Information About Starting Air System

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Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. 

The starting air system usually has interlocks to prevent starting if everything is not in order. Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. Opening of the cylinder air start valve will admit compressed air into the cylinder. 

The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever. When the air start lever is operated, a pilot air supply enables the remote valve to open. 

Pilot air for the appropriate direction of operation is also supplied to an air distributor. This device is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. 

Read Also:Control And Safety Devices Of Engine

The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder. An interlock is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged. 

The remote operating valve prevents the return of air further compressed by the engine into the system. Lubricating oil from the compressor will under normal operation pass along the air lines and deposit on them. If a cylinder air starting valve leaks, hot gases would pass into the air pipes and ignite the lubricating oil. 

If starting air is supplied to the engine this would further feed the fire and could lead to an explosion in the pipelines. To prevent such an occurrence, cylinder starting valves should be properly maintained and the pipelines regularly drained. Also, oil discharged from compressors should be kept to a minimum, by careful maintenance.

In an attempt to reduce the effects of an explosion, flame traps, relief valves and bursting caps or discs are fitted to the pipelines. In addition, an isolating non-return valve (the automatic valve) is fitted to the system. 

The loss of cooling water from an air compressor could lead to an overheated air discharge and possibly an explosion in the pipelines leading to the air reservoir. A high-temperature alarm or a fusible plug which will melt is used to guard against this possibility.

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