Section
13 Air compressors
13.1 General requirements
13.1.1 The
requirements of this Section are applicable to reciprocating air compressors
intended for starting main engines and auxiliary engines providing
essential services.
13.1.2 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 1 hour from atmospheric pressure, to the
pressure sufficient for the number of starts required by Pt 5, Ch 2, 9.2 Air receiver capacity. 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 Pt 5, Ch 2, 9.1 Dead ship condition starting arrangements is to be ignored.
13.1.3 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.
13.1.4 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. It is recommended that compressors be cooled
by fresh water.
13.1.5 Each compressor is to be fitted with an alarm for failure of the lubricating oil supply
which will initiate an automatic shutdown.
13.2 Plans and particulars
13.2.1 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.
13.3 Materials
13.3.1 The specified minimum tensile strength of castings and forgings for
compressor crankshafts are to be within the limits given in Pt 5, Ch 2, 2.1 Crankshaft Materials 2.1.1 and for grey cast iron to be not less than 300 N/mm2.
13.3.2 Where
it is proposed to use materials outside the ranges specified in Pt 5, Ch 2, 13.3 Materials 13.3.1, details of the chemical
composition, heat treatment and mechanical properties are to be submitted
for approval.
13.4 Design and Construction
13.4.2 The diameter, dp, 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 MPa g, as defined in Pt 5, Ch 12, 1.3 Design symbols 1.3.1
|
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
|
σ
u
|
= |
specified minimum tensile strength of crankshaft material, in
N/mm2.
|
Table 2.13.1 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
|
—
|
—
|
13.4.3 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.
13.4.4 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 Pt 5, Ch 2, 13.4 Design and Construction 13.4.2, 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.
|
13.4.5 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,
13.4.6 Fillets
and oil holes are to be rounded to an even contour and smooth finish.
13.4.7 An
oil level sight glass or oil level indicator is to be fitted to the
crankcase.
13.4.8 The
crankcases of compressors are to be designed to withstand a pressure
equal to the maximum working pressure of the system.
13.4.9 Compressors
with shaft power exceeding 500 kW are to have torsional vibration
analysis determined in accordance with Pt 5, Ch 8, 2 Torsional vibration as applicable.
13.4.10 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.
13.4.11 The
cooler dimensions for air cooled stage air coolers are to be based
on an air temperature of not less than 45°C.
13.4.12 The
piping to and from the air compressor is to be arranged to prevent
condensation from entering the cylinders.
13.5 Testing
13.5.2 Cylinders, covers and liners of air compressors are to be subjected to hydraulic
pressure tests at 1,5 times the final pressure of the stage concerned.
13.5.3 The compressed air chambers of the intercoolers and aftercoolers of air compressors are
to be subjected to hydraulic pressure tests at 1,5 times the final pressure of the stage
concerned.
13.5.4 Manufacturer’s certification for materials and pressure testing will be accepted for air
compressors with a calculated crankpin diameter of less than 50 mm.
13.5.5 After construction, all compressors are to be subjected to a running test to the
satisfaction of the attending Surveyor.
13.6 Safety arrangements and monitoring
13.6.1 Air
compressors are to be arranged and located so as to minimise the intake
of air contaminated by oil or water.
13.6.2 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.
13.6.3 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.
13.6.4 Each
compressor stage shall be fitted with a suitable pressure gauge, the
scale of which must indicate the relevant maximum permissible working
pressure.
13.7 Crankcase relief valves
13.7.1 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.
13.7.2 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 0,02 MPa.
13.7.3 The
valve lids are to be made of ductile material capable of withstanding
the shock of contact with stoppers at the full open position.
13.7.4 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.
13.7.5 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.
13.7.8 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.
13.8 Number of crankcase relief valves
13.8.1 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.
13.8.2 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.
13.8.3 In compressors having cylinders exceeding 300 mm bore, at least one valve
is to be fitted in way of each main crankthrow.
13.8.4 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.
13.9 Size of crankcase relief valves
13.9.1 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.
13.9.2 The
free area of each relief valve is to be not less than 45 cm2.
13.9.3 The
free area of the relief valve is the minimum flow area at any section
through the valve when the valve is fully open.
13.9.4 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.
13.10 Vent pipes
13.10.1 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.
|