Appearance
A tube sheet for a pressurised water reactor steam generator is a forging of SA-508 Grade 3 steel, 600 mm thick and 4,000 mm in diameter, with 8,000 holes drilled through it to accept the Inconel 690 U-tubes. Each hole is 18.5 mm in diameter, drilled from the solid at an L/D ratio of 32:1, with a diameter tolerance of +0.10/0 mm, a surface finish of Ra ≤ 1.6 µm, and a positional accuracy of ±0.10 mm relative to the adjacent holes. The distance between the edges of two adjacent holes — the ligament — is typically 4.5 mm. If the drill drifts by 0.15 mm at mid-depth, the ligament is breached and the entire forging — worth €500,000 — is scrapped. The drilling of this single tube sheet takes weeks of continuous machining on a multi-spindle BTA drilling machine, with every hole position verified, every tool change logged, and every ligament measured at three depths by ultrasonic gauging. Tube sheet drilling is the highest-stakes production drilling operation in the heat exchanger industry, and the standards that govern it — TEMA and ASME — have been refined over a century of steam power generation.
Heat Exchanger and Boiler Applications
| Component | Typical Bore Diameter | Thickness / Length | Number of Holes | Material |
|---|---|---|---|---|
| Tube sheet (shell-and-tube) | 10–60 mm | 50–600 mm | 500–20,000 | SA-516 Gr.70, SA-508 Gr.3 |
| Tube support plate | 10–60 mm | 10–50 mm | 500–20,000 | Carbon or stainless steel |
| Boiler drum tube stub bore | 20–100 mm | 50–200 mm | 50–500 | SA-516 Gr.70 |
| Feedwater heater tube sheet | 12–25 mm | 100–300 mm | 1,000–5,000 | SA-516 Gr.70, SA-266 |
| Condenser tube sheet | 15–30 mm | 20–50 mm | 5,000–30,000 | Muntz metal, naval brass, titanium |
| Nuclear steam generator tube sheet | 15–25 mm | 400–600 mm | 5,000–15,000 | SA-508 Gr.3 Cl.2 |
| Air cooler tube sheet | 20–50 mm | 25–75 mm | 200–2,000 | Carbon steel |
Tube Sheet Drilling Fundamentals
Hole Geometry and Arrangement
| Parameter | Typical Value |
|---|---|
| Tube hole diameter | 10–60 mm |
| Tube sheet thickness | 50–600 mm |
| Drilling L/D ratio | 3:1–60:1 |
| Tube pitch | 1.25–1.50 × tube OD |
| Ligament width (between holes) | 3–10 mm |
| Hole pattern | Triangular (60°) or square (90°) |
| Surface finish (as-drilled) | Ra 0.8–3.2 µm |
TEMA Classification
| TEMA Class | Application | Quality Level |
|---|---|---|
| R | Refinery / severe service | Highest — full NDT, tight tolerances |
| C | General / commercial | Moderate — standard tolerances |
| B | Chemical process | High — between R and C |
Deep Hole Drilling Methods for Tube Sheets
Gun Drilling (Small Diameters: 5–30 mm)
For smaller tube holes in thinner tube sheets, gun drilling is the standard method:
| Parameter | Carbon Steel (SA-516) | Stainless Steel (304/316) | Titanium |
|---|---|---|---|
| Cutting speed | 60–90 m/min | 40–70 m/min | 25–40 m/min |
| Feed rate | 0.03–0.08 mm/rev | 0.02–0.06 mm/rev | 0.015–0.040 mm/rev |
| Coolant pressure | 80–150 bar | 80–150 bar | 60–120 bar |
| Tool grade | K15–K20, TiAlN | K15–K20, TiAlN or AlCrN | K10–K15, uncoated or DLC |
| Achievable tolerance | H8–H9 | H8–H9 | H8–H9 |
| Surface finish | Ra 0.8–1.6 µm | Ra 0.8–2.0 µm | Ra 0.6–1.6 µm |
BTA Drilling (Medium to Large Diameters: 15–60 mm)
For thicker tube sheets (≥ 100 mm) where productivity is critical, BTA single-tube drilling is preferred:
| Parameter | Carbon Steel | Stainless Steel | Nickel Alloy (Inconel) |
|---|---|---|---|
| Cutting speed | 50–80 m/min | 35–60 m/min | 15–25 m/min |
| Feed rate | 0.06–0.15 mm/rev | 0.04–0.10 mm/rev | 0.03–0.08 mm/rev |
| Coolant flow | 50–200 L/min | 50–200 L/min | 40–150 L/min |
| Coolant pressure | 20–60 bar | 20–60 bar | 30–80 bar |
| Insert grade | K20, TiAlN | K20, AlCrN | K20–K30, AlCrN |
| Achievable tolerance | H9–H10 | H9–H10 | H9–H10 |
Multi-Spindle Configurations
Production tube sheet drilling uses multi-spindle machines to drill multiple holes simultaneously:
| Machine Type | Spindles | Hole Diameter Range | Typical Application |
|---|---|---|---|
| Gantry-type CNC drilling machine | 1–4 | 10–60 mm | General tube sheets |
| Multi-spindle BTA drilling machine | 2–8 | 15–40 mm | Nuclear steam generators |
| Single-spindle deep hole drilling | 1 | 5–100 mm | Small batches, thick sheets |
| Radial arm drill (manual) | 1 | 10–80 mm | Low-volume, repair |
Process Sequence for Tube Sheet Drilling
- Positioning — Tube sheet blank positioned on machine table, levelled and clamped
- Centre reference — Reference hole or centre mark established
- Program loading — CNC program with hole pattern coordinates loaded
- Index drilling — Machine indexes through all hole positions in optimised sequence
- In-process gauging — Hole diameter and position measured at intervals
- Bond/ligament measurement — Ultrasonic ligament check at three depths
- Deburring — Entry and exit deburr, both sides
- Final inspection — Full dimensional verification, borescope if specified
Two-Step Precision Drilling
For thick tube sheets where positional accuracy is critical, the two-step method (Mitsubishi Heavy Industries, US Patent 9,321,110) compensates for thermal expansion and workpiece deformation.
The Problem
During continuous drilling of 5,000–15,000 holes in a thick tube sheet:
- Weight imbalance — as holes are drilled, the mass distribution changes, shifting the plate on its supports
- Thermal expansion — heat from continuous drilling causes differential expansion across the plate
- Residual stress relief — drilling through a forged or rolled plate releases internal stresses, causing local distortion
- Result — hole positions drift by 0.1–0.3 mm from programmed coordinates
The Two-Step Solution
| Step | Operation | Purpose |
|---|---|---|
| 1 | Drill temporary pilot hole (smaller diameter, shallow depth) | Establish location at the entry face |
| 2 | Measure distance from temporary hole to nearest completed hole | Calculate actual position offset |
| 3 | Correct drill coordinates | Compensate for thermal/mechanical shift |
| 4 | Drill final hole to full diameter and depth | Achieve accurate position |
| 5 | Repeat for each hole or group of holes | Continuous compensation |
The process is iterative — for very thick tube sheets (> 400 mm), multiple temporary holes at increasing depths may be drilled, each measured and corrected before proceeding.
TIP
The two-step method is standard practice for nuclear steam generator tube sheets, where the ligament tolerance is ±0.10 mm and the cost of a single drift error is a scrapped forging worth €500,000. The incremental cost of the additional drilling step is negligible compared to the insurance it provides against positional drift — estimated at 0.5–1.0% additional machine time for 100% positional verification.
Tube Sheet Materials
Material Selection Guide
| Material | Standard | Tensile Strength | Hardness | Application |
|---|---|---|---|---|
| SA-516 Gr.70 | ASME II | 485–620 MPa | 150–200 HB | General heat exchangers |
| SA-508 Gr.3 Cl.2 | ASME II | 550–690 MPa | 200–250 HB | Nuclear steam generators |
| SA-266 Gr.2 | ASME II | 485–655 MPa | 150–200 HB | Feedwater heaters |
| SA-240 304/316 | ASME II | 515–690 MPa | 150–220 HB | Corrosive service |
| SA-240 321 | ASME II | 515–690 MPa | 150–220 HB | High-temperature service |
| SB-265 Gr.2 (Ti) | ASME II | 345–450 MPa | 150–200 HB | Condenser, seawater |
| SB-171 (naval brass) | ASME II | 310–450 MPa | 100–150 HB | Condenser tube sheets |
Clad Tube Sheets
For corrosive service, tube sheets are often manufactured as clad plates — a carbon steel base plate with a corrosion-resistant alloy layer (stainless steel, Inconel, or titanium) explosively bonded or weld-overlaid on the tube-side face. Drilling parameters must account for the different machinability of the clad layer:
| Cladding | Layer Thickness | Drilling Challenge | Solution |
|---|---|---|---|
| Stainless steel | 5–15 mm | Work-hardening at entry | Increase speed by 20% in clad zone |
| Inconel | 5–10 mm | High cutting forces | Reduce feed by 30% in clad zone |
| Titanium | 5–10 mm | BUE at entry, galling | DLC coating, high coolant pressure |
Drill Drift and Ligament Control
Drill Drift Formula (TEMA)
TEMA specifies a drill drift tolerance that accounts for the natural tendency of a drill to deviate from its entry point as it progresses through the tube sheet:
Drill drift = 0.0016 × (tube sheet thickness / tube diameter) (in inches)
In metric units: Drill drift = 0.0016 × (t / d) × 25.4 (mm), where t = thickness in mm, d = tube diameter in mm.
| Tube Sheet Thickness | Tube Dia. | Drill Drift (TEMA) | Minimum Ligament |
|---|---|---|---|
| 100 mm | 20 mm | 0.20 mm | 3.5 mm |
| 300 mm | 20 mm | 0.61 mm | 4.5 mm |
| 500 mm | 20 mm | 1.02 mm | 5.5 mm |
Ligament Requirements
| Requirement | Value |
|---|---|
| Standard ligament (96% of all ligaments) | ≥ p − d_h − (2 × drill_drift + 0.030") |
| Minimum ligament (no hole in tube sheet) | ≥ -0.001 + 0.510 × (p − d_h) |
| Inspection frequency | 100% of ligaments at 3 depths |
| Inspection method | Ultrasonic (UT) or CMM probe |
| Acceptance | 96% ≥ standard, 100% ≥ minimum |
Burr and Surface Defects in Ligaments
| Defect | Cause | Consequence |
|---|---|---|
| Ligament breach | Drill drift beyond tolerance | Tube sheet scrapped |
| Burr in ligament | High feed at exit | Tube insertion interference |
| Rough surface in ligament | Worn drill | Tube-to-tube sheet joint leakage |
| Step in hole (two-step drilling) | Misalignment between pilot and final | Tube expansion defect |
Boiler Drum and Collector Drilling
Boiler drums and headers require drilled holes for tube stubs, instrument connections, and manway openings:
Boiler Tube Stub Hole Drilling
| Parameter | Typical Value |
|---|---|
| Hole diameter | 20–100 mm |
| Drum wall thickness | 50–200 mm |
| Hole pattern | Single or multiple rows (staggered) |
| Number of holes per drum | 50–500 |
| Positional tolerance | ±0.5 mm |
| Diameter tolerance | H10–H12 |
| Surface condition | Chamfered on inside and outside |
Drilling Method
Boiler drum holes are typically drilled with twist drills or annular cutters on radial arm or CNC drilling machines:
| Step | Operation | Purpose |
|---|---|---|
| 1 | Layout or CNC position | Establish hole locations |
| 2 | Centre drill | Prevent drill wander on curved surface |
| 3 | Pilot drill | Drill to 0.5× final diameter |
| 4 | Final drill or annular cut | Drill to full diameter |
| 5 | Chamfer (inside and out) | Remove sharp edges (ASME Section I) |
| 6 | NDT | Magnetic particle of drilled area |
Drum Material Considerations
| Material | Standard | Typical Hardness | Machinability |
|---|---|---|---|
| SA-516 Gr.70 | ASME II | 150–200 HB | Good |
| SA-299 | ASME II | 160–210 HB | Good |
| SA-302 Gr.B | ASME II | 180–230 HB | Good-moderate |
Quality Standards
Applicable Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| TEMA 10th Edition | Tubular heat exchangers | Hole tolerances, ligament widths, drilling quality |
| ASME Section VIII Div.1 | Pressure vessels | Tube sheet design, tube-to-tube sheet joints |
| ASME Section I | Power boilers | Boiler drum construction, tube hole drilling |
| ASME Section III | Nuclear components | Steam generator tube sheet requirements |
| ASME B16.5 | Flanges (for nozzle connections) | Bolt hole drilling tolerances |
Inspection Requirements
| Inspection | Component | Method | Frequency | Acceptance | |---|---|---|---|---|---| | Hole diameter | Tube sheet | Plug gauge or air gauge | 100% | +0.10/0 mm (typical) | | Hole position | Tube sheet | Coordinate measurement | 100% or sampling | ±0.10 mm | | Ligament width | Tube sheet | Ultrasonic at 3 depths | 100% of ligaments | Per TEMA table | | Surface finish | Tube sheet | Profilometer (sampling) | Per specification | Ra ≤ 1.6–3.2 µm | | Burr condition | Tube sheet | Visual / borescope | 100% | No burrs | | MPT | Boiler drum | Wet fluorescent | 100% of hole areas | No linear indications | | Pressure test | Completed unit | Hydrostatic | 100% | 1.3× design pressure |
Common Defects and Troubleshooting
| Defect | Cause | Corrective Action |
|---|---|---|
| Drill drift (ligament breach) | Excessive feed, worn drill | Reduce feed, replace drill at scheduled interval |
| Hole position drift across pattern | Thermal expansion of tube sheet | Implement two-step drilling, improve coolant cooling, sequence holes to balance heat |
| Oversize hole (entry only) | Drill chatter at start | Use drill bushing, improve entry support |
| Undersize hole (exit) | Tool wear at depth | Schedule tool change before wear limit |
| Burr at hole exit | Feed too high at breakthrough | Reduce feed in last 5 mm, back-up plate |
| Rough surface in hole | Built-up edge on stainless/Ni alloy | Increase coolant pressure, change coating |
| Out-of-round hole | Uneven cutting, worn guide pads | Replace guide pads, check drill condition |
| Tube insertion interference | Burr or out-of-tolerance hole | Ream or hone individual holes |
FAQ
Q: What standard governs heat exchanger tube sheet drilling? TEMA (Tubular Exchanger Manufacturers Association) Standards, 10th Edition, is the governing standard. It specifies hole diameter tolerances, ligament widths, surface finish, and drill drift allowances. ASME Section VIII Div.1 governs the pressure vessel design and tube-to-tube sheet joint requirements.
Q: How many holes are drilled in a typical tube sheet? From several hundred (small air coolers) to 30,000 (large condensers). A typical shell-and-tube heat exchanger has 500–2,000 holes. A nuclear steam generator tube sheet has 5,000–15,000 holes. A large power plant condenser tube sheet can have 20,000–30,000 holes.
Q: What drilling methods are used for tube sheets? Gun drilling (for smaller diameters, 5–30 mm) and BTA drilling (for larger diameters, 15–60 mm) are the primary methods. Multi-spindle CNC gantry machines and dedicated deep hole drilling machines are used for production. For very thick tube sheets (> 400 mm), BTA drilling with two-step position compensation is standard.
Q: What is drill drift in tube sheet drilling? Drill drift is the deviation of the drill from its intended path as it progresses through the tube sheet thickness. TEMA specifies a formula: drill drift = 0.0016 × (tube sheet thickness / tube diameter in inches). For a 500 mm thick × 20 mm hole, the allowable drift is approximately 1.0 mm.
Q: What is the ligament width in a tube sheet? The ligament is the wall of material between two adjacent tube holes. It must be wide enough to maintain structural integrity under operating pressure and tube expansion forces. TEMA specifies minimum ligament widths based on tube diameter, pitch, and tube sheet thickness.
Q: What materials are used for heat exchanger tube sheets? SA-516 Gr.70 carbon steel is the most common. For corrosive service, SA-240 304/316 stainless steel, titanium (SB-265), or clad materials are used. Nuclear steam generators use SA-508 Gr.3 Cl.2 low-alloy steel forgings with Inconel cladding.
Q: What is the two-step drilling method? A precision drilling method (US Patent 9,321,110 by Mitsubishi) that drills a temporary pilot hole, measures its actual position relative to adjacent completed holes, corrects the drilling coordinates to compensate for thermal expansion and workpiece deformation, then drills the final hole. Used for thick nuclear-grade tube sheets.
Q: How are tube sheet ligaments inspected? By ultrasonic gauging — a probe is passed through each drilled hole to measure the distance to adjacent holes at three depths (entry, mid-thickness, exit). The ligament reading must meet TEMA minimum requirements at all depths.
Q: What happens if a ligament is breached? If the drill drifts far enough to break through the ligament into the adjacent hole, the tube sheet is typically scrapped. The cost of scrapping a large tube sheet forging can exceed €500,000 for nuclear-grade components. This is why two-step drilling and frequent tool changes are standard practice for thick tube sheets.
Q: Can tube holes be repaired if out of tolerance? Minor out-of-tolerance holes can be reamed oversize with custom oversized tubes, provided the reduced ligament still meets TEMA minimum requirements. Holes that are out of position (causing ligament breach) or severely oversized cannot be repaired and require the tube sheet to be scrapped.