5.32 MAINTAINING THE TANK ATMOSPHERE
Throughout the discharge, the inert gas plant must be in operation,
delivering IGS with an oxygen content of not more than 5% to the cargo
tanks. The cargo tanks should each have an oxygen content of 8% or less.
The inert gas main and cargo tanks must be maintained under positive
pressure of at least 200 mm water gauge.
Tank gauging should be accomplished using remote indicating ullaging
systems or vapour lock, portable ullaging devices.
The inert gas main pressure sensor should be calibrated before each
discharge and the low pressure alarm function tested. Despite the availability
of the low pressure alarm, the IGS system pressure should be visually
monitored at least hourly by both the engineering watch officer and
the cargo watch officer. If the IGS low pressure alarm sounds, immediate
action must be taken to prevent any air from being drawn into the tanks.
Cargo discharge must be stopped while the engineering staff attempts
to determine the cause of the IGS generator defect. The deck watch should
be assigned to close the ship's manifold valves and IGS deck isolating
valve. The deck watch should then trace the length of the IGS deck main,
verifying that all vent outlets, including the mast riser isolating
valve, are closed. They should also check that all tank openings are
Cargo discharge cannot be resumed until the correct operation of the
IGS system is restored.
IGS pressure may be increased to assist stripping of high vapour pressure
cargo, but should be returned to the normal level immediately after
stripping has been completed.
At the end of the discharge, a low, positive IGS pressure will permit
open inspection of the tanks for the ROB survey and will facilitate
draining of the manifold drip tray to a cargo tank.