Lars Josefsson | Marine Boilers Heavy Fuel Oil

Marine Boilers Heavy Fuel Oil

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Marine Steam Boilers Heavy Fuel Oil System
N.B. this is just an overview;  lots of valves and other accessories are missing. Steam & water system

Fuel Oil flow diagram for two oil-fired marine steam boilers

This Marine Steam Boilers Heavy Fuel Oil System is designed to supply two oil fired marine steam boilers with Heavy Fuel Oil for normal operation and Diesel Oil for the start up procedure when no steam is available for fuel oil heating.

  1. Fuel Oil header
    The Fuel Oil header should be a standing up relatively large pipe with a deaeration pipe to a higher level than the fuel oil service tanks; some times it's connected to the top of the diesel oil service tank.
  2. Fuel Oil filters
    Clean the Fuel Oil filter frequently. If the filters are clogged the fuel oil might vaporize in the pump and cause damage. A differential pressure meter across the filter would be a good help to have the filters cleaned in time.
  3. Shut off valves
    Shut off valves for cleaning of the filters. The valves should normally be open.
  4. Fuel Oil pumps
    Each pump should have sufficient capacity to run both boilers on maximum load. Normally one pump is running and the other is standing-by. The stand-by pump should start upon low pressure in the process line rather than being started upon a pump motor failure.
  5. Shut off non return valves
    These valves have to be non-return type, or a shut off valve and a non-return valve. The valves should always be open on both pumps, so the stand-by pump can start automatically.
  6. Fuel Oil heater
    The fuel oil heater is a heat exchanger that must have a safety relief valve.
    Always install a safety relief valve if a fuel oil volume can be shut up and heated.
  7. Fuel Oil Burners
    The burners must be capable of running on diesel oil as well as heavy fuel oil.
  8. Make-up water pump
    From softener unit.
  1. Change over valve, Heavy Fuel Oil, HFO, to Diesel Oil, DO
    A three-way-valve to choose between the Heavy Fuel Oil service tank and the Diesel Oil service tank. The three-way valve ought to be provided with a limit switch that breaks the electric circuit and shut off the fuel oil pipe heating to avoid the diesel oil from being heated over its flashpoint.
  2. Fuel oil pressure control
    The pressure control loop adjusts the fuel oil pressure by means of the pressure controller and the fuel oil return valve. The pressure is measured after the fuel oil heater to get accurate pressure to the burners. The return valve before the fuel oil heater recycles cold fuel to protect the pumps from overheating.
  3. Heavy fuel oil temperature control
    The temperature control loop adjusts the fuel oil temperature by means of the temperature controller and the steam inlet control valve. The temperature controller's set point should be set to assure an adequate fuel oil viscosity for the actual burners.
    Sometimes the control valve is installed in the condensate outlet line. This requires a smaller control valve and the control function works even better. However, this installation is for some reason not so very common.
  4. Fuel oil flow control
    On large boilers the fuel oil flow controller gets its setpoint from the burner management system or the airflow controller to achieve a proper air-fuel mixture.
    On smaller boilers the fuel oil flow control valve often is connected to the combustion air damper by means of a metal rod or wire.
    In both cases the steam pressure in the boiler sets a suitable fuel oil flow.
  5. Fuel oil shut of valves
    Almost every classification society and other authorities requires two shut of valves for the fuel oil, mechanically in series and electrically in parallel.
  6. Fuel oil recirculation valve
    This valve does not always exist, but if it does, the valve should open when the fuel oil shut of valves closes. The recirculation has advantages and disadvantages.
    • The advantage is that it keeps the fuel oil line warm when the burner is off.
    • The disadvantages is that it will, in the long run, heat up the fuel oil passing through the oil pump that might cause problem with cavitations.
    Leading the recirculation fuel oil back to the fuel oil service tank wouldn't be wise. The fuel oil's temperature often exceeds 120ºC and water that might occur at the bottom of the tank will vaporize and in worst-case cause damage to the tank.
  7. Fuel oil drain valve
    Some authorities require a drain valve to be installed between the shut off valves. The burner management program mustn't open the drain valve before the shut off valves are totally closed and it should close the drain valve before the shut off valves open.
  8. Smoke density monitor
    High smoke density indicates deficiency of air.
    The smoke-density-monitor should be mounted higher up in the funnel than the oxygen-meter because the lenses of the smoke-density-monitor's light-emitter and its light-reciver are flushed clean with air and this air must not affect the oxygen meter.
  9. Oxygen meter
    The smoke duct from the boiler to the funnel is designed with respect to the pressure drop and smoke velocity at maximum firing. At minimum firing the smoke velocity can be very low and along the sides of the smoke duct a downdraught can occur. Therefore the O2 probe ought to be located as close to the centre of the smoke duct as possible.
  10. Boiler outlet temperature
    Normally a thermocouple or a resistance-thermometer. This temperature should be recorded since a slowly increasing temperature indicates accumulation of soot on the boilers heating surfaces and a rapid rise in temperature might indicate a soot fire.
  11. Exhaust gas boiler differential pressure
    This exhaust gas boiler differential pressure should be recorded since increasing difference pressure indicates accumulation of soot on the boilers heating surfaces.