Stainless Steel Coil Tube – Mechanical & Hydraulic Expansion Jointing System
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchanger tube designed specifically for coil-type heat exchange assemblies. This product falls under the Installation & Maintenance category, with a primary focus on the Expansion Jointing (胀接连接) process — the critical interface between the heat exchanger tube and the tube sheet (Tube Sheet / 管板). The mechanical and hydraulic expansion process ensures both sealing integrity and structural strength at the tube-to-tubesheet joint, which is the most failure-prone region in shell-and-tube and coil heat exchanger designs.
This product knowledge point (No. 221) addresses the core engineering challenge: achieving a reliable metallurgical and mechanical bond between the stainless steel coil tube and the tube sheet through controlled plastic deformation, without inducing micro-cracking or stress corrosion sensitivity.
Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, commonly SAE 304 (UNS S30400) or SAE 316L (UNS S31603), selected for their superior corrosion resistance, formability, and weldability in aggressive process environments. The coil geometry is achieved through cold-rolling and coiling processes that maintain the material's grain structure and mechanical properties throughout the forming cycle.
Key Material Characteristics
- Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and general acid attack in aqueous and chemical environments
- Mechanical Strength: Yield strength typically 205–310 MPa (depending on grade and cold-work condition)
- Formability: High ductility enabling reliable expansion jointing without cracking
- Thermal Compatibility: Low thermal expansion coefficient (16–18 × 10⁻⁶ /°C) suitable for high-temperature differentials
- Surface Finish: Ra ≤ 0.8 μm for optimal expansion contact and seal performance
Specifications
| Parameter | Specification / Value | Standard / Reference |
|---|---|---|
| Product Category | Coil Tube (盘管) – Heat Exchanger Tube | — |
| Material Grade | SAE 304 / 316L / 321 / 310S (Austenitic Stainless Steel) | AISI / ASTM |
| Tube Outer Diameter (OD) | 12 mm – 51 mm (typical range) | ASTM A213 / A269 |
| Wall Thickness | 0.5 mm – 3.0 mm | — |
| Coil Inner Diameter | Custom per design requirement | — | Jointing Method | Mechanical Expansion (机械胀管) / Hydraulic Expansion (液压胀管) | ASTM E1924 / ASME PTC-27 |
| Expansion Rate (Critical) | 4% – 6% (controlled range) | Manufacturer Specification |
| Over-Expansion Limit | > 6% → Risk of micro-cracking / fracture | — |
| Joint Seal Pressure Rating | Up to 2.5 MPa (design-dependent) | ASME VIII Div. 1 |
| Operating Temperature Range | −196 °C to +870 °C (grade-dependent) | — |
| Surface Treatment | Pickled & Passivated / Mirror Polished (optional) | ASTM A967 |
Manufacturing Process
3.1 Coil Tube Fabrication
- Hot Rolling: Stainless steel billets are hot-rolled into seamless pipe stock conforming to ASTM A213 or A269 specifications.
- Cold Drawing / Seamless Forming: Tubes are cold-drawn to achieve precise dimensional tolerances (±0.05 mm) and surface finish requirements.
- Coil Forming: The straight tube is cold-coiled into the required helical geometry using CNC-controlled coiling equipment. The coil pitch and diameter are set according to the heat exchanger design.
- Solution Annealing: Post-coiling solution annealing (1010–1150 °C, water quench) restores full ductility and eliminates cold-work hardening, ensuring the tube can undergo expansion jointing without cracking.
- Pickling & Passivation: Removal of mill scale and formation of a protective chromium oxide layer per ASTM A967.
3.2 Expansion Jointing Process (Core Technology)
The expansion jointing process is the defining quality-determining step for the coil tube-to-tubesheet connection. Two primary methods are employed:
Method A: Mechanical Expansion (机械胀管)
- A tapered mandrel or roller-type expander is inserted into the tube end protruding beyond the tube sheet.
- Radial outward force is applied, causing the tube wall to undergo controlled plastic deformation against the tube sheet hole wall.
- The expansion rate is precisely controlled to achieve 4%–6% radial interference (measured as the ratio of tube wall thickness increase to original wall thickness).
- Post-expansion, the joint is pressure-tested to verify seal integrity.
Method B: Hydraulic Expansion (液压胀管)
- The tube end is sealed at both ends and pressurized with hydraulic fluid (typically 50–200 MPa).
- Internal hydrostatic pressure causes uniform radial expansion of the tube wall into the tube sheet hole.
- Hydraulic expansion offers superior uniformity compared to mechanical expansion, particularly for thicker-walled tubes or hard-to-reach joints.
- Expansion rate is monitored in real-time via pressure-feedback systems.
3.3 Critical Process Control – Expansion Rate Management
Engineering Principle: The expansion rate must be maintained within the 4%–6% window. This represents the optimal balance between:
- Sealing Performance: Sufficient plastic deformation to create a gas-tight, liquid-tight seal against the tube sheet
- Structural Integrity: Adequate mechanical interlock to resist axial pull-out forces and thermal cycling fatigue
- Avoidance of Over-Expansion: Expansion beyond 6% induces excessive residual tensile stresses, leading to micro-cracking, stress corrosion cracking (SCC), and premature joint failure
3.4 Post-Jointing Quality Verification
- Pressure Testing: Hydrostatic or pneumatic test at 1.5× design pressure to verify seal integrity
- Dimensional Inspection: Measurement of expanded tube outer diameter to confirm expansion rate is within 4%–6%
- Non-Destructive Testing (NDT): Dye penetrant inspection (DPI) or eddy current testing to detect micro-cracks at the expansion zone
- Leak Testing: Helium leak testing for critical applications (leak rate ≤ 1×10⁻⁶ Pa·m³/s)
Applications
The coil tube with expansion-jointed tube sheet connection is widely deployed in the following industries:
- Oil & Gas Processing: Crude oil heat exchangers, gas-cooled condensers, and process fluid heaters
- Chemical & Petrochemical: Reactor cooling systems, distillation column reboilers, and evaporators
- Power Generation: Steam generators, feedwater heaters, and turbine cooling systems
- Food & Pharmaceutical: Pasteurization equipment, sanitary heat exchangers, and CIP (Clean-in-Place) systems
- Marine & Shipbuilding: Seawater coolers, condenser tubes, and boiler systems
- Nuclear Energy: Intermediate heat exchangers and secondary circuit heat transfer systems
Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System |
| ASME PTC-27 | Standard for heat exchanger tube-to-tubesheet joints (expansion jointing) |
| ASTM E1924 | Standard practice for hydraulic expansion of tubes into tubesheets |
| ASTM A213 / A269 | Standard specifications for seamless austenitic stainless steel boiler, heat-exchanger, and condenser tubes |
| EN 10216-5 | European standard for heat-exchanger tubes of non-ferrous metals and steels |
| GB/T 12771 | Chinese national standard for seamless stainless steel tubes for fluid transport |
| API 5L / API 5CT | American Petroleum Institute standards for line pipe and casing (where applicable) |
| CE Marking | European Conformity marking for pressure equipment (PED 2014/68/EU) |
Unique Selling Points
- Precision Expansion Control: Strict adherence to the 4%–6% expansion rate window ensures optimal joint performance while eliminating the risk of over-expansion cracking — a common failure mode in the industry
- Dual-Method Flexibility: Availability of both mechanical and hydraulic expansion options allows selection based on tube diameter, wall thickness, material grade, and access constraints
- Sealing + Strength Dual Assurance: The expansion jointing process simultaneously provides a hermetic seal and a mechanically robust connection, eliminating the need for secondary welding in many applications
- Full Traceability: Each coil tube batch is traceable from raw material mill certification through final expansion jointing inspection, providing complete quality documentation
- Corrosion-Resistant Integrity: The expansion process does not compromise the passive chromium oxide layer when performed correctly, maintaining long-term corrosion resistance at the critical joint interface
- Custom Engineering Support: Wenzhou Jixing Metal Materials Co., Ltd. provides design consultation for expansion parameters tailored to specific service conditions, including high-temperature, high-pressure, and cryogenic applications