Causes of various faults in plate exchangers and solutions

Thread Content

To ensure the proper operation of plate heat exchangers and extend the service life of their key components (such as plates and gaskets), it is particularly important to understand the faults that occur in these exchangers, their causes, and the methods for addressing them. 1 External leakage 　1.1 Causes 　① Inadequate clamping dimensions, uneven dimensions in different areas (the deviation in dimensions at any one location 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. Example: Multiple heat stations in places such as Beijing, Qinghai, and Xinjiang use saturated steam as the heat source on the primary side for heating. Due to the high temperature of the steam, when the system is not stable during its initial operation, the rubber gaskets fail under these high temperatures, resulting in steam leakage. 　1.2 Handling methods 　　　① Under pressure-free conditions, re-clamp the equipment using the clamping dimensions provided by the manufacturer; these dimensions must be consistent, and the deviation in the clamping force should not exceed ±0.2N (mm) (N₀. For the total number of plates, the parallelism between the two compression 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. 2. Leakage 2.1 Causes 　　① Corrosion of the plates due to improper selection of materials, resulting in cracks or holes. ②The operating conditions do not meet the design requirements. ③ Residual 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 (such as C1) to concentrate and corrode the plates, resulting in cross-connection of fluids. Example: In the sulfuric acid system of a certain aluminum company, a BR03 plate heat exchanger made of 254 SMo plate material developed corrosion and leakage in the carbon steel connections on the cooling water side after 5 months of operation, resulting in acid leaking into the cooling water side. Inspection revealed severe corrosion and cracking at the acid inlet of the plate and in the flow guidance area. On-site analysis revealed that process parameters such as system operating temperature, flow rate, and concentration all exceeded the design specifications, with the operating temperature far beyond the acceptable range for the materials. Plate heat exchangers that use saturated steam as the primary heat source are prone to plate corrosion during operation, which leads to product cross-contamination. This is because the steam temperature is high; during operation of the equipment, the rubber gaskets can easily fail due to such high temperatures, resulting in steam leakage and rapid condensation in the secondary sealing area. As the leakage continues, more and more condensation residue accumulates, creating areas with a high concentration of Cl, which meet the conditions for corroding the passivation layer on the surface of the plates. At the same time, due to the high internal stresses generated by cold stamping of the plates in this area, stress corrosion occurs as a result of the action of these internal stresses once the surface passivation layer is damaged. 　2.2 Treatment methods ① Replace the plates with cracks or holes; use a light-transmission method on-site to detect 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 is not the case that the smaller it is, the better. ④ The sheet materials are properly matched. 3 Excessive pressure drop 　3.1 Causes 　① The pipelines of the operating system were not properly flushed; in particular, many contaminants such as welding slag ended up inside the plate heat exchanger. Due to the narrow flow area within the plate heat exchanger, deposits and suspended particles accumulate in the corner areas and guide zones, which significantly reduces the flow area in those areas and results in the majority of the pressure loss occurring there. 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. Example: In 2000, our factory supplied BR10 type plate heat exchangers to users in Xinjiang for centralized heating systems using water-to-water heat exchange, with a designed supply water temperature of 130 degrees. C. When designing and selecting heat exchangers, the heat transfer coefficient is on the high side, approaching 5500 W/(m²·K), whereas it should actually be 3500 W/(m²·K). At the same time, the design team selected water pumps with an excessive flow margin, resulting in a flow velocity between the medium plates on the secondary side of the heat exchanger exceeding 1 m/s. The actual operating pressure drop was between 0.2 and 0.3 MPa, which caused severe disruption to the hydraulic balance of the secondary network. 　3.2 Treatment methods 　　① Remove dirt or scale from the flow channels of the heat exchanger; for newly operational systems, cleaning should be carried out once a week depending on actual conditions. Remove scale from the surface of the plates (mainly referring to CaCO3). ) At that time, a cleaning solution containing 0.3% amino sulfonic acid or an 0.8 M acid solution containing 0.3% urotropine, 0.2% aniline, and 0.1% potassium thiocyanate is used, with a cleaning temperature of 40–60°C. When performing chemical immersion cleaning without removing the equipment, it is necessary to open the inlet and outlet ports for the cold medium in the heat exchanger; or, when installing the equipment, DN25 cleaning ports should be installed on the medium inlet and outlet connections. The prepared cleaning solution is then poured into the equipment, and after immersion, the remaining acidic liquid is washed away with clean water to ensure that the pH level is ≥7. When disassembling for cleaning, soak the plate in the cleaning solution for 30 minutes, then gently brush off the scale with a soft brush, and finally rinse it thoroughly with clean water. During the cleaning process, care should be taken to avoid damaging the plates and rubber pads. If the mechanical backwashing method without disassembly is used, a fitting should be installed in advance on the inlet and outlet pipes of the medium to connect the equipment to a mechanical cleaning vehicle; the cleaning solution is then injected into the equipment in the opposite direction to the flow of the medium. The circulation cleaning time is 10–15 minutes, with the flow rate of the medium controlled at 0.05–0.15 m/s. Finally, rinse several times with clean water to keep the mass concentration of Cl in the water below 25 mg/L. ② For 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 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 system once can be adopted. 4 The heating temperature does not meet the requirements. 4.1 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. ② The temperature on the cold side is low, as are the temperatures at the cold and hot ends. ③ The flow distribution among multiple plate heat exchangers operating in parallel is uneven. ④Severe scaling inside the heat exchanger. 　4.2 Treatment methods 　　　① Increase the flow rate of the heat source or enlarge the diameter of the pipeline for the heat source medium. 　　 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.