Assuming the bundle is leaking or, based on past experience, that it will not last until the next turnaround, then some repairs will need to be made. The following areas need to be considered before deciding on the extent of repairs.
• Is bundle nearing its historical life?
If a bundle is leaking and it is not approaching its historical life, then a failure analysis should be performed to determine if design changes are required. The best way to determine the cause of failure is to pull the leaking tubes and inspect them.
• Can tubesheet be reused?
Normally, a tubesheet can only be used two or three times before it can no longer have a tube rolled into it. If a tubesheet cannot be reused, then a new bundle will be necessary.
• Can leaking tubes be plugged or replaced?
Individual leaking tubes can be the sign of a much larger problem. Simply plugging or replacing leaking tubes may be setting yourself up for another failure before the next shutdown.
The leaking tube should be pulled and inspected to determine the cause of failure. The position of the failed tube relative to baffles and nozzles should be determined and recorded. This can help identify the cause of the failure. If it is not obvious that repairing or replacing the tube will ensure a sound bundle, then the bundle should be retubed or replaced.
Sometimes, all of the leaks may be in one part of the bundle. In this case, a design change (i.e., replacing tubes with solid rods, changing inlet configuration, etc.) may eliminate the problem.
If a new bundle is required, should it be redesigned?
Simple design changes may substantially improve a bundle’s operation and service life. If a bundle needs to be retubed or replaced, the incremental cost for making these design changes may not be very high. However, the cause of the bundle deterioration should always be identified and process changes considered along with design changes.
Some examples of bundle configuration changes that may improve the long term heat transfer or bundle life are listed below. (Discuss these changes and improvements with a CRTC heat transfer specialist or the CRTC Process Design or Process Consultation groups.)
• Change inlet impingement design to eliminate tube vibration wear.
• Plug tubes to increase tube velocity. (This improves heat transfer if fouling is significant.)
• Change shell side pass configuration and baffling to improve heat transfer and
• Use different materials that will increase the bundle reliability and life. (As a general rule, even in corrosive services such as sea water, a bundle should last at least two operating runs. Contact the CRTC Materials and Equipment Engineering
Unit for help in this area.)
• Change from floating head to U-tube design.