Marine Engineering

These are positioned at each transverse girder. They are intended to keep the transverse girder in compression at all times thus minimising risk of fatigue cracking. Correct tension is therefore important and this should be checked regularly in accordance with the engine manufacturers instructions, this normally means retensioning the bolts in pairs from the center of the engine. alternately for'd and aft.

A plate, frame, or platform serving as a base or support for a machine.
A typical marine diesel engine of a normal sized ship easily resembles the appearance of building having several floors in height and a sufficient covered area. It would be interested to know about the components of this giant engine and we will discuss about bedplate in this article.

Introduction

A solid foundation is necessary for any structure, be it on the ground or meant for the sea. No doubt this axiom applies equally well to marine diesel engines which are huge and gigantic structures and have a lot of forces apart from their weight, such as the reactive forces when the huge piston rush up and down through the cylinders. It goes without saying that a very strong base is required to support such load and forces and the bedplate of the engine servers as the structural base of the engine. It acts as housing for the huge crankshaft while it also supports the cylinder block. In this article we will see about the role and functions of the bedplate and learn about its usefulness and purpose.

A chain is a method of transferring rotary motion between two parallel shafts. The chain drive is positive, efficient and high torques can be transmitted. The chain is generally made from steel although plastic chains have been developed

In order for a diesel engine to operate, all of its components must perform their functions at very precise intervals in relation to the motion of the piston. To accomplish this, a component called a camshaft is used. Figure 9 illustrates a camshaft and camshaft drive gear. Major Components of a Diesel Engine illustrate the location of a camshaft in a large overhead cam diesel engine.

A camshaft is a long bar with egg-shaped eccentric lobes, one lobe for each valve and fuel injector. Each lobe has a follower as shown on Figure 10. As the camshaft is rotated, the follower is forced up and down as it follows the profile of the cam lobe. The followers are connected to the engine's valves and fuel injectors through various types of linkages called pushrods and rocker arms. The pushrods and rocker arms transfer the reciprocating motion generated by the cam shaft lobes to the valves and injectors, opening and closing them as needed. The valves are maintained closed by springs.
As the valve is opened by the camshaft, it compresses the valve spring. The energy stored in the valve spring is then used to close the valve as the camshaft lobe rotates out from under the follower. Because an engine experiences fairly large changes in temperature (e.g., ambient to a normal running temperature of about 190°F), its components must be designed to allow for thermal expansion. Therefore, the valves, valve pushrods, and rocker arms must have some method of allowing for the expansion. This is accomplished by the use of valve lash. Valve lash is the term given to the "slop" or "give" in the valve train before the cam actually starts to open the valve.

The camshaft is driven by the engine's crank shaft through a series of gears called idler gears and timing gears. The gears allow the rotation of the camshaft to correspond or be in time with, the rotation of the crank shaft and thereby allows the valve opening, valve closing, and injection of fuel to be timed to occur at precise intervals in the piston's travel. To increase the flexibility in timing the valve opening, valve closing, and injection of fuel, and to increase power or to reduce cost, an engine may have one or more camshafts. Typically, in a medium to large V-type engine, each bank will have one or more camshafts per head. In the larger engines, the intake valves, exhaust valves, and fuel injectors may share a common camshaft or have independent camshafts.

Depending on the type and make of the engine, the location of the camshaft or shafts varies. The cam shaft (s) in an in-line engine is usually found either in the head of the engine or in the top of the block running down one side of the cylinder bank. Figure 10 provides an example of an engine with the camshaft located on the side of the engine. Figure 3, (Major Components of Diesel Engine) provides an example of an overhead cam arrangement as on a V-type engine. On small or mid-sized V-type engines, the camshaft is usually located in the block at the center of the "V" between the two banks of cylinders. In larger or multi-cam shafted V type engines, the camshafts are usually located in the heads.

The puncture valve consists of a piston which communicates with the control air system of the engine. In the event of actuation of the shut-down system, and when 'STOP' is activated, compressed air causes the piston with pin to be pressed downward and 'puncture' the oil flow to the fuel valve. As long as the puncture valve is activated, the fuel oil is returned through a pipe to the pump housing, and no injection takes place. I have also added a few more comments and attached a file to further explain the function(s) of the puncture valve.

MAN B&W reversing: Reversal of the fuel pump follower only takes place while the engine is rotating. If the engine has been stopped from running ahead and started astern, the fuel pump follower will move across as the engine starts to rotate and before fuel is admitted by venting the fuel pump via the "puncture" valve.

Exhaust emissions from marine diesel engines largely comprise nitrogen, oxygen, carbon dioxide and water vapour, with smaller quantities of carbon monoxide, oxides of sulphur and nitrogen, partially reacted and non-combusted hydrocarbons and particulate material. SOx and NOx emissions, together with carbon dioxide, are of special concern as threats to human health and the environment.

Dominating influences in the formation of NOx in the combustion chamber are temperature and the longer the residence time in the high temperature, the more thermal NOx will be created.

Fuel oil is a hydrocarbon consisting of hydrogen and carbon, together with other elements most of which are unwanted.

Hydrogen has a higher calorific value than carbon, therefore, more heat may be obtained from fuels containing higher Hydrogen/Carbon ratios.

The lower specific gravity of hydrogen than carbon allows a rough rule of thumb to be; the higher the Specific Gravity, the lower the Calorific Value (and quality) of the fuel. The presence of impurities clouds the issue slightly

For efficient combustion an ignition source and sufficient oxygen need be present to completely oxidise the Hydrogen to water vapour and the carbon to carbon-dioxide.

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Basic Theory

Distance
Viscosity
Size of particle
Gravitational Force

Rate of sedimentation

When a particle (solid or liquid) moves through a viscous
medium under the influence of gravitation, it will attain a constant velocity after a certain time.
This is known as the sedimentation velocity
or derived from Stokes' Law, the rate of sedimentation
can be calculated if the following physical parameters
are known:
Particle diameter d (m)
Particle density Pp (kg/m3)
Density of the continuous phase PI(kg/m3)
Viscosity of the continuous phase 1'/(kg/m,s)
Gravitational acceleration g (9.81 m/s2)



Simple centrifuging

A heat exchanger allows the transfer of heat from one fluid (liquid or gas) to another fluid
 
Reasons for heat transfer

To heat a cooler fluid by means of a hotter fluid
To reduce the temperature of a hot fluid by means of a cooler fluid
To boil a liquid by means of a hotter fluid
To condense a gaseous fluid by means of a cooler fluid
To boil a liquid while condensing a hotter gaseous fluid

Methods of Heat Transfer

Conduction
Convection
Radiation

Heat Exchanger Types

Recuperative
Regenerative
Evaporative

Evaporative Type

Cooling Tower
Direct contact heater

Heat Transfer depends on

Area
Mean Temperature difference between the liquids
Material through which heat transfer takes place
Time in contact
Thickness

Types of Flow

Parallel
Contra
Cross

Types of Shell & Tube Heat Exchanger

Expansion Bellow in shell
Floating Tube Arrangement
Fixed ‘U’ Tubes
Bayonet Tube ( Guided Flow)

Heat Exchanger Materials

Tube –Aluminium Brass,Cupro Nickel or Stainless Steel
Tube Plate – To suit tube material-Naval Brass
Shell & End Covers – Fabricated Mild Steel or Cast Iron

Shell &Tube Heat Exch. Tube faults

Cavitation corrosion -Vapour bubbles are produced in regions of high velocity and low pressure caused normally by constriction of flow, when these bubbles move into a region of high pressure where they increase in size and then implode, if the bubble is remote from the tube surface then the pressure is transmitted as a shock wave.
Dezincification this is a form of wastage which takes place in brasses that contain more than 15% zinc. Zinc is removed leaving a weak porous copper. Tubes often contain a small amount of arsenic to act as an inhibitor.
Impingement especially near inlet tube ends. This impingement causes the protective oxide film to be removed and corrosion takes place since this small area becomes anodic whilst the rest of the tube is cathodic. Reduced by fitting plastic inserts at tube ends.
Acidic water prevents the formation of a ferrite oxide film forming, thus the tube surface is under continuous attack and it thickness is gradually reduced until perforation takes place.
Erosion by abrasive solids such as sand in the sea water and high sea water velocities can cause metal from the tubes to be removed
Graphitisation occurs in cast iron or mild steel water boxes, iron is removed leaving a soft layer of carbon.
Tube vibration.can cause failure of tubes due to fatigue, tubes should be adequately supported. Additionally tubes may come into contact with one another, this hammering effect causing flats to be formed at the points of contact, eventually wearing through the tube.

Plate type heat exchanger

PROPERTIES

Elasticity
The elasticity of a metal is its power of returning to its original shape after deformation by force. Many materials behave to some extent like powerful elastic, and within limits, will recover their shape when the load on them is removed.

Hardness

The hardness of a metal is a measure of its ability to withstand scratching wear and abrasion, indentation by harder bodies, marking, by a file etc.
The Brinnel hardness of a metal is found by pressing a ball on to the surface of the metal, the hardness number being found by dividing the load on the ball by the surface area of the impression.

Malleability

This is the property of permanently extending in all directions without rupture by pressing, hammering, rolling, etc. It requires that the metal shall be plastic but is not so dependent on strength e.g. lead is malleable.

Plasticity

This is a rather similar property to malleability and involves permanent deformation without rupture.It is the extreme opposite to elasticity.

Strength

The strength of a metal is its ability to resist the application of force without rupture.In service a material may have to withstand tension,compression,or shear forces.The strength of material is measured by loading it in a testing machine until fracture.

Toughness

Is the amount of energy a material can absorb before it fractures or measure of toughness of a metal may be obtained by nicking it, placing it in a vice and striking it hard with a hammer

Steering Gears
The direction of the ship is controlled by the steering gear. As the ship moves through the water, the angle of the rudder at the stern determines the direction it will move. Modern ships are so big that moving the rudder necessitates the use of hydraulics or electrical power.

The steering starts at the Bridge. The required rudder angle is transmitted hydraulically or electrically from the steering wheel at the Bridge to the telemotor at the steering gear, just above the rudder.
There are a few common arrangements for using hydraulic power. There are the 4-rams, 2-rams, and rotary vane types. The heart of these hydraulic systems is the variable delivery pump. This type of pump can be controlled by just moving a spindle. The pump is driven by an electrical motor at constant speed. By moving the control spindle away from the central point, the pump stroke increases, and the hydraulic fluid is pumped in one direction. Moving the spindle more from the central point will cause more fluid to be pumped and consequently more pressure is generated to drive the rams. Moving the control spindle back to the original position and then away in the opposite direction causes the hydraulic fluid to be pumped in the reversed direction. The rams will also move in the reversed direction.

By using a floating lever feedback mechanism, when the rudder stock has reached the desired angle, the pump control lever moves back to the original position, and the pumping action stops. The rudder is stopped at the required angle. Moving the steering wheel to the opposite direction will cause the rudder to come back to the original zero position.

The Ship’s machinery systems are represented in diagrammatic form on the computer. On most views, the diagram of the system appears on the left, with controls on the right. With these controls, the system can be operated-
Machinery started and stopped, valves opened and closed, flow controllers adjusted, automatic temperature and pressure controllers set points altered etc- by means of the mouse.
Open valves and running machinery are indicated by a lighted green indicator on the diagram. The indicator not being Iit means that the valve is shut or the item of machinery is not running. If there is no indicator, it means that particular valve is not operable; i.e. it is permanently open .In some cases such as flow control valves, a numerical read-out will indicate, as a percentage, the extent to which the valve is open. This will correspond to the setting that you have chosen for that valve on the control panel. Temperatures and pressures are indicated numerically on the diagram, or by means of thermometers and pressure gauges on the control panel. Tank levels are indicated by a gauge glass on the tank. Most of the meters and gauges will indicate the alarm range, i.e. 'too low' or 'too high' in red, with the 'normal operating range’ in between as black or blue.
There are 'manual - auto' change-over- switches which enable you to control temperatures etc either manually, or automatically. When 'manual ' is selected, adjustment of the set point cannot be done as a 'set point is meaningless if the parameter is under- manual control. When ‘auto’ is selected the by-pass valves etc cannot be manually adjusted, as the computer will then control it. Initially, use the 'manual' setting on all controllers and learn to control the temperatures etc by manual use of the cooler flow control and by-pass valves.

The model shown is of a B & W, 6-unit, SMC engine. Familiarize yourself with the machinery system, and also with the operation of the simulator. Use the given manual, along with this hand-out, to learn about the various components shown, and how to operate the system. More instructions and assistance can be got from the 'online help', i.e. the '?' mark at the top of the screen. Please note that not all the systems are described in the printed manual, but information about these, too, are available in the 'online help'. No other specific task for today
Main Engine (two-stroke slow-speed) - Preparation and
Starting
You have been given 'l-hrs notice' of departure. Prepare the main engine for starting. All the engine's systems - fresh water, lube oil, fuel oil, compressed air etc - must be brought to the condition which allows the main engine to run. Leave the separators (purifiers) for now; they can be put in operation later. This engine can be maneuvered on heavy fuel oil or on diesel oil. All controllers to be in 'manual' position and temperatures must be controlled manually. Leave only the auxiliary blower switch on 'auto'; when the scavenge air pressure rises to a certain value, the blower will cut off. When the engine is ready bridge orders will be given by means of the telegraph. These must be answered by means of the handle on the telegraph. When full sea speed is reached, change over to heavy fuel oil if it is presently on diesel oil. Then change over all controllers to ‘auto’.

Marine engineering emerged as a discipline with the arrival of Marine Engines for propulsion,largely during the latter half of the 19th century.Early marine engineers were known as "stokers" as they 'stoked' the coal fires of steam engined ships more or less from the middle of the 19th to the middle of the 20th centuries,the term is still used affectionately by modern ship's engineering staff to describe their role.