Operational Information

Air Start Explosions

 

 

 

Air Start explosions occur during a start sequence, when oil, which can accumulate in the air start receivers or on the surface of the start air lines, becomes entrained with high pressure air in the air start manifold and is ignited. The most infamous incident happened onboard the Capetown Castle in 1960 which killed 7 men.

 

In 1999, a large container ship, built in 1981 and fitted with a large bore two-stroke engine,  suffered damage when the starting air manifold was blown apart by an internal explosion. This occurred during manoeuvring when berthing. Fortunately there were no casualties.

Reference to Lloyds Register database has shown that this was not an isolated incident – between 1987 and 1999, 11 incidents of explosions in air start systems have been reported and most have been attributable to unsatisfactory shipboard practices by ships staff,  resulting in the presence of oil or explosive vapour in the manifold.

The source of ignition for these explosions can be attributed to one of the following:

  • A leaking air start valve. Whilst the engine is running, the hot gases produced as the fuel burns in the cylinder (at above 1200°C) leak past a valve which has not re-seated correctly. The branch pipe to the air start manifold heats up to red heat. If the engine is stopped and restarted before the pipe has time to cool, any oil vapour in the air can be ignited and an explosion can result if the mixture of oil/air is correct.

  • Fuel leaking into the cylinder whilst the engine is stopped. When the engine then undergoes a start sequence, and builds up speed, the fuel which has leaked into the cylinder vaporises and the heat from the compression of the air in the cylinder, as the piston rises, ignites the fuel. When the air start valve opens as the piston comes over TDC, the pressure in the cylinder is higher than the air start pressure, and the burning combustion gases pass to the air start manifold, igniting the oil entrained in the air.

  • A recent theory by ClassNK has concluded that the principal cause of explosions in starting air manifolds of marine engines is probably the auto ignition of oil deposited on the inner surface of the manifold, not backfire from cylinders as previously thought. Auto-ignition conditions occur because of the high temperature generated by the rapid inflow of high-pressure air, says the research. This incoming air compresses air downstream of the main starting valve, causing its temperature to reach as high as 400°C which in some cases causes oil deposits in the manifold to self-ignite leading to an explosion. ClassNK has adapted its safety requirements for a starting system to account for the findings. It now requires the fitting of rupture discs to the manifold on engines with a flame arrester in each branch pipe leading to the cylinders. This is beyond IACS unified requirements, which account for cylinder backfire as the cause of starting air manifold explosions.

 

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To minimise the risk of explosions, the oil carry over from the compressor should be reduced to a minimum. Class regulations require that the air compressor’s air intakes are located in an oil-free atmosphere, and a drain/filter for intercepting oil/water mist is fitted between compressor discharge and air receiver. There must be complete separation of compressor discharge and starting air supply to engines at the receiver which is fitted with a drain and a relief valve.

The air start system must be protected with a non return valve at the starting air supply to each engine. This is normally part of the automatic valve which opens when an air start is initiated.

In addition to this IACS require that:

For direct reversing main engines >230 mm bore flame arresters or bursting discs are required for each cylinder fitted between the cylinder start air valve and the manifold.

For non-reversing and auxiliary engines >230 mm bore a single flame arrester or bursting disc is acceptable fitted at the supply inlet to the starting air manifold.

Although not part of IACS regulations, a relief valve may be fitted to the manifold where flame arrestors are used instead of bursting discs.

Unsatisfactory practices which have led to explosions in the air start system include:

  • ‘Tell tales/drains’ at each end of the starting air manifold found to have been blanked off with screwed plugs.

  • Failure to drain starting air receivers and starting air pipes at regular intervals or before manoeuvring.

  • Failure to check for leaking air start valves.

  • Failure to maintain starting air valves and systems strictly in accordance with manufacturers recommended practices.

  • Failure to maintain fuel valves correctly.

SAFETY DEVICES

Flame Arrestors

 

Flame Arrestor MAN B&W L58/64

Flame Arrestor Sulzer RTA

The flame trap is manufactured from brass or aluminium alloy which both have a high specific heat capacity. A number of holes are bored through the thick circular form to allow the air to pass through. They are fitted in the main air line immediately before the air start valve to restrict the risk of a flame in the cylinder propagating back to the main air start manifold, by dissipating the heat energy in the flame.

 

Bursting Disks

 

Bursting Disc MAN B&W MC

The safety cap consists of a bursting disk enclosed by a perforated cylinder and a perforated cover in order to protect any bystanders, in the event of a burst. The cover is fitted with a tell tale, which shows if the bursting disc has been damaged. If the bursting disc of the safety cap is damaged due to excessive pressure in the starting air line, overhaul or replace the starting valve which caused the burst, and mount a new disk

 

If a new disk is not available, or cannot be fitted immediately, then the cover can be turned in relation to the perforated cylinder, in order to reduce the leakage of starting air.

 

Relief Valve

 

The sketch shows a relief valve as fitted to the air start manifold of Sulzer RTA 2 stroke engines.  Its purpose is to relieve excess pressure in the air start manifold. It consists of a spring loaded valve disk which locates on a mating seat which is bolted to the end of the air start manifold. When the force exerted on the disk due to excessive pressure is greater than the spring force holding the valve closed, the valve will open.

 

In members section: Complete Air Start System with position of all safety devices.

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