After completing this chapter the learner will know the maintenance
requirements of a diesel engine cooling system now the reasons for using
chemical treatment in a diesel engine cooling system know the basic principles
of how chemical treatment protects the system
when a diesel engine cooling system is operated with good quality water which
is correctly treated then maintenance is minimal and the system should give long
term operation with few problems lack of or inadequate treatment will lead to
scale deposits and corrosion of the system components which may result in
failure or overheating the only general maintenance required is to keep the
coolant levels topped up and to maintain the correct concentration of treatment
in the system care must be taken when draining the system containing treatment
chemicals as they may pose a pollution risk disposal should be in accordance
with the material safety data sheet and any marple requirements the system
components such as the pumps and heat exchangers will require periodic
maintenance to ensure satisfactory performance circulating pump impellers
and sealing rings often referred to as wear rings shaft bearings and shafts
seals will need to be replaced periodically in accordance with
manufacturer's instructions it may also be necessary
to clear debris from impellers occasionally heat exchangers may need to
be manually or chemically cleaned periodically with seals and anodes being
replaced as required system valves will also require periodic overhaul including
reconditioning of seat phases and repacking of glands the quality of the
fresh water used for the first charge and topping up is critical for good
performance of the system normally the water will be produced on board in the
freshwater generator and should be good quality there should therefore be very
low levels of dissolved solids in the water so scale formation should not be a
problem the water will still require treatment to bring the alkalinity to a
suitable level to prevent corrosion in the system this will be achieved by the
correct concentration of a suitable water treatment it should be noted that
any of the treatments which are normally soluble in the water will actually cause
the total dissolved solids content of the water to increase excess
concentrations will therefore lead to possible deposit
the heat transfers surfaces which can cause overheating due to the insulating
effect as has been stated the quality of the water in the cooling system dictates
how well the system will perform even with good quality water the system still
needs to be treated to prevent scale and corrosion there are many different
treatments available which are usually corrosion inhibitors and all scale
preventers most treatments also have some antifreeze component to give low
temperature protection by lowering the freezing point of the coolant there is
also a need to protect against cavitation erosion particularly on
cylinder liners you can click on the button to learn more about cavitation
erosion when you have finished this page the actual treatment required for a
particular system is dependant to a large extent on the materials used in
the system some treatments do not protect certain materials and in fact
can accelerate the rate of corrosion when used with them most treatments are
therefore combinations of different chemicals which give protection to the
system as a whole you may be surprised to know that many of the treatment
chemicals nitrate and nitrite based for example give protection against
corrosion by forming passive scale coatings on the metal surfaces which act
as a protective film against further corrosive attack since these protective
films are very thin there is only minor insulating effect and heat transfer is
hardly affected cavitation erosion is a particular problem with wet cylinder
liners damaged caused by this mechanism can lead to corrosion problems on the
water side of the line of wall as the piston moves up and down in the cylinder
liner during the cycle the pressure in the cylinder varies the area of the
liner at which side thrusts from the piston acts' also varies this variation
in pressure and thrust causes the line of wool to vibrate this vibration causes
the line of wool to move in and out affecting the volume in the water space
this leads to local pressure variations on the water side of the liner as the
liner wall moves in toward the center pressure on the wet side of the line of
Walled reduces and vapor bubbles form as the line of wool moves back out it
creates rapid increase in local pressure which causes the vapor bubbles to
collapse the collapsing bubbles allow the water to impact on the liner with
great force this action can break down the protective films formed by the water
treatment and erode the liner metal the damaged areas are then subject to a high
risk of corrosion traditionally the chemical composition of cooling water
treatment was either chromate or nitrate bore rate based often with the addition
of silicates the treatments commonly used today include so called
supplemental cooling additives SC A's these are complex compounds of soluble
chemicals which provide good corrosion protection for a wide range of materials
as well as scale prevention they often use some of the traditional treatments
combined with other additives such as molybdate sand phosphates another
traditional treatment is soluble oil another group of treatments are referred
to as organic acid technology but we will not consider these here as they are
not common in marine use we don't need to consider the chemistry in detail but
you should be aware of the basic way in which the treatments work click on the
buttons for more information on the most common treatments chromate based
treatments work by forming a thin film of the surface of the metal components
in the system this film forms a passive barrier which isolates the metal from
any oxygen within the system and therefore prevents oxidation and
corrosion occurring they are often combined with phosphates which help
maintain the alkalinity of the system chromates will not prevent scale
formation it should be noted that chromates are highly toxic and are
calcio genic that is they can lead to cancer chromate based treatments are
therefore not allowed in systems which are used to heat freshwater generators
if the product is used as potable water nitrite based treatments work in a
similar way to chromates in that they form passive protective
layers to prevent corrosion the borate component acts to maintain
alkalinity are often included in the treatment to
improve protection of aluminium components in the system nitrites can
give rise to microbial problems and the treatment compound often includes a
biocide soluble oil when used in the correct concentration gives corrosion
protection by forming a thin film over the metal surfaces soluble oil will not
prevent scale formation as with the other treatments considered in this
module if the protective layer formed by the
treatment breaks down then concentrated pitting corrosion will result you should
always make sure that you are familiar with the content of the materials safety
data sheets for any chemicals that you have to handle on board the ship
particular care must be taken with storage personal protective equipment
requirements and first aid measures the code of safe working practices also
gives general guidance on handling chemicals many of the cooling water
system chemicals are alkaline based to ensure a correct condition of the water
acidity of the water will increase corrosion the concentration of the
treatment chemicals and cooling water systems can usually be checked by simple
comparative tests the treatments calls the water to color and the intensity of
the color measured against a color comparator will give a direct reading of
the concentration some treatments require the addition of an indicator
solution to the sample prior to carrying out the comparative tests other test
methods include the addition of tablets until the sample color changes a
conversion chart is then used to give the result in parts-per-million
concentration any inhibitor type treatments which use passive film
formation to prevent corrosion need to be maintained at the correct
concentrations if this is not achieved then partial breakdown of the passive
film can give rise to local anodic areas which will result in severe pitting of
the metal and lead to early failure of the components excess concentrations of
soluble oil can form insulating layers on the heat exchange surfaces which may
lead to overheating of the engine in very high concentrations the soluble oil
may come out of solution and corrosion protection may be lost altogether the
quiet constant and dosage instructions for all
treatments will be given in the material safety data sheet and should be followed
if adequate protection is to be achieved the sea water solid of the system may be
treated to minimize or completely avoid marine growth in the pipe work or on the
seawater side of the heat exchangers the treatment used may either be chemical
additives or electro chlorination which generates sodium hypochlorite from
seawater care must be taken to use the treatment in such a way that no damage
is done to the environment or marine ecology by excessive dosage or
concentrations you should always refer to the manufacturer's instructions when
using any treatments corrosion protection can be achieved by fitting
sacrificial anodes to the end covers of the individual heat exchangers typically
zinc is used for the anodes as the name suggests the anodes will corrode rather
than the system metals in this section we have looked at the problems
associated with cooling water systems and some of the treatments available to
overcome these problems we have also considered some of the basic corrosion
mechanisms and seen her of the various treatments act to prevent them you
should now have an understanding of the treatments and the necessity to maintain
correct concentrations