Marine boilers are steam generators where heat resulted from combustion of a fuel, from nuclear reaction or thermal effect of electrical current is transferred to the water which is heated and vaporized.
There are two types of boilers in marine industry:
- Fire tube boiler is the type of boiler where the exhaust gases resulted from combustion process are passing through inner surface of boiling system tubes and the outer surface is fully immersed into the water. This type of boiler, is generally a boiler with a large volume of water.
- Water tube boiler is the type where water is circulating through inner side of boiler system tubes and combustion gases are passing through outer side of tubes. This type of boiler is of low water volume.
Water tube boilers are preferred onboard vessels because of multiple advantages compared with fire tube boilers. From all these advantages we can mention:
- Less weight which in water tube boilers is nearly 3 times lesser than fire tube boilers.
- It is possible to use higher pressure and temperature, especially on vessel using steam propulsion.
- An increased mechanical flexibility as water tube boiler are not so sensitive to pressure fluctuations like fire tube boilers.
- Rapid increase of steam pressure
- Increased safety.
The steam generating plant consists, generally, of one auxiliary oil-fired boiler and one exhaust gas boiler. On some vessels there will be a composite type boiler which comprises of an oil-fired boiler and exhaust gas boiler integrated within.
In port the steam demand of the vessel is served by the oil-fired boiler, but while at sea the vessel’s steam demand is met by the exhaust gas boiler. In this case the oil-fired boiler can be left on automatic stand by ready to cut in if the steam pressure drop below a predetermined set point.
The oil-fired auxiliary boiler is of the steel welded, usually vertical, cylindrical design, with a closely spaced tube bank in the upper section. The tube bank rests in the combined steam/water drum which is cylindrical. Steam is generated by the hot gases flowing vertically, horizontally or mixing both ways (depending boiler type) through the tube bank which contains water. Modern marine boilers are oil-fired by a horizontal burner situated in the lower section.
Where necessary, manholes are provided for easy access and inspection. The combined steam/water drum has a feed water internal pipe, surface blow-off internal pipe and water sampling pipe. Insulation is provided on the outer surface of the boiler and the outermost surface is covered with galvanized steel casing.
There are many types of boiler burners available onboard vessels, but the most common ones are Pressure Jet and Rotary Cup types. Apart from these, burners can be of variable orifice pressure jet, spill pressure jet, ultrasonic or steam assisted types.
These burners are suitable for HFO (or LSHFO in ECA or SECA areas) with MDO used for the igniter. It is also possible to burn waste oil in the boiler, and usually the boiler control panel has a facility for changing to waste oil operation. In this case MDO is used when flashing-up from cold condition.
When the boiler is started for the first time or after repair and/or prolonged shutdown period, the following procedure generally applies:
- Ensure that all gas side heating surfaces are clean and all furnace refractory is in good condition.
- The furnace bottom and the burner wind box are clean of oil and any other debris.
- All safety valves should be free and in good condition,
- All manholes are tightened, blow-off and drain valves are close and root valves for all instruments and controls connected to the boiler are open.
- Open the vent valve of the combined water/steam drum located on the top of the boiler.
- Fill the boiler with water until about 25 to 50 mm of water appears in the gauge glasses (this allow for expansion and increase in level after firing). The selected feed water pump should be in manual mode and stopped when the desired level is reached.
- Start the boiler burner on MDO.
- Steam pressure should be raised slowly and evenly to avoid placing undue stress on the boiler components. When raising the pressure, keep the burner firing for 5 minutes and out of service for 15 minutes repeatedly for one hour. A guideline would be to aim for 6 bar after 1.5 hours firing. These guideline should not override the manufacture’s instructions with regard to start up of boiler from cold condition.
- When the drum pressure has risen to about 1.5 bar, close the drum air vent valve.
- When enough steam is available and HFO reaches the proper temperature, switch the boiler to HFO and keep increasing slowly the pressure in the boiler until nominal working pressure.
- When nominal working pressure has been reached, boiler can be turned to Auto mode.
In some cases a steam heating or warming through coil is provided in the boiler shell and water in the boiler may be warmed using this coil thus reducing the period of time taken to raise steam pressure by means of the burner. The steam necessary for this operation must be available from the exhaust gas boiler. If the boiler is shut down for a period of only few days it should be maintained in warm condition by means of heating coil.
Boiler control panel provides operation, control and interlock devices required for boiler normal operation. The control panel performs the automatic and manual operation of the boiler and it gives an alarm to warn the operator if an abnormality occurs during operation.
In the same time, the panel tops the boiler in case of an emergency, by immediately shutting down the fuel supply to the boiler burner, if such abnormality deemed to be too serious to continue running the boiler. Normally the control unit is provided with the following safety features:
- Low water level alarm.
- High water level alarm.
- Shut off fuel supply at low water level.
- Stop the feed water pump at high water level.
- Flame failure supervision.
- High and low steam pressure alarms.
- Burner shut down in case of purging air low pressure, overload combustion air fan and burner motor and burner swing out.
In case of automatic operation of the boiler, the starting up is quite simple and below procedure generally apply (manufacturer instruction manual must be consulted for specific type of boiler) :
- Check by pushing the Lamp test button that all lights are functional on the control panel.
- Check if the water level is within acceptable limits with one feed pump selected for operation and the second one in stand by.
- Select one fuel oil pump as the operating pump and the second one in stand by.
- Turn the boiler operation mode switch to AUTO mode.
- Turn the burner AUTO/OFF switch to the AUTO position. The burner will be started and stopped by the signal from the start/stop pressure switch or the steam pressure transducer. The start up sequence and flame supervision are controlled by the burner sequence controller.
- When the start-up sequence is complete turn the boiler modulation mode selection switch to the AUTO position and the burner will be controlled by the load controller.
The manual operation system allows for firing the boiler, in the event of faults in the automatic sequence control or components of safety monitoring equipment. The procedure for starting the boiler in manual mode is generally as described below (manufacturer instruction manual must be consulted for specific type of boiler):
- Turn the burner AUTO/OFF switch to OFF position.
- Turn the boiler modulation mode selection switch to the MANUAL position.
- On some of the boilers the auto flame scanners must be removed and replaced with manual flame scanners.
- Turn the boiler operation mode switch to MANUAL position. The burner will move to the minimum position and when will reach that position the burner motor and the combustion air fan motor are started. In this moment “increase burner load” and “decrease burner load” buttons become active.
- Press the “increase burner load” pushbutton until the air damper reaches its maximum position.
- Allow the boiler furnace purge to operate for at least 60 seconds. It is essential that the boiler furnace be purged correctly in order to reduce the risk of furnace explosion.
- After purging the furnace the air damper must be brought to minimum position.
- Press the ignition pushbutton on the manual operation box and keep it pressed until the ignition flame is established.
- If there is a good ignition flame, press the oil valves pushbutton and keep both the ignition and oil valves pushbuttons pressed for maximum period of 5 seconds.
- Release both buttons. A steady flame should be established and the oil valve and flame will be supervised by the manual flame scanner. The burner modulation may be selected as AUTO or MANUAL.
- If no flame is established when the buttons are released the stat-up sequence must be repeated.
It is important to remember that during boiler manual operation there is no function of burner sequencing control unit and during burner off periods the flame supervision circuit has a voltage applied in order to carry out the detector and extraneous light test.
It is essential that the boiler is constantly supervised by a competent engineer when operating in manual mode, as in this situations the safety interlocks are reduced to:
- Too low water level
- High steam pressure
- Burner swing out
- Overload combustion air fan and burner motor
- Flame failure
Marine boilers are very important and sensitive equipment onboard vessels, therefore special care and attention should be paid. The are some situations when engineers must pay proper attention and they must be properly trained for such.
For example in case of a flame failure, make sure that the oil valve is closed and reduce the air pressure to prevent furnace over cooling (usually these processes are automatic). Purge the furnace before relighting the burner as you must make sure that no remnant fuel is trapped in the furnace as this could lead to a blow back of hot gases. Never attempt to relight the burner from the hot furnace refractory.
Another case can be when boiler trips due an emergency, like “Too low water level” alarm. In this case shut the steam main valve, feed valve and forced draught fan after purging the furnace. Never attempt to fill the boiler with feed water until the boiler has cooled sufficiently.
When taking a boiler out of service, the wet lay-up method is preferable, because it requires less preparation and the boiler can be returned quickly to service. When is in the cooling down process following shutdown, treatment chemical must be injected using the boiler dosing unit in order to ensure adequate protection of the boiler. Always follow the guidelines chemical supplier as excessive chemical may be more detrimental than insufficient chemical.
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