Maintenance Precautions for Heat Exchangers in the Chemical Industry

I. Safety Preparations Before Maintenance

1. Thorough Isolation and Pressure Relief
   • The heat exchanger must be physically isolated from the upstream and downstream process pipelines. After closing the inlet and outlet valves, blind flanges shall be installed. Isolation solely by valves is strictly prohibited (to prevent medium channeling caused by internal leakage).
   • Pressure relief and complete drainage of the medium inside the heat exchanger shall be performed. If the medium is toxic, flammable, or corrosive, nitrogen replacement or steam purging shall be conducted. The oxygen content and toxic gas concentration inside the equipment shall be tested and meet the standards before starting any work.
2. Power Cut, Tagging, and Warning
   • Cut off the power supply of auxiliary equipment of the heat exchanger (such as pumps, agitators, and heaters), hang the warning sign of "No Switching On, Personnel at Work", and implement the Lockout-Tagout (LOTO) system.
   • If it is necessary to enter the inside of the heat exchanger for maintenance (such as inspection of the tube bundle of a shell-and-tube heat exchanger), a permit for confined space operation shall be obtained, and ventilation, monitoring, and emergency rescue equipment shall be provided.
3. Medium Protection Measures
   • Maintenance personnel shall wear protective equipment according to the characteristics of the medium: wear chemical-resistant gloves and goggles when contacting corrosive media; wear heat-insulating clothing when contacting high-temperature media; wear gas masks when contacting toxic media.

II. Precautions for Troubleshooting and Dismantling

1. Accurate Fault Location
   • Common faults include fouling and blockage, leakage (channeling between tube side and shell side), corrosion and perforation, seal failure, and abnormal vibration noise. Fault locations shall be determined through process parameters (temperature difference reduction, pressure drop increase), air tightness test, endoscope inspection, and other methods.
   • Blind dismantling shall be avoided. For example, if the pressure drop on the tube side increases, priority shall be given to checking the fouling inside the tubes rather than directly disassembling the head cover.
2. Standardized Dismantling Procedures
   • The dismantling sequence shall comply with the equipment drawings. First, remove the thermal insulation layer and accessories (pressure gauges, thermometers), then remove the head cover and flange bolts. Violent dismantling is strictly prohibited.
   • When disassembling seals (gaskets, packing), pay attention to observing the condition of the sealing surface. If there are scratches or deformation, record them as the basis for subsequent repair.
   • For the extraction of the tube bundle of a shell-and-tube heat exchanger, special lifting tools shall be used for smooth lifting to prevent bending, deformation, or collision with the tube sheet of the tube bundle.

III. Key Operational Points During Maintenance

1. Precautions for Fouling Cleaning
   • Physical cleaning: When using high-pressure water jet cleaning, control the water pressure (adjust according to the material of the heat exchanger, generally ≤30 MPa for carbon steel and ≤20 MPa for stainless steel) to avoid erosion of the tube wall due to excessive water pressure. Mechanical pigging is suitable for tube bundles with larger diameters to prevent steel ball jamming.
   • Chemical cleaning: For insoluble scales (such as carbonate and sulfate scales), select matching cleaning agents (such as hydrochloric acid + corrosion inhibitor), control the cleaning temperature and time to avoid excessive corrosion of the base material. After cleaning, rinse and neutralize with clean water thoroughly, and test the pH value until it reaches neutrality.
2. Precautions for Leakage and Corrosion Repair
   • Channeling between tube side and shell side: If a single heat exchange tube leaks, the tube plugging method can be adopted (sealing both ends with special plugs). The number of plugged tubes shall not exceed 10% of the total number of tubes (otherwise, the heat exchange efficiency will be affected). If multiple tubes leak or the tube sheet is corroded, the tube bundle shall be replaced or the tube sheet shall be repaired.
   • Flange seal leakage: Check whether the sealing surface is flat, replace qualified gaskets (select materials according to the medium temperature and pressure, such as metal spiral wound gaskets for high temperature and PTFE gaskets for corrosive media). Tighten the bolts symmetrically and evenly to avoid eccentric load.
   • Shell corrosion: Local corrosion can be repaired by patch welding. Before patch welding, conduct flaw detection on the base material. The welding material shall match the base material. Post-weld heat treatment shall be carried out to eliminate stress. For large-area corrosion, the shell shall be replaced.
3. Material Protection and Damage Prevention
   • During maintenance, avoid scratching the inner wall of the heat exchanger and the sealing surface of the tube sheet with sharp tools. For heat exchangers made of stainless steel, it is forbidden to strike with carbon steel tools to prevent pitting corrosion caused by iron ion contamination.
   • When assembling the tube bundle, ensure uniform clearance between tubes to prevent vibration wear caused by fluid disturbance during operation.

IV. Acceptance and Trial Operation After Maintenance

1. Static Acceptance
   • After assembly, conduct air tightness test or hydrostatic test on the heat exchanger: the test pressure shall be 1.25–1.5 times the design pressure. During the pressure holding period, check for leakage and deformation. For heat exchangers operating under vacuum, a vacuum degree test shall be performed.
   • Check whether the thermal insulation layer and accessories are installed properly and whether the bolt tightening torque meets the standards.
2. Dynamic Trial Operation
   • Remove the blind flanges, slowly connect the process pipelines, and gradually increase the temperature and pressure. Sudden cooling and heating are strictly prohibited (to prevent seal failure or tube bundle deformation caused by thermal expansion and contraction of the heat exchanger).
   • Monitor the operating parameters: inlet and outlet temperature, pressure drop, abnormal noise, and leakage. Only after continuous and stable operation for 24–48 hours without abnormalities can the maintenance be judged as qualified.

V. Other Special Precautions

1. Maintenance of Heat Exchangers for Special Media
   • If the medium is strongly corrosive (such as strong acid, strong alkali) or easily polymerizable (such as ethylene, propylene), passivation treatment shall be carried out on the inside of the heat exchanger after maintenance to prevent secondary corrosion or polymerization blockage caused by residual medium.
   • When the heat exchange medium is flammable and explosive, hot work is strictly prohibited at the maintenance site. If welding is required, a hot work permit shall be obtained, and fire-fighting equipment shall be provided.
2. Record Filing
   • Detailed records shall be made during maintenance: fault phenomena, troubleshooting process, maintenance plan, model specifications of replaced parts, test data, etc. A maintenance file shall be established to provide a basis for subsequent preventive maintenance.

VI. Extended Precautions for Preventive Maintenance