Composite boiler shuts down on flame failure alarm during starting sequence. The smoke-tube boiler runs for

Composite boiler shuts down on flame failure alarm during starting sequence.

The smoke-tube boiler runs for about 30 minutes before shutting down with a flame failure alarm, and now it fails to go beyond the pilot burner stage during startup.

Composite boiler was fired when we were approaching Singapore.

1. The boiler was started up and initially fired successfully.
2. It operated normally for about 30 minutes before giving flame failure alarm.
3. We reset it and tried again. It fired, but another flame failure alarm occurred after another 30 minutes.
4. The issue has now worsened with the pilot burner firing for 6–10 seconds before triggering a flame failure.
5. The fuel was changed over to diesel, but the exact same issue persists.

Measures taken:

1. Fuel quality/supply was suspected initially.
2. The fuel lines, filters, and burner components were thoroughly checked and cleaned.
3. The fuel pump was verified to be good and is maintaining a steady pressure of 18 bar.
4. All burners and boiler tubes were cleaned.
5. The air damper flap was checked.
6. Solenoid valves (low firing main burner valve and shut-off valve) were checked, confirmed to be getting a 220V signal during sequence, and oil flow also physically verified.
7. The low firing fuel valve to the boiler was renewed.

According to sequence after the 5th second the low firing main burner will open along with shut valve. Both these valves were checked and oil is flowing out correctly.

Still same condition. 6-8 seconds into the sequence boiler stops on flame failure.