MAN B&W S65ME-C8.2-GI-TII, Project Manual

Page: 358 / 395

MAN B&W
Page 1 of 2
MAN Diesel
18.08
MAN B&W ME-GI TII engines 198 50 60-7.3
ME-GI Safety Aspects
The normal safety systems incorporated in the
fuel oil systems are fully retained also during dual
fuel operation. However, additional safety devices
will be incorporated in order to prevent situations
which might otherwise lead to failures.
Safety Devices – External systems
Leaking valves and fractured pipes are sources
of faults that may be harmful. Such faults can be
easily and quickly detected by a hydrocarbon (HC)
analyser with an alarm function. An alarm is given
at a gas concentration of max. 30% of the Lower
Explosion Limit (LEL) in the vented duct, and a
shut down signal is given at 60% LEL.
The safety devices that will virtually eliminate such
risks are double-wall pipes and encapsulated
valves with ventilation of the intervening space.
The ventilation between the outer and inner walls
is always to be in operation when there is gas in
the supply line, and any gas leakage will be led
to the HC sensors placed in the outer pipe dis-
charge.
Another source of fault could be a malfunction-
ing sealing oil supply system. If the gas sealing oil
differential pressure becomes too low in the gas
injection valve, gas will flow into the control oil
activation system and, thereby, create unintended
mixing of gas in the hydraulic oil system and cre-
ate gas pockets and prevent the ELGI valve from
operating the gas injection valve. Therefore, the
sealing oil pressure is measured by a set of pres-
sure sensors, and in the event of a too low pres-
sure, the engine will shut down the gas mode and
start running in the fuel oil mode.
Lack of ventilation in the double-wall piping
system prevents the safety function of the HC
sensors, so the system is to be equipped with
a set of flow switches. If the switches indicate
no flow, the engine will be shut down on gas
mode. The switches should be of the normally
open (NO) type, in order to allow detection of a
malfunctioning switch, even in case of an electric
power failure.
As natural gas is lighter than air, non-return valves
are incorporated in the gas system’s outlet pipes
to ensure that the gas system is not polluted, i.e.
mixed with air.
Safety Devices – Internal systems
During normal operation, a malfunction in the pilot
fuel injection system or gas injection system may
involve a risk of uncontrolled combustion in the
engine.
Sources of faults are:
• defective gas injection valves
• failing ignition of injected gas.
These aspects will be discussed in detail in the
following together with the suitable countermeas-
ures.
Defective gas injection valves
In case of sluggish operation or even seizure of
the gas valve spindle in the open position, which
will be detected by the gas pressure transmitter
in the gas block, a limited amount of gas may be
injected into the cylinder before the window valve
closes. Therefore, when the exhaust valve opens,
a hot mixture of combustion products and gas
flows out and into the exhaust pipe and further on
to the exhaust receiver.
The temperature of the mixture after the valve
will increase considerably, and it is likely that the
gas will burn with a diffusion type flame (without
exploding) immediately after the valve where it
is mixed with scavenge air/exhaust gas (with ap-
prox. 15 per cent oxygen) in the exhaust system.
This may set off the high exhaust gas temperature
alarm for the cylinder in question.