Fire tube boiler tube

The fire tube boiler is a classic and cost-effective boiler design that has been in use since the 18th century. It operates by burning fuel within a furnace to heat water inside tubes, generating steam. Among its key components—furnace body, tubing system, flue, and control system—the boiler tube plays a critical role in determining overall performance.

Structure of Fire Tube Boiler Tubes

The tube system in a fire tube boiler features a relatively simple design, typically comprising straight tubes, bent tubes, and expansion joints.

1. Straight Tubes

These are the most basic elements, often constructed from medium-thick or thin-walled seamless steel pipes or mild steel tubes. An external insulation layer is typically added to reduce heat loss and ensure safety.

Material options: 20# carbon steel / 304 stainless steel

Wall thickness: 3mm, 5mm, or 8mm

Insulation structure: Aluminum silicate fiber + galvanized iron sheet (anti-scalding design)

2. Bent Tubes

Bent tubes are used primarily to form the membrane wall structure of the boiler. Their curvature ensures smoother water circulation, minimizing flow resistance. They are commonly produced via cold-drawing processes to ensure inner wall smoothness and dimensional accuracy.

Minimum bending radius: ≥3× pipe diameter (ASME compliant)

Welding method: Full argon arc welding (to ensure sealing and prevent leaks)

3. Expansion Joints

These consist of two pipe flanges connected by a flexible metal hose. They absorb the thermal expansion of pipelines caused by temperature changes.

Thermal compensation rule: 5mm allowance for every 100℃ rise in temperature

Recommended replacement cycle: Every 3 years or 5,000 operating hours

Key Features of Fire Tube Boiler Tubes

Compact and Lightweight
The tubing system is designed to be small and lightweight, especially beneficial in high-pressure conditions—similar to lightweight hydraulic tubes.

Rapid Heat Transfer
The design allows direct exposure of water tubes to high-temperature gases, enabling quick steam generation.

Space-Efficient Structure
Tubes are densely arranged to maximize heat exchange within limited space.

Elevated Fire Risk
Due to direct contact with combustion gases, there's a heightened fire hazard. Without routine inspections, risks such as tube rupture can increase.

Common Issues and Emergency Handling

1. Emergency: Tube Water Leakage

Symptoms: Wall seepage > 200 ml/min, steam pressure drops ≥ 15%

Action plan:

Shut down the burner and depressurize to 0.2 MPa

Apply three layers of high-temp epoxy repair tape (e.g., 3M 2216)

Secure with stainless steel external clamps (≤150 mm spacing)

2. Decline in Heat Transfer Efficiency

Indicators:

Fuel usage ↑ 20%

Steam output ↓ 30%

Inspection protocol:

Internal scaling: Endoscopic check; if deposit > 2mm, perform 10% HCl pickling

Flue ash: Use U-type differential pressure gauge; clean if ΔP > 150Pa

Insulation failure: Thermal imaging; replace if surface temp ↑ > 40℃

3. Signs of Imminent Tube Failure

Sound warning:

Irregular metallic knocking > 85 dB

2–5Hz low-frequency vibrations (measurable via smartphone app)

Temperature anomalies:

Adjacent tube temp difference > 30℃

Localized overheating and reddening (> 450℃)

Conclusion

Boiler tubes are the core heat exchange component of fire tube boilers, directly influencing their efficiency and service life. Understanding their construction, materials, and operational behavior is essential for optimizing performance and conducting timely maintenance. Proper inspection and emergency response strategies can significantly extend the boiler’s operational lifespan and ensure safe, reliable operation.