Introduction to the Marine Boiler Fuel Oil Combustion System

A typical marine boiler today uses a burner system that consists of two distinct parts: the ignition (pilot) burner and the main burner. The pilot burner’s job is to initiate combustion, usually with diesel fuel, while the main burner sustains the fire and can run on diesel or Very Low Sulfur Fuel Oil (VLSFO).

Fuel Supply Lines

Pilot Burner Fuel Line – This line is dedicated to diesel. It includes its own pump, pressure gauges, a strainer, and solenoids that control fuel flow.

Main Burner Fuel Line – The main line can accept diesel or VLSFO. When VLSFO is used, a heater raises the oil temperature (often to about 120 °C) to reduce viscosity. The line also contains a fuel pump, pressure gauges, solenoids (SV3, SV1, SV2), a recirculation line, and a strainer that removes fine impurities before the fuel reaches the burner nozzle.

Key Components and Their Functions

• Pilot Burner – Provides the initial flame using diesel.
• Main Burner – Maintains combustion with diesel or heated VLSFO.
• Fuel Pump – Delivers fuel oil to the burners.
• Heater – Keeps VLSFO at a temperature that allows smooth flow.
• Thermostats – Control heater cut‑in and cut‑out to maintain the set temperature.
• Thermocouple – Measures fuel line temperature and feeds the information to the control system.
• Strainers – Prevent nozzle choking by filtering out debris.
• Solenoids (SV3, SV1, SV2) – Open and close to regulate fuel flow for low and high firing modes.
• Air Separator / Mixing Chamber – Vents air and vapors from the fuel before it reaches the burner.
• Recirculation Line – Keeps fuel moving during standby, especially important for VLSFO to avoid stagnation.
• Safety Valves, V‑lines, Drain Lines, Vent Lines – Provide additional control and emergency venting.

Boiler Firing Operation (Automatic Mode)

When steam demand rises and boiler pressure drops, a pressure switch sends a signal to the combustion circuit. The sequence then proceeds as follows:

1. Pilot Burner Activation – The pilot fuel pump cuts in, a solenoid opens, diesel flows, and an igniter spark lights the pilot flame.
2. Main Fuel Supply – Simultaneously, the main fuel line is pressurized. If VLSFO is used, it passes through the strainer, air separator, and heater to reach the required temperature.
3. Main Burner Ignition – Depending on the firing mode, solenoids SV3, SV1, and SV2 open in a specific order. In low firing, only one solenoid operates; in high firing, both SV1 and SV2 open, delivering more fuel while the draft fan modulates the combustion air.
4. Recirculation and Deaeration – Excess or standby fuel is recirculated back to the air separator, where air and vapors are vented.

Shutdown and Safety Procedures

If the pilot burner fails to ignite, all solenoids close, an alarm sounds, and the boiler trips to a safe state. Before any firing attempt, the system checks draft fan operation, water level, and fuel pressure. An alarm indicates any failure, and a manual reset is required after the cause is corrected.

Detailed Flow Process for Boiler Firing

Firing Modes

• Low Firing Mode – Only one solenoid (typically SV3) opens, delivering a modest fuel flow.
• High Firing Mode – Both SV1 and SV2 open, increasing fuel flow; the draft fan adjusts to provide the higher air demand.

Maintenance and Checks

• Strainer Cleaning – Regular cleaning prevents nozzle blockage.
• Thermostat Probes & Heater Elements – Inspect for deposits that could reduce heating efficiency.
• Line Connections – Ensure correct nozzle connections during overhauls; mixing up low‑fire and high‑fire lines can affect combustion.
• Manual Purging – Use air vent and drain lines to remove trapped air or vapor before starting the boiler.