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Air compressor arrangement - working principles and operational guideline
Use of compressed air for ships machinery :
Compressed air has many uses on board ship, ranging from diesel
engine starting to the cleaning of machinery during maintenance. The
air pressures of 25 bar or more are usually provided in multi-stage
machines. Here the air is compressed in the first stage, cooled and
compressed to a higher pressure in the next stage, and so on. The
two-stage crank machine is probably the most common.
Air is drawn in on the suction stroke through the first-stage suction
valve via the silencer/filter. The suction valve closes on the piston
upstroke and the air is compressed. The compressed air, having reached
its first-stage pressure, passes through the delivery valve to the
first-stage cooler. The second-stage suction and compression now take
place in a similar manner, achieving a much higher pressure in the
smaller, second-stage cylinder.
After passing through the second-stage
delivery valve, the air is again cooled and delivered to the storage system.
The machine has a rigid crankcase which provides support for the
three crankshaft bearings. The cylinder block is located above and
replaceable liners are fitted in the cylinder block. The running gear
consists of pistons, connecting rods and the one-piece, two-throw
Fig: Two stage air compressor
The first-stage cylinder head is located on the cylinder block
and the second-stage cylinder head is mounted on the first: each of the
heads carries its suction and delivery valves. A chain-driven rotary-gear
pump provides lubricating oil to the main bearings and through
internally drilled passages in the crankshaft to both connecting rod
bearings. Cooling water is supplied either from an integral pump or the
machinery space system. The water passes into the cylinder block which
contains both stage coolers and then into the first and second stage
A water jacket safety valve prevents a
build-up of pressure should a cooler tube burst and compressed air
escape. Relief valves are fitted to the first and second-stage air outlets
and are designed to lift at 10% excess pressure. A fusible plug is fitted
after the second-stage cooler to limit delivered air temperature and thus
protect the compressed-air reservoirs and pipework.
Cooler drain valves are fitted to compressors. When these are open
the machine is 'unloaded' and does not produce compressed air. A
compressor when started must always be in the unloaded condition. This
reduces the starting torque for the machine and clears out any
accumulated moisture in the system. This moisture can affect lubrication
and may produce oil/water emulsions which line the air pipelines and
could lead to fires or explosions.
The compressor motor is started and the machine run up to speed.
The lubricating oil pressure should be observed to build up to the
correct value. The first-stage drains and then the second-stage drains
are closed and the machine will begin to operate. The pressure gauge
cocks should be adjusted to give a steady reading. Where manual drains
are fitted they should be slightly opened to discharge any moisture
which may collect in the coolers. The cooling water supply should be
checked, and also operating temperatures, after a period of running
To stop the compressor, the first and second-stage cooler drain valves
should be opened and the machine run unloaded for two to three
minutes. This unloaded running will clear the coolers of condensate.
The compressor can now be stopped and the drains should be left open.
The cooling water should be isolated if the machine is to be stopped for a
Automatic compressor operation is quite usual and involves certain
additional equipment. An unloader must be fitted to ensure the machine
starts unloaded, and once running at speed will load' and begin to
produce compressed air. Various methods of unloading can be used but
marine designs favour either depressors which hold the suction valve
plates on their seats or a bypass which discharges to suction. Automatic
drains must also be fitted to ensure the removal of moisture from the
stage coolers. A non-return valve is usually fitted as close as possible to
the discharge valve on a compressor to prevent return air flow: it is an
essential fitting where unloaders are used.
Control or instrument air supplies have particular requirements with regard to being moisture and oil free and without impurities. A special type of oil-free compressor may be
used to supply control air or it may be treated after delivery from an
ordinary air compressor. This treatment results in the air being filtered
and dried in order to remove virtually all traces of oil, moisture and any
Maintenance involves the usual checks and overhauls common to
reciprocating machinery, e.g. crankcase oil level, cooling water system,
operating temperatures and pressures, etc. The suction and delivery air
valves for each stage will present the most work in any maintenance
schedule. These valves are automatic, requiring a small pressure
differential to operate.
The constant rapid opening and closing action of the valves may
require the seats to be refaced. Overheating, use of incorrect lubricating
oil, or the presence of dirt may result in sticking or pitting of the
surfaces. The various buffer plates, spring plates, valve plate and seat
which make up a suction or delivery valve can be seen in Figure. The
valves should be stripped and all parts carefully cleaned and examined,
any worn parts replaced and the valve seat and plate lightly lapped
separately on a flat surface before reassembly to ensure a good seal.
Fig:Air compressor automatic valve
The steam turbine is a device for obtaining mechanical work from the
energy stored in steam. Steam enters the turbine with a high energy
content and leaves after giving up most of it. The high-pressure steam
from the boiler is expanded in nozzles to create a high-velocity jet of
steam. The nozzle acts to convert heat energy in the steam into kinetic
energy. This jet is directed into blades mounted on the periphery of a
wheel or disc .
The steam does not 'blow the wheel around'.
The shaping of the blades causes a change in direction and hence
velocity of the steam jet. Now a change in velocity for a given mass flow
of steam will produce a force which acts to turn the turbine wheel, i.e.
mass flow of steam (kg/s) x change in velocity (m/s) = force (kgm/s2).
This is the operating principle of all steam turbines, although the
arrangements may vary considerably. The steam from the first set of
blades then passes to another set of nozzles and then blades and so on
along the rotor shaft until it is finally exhausted. Each set comprising
nozzle and blades is called a stage.
- Compressed Air Systems for various shipboard operations
The main aim of a compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. Some of the main applications of compressors onboard ships are main air compressor, deck air compressor, AC compressor and refrigeration compressor. Failure to start or control air compressor can be inconvenient, costly and can carry risks, which need to be managed.....
- Marine air compressors working principles
Control or instrument air supplies have particular requirements with regard to being moisture and oil free and without impurities. A special type of oil-free compressor may be used to supply control air or it may be treated after delivery from an ordinary air compressor. This treatment results in the air being filtered and dried in order to remove virtually all traces of oil, moisture and any atmospheric impurities.....
- Coolers at sea- Shell and tube type coolers and plate type coolers Heat exchangers on board ship are mainly coolers where a hot liquid is cooled by sea water. There are some instances where liquid heating is required, such as heavy fuel oil heaters and sea water heaters for tank cleaning. Although being heat exchangers, the main condenser for a steam ship and the evaporator/distiller are dealt with separately .....
- Distillation system- Production of distilled water for ships use - The evaporation process
Distillation is the production of pure water from sea water by evaporation and re-condensing. Distilled water is produced as a result of evaporating sea water either by a boiling or a flash process. This evaporation enables the reduction of the 32000 parts per million of dissolved solids in sea water down to the one or two present in distilled water. The machine used is called an 'evaporator', although the word 'distiller' is also used.....
- Oily water separator and filter unit for 15 parts per million purity
Oil/water separators are used to ensure that ships do not discharge oil when pumping out bilges, oil tanks or any oil-contaminated space. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.....
Sewage treatment on board- biological and chemical sewage treatment plantThe discharge of untreated sewage in controlled or territorial waters is usually banned by legislation. International legislation is in force to cover any sewage discharges within specified distances from land. As a result, and in order to meet certain standards all new ships have sewage treatment plants installed.....
- Incinerator for ships Stricter legislation with regard to pollution of the sea, limits and, in some instances, completely bans the discharge of untreated waste water, sewage, waste oil and sludge. The ultimate situation of no discharge can be achieved by the use of a suitable incinerator. When used in conjunction with a sewage plant and with facilities for burning oil sludges, the incinerator forms a complete waste disposal package.....
Marine machineries - Useful tags
Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump ||
Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor ||
Marine incinerator ||
Lub oil filters ||
MAN B&W engine ||
Marine condensers ||
Oily water separator ||
Overspeed protection devices ||
Piston & piston rings ||
Crankshaft deflection ||
Marine pumps ||
Various refrigerants ||
Sewage treatment plant ||
Starting air system ||
Steam turbines ||
Steering gear ||
Sulzer engine ||
Turbine gearing ||
Two stroke engines ||
UMS operations ||
Drydocking & major repairs ||
Critical machinery ||
Deck machineries & cargo gears
|| Control and instrumentation
||Engine room safety ||
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