Section 13 Air compressors

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 one hour from atmospheric pressure, to the pressure sufficient for the number of starts required by Pt 10, Ch 1, 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 10, Ch 1, 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^oC in service. A small fusible plug or an alarm device operating at 121^oC 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.

13.2.1 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.

13.2.2 Where compressors of a special type or design are proposed, the requirements of Pt 5, Ch 14 Machinery Piping Systems of the Rules and Regulations for the Classification of Ships, July 2022 are to be applied.

13.3.1 The specified minimum tensile strength of castings and forgings for compressor crankshafts are to be within the limits given in Pt 10, Ch 1, 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 10, Ch 1, 13.3 Materials 13.3.1, details of the chemical composition, heat treatment and mechanical properties are to be submitted for approval.

13.3.3 For compressors crankshafts with a calculated crank pin diameter equal to or greater than 50 mm, materials for components are to be manufactured and tested in accordance with the requirements of the LR Rules for Materials and Table 1.2.1 Summary of testing and associated documentation for engine components. For calculated crank pin diameters less than 50 mm, a manufacturers’ certificate may be accepted, seeCh 1, 3.1 General 3.1.3 of the Rules for the Manufacture, Testing and Certification of Materials, July 2022.

13.4.1 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 Pt 10, Ch 1, 13.4 Design and Construction 13.4.2Pt 10, Ch 1, 13.4 Design and Construction 13.4.2 to Pt 10, Ch 1, 13.4 Design and Construction 13.4.6 may be used.

13.4.2 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:

d

=

mm

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
dp	=	proposed minimum diameter of crankshaft, in mm
p	=	design pressure, in MPa g, as defined in Pt 15, Ch 1, 4.1 Design symbols 4.1.1
D	=	diameter of cylinder, in mm
S	=	length of stroke, in mm

Vc

=

1,0 for shafts having one cylinder per crank, or

for the shaft and cylinder arrangements as detailed in Table 1.13.1 Angle between cylinders.

Z	=	for steel
Z	=	for spheroidal or nodular graphite cast iron
Z	=	for grey cast iron
σu	=	specified minimum tensile strength of crankshaft material, in N/mm2.

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 Bt² is to be not less than given by the following formulae:

0,4d³ for the web adjacent to the bearing

0,75d³ for intermediate webs

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 13, Ch 1 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.

13.5.1 Cast Iron crankshafts for air compressors are to be manufactured and tested in accordance with the Rules for the Manufacture, Testing and Certification of Materials, July 2022, Ch 7, 5 Iron castings for crankshafts. Crankshafts for air compressors manufactured from other materials are to be tested in accordance with the applicable requirements of the Rules for the Manufacture, Testing and Certification of Materials, July 2022. Ultrasonic testing is not required for components manufactured from cast iron.

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.

13.6.1  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.

13.7.1 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.

13.7.6 A copy of the installation and maintenance manual required by Pt 10, Ch 1, 13.7 Crankcase relief valves 13.7.5 is to be provided on board the ship.

13.7.7 Plans showing details and arrangements of the crankcase relief valves are to be submitted for approval, see Pt 10, Ch 1, 1.4 Submission requirementsPt 10, Ch 1, 1.4 Submission requirements.

13.8.1 In compressors that have cylinders exceeding 200 mm but do not exceed 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 that have cylinders exceeding 250 mm but do not exceed 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 that have 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.

13.9.1 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.

13.10.1 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.