3.9.1 The flue gas uptake point should be so selected
that the gas is not too hot for the scrubber, nor causes hard deposits
on the flue gas isolating valves. It should not be so close to the
uptake outlet that air can be drawn into the system. When boilers
are fitted with rotary air heaters, the offtake point should be before
the air heater inlet.
3.9.2 The materials used for flue gas isolating
valves should take into account the temperature of gas at the offtake.
Cast iron is acceptable for temperatures below 220°C. Valves exposed
to a temperature exceeding 220°C should be made from a material
not only compatible with the temperature but also resistant to the
corrosive effect of stagnant flue gases.
3.9.3 Flue gas isolating valves should be provided
with facilities to keep the seatings clear of soot unless the valve
is designed to close with a seat cleaning action. Flue gas isolating
valves should also be provided with air sealing arrangements.
3.9.4 If expansion bellows are considered necessary
they should have a smooth internal sleeve and preferably be mounted
so that the gas flow through them is vertical. They should be constructed
of material resistant to stagnant damp acidic soot.
3.9.5 The pipework between the flue gas isolating
valve and the scrubber should be made from heavy gauge steel resistant
to corrosion and so arranged as to prevent the accumulation of damp
acidic soot by the avoidance of unnecessary bends and branches.
3.9.6 The inlet piping to the scrubber should
be so arranged as to permit positive isolation from the flue gases
prior to gas-freeing the scrubber for entry for maintenance purposes.
This may be accomplished by the removal of a suitable length of pipe
section and blanking, by spectacle flanges or by a water seal which
would prevent any leakage of gas from the boiler when the flue gas
isolating valve is shut and the scrubber is opened up for inspection
and maintenance. In the event that the drainage of the water seal
is itself required for inspection purposes, then isolation should
be achieved either by removal of the suitable lengths of pipe sections
and blanking, or by the use of spectacle flanges.
3.9.7 The gas outlet piping from the scrubber
to the blowers and recirculating lines should be made from steel suitably
coated internally.
3.9.8 Suitable isolating arrangements should be
incorporated in the inlet and outlet of each blower to permit safe
overhaul and maintenance of a blower while permitting the use of the
inert gas system using the other blower.
3.9.9 The regulating valve required by regulation 62.9.1 should be provided with
means to indicate whether the valve is open or shut. Where the valve
is used to regulate the flow of inert gas it should be controlled
by the inert gas pressure sensed between the deck isolating valve
and the cargo tanks.
3.9.10 Deck lines should be of steel and be so
arranged as to be self draining and should be firmly attached to the
ship's structure with suitable arrangements to take into account movement
due to heavy weather, thermal expansion and flexing of the ship.
3.9.11 The diameter of the inert gas main, valves
and branch pipes should take account of the system requirements detailed
in 3.5.9. To avoid excessive pressure drop,
the inert gas velocity should not exceed 40 m/s in any section of
the distribution system when the inert gas system is operating at
its maximum capacity. If the inert gas main is used for venting during
loading, other factors may need to be taken into consideration as
developed in regulation 58 of chapter II-2 of
the 1974 SOLAS Conventionfootnote for cargo tank
venting systems.
3.9.12 All pressure and vacuum relief openings
should be fitted with flame screens with easy access for cleaning
and renewal. The flame screens should be at the inlets and outlets
of any relief device and be of robust construction sufficient to withstand
the pressure of gas generated at maximum loading and during ballasting
operations while presenting minimum resistance.