Tube-sheet ligament: in shell-and-tube exchangers, the minimum clear metal between adjacent tube holes (“ligament”) should be what fraction of tube outside diameter, but in no case less than 4.5 mm?

Introduction / Context:
The tube sheet of a shell-and-tube exchanger is drilled with a precise pattern of holes. The remaining web of material between neighboring holes is called the ligament. Adequate ligament prevents weakening of the tube sheet and provides sealing strength for tube-to-tubesheet joints.

Given Data / Assumptions:

• Conventional exchangers with drilled tube sheets and standard tube pitches.
• Tube outside diameter (OD) is known; ligament = pitch − OD.
• Design follows common industry practice; detailed code checks still apply.

Concept / Approach:
Too small a ligament raises risks of leakage, plastic deformation, and cracking around tube holes. A widely used preliminary rule is to keep the minimum ligament at roughly one-fourth of tube OD, subject to a hard minimum (about 4.5 mm) to preserve structural integrity and manufacturability for small tubes.

Step-by-Step Solution:

Verification / Alternative check:
Fabrication standards and layout practices ensure adequate bridge strength around holes; FEA or code formulas can refine this further for high load or thermal cycling services.

Why Other Options Are Wrong:

• Equal to 1.0× OD or 0.75× OD: overly conservative and impractical for common pitches.
• 0.5× OD: more conservative than necessary in many services, increasing shell diameter and cost.

Common Pitfalls:
Ignoring corrosion allowance (which effectively reduces ligament); using very small pitch that complicates rolling/welding; not checking for tube-sheet bending stresses under differential pressure.

Final Answer:
One-fourth of the outside diameter