Boiler accidents: comprehensive cause analysis and preventive measures (Part 3)

In the last part, we will explore catastrophic events such as furnace explosion, flue gas re-ignition and structural failure, as well as human error and design defects that lead to boiler accidents.

1. Furnace explosion and flue gas re-ignition

1.1 Furnace explosion

Mechanism: The combustible mixture in the furnace (e.g., unburned fuel vapor, coal powder) ignites, resulting in rapid pressure fluctuations and structural damage. Common in oil/coal-fired boilers.

Causes:

Accumulation of fuel that was not discharged before starting (e.g., burner leakage, ignition failure).

Continued fuel supply after flameout, forming an explosive mixture.

Inadequate ventilation (e.g., insufficient pre-purge cycle).

Handling:

Shut down immediately; cut off fuel supply and close damper to isolate oxygen.

Use steam fire extinguisher; check for structural damage (e.g., wall cracks, pipe bends).

After cooling, ventilate for 10-15 minutes before restarting.

Preventive measures:

Forced pre-ignition purge (5-10 minutes of airflow).

Automatic flame failure detection and fuel cut-off system.

Train operators on safe ignition procedures (e.g., "light the torch first, then ignite the fuel").

1.2 Flue gas re-ignition (secondary combustion)

Risk: Unburned fuel (e.g., soot, oil droplets) in the tail flue ignites due to high temperature and oxygen ingress, damaging the economizer or air preheater.

Causes:

Poor combustion (e.g., poor atomization, coarse coal powder), resulting in unburned particles.

Low load operation or frequent starts, resulting in soot accumulation.

After shutdown, the damper leaks, allowing air to enter.

Treatment:

Emergency shutdown; seal the damper to suppress the flame.

Start the steam soot blower or fire extinguishing system.

Monitor the feed water temperature to protect the economizer.

Prevention:

Optimize combustion parameters (e.g., fuel-air ratio, coal fineness).

Check dampers regularly during sootblowing and after shutdown.

Install temperature alarms in flue gas ducts.

2. Structural failures: furnace walls and water walls

2.1 Damage to furnace walls

Problems: Cracks, collapse, or deformation due to thermal stress, poor masonry, or slagging.

Causes:

Inadequate curing of refractory materials; rapid temperature changes during startup/shutdown.

High-temperature corrosion of slag (e.g., in furnaces without water walls).

Improper deslagging damages walls.

Prevention:

Use high-quality refractory materials and proper curing; gradual heating/cooling schemes.

Regularly check for slag accumulation; safe deslagging procedures.

2.2 Water wall leakage

Symptoms: Drop in drum level, increased feedwater flow, unusual noise, or visible leakage.

Causes:

Circulation failure (e.g., header blockage, design flaw).

Mechanical damage from slag falling or improper sootblowing.

Corrosion caused by poor water treatment or hydrogen embrittlement.

Treatment:

Minor leaks: Reduce load, monitor liquid level, and prepare for shutdown.

Major leaks: Emergency shutdown; isolate affected area and inspect for repairs.

Preventive measures:

Complete water treatment; regular non-destructive testing (NDT) of pipelines.

Proper slag management and adjustment of sootblowing angles.

3. Human and design factors

3.1 Operator error

Common mistakes:

Ignoring water level monitoring or misinterpreting pressure gauge readings.

Improper use of drain valves (e.g., leaving them open for too long, forgetting to close them).

Ignoring safety alarms or bypassing interlocks.

Solutions:

Strict training through simulated accident drills.

Implementing a shift system to prevent fatigue; clear identification of critical valves.

3.2 Design flaws

Key issues:

Inadequate steam-water separators leading to startup.

Improper safety valve or pressure gauge connection locations.

Unsupported pipes leading to stress fractures.

Mitigation:

Comply with ASME/ISO standards for pressure parts and safety devices.

Stress analysis of critical components; easy-to-maintain designs.

4. Holistic prevention strategy

Regulatory compliance: Regular inspections by certified engineers; compliance with local safety codes.

Predictive maintenance:

Annual inspections of boiler thickness, cracks, and corrosion.

Ultrasonic testing of pipes; chemical analysis of boiler water.

Technology upgrades:

Automatic combustion control systems.

Real-time monitoring of pressure, temperature, and water levels through IoT sensors.

Emergency preparedness:

Post clear shutdown procedures near control panels.

Perform regular fire-fighting and evacuation drills.

5. Conclusion

Boiler safety requires a multi-layered approach: technical precision in design and maintenance, rigorous operator training, and proactive hazard identification. By addressing root causes, from water quality to human factors, industries can significantly reduce the risk of catastrophic failures. As this series has emphasized, vigilance and compliance are non-negotiable in public relations.