Answer EKM Question 14

Q14. State how EACH of the following defects becomes apparent, describe its effect on engine operation or safety and indicate the corrective action required to restore normal engine condition;

A. Leaking air inlet and exhaust valves;

B. Leaking air start valve

C. Cracked cylinder liner;

D. Broken piston rings.

Answer: A. Leaky air Inlet valve 

• compression pressure drops. 
• Peak pressure drops. 
• Air available for combustion reduces hence increased exhaust temperature.
• Continuous leakage of combustion product into scavenges spate leads to scavenge fire and explosion. 
• Air inlet manifold carbonization takes place.
• Turbocharger surging. 
• Hissing noise from the air intake valve.
• After burning and black smoke. 
• Action to be taken: Unit has to be isolated; upon next available opportunity the valves and the seat shall be inspected and renewed as required. 
• Leaky Exhaust valve
• Gradual increase in exhaust temperature will be observed. 
• Black smoke and sparks might Indicate the same. 
• The leaky exhaust v/v will create wire-drawing effect between v/v and the seat. 
• Compression pressure will reduce, in four stroke engine the peak pressure and combustion pressure will start dropping, neglecting this would result in major breakdowns.
• The leakage can be confirmed by taking an indicator card from the cylinder with the leaking exhaust valve. The card should be taken with the fuel on and off the unit.
• Air starvation due to very dirt y scavenge ports will also give similar indications on indicator diagrams, but in the absence of any abnormal scavenge air or supercharge pressure increase, it will usually he found that the exhaust valve is at fault.
• In four - stroke naturally aspirated engines similar indications will be given on the indicator diagram if the air inlet silencers become dirty.
• In these circumstances the maximum pressure indicator should be used and the maximum firing pressure and compression pressure checked and compared with normal valve.

 

B. Leaking air start valve 

• Causes 
• Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully.
• Sluggish valve action may be caused by dirty pistons or valve spindle guides. 
• In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances. 
• Indication 
• Starting airline paint peels off, sometimes a burning smell, this is the primary indication that the starting air v/v is leaking. 
• The starting air manifold becomes hot this could be felt by touching the pipe. 
• The air starting line drain will give out smoke. 
• Consequences 
• The leakage of starting air valve could prove fatal if left un-noticed, In most engines there is no direct indication or alarm that the manifold is over heated. 
• The Continuous raise in temperature could lead to bursting of bursting disc in worst case there could he a starting airline explosion. 
• Relief valve provided on the line may lift. 
• Action to be taken 
• If ship is maneuvering and it is not possible to stop the engine take first aid measures to see if the leakage stops by a tap on the valve, if stuck it may release. 
• Stop the engine to give a kick to see if the valve which Is tuck turns to its position. 
• Sulzer engine spindle is visible front the top use a spanner and turn it to shut the valve. 
• The fuel pump should be lifted (fuel rack zeroed, puncture valve operated) on the affected unit and the bridge informed. 
• The load should be kept at a minimum, as one unit is now out of operation. Stop engine on a next immediate possible occasion and replace the valve.

 C. Cracked liner 

• Causes 
• Vertical cracks in the liner are usually due to creep because the material has been overheated. Overheating can be due to several causes. 
• Lack of cooling caused by events such as a loss of coolant flow, heavy scaling of cooling surfaces or a fouled cooler. 
• Combustion problems such as flame impingement. This leads to localized overheating due to faulty fuel injectors or poor quality fuel. 
• Afterburning which is caused by poor quality fuel. 
• If the crack is on top: 
• During the firing stroke, the high pressure exhaust gases and combustion products will leak through the cylinder line Into the Jacket space and the jacket water temperature of that particular unit may rise. 
• Fluctuation in the jacket water pressure. 
• In some cases there is an alarm provided on the de-aerator indicating smoke.
• The color of the water will turn black and there will be bubbles in the header tank. 
• If the crack is at the bottom: 
• If the crack is at the bottom water may enter the combustion space and evaporate as the engine Is running. If the leak is below TDC the water will leak into the scavenge space. 
• Action to be taken 
• Stop the engine and replace the cylinder head and the liner if the vessel cannot be stopped the cut off the fuel supply to that particular unit, shut off the JCW v/v and Increase the lubrication and run the engine on reduced load. 

D. Broken piston rings

• Piston ring offers leak proof combustion space. It is necessary that the air should be suitably compressed to a sufficient compression pressure for a proper combustion.
• Broken piston ring causes the following
• Proper sealing will be affected
• Blow past
• Low compression and peak pressure
• Under piston space fire
• Black smoke
• Scavenge fire
• Reduced power
• Imbalanced engine operation
• SFOC will increase
• Fuel consumption will increase
• Thermal efficiency drops
• Line wear increases.