EN
A Leak That Could Have Been Prevented
A Gulf Coast chemical plant scheduled a turnaround to replace thirty-six heat exchanger tubes in a critical service unit. The maintenance team used a tube expander with the standard roll length that had worked on previous jobs. They rolled every tube, passed the hydrostatic test, and buttoned up the unit. Two weeks after startup, a tube joint started weeping—just a few drops per minute, but enough to flag a problem. Within a month, six more joints were leaking. The unit had to be taken offline for emergency repair, costing the plant nearly $90,000 in lost production and replacement labor.
The culprit? The tube sheet on that exchanger was thicker than the previous ones the team had worked on. The standard roll length didn't cover the full tube sheet thickness. The rolling action never reached the back face of the joint, leaving an unexpanded section that eventually developed a leak path.
Projection length—the amount the tube extends beyond the tube sheet—isn't a detail that can be guessed. It's a critical dimension that determines whether a rolled joint stays leak-free for years or fails within weeks.
What Projection Length Actually Means
Tube projection is the distance the tube extends past the face of the tube sheet. This dimension matters because the expander rolls need to cover the full thickness of the tube sheet plus the projection, with a small amount of roll beyond the tube seat to ensure complete expansion.
If the projection is too short, the rolls don't have enough material to engage properly. The rolling action starts inside the tube sheet rather than at the face, which can lead to insufficient expansion at the front of the joint. If the projection is too long, the rolls extend into the free span of the tube beyond the tube sheet, thinning the tube wall in an unsupported area and creating a stress riser that can lead to cracking under thermal cycling.
The typical projection range for shell and tube heat exchangers falls between 3/16 inch and 9/32 inch, with 3/16 inch serving as a common starting point. But these are general guidelines—the actual projection required depends on tube diameter, wall thickness, tube sheet thickness, and the specific design of the joint.
Calculating the Required Roll Length
The required roll length isn't arbitrary. It's calculated from three components: the length of the tube seat in the tube sheet, the tube projection beyond the tube sheet, and the amount of roll that extends beyond the tube seat.
A practical formula looks like this: roll length = tube seat length + tube projection + roll beyond tube seat. The roll beyond the tube seat typically ranges from 1/8 inch to 1/4 inch, ensuring that the expansion extends past the back face of the tube sheet.
Skipping any of these components means the expander won't fully cover the joint. The result is a partially expanded tube that looks fine on the surface but has a weak section where leaks can develop. In some cases, this weak section doesn't show up on hydrostatic testing but reveals itself after thermal cycling or pressure fluctuations in service.
| Projection Condition | Likely Outcome | Field Consequence |
|---|---|---|
| Correct projection (3/16"–9/32") | Full tube sheet coverage, complete expansion | Leak-free joint, long service life |
| Projection too short | Rolls engage inside tube sheet, incomplete front expansion | Leak path develops at tube sheet face |
| Projection too long | Rolls extend beyond tube sheet, wall thinning | Stress cracking, potential tube failure |
Wall Reduction: The Hidden Variable
Projection length works in conjunction with wall reduction—the percentage by which the tube wall thickness decreases during expansion. Industry practice typically targets wall reduction between 4 and 10 percent, with 5 to 8 percent being common for carbon steel and austenitic materials.
If the projection is too long, the rolls thin the tube wall in the unsupported free span beyond the tube sheet. This creates a localized reduction in wall thickness that may not fail immediately but becomes a weak point under vibration or thermal cycling. Conversely, if the projection is too short, the expansion doesn't achieve the required wall reduction at the front of the joint, leaving the connection mechanically weak.
The relationship between projection and wall reduction isn't linear. A small change in projection can produce a significant change in the final wall thickness at the joint interface. That's why measuring and controlling projection before rolling is as important as monitoring rolling torque or wall reduction during the process.
Measurement and Verification
Projection isn't something to eyeball. A simple depth gauge or caliper provides accurate measurement of how far the tube extends beyond the tube sheet face. On new installations, this measurement is straightforward. On replacement work, the existing tube stubs may need to be removed and the tube sheet surface cleaned to get an accurate reading.
Verification doesn't stop at measuring projection before rolling. After expansion, checking the tube ends for proper flare or bead formation provides visual confirmation that the expansion covered the full joint. A tube that has been properly expanded will show a smooth transition from the tube sheet face to the expanded section, with no abrupt changes in diameter or visible gaps.
Ultrasonic measurement techniques can also verify wall reduction after rolling, providing quantitative confirmation that the expansion achieved the specified parameters without creating defects. This technique is particularly valuable on critical service units where joint failure carries significant safety or operational consequences.
Material Considerations and Their Impact
Different tube materials behave differently under expansion. Carbon steel work-hardens during rolling, which means the tube becomes harder and more resistant to further deformation as the expansion progresses. Stainless steel and other austenitic materials have different work-hardening characteristics and may require adjustments to projection or rolling technique.
Tube wall thickness also affects the required projection. Thin-wall tubes are more sensitive to over-expansion—a projection that's too long can thin the wall past an acceptable limit much faster than in a thick-wall tube. Thick-wall tubes require more rolling force and may need a longer roll to achieve the same relative wall reduction.
The tube sheet material matters too. Harder tube sheet materials resist deformation, requiring more rolling force and potentially affecting how the projection interacts with the expansion process. Softer materials may allow the tube to expand more readily, changing the effective wall reduction for a given projection length.
When Things Go Wrong: Diagnosing Projection Problems
Leaking rolled joints often trace back to projection issues, but diagnosing the specific problem requires looking at the failure pattern. Leaks at the front face of the tube sheet—where the tube enters the sheet—usually indicate insufficient projection or a roll length that didn't cover the full tube sheet thickness.
Leaks at the back face or weeping from the joint interface suggest that the projection was adequate but the wall reduction was insufficient, or that the roll didn't extend far enough beyond the tube seat. Cracking in the tube just beyond the tube sheet often points to excessive projection that caused wall thinning in the free span.
In all cases, documenting the projection length used, the roll length, and the resulting wall reduction creates a record that helps diagnose problems and prevent repeats. When a joint fails, knowing what was done—not just that it failed—makes the difference between fixing the symptom and fixing the root cause.
For operations that require reliable, leak-free rolled joints across a range of tube sizes and materials, having access to precision expanders and knowledgeable support matters. Manufacturers like those in the Taizhou Ruiqi Tools network have been supplying tube expansion equipment to industrial clients for years, with a focus on the dimensional accuracy and consistent quality that prevent projection-related failures before they start.
