Heat exchangers are among the critical pieces of equipment in process units and play an essential role in heat transfer between different process streams. The safe and optimal operation of these exchangers requires strict adherence to procedures for handover to maintenance, repair activities, and recommissioning after repair. The handover process for maintenance includes steps such as draining, washing, and nitrogen purging to ensure that the exchanger is free of any process materials and fully prepared for repair. After maintenance is completed, returning the exchanger to service requires technical inspections, pressure testing, and verification of auxiliary equipment and instrumentation to ensure safe operation and proper performance. Strict compliance with these steps not only increases the service life of the exchanger but also prevents leakage, potential damage, and process disruptions.
Handover of the Exchanger for Maintenance
To hand over a heat exchanger for maintenance, a series of safety and preparation actions must be carefully carried out. The detailed steps and their proper sequence are described below.
Draining and Blinding
For handing over the exchanger to maintenance, the unit must first be taken out of service under normal shutdown conditions. Accordingly, the hot stream is gradually shut off first, followed by the cold stream.
For example, when a steam-heated reboiler is taken out of service, the steam inlet flow is slowly stopped first, and then the process stream is isolated.
After removing the exchanger from service and fully closing the inlet and outlet valves, all exchanger inlet and outlet lines must be blinded to prevent any fluid from entering the exchanger.
In the next step, any remaining process materials inside the exchanger are drained through drain lines to underground tanks, slop tanks, or downstream facilities. After complete draining, residual liquids are displaced using nitrogen pressure and transferred to downstream systems or slop tanks.
Purging and Washing of the Exchanger
After the heat exchanger has been drained of process materials, nitrogen purging or water washing is performed depending on the type of process fluid. If the fluids are flammable or toxic, nitrogen purging is carried out first to ensure full safety.
In this method, a temporary nitrogen line is connected to the top of the exchanger, and another line is connected to the flare system or a safe disposal route. Purging is continued for several hours. Afterward, the temporary connections are switched to water, allowing water to enter from the top and exit from the bottom so that the process passages of the exchanger are thoroughly washed. This washing operation is repeated several times, followed by another nitrogen purge to ensure the exchanger is completely ready for handover to maintenance.
It should be noted that these steps are required for major exchanger maintenance. For minor repairs, depending on conditions and the scope of work, not all steps may be necessary, provided that all safety and operational procedures are strictly followed.
Handover of the Exchanger after Maintenance
After completion of maintenance activities, returning the exchanger to the system requires a series of inspections, tests, and preparation steps. These include obtaining technical inspection approval, conducting pressure tests, checking connected lines and auxiliary equipment, and preparing the exchanger for recommissioning.
Obtaining Technical Inspection Approval
The plant technical inspection department is responsible for issuing inspection procedures for heat exchangers, including checks for corrosion, wall thickness, hydrostatic testing, and other safety-related items. For heat exchangers, hydrostatic test procedures are issued by technical inspection and executed by the maintenance team. Any exchanger that has been opened and reassembled during maintenance must undergo this inspection.
After successful hydrostatic testing and receipt of written approval from technical inspection, the exchanger may be handed over from maintenance. Failure to obtain approval indicates the possibility of leakage, improper connections, or other issues that could cause operational problems. All technical inspection approvals are formally documented to ensure that the exchanger is safe and ready for return to service.
Technical Inspection Tests
Technical inspection tests for heat exchangers generally include two main methods: hydrostatic testing and pneumatic testing. The procedures for these tests are issued by the technical inspection department.
Hydrostatic Test
Hydrostatic testing is commonly used for heat exchangers, as it involves pressurizing the exchanger with water for a specified period to detect potential leaks or defects.
In this method, the exchanger is pressurized using dedicated test pumps, and the applied pressure is selected based on applicable standards and the exchanger design pressure. Before pressurization, all measuring instruments and instrumentation are disconnected from the pressurized paths and replaced with blinds to prevent damage. Additionally, all lines and flanges leading to the exchanger, upstream of valves, are blinded, as valve isolation alone is not sufficient for pressure testing and may allow leakage.
After preparation, the test pump is connected to one of the exchanger nozzles, such as a drain connection, and water is injected into the shell or tube side to achieve the required pressure. Pressure is monitored using pressure gauges for a specified duration, and the allowable pressure drop is defined by applicable standards. If excessive pressure drop is observed, new instructions are issued by technical inspection, and any leaks are repaired. The exchanger is approved for service only when the pressure drop falls within acceptable limits.
In older exchangers, some tubes may rupture or leak during hydrostatic testing. In such cases, technical inspection may issue instructions for tube plugging. Typically, the maximum allowable number of plugged tubes is about 10% of the total tube count; if this limit is exceeded, tube replacement is required.
Tube plugging is performed by inserting conical metal plugs into both ends of the tube and driving them into place using a hammer. For additional assurance, these plugs are sometimes welded to the tube sheet to prevent leakage and potential damage.
Pneumatic Test
Pneumatic testing is used when the process fluid is a gas, such as helium, oxygen, or air. Due to the potential explosion hazards associated with pneumatic testing, hydrostatic testing is generally preferred for conventional heat exchangers to ensure equipment safety and operational reliability.
Checking and Pressurizing Connected Lines
During handover of the exchanger from maintenance, some lines connected to the exchanger may have been disconnected. These lines must be reconnected and checked for proper installation, correct gasket placement, and proper functioning of associated auxiliary equipment such as steam traps. The best reference for verifying correct connections is the unit P&ID, which should be compared with the actual field configuration to identify any discrepancies.
After installation, the lines can be pressurized with nitrogen gas to identify potential leakage points. For leak testing, flange joints are covered with adhesive tape, and leaks are detected by applying soap solution through a small opening in the tape. If leakage is observed, connections must be retightened, or if the gasket is defective, the joint must be dismantled and the gasket replaced.
Checking Instrumentation and Control Devices
After maintenance is completed and when the exchanger is completely free of pressure and liquid level, the optimal time to check and calibrate instrumentation and control devices begins. This activity, known as zero adjustment, is carried out by the instrumentation maintenance department after issuance of a work permit by operations to ensure proper equipment performance.
To verify valve operation, open and close commands are sent virtually from the control room, and the valve opening percentage is checked in the field. This process is performed simultaneously by the field operator and control room operator, and valve performance is verified at opening positions of 0%, 25%, 50%, 75%, and 100%. In addition, block valves, check valves, and valves on inlet and outlet lines are inspected to ensure proper installation and operation.
Manual valves and body check valves are also checked to confirm correct installation orientation and proper functionality. Failure to inspect and correct these components may lead to improper exchanger performance during operation.
Removal of Blinds
In the final stage of returning the exchanger to service, all blinds must be removed from the connecting lines. After removing the blinds, flange connections are carefully reassembled and properly tightened. During reassembly, ensure that all bolts are fully installed and adequately tightened to prevent leakage and potential problems.
Gaskets between flanges and line connections must also be carefully inspected to ensure they are free of defects. If the fluids entering the exchanger are sensitive to oxygen, nitrogen purging may be performed again after blind removal to ensure that oxygen presence does not cause issues during start-up.
Conclusion
Strict adherence to the procedures for handing over heat exchangers to maintenance and recommissioning after repair—including draining, purging, washing, technical inspection, and verification of auxiliary equipment—ensures safety, proper performance, and extended service life of the exchanger. Implementing these procedures prevents leakage, damage, and process disturbances and prepares the exchanger for long-term, reliable operation.
