it is necessary to confirm the process conditions and carry out calculations using software such as ASPEN, to ensure that the actual operating conditions match the designed conditions. 4. Acidic corrosion. Circulating water is acidic, which can cause acidic corrosion. 5. Electrochemical corrosion. In heat exchangers, an electrochemical corrosion occurs due to the potential difference existing in different parts of the metal surface, which creates countless tiny galvanic cells. Especially when the flow rate of the circulating water is low, dissolved oxygen and sludge in the water deposit on the surfaces of the tube sheet and tube bundle, forming corrosion cells that accelerate corrosion. 6. Biological corrosion. The acids produced by the metabolism of living organisms damage the metal’s corrosion-resistant coating, while the oxygen consumption associated with biological metabolism leads to uneven O2 concentrations on the metal surface, thereby causing oxygen concentration cell corrosion. 7. Chloride corrosion. Chloride stress corrosion cracking (CISCC) occurs when austenitic stainless steel is exposed to an aqueous environment containing chloride ions. If the temperature is above 54–79 °C and the pH value is very low, or if there is dissolved oxygen present, chloride stress corrosion cracking can occur even with trace amounts of chlorides. 8. Ammonia corrosion. Some heat exchangers in these devices require ammonia leak testing; after the testing, some ammonia remains on the coolant side. If there are heat exchangers made of copper on that coolant side, it can lead to internal leakage in those copper-containing heat exchangers. For example, the heat exchanger of the ice machine can be corroded by ammonia remaining from the leak testing of the process heat exchanger in the production equipment. 9. Hydrogen embrittlement. In high-temperature, high-pressure hydrogen-related heat exchangers, hydrogen atoms diffuse within the metal lattice, resulting in a decrease in the metal’s toughness and an increase in its brittleness, which leads to leaks. 10. Stress corrosion. The deformation of metal materials, such as expansion, welding stress, contraction, or twisting, can lead to stress corrosion. (Author: Fan Zhitao)