Transcript Cooling system

Jacket Water System
HT/LT systems
Sea Water Cooling system
Jacket cooling system
Jacket Water System
Jacket Water Heater. Unheated engine can lead to poor combustion, poor
lubrication and thermal shock, hence a water heater. A modern variation on
this is the "blend" water from the stand-by auxiliary alternator engines into the
main engine circuit increasing plant efficiency. The water enters and leaves
the engine via a series of cylinder isolating valves. In this way each cylinder
may be individually drained to prevent excessive water and chemical loss. In
addition dual level drains may be fitted which allow either full draining or
draining of the head only. A portion of the water is diverted for cooling of the
turbocharger.
De aerator. Air or gas entering the system can lead to unstable and even
total loss of cooling water pressure as the gas expands in the suction eye of
the circulating pumps. In the event of gas leakage via the head or cracked
liner rapid loss of jacket water pressure can occur. The deaerator is a method
to try to slow this process sufficiently. This system also allows the vessel to
operate with minor gas leakage.
.
Jacket Water System
• Jacket Water Cooler The hot water leaving the engine passes to a
temperature control valve were a portion is diverted to a cooler.
Temperature is controlled using both a feedback signal (temperature
measured after the cooler) and a feed forward signal (temperature
measured at outlet from the engine). In this way the system reacts
more quickly to engine load variations.
• Evaporator Increases plant efficiency . Modern systems sometimes
rely on the evaporator to supplement a reduced size main cooler.
• Expansion or header tank Maintains a constant head on the
circulation pumps reducing cavitations at elevated temperatures.
Allows the volume of water in the system to vary without need for
dumping. Acts as a reserve in the event of leakage
Scaling of Jacket Water System
Scale and deposit formation
In areas of deposit formation, dissolved solids, specifically Calcium and magnesium
hardness constituents can precipitate from cooling water as the temperature
increases. Deposits accumulate on the heat transfer surfaces as sulphates and
carbonates, the magnitude of which is dependent on the water hardness, the
dissolved solid content, local temperatures and local flow characteristics. Scales can
reduce heat transfer rates and lead to loss of mechanical strength of component parts,
this can be exacerbated by the presence of oils and metal oxides
Temperature solubility curves for CaSO4
Scale and deposit formation
The degree and type of scaling in a cooling water circuit are determined by;
System temperatures, Amount of leakage/makeup, quality of make up, quality
of treatment
Calcium Carbonate
Appears as a pale cream, yellow deposit formed by the thermal decomposition of
calcium bi-carbonate ; Ca(HCO3)2 + Heat becomes CaCO3 + H2O + CO2
Magnesium Silicate
A rough textured off-white deposit found where sufficient amounts of Magnesium are
present in conjunction with adequate amounts of silicate ions.
Silicate deposit is a particular problem for systems which utilize silicate additives for
corrosion protection. This is typical of system with aluminum metal in the cooling system.
The silicate forms a protective barrier on the metal surface. A high pH (9.5 - 10.5) is
required to keep the silicate in solution. In the event of sea water contamination or some
other mechanism that reduces the pH the silicate is rapidly precipitated and gross
fouling can occur.
Copper
The presence of copper within a cooling system is very serious and it can lead to
aggressive corrosion through galvanic action. Specific corrosion inhibitors are contained
with cooling water system corrosion inhibitors.
Effects of scale deposition
• The effects of scale deposition can be both direct or
indirect, typically but not specifically
• Insulates cooling surfaces leading to;
– increased material temperatures as the temperature
gradient must increase to ensure maintain heat flow.
– Loss of efficiency as exhaust gas temperatures from
cylinders increases
– Increased wear due to lubrication problems on overheated
surfaces
• Indirectly;
– Lead to caustic attack be increasing the OH- ion
concentration
Corrosion inhibitors used in
Jacket Water System
In order to maintain mechanical strength the components surrounding the
combustion zone must be cooled. The most convenient cooling medium is water,
the use of which could lead to possible problems of corrosion and scaling if not
properly treated.
Within the jacket water system a number of corrosion cells are available but the
two most common and most damaging are due to dissimilar metals and
differential aeration. In both types of cell there exists an anode and a cathode,
the metals which form part of the jacket system, and an electrolyte which is the
cooling water. The rate at which corrosion takes place is dependent upon the
relative areas of the cathode and the anode and the strength of the electrolyte. It
is the anode that wastes away. Corrosion due to temperature differences is
avoidable only by the use of suitable treatments. Dissimilar metals-a galvanic cell
is set up where two different metals and a suitable liquid are connected together
in some way. All metals may be placed in an electro-chemical series with the
more noble at the top . Those metals at the top are cathodic to those lower
down. The relative positions between two metals in the table determined the
direction and strength of electrical current that flows between them and hence,
the rate at which the less noble will corrode
Galvanic Action
Corrosion within cooling systems can occur if the coolant, i.e. water,
has not been properly treated. The corrosion can take the form of
acid attack with resultant loss of metal from a large area of the
exposed surface, or by Oxygen attack characterized by pitting. A
primary motive force for this corrosion is Galvanic action
The metals closer to the anodic end of the list corrode with
preference to the metals towards the cathode end.
A galvanic cell can occur within an apparently Homogeneous
material due to several processes on of which is differential
aeration where one area is exposed to more oxygen than
another. The area with less oxygen becomes anodic and will
corrode.
The Galvanic Series.
Cathode
17
Brasses
1
Gold and Platinum
18
Nickel
2
Titanium
19
Stainless-Steel 18-8 (Active)
3
Silver
20
Stainless Steel 18-8-3 (Active)
4
Silver solder
21
Chromium Iron (Active)
5
Chromium-Nickel-Iron (Passive)
22
Chromium-Nickel-Iron (Active)
6
Chromium-Iron (Passive)
23
Cadmium
7
Stainless Steel (Passive)
24
Iron
8
Copper
25
Steel
9
Monel
26
Cast Iron
10
70/30 Cupro-Nickel
27
Chromium
11
67-33 Nickel-Copper
28
Zinc
12
Hydrogen
29
Aluminum
13
lead
30
Aluminum Alloys
14
Tin
31
Magnesium
15
2-1 Tin lead Solder
16
Bronzes
Anode
Metals closer to the anodic end of the list corrode with
preference to the metals towards the cathode end
Galvanic Action
Galvanic action within metal
This situation can exist in cooling water systems with complex layout of heat
exchangers and passage ways within the diesel engine. Systems containing
readily corrodible metals such as zinc, tin and lead alloys can complicate and
intensify problems by causing deposit formations
Differential Aeration
-Where only a single metal exists within a system corrosion can still
take place if the oxygen content of the electrolyte is not homogenous.
Such a situation can occur readily in a jacket water system as regions
of stagnant flow soon have the oxygen level reduced by the oxidation of
local metal. The metal adjacent to water with reduced levels of oxygen
become anodic to metals with higher oxygen content electrolyte in
contact with it.. Generally, the anodic metal is small in comparison the
cathode i.e. the area of stagnant flow is small compared to the area of
normal flow of electrolyte, and high rates of corrosion can exist. One
clear case of this is the generation of deep pits below rust scabs.
Water treatment
To remove the risk of corrosion it is necessary to isolate the metal surface form
the electrolyte. One method would be by painting, but this is impractical for
engine cooling water passages. A better solution would be a system which not
only search out bare metal, coating it with a protective barrier, but also repair
any damage to the barrier.
For corrosion to occur four conditions must be met;
There must be an Anode
There must be a cathode
An electrolyte must be present
An electron pathway should exits
Corrosion Inhibitors
• Corrosion inhibitors are classified on how they affect the corrosion cell
and are placed into three categories;
– Anodic Inhibitors
– Cathodic Inhibitors
– Combination inhibitors/organic inhibitors
•
Common Corrosion Inhibitors
Principally Anodic
Inhibitors
Principally Cathodic
Inhibitors
Chromate
Carbonate
Polyphosphate
Phosphonates
Zinc
Nitrite
Orthophosphate
Bicarbonate
Silicate
Molybdate
Both Anodic and
Cathodic Inhibitors
Soluble Oils
Mercaptobenzothiaz
ole (MBT)
Benzotriazole (BZT)
Tolytriazole (TTZ)
Anodic Inhibitors
Nitrite (NO2- )- These are the most commonly used form of treatment and operate by
oxidizing mild steel surfaces with a thin, tenacious layer of corrosion product
(magnetite Fe3O4). Relatively high volumes of treatment chemical are required so
this method is only viable on closed systems
Sodium Nitrite- (sometimes with Borate added)-effective with low dosage,
concentration non-critical. It is non toxic, compatible with anti freezes and closed
system cooling materials. It does not polymerize or breakdown. However protection
for non-ferrous materials is low. An organic inhibitor is thus required. Although will not
cause skin disease it will harm eyes and skin. Approved for use with domestic fresh
water systems.
Sodium Nitrite is a passivator which chemically produce an insulating layer on the
metal surface. Whenever corrosion takes place the corrosion products including
bubbles of gas, are released from the metal surfaces. Passivating chemicals act on
the corrosion products preventing release from the metal surface and thus stifling
further corrosion. If the insulating layer becomes damaged, corrosion begins again
and the passivator acts on the new products to repair the layer.
Sodium Chromate which was an excellent inhibitor. Not allowed. Due to its toxicity.
Sometimes used as a biocide in such places as brine in large Reefer plants.
Anodic Inhibitors
• Silicates- react with dissolved metal ions at the anode. The
resultant ion/silicate complex forms a gel that deposits on anodic
sites. This gel forms a thin, adherent layer that is relatively
unaffected by pH in comparison to other inhibitors. The inhibiting
properties increase with temperature and pH, normal pH levels are
9.5 to 10.5.
• Care should be taken with the use of silicates, which are often used
for the protection of systems containing alumiinium. In the event of
boiling increased concentrations and lead to aggressive corrosion
due to the high pH.
• Orthophosphate Forms an insoluble complex with dissolved ferric
ions that deposit at the anodic site. It is more adherent and less pH
sensitive than other anodic inhibitors. The film forms in pH of 6.5 to
7.0. Dosage is typically 10ppm in neutral water
Cathodic Inhibitors
Cathodic Inhibitors
Polyphosphate- Forms complexes with Calcium, Zinc and other divalent ions, this
creates positively charged colloidal particles. These will migrate to the cathodic site
and precipitate to form a corrosion inhibiting film. The presence of calcium is
required at a typical minimum concentration of 50ppm.
Extreme variations in pH can upset the film and a reversion to orthophosphate will
occur with time and temperature.
Positively charged zinc ions migrate to the cathodic site and react with the free
hydroxyl ions to form a zinc hydroxide stable film at pH 7.4 to 8.2. If the water is too
acidic the film will dissolve and not reform. If it is too alkaline the zinc hydroxide will
precipitate in bulk and not at the cathodic site.
Phosphonates Initially introduced as scale inhibitors to replace polyphosphates,
they exhibit absorption at the metal surface especially in alkaline hard water.
Generally used with other inhibitor types
Both Anodic and Cathodic Inhibitors
Benzotriazole and Triazole Specific corrosion inhibitor for copper. They
break the electrochemical circuit by absorbing into the copper surface.
They are generally added to standard treatments.
Soluble and dispersible oils. Petroleum industry recognized that
emulsifying cutting oils (erroneously called soluble oils) were able to
reduce corrosion on metals by coating the surface. There were
disadvantages though, if the coating became too thick then it could retard
the heat transfer rate. Adherent deposits form as organic constituents
polymerize or form break down products which can accumulate and
disrupt flow. MAN-B&W recommend it not to be used.
It is effective in low dosages, safe to handle and safe with domestic water
production. Effectiveness is reduced by contamination with carbon, rust,
scale etc. Difficult to check concentration, overdosing can lead to
overheating of parts
Oils are classed as a barrier layer type inhibitor. The surfaces being
coated in a thin layer of oil.
Modern treatment
Nitrite-Borate treatment is most effective with a high
quality water base. This treatment has no scale
prevention properties and its effectiveness is reduced by
high quantities of dissolved solids.
A modern treatment will be a Nitrite -Borate base, with a
complex blend of organic and inorganic scale and
corrosion inhibitors plus surfactants, alkali adjusters,
dispersants and foam suppressers. A high quality water
supply is still strongly recommended.
Electrolytic protection for the whole system by the use of
sacrificial anodes is impractical. Parameters such as water
temperature, relative surface area of anode and cathode,
activity of metals in system and relative positions in galvanic
series come into play. Anodic protection has become out of
favor for cooling water systems as it can lead to local attack,
causes deposits leading to flow disturbance and it has no
scale protection
Preparation for cooling water treatment
-All anodes should be removed and the system inspected. No
galvanized piping is to be used (old piping can be assumed to have had
the Galvanizing removed). High quality water should be used and
chemicals measured and added as required. A history log should be
kept
Microbiological Fouling
Under certain conditions bacteria found in cooling water systems can
adapt to feed on the nitrite treatment. This can lead to rapid growth,
formation of bio-films, fouling and blockages.
Typical evidence is a loss of nitrite reserve but a stable or rising
conductivity level as the nitrate formed still contributes to the
conductivity,
Problems of this sort are rare due to the elevated temperatures and pH
levels. Should it occur treatment with a suitable biocide is required.
Piston cooling
Piston crowns attain a running temperature of about 450oC and in this
zone there is a need for high strength and minimum distortion in order
to maintain resistance to gas loads and maintain the attitude to the
rings in relation to the liner. The heat flow path from the crown must
be uniform otherwise thermal distortion will cause a non-circular
piston resulting in reduced running clearance or even possible contact
with the liner wall.
Efficient cooling is required to ensure the piston retains sufficient
strength to prevent distortion.
For medium and high speed engines the weight of the material
becomes important to reduce the stresses on the rotating parts. The
high thermal conductivity of aluminum alloys allied to its low weight
makes this an ideal material. To keep thermal stresses to a
reasonable level cooling pipes may be cast into the crown, although
this may be omitted on smaller engines. Where cooling is omitted, the
crown is made thicker both for strength and to aid in the heat removal
from the outer surface.
Piston cooling
Water Cooled
Oil Cooled
High specific heat capacity therefore
removes more heat per unit volume
Low specific heat capacity
Requires chemical conditioning treatment
to prevent scaling
Does not require chemical treatment but
requires increased separate and
purification plant
Larger capacity cooling water pump or
separate piston cooling pump and
coolers although less so than with oil
Larger capacity Lube oil pump, sump
quantity and coolers
Special piping required to get coolant to
and from piston without leak
No special means required and leakage
not a problem with less risk of
hammering and bubble impingement.
Coolant drains tank required to collect
water if engine has to be drained.
Increased capacity sump tank required
Pistons often of more complicated design
Thermal stresses in piston generally less
in oil cooled pistons
Cooling pumps may be stopped more
quickly after engine stopped
Large volumes of oil required to keep
oxidation down and extended cooling
period required after engine stopped to
prevent coking of oil
Pistons may be cooled by oil or water. Oil has the
advantage that it may be supplied simply from the
lubrication system up the piston rod. Its disadvantage
are that maximum temperatures is relatively low in
order to avoid oxidized deposits which build up on the
surfaces. In addition the heat capacity of oil is much
lower than that of water thus a greater flow is required
and so pumps and pipe work must be larger. Also, if
the bearing supply oil is used as is mainly the case a
greater capacity sump is required with more oil in use.
Water does not have these problems, but leakage into
the crankcase can cause problems with the oil (such
as Microbial-Degradation). The concave or dished
piston profile is used for most pistons because it is
stronger than the flat top for the same section
thickness
Sulzer water-cooled piston (rnd)
Sulzer water-cooled piston (rnd)
Increasing section thickness would result in higher thermal stress.
Sulzer piston require a flat top because of the scavenging and exhaust flow
arrangement (loop scavenging of RND etc). in order to avoid thicker sections internal
support ribs are used. However these ribs cause problems in that coolant flow is
restricted. With highly rated engines overheating occurred in stagnant flow areas
between the ribs and so a different form of cooling was required. The cocktail shaker
effect has air as well as water in the cooling cavity as the piston reciprocates water
washes over the entire inner surface of the piston just as in a cocktail shaker.
Unfortunately air bubbles become trapped in the water and flow to outlet reducing the
air content and removing the cocktail shaker effect. To avoid this problem air must be
supplied to the piston some engine builders use air pumps feeding air to the inlet flow.
The sulzer engine allows air to be drawn into the flow at a specially designed
telescopic transfer system. The telescopic arrangement is designed to prevent
leakage and allows air to be drawn into the coolant flow to maintain the cocktail
shaker effect.. Small holes allow connection from the main seal to the space between
the nozzles. Water flowing through the lower nozzle is subject to pressure reduction
and a velocity increase. The space between the nozzles is therefore at a lower
pressure than other parts of the system. Any water which leaks past the main seal is
drawn through the radial holes into the low pressure region and hence back into the
coolant flow. The pumping action of the telescopic draws air past the lower seal and
this is also drawn through the radial holes into the coolant flow..
B&W LMC oil cooled piston
The piston has a
concave top. This is
near self supporting
and reduces the need
for internal ribbing. It
prevents the cyclic
distortion of the top
when under firing
load. This distortion
can lead to fatigue
and cracking
Sulzer water-cooled piston (rnd)
The sulzer water cooled piston
differs from that of the Oil cooled
variety by the method it uses for
distributing the cooling medium. In
this case the piston is not
continually flooded but instead
contains a level governed by the
outlet weir. Cooling of the crown
occurs during change of direction
at the top of the stroke by so
called 'Cocktail shaker' action.
Thermal distortion of Piston
Shell and Tube Heat exchangers
• Tubes
aluminum –brass (Cu-76%,Zn-22%,Al-2%)
• Naval brass end plate (alpha-beta brass containing tin)
• End covers
cast iron (unprotected suffers graphitization)
• Shell
CI or MS
water velocity max.2.5 m/s
PHE
• Corrugation promotes turbulence and good heat
transfer due to breaking of boundary layer
• Corrugation makes the plates stronger
• The corrugation increases plate area
• Titanium and SS plates are the most common
based on duties.
• Nitrile rubber is found suitable up to 110’C, retightening as advised by makers are very
important as this can damage rubber, plate etc.
Heat exchanger maintenance
• Maintain cleanliness at heat transfer surfaces w/o
any flow restriction
• Corrosion of heat transfer surfaces due to sea water
is not uncommon. Sea water pressure is normally
maintained lower than the liquid being cooled to
avoid major problems
• Un protected steel will, in the presence of sea water
shall waste due to galvanic corrosion.
Electroplating with nobler metal, cathodic protection by
sacrificial anodes, impressed current system are the
remedies
Cooling water protection
• Effective protection against corrosion by
adding corrosion inhibitor
• Maintaining correct water quality
• Effective air vents
• Check and monitor of the water during service
• Using the correct procedure of cleaning and
maintenance
Corrosion
Cooling water quality
• Distilled water is common
This prevents deposits on hot surfaces which can lead to very high
operating temperatures
Water losses and overhauling
Sp heat capacity of some metals
Metal
Specific
Heat
cp
al/g° C
Thermal
Conductivity
c
k
watt/cm
K
Density
g/cm3
Electrical
Conductivity
1E6/Ωm
Brass
0.09
1.09
8.5
Iron
0.11
0.803
7.87
11.2
Nickel
0.106
0.905
8.9
14.6
Copper
0.093
3.98
8.95
60.7
Aluminu
m
0.217
2.37
2.7
37.7
Lead
0.0305
0.352
11.2
alpha-beta brass - a brass that has more zinc and is stronger than alpha brass;
used in making castings and hot-worked products
System Volume in relation to centrifuging process
System Volume in relation to centrifuging process
1. High operating temperature
2. Air Admixture
the total oil quantity is such that it circulates 15-18 times per hr.
3. Catalytic action
due to copper, iron, varnish, lacquer rust etc
being present in oil