Stainless Steel Radiation Coil Tube — Ethylene Cracking Furnace (Inlet/Outlet Insulation Design)

Product Code: 377  |  Category: Coiltube  |  Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.

1. Product Overview

The Radiation Coil Tube is a critical heat-exchange component installed within the radiation zone of an ethylene cracking furnace. Unlike convection-section tubes, radiation coil tubes are subjected to direct radiant heat flux from the furnace burner, operating under extreme thermal conditions that demand exceptional material integrity, dimensional precision, and reliable insulation at the tube inlet and outlet transition zones.

This product line is specifically engineered for the radiation section of steam-cracked ethylene furnaces, where hydrocarbon feedstocks are pyrolyzed at temperatures ranging from 750 °C to 900 °C under high-pressure conditions. The inlet and outlet sections of the radiation coil tube—where the tube transitions between the convection zone and the radiation zone—are particularly vulnerable to thermal stress concentration, differential expansion, and insulation degradation. Proper insulation design at these transition points is essential to minimize heat loss, prevent thermal shock, and ensure uniform temperature distribution across the coil tube wall.

Key Design Principle: The insulation at the radiation coil tube inlet and outlet must be designed to accommodate the maximum thermal expansion differential between the coil tube (expanding at up to ~12 mm per meter at 900 °C) and the surrounding furnace structure, while maintaining a continuous thermal barrier to prevent radiation heat bypass.

2. Material & Properties

2.1 Tube Body Material

The coil tube body is manufactured from austenitic stainless steels or nickel-based superalloys selected for their outstanding high-temperature oxidation resistance, thermal fatigue resistance, and creep strength under cyclic thermal loading conditions.

2.2 Inlet/Outlet Insulation Materials

The insulation system at the coil tube inlet and outlet is a multi-layer composite assembly designed to withstand the extreme thermal gradient between the hot radiation tube surface and the cooler furnace wall structure.

2.3 Key Mechanical and Thermal Properties

Property Grade 310 / 310S Alloy 601 Alloy 617
Maximum Continuous Service Temperature 1150 °C 1200 °C 1250 °C
Thermal Expansion (20–900 °C) ~11.5 mm/m ~12.0 mm/m ~12.2 mm/m
Tensile Strength at 900 °C ≥250 MPa ≥350 MPa ≥400 MPa
Thermal Conductivity at 800 °C ~25 W/(m·K) ~27 W/(m·K) ~28 W/(m·K)
Oxidation Resistance (Air, 1000 °C) Excellent Excellent Outstanding
Thermal Fatigue Resistance Good Very Good Excellent

3. Specifications

Parameter Specification / Range
Tube Outer Diameter (OD) 19.05 mm – 114.3 mm (¾" – 4½")
Tube Wall Thickness 2.11 mm – 6.35 mm (0.083" – 0.250")
Tube Length (Straight Segment) 6000 mm – 12000 mm
Coil Bending Radius ≥ 3 × OD (typical); minimum 2.5 × OD with special tooling
Operating Temperature (Radiation Zone) 750 °C – 900 °C
Operating Pressure 0.5 – 2.5 MPa (gauge)
Feedstock Naphtha, Ethane, Propane, Propane Dehydrogenation (PDH) feed
Insulation Thickness (Inlet/Outlet) 100 mm – 150 mm (multi-layer composite)
Insulation Maximum Service Temperature 1260 °C (ceramic fiber layer); 1400 °C (HPA board layer)
Heat Loss Through Insulation ≤ 50 W/m² at steady-state (surface temperature ≤ 150 °C)
Thermal Expansion Accommodation Up to 25 mm differential expansion at 900 °C
Surface Finish Pickled and passivated; Ra ≤ 3.2 μm (inner surface)
End Preparation Beveled ends (75°±5°) or flat ends per ASME B16.25

4. Manufacturing Process

4.1 Tube Fabrication

  1. Hot-Rolled Billet Preparation: Starting material is hot-rolled into seamless tube billets of the appropriate grade, with composition verified by spectroscopic analysis to meet ASTM/EN standards.
  2. Piercing and Hot Rolling: Billets are heated to 1200–1250 °C and pierced on a Mannesmann-type piercing mill, then hot-rolled to the target OD and wall thickness on a plug mill or rotary piercing mill.
  3. Heat Treatment (Solution Annealing): Tubes are solution-annealed at 1100–1150 °C followed by controlled water quenching to achieve a fully austenitic, strain-free microstructure with maximum corrosion and thermal fatigue resistance.
  4. Pickling and Passivation: Surface scale is removed by pickling in mixed acid (HNO₃ + HF or H₂SO₄ + HNO₃), followed by passivation treatment to form a protective chromium oxide layer.
  5. Coil Bending (Cold or Hot): Tubes are formed into the required coil geometry using CNC-controlled bending equipment. Hot bending at 1100 °C may be employed for tight radii to prevent work hardening and cracking.
  6. Welding of Butt Joints: Where coil tubes are fabricated from multiple straight segments, butt welds are performed using GTAW (Gas Tungsten Arc Welding) with matching filler metal (e.g., ER310 orERNiCrFe-6). Welds are 100% radiographically inspected per ASME Section V.

4.2 Inlet/Outlet Insulation Design and Installation

  1. Thermal Stress Analysis: Finite Element Analysis (FEA) is conducted to determine the thermal expansion profile of the coil tube at the inlet and outlet transition zones, identifying maximum stress concentrations and displacement vectors.
  2. Insulation Layer Layout Design: A multi-layer insulation system is designed with:
    • Inner RCF blanket layer (50–75 mm) for primary thermal resistance;
    • HPA insulation board layer (25–50 mm) for structural rigidity and thermal stability;
    • Outer refractory castable/mortar shell (25–50 mm) for mechanical protection and windage resistance;
    • Expansion joint provisions at the tube-penetrating interface.
  3. Expansion Joint Fabrication: Flexible metallic bellows or ceramic fiber rope packing is fabricated to accommodate the differential thermal expansion between the coil tube and the furnace wall, ensuring a continuous thermal seal without mechanical binding.
  4. Installation and Anchoring: The insulation assembly is installed with refractory anchors and ceramic fiber tie-wires to prevent sagging and displacement under furnace vibration and thermal cycling. Anchor spacing is calculated to maintain insulation integrity over the design service life.
  5. Sealing and Surface Finishing: The outer surface is sealed with high-alumina refractory mortar and finished to a smooth surface to minimize windage heat loss and facilitate furnace cleaning.
  6. Post-Installation Inspection: Thermographic scanning and infrared temperature mapping are performed to verify uniform insulation performance and identify any thermal bridges or defects.

4.3 Non-Destructive Testing (NDT)

5. Applications

5.1 Primary Applications

5.2 Typical Furnace Configurations

6. Quality Certifications & Standards

6.1 Applicable Standards

Standard Scope
ASTM A213 Standard Specification for Seamless and Welded Austenitic Cr-Ni Stainless Steel Heat-Exchange Tubes
ASTM A313 Standard Specification for Seamless and Welded Austenitic Chromium-Nickel-Iron Alloy Pipes and Tubes for High Temperature Service
ASTM A336 Standard Specification for Seamless Ferritic and Austenitic Alloy Steel Pipes for High-Temperature Service
ASME B16.25 Butt-Welding Pipe Fittings (End Preparation)
ASME Section V Non-Destructive Examination (RT, UT, PT)
ASME Section VIII Div. 1 Pressure Vessel Code (Design and Fabrication)
EN 10216-5 Seamless Steel Tubes for Heat Exchangers and Heat Sinks — Part 5: Austenitic Steel Tubes
ISO 4413 Hydraulic Fluid Power — General Rules for Systems and Components
GB/T 14976 Chinese National Standard — Seamless Steel Tubes for Cold-Rolled or Cold-Drawn Precision Tubes
GB/T 13296 Chinese National Standard — Seamless Steel Tubes for Heat Exchangers and Heat Sinks

6.2 Manufacturer Certifications