Stainless Steel Coil Tube for Ammonia Synthesis Heating Furnace

Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product Code: 952 | Classification: Process Engineering Literature & Technical Reference

1. Product Overview

The stainless steel coil tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a specialized heat exchanger component designed for high-temperature, high-pressure service environments, particularly within ammonia synthesis plant heating furnaces. This product represents a significant engineering innovation in the field of process piping and heat transfer systems, developed through extensive technical improvement programs targeting reliability, thermal efficiency, and extended service life under demanding operating conditions.

Unlike conventional straight pipe assemblies, coil tubes are formed into continuous helical or serpentine configurations that maximize the heat transfer surface area within a compact footprint. This geometric advantage is critical in ammonia synthesis processes, where precise temperature control of process gases (hydrogen, nitrogen, and ammonia mixtures) directly impacts conversion efficiency and catalyst performance. The coil tube design eliminates the need for numerous bends, reducers, and connections, thereby reducing potential failure points and enhancing overall system integrity.

Engineering Significance: The development and manufacturing technology improvement of heating furnace coil tubes for ammonia synthesis units represents a pivotal innovation in petrochemical process engineering, addressing long-standing challenges related to thermal fatigue, oxidation resistance, and mechanical deformation under cyclic thermal loading.

2. Material & Properties

2.1 Base Alloy Composition

The coil tubes are fabricated from austenitic stainless steel grades engineered for exceptional resistance to high-temperature oxidation, thermal cycling, and internal scale formation. The primary material specifications include:

• Grade 310S (UNS S31008): High-temperature austenitic stainless steel with elevated nickel and chromium content, offering superior oxidation resistance up to 1150°C.
• Grade 309 (UNS S30900): General-purpose high-temperature alloy with excellent weldability and resistance to thermal cracking.
• Grade 347H (UNS S34700): Stabilized austenitic grade with niobium addition, resistant to intergranular corrosion and sensitization during prolonged heat exposure.
• Custom Alloy Grades: Modified compositions tailored to specific ammonia synthesis operating parameters, including enhanced silicon and aluminum additions for improved internal scale resistance.

2.2 Mechanical Properties

Property	Specification	Test Standard
Tensile Strength (Rm)	≥ 520 MPa (at 20°C)	ASTM A213 / GB/T 12771
Yield Strength (Rp0.2)	≥ 205 MPa	ASTM A213 / GB/T 12771
Elongation (A)	≥ 35%	ASTM A213
High-Temperature Creep Strength (1000°C / 1000h)	≥ 35 MPa	ASTM E139
Thermal Conductivity (at 600°C)	24–26 W/(m·K)	ASTM E1461
Thermal Expansion Coefficient (20–800°C)	16.5–18.5 × 10−6 /°C	ASTM E228
Hardness (HBW)	150–200	ASTM E10

2.3 Chemical Composition (Typical — Grade 310S)

Element	C	Mn	S	P	Cr	Ni	Si	N	Fe
Max/Min (%)	0.08	2.0	0.03	0.045	24.0–26.0	19.0–22.0	1.5	0.10	Balance

3. Technical Specifications

Parameter	Specification Range	Notes
Outer Diameter (OD)	25 mm – 168 mm	Custom sizes available upon request
Wall Thickness	2.0 mm – 8.0 mm	Tolerance: ±10% per ASTM A213
Coil Pitch (Helical)	80 mm – 350 mm	Designed per furnace geometry
Coil Diameter	300 mm – 2000 mm	Dependent on furnace loading arrangement
Tube Length (Developed)	Up to 30,000 mm (continuous)	Seamless manufacturing capability
Operating Temperature	−40°C to 1150°C	Continuous service range
Operating Pressure	Up to 35 MPa (gauge)	Design per ASME B31.3
Surface Finish	Mill finish / Polished (Ra ≤ 0.8 μm)	Internal and external
End Preparation	Beveled, flat-cut, or threaded	Per customer specification
Heat Treatment	Solution annealed + Pickle	1050–1100°C, water quench
Welding Compatibility	GTAW, SMAW, SAW, FCAW	WPS per ASME Section IX

4. Manufacturing Process

4.1 Process Flow

1. Raw Material Selection & Inspection: Certified seamless stainless steel tubes are procured from qualified mills, with full chemical analysis and mechanical property verification prior to processing.
2. Dimensional Inspection & Sorting: Incoming tubes undergo ultrasonic testing (UT) for internal defects and optical measurement of OD, wall thickness, and straightness per ASTM A213 requirements.
3. Coil Forming (Cold or Hot): Tubes are formed into the required helical or serpentine geometry using precision CNC coil-bending machines. Hot forming is employed for larger diameters or tighter bend radii to avoid work-hardening and residual stress accumulation.
4. Stress Relief Annealing: Formed coils are subjected to controlled heat treatment (950–1050°C) to relieve residual stresses introduced during forming, restoring full ductility and preventing distortion during subsequent welding or service.
5. Welding & Assembly: Coil tubes are welded to headers, manifolds, and support structures using qualified welding procedures (WPS) compliant with ASME Section IX and applicable Chinese national standards (GB/T 985).
6. Non-Destructive Testing (NDT): All weld joints undergo radiographic testing (RT) or ultrasonic testing (UT) per ASME Section V, Article 4 or Article 5, with acceptance criteria per ASME Section VIII or customer specifications.
7. Hydrostatic Pressure Testing: Completed coil assemblies are pressure-tested at 1.5× the design pressure (minimum 52.5 MPa) with a hold time of not less than 30 minutes, per ASME B31.3 and GB 50316.
8. Final Surface Treatment & Packaging: Tubes receive pickling and passivation treatment to remove oxidation scale and restore corrosion-resistant chromium oxide layer. Final products are protected with desiccant, wrapped in moisture-barrier material, and packaged for shipment.

4.2 Key Technology Innovations

• Optimized Coil Geometry Design: Computational fluid dynamics (CFD) modeling is employed to optimize coil pitch, diameter, and number of passes, maximizing heat transfer efficiency while minimizing pressure drop and thermal stress concentrations.
• Advanced Forming Techniques: Multi-axis CNC forming equipment ensures dimensional accuracy within ±0.5 mm tolerance for coil pitch and ±1 mm for coil diameter, critical for proper furnace loading and alignment.
• Microstructure Control: Solution annealing parameters are precisely controlled to achieve a fully austenitic microstructure with optimal grain size (ASTM Grain Size 6–8), enhancing both high-temperature strength and thermal fatigue resistance.
• Welding Process Optimization: Pre-weld heat input control and interpass temperature monitoring (≤ 250°C) prevent sensitization and ensure uniform weld metal properties throughout the assembly.

5. Applications

5.1 Primary Applications

• Ammonia Synthesis Heating Furnaces: Primary application — coil tubes serve as process gas heaters in ammonia synthesis loop furnaces, where hydrogen-nitrogen-ammonia mixtures are heated to reaction temperatures (300–500°C) prior to catalytic conversion.
• Petrochemical Reforming Furnaces: Steam methane reforming (SMR) and secondary reforming furnace coil tubes for hydrogen production.
• Cracking Furnaces: Ethylene cracking and propylene production furnace tubes in olefin plants.
• Gas Turbine Combustors: Fuel heating coils and air preheat tubes in industrial gas turbine systems.
• Pyrolysis and Calcination Systems: High-temperature process heating in lime kilns, cement calciners, and waste-to-energy facilities.

5.2 Operating Environment Characteristics

Parameter	Typical Ammonia Synthesis Furnace	Reforming Furnace
Tube Wall Temperature	900–1100°C	850–1050°C
Internal Gas Temperature	350–500°C	800–950°C
Internal Pressure	15–35 MPa	2.5–4.0 MPa
Internal Atmosphere	H2/N2/NH3 (reducing)	CH4/H2/H2O (reducing)
External Atmosphere	Flue gas (oxidizing)	Flue gas (oxidizing)
Design Life	10–15 years	10–20 years

6. Quality Certifications & Compliance

6.1 International Standards Compliance

• ASTM A213: Standard Specification for Seamless Austenitic Chromium-Nickel Stainless Steel Tube for High-temperature Service.
• ASME BPV Code Section I & VIII: Boiler and Pressure Vessel Code — applicable for pressure-retaining components.
• ASME B31.3: Process Piping Code — design, materials, and construction requirements.
• EN 10216-5: European standard for welded and seamless heat-resistant steel tubes.
• GB/T 12771: Chinese National Standard for seamless stainless steel tubes for heat exchangers.
• GB/T 5310: Chinese National Standard for seamless steel tubes for high-pressure boiler tubes.

6.2 Manufacturer Quality Certifications

• ISO 9001:2015: Quality Management System certification for manufacturing processes.
• ISO 3834-2: Quality requirements for welding of metallic materials — comprehensive certification.
• ASME "U" Stamp: Authorization for fabrication of pressure vessels and components.
• API Q1: Quality Management System for oil and gas industry suppliers.
• NADCAP: Special processes accreditation (if applicable for specific aerospace-adjacent applications).
• CE Marking: Conformity with European Pressure Equipment Directive (PED 2014/68/EU).

6.3 Inspection & Test Protocol

Test Method	Standard Reference	Scope
Chemical Analysis (Spectroscopy)	ASTM E415 / E1410	Every heat lot
Hydrostatic Testing	ASTM A213 / A269	100% of tubes
Ultrasonic Testing (UT)	ASTM E213 / E309	100% of tubes
Radiographic Testing (RT)	ASME V, Article 4	All weld joints
Penetrant Testing (PT)	ASME V, Article 6	Welds & formed bends
Hardness Testing	ASTM E10 / E18	Heat-affected zones
Corrosion Testing (Salt Spray)	ASTM B117	Type approval / periodic
Intergranular Corrosion Test	ASTM A2