Manifold Fires and Explosions
Explosions and fires in the exhaust
manifold of large two stroke engines can occur if cylinder oil
or unburnt fuel passes to the manifold an subsequently ignites.
LOW LOAD RUNNING
With some 2
stroke engines, if the engine is running at loads below
75% for periods in excess of 4 hours then the engineer
has to reduce the cylinder oil consumption manually.
If the engine
runs on reduced load without reduction of the cylinder
oil consumption, a lot of the excessive cylinder oil can
end up in the exhaust manifold, so that when the load is
increased, the rise in temperature in the exhaust
channel will ignite the cylinder oil. The heat energy
released from combustion can result in extreme overspeed
on the turbochargers, which very often end up with
serious damage and total breakdown of the turbochargers
(damaged rotors, nozzle rings, compressor and exhaust
wheels, housing, etc.).
Choked scavenge ports
will result in poor scavenging of the cylinder, with partially
burnt or unburnt fuel passing into the exhaust manifold. Poor
atomisation from injectors may have the same result. A scavenge
fire could cause depletion of oxygen in the inlet air to the
affected cylinder, and in turn cause incomplete combustion of
the fuel injected into the affected cylinder. This unburnt fuel
would then pass into the exhaust and mix with oxygen from the
scavenging air from other cylinders. Combustion
of this fuel would then be possible. If a fire, or sufficient
after burning occurs within the exhaust manifold, the manifold
becomes a high volume combustion chamber with the fire rapidly
heating and expanding the hot gases exhausted from the cylinders
before they enter the turbocharger. The energy from even a small
amount of unburnt fuel when added to the exhaust gas flow
entering the turbocharger would be sufficient to cause a
significant energy imbalance, leading to uncontrolled
acceleration of the turbocharger rotor. This can lead to
disintegration of the compressor wheel, loss of blading due to
high centrifugal forces on the turbine wheel and subsequent
bursting of the casing.
Examples of Fires in exhaust manifolds leading to turbocharger
MV Marine Express, 24 August 2000
- Combustible gas between the cylinder cover and piston
crown flowed out through deep scratches in the surface of
the No. 8 cylinder liner. [There were two such scratches
vertically from the lubricating holes to the scavenging
ports. One piston ring was broken and the others worn.]
- There was a fire in the scavenging space.
- The fire consumed the air in the scavenging space.
- Lack of air made fuel combustion worse.
- The main engine governor activated to increase
revolutions (to maintain a certain range).
- More fuel was supplied to the combustion chamber.
- Non-combusted/rich fuel gas discharged to exhaust gas
- There was an explosion in exhaust gas manifold due to
- The turbochargers over-sped.
- The turbo charger impeller touched the air casing and
the turbine blades were torn off.
- The bearing around the rotor shaft was stuck.
MV Goliath, 22 September
2002 and 12 February 2003
The similarity of the
compressor disc failures supports the conclusion that both
turbochargers underwent an uncontrolled transient overspeed
event similar to the second failure.
While it is not
possible to state with certainty, the two possible
mechanisms which led to the turbocharger failure were a
slight slip of the compressor disc or a scavenge fire but
was more likely to have been a scavenge fire based on the