Expanding Thicker Tube Sheets in Heat Exchangers

Tube sheet thickness is determined by several factors, including pressure requirements, thermal expansion considerations, corrosion allowance, and the necessary tube support. Industry standards like TEMA, API, and ASME provide specific guidelines for determining appropriate thicknesses. However, thicker tube sheets introduce complexities in the expansion process:

• Increased Torque Requirements: Expanding a thicker tube sheet requires long reach tooling, leading to significantly higher torque being applied. This increased torque can damage tooling through galling, pitting, or heat damage.
• Uneven Expansion: The stepped expansion process used with thicker tube sheets can result in uneven expansion if not performed carefully. Missed areas can create ridges or “catch points” in the tube, which become potential failure points during operation.
• Work Hardening: Overlapping expansion areas or multiple operations in the same location can lead to work hardening of the tube material. This can cause elongation and embrittlement, reducing the tube’s lifespan.
• Increased Time and Labor: Expanding thicker tube sheets can require 2 – 4Xs the labor compared to thinner sheets. This is due to the step rolling process, the increased number of tools needed, higher costs, and more extensive setup time for each expansion step.

Selecting the appropriate expander is crucial for successful tube expansion. Consider the following:

• Tube and Tube Sheet Material Compatibility: If the tube and tube sheet materials have similar yield strengths, a multi-roll tool is generally recommended. For high-yield tube materials (e.g., super duplex stainless steel), a short-roll tool is often preferred due to the lower torque applied to the tool.
• Double Radius Rolls: Using double radius rolls can minimize the formation of ridges during expansion, simplifying the overlap between expansion steps.

Figure 1. SA350 LF2 Tube Sheet, Thickness 8.75″ x 66″ diameter.

Image provided by SHECO – Southern Heat Exchanger

Figure 2. Step Rolling Process

Step rolling is essential for achieving uniform expansion in thicker tube sheets. Follow these steps:

• Expander Setup: Begin with a long-reach expander. Set the tool so that the roll ends approximately 1/8″ before the back face of the tube sheet. For the next step, use another long-reach tool and set it to overlap the previous expansion by about 1/4″. Continue this process until you reach the operator side of the tube sheet.
• Sequential Expansion: Expand all tubes at the deepest setting first, working from the back face to the front face. Then, use the next shorter reach expander in the series and expand all tubes again. Repeat this process, progressively shortening the expander reach. Avoid using the long-reach tool for all expansions, as this increases the load on the tool and the risk of damage.
• Tool Maintenance: Maintain the tools in optimal condition. Keep them cool and clean, lubricate frequently, and remove any debris. Regularly swap out expanders to prevent overheating and premature wear.

• Proper Lubrication: Use a high-quality lubricant specifically designed for tube expansion to reduce friction and minimize tool wear.
• Controlled Expansion: Avoid over-expanding the tubes. Follow the manufacturer’s recommendations for torque and expansion rates.
• Inspection: Thoroughly inspect each expanded tube for any defects, such as cracks, ridges, or uneven expansion.
• Training: Ensure that operators are adequately trained on the proper techniques for expanding tubes in thicker tube sheets.

Expanding tubes into thicker tube sheets requires careful planning, the right tools, and a thorough understanding of the challenges involved. By following the guidelines outlined in this article, operators can achieve consistent, high-quality expansions, ensuring the long-term reliability and performance of heat exchangers in demanding applications. Remember to always consult relevant industry standards (TEMA, API, ASME) for specific requirements and best practices.