5.16 PRESSURE/VACUUM VALVES AND IGS PRESSURE

The pressure/vacuum (P/V), valves must be properly set before discharging commences, or the inert gas system (IGS), supply to the tanks ensured to prevent under-pressure of the cargo tanks.

5.16.1 P/V valve setting
Tankers without IGS must manually lift the relief valve weights, using the handwheel or lever, to provide for free flow of air into the tanks.
If the valves are not lifted for discharging, a cargo tank may be under-pressured, causing structural collapse of the deck. If a P/V valve is left closed when pumping begins, it will 'chatter' as it attempts to relieve the vacuum created in the tank by the removal of cargo. The deck watch should be instructed to be alert for this sound and to call for a pump stop if they hear it.
Immediately after a tank has finished stripping, the P/V valve should be reset to the normal automatic position.

5.16.2 Inert gas system
On tankers with an IGS system, all cargo tanks should be fully inerted before discharging begins. The engine room must be advised of the approximate time cargo pumping is expected to begin. It is then the watch engineer's job to start up the IGS system in good time to have a full supply of inert gas available when the pumps are started. All IG system alarm sensors should be verified at regular intervals as part of the vessel's preventive maintenance programme. Before discharge begins, ensure that:
• All cargo and slop tanks are connected to the IGS main, with tank isolating valves locked open.
• All other cargo and slop tank openings are closed.
• The IGS main is isolated from the atmosphere; vent valve and mast riser are closed.
• The deck isolating valve is open.
• The IGS plant is ready to produce inert gas with 5% or less oxygen.

After discharging begins, the inert gas system must be operated to maintain a positive gas pressure on all cargo tanks. In some cases, due to defects in the IGS or exceptionally high discharging rates, the IGS main deck pressure will begin dropping. If this occurs, the cargo watch officer should check with the engineer to verify that the IGS is operating at full capacity. If the engineer confirms that the IGS is operating at best possible efficiency, then the cargo pumping rate must be reduced so that the IGS can maintain a positive gas pressure of at least 200 mm water gauge on all cargo tanks while discharging.
The inert gas supplied to the tanks must contain less than 5% oxygen and the cargo tanks must be maintained at less than 8% oxygen to ensure that the there is no possibility of ignition of the hydrocarbon vapours in the tank. Dedicated IGS generators are capable of supplying gas with an oxygen content of between 1.5% and 2.5%. Boiler flue gas IGS installations provide IG with 2% - 4% oxygen, along with 77% - 80% nitrogen, 12% - 14% carbon dioxide, 5% water and traces of nitrogen oxides, sulphur dioxide and carbon monoxide. IGS processed from boiler exhaust gasses varies in quality with the boiler load. When several cargo pumps are operating, boiler load will be high and the inert gas quality will be good.
Both the IGS pressure and oxygen content must be continuously recorded during the cargo discharge. On a crude oil carrier, if the IGS plant fails during the discharge, the pumps must be stopped and discharge of cargo suspended until the IGS service is restored or a substitute source of inert gas obtained. This is a critical precaution on tankers whose cargo is a crude oil with high hydrogen sulphide content. The hydrogen sulphide will react with rusted steel surfaces in the tanks to form pyrophoric iron sulphide deposits on the steel. If these deposits are exposed to air, they may heat spontaneously to incandescence. Thus the urgency of maintaining positive IGS pressure. The IGS system will automatically shut down in the event of:
• Boiler failure.
• Combustion control failure.
• Low water pressure or low flow at the scrubber unit.
• High water level in the scrubber unit.
• High gas temperature.

On a product tanker whose IG plant has failed, offloading may resume after all participants agree that it is safe to do so. If discharge is resumed, is must be with strict observance of static electricity precautions. Valves to the vent mast risers must be opened and all flame screens checked to verify that they are intact.
The liquid levels in the deck water seal and the P/V breaker and the proper function of the IGS deck non-return valve should be verified before discharge begins and regularly during discharge. Log all inspection time and results.
IG generated from boiler flue gas can have a considerable static electric charge. With the exception of a rubber gas test unit sampling hose, no sounding, ullaging, or sampling equipment should be lowered into the tank during inerting, or for three hours after inerting breakdown or completion. After three hours, properly grounded probes can be introduced into the tanks.

5.16.3 IGS tank block valves
Most tankers with IGS are provided with valves or blinds which can be used to shut off IGS flow to the individual tanks. Each of these valves must be verified to be open before discharging commences. If the vessel is CCR operated, there may be no remote indication of the position of the IGS branch line valves in the CCR. In that case, the cargo officer must confirm the correct position of these valves personally before taking over the watch. Before discharging a tank, the deck watch must confirm that the IGS block valve is open. This is particularly important on ships where the IG supply line is also part of the common tank venting system, ie. on ships where there are no individual tank P/V valves.