Air Distribution Systems
Air Start Systems
There must be at least two starting air receivers, the total capacity of which will give 12 starts for a reversing engine or 6 starts fo a non-reversing engine with CPP.
There must be at least two compressors
In addition to these there must be a compressor which can be started by hand i.e. with a dead ship. Note: this is not necessary if one of the compressors is run off the emergency switchboard
A relief valve must be fitted to the HP discharge and be sufficient size to ensure that the pressure rise does not exceed 10% of the w.p. when the compressor is running and the outlet valves on the bottle are shut.
A relief valve or bursting disc on the hp cooler casing in order to protect the casing from overpressure in the event of cooler tube failure Note: Bursting discs are generally preferred because they fail and stay failed giving complete protection. A relief valve will reseat when the compressor is stopped allowing water to enter the air side.
A drain must be fitted at each stage
Diesel start air system
The components of the air start system are taken to include compressors and storage bottles in addition to the engine air start arrangement. The minimum of two compressors should be matched to the starting air requirements of the engine. The compressor after coolers should be protected by a bursting disc. All high pressure lines in the system to be of solid drawn pipe.
There must be a means of access to allow cleaning and inspection of internals. The internal surface should be protected by a coating which is flexible enough to move when the metal distorts. Copal varnish is generally used because it has these properties and willnot easily oxidise. Usually precautions are taken the same as for an enclosed space when entering. Ventilation is required to the solvent fumes in the varnish
Drains must be fitted in the lowest part of the receiver
Receivers must be protected by means of a relief valve, if the relief valve can be isolated from the receiver than a fusible plug or plugs must be fitted. These are usually fitted because in the event of a fire near to the bottle they will fail and release the entire contents of the bottle rapidly. A relief valve however will only release air down to its closing pressure which is set point less blowdown. If the structure of the bottle becomes weakened by the heat then its ability to withstand even the reduced pressure is weakened an possible rupture could occur.
The inlet and outlet valves are to be arranged to prevent direct flow through the bottle with insufficient residence time for moisture to precipitate. Valves to be of the slow opening type to prevent excessive pressure rises. All attachments should be via a support plate
The automatic valve (Main air start block valve) prevents connection between the air receiver and air start manifold unless actually in the process of starting.
This minimises the risk of an explosion in the air manifold actually propagating back to the air receiver where a much more severe explosion is possible. Safety devices are incorporated in the air start manifold in order to dissipate the energy of an explosion thus keeping its effects local.
Such devices include flame traps, relief valves and bursting discs Loss of air can be kept to a minimum by rotating moveable outer hood to blank off relief ports. The failed cap should be replaced as soon as possible.
Starting air explosions
Causes-continuous leaking of start valve followed by it sticking open on start.
An oil film may build up on the start air pipe due to oil discharge from the compressor. This oil may come from general lubrication or sticky scraper ring or from the engine room air.
With a continuous leaking valve hot gasses with unburnt fuel will enter the pipe and turn the oil film into a hot incandescent carbon. When high pressure air is put on the pipe line an explosion can occur with resultant high speed high pressure shock wave.
Alternately, air discharged into cylinder during starting may have an oil mist which can ignite in a hot cylinder. The hot gasses can return through the start valve. To prevent this the non return valve should be properly maintained, oil discharge from the compressors should be kept to a minimum and pipelines inspected and cleaned when necessary.
To minimise effects a flame gauze should be fitted to the start valve and ample relief valves, bursting discs or caps fitted. An isolated valve on the discharge side of the manoeuvring control valve.
Starting air valve.
Starting Air Regulations
First start requirements
Equipment for starting the main and auxiliary engines is to be provided so that the necessary initial charge of starting air or initial electric power can be developed on board ship without external aid. If for this purpose an emergency air compressor or electric generator is required, these units are to be power driven by hand starting oil engine or steam engine, except in the case of small installations where a hand operated compressor of approved capacity may be accepted. Alternatively, other devices of approved type may be accepted as a means of providing the initial start
Air Compressor requirements
Air Compressor number and capacities
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 require 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 first start is to be ignored.
Maximum discharge air temperature
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 121АC is to be provided on each compressor to give warning of excessive air temperature. The emergency air compressor is excepted from these requirements.
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.
Air Receiver requirements
Air Receiver capacity
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
For multi-engine installations, the number of starts required for each engine will be specially considered.