Stainless Steel Coil Tube (Coil Tube) for Once-Through Steam Generator

Product Overview

The Stainless Steel Coil Tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component designed for use in Once-Through Steam Generators (OTSG) and advanced nuclear/fossil-fuel power generation systems. The coil tube is formed into a continuous helical geometry that serves as the primary heat transfer surface within the steam generator, where feedwater is heated, vaporized, and superheated in a single pass through the two-phase flow region.

This product is specifically engineered to withstand the rigorous thermal-hydraulic conditions of the two-phase flow section of a once-through steam generator, where accurate prediction of heat transfer coefficients, pressure drop, and flow stability is critical to safe and efficient operation. The coil tube design eliminates the need for internal circulation pumps, offering a compact, high-efficiency alternative to conventional boiling water reactor (BWR) or pressurized water reactor (PWR) steam generator geometries.

Material & Properties

The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional corrosion resistance, thermal stability, and mechanical integrity under cyclic thermal loading conditions.

• Base Material: Type 304 (UNS S30400) or Type 316L (UNS S31603) austenitic stainless steel, conforming to ASTM A213 / ASTM A269 specifications
• Alternative Alloy Options: Inconel 690 (UNS N06690), Incoloy 800H (UNS N08810), or Alloy 625 (UNS N06625) for enhanced resistance to stress corrosion cracking (SCC) and high-temperature oxidation
• Key Mechanical Properties: Yield strength ≥ 205 MPa (304 grade); Tensile strength ≥ 515 MPa; Elongation ≥ 40%
• Thermal Conductivity: Approximately 16.3 W/(m·K) at 100°C, ensuring efficient heat transfer across the tube wall
• Corrosion Resistance: Excellent resistance to pitting, crevice corrosion, and intergranular corrosion in aqueous and steam environments
• Maximum Operating Temperature: Up to 870°C (oxidation limit for 304 grade); up to 1100°C for specialty alloy variants

Specifications

Parameter	Specification
Product Name	Stainless Steel Coil Tube (Coil Tube)
Product Code / Serial No.	1080
Manufacturer	Wenzhou Jixing Metal Materials Co., Ltd.
Tube Outer Diameter (OD)	15.0 mm – 50.0 mm (customizable per design requirement)
Tube Wall Thickness	1.0 mm – 3.0 mm
Coil Pitch (Helix Spacing)	15 mm – 100 mm (adjustable per thermal design)
Coil Diameter (Helix Mean Diameter)	0.5 m – 4.0 m (project-specific)
Tube Length per Coil	Up to 200 m continuous length
Material Grade	ASTM A213 TP304 / TP316L / Inconel 690 / Incoloy 800H
Welding Process	Gas Tungsten Arc Welding (GTAW) / Orbital TIG Welding
Surface Finish	Electropolished (Ra ≤ 0.4 μm) or Pickled & Passivated
Pressure Rating	Up to 25 MPa (design dependent)
Applicable Standards	ASTM A213, ASTM A269, ASME BPV Section III, EN 10216-5, GB/T 14976
Heat Treatment	Solution Annealing (1010°C – 1150°C) followed by water quenching
Non-Destructive Testing (NDT)	100% Radiographic Testing (RT), Ultrasonic Testing (UT), Helium Leak Testing
Operating Environment	Two-phase flow region of Once-Through Steam Generator (OTSG)
Design Temperature Range	−50°C to 650°C (standard); up to 900°C (specialty alloys)

Manufacturing Process

The production of the Stainless Steel Coil Tube follows a rigorous multi-stage manufacturing workflow designed to ensure geometric precision, metallurgical integrity, and compliance with nuclear-grade quality standards.

1. Raw Material Selection & Mill Certification:

   Virgin stainless steel seamless tubes are sourced from certified mills with full material traceability. Each heat lot is accompanied by a Mill Test Certificate (MTC) verifying chemical composition (per ASTM E1/E4) and mechanical properties.

2. Material Verification & Incoming Inspection:

   Incoming tubes undergo spectroscopic analysis (OES), hydrostatic pressure testing, and dimensional inspection to confirm compliance with specified grade and tolerance requirements.

3. Heat Treatment (Solution Annealing):

   Tubes are solution annealed at 1010°C – 1150°C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses, and ensure maximum ductility and corrosion resistance.

4. Coil Forming / Helical Bending:

   The straight tube is precisely formed into a helical coil geometry using CNC-controlled rotary draw bending machines. The forming process maintains uniform wall thickness reduction (≤ 10% thinning) and controls springback through calibrated tooling and compensating die profiles. Coil pitch and helix diameter are held within ±0.5 mm tolerance.

5. Welding & Joining:

   End connections and intermediate joints are fabricated using Orbital Gas Tungsten Arc Welding (GTAW) with inert gas shielding (Argon) on both inside and outside surfaces. Weld parameters are optimized for full penetration and minimal heat-affected zone (HAZ) distortion. Weld beads undergo 100% radiographic inspection per ASME Section V, Article 2.

6. Post-Weld Heat Treatment (PWHT):

   Where required by applicable codes (e.g., ASME BPV Section III, Division 1), post-weld heat treatment is performed to relieve welding residual stresses and restore mechanical properties in the HAZ region.

7. Surface Treatment:

   Tubes receive electropolishing (to achieve surface roughness Ra ≤ 0.4 μm) or pickling and passivation treatment to remove scale, contamination, and to establish a protective chromium oxide layer. This is critical for preventing flow-induced corrosion and ensuring clean heat transfer surfaces in the two-phase flow region.

8. Non-Destructive Examination (NDE):

   Comprehensive NDE includes 100% radiographic testing of all welds, ultrasonic thickness measurement at defined intervals, and helium mass spectrometry leak testing (leak rate ≤ 1×10⁻⁹ atm·cc/s) to verify pressure boundary integrity.

9. Final Dimensional Inspection & Hydrostatic Testing:

   Each coil assembly undergoes final dimensional verification (coil pitch, helix diameter, overall length) and hydrostatic pressure testing at 1.5× design pressure for a minimum hold time of 30 minutes.

10. Packaging & Shipping:

    Certified coils are wrapped in protective materials, secured on wooden or steel cradles, and shipped with complete documentation including MTCs, NDE reports, and dimensional certificates.

Applications

The Stainless Steel Coil Tube from Wenzhou Jixing Metal Materials Co., Ltd. is designed for demanding high-temperature, high-pressure thermal systems where reliable heat transfer and structural integrity are paramount:

• Once-Through Steam Generators (OTSG):

  Primary application in advanced power generation systems where feedwater undergoes complete vaporization in a single pass. The coil geometry provides uniform heat distribution and eliminates the need for recirculation, improving overall plant thermal efficiency.

• Nuclear Power Plants:

  Used in Generation III+ and Generation IV reactor designs (e.g., supercritical water reactors, high-temperature gas reactors) as fuel channel tubes or heat exchanger elements. The two-phase flow calculation model referenced in the product documentation is critical for predicting boiling heat transfer coefficients, critical heat flux (CHF) limits, and pressure drop characteristics within the coil geometry.

• Fossil Fuel Power Plants:

  Supercritical and ultra-supercritical (USC) boiler systems where steam is generated at pressures exceeding 25 MPa and temperatures above 600°C. Coil tubes serve as evaporator and superheater elements.

• Industrial Process Heat Exchangers:

  Chemical processing, petroleum refining, and pharmaceutical manufacturing facilities requiring clean, corrosion-resistant heat exchange surfaces for high-purity applications.

• Marine & Offshore Platforms:

  Boiler systems and waste heat recovery units on naval vessels and offshore oil/gas production platforms.

• Renewable Energy Systems:

  Concentrated solar power (CSP) plants and geothermal energy conversion systems where stainless steel coil heat exchangers interface with high-temperature heat transfer fluids.

Two-Phase Flow Design Considerations

The design of coil tubes for the two-phase flow section of a once-through steam generator requires rigorous thermal-hydraulic modeling. Key parameters include:

• Heat Transfer Coefficient: Determined using correlations such as Chen's correlation or Kim et al.'s modified correlation for helical coil geometries, accounting for enhanced heat transfer due to secondary flow (Dean vortices) induced by tube curvature.
• Pressure Drop: Calculated using homogeneous flow model or separated flow model (Lockhart-Martinelli correlation) to account for frictional and gravitational components in the helical geometry.
• Critical Heat Flux (CHF) / Departure from Nucleate Boiling (DNB): Evaluated using correlations such as Bowring, W-3, or Katto-Ohno to ensure adequate heat flux margins and prevent film boiling or dryout.
• Flow Stability: Analysis of density-wave oscillations and thermohydraulic instabilities inherent to once-through designs, particularly at high heat flux conditions in the transition and supercritical flow regimes.

Quality Certifications & Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring full traceability and compliance with international standards:

• ISO 9001:2015 — Quality Management Systems certification for manufacturing excellence
• ISO 3834-2 — Quality requirements for fusion welding of metallic materials (welding process certification)
• ASME "N" / "NPT" Stamp — Authorization for nuclear components and nuclear piping (where applicable)
• ASME BPV Section III — Compliance with rules for construction of nuclear power plant components
• EN 10204 Type 3.1 Certificate — Third-party inspection and testing certificates
• NADCAP (National Aerospace and Defense Contractors Accreditation Program) — For aerospace-qualified welding and heat treatment processes (where applicable)
• PED (Pressure Equipment Directive) 2014/68/EU — European conformity marking for pressure equipment
• CRN (Certification of Registration Number) — Canadian certification for pressure equipment
• 100% NDE Coverage — Radiographic Testing (RT), Ultrasonic Testing (UT), Dye Penetrant Testing (PT), and Helium Leak Testing per ASME Section V

Unique Selling Points

• Specialized Two-Phase Flow Expertise: Products are designed with reference to validated two-phase flow calculation models for once-through steam generators, ensuring optimal thermal-hydraulic performance in the critical boiling and superheating zones.
• Precision Helical Geometry: CNC-controlled coil forming ensures consistent pitch and diameter tolerances, critical for uniform heat distribution and predictable flow distribution in parallel flow channels.
• Full Material Traceability: From virgin mill ingot to finished coil, every component is traceable with complete documentation of chemical composition, mechanical properties, and heat treatment history.
• Nuclear-Grade Manufacturing: Processes and quality systems aligned with ASME BPV Section III requirements, suitable for safety-critical applications in nuclear and power generation industries.
• Custom Alloy Selection: Flexibility to supply standard austenitic stainless steels (304, 316L) or advanced alloys (Inconel 690, Incoloy 800H, Alloy 625) tailored to specific environmental and performance requirements.
• Comprehensive NDE Protocol: 100% non-destructive examination coverage ensures zero-defect pressure boundary integrity, exceeding minimum code requirements.