Q. What is crankcase explosion and how it occurs?
Ans – We know that large amount of oil droplets are present everywhere in the crankcase and normally the size of droplet is 200 microns which is not harmful at all.
Note – Flash point of oil is normally 230ºC(pure lube oil but when it mix with diesel it drops) but when hot spot occur may be the reason given below :-
- Too much clearance between the bearing so oil film may breakdown
- Crankshaft is not properly aligned
- Diaphragm not sealing properly
- If piston crown cracks then exhaust gas may leak to the crankcase
- In man b&w if chain is too loose or too tight then chain may run hot which act as the hot spot
- If matching gear don’t have proper backlash then lubrication is not so effective and generate a hot spot
- Circulation of lube oil is not proper
So due to all these reason which are given above create a hot spot in the crankcase and when oil comes into the contact of this hotspot it start to vaporize. Vapours are lighter so it goes up in the crankcase and form the white cloud just below the stuffing box (now the size of oil droplets becomes 10 microns because the formation of vapours), when this vapour comes down and again come in contact with the hotspot primary explosion takes place.
After primary explosion pressure inside the crankcase increases suddenly, so due to this crankcase relief valve activates and releases all the gases into the engine room and due to this crankcase again goes back to the vaccum. If at that time fresh air come from engine room to the crankcase then it will lead to the secondary explosion which is a big disaster in the engine room.
So our main objective is to avoid primary explosion but if in case it occurs then we will have to make sure that secondary explosion should not occur
How to avoid secondary explosion –
- All the main bearing and thrust bearing have high temperature alarm which give you the indication of increasing temperature inside the crankcase
- In new engine we have sensor which tell you how much wear down of main bearing will take place. Note – If unequal wear down takes place it mean oil flow is not same in all the bearings)
- We have NRV type relief valve whose function is to relief all the pressure which is generated during the primary explosion inside the crankcase
Q. What is crankcase relief valve and how to pressure test it?
Ans – As a practical safe guard against these explosions, explosion relief valves and doors are fitted. These valves serve two functions.
- To relieve excess pressure inside the crankcase thereby normalizing the pressure.
- To prevent the flames inside the crankcase from coming out and counter damage
- Even it won’t allow the outside flame to come in because of the flame arrester.
Note – In sulzer engine opening pressure is 0.2bar above atmospheric pressure.
In man b&w opening pressure is 0.05 bar above atmospheric pressure.
Now the main question is how to check the opening pressure of crankcase relief door
For that we have the spring scale and rod which we fit on the crankcase door and
pull the rod you will get the reading on the scale that at what point it will open (from scale you get reading in KG)
Pressure = Force / Area
Force = the value of force you will get from the scale in KG
Area = area of the door you will get to know out from the manual
So with this you can able to calculate the pressure setting of the crankcase relief door
Note – A deflector is fitted on the outside of the engine to safeguard personnel from the outflowing gases, and inside the engine, over the valve opening, an oil wetted gauze/flame arrestor acts as a flame trap to stop any flames leaving the crankcase. After operation the valve will close automatically under the action of spring.
Note – Sometime surveyor ask why crankcase relief doors are there?
Above we already told you so many reasons but sometime surveyors wants to listen about the regulation related to the relief door-
- The internal combustion engine of the cylinder diameter of 200 mm or a crankcase volume of 0.6 m³ & above shall be provided with the crankcase relief valve of a suitable type with sufficient relief area.
- In small engine cylinder diameter does not more than 300mm, crankcase door of which are normally very strong. It may have relief valve or valves at its end.
- In large engine, cylinder dia. over 300mm, It required one relief valve to be placed on each crankcase door.
- Its free area should not smaller than 45cm² and there shall be minimum of 115cm²/m³ of the gross crankcase volume.
- Spring setting for the opening pressure is 0.07 bar at an internal pressure & will close when the pressure has been relieved.
- The valves open smartly and close positively and rapidly.
Q. As a 4th Engineer you are on the watch and one unit OMD activates what will be your action?
Ans – Every people have different opinion on this question that was the reason surveyor won’t accept your answer most of the time, so below we try to give you the standard procedure that what to do in this kind of scenario:-
1. Inform to the wheel house
2. Press engineer’s call alarm (to call all for help)
3. As soon as possible stop the engine
4. But make sure not to stop any lubricating lube oil pump
5. But fuel pump to be stopped if it was not common for both the main engine and generator.
6. Open ventilation to be run which create the vaccum inside the crankcase
7. Don’t stand in front of the crankcase door
8. Connect all the fire hoses and make sure to ready all the equipment near to the bottom platform
9. Leave engine room for some time
10. Come down to engine room after 1 hour just to give the sufficient time so that the engine should be cool down)
11. With the help of the palm and hand check the condition of the crankcase door if it is still hot, we recommend you not to open the door at this instant of time)
12. If crankcase door is not too hot then you can open the crankcase door but make sure everything is ok else you may lead to the secondary explosion.
Note – Just slack open the door not to be completely open, just to confirm that crankcase should not be pressurize from inside.
13. If everything seems to be ok then stop the lube oil pump and ventilate the crankcase fully before man entry into the crankcase
14. Enter into the crankcase by taking one torch with you
15. If any bearing temperature is high it gives you the white metal (which give you the indication of hotspot)
16. If you find any bluish tint on the surface it mean that was the hot spot and due to the lube oil quenching action it changes the colour.
17. After complete inspection if you did not find anything then check all the OMD alarms are working satisfactorily or not (sometime OMD may give you the wrong indication)
18. Start the engine and slowly increase the speed and do continuous monitoring.
Q. what is OMD, explain its construction & working. What maintenance is to be done on OMD.
Ans – An overheated diesel engine can become a source of fire & extreme Havoc if periodic maintenance & proper practices are not performed. Oil Mist is formed in the crankcase when the lubricating oil is splashed by moving and rotating parts of the engine. This oil mist reduces the flash point of the oil, allowing it to catch fire in presence of a hotspot. It is important that this concentration of oil is kept under control and in case its presence is detected, the engine should be stopped or the speed lowered. But how will an engine detect or predict that the level of oil mist has increased in the crankcase?
Oil Mist detectors are used for this purpose. Crankcase of each of the cylinder is connected to the OMD, which continuously checks the air sample from each cylinder. If the amount of the mist increases, OMD raises an alarm. Let’s see how it detects the mist.
Generally only OMD is fitted in each engine. OMD does not reduce the formation of mist, but only give warning in case the concentration rises above the level at which an explosion can take place. The arrangement of the OMD consist of two tubes of the equal sizes. Both these tubes are places parallel to each other. At one end of each tube, a photo-electric cell is fixed. Photo-electric cells produce an electric current when light falls on their surface. The amount of the electric current produced is directly proportional to the intensity of light falling on it. The other ends of both the tubes are sealed by fitting lens that permits light to pass through them.
Equal intensity of light is reflected on the photo-electric cells using a lamp. Light passes through the lenses after being reflected by the mirrors. One of the tube has an inlet & outlet connection for introducing oil mist.
Out of the two tubes, one is called the reference tube & the other is called the measuring tube. Measuring tube has a connection for the oil mist, which is extracted from the crankcase with the help of an electric extractor fan. The reference tube is filled with clean air & is used as a reference for measuring the level of mist in the measuring tube. Samples from each cylinder is observed by using a rotating selector valve, which connects each cylinder in sequence to the OMD.
If the concentration of Oil mist in the measuring tube rises, the intensity of light reaching the photo-electric cell decreases. Now as both the tubes are electrically connected, reduction in the generation of electric current will create an electrical imbalance between the two cells, which will lead to ringing of the alarm.
When Oil mist is identified, the rotating selector valve immediately stops to indicate the cylinder with high concentration of the oil mist. On indication of an alarm, the engine should be slowed down or stopped to avert damage or explosion.
It is essential to carry out routine maintenance of the OMD to prevent false alarms. The sensitivity of OMD should be checked on the regular basis. As all the samples contain a small amount of the mist, the lenses & mirrors tend to get dirty & thus require periodic cleaning. The extractor fan & the rotating valve should be checked to avoid chocking of a particular sampling tube. The sampling tubes that connect cylinders to the OMD should not have any loops & also shouldn’t be of length more than(>) 12.5 meters.
Q. What do you understand by the term scavenge fire and reasons for the same and what action you will take.
Ans – For the scavenge fire to begin there should be a combustible material, oxygen or air to help in combustion, & a source of heat at a temperature high enough to start combustion. In the case of scavenge fires the combustible material is the 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 the faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from the defective injectors, injectors with incorrect pressure setting fuel particles striking the cylinders & other similar causes. The oxygen necessary for the combustion comes from the scavenge air which is in plentiful supply for the operation of the engines. The source of heat for the ignition comes from the piston blowby, slow ignition & after burning, or extreme exhaust back pressure, which leads to the blowback by the scavenge ports.
Causes of scavenge fire
There are many reasons for the scavenge fire. But, the main ones are as below:
1. Blow past of the combustion products caused by the leaky, sticky or broken piston rings, worn out liner, faulty cylinder lubrication, or insufficient axial clearance of the piston rings.
2. Overheated piston emits heat to the under piston area caused by the faulty atomization & injection pressure, faulty fuel pump timing, loss of compression engine overload, failure of the coolant circulation or insufficient cooling due to formation of scale.
3. Blow back of exhaust gases generated by the exhaust back pressure or deposits on the exhaust ports, fouling of grid before turbine inlet, fouling of turbine blades, choking of the EGB or economiser gas outlet
4. Presence of fuel oil in the scavenge spaces due to the defective fuel injectors, incorrect pressure setting of injectors or the fuel particles landing on the cylinder liner due to excessive penetration.
5. Excessive cylinder lubrication which is drained down to the scavenge spaces.
6. Oxygen is plenty during engine operation.
Indications of the scavenge fire
There are a few signs which specify a scavenge fire. One should be very cautious in case any of the below mentioned conditions are noticed.
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 the back pressure is generated under the piston space due to the fire.
5. Smoke coming out of the scavenge drains.
6. The paint blisters will be formed on the scavenge doors due to the high temperature but this will occur only in large fires & extreme cases.
Actions to be taken
Action taken in case of the scavenge fire rely on the type of the fire, whether small or large. In case of large fire the following signs will be easily visible – the peeling or blistering of the paint, large reduction in the engine rpm & surging of the turbocharger.
For small fires
1. Start reducing the engine rpm & reduce it to slow or dead slow.
2. Increase the cylinder lubrication of the affected unit. Special attention is to be given for this as this does not feed the fire. In case of increase of fire do not increase the lubrication.
3.The fire can be due to the leaky fuel valves, so lift up the pump of the affected unit. Keep the scavenge drain closed.
5. Keep monitoring the scavenge & exhaust temperatures & let the fire starve & wait for it to burn itself out.
6. After finding that the fire is out start increasing the rpm slowly.
7. Keep observing the scavenge temperature for any signs of re-ignition.
For large fires
1. Stop the engine immediately & engage turning gear, & keep engine rotating with the turning gear.
2. Extinguish the fire with fixed fighting system for the scavenge fire. This may be CO2 system or a steam connection for smothering the fire.
3. In case fixed system is not present on very old ships an external cooling is provided to prevent distortion due to the heat.
4.Once after finding that the fire is extinguished. The scavenge space is enabled to cool down & later opened for inspection & cleaning of the scavenge space.
Inspection after Scavenge Fire
1.Intense fire can cause distortion & may upset piston alignment
2.Check by turning the engine & watch movement of the piston in the liner, check for any sign of binding at part of stroke (Binding indicates misalignment of piston)
3.Check spring on the scavenge space relief device, if the device was near the set of fire
4. Piston rod packing spring also be checked, which may have become weakened by overheating
5.Check piston rings & liner for any distortion or reddish burning mark
6. Check diaphragm & frame near the affected part
7.Check guides & guide shoes
8. Check tension of the tie bolts.
Prevention of scavenge fire
1.Clean scavenge space & drain at regular intervals
2.Keep the scavenge space drain open at regular intervals
3.Excess cylinder lubrication must be avoided.
4. In case of timed lubrication, the time has to be checked as per PMS
5. Piston rings should be properly maintained & lubricated adequately
6. Piston rod stuffing box must be maintained to avert oil ingress in to the scavenge space.
7. Prolong engine running or any cylinder over loading to be avoided.
8.Cylinder liner wear must be within admissible limits
Scavenge Space protection Devices
1. Electrical temperature sensing device installed within the trunking, which will automatically sound an alarm in the event of excessive rise in the local temperature.
2. Pressure relief valves consisting of the self closing spring loaded valves are installed & should be examined & tested periodically.
3.Fixed fire extinguishing system may be CO2, Dry Powder or the Steam.
Note – Before oiler start cleaning the scavenge space, 2nd engineer always go inside the scavenge manifold to check maximum oil is collected near to which unit which help you to make out that something is not right with that unit and we normally fed the lube oil with respect to the sulphur content in the fuel oil.
Q – What all things you will inspect during the scavenge space inspection?
Ans – There could be number of things which need to be checked during the inspection of scavenge space: –
1. Check the main blower inlet flap valves. These should be free to open and should close when released
2. Turn the engine so that the piston is viewable through the scavenge ports. Inspect the piston skirt for signs of scuffing or abrasion. Inspect the rings for damage and the ring grooves for evidence of carbon build up. These rings are in good condition.
Note – At the time of inspection if you ever go with the 2nd engineer in scavenge space inspection then you notice that with the brass rod he try to push the piston ring just to check the elasticity of the piston ring if you feel hard to move it then it means either it is stuck or break
3. Turn the pistol down, so that the crown can be inspected. Look for excessive deposits, burning sign of poor injection, or cracking.
Note – If you find oil droplets on the top of the piston crown it simply give you indication that you injector is dripping which may lead to crown failure also.
4. Then look up the liner using the mirror. Look for evidence of corrosion, scuffing & abrasion.
5. Check all the scavenge drains are cleared or not
6. Check the condition of sludge by squeezing the oil in your finger because sometime you find a very fine metal particles in that.
7. Check the condition of locking wire below the piston which hold the skirt and crown together
8. Check the relief valve at the manifold of the scavenge space at the entrance (normally pressure setting is 10 percent more than the scavenge pressure)
9. If you found black spot on the piston ring it simply” means blowpast from that particular unit
10. If you found any oil is dripping from the underside of the piston it means your o-ring inside the piston cooling space is leaking.