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.
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
A large number of ships are fitted with deck cranes.
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.
On container ships using ports without special container handling
facilities, cranes with special container handling gear are essential.
Deck-mounted cranes for both conventional cargo handling and grabbing
duties are available with lifting capacities of up to 50 tonnes.
Ships specialising in carrying very heavy loads，however, are
invariably equipped with special derrick systems such as the Stulken
(Figure 9.11).
These derrick systems are capable of lifting loads of up to 500 tonnes
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)
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．
Deck cranes require to hoist, luff and slew and separate electric or hydraulic motors will be required for each motion.
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.
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.
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)).
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
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.
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.
Swinging of the hook will produce similar effects to inclination of the crane.
In the Stothert ＆ Pitt ‘Stevedore’ electro-hydraulic crane the jib is luffed by one or two hydraulic rains.
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.
When the jib is to be stowed the operator can override the limiting device.
In the horizontal stowed position the cylinder rods are fully
retracted into the rams where they are protected from the weather .
Some cranes are mounted in pairs on a common platform which can be rotated through 360ยบ .
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.
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.
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.
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.
On half loads hoisting speeds increase two to three times.
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.
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.
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.
Commonimpurities
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
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.
