Tuesday 26 June 2012

Fire fighting and safety

Fire fighting and safety
Fire fighting and safety
Fire is a constant hazard at sea. It results in more total losses of ships than any other form of casualty. Almost all  ire’s are the result of negligence or carelessness. Combustion occurs when the gases or vapours given off by a substance are ignited: it is the gas given off that burns, not the substance. The temperature of the substance at which it gives off enough gas to continue burning is known as the 'flash point'.
Fire is the result of a combination of three factors:
1. A substance that will burn.
2. An ignition source.
3. A supply of oxygen, usually from the air.
These three factors are often considered as the sides of the fire triangle. Removing any one or more of these sides will break the triangle and result in the fire being put out. The complete absence of one of the three will ensure that a fire never starts.
Fires are classified according to the types of material which are acting as fuel. These classifications are also used for extinguishers and it is essential to use the correct classification of extinguisher for a fire, to avoid spreading the fire or creating additional hazards. The classifications use the letters A, B, C, D and E.
Class A Fires burning wood, glass fibre, upholstery and furnishings.
Class B Fires burning liquids such as lubricating oil and fuels.
Class C Fires burning gas fuels such as liquefied petroleum gas.
Class D Fires burning combustible metals such as magnesium and aluminium.
Class E Fires burning any of the above materials together with high voltage electricity.
Many fire extinguishers will have multiple classifications such as A, B and C.
Fire fighting at sea may be considered in three distinct stages, detection—locating the fire; alarm—informing the rest of the ship; and control—bringing to bear the means of extinguishing the fire.

Sprinkler systems

Sprinkler systems
Sprinkler systems
Must be fitted to passenger ships carrying less than 36 passengers in the accommodation spacesand other areas considered necessary be the administration. For pasenger ships carrying greater than 36 passengers it must be fitted to accommodation spaces, corridors, stairwells and to control stations ( the latter may be served by an alternative system to prevent damage). The system must be of an approved type. See below for full requirements.
Generally takes the form of a wet pipe (line continuosly flooded) on to which are connected a number of sprinkler head. These heads consist of a valve held shut by a high expansion fluid filled quartzoid bulb.A small air space is incorporated.
When a fire occurs in an adjacent area to this bulb the fluid expands until the air space is filled, increasing internal pressure causes the bulb to fracture. The size of the air gap determines the temperature at which this failure occurs. The valve plug falls out and a jet of water exits , striking the spray generator where it is then distributed evenly over the surrounding area. In acting this way only the area of the fire is deluged and damage is minimised.

Water is supplied from an air pressurised water tank ( thus the system functions without electrical power), this water is fresh water to minimise damage. The tank is half filled with water and the rest is compressed air at pressure sufficient to ensure that all the water is delivered to the highest sprinkler at sprinkler head working pressure. Once this source of water is exhausted, falling main pressure is detected by a pressure switch. This activates a sea water supply pump. A valve is fitted on the system to allow proper testing of this function. After sea water has entered the system proper flushing with fresh water is required to prevent corrosion
A shore connection may be connected to the system to allow function during dry-dock

DECK CRANES

DECK CRANE
  1. A large number of ships are fitted with deck cranes.
  2. These require less time to prepare for working cargo than derricks and have the advantage of being able to accurately place (or spot) cargo in the hold.
  3. On container ships using ports without special container handling facilities, cranes with special container  handling gear are essential.
  4. Deck-mounted cranes for both conventional cargo handling and grabbing duties are available with lifting capacities of up to 50 tonnes.
  5. Ships specialising in carrying very heavy loads,however, are invariably equipped  with special derrick systems such as the Stulken (Figure 9.11).
  6. These derrick systems are capable of lifting loads of up to 500 tonnes
 
 
  1. Although crane motors may rely upon pole changing for speed variation, Ward Leonard and electro-hydraulic controls are those most widely used. ( Induction motors with PWM control also have been developed)
  2. One of the reasons for this is that pole-changing motors can only give a range of discrete speeds but additional factors favouring the two alternative methods include less fierce power surges since the Ward. Leonard motor or the electric drive motor in the hydraulic system run continuously and secondly the contactors required are far simpler and need less maintenance since they are not continuously being exposed to the high starting currents of  pole-changing systems.
  3. Deck cranes require to hoist, luff and slew and separate electric or  hydraulic motors will be required for each motion.
  4. Most makes of crane incorporate a rope system to effect luffing and this is commonly rove to give a level luff—in other words the cable geometry is such that the load is not lifted or lowered by the action of luffing the jib and the luffing motor need therefore only be rated to lift the jib and not the load as well.
  5. Generally, deck cranes of this type use the ‘ Toplis ’ three-part  reeving system for the hoist rope and the luffing ropes are rove between the jib head and the superstructure apex which gives them an approximately constant load, irrespective of the jib radius.
  6. This load depends only on the weight of the jib, the resultant of loads in the hoisting rope due to the load on the hook passes through the jib to the jib foot pin (Figure 9.12(a)).
  1. If the crane is inclined 5 in the forward direction due to heel of the ship the level-luffing geometry is disturbed and the hook load produces a considerable moment on the jib which increases the pull on the luffing rope
  2. In the case of a 5 tonne crane the pull under these conditions is approximately doubled and the luffing ropes need to be over-proportioned to meet the required factor of safety.
  3. If the inclination is in the inward direction and the jib is near minimum radius there is a danger that its weight moment will not be sufficient to prevent it from luffing up under the action of the hoisting rope resultant.
  4. Swinging of the hook will produce similar effects to inclination of the crane.
  5. In the Stothert & Pitt ‘Stevedore’ electro-hydraulic crane the jib is luffed by one or two hydraulic rains.
  6. Pilot operated leak valves in the rams ensure that the jib is supported in the event of hydraulic pressure being lost and an automatic limiting device is incorporated which ensures that maximum radius can not be exceeded.
  7. When the jib is to be stowed the operator can override the limiting device.
  8. In the horizontal stowed position the cylinder rods are fully retracted into the rams where they are protected from the weather .
  9. Some cranes are mounted in pairs on a common platform which can be rotated through 360ยบ .
  10. The cranes can be operated independently or locked together and operated as a twin-jib crane of double capacity, usually to give capacities of up to 50 tonnes.
  11. Most cranes can, if required, be fitted with a two-gear selection to give a choice of a faster maximum hoisting speed on 1ess than half load.
  12. For a 5 tonne crane full load maximum hoisting speeds in the range 50-75 m/min are available with slewing speeds in the range1-2 rev/min.
  13. For a 25 tonne capacity crane, maximum full load hoisting speeds in the range 20-25 m/min are common with slewing  speeds again in the range 1-2 rev/min.
  14. On half loads hoisting speeds increase two to three times.

Bunkering

Bunkering
Bunkering
The loading of fuel oil into a ship's tanks from a shoreside installation or bunker barge takes place about once a trip. The penalties for oil spills are large, the damage to the environment is considerable, and the ship may well be delayed or even arrested if this job is not properly carried out.
Bunkering is traditionally the fourth engineer's job. He will usually be assisted by at least one other engineer and one or more ratings. Most ships will have a set procedure which is to be followed or some form of general instructions which might include:

1. AH scuppers are to be sealed off, i.e. plugged, to prevent any minor oil spill on deck going overboard.
2. All tank air vent containments or drip trays are to be sealed or plugged.
3. Sawdust should be available at the bunkering station and various positions around the deck.
4. All fuel tank valves should be carefully checked before bunkering commences. The personnel involved should be quite familiar with the piping systems, tank valves, spill tanks and all tank-sounding equipment.
5. All valves on tanks which are not to be used should be closed or switched to the 'off position and effectively safeguarded against opening or operation.
6. Any manual valves in the filling lines should be proved to be open for the flow of liquid.
7. Proven, reliable tank-sounding equipment must be used to regularly check the contents of each tank. It may even be necessary to 'dip' or manually sound tanks to be certain of their contents.
8. A complete set of all tank soundings must be obtained before bunkering commences.
9. A suitable means of communication must be set up between the ship and the bunkering installation before bunkering commences.
10. On-board communication between involved personnel should be by hand radio sets or some other satisfactory means.
11. Any tank that is filling should be identified in some way on the level indicator, possibly by a sign or marker reading 'FILLING'.
12. In the event of a spill, the Port Authorities should be informed as soon as possible to enable appropriate cleaning measures to be taken.

Watchkeeping

Watchkeeping
Watchkeeping 
The 'Round the clock' operation of a ship at sea requires a rota system of attendance in the machinery space. This has developed into a system of watchkeeping that has endured until recently. The arrival of 'Unattended Machinery Spaces' (UMS) has begun to erode this traditional practice of watchkeeping. The organisation of the Engineering Department, conventional watchkeeping and UMS practices will now be outlined.
 

The Engineering Department

The Chief Engineer is directly responsible to the Master for the satisfactory operation of all machinery and equipment. Apart from assuming all responsibility his role is mainly that of consultant and adviser. It is not usual for the Chief Engineer to keep a watch.
The Second Engineer is responsible for the practical upkeep of machinery and the manning of the engine room: he is in effect an executive officer. On some ships the Second Engineer may keep a watch. The Third and Fourth Engineers are usually senior watchkeepers or engineers in charge of a watch. Each may have particular areas of responsibility, such as generators or boilers.
Fifth and Sixth Engineers may be referred to as such, or all below Fourth Engineer may be classed as Junior Engineers. They will make up as additional watchkeepers, day workers on maintenance work or possibly act as Refrigeration Engineer.
Electrical Engineers may be carried on large ships or where company practice dictates. Where no specialist Electrical Engineer is carried the duty will fall on one of the engineers.
Various engine room ratings will usually form part of the engine room complement. Donkeymen are usually senior ratings who attend the auxiliary boiler while the ship is in port. Otherwise they will direct the ratings in the maintenance and upkeep of the machinery space. A storekeeper may also be carried and on tankers a pump man is employed to maintain and operate the cargo pumps. The engine room ratings, e.g. firemen, greasers, etc., are usually employed on watches to assist the engineer in charge.

The watchkeeping system
The system of watches adopted on board ship is usually a four hour period of working with eight hours rest for the members of each watch. The three watches in any 12 hour period are usually 12-4, 4—8 and 8-12. The word 'watch' is taken as meaning the time period and also the personnel at work during that period.
The watchkeeping arrangements and the make up of the watch will be
decided by the Chief Engineer. Factors to be taken into account in this matter will include the type of ship, the type of machinery and degree of automation, the qualifications and experience of the members of the watch, any special conditions such as weather, ship location, international and local regulations, etc. The engineer officer in charge of the watch is the Chief Engineer's representative and is responsible for the safe and efficient operation and upkeep of all machinery affecting the safety of the ship.
 
Operating the watch

An engineer officer in charge, with perhaps a junior engineer assisting and one or more ratings, will form the watch. Each member of the watch should be familiar with his duties and the safety and survival equipment in the machinery space. This would include a knowledge of the fire fighting equipment with respect to location and operation, being able to distinguish the different alarms and the action required, an understanding of the communications systems and how to summon help and also being aware of the escape routes from the machinery space.
At the beginning of the watch the current operational parameters and the condition of all machinery should be verified and also the log readings should correspond with those observed. The engineer officer in charge should note if there are any special orders or instructions relating to the operation of the main machinery or auxiliaries. He should determine what work is in progress and any hazards or limitations this presents. The levels of tanks containing fuel, water, slops, ballast, etc., should be noted and also the level of the various bilges. The operating mode of equipment and available standby equipment should also be noted.

At appropriate intervals inspections should be made of the main propulsion plant, auxiliary machinery and steering gear spaces. Any routine adjustments may then be made and malfunctions or breakdowns can be noted, reported and corrected. During these tours of inspection bilge levels should be noted, piping and systems observed for leaks, and local indicating instruments can be observed.
Where bilge levels are high, or the well is full, it must be pumped dry. The liquid will be pumped to an oily water separator, and only clean water is to be discharged overboard. Particular attention must be paid to the relevant oil pollution regulations both of a national and international nature, depending upon the location of the ship. Bilges should not be pumped when in port. Oily bilges are usually emptied to a slop tank from which the oil may be reclaimed or discharged into suitable facilities when in port. The discharging of oil from a ship usually results in the engineer responsible and the master being arrested.

Bridge orders must be promptly carried out and a record of any required changes in speed and direction should be kept. When under standby or manceuvring conditions with the machinery being manually operated the control unit or console should be continuously manned.  Certain watchkeeping duties will be necessary for the continuous operation of equipment or plant—the transferring of fuel for instance.

In addition to these regular tasks other repair or maintenance tasks may be required of the watchkeeping personnel. However no tasks should be set or undertaken which will interfere with the supervisory duties relating to the main machinery and associated equipment.  During the watch a log or record will be taken of the various parameters of main and auxiliary equipment. This may be a manual operation or provided automatically on modern vessels by a data logger.  A typical log book page for a slow-speed diesel driven vessel is shown in Figure 17.1.

The hours and minutes columns are necessary since a ship, passing through time zones, may have watches of more or less than four hours.  Fuel consumption figures are used to determine the efficiency of operation, in addition to providing a check on the available bunker quantities. Lubricating oil tank levels and consumption to some extent indicate engine oil consumption. The sump level is recorded and checked that it does not rise or fall, but a gradual fall is acceptable as the engine uses some oil during operation. If the sump level were to rise this would indicate water leakage into the oil and an investigation into the cause must be made. The engine exhaust temperatures should ail read about the same to indicate an equal power production from each cylinder. The various temperature and pressure values for the cooling water and lubricating oil should be at, or near to, the manufacturer's designed values for the particular speed or fuel lever settings. Any high


outlet temperature for cooling water would indicate a lack of supply to that point.Various parameters for the main engine turbo-blowers are also logged. Since they are high-speed turbines the correct supply of lubricating oil is essential. The machine itself is water cooled since it is circulated by hot exhaust gases. The air cooler is used to increase the charge air density to enable a large quantity of air to enter the engine cylinder. If cooling were inadequate a lesser mass of air would be supplied to the engine, resulting in a reduced power output, inefficient combustion and black smoke.

Various miscellaneous level and temperature readings are taken of heavy oil tanks, both settling and service, sterntube bearing temperature, sea water temperature, etc. The operating diesel generators will have their exhaust temperatures, cooling water and lubricating oil temperatures and pressures logged in much the same way as for the main engine. Of particular importance will be the log of running hours since this will be the basis for overhauling the machinery.

Other auxiliary machinery and equipment, such as heat exchangers, fresh water generator (evaporator), boiler, air conditioning plant and refrigeration plant will also have appropriate readings taken. There will usually be summaries or daily account tables for heavy oil, diesel oil, lubricating oil and fresh water, which will be compiled at noon. Provision is also made for remarks or important events to be noted in the log for each watch.

The completed log is used to compile a summary sheet or abstract of information which is returned to the company head office for record purposes.

The log for a medium-speed diesel driven ship would be fairly similar with probably greater numbers of cylinder readings to be taken and often more than one engine. There would also be gearbox parameters to be logged.

For a steam turbine driven vessel the main log readings will be for the boiler and the turbine. Boiler steam pressure, combustion air pressure, fuel oil temperatures, etc., will all be recorded. For the turbine the main bearing temperatures, steam pressures and temperatures, condenser vacuum, etc., must be noted. All logged values should correspond fairly closely with the design values for the equipment.

Where situations occur in the machinery space which may affect the speed, manoeuvrability, power supply or other essentials for the safe operation of the ship, the bridge should be informed as soon as possible. This notification should preferably be given before any changes are made to enable the bridge to take appropriate action.

The engineer in charge should notify the Chief Engineer in the event of any serious occurrence or a situation where he is unsure of the action to take. Examples might be, if any machinery suffers severe damage, or a malfunction occurs which may lead to serious damage. However where immediate action is necessary to ensure safety of the ship, its machinery and crew, it must be taken by the engineer in charge.

At the completion of the watch each member should hand over to his relief, ensuring that he is competent to take over and carry out his duties effectively.

Boilers


Boilers
Boilers
A boiler in one form or another will be found on every type of ship.  Where the main machinery is steam powered, one or more large watertube  boilers will be fitted to produce steam at very high temperatures and pressures. On a diesel main machinery vessel, a smaller (usually firetube type) boiler will be fitted to provide steam for the various ship services. Even within the two basic design types, watertube and firetube, a variety of designs and variations exist.

A boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.
A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.
In the steam generation process the feedwater enters the boiler where it is heated and becomes steam. The feedwater circulates from the steam drum to the water drum and is heated in the process. Some of the feedwater passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum. Large-bore downcomer tubes are used to circulate feedwater between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and may be drawn off for use from here. It is known as 'wet' or saturated steam in this condition because it will contain small quantities of water, Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and 'dried', i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An 'attemperator', i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature.
The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel. The arrangement is shown in Figure 4.1.
Two basically different types of boiler exist, namely the watertube and the firetube. In the watertube the feedwater is passed through the tubes and the hot gases pass over them. In the firetube boiler the hot gases pass through the tubes and the feedwater surrounds them.


The watertube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.
Watertube boilers
The construction of watertube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising

Figure 4.2 Foster Wheeler D-Type boiler
process is much quicker. Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the watertube boiler has replaced the firetube boiler as the major steam producer.  Early watertube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers.
The hot gases from the furnace passed over the tubes, often in a single pass,
A later development was the bent tube design. This boiler has two drums, an integral furnace and is often referred to as the 'D' type because of its shape (Figure 4.2). The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These waterwall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted.

Figure 4.3 Foster Wheeler Type ESD I boiler

These provide the main heat transfer surfaces for steam generation.  Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water. In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner, The need for a wider range of superheated steam temperature control led to other boiler arrangements being used. The original External Superheater 'D' (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank (Figure 4.3). An attemperator located in the combustion air path was used to control the steam temperature.
The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economiser) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low temperature superheating was required.
In the ESD III boiler the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace. In the boiler shown in Figure 4.4, the furnace is fully water-cooled and of monowali construction, which is produced from finned tubes welded together to form a gaslight casing. With monowali construction no refractory material is necessary in the furnace.
The furnace side, floor and roof tubes are welded into the steam and water drums. The front and rear walls are connected at either end to upper and lower water-wall headers. The lower water-wall headers are connected by external downcomers from the steam drum and the upper water-wall headers are connected to the steam drum by riser tubes.
The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over the primary superheater and the economiser before passing to exhaust. The dry pipe is located in the steam drum to obtain reasonably dry saturated steam from the boiler. This is then passed to the primary superheater and then to the secondary superheater. Steam temperature control is achieved by the use of an attemperator, located in the steam drum, operating between the primary and secondary superheaters.
Radiant-type boilers are a more recent development, in which the radiant heat of combustion is absorbed to raise steam, being transmitted 


Figure 4.4 Foster Wheeler Type ESD III monowall boiler

by infra-red radiation. This usually requires roof firing and a considerable height in order to function efficiently. The ESD IV boiler shown in Figure 4.5 is of the radiant type. Both the furnace and the outer chamber are fully watercooled. There is no conventional bank of generating tubes. The hot gases leave the furnace through an opening at the lower end of the screen wall and pass to the outer chamber. The outer chamber contains the convection heating surfaces which include the primary and secondary superheaters. Superheat temperature control is by means of an attemperator in the steam drum. The hot gases, after leaving the primary superheater, pass over a steaming economises This is a heat exchanger in which the steam—water mixture

is flowing parallel to the gas. The furnace gases finally pass over a conventional economiser on their way to the funnel.  Reheat boilers are used with reheat arranged turbine systems. Steam after expansion in the high-pressure turbine is returned to a reheater in the boiler. Here the steam energy content is raised before it is supplied to the low-pressure turbine. Reheat boilers are based on boiler designs such as the 'D' type or the radiant type.

The problems associated with furnace refractory materials, particularly on vertical walls, have resulted in two water-wall arrangements without exposed refractory. These are known as 'tangent tube' and 'monowall' or 'membrane wall'.
In the tangent tube arrangement closely pitched tubes are backed by refractory, insulation and the boiler casing (Figure 4.6(a)), In the monowall or membrane wall arrangement the tubes have a steel strip welded between them to form a completely gas-tight enclosure (Figure 4.6(b)). Only a layer of insulation and cleading is required on the outside of this construction.

The monowall construction eliminates the problems of refractory and expanded joints. However, in the event of tube failure, a welded repair must be carried out. Alternatively the tube can be plugged at either end, but refractory material must be placed over the failed tube to protect the insulation behind it. With tangent tube construction a failed tube can be plugged and the boiler operated normally without further attention.
Firetube boilers
The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler* is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler* are also in use.

Package boilers
Most firetube boilers are now supplied as a completely packaged unit. This will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings.
A single-furnace three-pass design is shown in Figure 4.7. The first pass is through the partly corrugated furnace and into the cylindrical wetback combustion chamber. The second pass is back over the furnace through small-bore smoke tubes and then the flow divides at the front central smoke box. The third pass is through outer smoke tubes to the gas exit at the back of the boiler.
There is no combustion chamber refractory lining other than a lining

Figure 4,7 Package boiler
to the combustion chamber access door and the primary and secondary quart.
Fully automatic controls are provided and located in a control panel at the side of the boiler.

Cochran boilers
The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid' (Figure 4.8). The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.
Composite boilers
A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs.  The Cochran boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.

Other boiler arrangements
Apart from straightforward watertube and firetube boilers, other steam raising equipment is in use, e.g. the steam-to-steam generator, the double evaporation boiler and various exhaust gas boiler arrangements.
The steam-to-steam generator
Steam-to-steam generators produce low-pressure saturated steam for domestic and other services. They are used in conjunction with watertube boilers to provide a secondary steam circuit which avoids any possible contamination of the primary-circuit feedwater. The arrangement may be horizontal or vertical with coils within the shell which heat the feedwater. The coils are supplied with high-pressure, hightemperature steam from the main boiler. A horizontal steam-to-steam generator is shown in Figure 4.9.

Double evaporation boilers
A double evaporation boiler uses two independent systems for steam generation and therefore avoids any contamination between the primary and secondary feedwater. The primary circuit is in effect a conventional watertube boiler which provides steam to the heating coils of a steam-to-steam generator, which is the secondary system. The complete boiler is enclosed in a pressurised casing.

Exhaust gas heat exchangers
The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant
efficiency.
An exhaust gas heat exchanger is shown in Figure 4.10. It is simply a row of tube banks circulated by feedwater over which the exhaust gases flow. Individual banks may be arranged to provide feed heating, steam generation and superheating. A boiler drum is required for steam generation and separation to take place and use is usually made of the drum of an auxiliary boiler.

Figure 4.10 Auxiliary steam plant system
The auxiliary steam installation provided in modern diesel powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two watertube boilers (Figure 4.10).  Saturated or superheated steam may be obtained from the auxiliary boiler. At sea it acts as a steam receiver for the exhaust-gas heat exchanger, which is circulated through it. In port it is oil-fired in the usual way.

Auxiliary boilers on diesel main propulsion ships, other than tankers, are usually of composite form, enabling steam generation using oil firing or the exhaust gases from the diesel engine. With this arrangement the boiler acts as the heat exchanger and raises steam in its own drum.
Boiler mountings
Certain fittings are necessary on a boiler to ensure its safe operation. They are usually referred to as boiler mountings. The mountings usually found on a boiler are:
Safety valves. These are mounted in pairs to protect the boiler against overpressure. Once the valve lifting pressure is set in the presence of a Surveyor it is locked and cannot be changed. The valve is arranged to open automatically at the pre-set blow-off pressure.
Mam steftm stop valve. This valve is fitted in the main steam supply line and is usually of the non-return type.
Auxiliary steam stop valve. This is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non-return type.
Feed check or control valve. A pair of valves are fitted: one is the main valve, the other the auxiliary or standby. They are non-return valves and must give an indication of their open and closed position.
Water level gauge. Water level gauges or 'gauge glasses' are fitted in pairs, at opposite ends of the boiler. The construction of the level gauge
depends upon the boiler pressure.
Pressure gauge connection. Where necessary on the boiler drum, superheater, etc., pressure gauges are fitted to provide pressure readings.
Air release cock. These are fitted in the headers, boiler drum, etc., to release air when filling the boiler or initially raising steam.
Sampling connection. A water outlet cock and cooling arrangement is provided for the sampling and analysis of feed water. A provision may also be made for injecting water treatment chemicals.
Blow down valve. This valve enables water to be blown down or emptied from the boiler. It may be used when partially or completely emptying
the boiler.
Scum valve. A shallow dish positioned at the normal water level is connected to the scum valve. This enables the blowing down or removal
of scum and impurities from the water surface.
Whistle stop valve. This is a small bore non-return valve which supplies the whistle with steam straight from the boiler drum.

Boiler mountings (water-tube boilers)
Watertube boilers, because of their smaller water content in relation to their steam raising capacity, require certain additional mountings:
Automatic feed water regulator. Fitted in the feed line prior to the main check valve, this device is essential to ensure the correct water level in.the boiler during all load conditions. Boilers with a high evaporation rate will use a multiple-element feed water control system (see Chapter 15).
Low level alarm. A device to provide audible warning of low water level conditions.
Superheater circulating valves. Acting also as air vents, these fittings ensure a flow of steam when initially warming through and raising steam
in the boiler.
Sootblowers, Operated by steam or compressed air, they act to blow away soot and the products of combustion from the tube surfaces.
Several are fitted in strategic places. The sootbiower lance is inserted, soot is blown and the lance is withdrawn.

The water level gauge provides a visible indication of the water level in the boiler in the region of the correct working level. If the water level were too high then water might pass out of the boiler and do serious damage to any equipment designed to accept steam. If the water level were too low then the heat transfer surfaces might become exposed to excessive temperatures and fail. Constant attention to the boiler water level is therefore essential. Due to the motion of the ship it is necessary to have a water level gauge at each end of the boiler to correctly observe the level.
Depending upon the boiler operating pressure, one of two basically different types of water level gauge will be fitted.
For boiler pressures up to a maximum of 17 bar a round glass tube type of water level gauge is used. The glass tube is connected to the boiler shell by cocks and pipes, as shown in Figure 4.11. Packing rings are positioned at the tube ends to give a tight seal and prevent leaks. A guard is usually placed around the tube to protect it from accidental damage and to avoid injury to any personnel in the vicinity if the tube shatters. The water level gauge is usually connected directly to the boiler. Isolating cocks are fitted in the steam and water passages, and a drain cock is also present. A ball valve is fitted below the tube to shut off the water should the tube break and water attempt to rush out.
For boiler pressures above 17 bar a plate-glass-type water level gauge is used. The glass tube is replaced by an assembly made up of glass plates within a metal housing, as shown in Figure 4.12. The assembly is made

up as a 'sandwich' of front and back metal plates with the glass plates and a centre metal plate between. Joints are placed between the glass and the metal plate and a mica sheet placed over the glass surface facing the water and steam. The mica sheet is an effective insulation to prevent the glass breaking at the very high temperature. When bolting up this assembly, care must be taken to ensure even all-round tightening of the bolts. Failure to do this will result in a leaking assembly and possibly shattered glass plates.
In addition to the direct-reading water level gauges, remote-reading level indicators are usually led to machinery control rooms.
It is possible for the small water or steam passages to block with scale or dirt and the gauge will give an incorrect reading. To check that

passages are dear a 'blowing through' procedure should be followed.  Referring to Figure 4.11, close the water cock B and open drain cock C. The boiler pressure should produce a strong jet of steam from the drain.  Cock A is now closed and Cock B opened. A jet of water should now pass through the drain. The absence of a flow through the drain will indicate that the passage to the open cock is blocked.
Safety valves
Safety valves are fitted in pairs, usually on a single valve chest. Each valve must be able to release all the steam the boiler can produce without the pressure rising by more than 10% over a set period.
Spring-loaded valves are always fitted on board ship because of their positive action at any inclination. They are positioned on the boiler drum in the steam space. The ordinary spring loaded safety valve is shown in Figure 4.13. The valve is held closed by the helical spring

whose pressure is set by the compression nut at the top. The spring pressure, once set, is fixed and sealed by a Surveyor. When the steam exceeds this pressure the valve is opened and the spring compressed.
The escaping steam is then led through a waste pipe up the funnel and out to atmosphere. The compression of the spring by the initial valve opening results in more pressure being necessary to compress the spring and open the valve further. To some extent this is countered by a lip arrangement on the valve lid which gives a greater area for the steam to act on once the valve is open. A manually operated easing gear enables the valve to be opened in an emergency. Various refinements to the ordinary spring-loaded safety valve have been designed to give a higher lift to the valve.
The improved high-lift safety valve has a modified arrangement around the lower spring carrier, as shown in Figure 4.14. The lower

spring carrier is arranged as a piston for the steam to act on its underside. A loose ring around the piston acts as a containing cylinder for the steam. Steam ports or access holes are provided in the guide plate. Waste steam released as the valve opens acts on the piston underside to give increased force against the spring, causing the valve to open further. Once the overpressure has been relieved, the spring force will quickly close the valve. The valve seats are usually shaped to trap some steam to 'cushion' the closing of the valve.
A drain pipe is fitted on the outlet side of the safety valve to remove any condensed steam which might otherwise collect above the valve and stop it opening at the correct pressure.


Combustion is the burning of fuel in air in order to release heat energy. For complete and efficient combustion the correct quantities of fuel and air must be supplied to the furnace and ignited. About 14 times as much air as fuel is required for complete combustion. The air and fuel must be intimately mixed and a small percentage of excess air is usually supplied to ensure that all the fuel is burnt. When the air supply is insufficient the fuel is not completely burnt and black exhaust gases will result.
Air supply
The flow of air through a boiler furnace is known as 'draught'. Marine boilers are arranged for forced draught, i.e. fans which force the air through the furnace. Several arrangements of forced draught are possible. The usual forced draught arrangement is a large fan which supplies air along ducting to the furnace front. The furnace front has an enclosed box arrangement, known as an 'air register', which can control the air supply. The air ducting normally passes through the boiler exhaust where some air heating can take place. The induced draught arrangement has a fan in the exhaust uptake which draws the air through the furnace. The balanced draught arrangement has matched forced draught and induced draught fans which results in atmospheric pressure in the furnace.

Fuel supply
Marine boilers currently burn residual low-grade fuels. This fuel is stored in double-bottom tanks from which it is drawn by a transfer pump up to settling tanks (Figure 4.15). Here any water in the fuel may settle out and be drained away.
The oil from the settling tank is filtered and pumped to a heater and then through a fine filter. Heating the oil reduces its viscosity and makes it easier to pump and filter. This heating must be carefully controlled otherwise 'cracking' or breakdown of the fuel may take place. A supply of diesel fuel may be available to the burners for initial firing or low-power operation of the boiler. From the fine filter the oil passes to the burner where it is 'atomised', i.e. broken into tiny droplets, as it enters the furnace. A recirculating line is provided to enable initial heating of the oil.

Fuel burning
The high-pressure fuel is supplied to a burner which it leaves as an atomised spray (Figure 4.16). The burner also rotates the fuel droplets by the use of a swirl plate. A rotating cone of tiny oil droplets thus leaves the burner and passes into the furnace. Various designs of burner exist, the one just described being known as a 'pressure jet burner' (Figure 4.16(a». The 'rotating cup burner' (Figure 4.14(b)) atomises and swirls the fuel by throwing it off the edge of a rotating tapered cup. The 'steam blast jet burner', shown in Figure 4.16(c), atomises and swirls the fuel by spraying it into a high-velocity jet of steam. The steam is supplied down a central inner barrel in the burner.
The air register is a collection of flaps, vanes, etc., which surrounds each burner and is fitted between the boiler casings. The register provides an entry section through which air is admitted from the windbox. Air shut-off is achieved by means of a sliding sleeve or check.  Air flows through parallel to the burner, and a swirler provides it with a rotating motion. The air is swirled in an opposite direction to the fuel to ensure adequate mixing (Figure 4.17(a)). High-pressure, higb-0i»tput marine watertube boilers are roof fired (Figure 4.17(b)). This enables a long flame path and even heat transfer throughout the furnace.

The fuel entering the furnace must be initially ignited in order to burn.
Once ignited the lighter fuel elements burn first as a primary flame and provide heat to burn the heavier elements in the secondary flame. The primary and secondary air supplies feed their respective flames.  The process of combustion in a boiler furnace is often referred to as 'suspended flame' since the rate of supply of oil and air entering the furnace is equal to that of the products of combustion leaving.
Purity of boiler feedwater
Modern high-pressure, high-temperature boilers with their large steam output require very pure feedwater.
Most 'pure* water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as 'caustic cracking'. This is actual cracking of the metal which may lead to serious failure.

The presence of dissolved oxygen and carbon dioxide in boiler feedwater can cause considerable corrosion of the boiler and feed systems. When boiler water is contaminated by suspended matter, an excess of salts or oil then 'foaming' may occur. This is a foam or froth which collects on the water surface in the boiler drum. Foaming leads to 'priming' which is the carry-over of water with the steam leaving the boiler drum. Any water present in the steam entering a turbine will do considerable damage.
Common impurities
Various amounts of different metal salts are to be found in water. These include the chlorides, sulphates and bicarbonates of calcium, magnesium and, to some extent, sulphur. These dissolved salts in water make up what is called the 'hardness' of the water. Calcium and magnesium salts are the main causes of hardness, The bicarbonates of calcium arid magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as 'temporary hardness'. The chlorides, sulphates and nitrates are not decomposed by boiling and are known as 'permanent hardness*. Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the boiler feedwater.

Feedwater treatment deals with the various scale and corros

Pressure measurement


Pressure measurement
Pressure measurement

The measurement of pressure may take place from one of two possible datums, depending upon the type of instrument used. Absolute pressure is a total measurement using zero pressure as datum. Gauge pressure is a measurement above the atmospheric pressure which is used as a datum. To express gauge pressure as an absolute value it is therefore necessary to add the atmospheric pressure.

Manometer
A U-tube manometer is shown in Figure 15.1. One end is connected to the pressure source; the other is open to atmosphere. The liquid in the tube may be water or mercury and it will be positioned as shown. The excess of pressure above atmospheric will be shown as the difference in liquid levels; this instrument therefore measures gauge pressure. It is usually used for low value pressure readings such as air pressures. Where two different system pressures are applied, this instrument will Measure differential pressure.

Barometer
The mercury barometer is a straight tube type of manometer. A glass capillary tube is sealed at one end^ filled with mercury and then inverled in a small bath of mercury (Figure 15.2). Almost vacuum conditions; exist above the column of mercury, which is supported by atmospheric

pressure acting on the mercury in the container. An absolute reading of atmospheric pressure is thus given.The aneroid barometer uses an evacuated corrugated cylinder to detect changes in atmospheric pressure (Figure 15.3). The cylinder centre tends to collapse as atmospheric pressure increases or is lifted by the spring as atmospheric pressure falls. A series of linkages transfers the movement to a pointer moving over a scale.
Bourdon tube
This is probably the most commonly used gauge pressure measuring instrument and is shown in Figure 15.4. It is made up of an elliptical
section tube formed into a C-shape and sealed at one end. The sealed end, which is free to move, has a linkage arrangement which will move a pointer over a scale. The applied pressure acts within the tube entering through the open end, which is fixed in place. The pressure within the tube causes it to change in cross section and attempt to straighten out with a resultant movement of the free end, which registers as a needlemovement on the scale. Other arrangements of the tube in a helical or spiral form are sometimes used, with the operating principle being the same.
While the reference or zero value is usually atmospheric, to give gauge pressure readings, this gauge can be used to read vacuum pressure values.
Other devices
Diaphragms or bellows may be used for measuring gauge or differential pressures. Typical arrangements are shown in Figure 15.5. Movement of the diaphragm or bellows is transferred by a linkage to a needle or pointer display.




The piezoelectric pressure transducer is a crystal which, under pressure, produces an electric current which varies with the pressure. This current is then provided to a unit which displays it as a pressure value.
Temperature measurement by instruments will give a value in degrees Celsius (°C). This scale of measurement is normally used for all readings and temperature values required except when dealing with theoretical calculations involving the gas laws, when absolute values are required (see Appendix).
+Temperature Measurement
Liquid-in-glass thermometer
Various liquids are used in this type of instrument, depending upon the temperature range, e.g. mercury -35°C to +350°C, alcohol -80°C to 4-70°C. An increase in temperature causes the liquid to rise up the narrow glass stem and the reading is taken from a scale on the glass (Figure 15.6). High-temperature-measuring mercury liquid thermometers will have the space above the mercury filled with nitrogen under pressure.

Liquid-in-metal thermometer
The use of a metal bulb and capillary bourdon tube filled with liquid offers advantages of robustness and a wide temperature range. The useof mercury, for instance, provides a range from —39°C to +650°C. The bourdon tube may be spiral or helical and on increasing temperature it tends to straighten. The free end movement is transmitted through linkages to a pointer moving over a scale.
Bimetallic strip thermometers
A bimetallic strip is made up of two different metals firmly bonded together. When a temperature change occurs different amounts of expansion occur in the two metals, causing a bending or twisting of the strip. A helical coil of bimetallic material with one end fixed is used in one form of thermometer (Figure 15.7). The coiling or uncoiling of the


helix with temperature change will cause movement of a pointer fitted to the free end of the bimetallic strip. The choice of metals for the strip will determine the range, which can be from — 30°C to +550°C.

Thermocouple
The thermocouple is a type of electrical thermometer. When two different metals are joined to form a closed circuit and exposed to different temperatures at their junction a current will flow which can be used to measure temperature. The arrangement used is shown in Figure 15.8, where extra wires or compensating leads are introduced to complete the circuit and include the indicator. As long as the two ends A and B are at the same temperature the thermoelectric effect is not influenced. The appropriate choice of metals will enable temperature ranges from ~200°C to +1400°C.

Radiation pyrometer
A pyrometer is generally considered to be a high-temperature measuring thermometer. In the optical, or disappearing filament, type shown in Figure 15.9, radiation from the heat source is directed into the unit. The current through a heated filament lamp is adjusted until, when viewed through the telescope, it seems to disappear. The radiation from the lamp and from the heat source are therefore the same. The current through the lamp is a measure of the temperature of the heat source,



and the ammeter is calibrated in units of temperature. The absorption screen is used to absorb some of the radiant energy from the heat source and thus extend the measuring range of the instrument. The monochromatic filter produces single-colour, usually red, light to simplify filament radiation matching.
Thermistor
This is a type of electrical thermometer which uses resistance change to measure temperature. The thermistor is a semi-conducting material made up of finely divided copper to which is added cobalt, nickel and manganese oxides. The mixture is formed under pressure into various shapes, such as beads or rods, depending upon the application. They are usually glass coated or placed under a thin metal cap. A change in temperature causes a fall in the thermistor resistance which can be measured in an electric circuit and a reading relating to temperature can be given. Their small size and high sensitivity are particular advantages. A range of measurement from — 250°G to + 1500°C is possible.

+ Level measurement
Float operated
A float is usually a hollow ball or cylinder whose movement as the liquid surface rises or falls is transmitted to an indicator. A chain or wire usually provides the linkage to the indicator. Float switches may be used for high or low indication, pump starting, etc., where electrical contacts are made or broken, depending upon the liquid level.
Sight or gauge glasses
Various types of sightglass are used to display liquid level in storage tanks. The simple boiler gauge glass referred to in Chapter 4 is typical of such devices.
Pneumatic gauge
This is a device which uses a mercury manometer in conjunction with a hemispherical bell and piping to measure tank level. The arrangement is shown in Figure 15.10. A hemispherical bell is fitted near the bottom of the tank and connected by small bore piping to the mercury manometer.
A selector cock enables one manometer to be connected to a number of tanks, usually a pair. A three-way cock is fitted to air, gauge and vent positions. With the cock at the 'air' position the system is filled with compressed air. The cock is then turned to 'gauge' when the tank contents will further pressurise the air in the system and a reading will be given on the manometer which corresponds to the liquid level. The cock is turned to Vent' after the reading has been taken.
Flow measurement can be quantity measurement, where the amount of liquid which has passed in a particular time is given, or a flow velocity which, when multiplied by the pipe area, will give a rate of flow.
+ Quantity measurement
A rotating pair of intermeshing vanes may be used which are physically displaced by the volume of liquid passing through (Figure 15.1 l(a)). The


number of rotations will give a measure of the total quantity of liquid that has passed. The rotation transfer may be by mechanical means, such as gear wheels, or the use of a magnetic coupling. Another method is the use of a rotating element which is set in motion by the passing liquid (Figure 15.1 l(b)). A drive mechanism results in a reading on a scale of total quantity. The drive mechanism may be mechanical, using gear wheels or electrical where the rotating element contains magnets which generate a current in a pick-up coil outside the pipe.
+flow velocity measurement
The vmturi tube
This consists of a conical convergent entry tube, a cylindrical centre tube and a conical divergent outlet. The arrangement is shown in Figure 15.12. Pressure tappings led to a manometer will give a difference in

head related to the fluid flow velocity. The operating principle is one of pressure conversion to velocity which occurs in the venturitube and results in a lower pressure in the cylindrical centre tube.
The orifice plate
This consists of a plate with an axial hole placed in the path of the liquid. The hole edge is square facing the incoming liquid and bevelled on the




outlet side (Figure 15.13). Pressure tappings before and after the orifice plate will give a difference in head on a manometer which can be related to liquid flow velocity.
+ Other variables
Moving coil meter
Electrical measurements of current or voltage are usually made by a moving coil meter. The meter construction is the same for each but its arrangement in the circuit is different. A moving coil meter consists of a coil wound on a soft iron cylinder which is pivoted and free to rotate (Figure 15.14). Two hair springs are used, one above and one below, to provide a restraining force and also to



conduct the current to the coil. The moving coil assembly is surrounded by a permanent magnet which produces a radial magnetic field. Current passed through the coil will result in a force which moves the coil against the spring force to a position which, by a pointer on a scale, will read current or voltage.



The instrument is directional and must therefore be correctly connected in the circuit. As a result of the directional nature of alternating current it cannot be measured directly with this instrument, but the use of a rectifying circuit will overcome this problem.
Tachometers
A number of speed measuring devices are in use utilising either mechanical or electrical principles in their operation.
Mechanical
A simple portable device uses the governor principle to obtain a measurement of speed. Two masses are fixed on leaf springs which are fastened to the driven shaft at one end and a sliding collar at the other (Figure 15.15). The


sliding collar, through a link mechanism, moves a pointer over a scale. As the driven shaft increases in speed the weights move out under centrifugal force, causing an axial movement of the sliding collar. This in turn moves the pointer to give a reading of speed.