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
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.
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
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
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
In addition to this IACS require that:
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.
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
practices which have led to explosions in the air start system
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.
Flame Arrestor MAN B&W L58/64
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.
MAN B&W MC
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
cylinder, in order to reduce the leakage of starting air.
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
In members section:
Complete Air Start System with position of all safety devices.