Section
12 Air compressors and air starting arrangements
12.1 General requirements
12.1.1 The requirements of this Section are applicable to reciprocating air
compressors intended for starting main engines and auxiliary engines providing essential
services and are applicable to both reciprocating internal combustion engines and gas
turbine engines.
12.1.2 Two
or more air compressors are to be fitted having a total capacity,
together with a topping-up compressor where fitted, capable of charging
the air receivers within one hour from atmospheric pressure, to the
pressure sufficient for the number of starts required by Vol 2, Pt 7, Ch 3, 12.12 Air receivers. At least one of the air compressors
is to be independent of the main propulsion unit and the capacity
of the main air compressors is to be approximately equally divided
between them. The capacity of an emergency compressor which may be
installed to satisfy the requirements of Vol 2, Pt 7, Ch 3, 12.11 Dead ship condition starting arrangements is to be ignored.
12.1.3 The
compressors are to be so designed that the temperature of the air
discharged to the starting air receivers will not substantially exceed
93°C in service. A small fusible plug or an alarm device operating
at 121°C is to be provided on each compressor to give warning
of excessive air temperature. The emergency air compressor is excepted
from these requirements.
12.1.4 Each
compressor is to be fitted with a safety valve so proportioned and
adjusted that the accumulation with the outlet valve closed will not
exceed 10 per cent of the maximum working pressure. The casings of
the cooling water spaces are to be fitted with a safety valve or bursting
disc so that ample relief will be provided in the event of the bursting
of an air cooler tube.
12.1.5 Each
compressor is to be fitted with an alarm for failure of the lubricating
oil supply which will initiate an automatic shutdown.
12.2 Plans and particulars
12.2.1 Detailed plans, particulars, dimensional drawings and material
specifications for compressor crankshafts are to be submitted. Plans and particulars for
other parts and calculations, where applicable, are to be submitted to LR upon
request.
12.3 Materials
12.3.2 Where it is proposed to use materials outside the ranges specified in Vol 2, Pt 2, Ch 1, 2.1 Crankshaft materials, details of the chemical composition, heat treatment and mechanical
properties are to be submitted for approval.
12.3.4 For compressors with crankshafts with a calculated crank pin diameter equal
to or greater than 50 mm, materials for components are to be tested as indicated in
Vol 2, Pt 2, Ch 1, 2.2 Testing and inspection. For calculated crank pin diameters less than 50 mm, a manufacturer’s
certificate may be accepted, see
Ch 1, 3.1 General 3.1.3.(c) of the Rules for Materials .
12.4 Design and construction
12.4.1 A
fully documented fatigue strength analysis is to be submitted, indicating
a factor of safety of 1,5 at the design loads, based on a suitable
fatigue strength criterion. Alternatively, the requirements of Vol 2, Pt 7, Ch 3, 12.4 Design and construction 12.4.2 may be used.
12.4.2 The
diameter, d
p, of a compressor crankshaft is
to be not less than d, determined by the following formula,
when all cranks on the shaft are located between two main bearings
only:
| where:
|
|
a
|
=
|
distance
between inner edge of one main bearing and the centreline of the crankpin
nearest the centre of the span, in mm
|
|
b
|
=
|
distance
from the centreline of the same crankpin to the inner edge of the adjacent
main bearing, in mm
|
|
a + b
|
=
|
span
between inner edges of main bearings, in mm
|
|
d
p
|
=
|
proposed
minimum diameter of crankshaft, in mm
|
|
p
|
=
|
design
pressure, in bar g, as defined in Vol 2, Pt 7, Ch 1, 3.3 Design pressure 3.3.1
|
|
D
|
=
|
diameter
of cylinder, in mm
|
|
S
|
=
|
length of
stroke, in mm
|
|
V
c
|
=
|
1,0 for
shafts having one cylinder per crank, or
|
|
|
=
|
1,05 for 90°
|
between adjacent cylinders on the
same crankpin
|
|
|
=
|
1,18 for 60°
|
between adjacent cylinders on the
same crankpin
|
|
|
=
|
1,25 for 45°
|
between adjacent cylinders on the
same crankpin
|
for the shaft and cylinder arrangements
as detailed in Table 3.12.1 Angle between cylinders
|
σ
u
|
= |
specified minimum tensile strength of crankshaft material, in
N/mm2.
|
Table 3.12.1 Angle between cylinders
Number of
crankpins
|
Number of
cylinders per crank
|
Angle between cylinders,
in degrees
|
| 1 or 2
|
2
|
45
|
60
|
90
|
| 3
|
2
|
45
|
60
|
—
|
| 4
|
2
|
45
|
60
|
—
|
| 1
|
3
|
45
|
60
|
90
|
| 2
|
3
|
45
|
60
|
—
|
| 3
|
3
|
45
|
—
|
—
|
| 1
|
4
|
45
|
60
|
—
|
| 2
|
4
|
45
|
—
|
—
|
12.4.3 Where
the shaft is supported additionally by a centre bearing, the diameter
is to be evaluated from the half shaft between the inner edges of
the centre and outer main bearings. The diameter so found for the
half shaft is to be increased by six per cent for the full length
shaft diameter.
12.4.4 The
dimensions of crankwebs are to be such that Bt
2 is
to be not less than given by the following formulae:
0,4d
3 for the web adjacent to the bearing
0,75d
3 for intermediate webs
|
|
= |
where: |
|
B
|
= |
breadth
of web, in mm |
|
d
|
= |
minimum
diameter of crankshaft as required by Vol 2, Pt 7, Ch 3, 12.4 Design and construction 12.4.2, in mm
|
|
t
|
= |
axial
thickness of web, which is to be not less than 0,45d for
the web adjacent to the bearing, or 0,60d for intermediate
webs, in mm.
|
12.4.5 Fillets
at the junction of crankwebs with crankpins or journals are to be
machined to a radius not less than 0,05d. Smaller fillets,
but of a radius not less than 0,025d, may be used, provided
the diameter of the crankpin or journal is not less than cd:
12.4.6 Fillets
and oil holes are to be rounded to an even contour and smooth finish.
12.4.7 An
oil level sight glass or oil level indicator is to be fitted to the
crankcase.
12.4.8 The
crankcases of compressors are to be designed to withstand a pressure
equal to the maximum working pressure of the system.
12.4.10 The
cooler dimensions for sea-water cooled stage air coolers are to be
based on an inlet temperature of not less than 32°C. Where fresh
water cooling is used, the cooling water inlet temperature is not
to be greater than 40°C.
12.4.11 The
cooler dimensions for air cooled stage air coolers are to be based
on an air temperature of not less than 45°C.
12.4.12 The
piping to and from the air compressor is to be arranged to prevent
condensation from entering the cylinders.
12.5 Testing
12.6 Safety arrangements and monitoring
12.6.1 Air
compressors are to be arranged and located so as to minimise the intake
of air contaminated by oil or water.
12.6.2 Where
one compressor stage comprises several cylinders which can be shut
off individually, each cylinder shall be equipped with a safety valve
and a pressure gauge.
12.6.3 After
the final stage, all air compressors are to be equipped with a water
trap and after cooler. The water traps, after coolers and the compressed
air spaces between the stages are to be provided with discharge devices
at their lowest points.
12.6.4 Each
compressor stage shall be fitted with a suitable pressure gauge, the
scale of which must indicate the relevant maximum permissible working
pressure.
12.7 Crankcase relief valves
12.7.1 In
compressors having cylinders not exceeding 200 mm bore or having a
crankcase gross volume not exceeding 0,6 m3, crankcase
relief valves may be omitted.
12.7.2 Crankcases
are to be provided with lightweight spring-loaded valves or other
quick-acting and self-closing devices to relieve the crankcases of
pressure in the event of an internal explosion and to prevent any
inrush of air thereafter. The valves are to be designed and constructed
to open quickly and be fully open at a pressure not greater than 0,2
bar.
12.7.3 The
valve lids are to be made of ductile material capable of withstanding
the shock of contact with stoppers at the full open position.
12.7.4 Each
valve is to be fitted with a flame arrester that permits flow for
crankcase pressure relief and prevents the passage of flame following
a crankcase explosion.
12.7.5 The
valves are to be provided with a copy of the manufacturer’s
installation and maintenance manual for the size and type of valve
being supplied. The manual is to contain the following information:
-
Description of
valve with details of function and design limits;
-
Copy of type
test certification;
-
Installation
instructions;
-
Maintenance and
in-service instructions to include testing and renewal of any sealing
arrangements;
-
Actions required
after a crankcase explosion.
12.7.8 The
valves are to be provided with suitable markings that include the
following information:
-
Name and address
of manufacturer;
-
Designation and
size;
-
Month/Year of
manufacture;
-
Approved installation
orientation.
12.8 Number of crankcase relief valves
12.8.1 In
compressors having cylinders exceeding 200 mm but not exceeding 250
mm bore, at least two relief valves are to be fitted. Where more than
one relief valve is required, the valves are to be located at or near
the ends of the crankcase.
12.8.2 In compressors having cylinders exceeding 250 mm but not exceeding 300 mm
bore, at least one relief valve is to be fitted in way of each alternate crankthrow with
a minimum of two valves. For compressors having 3, 5, 7, 9, etc. crankthrows, the number
of relief valves is not to be less than 2, 3, 4, 5, etc. respectively.
12.8.3 In compressors having cylinders exceeding 300 mm bore, at least one valve is
to be fitted in way of each main crankthrow.
12.8.4 Additional
relief valves are to be fitted for separate spaces on the crankcase,
such as gear or chain cases, when the gross volume of such spaces
exceeds 0,6 m3.
12.9 Size of crankcase relief valves
12.9.1 The
combined free area of the crankcase relief valves fitted on a compressor
is to be not less than 115 cm2/m3 based on the
volume of the crankcase.
12.9.2 The
free area of each relief valve is to be not less than 45 cm2.
12.9.3 The
free area of the relief valve is the minimum flow area at any section
through the valve when the valve is fully open.
12.9.4 In
determining the volume of the crankcase for the purpose of calculating
the combined free area of the crankcase relief valves, the volume
of the stationary parts within the crankcase may be deducted from
the total internal volume of the crankcase.
12.10 Vent pipes
12.10.1 Where
crankcase vent or breather pipes are fitted, they are to be made as
small as practicable and/or as long as possible to minimise the inrush
of air after an explosion.
12.11 Dead ship condition starting arrangements
12.11.1 Means
are to be provided to ensure that machinery can be brought into operation
from the dead ship condition without external aid.
12.11.2 Dead
ship condition for the purpose of Vol 2, Pt 7, Ch 3, 12.11 Dead ship condition starting arrangements 12.11.1 is to be understood to mean a condition under which the main
propulsion plant, boilers and auxiliaries are not in operation. In
restoring propulsion, no stored energy for starting and operating
the propulsion plant is assumed to be available. Additionally, neither
the main source of electrical power nor other essential auxiliaries
is assumed to be available for starting and operating the propulsion
plant.
12.11.4 Where
the emergency source of power is an emergency generator which fully
complies with the requirements of Vol 2, Pt 9, Ch 2 Electrical Power Generator and Energy Storage of the Rules, this generator may be used for restoring
operation of the main propulsion plant, boilers and auxiliaries where
any power supplies necessary for engine operation are also protected
to a similar level as the starting arrangements.
12.11.5 Where there is no emergency generator installed or an emergency generator
does not comply with Vol 2, Pt 9, Ch 2 Electrical Power Generator and Energy Storage of the Rules, the arrangements for bringing main and
auxiliary machinery into operation are to be such that the initial charge of starting
air or initial electrical power and any power supplies for engine operation can be
developed on board ship without external aid. If for this purpose an emergency air
compressor or an electric generator is required, these units are to be powered by a
hand-starting engine or a hand-operated compressor. The arrangements for bringing main
and auxiliary machinery into operation are to have capacity such that the starting
energy and any power supplies for engine operation are available within 30 minutes of a
dead ship condition.
12.12 Air receivers
12.12.1 Where
the main engine is arranged for air starting, the total air receiver
capacity is to be sufficient to provide without replenishment, not
less than 12 consecutive starts of the main engine, alternating between
ahead and astern if of the reversible type and not less than six consecutive
starts if of the non-reversible type. At least two air receivers of
approximately equal capacity are to be provided. For scantlings and
fittings of air receivers, see
Vol 2, Pt 8, Ch 2 Other Pressure Vessels.
12.12.2 For
multi-engine installations, where more than one engine is driving
each propulsion shaft line, the following requirements apply:
-
Twin engine
installations driving fixed pitch propeller, where one of the engines
can be reversed, six consecutive starts per engine are required.
-
For all other
types of multi-engine installations three consecutive starts per engine
are required.
12.12.3 Each
air receiver is to be fitted with a drain arrangement at its lowest
part, permitting oil and water to be blown out.
12.12.4 Each
receiver which can be isolated from a relief valve is to be provided
with a suitable fusible plug to discharge the contents in case of
fire. The melting point of the fusible plug is to be approximately
150°C.
12.12.5 Receivers
used for the storage of air for the control of remotely operated valves
are to be fitted with relief valves and not fusible plugs.
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