Tuesday, 12 June 2012

Air Start Systems

Air Start Systems
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Regulations

    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 tow 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.
Air Receivers
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 reciever than a fusible plug or plugs must be fited. 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 rpecipitate. Valves to be of the slow opening type to prevent excessive pressure rises. All attachments should be via a support plate opening into a bottle
Safety devices
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 encorporated in the air start manifold in order to dissapate the energy of an explosion thus keeping its effects local.

Such devices include flame traps, relief valves and bursting discs Bursting disc 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 dscharge 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 compresors should be kept to a minmum and pipelines inspected nad 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 manouervring control valve.
Starting air valve.
Starting air valve

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