Zeeshan-Ahmed

2012-11-28T22:20:00.001-08:00

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

The Funnel of ship Funnel of a ship is usually located at aft of the ship. It can be located at amidships and fore ship. This depends upon on what type of ship it is and from where the exhaust pipe lines are drawn out from ship. The information located below can be treated for tanker ships. Ship funnel play a vital role, the lighter gases (warm gases of engine room) travels up and exhausted by grills at funnel. It is also a survey job to check the flap opening and closing at the back of funnel. The exhaust of Main Engine, Generators, Boilers, incinerator and safety Valve safely uptake from funnel. The material used for the construction of this is mild steel. Four to six pipelines are present there. The one which have large diameter is the exhaust of main engine. Three to four pipeline present for the auxiliary engine (generator). Two uptakes present for the exhaust of port and starboard boiler. Two smaller diameter pipelines present for the safety valve of port and starboard boiler. Some of the vents are also located at the funnel grills position (Lubricating oil, Fuel oil). On the exhaust pipeline there present is a spark arrestor (Flame Arrestor). The function of the spark arrestor is to limit the un-burnt carbon in the exhaust. Spark arrestor is used for safety purpose. In case it is not present or damage un burnt carbon may fall on the main deck could cause fire. Lagging is also done on exhaust so that maximum heat remains in pipe and don’t radiate in ambient atmosphere. One main thing I forget to explain is the gong (Pneumatic whistle) located on the front of the funnel. The logo of the company is also painted on the funnel of the ship.
Author
Zeeshan Ahmed
46th Batch Engineer
PAKISTAN MARINE ACADEMY
http://zeetec4.googlepages.com
http://shipengineer.blogspot.com

Any question and suggestion are welcome

Control technology

2011-08-11T22:24:00.000-07:00

1.Describe following Control Terminalogy

Controlled Condition :

Temperature, Flow, Pressure, Level

Detecting Element:

Thermometer bulb, Bellows, Orifice plate

Measuring Element :

Receives and evaluates the signal from the detecting element

Correcting Unit (regulating unit) :

A device capable of effecting change in the controlled condition

Controlling Unit :

Means of operating the correcting unit according to the difference between the desired and the measured value

Desired Value :

The value of the controlled condition that the operator desires to obtain

Set Value :

The command signal to a process system

Deviation :

Difference between the measured value of the controlled condition and the set value

Dead Time :

Time interval between a change in signal to the element and the initiation of a response to the change

Dead Zone (band) :

A range of values over which a signal may vary without initiating any response

Transmission of a signal from a late stage to an earlier stage in a control system

Feedback

Gain :

The ratio of output signal to input signal change in any part of a control system

Proportional Band :

Range of values of deviation corresponding to the full operating range of output signal from the controller due to proportional action only

Proportional Action :

Action of controller whose output signal is directly proportional to the deviation

Offset :

Sustained deviation of the controlled variable from the set value occurring when proportional control action is used alone

Overshoot :

Amount by which a changing process variable exceeds the desired value following a change in the system

Open Loop :

No feedback and the controller action has no relation to thefinal result

Closed Loop :

A combination of units by which the controlled condition is measured and compared with the desired value. If there is a deviation from the desired value, a signal is sent back to the final control element to correct or limit such deviation

Cascade Control :

Control in which one controller provides the command signal to one or more controllers.

Actuator :

Servo-motor or a complete self contained servomechanism producing a limited output motion

Reference Input Signal :

(desired or set point) information required before action can be taken

Gate :

Automatic switch allowing a pulse to pass to output only if specific combinations of pulse signals arrive simultaneously at its input

Address :

The identity of a particular store location or placing of data in a store

Positioner :

Part of the regulating unit, using power external to the loop, ensures precise adjustment of the regulating unit in accordance with controller output requirement

2.Explain with figure Closed Loop and Open Loop Control System

CLOSED LOOP

The input signal to the controller is dependent on the "ERROR" that is the difference between the system input and its output. A diesel generator fined with a governor will give small variations in speed with load changes.

OPEN LOOP :

In this the input signal to the controller is independent of the output from the controller, for example, a diesel generator without a governor, changes in load will give large speed variations.

3.State The advantages and disadvantages of Electrical and Pneumatic control system

Electrical Control System:

Advantages

a) No compressor and associated equipment required

b) Cables are cheap and easy to lay.

c) High efficiency (no leakage) .

d) Virtually instantaneous response .

e) Normal temperatures have little effect on operation.

f) Very accurate .

Disadvantages

a) Airborne moisture may cause damage .

b) Damage readily occurs in the event of a fire

c) Some risk of fire due to overheating .Intrinsically safe and/or explosion proof

equipment may be required

d) Equipment is affected by voltage fluctuations and/or supply failure

Therefore D.C supply from transformer / rectifier with fully charged batteries for

standby

Pneumatic Control System:

Advantages

a) Unaffected (in short term) by variations in ships power supply

b) Piping simple and easily installed

c) No heat generated therefore virtually no fire risk

d) Actuators are cheaper and have a faster response than electrical

Disadvantages

a) Transmission lags, especially in large systems

b) A supply of clean, dry , oil free air is required .Therefore filters, dryers, carefully

installed pipe work with drainage points are required .

4.Write Short Notes on

Control Air Compressor

Refrigeration type Air Driers

Adsorption type air driers

COMPRESSOR

It is normal to have two oil free compressors, one of which is on standby and arranged to cut in at a preset pressure. The compressors will be fitted with unloading devices to allow light running when the desired pressure is reached, alternatively they may have preset cut in and cut out switches.

Typical pressures are:

Air supply from compressor 4 - 7 bar

Air control pressure 1.2 - 1.5 bar

Refrigeration Type Air dryers

This incorporates a small domestic refrigeration unit and contains controls that ensure that water is drained off does not freeze. The cooled air is reheated by the incoming air cooling the latter and increasing the volume of the former.

Adsorption Type

This type-of drier consists of twin chambers holding a drying chemical (silica gel) which has a fine honeycomb texture, this offers a very large surface area to which the water droplets in the air cling. The chemical is recharged by either heating or blowing through with dry air.

The unit maybe arranged to change over chambers automatically on a time basis.

Operation

With the valves in the positions shown, air flows through the valve A and through dryer '1'. Approximately 90% of the air flows through the outlet valve F. Valves 'C' and 'E' are open whilst valves 'B' and 'D' are closed allowing the remaining 10% of the dry air to flow through dryer '2 to reactivate the chemical Before the chemical in dryer '1' is saturated all the valves are switched and the main flow is now through dryer '2'. As previously stated this change over is done automatically on a time basis.

5.Draw a Control air supply system.

Explain why controlled air need to be oil, dirt free and dry.

CONTROL AIR SUPPLY SYSTEM CLEANLINESS :

The most important factor in obtaining a reliable, trouble :free control system is to ensure a supply of CLEAN DRY AIR.Free of dirt, water and oil The nozzle restrictor hole (can be as small as O.2mm diameter) can easily become blocked with dirt, whilst oil can react with rubber diaphragms and water will corrode the system.

Air lines should have a gradient of 1:50 and the instrument tapping are taken off the top of headers. Seamless copper, polythene and P.V.C tubes are used

The air used for a pneumatic control system should always be separate from the general service air system, since if either system fails then an alternative supply is available.

The air should be 100% oil free, having a dew point of between -20°C and -40oC under pressure and also having no solid panicles of a size that could cause damage to the instruments .

6.Explain with sketches

Proportional Action

Booster Relay

Draw Simple pneumatic proportional controller with booster relay.

PROPORTIONAL ACTION

Introducing a feedback (proportional) bellows into the flapper nozzle arrangement reduces flapper movement per unit error movement. This is termed negative feedback and increases the proportional bandwidth i.e decreasing the sensitivity of the unit.

The output of the unit is proportional to the difference between the desired value signal and the measured value signal (error)

The error signal now -moves the position of the flapper relative to the nozzle, varying the air leakage to atmosphere and hence the output signal.

PROPORTIONAL ACTION results in a steady hold after a change, but at a different value to the desired value.

BOOSTER RELAY

The output from the flapper / nozzle arrangement is very small and cannot be transmitted over long distances or operate large equipment, hence a booster or pilot relay valve is often fitted into the system to amplify the output signal.

The above booster relay shows a direct acting relay, i.e an increase in signal pressure from the controller results in an increased output signal from the relay.

An increased input signal moves the valve to the right thus reducing the leak- off at "D" and increasing the operating air flow at "B". This results in increased output signal at

SIMPLE PNEUMATIC PROPORTIONAL CONTROLLER WITH BOOSTER RELAY

7.Write Short notes on

Multi function Relay

Air to open and Air to close control valve

MULTI-FUNCTION RELAY

A multi-function relay combines the requirements for adding, averaging, subtracting and ratio ing in one relay.

The relay contains four bellows and a movable fulcrum.. which can be accurately positioned and clamped in the central position for all cases, except when it is used as a ratio relay.

The bellows required are connected to the input signals, any bellows not required being

vented to atmosphere.

Addition: Multiplication

P3= Zero P3=P4=Zero

A=B P1= A/B X P2

P1= P2 +P4

Substruction: Averaging

P2= Zero P1=P3, A=B

A=B 2P1=P2+P4

P1=P4-P3 P1=P2+P4

AIR TO OPEN CONTROL VALVES

Control air is used to open the control valve.Failure of control air will cause the valve to close by spring pressure.

AIR TO OPEN CONTROL VALVES

Control air is used to close the control valve.Failure of control air will cause the valve to remain full open by spring pressure.

It is normal to have a means of operating the valve in the event of complete loss of operating au

8.Explain with Sketches

Proportional & Intregal Action ( Reset Action)

Proportional & Intregal & derivative Action.

PROPORTIONAL & INTEGRAL ACTION (RESET ACTION)

To reduce "offset integral action is added by placing another bellows opposite the proportional bellows this gives positive feedback.

A variable restrict or is placed in the line before the integral bellows so that the integral action time can be changed.

When integral action is included in the controller it gives an output whose rate of change is proportional to the difference between the measured value and the desired value signals

PROPORTIONAL & INTEGRAL & DERIVATIVE ACTION Also called a three term controller.

Derivative action is obtained by placing a variable restrictor in the line into the proportional bellows. This gives a faster response from the controller, the output being proportional to the rate of change of the deviation

Derivative action has a stabilizing effect on the system and reduces overshoot.

9.Sketch a Single seated and double seated valve.

State advantages of above

State the seat materials

The valve plugs can be either Single or Double seated.

Single Seated when the valve is small or where the effects of fluid forces on the valve are small. They are cheap and tight seating can be readily achieved.

Double Seated (Double Ported) are arranged so that the fluid force across the plug are balanced. These usually require a smaller valve movement, or for a given movement, have larger flow capacities. If metal seats are used with double ported valves then complete shut off is not possible and to 2 % leakage is normal.

Seat materials can be metal (usually stainless steel or monel) which give high wear resistance, can be ground in, can cope with most fluids and can handle high temperatures. Metal seats are required for emergency shut off duties.

Alternatively soft seats can be used, where the material may be Nitrile Rubber (or similar). These can allow complete shut off with double ported valves and produce a softer action on closing.

10.Explain three types flow characteristics of Control valves

QUICK OPENING (Poppet Type)

This characteristic is common in conventional valves, the lift is small and the valve offers minimum resistance to flow. Use in control applications limited to ON / OFF situations, The flow-lift characteristic is geaually linear.

LINEAR FLOW TYPE

These are used when a large proportional of the systems pressure drop occurs across the valve its self

They give a flow - lift characteristic which is linear if plotted on a graph, 40% lift will give 40% flow

The plug can be modified to give a parabolic characteristic to get "fine" control at low flow rates .

EQUAL PERCENTAGE TYPE

It is also called the Logarithmic valve plug

The equal percentage plug is one of the common types in use in control systems as it gives good control over a wide range of pressure changes.

It is particularly useful when flow conditions are difficult to measure, as it has the flexibility for matching (or sizing) to systems.

It is used when most of the pressure drop take place in the system and where there are high system motion losses.

Equal increments of travel give equal percentage changes in existing flow., hence at low lift the flow rate will be low and changes in flow are also small, at large flows then changes in flow are also large.

11.Describe with sketches different type control valve glands.

CONTROL VALVE GLANDS

Control valve glands are fairly long to give good sealing with minimum friction. The type of gland and the packing will depend on the control valve application. For oil, packings are usually rubber proofed cotton,

For low pressure water, greasy packing of Indian Hemp is used.

With hazardous fluids or were the fluid is very expensive, a bellows seal is used, giving a completely tight seal.

Where P. T.F.E materials are used the packing is chevron shaped with the vee expanded against the gland (up to 230 degrees C no lubrication is required).

Where the temperature is very high a lubricant may be required as the temperature may quickly destroy any built in lubricant. The packing is normally asbestos, wire wound and impregnated with PTFE. The gland housing may also have cooling fins around it to help dissipate heat thus preventing expansion imposing increased resistance to valve movement.

12.State why valve Positioner is Used.

State where they are normally fitted

State their advantages.

VALVE POSITIONERS

In some systems (single ported valve with high pressure drop and high gland pressure) the direct signal from the controller may not always be positioned exactly each time it moves. for example at O.6b the valve may be at a different position when it is opening to that when it is closing with a O.6b signal. To overcome this problem a valve positioner is fitted into the system.

The valve positioner compares the valve position and the air signal from the controller to position the valve stem and valve quickly to a position proportional to the controller air signal, independent of counter acting forces.

Valve positioners are fitted where

1) there is a high pressure drop across the valve

2) the valve is remote from the controller

3) the medium being controlled is viscous

4) there are high gland pressures required

The advantages of fitting a valve positioner are:

1) faster response

2) helps overcome motion between the valve spindle and the gland packing

3) helps to overcome out of balance forces on the valve plug .

.14.Explain with a sketch operation of a valve positioner.

An increase in instrument air pressure expands the bellows causing the flapper to cover the nozzle. Through the relay action pressure to the control valve diaphragm increases. The valve stem moves downwards, this movement is fed back to the beam by the earn which moves the flapper away from the nozzle. The relay supply valve closes and the positioner is again in equilibrium but at a higher instrument pressure and a new valve stem position.

On a decreasing instrument pressure the bellows contracts and the flapper moves away from the nozzle. Relay operation causes a decrease in air pressure above the diaphragm by exhausting this air to atmosphere, allowing the valve stem to move upwards due to the force of the return spring. When equilibrium has been reached then the relay exhaust valve closes to prevent any further decrease in diaphragm air pressure.

15.Explain the terms

a. Fail safe

b. Fail set

Fail safe

On failure of the control air supply the valve maybe arranged to move to a position that allows the plant to continue to operate safely, that is fully open or fully closed this is known as fail safe.

Fail set

In some systems failure of the control air supply will cause the valve to be locked in the position at failure this is known as fail set this has the advantage that the system will remain stable and allow time for the plant to be shut down in a methodical method or the air supply to be re-established.

16.Explain the meaning of the following terms:

a.two step control action

b.proportional plus integral control

c.ratio control

d.Give a circumstance or condition where each of the above control modes might be suitable and give the reasons for its choice.

ANSWER

a i) Two step control action is the simplest form of control, in that it assumes one of two preset positions, i.e a switch is ON or OFF, in the case of a valve it is open or closed.

ii) Ratio control is one in which one variable is kept at a fixed ratio to another variable

iii) In a proportional plus integral control controller, the proportional element acts if there is any deviation between the output and the desired value, however the action results in a permanent difference between the desired value and the new output value (offset). The integral element of the controller acts to remove this offset

b) Typical areas of application of the above forms of control may be as follows:

i) A pressure switch controlling the Air start compressor, this is an acceptable form of control since there can be a fairly large deviation from the desired value

ii)Ratio control is found in a boiler combustion control system where the ratio between

the air / fuel must be controlled to ensure good combustion. The air flow is controlled by dampers to match the fuel flow.

iii)A boiler water level control system will employ P + I control, to maintain the water

level at the desired value at all times, removing offset and instability.

17.Suggest with reasons the effect of the following faults in pneumatic control systems:

a) A punctured diaphragm in the nozzle relay valve

b) Grit on the seat of the nozzle relay valve

c) Leaking proportional bellows in valve controller

d) Leaking measurement bellows in valve controller .

ANSWER

a)Because of the leaking diaphragm / bellows the valve will not close, hence the control valve will be unable to fully close

b)Grit on the seat of valve "E" will allow continuous leak of to atmosphere to take place therefore maximum operating pressure will not be obtained, hence the control valve will be unable to open fully

c)A leaking proportional bellows will reduce the ability of the bellows to introduce negative feedback. Hence the gain of the controller is increased. The control valve will move to extreme positions leading to instability

d)A leaking measurement bellows will binder the flapper from moving towards the nozzle. Hence the control valve will be moving from its equilibrium position, causing the system to hunt.

18.With sketch describe a Cascade control system used on Board

JACKET WATER TEMPERATURE CONTROL SYSTEM

In this system there are two measuring elements,

a)the primary measured value being the Jacket Water outlet temperature

b)the secondary measured value the coolant outlet temperature

The secondary measuring element detects changes in the cooler or heater, at the jacket water inlet to enable corrective action to be taken before significant temperature change in the engine occurs, to achieve this a second controller is needed.

The primary measuring element detects changes in JW outlet temperature, the signa] being sent to the master controller where it is compared with the set point. The output of the master controller passes to the slave to control the cooling or heating valve.

The secondary measurement, also passes to the slave controller, so that the value of the slaves output signal depends upon the error between the master controller output and JW inlet temperature, hence the heating / cooling control valves can be adjusted to compensate for changes in sea water or steam temperature, which leads to much quicker response.

The output from the slave controller fed to both the heating and cooling control valve, its value being split, so that only one valve will be open at a particular time.

To comply with the system fail safe requirements;

a) the cooling valve must be air to dose (ATC), so that in the event of a power loss the engine is cooled rather than heated.

b) the steam valve must be air to open (ATO)

The cooling valve (ATC) must therefore use the lower part of the slave output range and the steam valve the upper part of the signal.

To ensure that that the valves are never both open at once a small amount of dead band is introduce.

CASCADE: A number of controllers in series

SPLIT RANGE: Final controllers output signal range is split between two control

valves, one A TO, one A TC

TWO ELEMENT: There are two measured values

19.Sketch and describe automatic Package boiler control system

AUTOMATIC PACKAGE BOILER

On starting the circulating solenoid recirculates fuel until the correct temperature is reached. The solenoid then switches over to put the low flame spill line into operation. Once the flame is established the steam pressure switch operates the solenoid valve to change over between high and low flame as the steam pressure varies.

A shut down solenoid which shuts the fuel supply down is operated by:

a) High and low water level

b) High steam pressure

c) Flame failure

d) Low furnace air pressure

20.Sketch and describe Boiler water level controller .

Sketch and describe a control system which employs P +I control action.

BOlL.ER WATER LEVEL CONTROL

In simple single element boiler water level controllers, the boiler water level alone is measured, this can lead to the boiler tripping on high or low level due to the effects of boiler water shrink or swell.

On medium sized boilers it is common to employ two element controllers to control the boiler water level.

The primary measured value is the water level which may be measured by a D.P cell. the signal being compared to the set point, any deviation being fed into the slave controller as its set point.

The secondary measured value is the steam flow from the boiler, this being measure and the signal being sent to the slave controller where it is compared, with the set point (master controller output).

During the swell period the level signal calls for less water, even though the actual amount of water has actually decreased. Simultaneously the increase in steam flow calls for an increase in feed flow, this combination of signals will usually prevent the boiler from tripping.

21.Sketch a simple Hydraulic deck crane circuit

HYDRAULIC DECK CRANE CIRCUIT

The circuit is typical of a system using variable stroke pump and a fixed stroke hydraulic motor. The complete system would require three such circuits, one for hoist, luff and slew.

SAFETY FEA TIJRES

Braking _

The requirements are that it win hold the load securely and allow a smooth change over from the static position to winding and fail safe.

Normally the brake is spring loaded to the "ON" position, it being released by hydraulic pressure. The signal to operate the brake coming from a micro switch on the swash plate control lever, which opens the solenoid valve.

Override Controls

These are required to prevent damage or operating the equipment in unsafe areas, e.g slew angle, max and min luff angle, and slack rope protection

The operating pressure of the system will depend upon the position of the swash plate and the size of the load being lifted.

22.Sketch and Describe Constant pressure hydraulic system

CONSTANT PRESSURE HYDRAULIC SYSTEM

Hydraulic systems on deck normally use variable delivery pressure compensated pumps to supply the system with constant pressure oil.

The pump takes suction from a sump through a non-return valve and a filter, before discharging it into the main.

When the pumping capacity exceeds the load requirements, the systems pressure will increase to a predetermined value. Once this value is reached the pressure compensator acts to take the pump off stroke. Should the pressure compensator fail then the system is protected by a relief valve which returns the oil to the sump. Once the main line pressure has reduced to a preset value the compensator acts to put the pump back on stroke, increasing the system pressure.

To allow for slight fluctuations in pressure an accumulator is fitted into the system.

Hydraulic fluid is directed to the load via a 3-way valve.

23.Describe the components of simple hydraulic system

SIMPLE HYDRAULIC SYSTEM

All hydraulic power systems comprise of at least the following components

(1) Tank -Used to store the hydraulic fluid that is not currently in use

(2) Pump - Used to force the hydraulic fluid through the system, acts as the

pressure source

(3) Prime mover - The power source for the pump. b hydraulic systems the prime mover is usually an electric motor

(4) Valves - Installed to control liquid direction, pressure, and flow rates.

(5) Actuator - Devices that convert the energy of the liquid into mechanical

force or torque. Typically, single piston and cylinder arrangement that results in linear motion. A ship's steering system uses this design.

(6) Piping - Used to contain and direct hydraulic fluid from one point to

another.

Additional Components

Filters/strainers - Used to remove foreign particulate matter from hydraulic fluid

that could damage (by scratching close tolerance components) or clog the system.

Pressure regulator - A device that vents off or unloads hydraulic fluid from the high pressure side (pump outlet) when the pressure in the system exceeds set point (design pressure). The unloaded fluid usually . is returned to the low-pressure side of the system or the sump. By unloading hydraulic fluid, pressure is reduced. When hydraulic pressure returns to set point regulator stops unloading. By constantly loading or unloading, the pressure regulator maintains the pressure at set point.

Accumulator - A device that acts as a hydraulic shock absorber for the system.

It basically consists of a container, divided into two sections by a flexible divider or membrane. One section is open to the hydraulic system's liquid and the other section contains a gas (often nitrogen) under an appropriate pressure. It is used to store a certain volume of hydraulic fluid under pressure. The liquid side experiences the same pressures as the fluid in the hydraulic system. When the hydraulic system experiences a sudden pressure increase, the effect is reduced or dampened by fluid in the accumulator compressing the gas on the other side of the membrane. When the system experiences a sudden pressure decrease, the gas forces fluid out of the accumulator to increase pressure. The accumulator is designed to handle _brief system transients, not system casualties or long term degradations.

Relief valve - A device that protects Systems from excess pressure. A relief valve is much the same as a pressure regulator in that it unloads excess pressure. However, a relief valve has a higher set point than the pressure regulator and only operates under abnormal conditions such as a failure in the pressure regulator.

Check valve - A device that permits flow in only one direction.

Sequence valve - A device that controls the sequence of operation of one or more

other components.

Servo - A device that measures output or state such as the rotation of a motor or the distance of travel of a piston in a cylinder. The servo is output is compared to a standard or output signal. The results of the comparision are used to control a systems or actions.

24.Why it is necessary to have hydraulic system free of dirt used in Deck Machinery

What are the sources of contaminants.

HYDRAULIC DECK MACHINERY

The most important factor in obtaining a reliable, trouble free hydraulic system is to ensure a supply of clean hydraulic oil free of dirt, water The nozzle restrictor hole (can be as small as O.2mm diameter) can easily become blocked with dirt, causing malfunction of hydraulic control valves. water will corrode the system.

SOURCES OF CONTAMINATION

1 RESIDUES FROM THE MANUFACTURING PROCESS

a. Metallic particles swarf, grinding dust from machining process ,

b. Scale from heat processes such as forging welding rolling bending

c. Slag and sand from castings

d.Debris from sealants

e. Fibres from cleaning materials

f. Ingress of S.W. causing rust particles

g. Particles produced from fretting and vibration

h. Particles produced by the oxidation of oil, breakdown of additives g. Dirt from human sources hair and skin

CONTAMINATION OF THE HYDRAULIC SYSTEM BYWATER

The only source of contamination by water is from the leakage at the

water / oil interface.

Oil is capable of dissolving water in the order of hundreds of p.p.m.and

still remains clear and bright. This may reappear as condensate and cause rusting of ferrous components.

25.Describe Hydrostatic & pneumercator level gauge.

Hydrostatic level gauge

Hydrostatic pressure sensors can be used to measure liquid levels directly in tanks.

A pressure gauge located at the zero or empty level of a tank will register zero when the tank is empty and as the tank is filled with liquid a rise in pressure will occur due to the increased head of liquid acting on a diaphragm or bellows this pressure is then transmitted to a bourdon tube type gauge. The gauge is normally calibrated in tonnes, (if calibrated in meters then an allowance must be made for the relative density of the tank liquid)

The Pneumercator gauge

This instrument is in common use on fuel oil tanks and uses a mercury manometer and hemispherical bell. The hemispherical bell is fitted near the bottom of the tank and is connected by small bore piping to the mercury manometer. Compressed air pressurizes the system, the air supply is isolated and the system is switched to connect the hemispherical bell to the manometer via selector cocks, the head of liquid further increases the pressure of the compressed air in the system

which is measured on the mercury manometer.

The bubbler level gauge works by adjusting the air pressure entering a open ended tube immersed inside a tank, so that bubbles can just be seen leaving the bottom of the tube. A pressure gauge then measures the air pressure that is required to overcome the pressure of the liquid head above the bottom of the tube. The diaphragm bubbler gauge is a slight modification where a diaphragm isolates the air from the liquid, the air escaping to atmosphere. The varying head of oil acting on the diaphragm will vary the air escaping to atmosphere and hence the back pressure in the tube will alter. This type of gauge would be used lubricating oil tanks where the air may oxidize the oil

26.Explain Diesel engine Bridge Control System

DIESEL ENGINE BRIDGE CONTROL SYSTEM

The classification societies require that speed of the main engine must be able to be controlled from the bridge by means of a single lever, if:

a)the machinery spaces are periodically unmanned

b) only one man is on watch within the engine room.

The telegraph acts as a reference transmitter for the propeller speed and is graduated in r.p.m. Alterations of the speed of the main engine are carried out according to fixed programmes. These programmes are set up to meet the requirements of each engine and of the shi;> with respect to overloading and maneuverability.

It is desirable to include safety functions, e.g. low lub oil pressure etc. in order to obtain automatic speed reduction or to stop the machinery.

Starting and reversing of the engine are provide for by logic circuits, which control the injection of starting air, positioning of the camshaft and fuel injection.

The output from the logic circuits is converted to a suitable power level for control of the mechanical actuators on the main engine.

27. Describe following two types of torsionmeter

a.Angular Displacement

b. Magnetic stress sensitivity

Angular Displacement

Torsion meters are used for the measurement of power transferred through a propulsion shaft.

Principle

A torque of value T is applied to a shaft of fixed length L and radius r. An angle of twist θ is generated and is dependent of the modulus of torsional rigidity G and given by

T/r = Gθ/L

The modulus of rigidity, the raidus and the length of the shaft are all fixed thus the torque on the shaft is proportional to the angle of twist

Typical system

Two AC generators are mounted so that they are driven by the main shaft and area at set distance apart L. A sinusoidal waveform is produced. One of the generators is adjusted so that at minimum torque the generated waveforms are 180' out of phase. The outputs from the two generators are then added and the resultant voltage is used as the measurement of torque

As the torque is applied to the shaft so the twist causes the waveforms to shift in phase. When the two waveforms are now added an output ac current is produced which may be amplified and rectified to give an ouput voltage proportional to the torque appled to the shaft.

Another method of achieving this is to replace the generators wit sensors and toothed ring.

Power Calculation

Power is a product of the Torque and revs of the shaft, one of the generator outputs is used to measure the shaft rev/s and a calculation performed

Magnetic Stress Sensitivity

This type measures magnetic fields in the shaft surface, the distortion of these fields gives and indication of the torque. The principle behind this is that in some ferromagnetic materials reluctance ( magnetic resistance) is less along the plane of stress than across it.

In the torductor three rings are fitted around the shaft each ring having four electromagnetic poles. The centre ring acts as a transformer primary with the two outer secondary rings having their poles arranged 45' apart to the primary poles but in line with each other. The poles are held in a frame so that there is no contact between the poles and the shaft which have a gap of about 3 mm between them.

An alternating current is fed to the centre ring thus generating a magnetic field. This educes an emf in the outer two rings. The outer two ring coils are connected in series in such away that at zero stress the emf generated in each ring is opposite and equal in value giving an output of zero volts.

When torque is applied to the shaft the stress lines are distorted to to the axial. The distorion of this field affects the emf induced in the coils increasing on one side and reducing on the other. Thus a resultant emf of a few milivolts is available at the output. The size of the output is proportional to the stress applied.

28.Explain following terms

a. Two step control action

b. Proportonal action

c. Split range action

Two step control action

This can be defined as 'the action of a controller whose output changes from one state to another due to a variation in its input' One example of this control is that of a float operated filling v/v say for a cistern. In normal condition the output of the float is nil and no water passes through the valve, should the water level drop the float detects this and operates the valve to change to its second state which is open and water flows. When the level re-establishes then the float controls the valve to return to its primary state which is closed. In this way the float is controlling the water level by changing the valve between two different states. A more realistic system is shown below.

The system works as follows; the level drops until the lower float is uncovered, the controller detects this and opens the filling valve, the filling v/v remains open until the top float is covered and then the controller shuts the valve

The distance between the floats is termed the 'Overlap' i.e. the distance between the high and low controlling values ( on some systems this can be altered by altering the high or low set point of the controller, in the above system this would mean altering the position of the floats )

Proportional control action

This where the change in output signal from a controller is proportional to the change in input signal

The control can be summed up in the following;

Output = Constant x Deviation

Output - this is the output from the controller and goes to the control element ( say the filling v/v on the previous example i.e. the piece of equipment that actually alters the process.

Constant- This is the 'Gain' of the controller, as the output varies with the deviation, the amount it varies can be altered.

Say if the deviation changes by one unit the output changes by one unit, hence the gain is one. If the output varied by two for the same one change in deviation then the gain would be two. Similarly if the change in output was one half a unit for a one unit change in input then the gain would be half. Another way used to describe Gain is 'Proportional band', here a gain of one is described as a proportional band (Pb)of 100%. For a gain of two the Pb is 50%, and for a gain of a half the Pb is 200%, hence it can be seen that the magnitude of the Pb is opposite to the gain.

Deviation- This is the difference between the set point of the controller and the measured value. If the set point was one unit and the measured value was two units the deviation would be one unit.

Deviation = Set point - Measured value

The important think to remember is that the narrower the Proportional band the higher the gain and hence the higher the output varies for a change in deviation, this has the effect of making the controller control the process quicker by operating the controlling element more for smaller variations measured value. This has the negative

Split range control( negative and positive offset)

A system could be designed to control both the outlet valves and inlet valves (this is what is seen on the feed water system level control with the spill and filling being controlled from the one controller) ; here the controller would be set up so that when the level is at the set point its output is mid range ( say for a controller operating in the 3 to 15 psi range this would be 9 psi)

The control valves would be set up so that one, say the filling v/v would go from close at 9.5 psi to open at 15 psi, and the spill v/v would go from close at 8.5 psi to open at 3 psi. The 1 psi in the middle is called the 'Deadband' and is there to ensure both v/vs are not open at the same time.( The v/v acting to open with increasing input signal is called 'Direct Acting' and the v/v closing with increasing pressure is called 'Reverse Acting')

It can be seen that there can now be an offset in the positive i.e. water being used and hence the make up v/v has to be open and in the negative i.e. there is too much water entering the system and the spill v/v's have to be opened.

Offset is not a desired result of the control of a system, however for proportional only controllers this is a direct consequence. That is why for all controllers performing important functions; including the make up/spill system controller above other types of controlling action are added to remove the offset

Electrical-Three-Phase Systems

2011-08-11T22:14:00.000-07:00

Three-Phase Systems
Outputs from the three sets of conductors in the alternator are delivered to three separate bus-bars in the switchboard. This is necessary because of the voltage and current disparity between them at any instant.
Three-phase, four systems use a single return wire which is connected to the neutral point of the star windings. Current in the turn wire is the sum of currents in the individual phases. If loads on each phase are balanced with voltages equal and at .1 20 apart, the three currents will sum to zero and the return wire will carry no current The fourth (return) wire will carry a small current it there is imbalance.
Three-phase, three-wire systems have no return wire. This is acceptable for snips where, direct from the main switchboard, three-phase motors make up much of the load and unless there is a fault they take current equally from the phases. Also some imbalance is acceptable with a three-phase, three-wire system provided load is connected in delta Supplies for lighting, heating, single-phase motors and other loads are taken through delta-star or delta-delta transformers.
The Neutral Point
The majority of British ships use three-phase, three-wire distribution with the neutral points of alternators insulated (Fig. Very little current will flow through an earth fault on one phase, because there is no easy path for it back to the electrical system. With such a system, an essential electric motor with an earth fault can be kept running until stoppage for repair is convenient This would be as soon as possible to avoid a full-phase fault that would result if an earth occurred on another phase as well.
Although fault current is negligible with an insulated/unearthed neutral point, over voltages are high. The transient likely is 2.5 X line voltage. Such a voltage surge is within the capability of the main insulation of marine electrical equipment which is tested to 2 X line voltage + 1000 volts.

A few British vessels have electrical distribution systems with an earthed neutral . This is a connection of the system, via the neutral point of the alternator, to the hull steel. The result of not isolating the electrical system from the hull is that current flow from an earth fault on any phase has a path through the hull steel and earthed neutral back to the system. The availability of the path encourages higher fault current flow than is the case where the neutral is insulated or connected by resistance. Equipment with an earth fault, where-the system is earthed, must be disconnected immediately if a fault develops. This can be effected automatically with an earthed neutral system because the level of fault current is ugh enough to operate a trip.
Earth fault current is high with earthed neutral systems, but over- voltages due specifically to earth faults are lower. The earthed system is chosen to limit over voltages and to give automatic earth fault location and disconnection.
Over voltages due to switching are not affected by choice of earthing or insulating the neutral. These high surges, and the possibility of others from failure of the voltage regulator, means that the same standard of equipment insulation is required for both arrangements.

Electrical-Static Automatic Voltage Regulator

2011-08-11T22:13:00.000-07:00

Static Automatic Voltage Regulator
The carbon pile regulator uses a magnetic coil powered from the alternator output Strength of the field vanes with alternator voltage and this strength is tested against springs which are the voltage reference. The moving contact regulator employs a similar matching of alternator output effect through a magnetic coil against springs.
The availability of a small transformed, rectified and smoothed power supply from the alternator output makes possible the matching or it directly against an electronic reference in the static automatic voltage regulator. The direct current derived from the alternator output is applied to a bridge which has fixed resistances on two arms and variable resistances (zener diode voltage references) on the other two. The zeners operate in the reverse breakdown mode having been manufactured with a zener breakdown voltage of very low value. As can be seen from the earlier description of zener diodes, voltage remains constant once breakdown has occurred despite change in current. This implies, however, that changes in applied voltage while not affecting voltage across the diode will cause a change in resistance which permits change in current. .As with a Wheatstone bridge, imbalance of the resistances changes the flow pattern and produces in the voltage measuring bridge an error signal
The error signal can be amplified and used to control alternator excitation in a number of different ways. Thus it can control the firing angle of Thyristors (Fig. 3.14) through a triggering circuit to give the desired voltage in the brushless alternator described. It can be used in the statically excited alternator to correct small errors through a magnetic amplifier arrangement-The error signal has also been amplified through transistors in series, for Excitation control.

Electrical-D.C. Generators

2011-08-11T22:12:00.000-07:00

D.C. Generators

Operation of a d.c. generator relies (as with alternators) on the principle that when magnetic lines of force are cut by a conductor(Fig. 6.1) a voltage is induced in the conductor. Size of induced voltage and resulting current are dependent on magnetic field strength, length of conductor and speed of cutting.
The direction of current flow is dictated by the relationship between magnetic field and direction of movement of the conductor. Ii can be found from Fleming’s Right Hand Rule, which is applied to give direction of conventional current flow during generation,A simple generator can be constructed from a loop or coil of wire mounted on a spindle and arranged for rotation between opposite magnetic poles The field-cutting action of the straight sides will cause current flow as the result of induced voltage. Direction of flow is shown by the arrows (found from the Right Hand Rule) and can be seen to be continuous around the loop. The voltages generated are in series and therefore add to give twice the voltage produced in one side.
Direct current can be collected from the wire ends through the commutator which consists of two half-rings with brushes. Each brush takes current from one half-ring in turn, so that current flow is always in the same direction f or each collecting brush. The out put is not steady but has a wave form.

Electrical-Fuses

2011-08-11T22:11:00.000-07:00

Fuses
High current flow through a thin fuse wire will raise its temperature causing it to melt and break the circuit before the current excess reaches a level sufficient to damage other, more substantial, parts of the system. Melting temperature depends on the material used (tinned copper in rewireable fuses melts at 1080ºC, the silver in cartridge fuses at 960ºC). The wire is sized so that the normal current is carried without overheating, but due to the resistance of the relatively small wire, that excess current will produce heat sufficient to melt it. Current rating gives the normal current that may be carried: minimum fusing current is the smallest current that will cause melting.

A fuse will melt much quicker with very large fault current than when the value of fault current is only just above the minimum fusing current. Time/current characteristics are found by testing six or more of the same type of fuse at different currents and plot ting the results. The bottom current for the test is not more than 1.05 X minimum fusing current, and the top current is one that will melt the wire in not more than 0.5 second. The other test currents are equally spaced between these. Fuses are rated for particular ac. and/or d.c. voltages

Cartridge Fuses
High Rupture Capacity (HRC) fuses have silver wire enclosed in a quartz powder filled ceramic tube with metal end caps (Fig. 9.1). Arcing when this type of fuse blows is buried in the powder, fusion of which in the arc path helps to extinguish it.
HRC fuses can be used for very high fault levels: deterioration is negligible; they have accurate time/current characteristics and reliability for discrimination; they are safer if accidentally inserted on a fault: there is no issue of smoke or flame; cartridges are sized to ensure that the correct value fuse is fitted.
Semi-enclosed Fuses
The rewireable fuse has an insulated carrier for safe handling and containment of the wire in an asbestos lined tube
The wire is easily replaced after operation, but the des is open to abuse as too heavy a wire can be used which could mask a fault and also cause severe arcing if it did operate. Another fault is that of premature failure if the wire is made thinner by oxidation or con tact with air, or by being stretched when fitted (a problem with wire made of lead, tin or an alloy of the two).
Fuses in Service
Fuses may be used as the only protection in a steady load circuit, such as for lighting. An ac. motor with its high starting current and varying load has fuses in each of the supply conductors, but fitted as a backup for the other forms of protection and to break the circuit in the event of a short-circuit current greater than that which the ordinary contact breakers are designed to interrupt without damage. Very accurate time/current characteristics are needed for fuses used in conjunction with other safety devices, to ensure that the overload trip is allowed time to operate for moderate over- current but that the fuse blows first if there is very high short-circuit current.
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Electrical-D.C. Switchboards-MCB

2011-08-11T22:10:00.000-07:00

D.C. Switchboards
The switchgear of d.c switchboards, unlike that of enclosed a.c. equipment was mounted on what was literally a board or screen of panels. On the front the open main circuit breakers, distribution breakers, knife switches and instruments were easily accessible for inspection and maintenance. Safeguards against contact were only an insulated handrail and a rubber mat’ closed gate normally prevented entry to the passage behind the board where the copper bus-bars and rheostats were mounted.
Enclosed or dead-front boards are only required for d.c. installations if the voltage is greater than 250 volts. Most of the D.C. switchboards on British vessels just prior to the general change to ac. were for 220 volt supplies.

Main Circuit Breaker
The brush type moving contacts of the generator breaker are closed against a resisting spring by a lift-and-latch handle. The handle, when raised, slips the operating pin over the latch, where it is clipped into position. Downward movement of handle pushes
the cam plate against the hinged contact holder. The leverage and pressure applied forces the brush contacts to spread on the fixed studs. This brushing action is beneficial in that contacts are wiped and any film or deposit removed. Vibration with consequent burning or welding is prevented by the pressure, sustained by the resilience of copper strips, which are clamped In the manner of a leaf 3oring. Conductivity of the copper is slightly reduced by a small amount of alloying material added to prevent work-hardening and to give the necessary resilience.
The tripping of a main breaker due to overload or when load current was being carried would causes arcing and damage to the main contacts. Arcing contacts are therefore fitted and arranged to make first and break last to protect main contacts from burning. They are of metallized carbon, sintered silver-tungsten or other material which does not weld at high temperature; such compositions are not suitable for carrying high current over long periods.
There is a set of main and arcing contacts for each connection to be made from the generator to the switchboard bus bars. They are fixed together by bars for simultaneous movement by one handle. Thus, for a two-wire insulated d.c. system the triple pole circuit breaker connects the positive, negative and equalizing connections in one operation. The equalizing contact is given a lead so that it closes first Short-circuit’ between poles is prevented by insulation.
The breaker is held in the closed position by the latching device but is easily opened by the trip. Tripping is automatic in the event of short-circuit, overload, reverse current or low-voltage faults. For normal opening, the trip can be operated manually after reducing the load with the rheostat.

Handbook of Switchgears (Mcgraw-Hill Handbooks)
Distributed Switchgear (IEE Power & Energy Series)
1953-54 Whipp Bourne Switchgear Bethlehem Steel Builder
Handbook of Switchgears
Switchgear and Control Handbook

Emergency power supply for ships machinery operation

2011-08-11T22:05:00.000-07:00

Use of emergency generator

In the event of a main generating system failure an emergency supply of electricity is required for essential services. This can be supplied by batteries, but most merchant ships have an emergency generator. The unit is diesel driven and located outside of the machinery space .

The emergency generator must be rated to provide power for the driving motors of the emergency bilge pump, fire pumps, steering gear, watertight doors and possibly fire fighting equipment. Emergency lighting for occupied areas, navigation lights, communications systems and alarm systems must also be supplied. Where electrical control devices are used in the operation of main machinery, these too may require a supply from the emergency generator.

Fig: A.C. distribution system

A switchboard in the emergency generator room supplies these various loads . It is not usual for an emergency generator to require paralleling, so no equipment is provided for this purpose. Automatic start up of the emergency generator at a low voltage value is usual on modern installations.

Ward—Leonard motor speed control system for ships machinery operation

2011-08-11T22:04:00.000-07:00

Motor speed control

Fig: Ward-Leonard speed control

As a very flexible, reliable means of motor speed control the Ward-Leonard system is unmatched.

The system is made up of a driving motor which runs at almost constant speed and powers a d.c. generator . The generator output is fed to a d.c. motor. By varying the generator field current its output voltage will change. The speed of the controlled motor can thus be varied smoothly from zero to full speed. Since control is achieved through the generator shunt field current, the control equipment required is only for small current values. A potentiometer or rheostat in the generator field circuit enables the variation of output voltage from zero to the full value and also in either direction. The controlled motor has a constant excitation: its speed and direction are thus determined by the generator output.

Depending upon the particular duties of the controlled motor, series windings may be incorporated in the field of the motor and also the generator. This may result in additional switching to reverse the controlled motor depending upon the compounding arrangements. The driving motor or prime motor for the Ward—Leonard system can be a d.c. motor, an a.c. motor, a diesel engine, etc. Any form of constant or almost constant speed drive can be used, since its function is only to drive the generator.

Direct current supply, distribution, D.C. generators field connection & earth lamp circuit

2011-08-11T22:03:00.000-07:00

How a D.C. current is produced ?

A current is produced when a single coil of wire is rotated in a magnetic field. When the current is collected using a ring which is split into two halves (a commutator), a direct or single direction current is produced. The current produced may be increased by the use of many turns of wire and additional magnetic fields.

With many coils connected to the commutator, sparking will occur as the current collecting brushes move across the insulated segments. Commutating poles or interpoles are used to reduce this sparking. They are in fact electromagnets having a polarity the same as the main pole which follows in the direction of rotation.

The magnetic field between the poles is produced by what are known as 'field coils'. These coils are excited or energised by the current produced in the machine. The soft iron core of the field coils retains some magnetism which enables a preliminary current generation to build up eventually to the full machine output. The field windings can be connected to the output current in a number of ways—shunt, series or compound. The compound wound arrangement is usual since it provides the best voltage characteristics.

Fig:D.C. generators field connection

The compound wound generator has two sets of field coils . The shunt coil has many turns of fine wire and the series coil has a few turns of heavy wire. The shunt field produces full voltage on no-load which falls off as the load current increases. The series field creates an increase in voltage as the load increases. Properly combined or compounded the result is a fairly constant voltage over a range of load (Figure below).

Fig:D.C. generators characteristic curves

Direct current distribution

The generated supply is provided to conductors known as 'bus-bars' which are located behind the main switchboard. The supply then passes through circuit breakers to auxiliaries directly or to section or distribution boards. A circuit breaker is an isolating switch. A section board is a grouping of electrical services fed from the main board. A distribution board feeds minor supplies such as lighting and may itself be fed from the main board or a section board. The distribution system is shown in diagram

A two wire system is usual to provide a supply and return to each item of equipment. An earth lead would be the only electrical connection between any item of equipment and the ship's structure. With compound wound generators a third bus-bar would be introduced as the equalising connection between machines.

A fuse is a type of switch which isolates a circuit if an excessive current flows. To reconnect the circuit, after discovering the cause of the overload, the fuse must be rewired or replaced. The fuse is in effect a weak link in the circuit designed to break and protect equipment from damaging high currents. A semi-enclosed or rewirable fuse will have provision for a wire to be replaced after it has burnt out. The correct rating of fuse wire should be replaced within the holder to reinstate the circuit. A cartridge fuse has the wire enclosed within a ceramic body and it is not rewirable. A 'blown' cartridge fuse must be replaced by a new one, The cartridge fuse is to be preferred since the fusing current value is more reliable than for a rewirable type.

A circuit breaker is an isolating switch which also functions as a fuse. It has two designed ratings: one of the normal safe working current, the other the overload current. The breaker is closed against the action of a spring to make the circuit and supply the section board or auxiliary. A trip mechanism opens the breaker, a fast opening being ensured by the spring. When desired the breaker is tripped or opened manually. It will also open if the overload current rating is exceeded for a period of time.

A delay mechanism prevents the breaker opening for short-period overload currents. The circuit breaker opens or closes both supply and return leads in the circuit. Where a circuit breaker feeds the generator supply to the bus-bars a third 'make-or-break' arm will be provided for the equaliser connection.

Preferential tripping is a means of retaining essential electrical supplies. In the event that a generator cannot supply all the load then non-essential loads are disconnected by preferential trips. The intention is to reduce the generator load while ensuring essential equipment such as steering gear, navigation lights, etc., retains its electrical supply. Various circuit faults can occur as a result of either a break in the conductor (cable) or a break in the insulation. An open-circuit fault results from a break in the conductor and no current flow will take place, A short-circuit fault is due to two breaks in the insulation on, for example, adjacent conductors. The two conductors are connected and a large current flow takes place. An earth fault occurs when a break in the insulation permits the conductor to touch an earthed metal enclosure (or the hull).

Fig:D.C. generators Earth lamp circuit

Earth faults are usually detected by the use of earth indicating lamps. Two lamps are used, each rated for the full system voltage, but connected in series across the system with the mid point earthed , If the system is correctly insulated then both lamps will glow at half brilliance. The lamps are placed close together to enable a comparison to be made. A direct earth in one pole will short circuit its lamp, causing the other to shine brighdy. A slight insulation breakdown would produce a difference in bulb brightness between the two. Where an earth fault is detected the circuit breakers for each separate circuit must be opened in turn until the fault location is discovered. The particular section or distribution box would then have to have its circuits investigated one by one to locate the fault and enable its correction.

Direct current supply

The supply to a distribution system will usually come from two or more generators operating in parallel. Each generator must be provided with certain protective devices to ensure against reverse currents, low voltage or an overcurrent. There must also be ammeters and voltmeters in the circuits to enable paralleling to take place.

Fig:Protective trips for the parallel operation of two d,c. generators

The circuit for two generators operating in parallel is shown in Figure above. A triple-pole circuit breaker connects the supply to the bus-bars and also the equaliser bus-bar. The arrangement of the various protective trips can be seen, with excess current protection being provided in each pole. The reverse current trip prevents the generator operating as a motor if, for instance, the prime mover stopped.

The voltmeters and ammeters are provided in the generator supply circuits for paralleling purposes. A voltmeter is positioned across the bus-bars to indicate their voltage. Consider the situation where one generator is supplying the bus-bar system and a second generator is to be paralleled with it. The second machine is run up to speed and its field current adjusted until the two machines are at the same voltage. The circuit breaker connecting the second machine to the bus-bar can now be closed and the Field current adjusted to enable the generator to take its share of the load. When the load is evenly shared the two machines can then be left to operate in parallel. The equalising connection will cater for any slight changes in load sharing that occur.

Choice of batteries for ships machinery spaces - Lead acid and alkaline batteries

2011-08-11T22:00:00.000-07:00

Requirement of appropriate batteries on board

The battery is a convenient means of storing electricity. It is used on many ships as an instantly available emergency supply. It may also be used on a regular basis to provide a low-voltage d.c. supply to certain equipment. To provide these services the appropriate size and type of battery must be used and should be regularly serviced. Two main types of battery are used on board ship: the lead—acid and the alkaline type, together with various circuits and control gear.

Lead-acid battery

The lead - acid battery is made up of a series of cells. One cell consists of a lead peroxide positive plate and a lead negative plate both immersed in a dilute sulphuric acid solution. The sulphuric acid is known as the 'electrolyte'. A wire joining these two plates will have a potential or voltage developed across it and a current will flow. This voltage is about 2.2V initially with a steady value of about 2V. A grouping of six separate cells connected in series will give a 12V battery. The word 'accumulator* is sometimes used instead of battery.

Actual construction uses interleaved plates in the cell in order to produce a compact arrangement with a greater capacity. The complete battery is usually surrounded by a heavy-duty plastic, hard rubber or bitumen case.

In the charged condition the battery contains lead, lead peroxide and sulphuric acid. During discharge, i.e. the providing of electrical power, some of the lead peroxide and the lead will change to lead sulphate and water. The sulphuric acid is weakened by this reaction and its specific gravity falls.

When the battery is charged, i.e. electrical power is put into it, the reactions reverse to return the plates to their former material and the water produced breaks down into hydrogen gas which bubbles out.

Alkaline battery

The basic cell of the alkaline battery consists of a nickel hydroxide positive plate and a cadmium and iron negative plate immersed in a solution of potassium hydroxide. The cell voltage is about 1.4V. A grouping of five cells is usual to give about seven volts.

An interleaved construction is again used and each cell is within a steel casing. This casing is electrically 'live' being in contact with the electrolyte and possibly one set of plates. A battery consists of a group of cells mounted in hardwood crates with space between each. The cells are connected in series to give the battery voltage.

In the charged condition the positive plate is nickel hydroxide and the negative plate cadmium. During discharge oxygen is transferred from one plate to the other without affecting the specific gravity of the potassium hydroxide solution. The negative plate becomes cadmium oxide and the positive plate is less oxidised nickel hydroxide. Charging the battery returns the oxygen to the positive plate.

Direct current motors ,Shunt wound d.c. motor,Series wound d.c. motor & d.c. motor starter

2011-08-11T21:59:00.000-07:00

Direct current motors

When a current is supplied to a single coil of wire in a magnetic field a force is created which rotates the coil. This is a similar situation to the generation of current by a coil moving in a magnetic field. In fact generators and motors are almost interchangeable, depending upon which two of magnetic field, current and motion are provided.

Additional coils of wire and more magnetic fields produce a more efficient motor. Interpoles are fitted to reduce sparking but now have opposite polarity to the next main pole in the direction of rotation, When rotating the armature acts as a generator and produces current in the reverse direction to the supply. This is known as back e.m.f. (electromotive force) and causes a voltage drop across the motor. This back e.m.f. controls the power used by the motor but is not present as the motor is started. As a result, to avoid high starting currents special control circuits or starters are used.

The behaviour of the d.c. motor on load is influenced by the voltage drop across the armature, the magnetic field produced between the poles and the load or torque on the motor. Some of these factors are interdependent. For example, the voltage drop across the armature depends upon the back e.m.f. which depends upon the speed of the motor and the strength of the magnetic field. Shunt, series and compound windings are used to obtain different motor characteristics by varying the above factors.

Fig: Shunt wound d.c. motor

The shunt wound motor has field windings connected in parallel with the armature windings. Thus when the motor is operating with a fixed load at constant speed all other factors are constant. An increase in load will cause a drop in speed and therefore a reduction in back e.m.f. A greater current will then flow in the armature windings and the motor power consumption will rise: the magnetic field will be unaffected since it is connected in parallel. Speed reduction is, in practice, very small, which makes the shunt motor an ideal choke for constant-speed variable-load duties.

Fig: Series wound d.c. motor

The series motor has field windings connected in series with the armature windings . With this arrangement an increase in load will cause a reduction in speed and a fall in back e.m.f. The increased load current will, however, now increase the magnetic field and therefore the back e.m.f. The motor will finally stabilise at some reduced value of speed. The series motor speed therefore changes considerably with load.

Control of d.c. motors is quite straightforward. The shunt wound motor has a variable resistance in the field circuit, as shown in Figure . This permits variation of the current in the field coils and also the back e.m.f., giving a range of constant speeds. To reverse the motor the field current supply is reversed, as shown in Figure .

One method of speed control for a series wound motor has a variable resistance in parallel with the field coils. Reverse operation is again achieved by reversing the field current supply as shown in Figure .

In operation the shunt wound motor runs at constant speed regardless of load. The series motor runs at a speed determined by the load, the greater the load the slower the speed. Compounding—the use of shunt and series field windings—provides a combination of these characteristics. Starting torque is also important. For a series wound motor the starting torque is high and it reduces as the load increases. This makes the series motor useful for winch and crane applications. It should be noted that a series motor if started on no-load has an infinite speed. Some small amount of compounding is usual to avoid this dangerous occurrence. The shunt wound motor is used where constant speed is required regardless of load; for instance, with fans or pumps. The starting of a d.c. motor requires a circuit arrangement to limit armature current. This is achieved by the use of a starter .

Fig: d.c. motor starter

A number of resistances are provided in the armature and progressively removed as the motor speeds up and back e.m.f. is developed. An arm, as part of the armature circuit, moves over resistance contacts such that a number of resistances are first put into the armature circuit and then progressively removed. The arm must be moved slowly to enable the motor speed and thus the back e.m.f. to build up. At the final contact no resistance is in the armature circuit. A 'hold on' or 'no volts' coil holds the starter arm in place while there is current in the armature circuit.

If a loss of supply occurs the arm will be released and returned to the 'off position by a spring. The motor must then be started again in the normal way. An overload trip is also provided which prevents excess current by shorting out the 'hold on* coil and releasing the starter arm. The overload coil has a soft iron core which, when magnetised sufficiently by an excess current, attracts the trip bar which shorts out the hold on coil. This type of starter is known as a 'face plate'; other types make use of contacts without the starting handle but introduce resistance into the armature circuit in much the same way.

Maintenance requirement for ships electrical equipment

2011-08-11T21:56:00.000-07:00

Guideline for electrical plant on board

With all types of electrical equipment cleanliness is essential for good operation. Electrical connections must be sound and any signs of sparking should be investigated. Parts subject to wear must be examined and replaced when necessary. The danger from a.c. equipment in terms of electric shocks is far greater than for similar d.c. voltages. Also a.c, equipment often operates at very high voltages. Care must therefore be taken to ensure isolation of equipment before any inspections or maintenance is undertaken.

The accumulation of dirt on electrical equipment will result in insulation breakdown and leakage currents, possibly even an earth fault. Moisture or oil deposits will likewise affect insulation resistance. Regular insulation resistance measurement and the compiling of records will indicate the equipment requiring attention. Ventilation passages or ducts may become blocked, with resultant lack of cooling and overheating. Oil deposits from a direct-coupled diesel engine driving an open generator (usually d.c.) can damage windings and should therefore be removed if found. Totally enclosed machines should be periodically opened for inspection and cleaning since carbon dust will remain inside the machine and deposit on the surfaces.

Brushgear should be inspected to ensure adequate brush pressure and the springs adjusted if necessary. New brushes should be 'bedded in' to the commutator or slipring shape with fine glass paper. Sparking at the commutator will indicate poor commutation. This may require polishing of a roughened commutator surface. The mica insulation between commutator segments may require undercutting if it protrudes, or simply cleaning if deposits have built up.

Control equipment, such as starters, will require attention to contacts which may be worn or pitted as a result of arcing. Contactors usually have a moving or wiping action as they come together. This helps clean the surfaces to provide good electrical contact, and also the arc produced during closing and opening is not at the finally closed position. The contactor contact surfaces of frequently used equipment should therefore be subject to regular inspections.

Ships electrical plant and distribution system for the A.C. generators

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A coil of wire rotating in a magnetic field produces a current. The current can be brought out to two slip rings which are insulated from the shaft. Carbon bushes rest on these rings as they rotate and collect the current for use in an external circuit. Current collected in this way will be alternating, that is, changing in direction and rising and falling in value. To increase the current produced, additional sets of poles may be introduced.

How alternating current is produced onboard ?

A coil of wire rotating in a magnetic field produces a current. The current can be brought out to two slip rings which are insulated from the shaft. Carbon bushes rest on these rings as they rotate and collect the current for use in an external circuit. Current collected in this way will be alternating, that is, changing in direction and rising and falling in value. To increase the current produced, additional sets of poles may be introduced.

The magnetic field is provided by electromagnets so arranged that adjacent poles have opposite polarity. These 'field coils', as they are called, are connected in series to an external source or the machine output.

Fig: Three-phase alternator output

If separate coils or conductors are used then several outputs can be obtained. Three outputs are usually arranged with a phase separation of 120°, to produce a three-phase supply. The supply phasing is shown in fig . The three-phase system is more efficient in that for the same mechanical power a greater total electrical output is obtained. Each of the three outputs may be used in single-phase supplies or in conjunction for a three-phase supply. The separate supplies are connected in either star or delta formation .

The star formation is most commonly used and requires four sliprings on the alternator. The three conductors are joined at a common slipring and also have their individual siipring. The central or neutral line is common to each phase. The delta arrangement has two phases joined at each of the three sliprings on the alternator. A single-phase supply can be taken from any two sliprings.

Fig: Star and delta three-phase connections

So far, alternator construction has considered the armature rotating and the field coils stationary. The same electricity generating effect is produced if the reverse occurs, that is, the field coils rotate and the armature is stationary. This is in fact the arrangement adopted for large, heavy duty alternators.

The field current supply in older machines comes from a low-voltage direct current generator or exciter on the same shaft as the alternator. Modern machines, however, are either statically excited or of the high-speed brushless type. The exciter is required to operate to counter the effects of power factor for a given load.

The power factor is a measure of the phase difference between voltage and current and is expressed as the cosine of the phase angle. With a purely resistance load the voltage and current are in phase, giving a power factor of one. The power consumed is therefore the product of voltage and current. Inductive or capacitive loads, combined with resistance loads, produce lagging or leading power factors which have a value less than one. The power consumed is the product of current, voltage and power factor.

The alternating current generator supplying a load has a voltage drop resulting from the load. When the load has a lagging power factor this voltage drop is considerable. Therefore the exciter, in maintaining the alternator voltage, must vary with the load current and also the power factor. The speed change of the prime mover must also be taken into account.

Hand control of excitation is difficult so use is made of an automatic voltage regulator (AVR). The AVR consists basically of a circuit fed from the alternator output voltage which detects small changes in voltage and feeds a signal to an amplifier which changes the excitation to correct the voltage. Stabilising features are also incorporated in the circuits to avoid 'hunting' (constant voltage fluctuations) or overcorrecting. Various designs of AVR are in use which can be broadly divided into classes such as carbon pile types, magnetic amplifiers, electronic types, etc,

The statically excited alternator has a static excitation system instead of a d.c. exciter. This type of alternator will more readily accept the sudden loading by direct on-line starting of large squirrel cage motors. The static excitation system uses transformers and rectifiers to provide series and shunt components for the alternator field, that is, it is compounded. Brushes and sliprings are used to transfer the current to the field coils which are mounted on the rotor.

The terminal voltage from the alternator thus gives the no-load voltage arid the load current provides the extra excitation to give a steady voltage under any load condition. The careful matching of components provides a system which functions as a self regulator of voltage. Certain practical electrical problems and the compensation necessary for speed variation require that a voltage regulator is also built into the system.

The brushless high speed alternator was also developed to eliminate d.c. exciters with their associated commutators and brushgear. The alternator and exciter rotors are on a common shaft, which also carries the rectifiers. The exciter output is fed to the rectifiers and then through conductors in the hollow shaft to the alternator field coils. An automatic voltage regulator is used with this type of alternator.

Fig: Alternator construction

The construction of an alternator can be seen in Figure above. The rotor houses the poles which provide the field current, and these are usually of the salient or projecting-pole type. Slip rings and a fan are also mounted on the rotor shaft, which is driven by the auxiliary engine. The stator core surrounds the rotor and supports the three separate phase windings. Heat is produced in the various windings and must be removed by cooling. The shaft fan drives air over a water-cooled heat exchanger. Electric heaters are used to prevent condensation on the windings when the alternator is not in use.

In addition to auxiliary-engine-driven alternators a ship may have a shaft-driven alternator. In this arrangement a drive is taken from the main engine or the propeller shaft and used to rotate the alternator. The various operating conditions of the engine will inevitably result in variations of the alternator driving speed. A hydraulic pump and gearbox arrangement may be used to provide a constant-speed drive, or the alternator output may be fed to a static frequency converter. In the static frequency converter the a.c. output is first rectified into a variable d.c. voltage and then inverted back into a three-phase a.c. voltage. A feedback system in the oscillator inverter produces a constant-output a.c, voltage and frequency.

Fig: A.C. distribution system

Distribution system

An a.c. distribution system is provided from the main switchboard which is itself supplied by the alternators (Figure above). The voltage at the switchboard is usually 440 volts, but on some large installations it may be as high as 3300 volts. Power is supplied through circuit breakers to larger auxiliaries at the high voltage. Smaller equipment may be supplied via fuses or miniature circuit breakers. Lower voltage supplies used, for instance, for lighting at 220 volts, are supplied by step down transformers in the distribution network.

The distribution system will be three-wire with insulated or earthed neutral. The insulated neutral has largely been favoured, but earthed neutral systems have occasionally been installed. The insulated neutral system can suffer from surges of high voltage as a result of switching or system faults which could damage machinery. Use of the earthed system could result in the loss of an essential service such as the steering gear as a result of an earth fault. An earth fault on the insulated system would not, however, break the supply and would be detected in the earth lamp display. Insulated systems have therefore been given preference since earth faults are a common occurrence on ships and a loss of supply in such situations cannot be accepted.

In the distribution system there will be circuit breakers and fuses, as mentioned previously for d.c. distribution systems. Equipment for a.c. systems is smaller and lighter because of the higher voltage and therefore lower currents. Miniature circuit breakers are used for currents up to about 100 A and act as a fuse and a circuit breaker. The device will open on overload and also in the event of a short circuit. Unlike a fuse, the circuit can be quickly remade by simply closing the switch. A large version of this device is known as the 'moulded-case circuit breaker' and can handle currents in excess of 1000 A. Preferential tripping and earth fault indication will also be a part of the a.c. distribution system. These two items have been mentioned previously for d.c. distribution systems.

Alternating current supply

Three-phase alternators arranged for parallel operation require a considerable amount of instrumentation. This will include ammeters, wattmeter, voltmeter, frequency meter and a synchronising device. Most of these instruments will use transformers to reduce the actual values taken to the instrument. This also enables switching, for instance, between phases or an incoming machine and the bus-bars, so that one instrument can display one of a number of values. The wattmeter measures the power being used in a circuit, which, because of the power factor aspect of alternating current load, will be less than the product of the volts and amps. Reverse power protection is provided to alternators since reverse current protection cannot be used. Alternatively various trips may be provided in the event of prime mover failure to ensure that the alternator does not act as a motor.

The operation of paralleling two alternators requires the voltages to be equal and also in phase. The alternating current output of any machine is always changing, so for two machines to operate together their voltages must be changing at the same rate or frequency and be reaching their maximum (or any other value) together. They are then said to be 'in phase'. Use is nowadays made of a synchroscope when paralleling two a.c. machines. The synchroscope has two windings which are connected one to each side of the paralleling switch. A pointer is free to rotate and is moved by the magnetic effect of the two windings. When the two voltage supplies are in phase the pointer is stationary in the 12 o'clock position. If the pointer is rotating then a frequency difference exists and the dial is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency.

To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal Voltmeters are provided for this purpose. Secondly the frequencies must be brought into phase. In practice the synchroscope usually moves slowly in the FAST direction and the paralleling switch is closed as the pointer reaches the 11 o'clock position. This results in the incoming machine immediately accepting a small amount of load.

A set of three lamps may also be provided to enable synchronising. The sequence method of lamp connection has a key lamp connected across one phase with the two other lamps cross connected over the other two phases. If the frequencies of the machines are different the lamps will brighten and darken in rotation, depending upon the incoming frequency being FAST or SLOW. The correct moment for synchronising is when the key lamp is dark and the other two are equally bright.

Ships power generation equipment & distribution system

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Composition of an electrical plant

The complete electrical plant on board ship is made up of power generation equipment, a distribution system and the many power utilising devices. Electricity is used for the motor drive of many auxiliaries and also for deck machinery, lighting, ventilation and air conditioning equipment.

A constant supply of electricity is essential for safe ship and machinery operation, and therefore standby or additional capacity is necessary together with emergency supply equipment. Emergency equipment may take the form of an automatically starting emergency alternator or storage batteries may be used.

The complete range of electrical equipment will include generators, switch gear for control and distribution, motors and their associated starting equipment and emergency supply arrangements.

Alternating or direct current

Alternating current has now all but replaced direct current as the standard supply for all marine installations. The use of alternating current has a number of important advantages: for example, reduced first cost, less weight, less space required and a reduction in maintenance requirements. Direct current does, however, offer advantages in motor control using, for example, the Ward-Leonard system which provides a wide range of speed.

Machine rating

Motors and generators, both d.c. and a.c., are rated as Continuous Maximum Rated (CMR) machines. This means they can accept a considerable momentary overload and perhaps even a moderate overload for a longer duration.

Temperature affects the performance of all electrical equipment and also the useful life of the insulation and thus the equipment itself. The total temperature of an operating machine is a result of the ambient air temperature and the heating effect of current in the windings, Temperature rise is measured above this total temperature. Adequate ventilation of electrical equipment is therefore essential. Classification Societies have set requirements for the various classes of insulation. The usual classes for marine installations are E, B and F where particular insulation materials are specified and increasing temperature rises allowed in the order stated.

Enclosures

Depending upon the location, a motor or generator will have one of a number of possible types of enclosure. 'Drip-proof is most common and provides protection from falling liquids or liquids being drawn in by ventilating air. A 'watertight enclosure' provides protection for immersion under a low head of water for up to one hour.

'Weatherproof, 'hose proof and 'deck watertight' provide immersion protection for only one minute. 'Totally enclosed' can also be used or an arrangement providing ducted ventilation from outside the machinery space. A 'flameproof enclosure is capable of withstanding an explosion of some particular flammable gas that may occur within it. It must also stop the transfer of flame, i.e. contain any fire or explosion.

Operational guidance for ships navigational light circuit

2011-08-11T21:43:00.000-07:00

Use of navigational light circuits

The supply to the navigation lights circuit must be maintained under all circumstances and special provisions are therefore made. To avoid any possibility of accidental open circuits the distribution board for the navigation lights supplies no other circuit. A changeover switch provides an alternative source of supply should the main supply fail. If the navigation lights fail, a visual or audible indication must be given.

A navigation lights circuit is shown in Figure. Two sources of supply are available from the changeover switch. A double pole switch connects the supply to each light circuit, with a fuse in each line, A relay in the circuit will operate the buzzer if an open circuit occurs, since the relay will de-energise and the trip bar will complete the buzzer circuit. A resistance in series with the indicating lamp will ensure the navigation lights operate even if the indicating lamp fails. A main supply failure will result in all the indicating lamps extinguishing but the buzzer will not sound. The changeover switch will then have to be moved to the alternative supply.

Fig: Navigational light circuit

Alternating current motors for ships machinery spaces - Arrangement and distribution methods

2011-08-11T21:41:00.000-07:00

Supplying alternating current to a coil

Supplying alternating current to a coil which is free to rotate in a magnetic field will not produce a motor effect since the current is constantly changing direction. Use is therefore made in an induction or squirrel cage motor of a rotating magnetic field produced by three separately phased windings in the stator.

The rotor has a series of copper conductors along its axis which are joined by rings at the ends to form a cage. When the motor is started the rotating magnetic field induces an e.m.f. in the cage and thus a current flow. The current-carrying conductor in a magnetic field produces the motor effect which turns the rotor. The motor speed builds up to a value just less than the speed of rotation of the magnetic field.

Fig:Squirrel cage induction motor starting

The motor speed depends upon the e.m.f. induced in the rotor and this depends upon the difference in speed between the conductors and the magnetic field. If the load is increased the rotor slows down slightly, causing an increase in induced e.m.f. and thus a greater torque to deal with the increased load.

The motor is almost constant speed over all values of load. It will start against about two times full load torque but draws a starting current of about six times the normal full load current. The starting current can be reduced by having a double cage arrangement on the rotor. Two separated cages are provided, one below the other in the rotor. When starting, the outer high-resistance cage carries almost all the rotor current. As the motor accelerates the low-resistance inner winding takes more and more of the current until it carries the majority. A number of different fixed speeds are possible by pole changing. The speed of an induction motor is proportional to frequency divided by the numbers of pairs of poles. If therefore a switch is provided which can alter the numbers of pairs of poles, then various fixed speeds are possible. The number of poles affects the starting characteristics such that the more poles the less the starting torque to full load torque ratio. Only the induction type of a.c. motor has been described, since it is almost exclusively used in maritime work. Synchronous motors are another type which have been used for electrical propulsion systems but not auxiliary drives.
A number of different arrangements can be used for starting an induction motor. These include direct on-line, star delta, auto transformer and stator resistance. Direct on-line starting is usual where the distribution system can accept the starting current. Where a slow moving high inertia load is involved the starting time must be considered because of the heating effect of the starting current. The star delta starter connects the stator windings first in star and when running changes over to delta.

The star connection results in about half of the line voltage being applied to each phase with therefore a reduction in starting current. The starting torque is also reduced to about one-third of its direct on line value. A rapid change-over to delta is required at about 75% of full load speed when the motor will draw about three-and-a-half times its full load current. The auto transformer starter is used only for large motors. It uses tappings from a transformer to provide, for example, 40%, 60% and 75% of normal voltage (Figure below).

The motor is started on one of the tappings and then quickly switched to full voltage at about 75% full speed. The tapping chosen will depend upon the starting torque required with a 60% tapping giving about 70% of full load torque. A smaller percentage tapping will give a smaller starting torque and vice-versa. The stator resistance starter has a resistance in the stator circuit when the motor is started. An adjustable timing device operates to short circuit this resistance when the motor has reached a particular speed.

Modern electronic techniques enable a.c. induction motors to be used in speed-control systems. The ship's supply, which may not be as stable in voltage or frequency as that ashore, is first rectified to provide a d.c, supply. This is then used as the power supply of an oscillator using high-power electronic devices. These may be thyristors (for powers up to 1.5 M W or more) or transistors (for powers up to a few tens of kilowatts). The high-power oscillator output is controlled in frequency and voltage by a feedback system. The motor speed is varied by changing the oscillator output frequency. The motor current necessary to obtain the desired torque (at small angles of slip) is normally obtained by maintaining the voltage almost proportional to frequency.

Certain protective devices are fitted in the motor circuit to protect against faults such as single phasing, overload or undervoltage. Single phasing occurs when one phase in a three-phase circuit becomes open circuited. The result is excessive currents in ail the windings with, in the case of a delta connected stator running at full load, one winding taking three times its normal load current.

A machine which is running when single phasing occurs will continue to run but with an unbalanced distribution of current. An overload protection device may not trip if the motor is running at less than full load. One method of single phasing protection utilises a temperature-sensitive device which isolates the machine from the supply at some particular winding temperature.

Overload protection devices are also fitted and may be separate or combined with the single phase protection device. They must have a time delay fitted so that operation does not occur during the high starting current period. An undervoltage or 'no volts' protective device ensures that the motor is properly started after a supply failure.

Duties of Electrical Engineer Officer On Board Ship

2011-08-11T21:32:00.000-07:00

All the machinery onboard ship is a combination of mechanical and electrical systems. The modern day shipping is more reliable on automations and electronics whose knowledge and maintenance can only be handled by an engineer expert in the electrical field. Marine electrical officer engineers are perfect for such jobs and that’s why they hold an important role on ship and in offshore industry.

Electrical engineer is one of the most vital positions in the technical hierarchy of a ship and engineer is responsible for his assigned work under the chief engineer’s instructions.
Some shipping companies do not carry electrical officers on their ship to cut down the manning cost and the electrical duties are carried by some one from the engineer’s side, normally third engineer. However, many companies realized that electrical and electronic system requires some extra attention and therefore require an expert to attend them.

As the technology is advancing, more and more automations and electronic circuit is replacing conventional and electrical systems. Hence the international Maritime Organisation (IMO) amended STCW 95 on 25thJune 2010 known as Manila amendment, to introduce a certified position of Electro-technical officer in place of electrical officer.

The general duties of electrical engineer or Electro-technical officer are:

He is responsible for maintenance of all the electrical motors on ship i.e. in engine room and on deck.

He is in charge of maintenance of all switchboard including main switchboard and emergency switchboard.

He is responsible for maintenance of fire detectors and fire alarm system.

He has to maintain all the ship’s alarm system.

He is responsible for the electronic system fitted onboard ship.

He is responsible for the ship’s navigational lights and other navigational equipments.

He is responsible for all the batteries that are connected to machineries onboard. It includes:

-          Emergency batteries for alarm and lights
-          Lifeboat batteries
-          Batteries for emergency generator
-          Other batteries fitted onboard

He is responsible for maintaining refrigeration unit in the engine room

He has to take care of air conditioning unit of the vessel.

Electrical officer is responsible for maintaining refrigerated containers carried on container ship.

He is responsible for cargo and engine room cranes electrical system.

He has to carry out routine maintenance for main engine alarms and trips along with the chief engineer.

During the time of manoeuvring, he has to be present in the engine room along with other engineers to tackle any kind of electrical and other emergencies.

Electrical officer can assist in watch keeping routines at desired time by the chief engineer.

He has to assist ship’s engineer and deck officer in all kind of electrical problems.

Even the post of electrical officer is not a compulsion on a ship, but due to technicalities and complex knowledge requirement of the electrical and electronic system, they are extensively present in the shipping industry.

Duties of 2nd Engineer on Ship

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One of the most important positions in the hierarchy level of maritime professionals working on the ship is the position of Second engineer. As the position comes under the management level category, second engineer’s job is not only limited to carry out his own duties but also to look after the entire operation of the engine room and other technical aspects of the ship.

The 2nd engineer provides utmost assistance to the Chief engineer for running the ship efficiently.
Moreover, second engineer is also in-charge of all the operational engineers and the crew of the engine room.
He ensures for their personnel safety and routine duties. He also plans the overall maintenance of all the machinery present in the engine room of the ship.

Duties of 2nd Engineer
A brief description of the duties of second engineer is as follows:
Safety
1.        Safety of Personnel: Second engineer is responsible for risk assessment, briefing and safety training of the entire engine crew, especially for the junior engineers and fresh crew.
2. LSA and FFA: He is responsible for operation and maintenance of life saving appliances and fire fighting appliances.
3.     Emergency equipment: All the emergency machinery and equipments under SOLAS are responsibility of second engineer.
4.        Rest hours: The rest hour of all the individual working in the engine room has to be taken care of by the second engineer as per STCW.
Pollution Prevention
1.        Pollution Prevention Machineries: He is the person in-charge of Pollution prevention equipment onboard like OWS, Sewage plant, incinerator etc.
2.        Oil transfer: He is responsible for all oil transfer operations carried out onboard including bunkering.
3.        Pollution prevention plan: He is responsible for implementation of SOPEP and other the equipments involved with the same.
4.        Sludge and bilge: Second engineer has to keep the engine room bilge clean of oil and oily water. All tank parameters are to be recorded for sludge and bilge system.
5.        Regulations: All the rules and regulation related to MARPOL has to be applied and implemented by the second engineer.
Engine Room Management
1.        Responsibility: He is the in-charge for managing the engine room staff and carrying out duties of the engine room.
2.        Assistance: He directly reports to the chief engineer and also becomes the in-charge of the engine room on his absence/behalf.
3.        Job distribution: He distributes and assigns duties to all the engine crew members and acts as supervisor to them.
4.        Housekeeping: He is also the in-charge of engine room housekeeping and engine room garbage management.
5.       Spares and inventory: Second engineer is responsible for storing all the spares properly and keeping and maintaining record of the spares’ inventory.
Operation and Maintenance of Machinery
1.        Engine room and deck machinery: The second engineer is responsible for the maintenance of all the engine room and deck machinery.
2.        Safe operation: He is responsible to make sure all the machineries and safety systems are working safely, efficiently and within the provided parameters.
3.        PMS and BMS: Planned maintenance system(PMS) is to be implemented by the second engineer and in case of any breakdown maintenance (BMS), should be immediately taken care of by him.

Documentation
1. Record keeping: Different records of the engine room have to be maintained and updated regularly. This is the responsibility of the second engineer. A few examples of important records are log book, bell book, Saturday or Monday routine book etc.
2.   Spare Inventory: All the spares for engine room machineries like pumps, auxiliary engine etc are to be stored and inventory to be maintained and updated regularly.
3.   Oil inventory: The quantity for oil present in the tanks for machinery operation like lube oil, fuel or diesel oil etc has to be recorded in an oil record book. A track for the next bunker order also needs to be maintained.
4. PMS: The planned maintenance system paper work like updating the maintenance data etc. is to be completed under the second engineer supervision.
Training
1.        He is responsible for familiarising the crew member with safety features of the ship such as emergency escape, life boat, sopep etc.
2.        He should supervise the engine crew for training them to operate pollution prevention machineries like OWS, sewage plant etc.
3.        He is responsible to train the crew members for various emergency situations like flooding, fire, pirates attack and abandon ship etc.
This is a general overview of the duties of second engineer on board a ship. Second engineer is the most influencing personality in the engine room and needs to have great leadership quality apart from stoic and cool mindset.

Duties of a 4th Engineer on a Ship

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According to policies of all shipping companies, seafarers joining any vessel have to be competent, qualified and experienced. Joining a ship as a fourth engineer is the first step towards a sailing career on ship. It is only after becoming a fourth engineer that a mariner experiences real responsibility on a ship.

Though a fourth engineer, before joining the ship has cleared the required exams and training courses, it is appreciated that the training part becomes an ongoing process and there is always a requirement for refresher courses and periodical safety training.

Duties of 4th Engineer by Default
On all types of ships, irrespective of the shipping company, a 4th engineer shall report to the 2nd Engineer, who shall assign the duties to him, both at sea and at port and whose orders he or she shall consider effective and binding as though emanating from the Chief Engineer. Moreover, the 4th engineer shall also assist the 2nd engineer when necessary in carrying out duties of the later.

Duties of 4th Engineer upon joining a Vessel
A 4th engineer, upon joining a vessel, should carry out the following duties.

Upon joining a vessel, the relieving 4th engineer shall report his presence on board to the Chief Engineer or in his absence to the 2nd Engineer.

The 4th engineer should take a thorough round of the engine department with the signing off engineer and do a proper taking over of the duties.

He should check the inventory and location of all purifier and compressor spares.

He should check Inventory and location of pumps spares and tools.

He should check running hours and maintenance schedule of his designated machinery.

He should check general condition of machinery and special procedure for operation.

Condition and layout of bunkering system including valve operation, tanks and sounding pipe location should be checked.

Condition and layout of sludge and bilge discharge system including valves and pump operation should also be checked.

Thorough sounding of all fuel tanks, bilge and sludge tank should be taken.

Daily consumption of lube oil, fuel oil, marine diesel or gas oil and cylinder oil for daily record keeping in sounding log is to be checked.

Daily production of bilge and sludge on board for record keeping of waste water sounding log.

After completion of the engine room round together, the 4th engineer shall report the details to the 2ndengineer and notify discrepancies observed, if any.
The 4th Engineer as in charge of Purifier, Compressor, and machinery other than main engine, auxiliary engine and boiler, is responsible for:

Record keeping of machinery running hours under his charge.

For carrying out planned maintenance routine under the supervision of chief or second engineer.

Taking daily tank sounding of fuel oil, diesel oil, lube and cylinder oil on board ship and record keeping of the same.

Record keeping of sludge and bilge on board.

Acting as assistant in charge to chief engineer for operation during bunker of fuel, diesel and lube oil.

Completing the month end official papers for his designated machinery.

Compliance of environment policy and MARPOL requirements under the guidance of Chief Engineer.

Maintenance and up keeping of the engine room log book and all those files on board advised by the Chief Engineer.

The above mentioned duties are the basic requirements for any 4th engineer going on board. However, duties might be subtracted or added on the basis of the type of the ship and shipping company.

Jacket cooling water system

2011-06-24T07:18:00.000-07:00

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

2011-06-22T04:58:00.000-07:00

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:

Preparation for engine starting
Starting and stopping engines according to load demands
Sychronisation of incoming sets with supply
Circuit breaker closure
Load sharing between alternators
Maintenance of supply frequency and voltage
Engine/alternator fault protection
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

Start extraction pump
Start air ejectors
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

2011-06-22T04:56:00.000-07:00

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

2011-06-22T04:55:00.000-07:00

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

2011-06-22T04:53:00.000-07:00

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