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MAN B&W Diesel A/S 18.08
valve where it <strong>is</strong> mixed with scavenge air/exhaust
gas (with approx. 15 per cent oxygen) in the exhaust
system. <strong>Th<strong>is</strong></strong> will set off the high exhaust
gas temperature alarm for the cylinder in question.
In the unlikely event of larger gas amounts entering
the exhaust receiver without starting to burn
immediately, a later ignition may result in violent
burning and a corresponding pressure r<strong>is</strong>e. Therefore,
the exhaust receiver <strong>is</strong> designed for the
maximum pressure (around 15 bars).
However, any of the above-mentioned situations
will be prevented by the detection of defective
gas valves, which are arranged as follows:
The gas flow to each cylinder during one cycle will
be detected by measuring the pressure drop in
the accumulator. <strong>Th<strong>is</strong></strong> <strong>is</strong> to ensure that the injected
gas amount does not exceed the amount corresponding
to the MCR value.
It <strong>is</strong> necessary to ensure that the pressure in the
accumulator <strong>is</strong> sufficient for gas operation, so the
accumulator will be equipped with a pressure
switch and a differential pressure switch.
An increase of the gas flow to the cylinder which
<strong>is</strong> greater than corresponding to the actual load,
but smaller than corresponding to the MCR value,
will only give r<strong>is</strong>e to the above-mentioned exhaust
gas temperature alarm, and <strong>is</strong> not harmful. By th<strong>is</strong>
system, any abnormal gas flow, whether due to
seized gas injection valves or fractured gas pipes,
will be detected immediately, and the gas supply
will be d<strong>is</strong>continued and the gas lines purged with
inert gas.
In the case of slightly leaking gas valves, the
amount of gas injected into the cylinder concerned
will increase. <strong>Th<strong>is</strong></strong> will be detected when
the exhaust gas temperature increases. Burning in
the exhaust receiver will not occur in th<strong>is</strong> situation
due to the lean mixture.
Ignition failure of injected gas
Page 2 of 3
Failing ignition of the injected natural gas can
have a number of different causes, most of which,
however, are the result of failure to inject pilot oil
in a cylinder:
• Leaky joints or fractured high-pressure pipes,
making the fuel oil booster inoperative.
• Seized plunger in the fuel oil booster.
• Other faults on the engine, forcing the fuel oil
booster to ‘O-index’.
• Failing pilot oil supply to the engine.
Any such faults will be detected so quickly that
the gas injection <strong>is</strong> stopped immediately from the
first failure to inject the pilot oil.
In extremely rare cases, pilot fuel can be injected
without being ignited, namely in the case of a
sticking or severely burned exhaust valve. <strong>Th<strong>is</strong></strong>
may involve such large leakages that the compression
pressure will not be sufficient to ensure
ignition of the pilot oil. Consequently, gas and pilot
fuel from that cylinder will be supplied to the
exhaust gas receiver in a fully unburned condition,
which might result in violent burning in the receiver.
However, burning of an exhaust valve <strong>is</strong> a
rather slow process extending over a long period,
during which the exhaust gas temperature r<strong>is</strong>es
and gives an alarm well in advance of any situation
leading to r<strong>is</strong>k of m<strong>is</strong>firing.
A seized spindle in the pilot oil valve <strong>is</strong> another
very rare fault, which might influence the safety of
the engine in dual fuel operation. However, the still
operating valve will inject pilot oil, which will ignite
the corresponding gas injection, and also the gas
injected by the other gas valve, but knocking cannot
be ruled out in th<strong>is</strong> case. The cylinder pressure
monitoring system will detect th<strong>is</strong> condition.
As will appear from the above d<strong>is</strong>cussion, which
has included a number of very unlikely faults, it <strong>is</strong>
possible to safeguard the engine installation and
personnel and, when taking the proper countermeasures,
a most sat<strong>is</strong>factory service reliability
and safety margin <strong>is</strong> obtained.
ME-GI engines 198 50 60-7.0