3.10.1 Pressure control arrangements should be
fitted to fulfil two functions:
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.1 to prevent automatically any backflow of gas
in the event of either a failure of the inert gas blower, scrubber
pump, etc., or when the inert gas plant is operating correctly but
the deck water seal and mechanical nonreturn valve have failed and
the pressure of gas in the tank exceeds the blower discharge pressure,
e.g. during simultaneous stripping and ballasting operations;
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.2 to regulate the flow of inert gas to the inert
gas deck main.
3.10.2 A typical arrangement by
which 3.10.1.2 can be achieved is as follows:
Systems with automatic pressure control and a gas recirculating
line. These installations permit control of inert gas pressure in
the deck main without having to adjust the inert gas blower speed.
Gas not required in the cargo tanks is recirculated to the scrubber
or vented to atmosphere. Gas pressure regulating valves are fitted
in both the main and recirculating lines; one is controlled by a gas
pressure transmitter and regulator, while the other may be controlled
either in a similar manner or by a weight-operated valve. The pressure
transmitter is sited downstream of the deck isolating valve; this
enables a positive pressure to be maintained in the cargo tanks during
discharge. However, it does not necessarily ensure that the scrubber
is not overloaded during inerting and purging operations.
Alternative
methods which comply with regulation 62.9.1 may
be considered.