Wartsila ?31DF Series Product Manual page 97

Wärtsilä 31DF Product Guide
6.3.1.4 Gas valve unit (10N05)
Before the gas is supplied to the engine it passes through a Gas Valve Unit (GVU). The GVU
include a gas pressure control valve and a series of block and bleed valves to ensure reliable
and safe operation on gas.
The unit includes a manual shut-off valve, inerting connection, filter, fuel gas pressure control
valve, shut-off valves, ventilating valves, pressure transmitters/gauges, a gas temperature
transmitter and control cabinets.
The filter is a full flow unit preventing impurities from entering the engine fuel gas system. The
fineness of the filter is 5 μm absolute mesh size. The pressure drop over the filter is monitored
and an alarm is activated when pressure drop is above permitted value due to dirty filter.
The fuel gas pressure control valve adjusts the gas feed pressure to the engine according to
engine load. The pressure control valve is controlled by the engine control system. The system
is designed to get the correct fuel gas pressure to the engine common rail pipe at all times.
Readings from sensors on the GVU as well as opening and closing of valves on the gas valve
unit are electronically or electro-pneumatically controlled by the GVU control system. All
readings from sensors and valve statuses can be read from Local Display Unit (LDU). The LDU
is mounted on control cabinet of the GVU.
The two shut-off valves together with gas ventilating valve (between the shut-off valves) form
a double-block-and-bleed function. The block valves in the double-block-and-bleed function
effectively close off gas supply to the engine on request. The solenoid operated venting valve
in the double-block-and-bleed function will relief the pressure trapped between the block
valves after closing of the block valves. The block valves V03 and V05 and inert gas valve V07
are operated as fail-to-close, i.e. they will close on current failure. Venting valves V02 and V04
are fail-to-open, they will open on current failure. There is a connection for inerting the fuel
gas pipe with nitrogen, see figure "Gas valve unit P&I diagram". The inerting of the fuel gas
pipe before double block and bleed valves in the GVU is done from gas storage system. Gas
is blown downstream the fuel gas pipe and out via vent valve V02 on the GVU when inerting
from gas storage system.
During a stop sequence of DF-engine gas operation (i.e. upon gas trip, pilot trip, stop,
emergency stop or shutdown in gas operating mode, or transfer to diesel operating mode)
the GVU performs a gas shut-off and ventilation sequence. Both block valves (V03 and V05)
on the gas valve unit are closed and ventilation valve V04 between block valves is opened.
Additionally on emergency stop ventilation valve V02 will open and on certain alarm situations
the V07 will inert the gas pipe between GVU and the engine.
The gas valve unit will perform a leak test procedure before engine starts operating on gas.
This is a safety precaution to ensure the tightness of valves and the proper function of
components.
One GVU is required for each engine. The GVU has to be located close to the engine to ensure
engine response to transient conditions. The maximum length of fuel gas pipe between the
GVU and the engine gas inlet is 30 m.
Inert gas and compressed air are to be dry and clean. Inert gas pressure max 0.9 MPa (9 bar).
The requirements for compressed air quality are presented in chapter "Compressed air system".
DBAE248994
6. Fuel Oil System
6-13