Gard AS, February 2011
Your contacts
Senior Manager, Loss Prevention Terje R. Paulsen
[email protected]
Senior Loss Prevention Executive Marius Schnberg
[email protected]
Loss Prevention Circular No. 02-11
Exhaust boiler damages Introduction
Gard has experienced an increase in the severity of incidents
involving exhaust boilers, and in particular fire damages. The
causes of the boiler damages vary, however, slow steaming, fuel
quality and the frequency of manoeuvres can all be important
factors, and can lead to soot accumulating. This can increase the
risk of fire due to sparks from the exhaust gas igniting the soot.
The purpose of this circular is to address the risks involved and
to emphasise the importance of proper soot blowing, cleaning,
maintenance and treatment of boiler feed water. Problems caused by
soot on the exhaust side
The design of exhaust boilers is compact with very narrow
channels and fins on the exhaust side. In addition the design of
the main engine exhaust channel to the exhaust boiler on many ships
is restricting the easy flow of gas through the boiler. Combined
with e.g. slow steaming and reduced exhaust gas velocity the risk
for build up of soot deposits will increase. Unburned deposits
coming from the combustion process of the main engine due to
different fuels and different loads will cause ignitable soot
deposits to build up in the boilers. As the exhaust contains high
levels of oxygen (about 14%), the ignition of the soot deposit may
cause serious fire damage if preventive actions are not taken in
due time. For long-stroke diesel engine, the larger lub.oil quantum
required will also add to the build up of ignitable soot deposit.
High lubrication oil consumption in slow speed engines lower gas
temperatures and larger economizers in combination with low grade
residual oils will increase the risk for soot fires and in worst
case result in a complete meltdown of the boiler or economizer. The
soot blowing equipment fitted must be used according to
manufacturers recommendations, but the equipment provided does not
necessarily take into account the various design and layout of the
exhaust channel. It is therefore important that the condition of
the exhaust side is checked frequently in order to establish that
the soot is effectively removed, and if not, more frequent blowing
or manual cleaning will have to be performed. If the vessel
operates on a reduced load, we recommend increasing the load during
the soot blowing. It is also possible to fit additional equipment
to monitor pressure and temperature through the boiler/economiser
for optimization of cleaning intervals. Proper treatment of feed
water
As slow steaming appears to have become more common, the
importance of regular inspection, water washing, blow down and
proper feed water treatment should be emphasised. Impurities in the
feed water can result in increased scaling, corrosion and foaming.
Proper treatment of boiler feed water is therefore an important
part of the boiler maintenance program and will reduce the risk of
pitting and corrosion. To reduce the possibility of problems
arising, and to have control of the concentration of dissolved
solids in the feed water
1, it is necessary to carry out blow down or periodically
discharge the water or in some cases replace the water
completely. Surface water blow down is often done continuously to
reduce the level of dissolved solids, whilst bottom blow down is
performed periodically to remove sludge from the bottom of the
boiler. Regular testing and chemical dosing of the feed water is
therefore important to reduce the problem of corrosion and pitting.
Normally the feed water is produced by the vessels own fresh water
generator; however, it may become necessary to top up with shore
water which will be of a different quality. If the feed water has
been topped up with shore water, feed water tests, chemical dosing
and blow down are extremely important.
more
1 Important parameters of the feed and boiler water are PH,
chloride, hardness, oil, iron, carbon
dioxide concentration, silicates, dissolved solids.
Gard AS, February 2011
Your contacts
Senior Manager, Loss Prevention Terje R. Paulsen
[email protected]
Senior Loss Prevention Executive Marius Schnberg
[email protected]
Loss Prevention Circular No. 02-11
Exhaust boiler damages Problems caused by scale on the water
side
The biggest problem caused by scale is overheating and failure
of the boiler tubes. The thermal conductivity of porous boiler
scale is similar to insulating bricks. The scale acts as an
insulating layer and prevents an efficient transfer of heat through
the tubes to the circulating water. The reduction in thermal
conductivity means lower boiler efficiency which in turn leads to
overheating and may result in softening, bulging or even fracturing
of the boiler tubes. Boiler scale can also cause plugging or
partial obstruction of circulating tubes in a water tube boiler,
which again causes starvation and overheating of the tubes. It can
be necessary to plug some of the tubes from time to time. However,
this is only intended to be a temporary solution. In case of a
larger number of plugged tubes, the manufacturer/class society
should be consulted.
Picture: Lower side of middle evaporator tubes section with
melted metal. Recommendations
With the new generation of very efficient engines the soot
cleaning of exhaust gas boiler has become even more important for
efficient and safe operation.
It is therefore strongly recommended that all soot blowing/fixed
washing systems are used and tested regularly by skilled crew to
ensure that they are fully operational.
The importance of regular inspection, water washing, blow down
and proper feed water treatment should be emphasised. Acceptance
criteria for the condition after cleaning should be established
throughout the fleet. By this, any alteration in operational
conditions and its consequences for the boiler conditions will be
addressed.
Alarm and monitoring system to be regularly tested to ensure
proper function. The manufacturer to be contacted for further and
more detailed instructions, and in particular - If vessels normal
operational conditions alter. (i.e. such as slow steaming or change
of fuel quality )
The information is provided for general information purposes
only. Whilst we have taken every care to ensure the accuracy and
quality of the information provided at the time of original
publication, Gard AS can accept no responsibility in respect of any
loss or damage of any kind whatsoever which may arise from reliance
on this information. www.gard.no
