Forward and share: Heat exchanger maintenance

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I. Quality standards for heat exchanger maintenance 1. The heat exchange plates shall be free from defects such as cracks, scratches, and deformation. The deviation in plate thickness uniformity shall not exceed 5% of the plate thickness, and the warping deformation of flat plates shall not be greater than 0.5 mm. The bolt hole spacing deviation of the umbrella plate heat exchanger is less than ±0.3 mm. The conical surface of the umbrella plate should be flat and smooth, with a warping deformation amount not exceeding 1 mm. The perimeter and surface of the plate should be smooth and even; hammer marks, wrinkles, and other mechanical defects are not allowed. 2. Technical standards for gasket materials 1) Asbestos rubber gaskets must be manufactured from asbestos rubber raw materials with a storage period of no more than one year. Asbestos rubber sheets shall be used, and their physical and mechanical properties shall comply with the requirements of XB200 (JC125). The material of the sealing gasket must meet the design requirements, have the correct dimensions, uniform thickness, and a smooth surface free from defects such as scratches or wrinkles. No signs of seams such as overlaps or joints are allowed. Compressive deformation of 7-12% is allowed. 2) Rubber gaskets: When the temperature of the working medium is ≤140°C, EPDM rubber is generally used. ≤For oil media at 100°C, nitrile rubber is used. The surface must be smooth with uniform thickness, and there must be no transverse cracks (in the direction perpendicular to the grooves of the plate). If the depth of longitudinal (parallel to the plate groove) cracks is within 0.2–0.3 mm, it can be used further. There shall be no defects such as cracks, bubbles, missing material, aging, local hardening, or warping. No traces of overlapping or butted joints are allowed. 3) Overall assembly technical standards: The length of the assembled plate bundle shall not be less than the calculated value. During assembly, it must be carried out in the order specified by the disassembly sequence; reversal or misalignment is not allowed. A thorough recheck has been done and everything is correct. After the plates are assembled and compressed, the misalignment in the up, down, left, and right directions must not exceed 1 mm/m; otherwise, misplacement may occur. The center distance between the semi-circular notches of the end cap (skull) that fit with the upper and lower guide rods is 20 mm smaller than the center distance between the guide rods, and the diameter of the semi-circular notches should be 4 mm larger than the diameter of the guide rods. 4) Technical standards for end caps (head covers): The diameter of the bolt holes in the end caps (the notches) should be 0.3–0.5 mm larger than the diameter of the bolts. Otherwise, the heat exchange plates are prone to misalignment. End caps and hangers are generally not prone to damage; they should be regularly coated with anti-corrosion paint to prevent corrosion. The machining accuracy of the end face where the head contacts the plate must not be lower than △6. 5) Technical standards for clamping bolts: When the heat exchanger is installed indoors, carbon steel is generally used for the clamping bolts. If it is to be placed outdoors, stainless steel should be used. The exposed threaded portion should be coated with grease (calcium-based dry grease) or covered with a plastic tube. The bolt length should be the free length of the plate bundle in the plate heat exchanger, plus the thickness of the fixed and movable end caps, with an additional 20 mm left. II. Installation Technical Standards 1. Before installing the heat exchanger, a hydrostatic test must be conducted first, with the test pressure being 1.25 times the operating pressure under production conditions. During pressure testing, maintain the pressure for 30 minutes; it is considered successful if the pressure does not drop. During the airtightness test, the test pressure is 1.05 times the maximum operating pressure; it is considered successful if no bubbles appear when checking with soapy water. 2. All instruments shall be prepared and calibrated as required. 3. All valves shall be inspected and pressure-tested, and made available as required. 4. All pipes should be inspected and pressure-tested, with any scale or other debris inside them removed. If corrosive media are involved, anti-corrosion measures must be taken. Piping valves are pressure-tested at 1.5 times their operating pressure. 5. The anchor bolts should be grouted after the heat exchanger is placed on the foundation. 6. The base load of the heat exchanger is calculated based on the total weight of the liquid filling it. 7. When the medium contains solid particles, fibrous substances, or a large amount of impurities, resulting in high turbidity, a filter should be installed in front of the heat exchanger. Two filters are installed for each medium and used alternately; the open area ratio of the filter mesh is not less than 80%. 8. The nozzles of the heat exchanger should be protected from tensile forces, vibrational forces, and impact forces transmitted from the pipes. 9. The distance between the outer perimeter of the heat exchanger and the wall should generally be no less than 800 mm. 10. It is advisable to install the equipment indoors, as this ensures cleanliness and makes it easier to clean, disassemble, assemble, and repair. III. Common faults in heat exchange equipment and their troubleshooting (Table 1-1) Fault symptoms, causes, and solutions: Flange leakage – Flange leakage often occurs at the bolt tightening areas and at the points where bolts are screwed in; as the temperature rises, the bolts expand, causing the tightening areas to become loose. (1) Minimize the use of connecting flanges. (2) Ensure that tightening is easy to perform. (3) Use self-tightening structural bolts. Dirt reduces thermal efficiency; the fluid contains solids and suspended particles ； Algae, bacteria, and sediment in the cooling water can all cause severe scaling. (1) It is necessary to fully understand the areas prone to fouling, the substances that cause fouling, and the extent of the fouling, and to conduct regular inspections. (2) When scaling is likely to occur, equipment designs that are easy to inspect, disassemble, and clean must be used. Corrosion and wear of pipes: (1) Fouling-induced corrosion; (2) The fluid being corrosive; (3) Accumulation of foreign substances on the inner wall of the pipe, leading to localized corrosion; (4) Excess flow velocity inside the pipe, resulting in wear ； If the flow rate is too low, foreign substances are likely to adhere to the pipe walls, creating a potential difference that leads to corrosion. (5) Wear at the pipe ends. (1) Perform regular cleaning. (2) Improve the quality of the pipes; if suitable materials are not available, increase the thickness of the pipe walls or add corrosion inhibitors to the fluid. (3) Install screens or filters before the fluid enters to remove foreign substances. (4) Maintain an appropriate flow rate within the pipes. (5) Insert protective tubes made of long synthetic resins at the pipe inlets. Pipe vibration: (1) Resonance between the pipe and pumps/compressors. (2) Direct pulsating forces generated by rotating machinery. (3) Impact from high-speed steam entering from the sides on the pipes. (4) Pipe vibration is caused by a combination of factors such as flow rate, pipe wall thickness, baffle spacing, and tube arrangement. (1) Install buffer tanks before the fluid enters to prevent pulses. (2) Ensure a tight fit between the pipe holes in the baffles. (3) Reduce the baffle spacing to decrease the amplitude of pipe vibration. (4) Increase the thickness of the pipe walls and baffles. Leaks resulting from loose connections in the pipe assembly: (1) Pipe vibration. (2) Thermal shock caused by starting/stopping or emergency shutdowns. (3) Mechanical shock resulting from improper handling during regular maintenance. (1) Re-expand the pipes; when expanding a particular pipe during maintenance, it is necessary to re-expand the surrounding pipes as well to prevent loosening. (2) For equipment where leaks at the expanded sections are not acceptable, welding should be used for assembly. IV. Maintenance techniques for plate heat exchangers (I) Maintenance tasks 1. Intermediate maintenance tasks: (1) Remove the inlet and outlet pipes and clean them of debris. (2) Inspect the rubber lining of the inlet and outlet pipes; there should be no cracks or damage. (3) Check the preload of the measuring bolts and the overall dimensions of the plate. 2. Major repair contents (1) include the medium repair contents. (2) If the heat exchanger is scaled, it should be disassembled and cleaned, or separate piping should be installed for online chemical cleaning. (3) Examine the elasticity and compression deformation of the sealing strips under a magnifying glass; replace them if necessary. (4) Check the deformation of the heat transfer plates. (5) Check the heat transfer plates for defects such as corrosion and perforations. (II) Maintenance Techniques 1. Before disassembling a plate heat exchanger, the overall dimensions of the compression of the plates should be measured. And keep a record of it. 2. Remove the sealing rubber strips; to prevent puncturing the strips with a screwdriver, liquid nitrogen can be used for rapid cooling, causing the rubber strips to deform rapidly and then they can be torn off. 3. Remove the residual adhesive from the seal groove. 4. Use lighting or penetration methods to check for cracks or perforations in the heat transfer plates. 5. Rinse the plates with clean water. 6. Reassemble. 7. Airtightness and hydrostatic tests. (III) Reassembly 1. Before assembly, first clean the sealing groove with acetone and apply adhesive, then attach the sealing strip in a horizontal position. 2. For the panels with the sealing strip properly applied, group them in sets of 50 each, and clamp them using 20–30 mm thick steel plates. Allow them to cure for 24 hours at an ambient temperature ranging from 30–35°C before they can be hung. 3. After hanging the plate, gently attach the end covers and insert the fixing bolts. 4. Use a torque wrench to tighten the bolts evenly. 5. Install the inlet/outlet liner. 6. Overall pressure test. First, the inlet pipe of the fluid channel on one side of the plate is plugged off and filled with water; then, a blind flange equipped with a vent tube is installed on the outlet pipe of the working medium channel on the other side of the plate, and a pressure gauge is attached to the pressure testing side. After filling it with water, press the pump by hand to reach a pressure 1.25 times that of the operating pressure; maintain this pressure for 30 minutes without any drop, and then the outer tube can be connected. 7. Single-side test pressing. To be more certain of preventing internal leakage, a single-sided pressure testing method can also be employed, that is, filling the side that requires testing with water while leaving the other side unfilled, with the test pressure being the operating pressure. It is considered qualified if the pressure remains unchanged after 20 minutes of holding. Plate heat exchangers are generally not pressurized on a single side to prevent excessive deformation of the plates. Damaged plates and sealing strips. 8. Airtightness test. As required for plate heat exchangers, after the hydrostatic test, a airtightness test must also be conducted. The test pressure should be 1.05 times the operating pressure, and soapy water should be applied around the heat exchanger plates to check for any leaks. V. Maintenance Techniques for Shell and Tube Heat Exchangers 1. The most common cause of damage to heat exchangers in chemical plants is corrosion, which occurs mainly on the tubes, at the joints where the tubes meet the tube sheet, and on the shell. (1) Pipe corrosion: If corrosive fluid media flow inside the pipes, prolonged erosion and operation will cause the pipes to thin out significantly. It is therefore necessary to conduct regular eddy current thickness testing on such heat exchangers, calculate the annual corrosion rate, and carry out necessary replacements. Another type is local corrosion, particularly corrosion at the pipe end 10 mm from the inlet, which is related to vortex disturbances generated by the fluid in dead corners. During each maintenance check, this area must be inspected and its thickness measured; if severe corrosion is detected, it must be repaired using metal materials. (2) Corrosion at the joint between the tube and the tube sheet. The connection between the tube and the tube sheet is usually achieved through welding, expansion joining, or by first expanding and then welding, or vice versa. Due to the stresses from expansion bonding and welding, cracks are likely to occur in the tube sheet and the expansion welding area, usually in a circumferential serrated pattern. (3) Corrosion of the shell: Corrosion of the heat exchanger shell generally occurs at the welded areas. Furthermore, when the material of the baffle differs from that of the shell, electrochemical corrosion can easily occur at the contact points between the baffle and the shell, leading to perforation of the shell. 2. Maintenance techniques: The maintenance of heat exchangers is relatively simple; it mainly involves identifying the problems that arise during operation and carrying out targeted repairs based on those issues. (1) Tube plugging: When the tubes of a heat exchanger leak, the tube plugging method is generally used. First, the head of the heat exchanger is disassembled, and leaky pipe sections are inspected by filling them with water or gas or by using liquid ammonia for penetration testing. Then, pipe plugs are fabricated, usually in the form of tapered wedge-shaped plugs; in some cases, welding is performed after plugging. Depending on the arrangement of the tubes on the tube sheet, welding should not be carried out after plugging, as welding causes local heating of the tubes, which in turn easily heats the surrounding tubes and the tube sheet as well; this exerts a pulling force on the tubes, causing the adjacent tubes to become loose as well. This can lead to leaks once the system is put into operation. The method currently used is a pistol-type plugger tool that can generate a pressure of up to 10 Mp per use, in order to push the plug into the leaking pipe; once an opportunity arises, the plug can be removed and a new pipe installed. When plugging a pipe, it is essential to choose the right material; in principle, the plug should be made of the same material as the pipe to prevent electrochemical corrosion. (2) Corrosion prevention: Corrosion prevention primarily applies to the tubes of heat exchangers; generally, a corrosion-resistant liquid is used so that it adheres to the tubes, forming a thin protective layer on their surfaces that serves to prevent corrosion. Practice has shown that such measures can extend the lifespan of heat exchangers, and they are widely adopted in chemical enterprises. (3) Cleaning: In a circulating water system, heat exchangers located in different positions may suffer from overheating and scaling due to high elevation levels caused by untimely adjustments to the water flow rate, as various resistances within the system were taken into account during design. It is therefore necessary to clean these heat exchangers. Before cleaning, sample the scale for analysis to determine the cause of scaling, and then add chemicals to the circulating water appropriately to stabilize its quality. In the past, chemical cleaning was commonly used, requiring new piping to be installed on-site, which was time-consuming. Currently, high-pressure water flushing is the method most commonly used; different rotating water nozzles are employed for various heat exchangers, which can be either rigid or flexible, with the pressure adjustable freely from 10 Mp to 200 Mp. High-pressure water flushing is effective for cleaning heat exchangers. (4) Tube replacement: In fixed-plate heat exchangers, excessive blocked tubes reduce the heat transfer area, so tube replacement is necessary. First, special tools for removing tubes are used to extract the leaking tubes. For example: hydraulic propulsion type, threaded tension type. After the tube is pulled out, the holes in the tube sheet and the baffle plates are cleaned. It is checked whether the centerlines of these holes are misaligned, after which the tubes are inserted. The areas where the tubes are welded to the tube sheet are cleaned using acetone and alcohol. Welding must be carried out using TIG welding after replacing the tubes, in order to ensure that the tube sheet does not deform. The allowable deviations for the diameter of the holes in the tube sheet are shown in Table 1-2. Table 1-2: Deviations of tube sheet holes. Outer diameter of heat exchange tubes (Φ): Allowable deviation for diameter of tube sheet holes (Φ). Outer diameter of heat exchange tubes (Φ): Allowable deviation for diameter of tube sheet holes (Φ). 19: 19.4 + 0.20; 38: 38.5 + 0.30. 25: 25.4 + 0.24; 45: 45.5 + 0.40. 32: 32.5 + 0.30; 57: 57.7 + 0.40. The hardness after welding should be around HB30; otherwise, annealing treatment should be carried out within 150–200 mm from the tube ends. The expansion joint area of the heat exchange tubes shall be free of longitudinal scratches and pits, but circumferential grooves with a depth not exceeding 0.1 mm are permitted. For tubes in bundles that are relatively long, fittings can be used, but the butt weld at the fitting site shall not exceed 10% of the tube diameter; otherwise, it will be difficult to pass through the baffle holes. For 4U-shaped tubes, there can be two welds, with a distance between these two welds of not less than 300 mm. The butt welds should be smooth, and the misalignment between the tube ends should not exceed 15% of the tube wall thickness. The straightness of the pipe fittings should be such that the tube can pass through smoothly. For U-shaped tubes, a ball test using an object with a diameter equal to 0.85 times the inner diameter should be conducted; for replaced tubes, a hydrostatic test on a single tube should be carried out, with the test pressure being twice the design pressure for the tube circuit. After tube replacement, the tube fixation procedure must be followed strictly, especially regarding mechanical expansion; the pressure of the expander rollers must be tightly controlled. The working principle of mechanical tube expansion is relatively simple: it utilizes the pressure from the rollers of the expansion tool to compress the metal in the wall at the end of the tube, causing the tube to expand. Once the outer wall of the tube comes into contact with the holes in the tube sheet, the pressure is transmitted through the tube to the walls of those holes, resulting in deformation of the metal in those walls. Since the material of the tube is softer than that of the tube sheet, the tube undergoes greater deformation; this deformation exceeds the elastic limit and leads to permanent deformation (plastic deformation), while the holes in the tube sheet only experience elastic deformation. When the expansion tool is removed, the metal deformation of the tube sheet holes disappears and returns to its original state, whereas the permanent deformation of the tube remains. Due to the elastic contraction of the tube hole walls, the expanded portion of the tube is firmly pressed, thereby ensuring a tight fit between the tube and the tube sheet. To prevent the tube from being pulled out of the tube hole, it is necessary for the expansion tool to bend the protruding part of the tube into a flared shape. 3. Inspection and maintenance of heat exchangers: After one cycle of operation in chemical plants, the heat exchange capacity of heat exchangers decreases, and their thermal resistance increases. The tubes may also become deformed due to thermal expansion and contraction resulting from repeated startups and shutdowns, which can lead to leaks; furthermore, the tubes need to be inspected and cleaned due to defects such as rust and corrosion. For the maintenance of heat exchangers, a hydrostatic test is generally carried out before the maintenance to check for leaks in the tubes, tube sheets, and shell, with records kept, and the work is carried out in accordance with the maintenance plan. (1) Minor repair items a. Remove the end heads or tube boxes of the heat exchanger. b. Clean and remove foreign objects from the inner surface of the pipes and the housing. Also inspect the end caps and tube boxes of the heat exchanger for defects such as corrosion, rust, cracks, and sand holes. c. Perform pressure testing and leak testing on the tubesheet and shell. d. Check bolts as well as insulation and corrosion protection. e. Perform local thickness measurement. (2) Medium repair items a. Include minor repair items. b. Extract the tube bundle for cleaning, wiping, and washing, and inspect the deformation and bending of the heat exchange tubes. c. Check for corrosion and rust on the partitions and tie rod bolts. d. Inspect the sealing surfaces of the heat exchanger; there should be no scratches, dents, or pitting on the surface. (3) Major repair contents a. Include medium and minor repair contents. b. Thoroughly inspect the operation of the heat exchanger, and conduct a dye penetrant test on the welds between the tube sheet and the tubes.