The learning objective of this chapter is to introduce the learner to basic
boiler fuel oil systems and equipment
unlike combustion in a diesel engine cylinder the combustion process in a
boiler furnace is a continuous process once the flame is established the
combustion will continue until the steam pressure in the boiler reaches the
preset value for the burner to cut out in simple boiler combustion systems the
burner will operate with on/off control however in most marine boilers the
combustion rate will vary automatically depending on the steam pressure and the
boiler load you can think of the boiler load as the rate at which the generated
steam is being used also unlike for diesel engine boiler combustion takes
place with only a small excess of air so that there is little unused oxygen often
less than 5 percent in the boiler exhaust
this makes the boiler exhaust an ideal source of inert gas for cargo tank
inerting on tankers there are also a number of safety cutouts on the boiler
which will shut down the burner regardless of the steam pressure we will
consider some of these later in the chapter
due to the fact that the combustion process is continuous once the initial
flame is established boilers could operate on lower grades of fuel than
diesel engines the fuel systems are therefore much simpler and the fuel
requires much less preparation in most cases boilers will operate on heated
heavy residual fuel which has been centrifuge and then passed through
fairly coarse filters there will usually be a facility for cold start on
distillate fuel when steam heating is unavailable
a pilot flame is used to ignite the main burner the pilot flame is provided by a
diesel oil burner which is lit by spark ignition by electrodes and a high
voltage supply on LNG carriers operating with steam propulsion the boilers are
designed to operate either on liquid fuel or on cargo boil off gas as has
already been stated most marine steam boilers have automated combustion
control this is mainly due to safety considerations these combustion control
systems can be extremely complex we will only consider the automatic control
briefly in this module
you
steam boilers can either the bottom fire burner units located in the wall of the
boiler or top fire types with the burner units located in the roof of the boiler
usually for modern boilers small low rated types are wall fired and large
highly rated types are roof fired the burner itself can take one of a number
of forms pressure jet nozzle type
you
combustion air is usually supplied by a forced draft fan although induced draft
fans can be used the air enters the furnace through a flat valve or an air
register an air adjuster can take the form of a slide valve which controls the
supply of combustion air to the boiler furnace the position of the register is
controlled to give the air quantity required for the amount of fuel passing
through the burner at any given time the air flows through a diffuser and a swirl
plate as it exits the air register which controls the air movement and ensures
good mixing with the fuel as it leaves the burner this positive mixing means
that only minimal excess air is required to ensure good combustion
a steam boiler requires a certain degree of monitoring and alarm and shutdown
protection to ensure a safe operation the two main concerns are always low
water level and high steam pressure a very low water level can result in
overheating resulting in boiler failure this is often termed low low level high
steam pressure can lead to rupture of the boiler early detection of both of
these conditions will result in an emergency shutdown of the burner unit
before any damage could occur the list shows the main parameters that are
monitored for a boiler fuel oil system fault condition of any of these will
result in emergency shutdown of the burner unit
all boiler fuel systems that you sail with will have an automated sequence to
control the combustion operation a typical combustion control sequence will
have the following basic elements the sequence will be initiated when the
steam pressure falls below the burner cut in point the first part of the
sequence will be the furnace pre purged combustion air Pham starts their
register full open furnace purged for predetermined time once the pre purge is
complete the ignition burner is activated air register to ignition
setting fuel to ignition burner spark igniter operates once the ignition
period is complete the main burner will activate main burner fuel arm once the
main flame is correctly established the ignition burner will shutdown flame
established check ignition burner fuel off as the steam demand and pressure
valley the burner unit will modulate tear and fuel automatic adjustment to
suit load condition if the steam pressure reaches the cut out point the
burner unit will shut down with a fan continuing to run for a short time you
should always make sure you are familiar with the actual system fitted to the
boiler on your vessel
you
as with any fuel oil system routine mate of a boiler fuel oil system will require
regular cleaning of filters and prevention of leaks from joints and
valve glands in addition the boiler burners and combustion air equipment
such as diffusers and swirl plates have to be cleaned regularly care must always
be taken to ensure that burner nozzles are in good condition and that nozzle
holes are not blocked or damaged the various shutdowns should be tested on a
regular basis to ensure correct function remember correct maintenance and
operating procedures will ensure safe operation
we have mentioned previously that combust
fuel oil in a boiler furnace produces exhaust gas which is suitable for use as
the inerting medium for cargo oil tanks the exhaust gas still needs to be cooled
and treated before it can be used as in that gas and in particular the sulfur
based products of combustion need to be removed this is usually achieved using a
scrubber tower which use a seawater to effectively wash the sulfur based
compounds from the exhaust gas the clean exhaust gas can then be passed to the
inert gas system for distribution to the cargo tanks inert gas system operation
and control are covered in detail in other siegel training modules
you you
in this chapter we have looked at the basic
acquirements for a boiler fuel system and have considered some of the main
burner types that you are likely to sail with boilers and the associated systems
are considered as high-risk equipment because of the dangers of furnace
explosion and boiler overpressure the high temperatures and pressures involved
require that the operator is completely familiar with the safe operating
procedures for the equipment you should always make sure that you study the
manufacturers and shipboard instructions before operating such equipment before
moving on to the assessment chapter apply the questions and the following
pages to check your understanding so far