Basic knowledge of heat exchanger failures

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Today, we will summarize common heat exchanger failures and provide detailed solutions for them. I. Interleaving of the two media (internal leakage) 1 Causes ① Corrosion, perforation, or cracking of the heat exchange tubes. ②The expansion joint (weld) between the heat exchange tube and the tube sheet has cracked. ③Leakage at the floating head flange seal of the floating head heat exchanger. 2 Treatment methods: ① Replace or plug the leaking heat exchange tubes. ②The heat exchange tubes and tube sheet are over-expanded (res welded) or blocked. ③Tighten the bolts or replace the gasket. II. Sealing leakage at the flange 1 Causes: ① Insufficient pressure resistance of the gasket, corrosion, or deterioration. ②Insufficient bolt strength, looseness, or corrosion. ③Insufficient flange rigidity and defects in the sealing surface. ④The flange is uneven or misaligned, and the gasket quality is poor. 2 Treatment methods: ① Tighten the bolts and replace the gaskets. ②Upgrade the bolt material, tighten the bolts, or replace them. ③Replace the flange or address the defect. ④Realign or replace the flange, and replace the gasket. III. Poor heat transfer efficiency 1 Causes: ① Scaling on the heat exchange tubes. ②The water quality is poor, with a lot of oil and microorganisms. ③Solution for 2 cases of diaphragm short circuit: ① Chemically clean or use jet cleaning to remove deposits. ②Strengthen filtration and purification media, and improve water quality management. ③Replace the tube box gasket or replace the partition. IV. The pressure drop exceeds the allowable value. 1. Causes: Scaling inside the shell and on the inside and outside of the pipes. 2. Solutions: Use jets or chemical agents to remove the scale. V. Severe vibration. 1. Causes: ① Resonance caused by the frequency of the medium. ②Resonance caused by vibration of external pipes. 2 Treatment methods: ① Change the flow rate or change the natural frequency of the tube bundle. ②Strengthen the pipeline to reduce vibration. Analysis of common failure causes and treatment methods for plate heat exchangers. The common failures of plate heat exchangers include fluid cross-contamination, external leakage, excessive pressure drop, and heating temperatures that do not meet the required standards. I. Liquid leakage 1: Causes ① Corrosion of the plates due to improper selection of materials, resulting in cracks or perforations. ②The operating conditions do not meet the design requirements. ③Residual stresses resulting from the cold stamping of sheet metal, combined with overly tight clamping dimensions during assembly, cause stress corrosion. ④There is a slight leak at the plate leakage groove, which causes harmful substances in the medium to concentrate and corrode the plates, resulting in fluid leakage between them. 2 Treatment methods: ① Replace the cracked or perforated plates, and use a light-transmission method on-site to locate the cracks in the plates. ②Adjust the operating parameters to meet the design conditions. ③During the repair and assembly of heat exchangers, the clamping size must meet the required standards; it’s not the case that the smaller it is, the better. ④The sheet materials are properly matched. II. External leakage 1 Causes: ① Inadequate clamping dimensions, uneven dimensions in different areas (the deviation in dimensions at various points should not exceed 3 mm), or loose clamping bolts. ② Some gaskets have come loose from their sealing grooves; the main sealing surface of the gaskets is contaminated; the gaskets are damaged or the washers have aged. ③ Deformation of the plates and misalignment during assembly cause the running mat to shift. ④There are cracks at the plate seal groove or in the secondary sealing area. 2 Treatment methods: ① Under pressure-free conditions, re-clamp the equipment using the clamping dimensions provided by the manufacturer; these dimensions must be consistent, with the deviation in the clamping force not exceeding ±0.2N(mm) (where N is the total number of plates). The parallelism between the two clamping plates should be maintained within 2 mm. ② Mark the areas where there is leakage, then disassemble the heat exchanger to identify and address the issues one by one, reassembling or replacing the gaskets and plates. ③ Disassemble the heat exchanger, repair the deformed plates or replace them. When plate spare parts are unavailable, the deformed plate can be temporarily removed and reassembled for use. ④ When reassembling the separated plates, the plate surfaces should be cleaned to prevent dirt from adhering to the sealing surfaces of the gaskets. III. Excessive pressure drop 1. Causes: ① The system pipelines in operation were not properly flushed; in particular, many contaminants such as welding slag entered the interior of the plate heat exchanger. Due to the narrow flow area within the plate heat exchanger, deposits and suspended particles accumulate at the corner holes and in the flow guide areas, resulting in a significant reduction in the flow area there and causing the majority of the pressure loss to occur at these locations. ② When selecting a plate heat exchanger for the first time, if its area is too small, the flow velocity between the plates becomes too high, resulting in a large pressure drop. ③ After operating for a period of time, plate heat exchangers experience excessive pressure drop due to scaling on the surface of the plates. 2 Treatment methods: ① Remove dirt or scale from the flow channels of the heat exchanger; for newly operational systems, clean them once a week depending on actual conditions. ②For secondary circulating water, it is preferable to use softened water; generally, the mass concentration of suspended solids in the water should not exceed 5 mg/L, the diameter of impurities should not be greater than 3 mm, and the pH value should be ≥ 7. When the water temperature is not greater than 95°C, the Ca and Mg concentrations should not exceed 2 mmol/L ； When the water temperature is greater than 95°C, the concentrations of Ca and Mg should not exceed 0.3 mmol/L, and the mass concentration of dissolved oxygen should not exceed 0.1 mg/L. ③For central heating systems, the method of replenishing water to the secondary side once can be used. IV. The heating temperature does not meet the requirements. 1 Causes: ① Insufficient flow rate of the fluid on the primary side, resulting in a large temperature difference on the hot side and a low pressure drop. ②The temperature on the cold side is low, as are the temperatures at the cold and hot ends. ③The flow distribution among multiple plate heat exchangers operating in parallel is uneven. ④Severe scaling inside the heat exchanger. 2 Treatment methods: ① Increase the flow rate of the heat source or enlarge the diameter of the heat source medium pipeline. ② Balance the flow rates of multiple plate heat exchangers operating in parallel. ③Disassemble the plate heat exchanger to clean the scale on the surface of the plates. I. Tube bundle failures: 1. Corrosion and wear of the tube bundle can lead to leaks, while scaling inside the tube bundle can cause blockages and resulting failures. Cooling water contains metal ions such as iron, calcium, and magnesium, as well as anions and organic substances; these active ions increase the corrosiveness of the cooling water. The presence of metal ions induces depolarization reactions of hydrogen or oxygen, thereby causing corrosion of the tube bundle. At the same time, since the cooling water contains Ca2+ and Mg2+ ions, scaling is likely to occur over time at high temperatures, leading to blockages in the tube bundles. To improve heat transfer efficiency and prevent corrosion or blockage of the tube bundle, the following methods have been adopted: (1) Adding scale inhibitors to the cooling water and performing regular cleaning. For example, ion electrostatic processors can be used for the cooling water of gas coolers, or scale and corrosion inhibitors as well as biocides can be added to remove dirt and reduce the hardness of the cooling water, thereby minimizing scale formation in the tube bundles. (2) Maintain a stable flow rate of the fluid inside the pipe. If the flow rate increases, the thermal conductivity increases as well, but wear also increases accordingly. Minsheng Coal Chemical has carried out variable-frequency upgrades on the underground water pumps, resulting in more stable pressure in the underground water network, improved heat exchange efficiency of the heat exchangers, and reduced corrosion of the pipe bundles. (3) Use corrosion-resistant materials (stainless steel, copper) or increase the wall thickness of the tube bundle. (4) When the end of the tube is worn, a protective tube bundle such as synthetic resin can be inserted over a length of 200 mm at the inlet. 2. Failures caused by vibration: The causes of vibration include: vibration of the tube bundle resulting from the vibration of pumps and compressors ； Pulsations generated by rotating machinery ； The impact of the high-speed fluid (high-pressure water, steam, etc.) flowing into the tube bundle on the tube bundle. To reduce the vibration of the tube bundle, the following methods are commonly used: (1) Minimize the number of start-up and shutdown cycles. (2) At the inlet of the fluid, an adjustment groove is installed to reduce the vibration of the tube bundle. (3) Reduce the spacing between baffles to decrease the amplitude of the tube bundle. (4) Minimize the diameter of the holes in the baffle through which the tube bundle passes. II. Leakage at the flange: Leakage at the flange is caused by an increase in temperature, which leads to the expansion of the fastening bolts as they heat up, thereby creating gaps at the fastening points. Therefore, after the heat exchanger is put into use, the flange bolts need to be tightened again. The fluids in heat exchangers are often toxic, high-pressure, and high-temperature substances; leaks can easily lead to poisoning and fire accidents. In daily operations, special attention should be paid to the following points: minimize the use of gaskets and opt for metal gaskets instead ； Adopt the method of tightening the gasket with internal pressure ； Use a easily fastening working method.