This course is designed to provide marine engineers and support staff with
an introduction to water treatment for engine cooling systems it agin x' with
an introduction to engine cooling systems and basic water chemistry the
course goes on to look at the problems caused by corrosion and scaling and the
water treatment solutions that are available the course examines the
importance of correct chemical dosing and control and the procedures for safe
sampling and testing there is a section on monitoring water test results and a
look at the tools and service that are available at the end of the course there
is a troubleshooting guide
marine diesel engines are used to provide ship's propulsion and electrical
power these engines are typically fueled by heavy fuel oil that is preheated and
cleaned by centrifugal filtration large quantities of waste heat are evolved
from the combustion processes some of the energy from this waste heat is
recovered and used to produce steam via exhaust gas boiler systems the majority
of the heat produced by combustion must be dissipated to prevent overheating and
failure of the engines moving parts such as the Pistons valves and associated
seals for this reason all marine engines are fitted with a water-filled cooling
system this diagram shows a schematic of a typical engine cooling circuit the
engine cooling circuit is designed to be a closed cooling system that is the
makeup is normally less than 5% of the system capacity per week we will review
how the combination of high temperatures and water conditions can lead to
problems of fouling corrosion and scale formation that can adversely affect the
performance of the engine we will also describe how good water treatment
practice will prevent such problems occurring the engine cooling circuit is
a closed system and absorbs large amounts of heat from the engine to
reject this heat sea water is in turn used to provide secondary cooling for
the engine cooling water sea water is introduced into the ship through open
inlets and stored in containing tanks called sea chests within the body of the
hull these cooling duties utilize a variety of heat exchanger designs which
can become severely fouled due to impurities and contamination in the sea
water supply the two main problematic organisms are the muscle mytilus edulis
and the barnacle balanus balinese these are common shelled filter-feeding forms
of marine life found around the world they can grow up to 10 centimetres in
length and 5 centimetres in diameter and cause severe fouling where sea water is
used as a coolant fully grown mussels and barnacles can be a
eliminated by filters and strainers however it is the freshly spawned
species villagers that cause problems these villages start life in a
microscopic size and can easily find their way into the cooling circuit once
in the system pipe work they attach themselves to surfaces using strong
elastic threads of protein once attached they can readily feed and grow and as
their size and numbers increase fouling and blockage of waterways takes place
the best approach is to eliminate the mussels and barnacles when they are in
their most vulnerable state has villages and prevent them from attaching and
growing this can be accomplished by the routine application of a proprietary
biocide or anti foul and optimum engine performance is linked to good cooling
efficiency the engine cooling circuit must be kept free of corrosion damage
and scale deposition in general water is never pure normal tap water contains a
number of constituents that are important and beneficial to our
Constitution the table shows that sea water has a predominance of sodium
chloride with significant amounts of calcium and magnesium salts these
impurities will cause severe operational problems if not removed or if the system
metals are not protected for example dissolved gases such as oxygen and
carbon dioxide will cause corrosion and dissolved salts such as calcium
carbonate will lead to scale deposition corrosion is an electrochemical process
where metal in contact with an electrolyte water is oxidized to
corrosion products local difference is in chemistry metallurgy and oxygen
concentrations will induce potential differences on the metal surface and a
cathode and anode is formed at the anode corrosion will take place and metal will
be lost characteristic deposits of iron oxide
rust are also formed there are a number of factors that can affect corrosion in
the engine cooling system as previously discussed the corrosion rates of metals
with respect to pH can vary depending on their
electrochemical nature for example the corrosion rate of Steel is greatly
increased in acid conditions as they prevent the formation of a protective
oxide layer the corrosion rate of mild steel reduces as pH Rises up to around
pH 13 because of the reducing solubility of iron oxide in this range copper
behaves in a similar manner aluminium and zinc because of their amphoteric
nature exhibit increased corrosion rates at extremes of pH as a general rule each
10 degrees Celsius increase in temperature doubles the rate of most
chemical reactions therefore an increase in temperature will increase the speed
of corrosion because the reactions at the cathode will proceed faster the rate
of oxygen diffusion is also increased up to around 80 degrees Celsius in an open
system the increase in corrosion rate begins to reduce because of the reduced
solubility of oxygen however in a closed system such as an engine cooling circuit
the oxygen cannot escape an increased corrosion rates will continue to be
observed as the temperature rises small negatively charged ions such as chloride
ions tend to gather at the anodic sites to electro chemically balanced the
positive ions Fe 2 plus etc produced by corrosion of the metal chlorides
increase the localized conductivity which creates an accelerated environment
for corrosion to progress in the marine environment it is usual to recommend
higher levels of corrosion inhibitor when sea water contamination has
occurred stress corrosion cracking is a phenomenon mostly associated with
stainless steels here the chloride ions are just the right size to enter the
atomic matrix of the metal and their concentration greatly accelerates
corrosion and causes the propagation of cracks in the metal catastrophic failure
is often the outcome of such corrosion mechanisms this relatively common form
of corrosion occurs when two dissimilar metals are connected and exposed to an
aqueous environment one metal becomes anodic and the other cathodic thus
setting up what is to a galvanic cell the anodic metal will
exhibit corrosion in preference to the cathodic one the most common example of
this type of corrosion is when copper and mild steel are connected in water
the mild steel becomes a notic because it will give up electrons more readily
than copper and will rapidly corrode this diagram shows the galvanic series
of common metals and alloys the closer the metals are together in the series
the less the corrosion rate and vice-versa the rate of corrosion at the
anode is very much related to the relative surface areas of the anodic and
cathodic metals at the high temperatures experienced metal surfaces in the engine
cooling system cavitation damage can be experienced the mechanism of cavitation
can be described as the instantaneous formation and collapse of vapor bubbles
in a liquid that is subject to rapid and intense localized pressure changes when
these phenomena act at the metal surface the hydrodynamic forces created by the
collapsing vapor bubbles create microscopic torpedoes of water these
torpedoes can have velocities of up to 500 meters per second and on impact with
the metal surface dislodge the protective oxide coating and deform the
metal itself the damaging effect of cavitation is typically seen on engine
liners and pump impellers where high temperatures and pressures cause
localized boiling the effect of scale is to thermally insulated of the cooling
surface and retard the flow of heat from the metal to the water the only way the
heat can get out is by raising the metal temperature and by raising the exit gas
temperature from the engine the former may endanger the engines structure the
latter will reduce engine efficiency the presence of scale on the heat transfer
surfaces of the cooling circuit can lead to a situation where the alkalinity in
the system begins to concentrate by evaporation within the scale deposit
high concentrations of OAH alkalinity can attack pipe work and in particular
lead to rapid failure of aluminium pipe work and components the primary method
of scale control engine cooling circuits is to ensure
that the make up water is delivered from an evaporator or is suitably pretreated
to remove scale forming salts if the system is made up with shore water then
the amount of makeup should be minimized to less than five percent per week and
the corrosion inhibitor selected should contain a suitable polymeric scale
inhibitor to help prevent deposition options do exist for minimizing
corrosion by improved design material selection and improved construction
techniques economics dictate that most systems are designed and fabricated in
such a way that the application of a chemical inhibitor is the most cost
effective option for corrosion control in engine cooling as we have seen
corrosion is an electrochemical circuit therefore any chemical treatment applied
to the water that stops the anodic or cathodic reactions will inhibit
corrosion corrosion inhibitors are classified on how they affect the
corrosion cell and can be categorized as a notic inhibitors cathodic inhibitors
combination inhibitors engine cooling systems typically contains several types
of metals for best corrosion protection a corrosion inhibitor must be applied
that can protect all types of metal corrosion inhibitors used in engine
cooling circuits are typically blends of nitrite alkalinity and Hazel's and other
specialist chemicals once a suitable corrosion inhibitor has been selected
and is applied to an engine cooling circuit general testing of water quality
would be undertaken to measure the levels of inhibitor pH and chlorides has
a check for contamination and hardness salts the performance of the corrosion
inhibitor can be monitored by directly measuring corrosion rates it is best
practice to install chemical dosing and control systems for engine cooling
systems the now track 1000 control system works by directly monitoring the
conductivity of the cooling water conductivity is proportional to the
level of corrosion inhibitor and the product is dosed to meet a control
setpoint routine water sampling and testing is carried out to control a
performance of treatments for engine cooling water
systems most treatment chemicals have a set of control parameters that need to
be maintained for correct performance appropriate sampling frequency and
procedures are important to get consistent results click on the pictures
for more information about the correct sampling procedures weekly testing is
adequate if the system is tight more frequent
testing is advised if there are significant water losses if the systems
are experiencing problems higher testing frequency will be required typical
control parameters are shown in the table below it is important that test
results are recorded both for quality purposes and to help with the
interpretation of trends this enables corrective decisions to be made quickly
and effectively water testing is an essential shipboard activity that should
be performed on a regular basis water proof is software developed to enable
the crew onboard the vessels to electronically store the results from
the water tests when entering the test results the user immediately gets
notified if the results are within the set limits or if corrective actions are
needed the test results can be sent to Williamson ship service for review a
report from our water treatment experts will be returned within days after the
defined test period is closed
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