Jacket cooling water system

In the name of Allah who is the most beneficient and merciful

This system forms closed circuit water passing from the engine returns through cooler to the p/p suction and then back to engine. A header and expansion tank is placed at a reasonable height to allow venting and pressurizing of the sys. This is connected to both the engine discharge and the pump suction line. A heater is included with a bypass and the sys can warm the engine when necessary. Cooling water at the lower end of cylinder jackets passing upto connection from the top of the jacket to the cylinder cover and then exhaust valve(if there). A restricted amount of water is taken from this discharge and passes through turbocharger cooling spaces and air separator is fitted in the return which discharge any trapped air to expansion tank. Other venting connection are taken from top of the engine system, the turbocharger and the cooler.


WRITTEN BY S/C ZEESHAH AHMED
LECTURE BY: CHIEF ENGR. YASEEN SAHAB
EMAIL: ZEETEC4@YAHOO.COM
DATE: 12.july.2009

Control Engineering

In the name of Allah who is the most beneficient and merciful

The closer control of machinery operating conditions, eg cooling water temperature and pressure, permits machinery to be run at its optimum design conditions, making for fuel economy and reduced maintenance.

Automation can carry out some tasks far more effectively than men in other areas it is less effective. For example, the monitoring of machinery operating conditions such as temperatures and pressure can be carried out by a solid-state alarm scanning system at the rate of 400 channels/sec giving a degree of surveillance wihcih would be impossible by human observation. Conversely the detection of a noisy bearing a leaky gland or a cracked pipe is scarcely possibly by automatic means. The balance between the possible and the necessary would be achieved in this case by combining automatic monitoring of all the likely fault conditions, with routine machinery space inspection say twice daily.

Classification societies

The class notation of a ship granted by a classification society, is a mark of approval of its standard, and it indicates that the vessel has been built to specific rules and thereafter periodically surveryed. It is the practice to include in the notation a special mark for ships designed to operate with periodically unmanned engine rooms, e.g. Lloyds Register add U.M.S. to the 100A1 notation to signify approval of operation with unattended machinery spaces.

The granting of special notations is also subject to the ship being built in conformity with rules or recommendations concerning automation. The principal international classification societies are Lloyds Register of Shipping, the American Bureau of shipping, Detnorske Veritas, Bureau Veritas and Germanischer Lloyd.

CONTORLS FOR GENERATORS

In unattended machinery installations it is necessary to provide certain control facilities for the electrical generating plant. These may vary from simple load sharing and automatic starting of the emergency generator, to a fully comprehensive system in which generators are started and stopped in accordance with variations in load demand.

Medium-speed propulsion plants normally use all-diesel generating plant. Turbine ships obviously use some os the high quality steam generated in the main boilers in condensing or back pressure turbo charger generators, with a diesel generator for harbour use. The usual arrangement on large-bore diesel propulsion systems is a turbogenerator employing steam generated in a waste-heat boiler, plus diesel generator for manoeuvring, port duty, and periods of high electrical demand.

Diesel generators

The extent of automation can range from simple fault protection with automatic shut-down for lubricating oil failure, to fully automatic operation. For the latter case the function to be carried out are:

1. Preparation for engine starting
2. Starting and stopping engines according to load demands
3. Sychronisation of incoming sets with supply
4. Circuit breaker closure
5. Load sharing between alternators
6. Maintenance of supply frequency and voltage
7. Engine/alternator fault protection
8. Preferential tripping of non essential loads

When diesel generators are arranged for automatic operations, it is good policy to arrange for off-duty sets to be circulated with main engine cooling water so that they are in state od readiness when required. Pre-starting preparations are then simply limited to lubricating oil priming.

It is necessary to provide fault protection for lubricating oil and cooling services, and in a fully automatic system these fault signals can be employed to start a standby machine, place it on line, and stop the defective set. In some installations, automatic controls carry out the sequence as far as synchronisation, and leave final circuit breaker closure to the engineer.

Turbo-generators

The starting and shut-down sequences for a turbo-generator are more complex than those needed for a diesel-driven set, and fully automatic control is therefore less frequenctly encountered. However, the control facilities are often less frequently encountered. However, the control facilities are often centralized in the control room, together with sequence indicator lights to enable the operator to verify each step before proceeding to the next. Interlocks may also be employed to guard against error.

The start up sequence given below is necessarily general, but it illustrates the principal and may be applied to remote manual or automatic control:

• Reset governor trip lever
• Reset emergency stop valve
• Start auxiliary L.O. pump
• Start circulating pump
• Apply gland steam
•  

1. Start extraction pump
2. Start air ejectors
3. Open steam valve to run-up turbine

Where a waste heat boiler (economizer- a word form economy) is used to supply steam to a turbo-alternator, control of steam output is normally controlled by a three way valve in the exhaust uptake, the position of which is regulated in accordance with steam demand. Surplus waste-heat is then diverted to a silencer.

Written By: Zeeshan Ahmed

Oil lubricated stern tubes

In the name of Allah who is the most beneficient and merciful

Progress from sea water to early oil lubricated stern tubes involved exchange of the wood lined bronze carrier
for a while metal lined, cast iron bush. oil retention and exclusion of sea water. necessitated the fitted of an
external face type seal. The stuffing box was returned in many early oil lubricated stern tubes at the inboard end.

the latter designs with an extended length boss built into the stern frame provide better support for the white metal lined
bearing. A minimum bearing length of two times the shaft diameter will ensure that bearing load doesnot exceed 0.8 N/MM2

the tube is fbricated and welded direct to the extension of the stern tube frame boss at the after end and to the aft peak bulkhead at the forward end.

oil contaminated with in simplex type stern tube by lip seal. The elastic lip of each nitrile rubber seal liners at outboard and inboard end of the steel propeller shaft.
The outboard liner additionally protects the steel shaft from sea water contact and corrosion.

Heat produced by the friction will result in hardening and loss of elasticity of the rubber, should temperature of the seal material exceed 110c. cooling
at the outboard end is provided by the sea. Oil circulation aided by convention, is arrranged to maintain low temperature of seals at the inboard end.
connections  are fitted top and bottom between the two inboard seals.

The chrome steel liners act as rubbing surfaces for the rubber lip seals and grooving from fricitional wear has occurred. The problem has been overcome by using a ceramic
filler for the groove or alternately a distance piece to axially displace the seal and ring assembly. Allowance must be made for relative movement of shaft and stern tube due to
differential expansion. New seals are fitted by cutting and vulcanizing in position.

Lip seals will accept misalignment but a floating ring design was introduced by one maker.


WRITTEN BY S/C ZEESHAH AHMED
LECTURE BY: CHIEF ENGR. Tahir jamil
Phone no. 03344011232
EMAIL: ZEETEC4@YAHOO.COM
date: 03.nov.2009

Scavenge Fire

List the various factors which must be present for a scavenge fire to start?

For any fire to begin there must be present a combustible material, oxygen or air to support combustion and a source of heat of a temperature high enough to start combustion. In case of scavenge fires the combustible material is oil. The oil is usually cylinder oil which has drained down from the cylinder material is oil. The oil is usually cylinder oil which has drained down from the cylinder spaces, in some cases the cylinder oil residue may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinder, and other similar causes. The oxygen necessary for combustion comes from the scavenge air which is plentiful supply for the operation of the engines. The heat in the scavenge space, around the cylinder, brings the oil to a condition where it is easily ignited. The high temperature required to start combustion may arise from piston-ring blow past.

How would you become aware of a scavenge fire? How would you deal with a scavenge fire?

The first indication of a scavenge fire may be a slight reduction in the engine speed due to the reduction in power which comes about when a fire starts. Other indications are a higher exhaust temperature at the cylinders where the scavenge fire has started and irregular speed of turbo-blowers. External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles. If  the scavenge trunk is oily the fire may spread back from the space around or adjacent to the cylinders where fire started and will show itself as very hot spots on areas of the scavenge trunk surfaces. In ships where the engine room is periodically unmanned, temperature sensors are fitted at critical points within the scavenge spaces. On uniflow-scavenged engines the sensors are fitted round the cylinder liner just above the scavenge ports. A temperature higher than reference or normal then activates the alarm system.

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 fire (see note). 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 engine rooms. In allows the fire to burn itself out without damage. Once the fire is out and navigational circumstances allow it, the engine should be stopped and the whole of the scavenging port examined and any oil residues found round other cylinders removed. The actual cause of initiation of the fire should be investigated. If the scavenging fire is more major nature. It sometimes become necessary to stop the engine and use the steam or extinguishing arrangement fitted to the scavenging trunk. The fire is then extinguished before it can be spread to surfaces of the scavenging trunk. Where it may cause the paint to start burning if special non-flammable paint has not been used.

 How can the incidence of scavenge fires be prevented or reduced?

One of the first things that must receive attention is maintaining the scavenge space in as clean a condition as possible. This can be done by keeping scavenge drain pipes clear and using them regularly to drain off any oil which comes down into scavenge space drain pockets. The scavenge space and drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludge which don’t drain down so easily and which are a common cause of choke drain pipes. The piston rings must be properly maintained and lubricated adequately so that ring blow-by (blow-past) is prevented. At the same time one must guard against exceed cylinder oil usage. With timed cylinder oil injection the timing should be periodically checked. Scavenge ports must be kept clear. 

Written by: Zeeshan Ahmed

Microbial degradation of oil

In the name of Allah who is the most beneficient and merciful

Microbial degration of oil is the possible infection of an oil system by micro organisms. these live
by consuming hydrocarbons in the base oil togather with nitrogen, sulphur and phosphorus from additives.
if water is present they will multiply rapidly in the warm, agitated conditions in the oil system. contamins is
humids climates or accidental entry of dirt and water during overhaul.

infection by microorganisms will form organic acids and more water while depleting the additives. This will cause
corrosions and wear of metal surfaces(particularly bearing metals) and will create sludge and slime that choke oil filters.
The oil may be prone to emulsification and saponification.

Test for microbial infection make use of a special gel which is either dipped in the oil or has oil poured over it and is then
incubated to develop a growth or culture. the appearance of this culture is then compared visually with a standard coloured
chart, to indicate the degree of contamination if any. If the oil doesnot wet the get, it may be necessary to mix the sample with a
small quantity of sterile water.

contamination doesnot commonly occur if cleanliness and care are exercised and water is eliminated from the sytem. Recommendations are that a
maximum of 0.2 % of water content must not be exceeded when using detergent oils.


WRITTEN BY S/C ZEESHAH AHMED
LECTURE BY: CHIEF ENGR. Tahir jamil
Phone no. 03344011232
EMAIL: ZEETEC4@YAHOO.COM
date: 03.nov.2009

Slow, medium and high speed diesel engines

In the name of Allah who is the most beneficient and merciful

The term medium refers to diesel engines that operate with in the approximate speed range 250 to 800 revolutions per minute. High speed is usually 100 rpm and above, while slow speed diesel engines have 100 to 120 rpm. Each has their various advantages and disadvantages for various duties on board ship.

The slow speed two stroke cycle diesel is used for main propulsion units since it can be directly coupled to the propeller and shafting. It provides high power, can burn low grade fuels and has high thermal efficiency. The cylinders and crankcase are isolated, which reduces contamination and permits the use of specialized lubricating oil in cylinders and bearings. The use of two stroke cycle usually means there are no inlet and exhaust valves. This reduces maintenance and simplifies engine construction.

The medium speed diesel engines are generally to operate on the four stroke cycle and are trunk piston construction. These egines provide a better power to weight ratio and power to size ratio and there is also a lower initial cost for equivalent power.

The higher speed how ever requires the use of gearbox and flexible coupling for main propulsion use. These engine may be constructed in either “In-line” or “vee configuration” (shape). Vee types engines are constructed with two banks of cylinders arranged at an angle using a common crankcase and bed plate with considerable saving in size and weight. The most common method for connecting two pistons to each throw of the shaft is the side arrangement with two bottom end bearings.

Various makes of medium speed engines

Various make of diesel engines are as under

MAN B&W L58 ENIGNES

Engine of this type have cylinders bore of 580mm and 640mm, stroke operating at 400rpm. This is larger medium speed four stroke diesel engines; these are in six and nine cylinders. All are in line and operate on heavy oil and are designated ease in maintenance with extanded periods between overhauls.

SULZER ZA  40S ENIGNES

The engine has cylinders bore of 400mm and a stroke of 500mm and normal speed of 510 rpm. This engine produced in many cylinder numbers, ranging from six to nine for “In-line” and twelve to eighteen for “vee form” engines.

SEMT PIELSTICK PC26 ENGINES

It is one of the wide ranges of medium speed engines produced by the company for marine and land applications and available as “In-line” or “Vee configuration”. The cylinder bore is 400mm, with a stroke of 400mm and speed of 520 rpm. Cylinder numbers are from six to nine for “In-line” and from ten to eighteen for “Vee” versions. These are four stroke engines.

WARTSILA VASA 46 ENGINES

Wartsila R-46 engines are medium speed four stroke engines having trunk piston. The engine has cylinder bore of 460mm, a stroke of 580mm and designated speeds of 450, 500 or 514 rpm . it is available with between four and nine cylinders “In line” or with between twelve and sixteen cylinders in “Vee form”.

Fuel system of this engines uses twin or pilot injection. This gives easy starting and smooth efficient combustion of low quality fuel over the full ranges of engine power.

ENGINE DESIGN

The principle design parameters for a medium speed diesel engines are:

1. high power weight ratio
2. simple, strong, compact and space saving.
3. high reliability
4. able to maintain the fact that components are smaller and lighter than those for slow speed diesels makes for easier handling.
5. easily capable of adoption to un-manned operation
6. low fuel and lubricating oil consumption
7. high thermal efficiency
8. low cast and simple to install

TYPE OF ENGINE

Either 2 or 4 stroke cycle single acting turbo charged with inline or vee cylinder configuration. The main choice is certainly at present, for the four stroke engine due to the following reasons.

1. they are capable of operating satisfactory on same heavy oils which are used for slow speed two stroke engines.
2. effective scavenging.
3. higher mean piston speed which will give greater power.
4. engine can operate effectively with turbocharger out of commission, this would present a considerable problem with some two stroke engine of the medium speed type.
5. turbocharger power and size can be reduced.
6. it is also claimed that the fuel consumption would be reduced.
7. to reduce inertia forces use is made of aluminum alloy skirted piston or complete aluminum alloy pistons. Inertia forces must be taken into account for bearing loads. Important in trunk piston engines(which are the majority of medium and high speed diesels) where the guide surfaces is the cylinder liner, the smaller the side thrust the less  the friction and wear.

The following are the data’s of medium speed diesel engine currently in use:

RPM 600, cylinder bore 400mm, 4stroke, turbocharged upto 18 cylinders, developing approx: 700KW per cylinder, break mean effective pressure 23 bar, maximum cylinder pressure 160 bars, at approx: 600 rpm.

MAINTENANCE OF DIESEL ENGINES

The reliability and safe working of marine diesel engine depends upon the technical staff and in particular upon sea going engineers.

Planned maintenance of marine machinery can prolong its life. Engine builders, pump makers, in short all the makers of the equipment in ship provide instruction about the regular care and maintenance to be given to their equipment, with information and knowing the continuous survey requirement, the chief engineer plans the voyages maintenance work.

On board ship every engineer is delegated with the responsibility for the efficient running and maintenance of various items of machinery. Duties and responsibilities are clearly defined and are not to be left any uncertainity.

GENERAL MAINTENANCE INSTRUCTION

1.      trained and qualified personnel must operate diesel engines only. Unauthorized people must not allowed to operate the engine

2.      a logbook should be kept for each engine, in which all important data and work concerning the engine (eg number of running hours each day, actual fuel consumption, intervals between oil changes, pressure and temperature of engine and general performance remarks about machinery) are written .this provide useful information when irregularities or trouble occur.

3.      the safety regulations and instruction concerning safety measures to be observed during operation, maintenance and repair work.

4.      all essential parts of engine should be overhauled and cleaned up at regular intervals as per makers’ maintenance schedule. In order to keep the engine in good condition and constant operational readiness.

5.      A part from others factor the frequency of overhauls of various engine parts is determined by the load at which the engine is normally run and by the grade of fuel and lubricating oil used.

6.      overhaul jobs should be carried out only with the special devices and tools normally supplied with engine. The use of inappropriate tools will cause loss of time and damage to engine parts.

7.      all parts overhauls during maintenance work to be checked for correct functioning before they are put into operation again particularly pipes must be pressure tested for possible leaks.

8.      after the completion of the overhauls or maintenance work the engine should be cleaned with cleaning cloths (not by cotton rags or wool). Petrol should not be used for cleaning in closed rooms because of danger of explosion.

9.      for putting the engine into operation after overhauling or maintenance work, turn the engine over by hand or with the turning gear for checking that all moving parts are moving freely.

10.  to fill empty starting air bottle ( for starting engine) no other gases are to be used than air or carbon dioxide, even in emergency. In particular, oxygen or hydrogen must not be used, as this would expose the personnel and the engine  to very danger.

11.  engine must always have a reverse output hence it is advisable in continuous operation to limit the output to 85 to 90% of the rated output.

12.  if the engine had to be stopped on account of running gear components or bearing running hot the engine should be allowed to cool off for at least 10 minutes before opening the crankcase doors for only repair work or checking.

13.  if the carbon dioxide ( CO2) had been used in scavenging space or in any other compartment of the engine room for fire extinguishing, it must be thoroughly vented before going to work inside it.

DEVELOPMENT OF MEDIUM SPEED DIESEL ENGINES

The trend of the medium speed engine is towards higher power outputs per cylinder, with high  reliability, when operating on cheaper high viscosity fuels, manufacturers to improve the combustion process are carrying out much development work. This work focuses on the timing and duration of fuel injection to achieve reliable combustion and manufacturer are now testing engines operating with firing pressure in excess of 210 bar.

• Cylinder bore 580mm
• Stroke 600mm
• Speed 450rpm
• Power per cylinder 1250KW

The development in the medium speed engine has been such that it is now a serious competitor for applications, which were once only the domain of  slow speed 2 stroke engines. If the advantages and salient features of the medium speed diesel engines are examined then it will be appreciated that why this engine is dominating for certain vessels. Advantages and salient feature of medium speed diesel engine are as under.

1. compact and space saving
2. reduced weight of components
3. more cargo space available on ships having medium speed engines
4. higher power weight ratio of the engine a greater weight of cargo can be carried.
5. as using a reduction gear a useful marriage between ideal engine speed and ideal propellers speed can be achieved
6. modern tendency is to utilized unidirectional medium speed geared diesel engine coupled to either a reverse reduction gear or controllable pitch propeller which will give advantages of:
• less starting torque-reduced number of starts
• engine can be tested at full speed with the vessel alongside
• improved maneuverability engine can stop with in short time

Written By: Zeeshan Ahmed

Crankcase oil mist detector

In the name of Allah who is the most beneficient and merciful               

This device is fitted to monitor the presence of an oil mist in the crankcase. In this monitoring process samples of the air and vapours mixture taken continually from the crankcase of a diesel engine. This device will detect the level of oil mist concentration which must remain below level. At which explosion may occur. In case of coming to this level a warning is given to allow avoiding action to slow the engineer and prevent serious damages and explosion.

The presence of an oil mist in the crankcase is the result of oil vaporization caused by a “Hot spot” this hotspot is produced by other heating of any engine bearing or overheating of any other moving parts of the engine time chain or gear drives. Explosive condition can result if build up of oil mist is allowed. Hence continuous monitoring of vapours intensity is required. A diagrammatic view of an oil mist detector is shown in the sketch.

Measure any increase in oil mist density. The detector consists basically of two parallel tubes of equalize, each having an electric current directly proportional to the intensity of light falling on the surface. Lenses are fitted to seal the tube ends of each tube but allow light to pass. One tube is sealed to contain clean air and is termed the reference tube. The other tube is called Measuring tube, has connections through which samples of the crankcase vapours are drawn by extractor fan. If the concentration of explosive mixture is reached in the oil mist sample, light will be obscured(not clear) before reaching the cell of measuring tube, electrical balance between two cells will be disturbed and an alarm will be operated indicating excess concentrations in oil mist. Sampling points should be fitted to each cylinders crankcase and their connections are brought to a rotating selector valve which is driven from the fan motor. This repeatedly connects each sampling point  to the measuring tube in sequence.

In the event of oil mist being detected the rotator stops to indicate which sampling will commence again its sequence.
The detector should be checked daily and the sensitivity tested. Lenses and mirrors should be cleaned periodically. Two identical beams of light from a common lamp are reflected by mirrors to pass along the tube on to the cells which are then in electrical balance and if light will be obscured before reaching the cells, electrical balance will be distured and an alarm will be operated.


WRITTEN BY: S/C ZEESHAN AHMED
LECTURED BY: CH. ENGR. YASEEN SAHAB
DATE: 23/OCT/2009
TIME: 11:17 AM

An introduction to gas turbine (GT)

In the name of Allah who is the most beneficient and merciful       

The aim is to be able to identify and explain the function of gas turbine major engine component and discuss the mode of operation of simple and complex cycle gas turbine engines compared with diesel and steam plant.

Definition of a Gas Turine

A continuous cycle self contained heat engine using a gas as the working fluid.

Breakdown of definition

1.      Energy output. After providing the power to drive the compressor(self contained) the energy output may be in the form of :

·         Jet thrust

·         Shaft power

·         Compressed air from compressor

·         Heat

2.      Self contained. Once running it is not dependent on any other machine.

3.      Gas may be.

·         Air – Open cycle

·         Hydrogen, helium etc closed cycle . In this case heat is added by means of a heat exchanger.

4.      Energy source. This may be provided by

·         Oil fuel

·         Natural gas

·         Sewage gas

·         Waste gas

·         Waste heat (eg blast furnance)etc .

5.      Compressor. This is usually either axial or centrifugal, but free piston and reciprocating compressor have been tried, although the process then is no longer continuous.

6.      Continuous. Unlike a diesel which operates with gas in a similar way the GT uses a constant pressure process which is continuous.

Comparison of cycles

In the services we are concerned with three main forms of providing power (see below). These all have particular advantages and disadvantages, some of which can be readily appreciated by a quick comparision of the ideal cycle on a T-S chart.

The following factors show themselves:

1)      Steam Plant

• Operates across saturation envelope.
• Closed cycle – recovery process needed to condense and re-use steam, hence much additional machinery.
• Small work to compress fluid. Good work from expanding vapour- large exess.

2)      Diesel

·         Fairly good available work output after compressing gas.

·         Open cycle.

·         Non-continuous cycle means valves and moving parts. Stress problems.

3)      Gas turbines

• Very poor specific work output available after compressing gas. High powers only available if:
  
• Component efficiencies high
• High mass flow
• High mass flow possible because process is continuous.

Conclusions

From the above comparisons the factors which stand out about gas turbines are:

• Unlike The others, an overall power output is possible purely because the constant pressure lines diverge with increase in entropy.
• Its specific output is very small and dependent upon the efficient operation of the various components. Mass flows are high hence small increases in component efficiencies will give relatively large increases in output.

Marine gas turbine  (GT)

An aircraft can use the energy in the gas stream directly to provide the propulsive power eg in a jet. In the marine environment this is not feasible because of the very nature of the surroundings , the noise, the requirement to reverse and limitations imposed by the construction of the ship. The GT must be used to drive a propeller/water jet/generator. Because of this requirement we may consider the marine GT to be split into two main parts.

·         The gas generator- that providing the high energy gas stream.

·         The power producer- that converting the energy in the stream into a useful form of power eg shaft power(power turbine)

Hence it is possible to adapt an aircraft engine for  marine use by the addition of a power turbine eg Olympus SMIA.

In addition marine GTs are not nearly so restricted for space as the aero versions and therefore it is theoretically possible to attempt to improve the performance by using one or more of the following:

• Intercooling
• Reheat
• Heat exchanger
• Water injection
• Waste heat recovery

REASONS FOR THE ADOPTION OF GAS TURBINE

1. The aim to be able to explain the reasons why the royal navy and other navy is adopted gas turbines as main propulsion units in major surface warships, the typical problems associated with running them at sea and their solutions.

1. The resons for using gas turbine in warships:

·         High power/weight ratio.

·         Quick startup capability

·         Comparatively low development cost.(benefits from aero engine development)

·         Low onboard maintenance requirement.

·         Ease of upkeep by exchange of critical parts.

·         Reduced watchkeeping manpower.

·         Good SFC at high power.

·         Availability

·         Reduced underwater noise(fewe hull openings)

1. Typical problems and solutions

·         Distortion of combustion chambers- ongoing design effort, regular inspection.

·         Combustion of naval fuels-redesign of combustion system.

·         Compressor fouling-air filtration, regular washing.

·         Surge and rotating stall- air bleeds, variable geometry blades.

·         Turbine Disc failures – Design effort, defined service life.

·         Bearing failures- Uprated bearings, earlier detection of possible failure.

·         Fuel consumption at part load- multispool variable geometry engines, higher operating temperature.

·         Practical problem of a complex cycle- no solutions at present.

·         Poor life of aero types –rapid engine change capability-comprehensive repair/rebuild facilities, life continually being upgraded.

Written by Zeeshan Ahmed

Life Savings Appliances

  In the name of ALLAH who is the most beneficient and merciful.                                  

The life savings equipment carried on board a ship depends upon the number of persons carried and the normal service of the ship. A transatlantic passenger liner would carry considerably more equipment than a coastal cargo vessel.

There must be sufficient life boat accommodation on each side of the ship for the whole of the ship’s complement(number/quantity that make something complete). The lifeboats must be at least 7.3 long and may be constructed of wood, steel, aluminium or fiberglass. They carry rations for several days, together with survival and signaling equipment such as fishing lines, first aid equipment, compass, lights, distress rockets and smoke flares. One life boats on each side must be motor driven.

The lifeboats are suspended from davits which allow the boats to be lowered to the water when the ship is heeled to 15 degrees. Most modern ships is heeled to 15 degrees. Most modern ships are fitted with gravity davits, which when released allow the cradle carrying the boats to run out board until the boat is hanging clear of the ship’s side.(fig.1)

The boat is raised and lowered by mean of an electrically driven winch. The winch is manually controlled by a weighted lever(fig.2) know as dead man handle which releases the main brake. Should the operator lose control of  the brake the lever causes the winch to stop. The speed of descent is also controlled by a centrifugal brake which limits the speed to a 36 m/min. Both the centrifugal brake and the main brake drum remain stationary during the hoisting operation. If the main power fails while raising the boats, the main brake will hold the boat.

Each member of the crew is supplied with a lifejacket which is capable of supporting an unconscious person safely.

Lifebuoys are provided in case a man falls overboard. Some are fitted with self igniting lights for use at night and others fitted with smoke signals for pin-pointing positin during the day.  

WRITTEN BY S/C ZEESHAN AHMED (A/C 2988)
INSTRUCTOR: CHIEF ENGINEER YASEEN SAHAB
DATE: 25/10/2009
TIME: 10:05:00 AM
 

Conditions of assignment

In the name of ALLAH who is the most beneficient and merciful

The load line rules are based on the very reasonable assumption that the ship is built to and maintained at a high level of structural strength and will sail in a safe and seaworthy condition.

Until recently the rules laid down the standard of longitudinal and transverse strength. The classification societies usually found it necessary to increase these standards although in some design considered the rules excessive. It is now felt that the structural strength of the ship is more properly the function of the classification societies who may well be the assigning authority.

Standards of stability are given in the rules for both small and large angles of heel. Details of the information required to be carried on a ship are stated, together with typical calculations, all the information is based on an inclining experiment carried out on the completed ship in the presence of a DTp surveyor.

It is essential that all openings in the weather deck are water tight. Hatch coamings, hatch covers, ventilator coamings, air pipes and doors must be strong enough to resist the pounding from the sea and standards of strength are laid down. The rules also specify the height of coamings, air pipes and door sills above the weather deck, those at the force end being higher than the remainder.

It is important to remove the water from the deck quickly when a heavy sea is shipped. With completely open decks, the reserve buoyancy is sufficient to lift the ship and remove the water easily. When the bulwarks are fitted, however, they tend to hold back the water and this may prove dangerous. For this reason openings knows as freeing ports are cut in the bulwarks, the area of the freeing ports depend upon the length of the bulwark. If the freeing ports are wide, grids must be fitted to prevent crew being washed overboard. In addition, scuppers fitted to remove the surplus water from the deck. The scuppers on the weather deck are led overboard whilst those on intermediate decks are may led to bilges or, if automatic nonreturn valves are fitted, may be led overboard.

Type A ships, with their smaller freeboard are more likely to have water on the decks and it is a condition of assignment that open rails be fitted instead of bulwarks. If the vessel has midship accommodation, a longitudinal gangeway must be fitted to allow passage between the after end and midships with out setting foot on whether deck. In larger ships it is necessary to fit shelters along the gangway. Alternatively access may be provided by an underdeck passage, but while convenient for bulk carriers could prove dangerous in oil tankers.

WRITTEN BY S/C ZEESHAN AHMED (A/C 2988)
INSTRUCTOR: CHIEF ENGINEER YASEEN SAHAB
DATE: 25/10/2009
TIME: 10:05:00 AM

Scavenge Fire

  In the name of ALLAH who is the most beneficient and merciful

List the various factors which must be present for a scavenge fire to start?

 For any fire to begin there must be present a combustible material, oxygen or air to support combustion and a source of heat of a temperature high enough to start combustion. In case of scavenge fires the combustible material is oil. The oil is usually cylinder oil which has drained down from the cylinder material is oil. The oil is usually cylinder oil which has drained down from the cylinder spaces, in some cases the cylinder oil residue may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinder, and other similar causes. The oxygen necessary for combustion comes from the scavenge air which is plentiful supply for the operation of the engines. The heat in the scavenge space, around the cylinder, brings the oil to a condition where it is easily ignited. The high temperature required to start combustion may arise from piston-ring blow past.

How would you become aware of a scavenge fire? How would you deal with a scavenge fire?

The first indication of a scavenge fire may be a slight reduction in the engine speed due to the reduction in power which comes about when a fire starts. Other indications are a higher exhaust temperature at the cylinders where the scavenge fire has started and irregular speed of turbo-blowers. External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles. If  the scavenge trunk is oily the fire may spread back from the space around or adjacent to the cylinders where fire started and will show itself as very hot spots on areas of the scavenge trunk surfaces. In ships where the engine room is periodically unmanned, temperature sensors are fitted at critical points within the scavenge spaces. On uniflow-scavenged engines the sensors are fitted round the cylinder liner just above the scavenge ports. A temperature higher than reference or normal then activates the alarm system.

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 fire (see note). 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 engine rooms. In allows the fire to burn itself out without damage. Once the fire is out and navigational circumstances allow it, the engine should be stopped and the whole of the scavenging port examined and any oil residues found round other cylinders removed. The actual cause of initiation of the fire should be investigated. If the scavenging fire is more major nature. It sometimes become necessary to stop the engine and use the steam or extinguishing arrangement fitted to the scavenging trunk. The fire is then extinguished before it can be spread to surfaces of the scavenging trunk. Where it may cause the paint to start burning if special non-flammable paint has not been used.

 How can the incidence of scavenge fires be prevented or reduced?

One of the first things that must receive attention is maintaining the scavenge space in as clean a condition as possible. This can be done by keeping scavenge drain pipes clear and using them regularly to drain off any oil which comes down into scavenge space drain pockets. The scavenge space and drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludge which don’t drain down so easily and which are a common cause of choke drain pipes. The piston rings must be properly maintained and lubricated adequately so that ring blow-by (blow-past) is prevented. At the same time one must guard against exceed cylinder oil usage. With timed cylinder oil injection the timing should be periodically checked. Scavenge ports must be kept clear.

WRITTEN BY S/C ZEESHAN AHMED (A/C 2988)
INSTRUCTOR: CHIEF ENGINEER YASEEN SAHAB
DATE: 25/10/2009
TIME: 10:05:00 AM

oil lubricated stern tubes

oil lubricated stern tubes
Progress from sea water to early oil lubricated stern tubes involved exchange of the wood lined bronze carrier
for a while metal lined, cast iron bush. oil retention and exclusion of sea water. necessitated the fitted of an
external face type seal. The stuffing box was returned in many early oil lubricated stern tubes at the inboard end.

the latter designs with an extended length boss built into the stern frame provide better support for the white metal lined
bearing. A minimum bearing length of two times the shaft diameter will ensure that bearing load doesnot exceed 0.8 N/MM2

the tube is fbricated and welded direct to the extension of the stern tube frame boss at the after end and to the aft peak bulkhead at the forward end.

oil contaminated with in simplex type stern tube by lip seal. The elastic lip of each nitrile rubber seal liners at outboard and inboard end of the steel propeller shaft.
The outboard liner additionally protects the steel shaft from sea water contact and corrosion.

Heat produced by the friction will result in hardening and loss of elasticity of the rubber, should temperature of the seal material exceed 110c. cooling
at the outboard end is provided by the sea. Oil circulation aided by convention, is arrranged to maintain low temperature of seals at the inboard end.
connections are fitted top and bottom between the two inboard seals.

The chrome steel liners act as rubbing surfaces for the rubber lip seals and grooving from fricitional wear has occurred. The problem has been overcome by using a ceramic
filler for the groove or alternately a distance piece to axially displace the seal and ring assembly. Allowance must be made for relative movement of shaft and stern tube due to
differential expansion. New seals are fitted by cutting and vulcanizing in position.

Lip seals will accept misalignment but a floating ring design was introduced by one maker.


WRITTEN BY S/C ZEESHAH AHMED
LECTURE BY: CHIEF ENGR. Tahir jamil
Phone no. 03344011232
EMAIL: ZEETEC4@YAHOO.COM
date: 03.nov.2009

Marine electricity & ancillary equipments

In the name of Allah who is the most beneficient the most merciful

Brushless DC Motor Back EMF

Brushless DC motors (BLDC) are used where there are limitations in the use of the brush-type DC motors. In this article we discuss how it is possible operate a DC motor with no brush arrangement and also about the back EMF in a brushless DC motor (BLDC).

Introduction

A DC motor is a one which operates on supply from a DC source. The DC source may be either DC generator or from a battery. DC motors may be classified as:

·         Series wound DC motor

·         Shunt wound DC motor

·         Compound wound DC motor

·         Separately wound DC motor

In all types of DC motors, the supply is given to both stators to make it as an electromagnet. This supply is necessary because the operation of a DC motor depends on the attraction and repulsion principles of magnetism.

In the stator, the supply voltage from a DC source is given directly, and in the rotor of DC motor it is supplied by means of a brush arrangement. But in case of brushless DC motors, this supply voltage to the rotor should be supplied without any brush arrangement. Brushless DC motors are more complicated than ordinary DC motor with brush arrangements, but certain applications needs this brushless DC motor, and hence it exists.

In a brushless DC motor (BLDC), we have an exciter rotor mounted on the same shaft of the rotor of a DC motor. This exciter stator induces an EMF when a small voltage is applied to the stator of this exciter. The voltage induced in the exciter rotor is an AC voltage and this is rectified to DC by means of a rotating rectifier diode arrangements mounted on the same shaft of the motor. The rectified DC voltage is applied to the rotor of DC motor, and there is no brush required so the DC motor with this type of complicated arrangement is called a brushless DC motor (BLDC).
 

Back EMF in Brushless DC Motor (BLDC):

According to Faradays law of electromagnetic induction, when a current carrying conductor is placed in a magnetic field that is if the conductor cuts the magnetic field), an EMF is induced or produced in a conductor and if a closed path is provided current flows through it.

When the same thing happens in a brushless DC motor (BLDC) as a result of motor torque, the EMF produced is known as “back EMF.” It is so called because this EMF that is induced in the motor opposes the EMF of the generator.

This back EMF that is induced in the brushless DC motor (BLDC) is directly proportional to the speed of the armature (rotor) and field strength of the motor, which means that if the speed of the motor or field strength is increased, the back EMF will be increased and if the speed of the motor or field strength is decreased, the back EMF is decreased.

This back EMF created acts as a resistance and we all know that any resistance in a line reduces and opposes the current flow so if the speed of the DC motor or field strength is increases, the back EMF increases which it turn increases the resistance to the current flow in windings and hence only less amount of current is delivered to the armature of DC motor. Also if the speed of Dc motor armature or field strength decreases, the back EMF decreases, which in turn reduces the resistance and hence more amount of current flow to the armature of DC motor.

When the DC motor is first started, there is no back EMF induced and as discussed above there is maximum current flow from the DC generator or distribution lines to the motor armature and as a result the motor toque will be maximum. In this case there is no resistance offered by back EMF. The only resistance available is the motor winding resistance.

During normal operation (rated speed) of DC motor, the back EMF induced will be maximum which will reduces the motor armature current to its minimum level and as a result the motor torque will also be reduced.

When the load on the motor is increased, the motor speed (RPM) is decreased and this will reducs the back EMF. This decreases in back EMF will automatically increase the motor torque thereby bringing the motor to its rated speed.

Marine Generators – Starting Checks & Procedure

Starting of Generator Engine

Starting of an engine from “stop” state is something which needs to be done with care, especially if the interval of starting is sufficiently long. The following is a checklist of all the checks which ideally need to be carried out before starting the generator. In actual practice sometimes the engineers might take some of these for granted and skip, but it is advisable not to indulge in such a practice. In fact these checks are generic for any four stroke engine starting process

1.    Check the turbocharger sump oil level, governor, alternator, forward and aft lube oil levels, and diesel oil level in service tank

2.    Open the indicator cock

3.    Prime the lube oil to all parts by hand pump or by motor driven priming pump

4.    Ensure that all jacket cooler valves, lube oil cooler valves, air cooler valves should be in open position

5.    With use of the Turning bar turn the fly wheel and check for any resistance on the bottom end bearing and check any water / fuel coming out through indicator cocks

6.    While turning engine, check all visible lube oil points are lubricated

7.    Remove the turning bar from fly wheel and put in the place

8.    Drain the auxiliary air bottle

Blow through engine (i.e.: by turning engine with air). In order to ensure that no water is inside combustion chamber if it is present it may cause water hammering

9.    Close the indicator cocks and pull lever from stop to start

10.  When the needle in RPM indicator deflects to some value of (0-25 rpm) put the lever in run condition

11.  The engine will run on fuel oil once the generator picks up the rated speed

12.  Put generator on load by closing air circuit breaker

13.  For checking the alternator fore and aft bearing lube oil level by opening oil plug in the alternator and the ring bearing while rotating splash lube oil from the sump can be seen

14.  In order to synchronize the incoming generator with running generator syncroscope method/dark lamp method is used

Starting of generator

Checks to be made while running

Once the generator has actually started to run, there are several checks which must be performed before it is left on its own to continue running. These checks pertain to verifying various parameters related to lube oil levels, temperatures and so forth. Given below is a brief checklist related to the same.

Lube oil checks

1.    Sump lube oil level

2.    Governor lube oil level

3.    Rocker arm lube oil level

4.    Alternator forward and aft bearing lube oil level

5.    Lube oil in turbine & blower side of turbo charger

Temperature checks

1.    Exhaust gas temperature

2.    Turbocharger (inlet-outlet) temperature

3.    Booster air inlet temperature

Cooler temperatures

1.    Cooling sea water (inlet – out let) temperature in cooler

2.    Jacket cooling water (inlet – outlet) temperature

3.    Air cooler (inlet -outlet) temperature

Safety Devices

Once the above mentioned parameters have been checked and found within normal range, it is safe to continue running the generator. Yet a fault can develop even at a later stage, so for this very purpose various trips and alarms are situated on the generators. An alarm gives the signal of an impeding danger and requires quick action while a trip actually trips the generator immediately because of the nature of the fault.

The various trips and alarms are mentioned as follows

1.    Alternator bearing low oil level alarm & trip

2.    Alternator bearing high temperature lube oil alarm &trip

3.    Low sump oil level alarm and trip

4.    Lube low oil pressure alarm and trip

5.    Reverse current trip

6.    Over speed trip

7.    Over load trip

8.    High and low frequency trip

9.    Jacket cooling water low pressure alarm