High-Pressure CO2 Piping: Unapproved Copper and Brazed Repair
During a scheduled repair of a 2,500 GT cargo ship, the yard removes a 3 m section of the 25 mm nominal bore high-pressure CO2 discharge pipe serving the machinery space. The original pipe is seamless steel, butt-welded, with a design pressure of 15 MPa. The yard replaces it with seamless copper tube and joins it to the steel pipe with silver-brazed sleeves. The repair is tested with compressed air at 1.0 MPa for 5 minutes; soap bubbles show no leak. The yard manager argues that copper is corrosion-resistant and the low test pressure is safe because "CO2 is not under pressure until the cylinders are opened." The CO2 system is a fixed high-pressure system with 40 cylinders, and the engine room is normally unattended. The class surveyor arrives after the repair is complete and finds no material certificate, no brazing procedure and no NDT records. The ship is due to sail in 24 hours. The surveyor must decide whether to accept the repair, what testing is required, and what safety precautions apply.
1. Evaluate the acceptability of the copper material and brazed joints for this CO2 discharge line.
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Reject the material and joining as proposed until approved. High-pressure CO2 piping is governed by the FSS Code and class rules, which require approved materials and joining methods for the design pressure and temperature. The original seamless steel butt-welded pipe was approved; substituting seamless copper with silver-brazed sleeves is a major change. Copper may have insufficient pressure rating at 25 mm and 15 MPa, and brazed joints may not be approved for this service; dissimilar steel/copper joints also raise galvanic and thermal expansion concerns. The yard has no material certificate, no brazing procedure qualification and no NDT records. Do not accept "corrosion-resistant" as a justification. The correct action is to remove the copper section and replace it with approved steel pipe and qualified welding, or obtain written class approval supported by calculations and procedure qualification.
2. Specify the correct pressure test and inspection requirements.
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Test pressure must be based on the 15 MPa design pressure, not 1.0 MPa. Confirm the class rule; typically hydrostatic test at 1.5 × design = 22.5 MPa, using calibrated gauges and a safe test bay. A pneumatic test at 1.0 MPa is both inadequate and dangerous; compressed air stores large energy. If pneumatic testing is permitted, it requires class approval, reduced pressure, exclusion zones and appropriate safeguards. Perform NDT on the joints (visual, PT and/or RT as required), clean and dry the pipe after hydro, and verify supports and restraints. The system must also be functionally tested without discharging CO2: check alarms, isolation, ventilation and cylinder valve operation.
3. What documentation and functional checks are required before the system is returned to service?
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Documentation: material certificates, approved drawings, WPS/PQR and welder or brazier qualifications, NDT reports, hydrotest record, calibrated gauge certificates and class survey report. Functional checks: CO2 system alarm and release controls, cylinder room ventilation, warning signs and pressure gauge. The surveyor must approve the repair before the system is returned to service. Given the 24-hour sailing deadline, the safe decision is to keep the CO2 system isolated or out of service with a documented risk assessment and repair plan, or delay sailing; do not accept an unapproved high-pressure repair on an unattended machinery space system.