Tuesday, March 31, 2020

Crankcase explosion

The atmosphere inside a crankcase is stable and will not allow combustion or an 
explosion to occur as there is no ignition or fuel source. 
Hence the first event is the production of an explosive mixture. This will occur when 
the lube oil in the crankcase is heated by a “hot spot” and lube oil coming into contact 
with this will be evaporated. The evaporated oil then rises within the crankcase, and 
then condenses in a cooler part of the crankcase. The resultant white mist is within the 
explosive envelope, and is thus flammable. 

The second event is the ignition of this white mist by either the same or another hot 
spot within the crankcase. When the oil mist is ignited, a crankcase explosion will 
occur, which will raise the pressure within the crankcase. 
One of the common areas of overheating is the various bearings within the crankcase. 
Hence bearing temperature monitors could be used to indicate that a bearing is 
overheating and could be oil mist generation site. 

The rapid pressure rise within the crankcase can cause the engine structure to be 
blown apart, causing physical damage, and the resultant flame travelling across the 
engine room space causing personnel injury. This pressure rise is limited by the 
statutory use of relief doors fitted to the crankcase. These doors will open when the 
pressure rises above 0.02 – 0.1 bar, and prevent the over-pressure of the engine 
structure. The doors also perform the added function of preventing fresh air ingress 
into the crankcase where hot burning gases are present, by the quick closing action of 
the relief door. 

As the explosion is an uncontrolled event, then great care must be taken to ensure the 
safety of the engineers within the engine room. MAN B&W recommend that 
1. Move away from the crankcase doors immediately 
2. Reduce speed to slow, and ask the bridge to stop 
3. When the engine has stopped, close the fuel supply 
4. Stop the auxy blowers 
5. Open the skylight and/or stores hatch 
6. Leave the engine room 
7. Lock engine room entry doors and keep away from them 
8. Prepare the fire fighting equipment. 
9. Do not open the crankcase for at least 20 minutes after stopping the engine, and 
ensure that the oil mist detector alarm (or bearing temperature monitor) has reset 
10. Stop the LO circulating pump. Shut the starting air, and engage the turning gear. 
11. Locate the “hot spot” (source of the oil overheating) 
12. Make a permanent repair to the fault

Continous survey of machineries (CSM)

Classification Societies within IACS (International Association of Classification 
Societies) such as LR, ABS, GL, DnV, BV, etc require that all machinery under their 
Rules must be surveyed every five years. To reduce this heavy work load this would 
entail if all surveys were done at one time, all Class Societies will allow some items to 
be surveyed in rotation, over a number of annual cycles. Hence over the five year 
cycle of surveys, surveys are continuous, hence the term CSM

Microseizure and Cloverleafing

Cloverleafing will be caused when the supply of lube oil is not uniform around the 
radial bore of the liner. The normal effect is for the oil to reduce in alkalinity away 
from the injection point, thus if the oil becomes acidic then high corrosive wear rates 
will result. This will cause uneven bore wear rates, with heavy wear in the areas 
furthest away from the oil injection points 

Microseizure is caused when the liner and piston ring material is pressed together 
causing localised welding of the material in the absence of sufficient lube oil. The 
causes are insufficient oil and/or excessive cylinder pressures causes heavy ring/liner 
contact forces. The appearance is heavy scratching/tearing in the vertical direction, 
together with a localised hardening of the ring and liner material. 

The cylinder oil will require 
• Ample viscosity to separate the surfaces under high loading conditions 
• Sufficient alkaline reserve to neutralise the acids formed by combustion of 
residual fuel oil 
• High levels of detergent to maintain piston ring cleanliness and free ring 
movement 
• A level of anti wear properties to minimise micro seizure 
• The ability to burn without resid

Water hammer


Water hammer occurs when steam is admitted into a cold pipeline. The steam condenses 
producing both water, and a vacuum (when the water seals the pipeline from the steam 
supply). This vacuum causes the water plug to be drawn into the closed end of the pipe with  increasing velocity producing high impact forces on the pipework. This impact force can be  high enough to rupture the pipeline.

Water hammer is avoided by slowly admitting the steam into the cold line, and 
draining/venting the cold line to minimise the vacuum forming, and assist in draining the 
condensate. 
Testing of the boiler water is important to determine that the level of chlorides is within acceptable limits (below 200 ppm) to prevent hard scale 
and pitting   the reserve of boiler chemicals within the boiler water is accepted (P Alk above 100 ppm,  and Phosphate above 20ppm) 

Treatment of the boiler water with chemicals will ensure 
Alkalinity reserve against boiler water space corrosion 
Scale build-up is minimised 
Enough coagulant levels exist to minimise sludge build-up within the boiler 
The strength of the boiler drum is maintained
 
Boiler problems would be minimised by: 
Reducing the steam load and demand on the aux. boiler to reduce firing temperatures 
Ensuring hotwell feed temperatures are at the recommended 95o C to minimise oxygen 
levels in the feed 
Minimise blowdown of boiler water, whilst ensuring chloride level is not excessive, to 
avoid loss of the boiler chemicals

Bridge control of a large slow speed propulsion enigne how starting and reversal is achieved

Forr large propulsion engines the Bridge Control is achieved by using the telegraph to 
select the desired speed and direction. 
When the bridge telegraph is placed in Stop, the fuel is prevented from injecting as 
the fuel pump puncture valves are energised. 
When the bridge telegraph is moved to an ahead command, the air start and fuel cams 
are placed in the required direction, and the start command is given. This will admit 
starting air to the engine. Once the engine is turning above the starting speed, the air is 
closed and fuel is admitted. The quantity of fuel admitted will depend on the position 
of the telegraph handle, i.e. slow ahead, full ahead, etc. 
When the bridge telegraph is moved to an astern direction, the air start and fuel cams 
are reversed. Once the air start is reversed, the engine will start as detailed for the 
ahead start

Monday, March 23, 2020

Fuel index numbers

The fuel index is an indication of the active stroke of the fuel pump. This controlled  by push rods on the individual fuel pumps connected to the fuel rack , which in turn is controlled by the engine governer
When the engine is given a speed command the governer increases the fuel index , subkect to certain limitations , until the requested engine speed is reached

Galling

Galling usually refers to the adhesive wear  and transfer of material between metallic surfaces in relative converging contact during sheet metal forming and other industrial operations .

Sac volume of fuel injectors

The " sac" is a small volume within the fuel flow path of an electronic fuel injector . In this study , it is defined as the volume between the valve seat ( fuel shut off point ) and the entrance to the final metering orifice of the injector . This sac causes fuel injectors to deliver uncalliberated excess fuel when the engine is operated under closed throttle , high manifold vacuum conditions such as vehicle decelerations or idle.

Why Negative cam or Inward cam used in Air distributer of MAN B&W starting air system .why not a possitive cam ?

The cam for the air distributer serves its purpose only while starting the engine . Actually if you see there is no other requirement of this particular cam while the engine is continue in operation. Therefore if a cam ( normal possitive ) is used then wear of this particular cam will be abviated and will be expensive to repair / replace this cam . So when the engine has started and the vessel is enroute a voyage , then this particular cam turns without any running gear touching it. It comes into play only starting

Differance between floating ring and blowdown ring ?

Floating ring acts as a cylinder for waste steam piston while blow down ring is on the valve seat and is used to adjust the blowdown of the valve

why thrust pad in pieces , why not single piece?

  • By being in pieces each pad can generate its own hydrodynamic film  , so as compared to a normal journal bearing more area can be utilized for the film and taking load.
  • The wedge formed using a single piece would make it tilt in such aanner causing huge clearances at one end 
  • By single pieces you can get more or less even oil distributiom