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 tightly closed.
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.