(a) Define the indication of a leaking cylinder head exhaust valve.
(b) Suggest with reasons the likely result of permitting a leaking exhaust valve to remain in this condition for an extended period of time.
(c) Identify the factors leading to such leakage.
(d) State how the incidence of exhaust valve leakage can be reduced in manufacturing.
(a) Symptoms of Leaking exhaust valve
1. Exhaust high temperature
2. Exhaust backpressure slight build-up, due to uncomplete fuel oil burning in exhaust manifold.
3. Noise and smoky operation.
4. Low P max and Low P comp
5. Abnormal light spring diagram showing pressure down.
6. Engine rpm slight reduction, Engine efficiency Lowering
7. If engine is cold, starting difficult
(b) Results
1. Exhaust valve and valve seat burn by high velocity hot gas
2. Exhaust system fouling, Turbocharger surging
4. Uptake fire, Voyage delay
6. Lowering of engine power and engine efficiency
(c) Factors leading to exhaust valve leakage
1. Incorrect valve /seat material
2. Incorrect tappet clearance, Valve not closing properly
3. Incorrect hardening down of exhaust valve, in cylinder head, leading to valve seat distortion
4. Insufficient cooling
5. After burning, poor combustion of fuel
6. Excess design rating of cylinder power
(d) Exhaust valve leakage can be reduced
- Advanced designs, rotating system with air spring, wider seating and cooled bottom piece, etc.
- Exhaust valve with wider seating gives further improvements for the prolonged overhaul interval
- Many valve leakage problems which reduce seat temperature and keep seat clean
- In B&W engine, modification to recess chamber type
(a) What factors govern the position of the top piston ring from the top of the piston?
(b) Give the reason why pistons are tapered on top of the top ring groove.
(c) State with reasons, the immediate steps to be taken if pistons run hot.
(a)
Following factors are governed position of top piston ring from top of piston.
(1) Working temperature of the piston
(2) Live air ratio
- When their working temperature is lowest, piston rings give best results
- Top piston ring clear of hottest part of piston, limit of minimum distance from top of piston to uppermost groove
- If top piston ring is close to crown, high thermal stress and ring distortion due to lubricating failure
- If top piston ring is lower down, increase side leakage, drop less of air and live air ratio
- Carbon deposition between side of piston crown and liner above top ring.
- If carbon deposit flakes away, cause rapid abrasive wear between piston ring landings in piston ring groove.
(b)
- Piston crowns made taper on top of top ring groove.
- Piston crowns have several thermal and mechanical stresses.
- During running above piston ring is hottest part of piston and greater allowance made for radial expansion.
- At this point, greatest mass of metal occurs and expansion of this part is relatively great
- Least effectively cooled part of an internally cooled piston.
(c)
In case of the piston is running hot during operation, the following immediate steps must be taken.
1. Inform bridge and reduce engine speed.
2. Cut out fuel to affected cylinder to prevent addition of heat to piston.
3. Suddenly stopping cause in piston seizure.
4. If possible, don’t stop engine immediately and allow cooling down before stopping.
5. After the engine stopping, turn the engine by Turning Gear until the piston cool down to normal.
(a) Sketch one type of crankcase relief valve.
(b) Describe above and state SOLAS requirements of it.
(c) Describe oil mist detector and alarm limits.
(a) Crankcase relief valve
(b)
- Crankcase Relief valve prevents rise of pressure inside crankcase and reduces risk of crankcase explosion.
- Consists of a light spring loaded non-return disc valve.
- Non-sticks, oil and heat resisting rubber sealing rings fitted to the valve.
- After excess pressure released, the valve smartly open and positively close and rapidly, preventing entry of air and secondary explosion.
- Mild steel gauze flame trap is fitted and wetted with lube oil splash from bottom end bearing.
- The flame trap dissipates greater heat and prevents discharge of flame to the engine room.
SOLAS Requirements for Crankcase Relief Valve
- Cylinder diameter 200mm or crank case volume of 0.6 m3 and above provided with suitable type
- Cylinder diameter 30 cm and above requires at least one valve in each main crank door.
- Discharge is to minimize possibility of personnel injury
- Spring pressure is set to open at internal pressure of approximately 0.07bar, valve close when pressure relieved.
(c) Oil Mist Detector
- Designed for automatic and continuous evaluation of oil mist density from each crank unit in sequence.
- A comparator type oil mist samples are drawn through rotary valve to atmosphere by fan.
- When overheated part in the crankcase, different density in tube.
- The photocell output different. At predetermined value, alarm will initiate and rotary valve stops to know the over heating unit.
Purpose
- Detect, locate hot spots in crankcase.
- Early detection protects engine components before serious damage take place
- Danger of crankcase explosion by measuring increase in oil mist density
- Alarm at oil mist concentration of 2-5% of inflammable limit.
(a) Why cooling water quality is important for the efficient operation of the engine?
(b) What are the effects of contaminations in cooling water system?
(c) Explain implications of out of limit reading from water treatment tests and state corrective procedures?
(a) Important of cooling water quality
- It is important to use correct cooling water quality.
- Use de-ionized or distilled water
- Prevents formation of limestone on cylinder liners and cylinder covers, impair heat transfer, and result in high material temperatures.
Before use, check following values are not exceeded:
- Hardness max 10 Dh (10 ppm CaO) Ph 6.5-8.0 (at 20’C)
- Chloride ion content (50 mg/litre) Sulphate ion content (100 mg/litre)
- Silicate (150 mg/litre)
(b) The effects of contaminations in cooling water system are
(1) Scale
- When a compound deposit from water phase because of its solubility.
- Scale is a dense, adherent deposit of minerals, tightly bonded to itself and to metal surfaces.
(2) Corrosion
- Corrosion is process, metals return to their native states as, chemical compounds.
- In diesel engines containing dissimilar metals, main concern is galvanic corrosion.
- When exposed to water, acts as an electrolyte, one metal becomes anodic and the other cathodic, setting up a galvanic cell.
- E.g. when Mild Steel and copper connected in water, Mild Steel becomes Anode, Copper become Cathode, Mild Steel corrodes.
(3) Fouling
- Different from scaling; formed from material suspended in the water.
- Main cause of fouling in cooling water systems are- solid suspended and oil leaked into system.
- Controlled with effectiveness of corrosion inhibitors.
(4) Microbiological activity
- Nitrites act food source for some types of bacteria.
- Bacteria happen into cooling water system due to raw untreated water from ashore.
- When bacteria are active within system, bacteria attack rubber, seal rings and hoses.
- Biocides are used to kill these micro organisms.
(c) Implications of out of limit readings from water treatment tests and state the corrective procedures
1. Cleaning of the C.W. system
2. Filling up with fresh water
3. Adding corrosion inhibiter
4. Regular check and analysis of inhibitor content
- Before engine use, cleaning C.W. system. When engines in service, clean C.W system every 4 ~ 5 years
- Purpose of this operation is to remove any deposits of oil, sludge, rust, lime etc.
- To have clean surfaces and uniform protection by inhibiter.
Cleaning should comprise of
(a) Degreasing to remove oil sludge
(b) Descaling with acid, afterwards to remove rust and lime deposits
(c) Flushing with water, for neutralizing and removal of debris.
(a) Sketch a fuel viscosity control device and its system?
(b) Explain about working principal.
(c) How will you do if the viscosity controller is in operative?
(d) What is Injection viscosity of Large Marine Engines?
(a) Fuel Oil Viscosity controller- System Components
(b)
- Normally two fuel oil end heaters are fitted for alternate-use and stand-by purpose.
- Steam supply to end heaters is regulated by pneumatic controlled regulating valve. In some case, Electric heater is used.
- Viscotherm unit is fitted downstream of fuel oil heaters.
- The unit consists of slow speed (about 40 rpm) small gear pump driven by electric motor with reduction gear.
- Pump draws fuel oil from supply line and discharges with constant amount to pressure chamber through capillary tube.
- Liquid flowing, pressure difference before and after capillary tube is related to the oil viscosity.
- Measured by differential pressure transmitter and the signal obtained is directly proportional to oil viscosity
- This signal is converted into electric current and inputted to viscosity controller as measured viscosity value.
- Viscosity value (0 ~ 20 Cst) preset by selector. If measured value is lower or higher than preset limit, alarm will initiate.
- The output signal from viscosity controller regulate steam actuator valve
(c)
- If viscosity controller is in operative, fuel oil viscosity can be controlled as follows to operate engine satisfactorily.
- Use heating arrangement as normal operation
- By-pass the Viscotherm unit.
- Close steam regulating valve and manually control with its by-pass valve.
- Maintain heater temperature as normal (120~135˚C )
- Monitor the engine running condition (exhaust temp. , knocking sound , smoke. etc)
- Take draw card to determine engine performance.
(d)
Fuel injection viscosity for large slow speed engine using 380 Cst fuel oil is normally between 10~20 Cst. (Fuel oil temperature is about 120~135˚C depend on the fuel Cst).
(a) As a senior Engineer, Describe the term Continuous Machinery Survey.
(b) Explain how the classification societies have reduced the need for attendance by the surveyor
(c) Describe how a planned maintenance scheme may be used in conjunction with the CMS.
(d) Name TWO programs approved by classification societies so that physically opening up of the machinery is not necessary on every occasion.
(a) Continuous Machinery Survey(CMS)
- CMS is the survey of a part of machinery as per Class’s survey lists.
- By opening up within the range date
- Continuously to complete the whole machineries on board within 5-years cycle.
(b) Following systems approved by Classification Societies reduced the need for attendance by Class Surveyor for some works.
1. Classification society surveyor surveyed crankshaft, main bearings, bottom end bearings, and thrust bearings
2. Chief Engineer held his certificate of competency for more than 3-years & sailed as Chief Engineer for 1-year with company classification society agree to him surveyed other items such as crossheads, pistons, cylinder liners etc.
(1) Chief Engineer’s Open up, Carry out inspection and Confirmatory Survey System
- Most CMS items except ME coupling, reduction gear, turbine and one set of cargo refrigerating compressor, can be opened up, inspected and prepared survey reports by Chief Engineer within 3 months before due date.
- All these items can be credited by attending Class Surveyor as Confirmatory survey.
(2) Substitution for Open-up Inspection System
Pumps, heaters and deck machineries of CMS items visually inspected under operating conditions, reviewed the log book data and credited by attending Class Surveyor.
(3) Planned Maintenance Scheme (PMS)
Most of CMS inspections are carried out by Chief engineer, based on approved computerized Planned Maintenance Scheme jobs. Class surveyor needs to attend onboard only once a year to verify the CMS items.
(4) Harmonized Survey System
Harmonize intervals between surveys of main legitimate certificates. all main surveys can be carried out at one time and reducing the Class surveyor attendance.
(c) PMS consists of most of CMS items.
- If PMS is approved by Classification Society, PMS for machinery is as alternative to (CMS).
- Maintenance carried out as per PMS including CMS items and all data and history entry into PMS program by Chief engineer.
- Machinery components credited after carrying out the annual audit by Class Surveyor.
(d) (1) Lubricant Analysis
It is condition monitoring program of the machinery. If the analysis results show normal, the machinery is assumed in normal operating condition and not necessary for open up on every occasion.
(2) Vibration Monitoring Analysis
- It is the most common technique to determine the condition of rotating machinery.
- All vibration measurements compared to alarm set points and display in graphically changes in machine condition over time.
- not necessary for open up on every occasion.
