Marine Diesel Engines II

Failure Detection

Prof. Dr. Cengiz DENİZ

 Crankcase Explosions
The first major crankcase explosion was noted in 1947 when 28 people
lost their lives in an engine explosion.
Crankcase explosions can result in loss of life, shipboard fires, loss of
power, and expensive ship repairs.
In most cases, the damage has been limited to the engine proper.
However, in cases where the oil mist is ignited outside the crankcase,
the blast can cause severe damage in the engine room, especially to
the electrical equipment. In some cases, in narrow engine rooms, the
pressure wave has caused damage to mechanical items. Doors and
lifts have been damaged and, in rare cases, also floor plates have been
torn loose and thrown around.

I
Conditions required for crankcase explosion

• The following conditions are required for a

crankcase explosion to occur:
• Ignitable substance
• Oxygen
• Source of ignition
 Ignitable substance
Oil mist generation by lubricating oil
A mist of lubricating oil is always present in the crankcase of an
engine during normal operation. The oil mist formation is
influenced by engine design, gas and oil pressures,
temperatures and oil flow rates. The critical conditions required
to create a crankcase explosion however are the presence of
ahot spot and for the formation of specific sizes of oil mist
droplets.
Oil mist generated by oil with fuel contamination
As the ignitibility of fuel is almost equal to that of lubricating oil,
the same operating conditions are present. Therefore any
contamination of oil by fuel does not increase the danger of
explosion.
 Source of ignition

The source of ignition in general is a spark or a hot spot

caused by:
Damaged pistons
Damaged bearings
Sparking (metal against metal)
Damaged stuffing box seals (piston rod)
Damaged drive components of high pressure fuel pumps.
 Crankcase explosions (I)
• Crankcase explosions which are violent and cause the
maximum damage are thought to be caused by turbulent
flame fronts having enough room to accelerate to sonic
velocities thus raising the crankcase pressure high
enough in a very short time to have serious
consequences.
• A mist of lubricating oil is always present in the
crankcase of an engine during normal operation. The oil
mist formation is influenced by engine design, gas and
oil pressures, temperatures and oil flow rates.
• The critical conditions required to create a crankcase
explosion however are the presence of a hot spot and for
the formation of specific sizes of oil mist droplets.
 Crankcase explosions (II)
• As oil mist is highly explosive, there will be a risk that the hot spot
which created the oil mist will become the ignition point for an oil
mist explosion.
• A crankcase explosion is caused by the ignition of oil mist from a
hotspot, with the mist itself being caused by high temperatures on
an internal metal surface.
• Oil mist can provide the first signs of impending problems for all the
moving parts of the engine.
• Temperature and pressure measurements can also provide an early
warning of potential problems
• An oil mist detection system can avoid damage to the engine and
injury to the crew by allowing early detection of engine wear and
bearing damage that may create an explosive environment within
the engine.
Crankcase explosions (III)
 Crankcase explosions (IV)
• Under normal conditions the atmosphere in the crankcase
when the engine is running contains a large amount of
relatively large oil droplets (200 micron) in warm air. Because
of the droplets small surface area to volume ratio, the
possibility of ignition by a heat source is very low.
• Oil mists can be readily detected at concentrations well below
that required for explosions, therefore automated detection of
these oil mists can be an effective method of preventing
explosions.
• The Oil mist detector takes continuous samples from the main
engine crankcase and check whether the sample
concentrations of mist are well below the level at which a
crankcase explosion can take place.
• Oil mists are formed at temperatures of around 200-350 C
• Ignition occurs at under 500 C
 Crankcase explosion
• A mechanical fault, overload or rubbing of moving parts creates a
hot spot somewhere in the crankcase.
• When lube oil comes in contact with hot spot having a temperature
above 200°C, starts vaporizing.
• Movement of the engine component allows the vapours to moves
inside the engine when vapours pass to a cooler area; they re-
condense and forms a white mist of oil droplets of size 0.005-
0.01mm.
• The concentration of oil mist increases steadily and reached the
lower explosive limit of around 50 mg/l.
• When the hot spot reached its temperature 500 °C, ignite the oil mist
and fire take place.
• When oil mist concentration reached a certain limit and comes in
contact with a hot spot, it ignites and fire takes place as primary fire.
The extent of fire will depend upon the amount of oil mist formed
inside. This primary explosion might be sufficient to lift the
crankcase relief valve.
 Secondary Explosion
Following the venting of the explosion through the relief valves, there is
a drop in crankcase pressure to below atmospheric pressure. This can
cause air to enter the crankcase resulting in another flammable mixture
to be developed resulting in a secondary explosion to occur.
The secondary explosion is more violent and can result in crankcase
doors being blown off the engine, and fires starting in the engine room.
 Solas Chapter II-1 Regulation 47
• Internal combustion engines of 2,250 kW
and above or having cylinders of more
than 300 mm bore shall be provided with
crankcase oil mist detectors or engine
bearing temperature monitors or
equivalent devices.
 Crankcase relief valves
• The engines are equipped with crankcase relief valves sufficient for
protecting the engine against the impact of an oil mist explosion by
relieving the pressure inside the crankcase.
• Crankcase relief valve is the most significant safety component that
is used to minimize damage in case of crankcase explosion.
• Crankcase relief valves have two main functions;
• relieve the pressure in crankcase during explosion, and stop
prevent flames to exit out.
• In the event of an oil mist explosion in the crankcase, the very fast
propagating pressure wave will send large amounts of yet unburned
oil mist out into the engine room. The relief valve is equipped with a
flame arrester to quench the much slower propagating flame front,
thereby preventing any outside ignition and fire.
 It is fitted to release any sudden rise of internal pressure with large
free escape area thus to prevent secondary explosion.
Non-return valves is fitted on the crankcase doors
A non-return valve is fitted on the
crankcase door, which only allows the
inside excess pressure to escape. It will
not allow air to go inside the crankcase
space.

The relive valve consists of a light spring

that holds the valve closely against its
seat. In the case of excess pressure, the
spring gets compressed due to the
pressure force and allow to the opening
of the valve, which passes the excess
pressure. Outside of the valve
deflector(disc) is fitted which deflect the
pressurized gases and provide the
safeguards to the personal escaping.
When pressure goes down to the set
pressure spring expands and closes the
valve, which will prevent the air to go
inside.
 Oil mist detector (OMD)
• Oil mist detection system is one of commonly used safety aids to monitor
and detect engine’s critical conditions in early stages, to protect engines
from serious damage.
• SOLAS regulation II-1/47.2 issued by IMO required internal combustion
engines that produce power of 2,250 kW or more, or having cylinders bore
diameter of 300 mm or more, must be fitted with oil mist detectorsor
equivalent system
• A safety device, OMD is fitted in the crankcase to continuously monitor the
oil mist inside the crankcase by taking sample of air from the space(one at a
time from each unit and chain case). Whenever the oil mist level reached
set value it will give the alarm to control room.
• Oil mist detector is a safety device fitted in marine diesel engines to check
and monitor crankcase environments for the presence of oil mist. It can
detect the presence of oil mist within its flammable range.
 It is fitted to detect the oil mist concentration in the crankcase and to give
early warning.
 To prevent the primary explosion.
Oil mist detector
Oil mist detector
• Fan-draws the sample from the sample points through the reference
and measuring tubes via non-return valves.
• Reference tube-measures the average density of the mist within the
crankcase, as there will always be some mechanically generated
mist.
• Measuring tube- measures the opacity of the sample by means of a
photoelectric cell as with the measuring cell. To exclude variables in
lamps a single unit is used with beams directed down the tube by
mirrors.
• The photoelectric cell gives an output voltage proportional to the
light falling on it. In this way the opacity of the sample is measured,
the voltages generated in the cell in the measuring and reference
tubes are compared in an electronic circuit. The difference is
compared to a potentiometer varied setpoint which if exceed initiates
an alarm circuit.
LIGHT SCATTER
Oil mist detector
 Action in Case of OMD alarm (General)
• The action to be taken in the case of an alarm indicating a high oil mist in the
crankcase, high bearing temperature or other indication of the possibility of an
explosion occurring will depend on the age and type of installation, however the
general rule is to STOP THE ENGINE!!
• If the engine is manually controlled from the engine room, then on an alarm situation
occurring, the engine should be reduced immediately (ring telegraph to half ahead,
reduce load, contact bridge, ask to stop), and then stopped as soon as possible. Call
for assistance.
• Once the engine has been stopped, keep the lube oil pumps running. Engage turning
gear (if it can be approached without passing the crankcase). Evacuate personnel
from engine room. Do not approach crankcase and do not remove crankcase doors
for at least 20 minutes - a sudden inrush of air may cause an explosion.
• Otherwise the engine should be stopped and turned on gear until cooled.
• Once cooled the engine can be opened and ventilated (the crankcase is an enclosed
space).
• An inspection should be made to locate the hotspot, the engine should not be run
until the fault has been rectified.
 Action in Case of OMD alarm (I)
• Keep a safe distance from the crankcase door and engine. It may cause
personal injury.
• Reduce the rpm of the engine, if not engine will slow down automatically
after OMD alarm.
• Inform the bridge and ask them to stop the engine.
• After getting confirmation from bridge stop the engine
• Stop the fuel oil supply to the engine and maintain the cooling system.
• Open the skylight and stores hatch to release pressure from the engine
room
• Leave the engine room to avoid human injury.
• Lock the crankcase door and leave the space immediately
• Get prepared for the firefighting because secondary explosion may lead to a
large fire in engine room.
• Do not open the crankcase until at least 20 minutes after stopping the
engine, fire may come out, which may lead to another fire.
• Open the door carefully, do not use any naked light, lighter may cause re-
ignition and fire. All these should be done under the supervision of a
competent engineer.
 Action in Case of OMD alarm (II)
• Circulating oil pump to be stopped. Take off all the lowermost doors on the
side of the crankcase. Cut off the starting air supply and engage the turning
gear.
• Find out the hot spot.
• Keep possible bearing metal found at the bottom of the oil tray for later
analyzing of the incident.
• Prevent further hot spot by preferably making a permanent repair of the
component.
• Make sure that the respective sliding surfaces are in good condition.
• Circulation of the oil supply to be checked and should b in order.
• Start the circulating oil pump and turn the engine by means of the turning
gear to avoid seizing of the piston.
• Ensure the oil is flowing from all bearing, spray pipes and spray nozzles in
the crankcase and thrust bearing.
• Check if there are any leakages from piston or piston rods.
Injection Timing Early
Injection Timing Early
Scavenge fire
 Scavenge Fires
For a scavenge fire to begin there must be present a combustible
material, oxygen or air to support combustion, and a source of heat at a
temperature high enough to start combustion.
In the case of scavenge fires the combustible material is oil. The oil can
be cylinder oil which has drained down from the cylinder spaces, or
crankcase oil carried upwards on the piston rod because of a faulty
stuffing box. In some cases the cylinder oil residues may also contain
fuel oil. The fuel may come from defective injectors, injectors with
incorrect pressure setting, fuel particles striking the cylinders and other
similar causes.
The oxygen necessary for combustion comes from the scavenge air
which is in plentiful supply for the operation of the engines.
The source of heat for ignition comes from piston blowby, slow ignition
and afterburning, or excessive exhaust back pressure, which causes a
blowback through the scavenge ports.
 Causes of scavenge fire

There are many reasons for scavenge fire. However, the main ones are
as below:
1. Excessive wear of the liner.
2. The piston rings might be worn out or have loose ring grooves.
3. Broken piston rings or rings seized in the grooves.
4. Dirty scavenge space.
5. Poor combustion due to leaking fuel valves or improper timing.
6. Insufficient or excess cylinder lubrication.
 Causes of scavenge fire
• Blow past- leakage of combustion products from combustion chamber caused by
damage (sticky or broken) piston rings, worn out liner, improper cylinder lubrication,
or insufficient axial clearance (clearance between the inner part of ring and groove) of
the piston rings.
• Overheating of the piston– Failure of flow of piston cooling oil will cause overheating
of the piston, it will dissipate the heat to the under piston area. Other reasons may
include bad combustion because of faulty atomization, injection pressure, wrong fuel
pump timing, loss of compression, overload on the engine.
• Presence of the fuel oil –If the unburnt fuel oil is present in the scavenge spaces due
to defective fuel injectors, incorrect pressure setting of injectors or fuel particles
landing on the cylinder liner due to excessive penetration.
• Blowback of the exhaust gas –It will give heat to scavenge and it is caused by the
high back pressure of exhaust gas, deposits on the exhaust ports, fouling of grid
before turbine inlet, fouling of the turbine blades, choking of exhaust gas boiler or
economizer gas outlet.
Deposits under scavenge space include:
– Inadequate draining of the scavenge spaces.
– Excessive cylinder lubrication will cause deposits of
lube in scavenge spaces.
– Leakage of the stuffing box.
– Fouled scavenge manifold and space.
 Scavenge fire
• If a scavenge fire starts two immediate objectives arise- they are to contain
the fire within the scavenge space of the engine and to prevent or minimize
damage to the engine. The engine must be put to dead slow ahead and the
fuel must be taken off the cylinders affected by the fire. The lubrication to
these cylinders must be increased to prevent seizure and all scavenge
drains must be shut to prevent the discharge of sparks and burning oil from
the drains into the engine room. A minor fire may shortly burn out without
damage, and conditions will gradually return to normal. The affected units
should be run on reduced power until inspection of the scavenge trunking
and overhaul of the cylinder and piston can be carried out at the earliest
safe opportunity. Once navigational circumstances allow it, the engine
should be stopped and the whole of the scavenge trunk examined and any
oil residues found round other cylinders removed. The actual cause of the
initiation of the fire should be investigated.
 Indications of scavenge fire
There are a few signs which indicates a scavenge fire. One
should be extremely cautious in case any of the below
mentioned conditions are observed:
1. Scavenge temperature will start increasing.
2. The turbochargers will start surging.
3. High exhaust temperature.
4. Loss of engine power and reduction in rpm. This happens
because a back pressure is created under the piston space due to
fire.
5. Smoke coming out of the scavenge drains.
Poor Atomization
Poor Atomization
Poor Atomization
Poor Atomization
Injection Nozzle Wear
Injection Nozzle Wear
Injection Nozzle Wear
Exhaust Valve Open Late
Exhaust Valve Open Late
Exhaust Valve Open Late
Scavenge fire

• After extinguishing the fire and cooling down, the scavenge

trunking and scavenge ports should be cleaned and the
trunking together with cylinder liner and water seals, piston,
piston rings, piston skirt, piston rod and gland must be
inspected.
• Heat causes distortion and therefore checks for binding of
piston rod in stuffing box and piston in liner must be carried
out. Tightness of tie bolts should be checked before
restarting the engine.
• Fire extinguishers should be recharged at the first
opportunity and faults diagnosed as having caused the fire
must be rectified.
 Preventionof Scavenge fire
• To prevent scavenge fires good maintenance and correct adjustment must be carried
out.
• Scavenge trunking must be periodically inspected and cleaned and any buildup of
contamination noted and remedied.
• The drain pockets should also be cleaned regularly to remove the thicker carbonized
oil sludges which do not drain down so easily and which are a common cause of
choked drain pipes.
• Scavenge drains should be blown regularly and any passage of oil from them noted.
• The piston rings must be properly maintained and lubricated adequately so that ring
blow-by is prevented. At the same time one must guard against excess cylinder oil
usage. With timed cylinder oil injection the timing should be periodically checked.
• Fuel injection equipment must be kept in good condition, timed correctly, and the
mean indicated pressure in each cylinder must also be carefully balanced so that
individual cylinders are not overloaded.
• If cylinder liner wear is up to maximum limits the possibility of scavenge fires will not
be materially reduced until the liners are renewed.