Fully Drilled and Machined Tubesheet
In the heart of modern industrial installations such as petrochemical plants, power stations, marine vessels, and nuclear facilities, shell-and-tube heat exchangers perform the critical functions of energy transfer and temperature control. As the most critical pressure-retaining component within these systems, the design and manufacturing quality of the Tubesheet directly determines the long-term operational reliability of the entire equipment.
Fully Drilled and Machined Tubesheet
Function of the Tubesheet
A tubesheet is a metallic plate of specified thickness, containing thousands of precisely machined holes, which connects the channel and shell sides of a heat exchanger. Its primary function is to position and secure the heat exchange tubes, thereby physically isolating the tube-side and shell-side fluids. During operation, the tubesheet must withstand differential pressure, thermal stresses, and potential mechanical vibration. Consequently, its machining accuracy directly impacts the sealing integrity and strength of the tube-to-tubesheet joints. A qualified tubesheet must remain free from leakage or fatigue failure throughout its design life, typically exceeding 20 years. Achieving this objective begins with rigorous control over the drilling and machining processes.
Fully Drilled
Fully drilled encompasses not only the machining of heat exchanger tube holes but also the precision formation of all hole types, including tie-rod holes and lifting eye bolt holes. Each hole category serves a specific function and demands distinct accuracy requirements.
1.Tube Holes
Diameter Tolerances and Bore Surface Roughness
Tube holes are typically arranged in triangular or square patterns, with tube pitch determined by thermal calculations. Ligament width is controlled within ±0.1 mm to prevent stress concentration.
| Application | Tolerance Grade | φ25 mm Hole Range (mm) |
|---|---|---|
| Strength Expansion/Welding | H11/H12 | 25.000 – 25.130 |
| High-Pressure/Vibration | H9/H8 | 25.000 – 25.033 |
Bore roughness: Ra 6.3–12.5 μm (expansion), Ra ≤ 3.2 μm (welding) to avoid weld defects.
2.Tie-Rod Holes
Secure baffles/support plates to maintain tube bundle rigidity, distributed near the tubesheet periphery with H12 diameter tolerance.
Perpendicularity is mandatory; through-holes with counterboring/tapping on reverse side, thread accuracy controlled to 6H grade.
Machined on CNC coordinate boring machines to ensure position accuracy and avoid flow-induced vibration.
3.Lifting Eye Bolt Holes
For lifting heavy tubesheets (tens of tons) during fabrication/transport, symmetrically positioned near the periphery.
Thread sizes (M24, M30) based on lifting loads, with full thread engagement and 6H grade accuracy required.
Visual inspection + random plug gauge verification; designated as a critical manufacturing control point.
4.Deep Hole Drilling Process
For thick tubesheets (hundreds of mm), CNC deep hole drilling machines are used with pilot drilling → reaming → finishing.
Drilling parameters optimized by material hardness; high-pressure coolant for effective chip evacuation.
Sample hole inspection for diameter/ligament width/perpendicularity with full quality traceability records.
Precision Machining
Following drilling, the tubesheet possesses only its basic form; precision machining imparts the functional surfaces and final dimensions essential for service.
1.Sealing Surface Finish
Flanged joints with sealing face types: raised face, male-female, tongue-and-groove, RTJ.
Flatness ≤ 0.05 mm, surface roughness Ra ≤ 3.2 μm (Ra ≤ 1.6 μm for metallic gaskets/severe service).
Machined on vertical boring mills; flatness verified by straightedges/electronic levels, no radial tool marks to avoid leakage.
2.Pass Partition Groove Machining
Annular/rectangular grooves to segregate tube-side fluid passes, 2–5 mm below the sealing face.
Width: plate thickness + 0.5–1 mm clearance; groove flatness ≤ 0.1 mm, sidewall Ra ≤ 6.3 μm.
Layered milling + chamfering/deburring; radiused transitions for thermal stress applications to reduce welding residual stress.
3.Tube Hole Internal Grooving
Annular expansion grooves in mid-thickness for high-strength pull-out resistance, 2–5 mm width & 0.3–0.8 mm depth.
Machined by specialized internal grooving tools with smooth contours and no burrs.
Complies with ASME/GB/T 151 standards; tube material deforms into grooves for mechanical interlocking during expansion.
Common Questions on Quality Inspection and Engineering Challenges
1.Hole Diameter and Ligament Verification
All tube holes require individual diameter measurement (or air gauging sampling) to meet tolerance ranges. Ligament widths are verified by specialized calipers/optical projectors. Tool wear and thermal deformation cause deviations, requiring periodic in-process inspection and timely tool replacement.
2.Surface Finish Control
Excessive sealing surface roughness impairs gasket performance; insufficient roughness causes gasket creep. Inappropriate tube hole roughness reduces expansion joint strength. An overseas refining project saw inadequate pull-out strength from Ra 1.6 μm tube holes, corrected by adjusting reaming allowances and tool geometry.
3.Groove Machining Accuracy Verification
Post-machining, expansion grooves require borescopic inspection for contour integrity, with depth verification using specialized gauges or optical projection methods. Insufficient depth prevents effective mechanical interlocking during expansion; excessive depth may compromise tubesheet strength. During nuclear equipment fabrication, groove position deviation from design led to localized tubesheet deformation after expansion, subsequently corrected through CNC program modification and implementation of in-process measurement.
Dimensional Inspection of Tubesheet
Why Fully Drilled and Machined Tubesheets?
Structural Integrity Assurance
Precision-machined tubesheets ensure uniform load distribution among all heat exchange tubes, preventing localized overloading that could precipitate fatigue failure. Expansion grooves particularly enhance resistance to vibration and thermal transients.
Heat Transfer Efficiency Improvement
Accurate hole positioning and smooth bore surfaces promote uniform fluid distribution, minimizing bypass flow and maximizing effective heat transfer surface utilization.
Compliance with International Standards
Codes including ASME BPVC Section VIII Division 1, GB/T 151, and TEMA prescribe detailed requirements for tubesheet fabrication. Fully drilled and machined tubesheets represent prerequisites for code compliance and successful third-party inspection.