151.Starting air line explosion

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 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