#4 – Tube to Tube Sheet Joint Failure…
Typically, the tube to tube sheet joint is one of the most critical aspects of heat exchanger design, right up there with size and materials. Joint design is known to make or break the longevity of the unit and is highly dependent upon the proper specification and execution of that manufacturing process.
Generally speaking, there are three different basic joint designs:
- Expanded and Seal Welded
- Expanded and Strength Welded (sometimes Strength Welded only)
Expanding tubes requires specially-calibrated equipment that fits the ID of the tube and presses the tube out into the tube sheet. Often, the tube sheet often has grooves cut in the holes to allow room for the tubes to expand to get a better joint seal than if it were in a plain, smooth hole. Meticulous care must be taken to calculate the proper expansion that achieves the proper percentage of tube wall reduction. Dimensions are taken to three decimal places to be certain of the proper tube to tube sheet engagement.
Seal welding tubes is common in the industry, but shouldn’t be taken lightly. The more experience your fabricator has with tube end welding, the better the end product. It is critical to ask lots of questions about QC and non-destructive examination (NDE) that will be employed. That is often the true sign that you will receive the best final product.
Strength welding is a very specialized detail that are used for the most critical applications to prevent leaks. Gaining the experience to strength weld tube ends comes through decades of research and development, training, continuous improvement and – of course – production units. Mock-ups are often required, which are very time-consuming and often the only way to ensure strength welds are done correctly. Although it is developing technology, there is now a way to perform 100% Radiographic Testing (RT) on tube end welds. Many customers request this inspection to be 100% confident that their equipment will perform as required.
In the end, it’s always best practice to trust your heat exchanger design to a firm who can handle every aspect of the project, from thermal design to mechanical design to mock-ups to manufacturing and properly testing the unit. Experienced, reputable firms handle all facets of the project in-house to ensure you get the design you need and the manufacturing and QC expertise that is required to provide long life to your equipment.