Inert gas (IG) piping Fitted on all tankers over 20,000 dwt and on all tankers fitted with crude oil washing (COW) systems. IG piping is usually large diameter low-pressure mild steel, with smaller diameter branch lines. The internal surface of inert gas piping does not usually corrode. The external surface is painted but will corrode if the paint coating deteriorates.
Using inert gas system on board tankers required some careful consideration.Below guideline should be followed when operating the inert gas system.
During Operation Of Inert Gas System (IGS) : The oxygen content of the Inert Gas (hereinafter called IG) supplied to cargo tanks should be 5% or less. However, it is to be noted that too less content of oxygen in the IG would introduce other impurities into the cargo tanks.
During operation of the Inert Gas system (IGS) the automatic Recorder for Oxygen (O2) and IG Pressure on main line must be operational. The details of the start of operation (such as discharge at XXX port, date and time of mark, etc. should be noted on the recording).
Prior to start of the IGS, ensure safety confirmation as per the Operational Record of the GS. Inspection of the deck seal and PV breaker must be confirmed in good condition. Also, the status of alarm, indication and proper operation and sequence of related equipment must be observed.
Prior arrival discharge port, follow companys designated Tanker Discharging Checklist for IGS preparation and checks / tests. * For operation of the IGS and precautions, refer to IGS operation makers manual which is provided to each vessel.
Starting of Inert Gas System before entering discharge ports of environmentally sensitive nature, where the air pollution and scrubber discharge flushing could interfere with the ecosystem, the Inert Gas System shall be run before embarking the harbor pilot at time of entering the port. This is done so that the IGS plant can settle down & avoid dark funnel smoke emissions on start-up.
Cargo Discharging Operations
Operate and test the IGS before arrival at discharge port. Ensure that O2 level in all cargo oil tanks is less than 8% (preferably less than 5%) using Double Hull (D/H) Operational Record – Voyage record of COT Oxygen / H2S content. Also, the cargo tank pressure shall be reduced to minimum positive IG Pressure (pressure minimum 100 mmAq.)
Prior to starting of Discharge Operations, re-confirm all cargo Tank openings are tightly closed (eg Vapor locks used for tank gauging, etc) and cargo tank IG supply valves are correctly set, prior to starting IG on deck (ie opening IG main supply valve). The individual Cargo Tank IG valve key is to be kept within the control of the Chief Officer.
Monitor the cargo oil tank O2 level, Pressure parameters and Temperature throughout the cargo oil discharge operations.
During discharging, the monitoring of the cargo tanks not connected to the IGS shall be done carefully and more frequently. Watch the temperature.(in case of heated cargoes)
On completion of cargo discharge and before shutting down the IGS, increase the IG pressure in all oil cargo tanks sufficiently. (Allow for cooling down of temperature inside cargo tanks).
During Ballast Voyage
Maintain the cargo oil tank pressure in all cargo oil tanks at minimum 100 mmAq during the ballast voyage. Start IGS plant if required to raise tank pressure. However, if the temperature was raised sufficiently at discharge port, the above may not be generally necessary.
Be aware that during a sudden drop in the ambient temperature, may cause air to enter the cargo oil tanks through the P/V valves, increasing the O2 level, thereby causing an explosive atmosphere inside the cargo tanks.
Leave individual cargo tanks IG supply valves open during the ballast voyage and monitor the IG main line pressure from the cargo control room or the bridge.
Cargo Loading Operation
Ensure that the O2 level in all cargo oil tanks is less than 8% and that the tank pressure is minimum 100 mmAq upon arrival at load port, using Double Hull (D/H) Operational Record Voyage record of COT Oxygen / H2S content.
In certain ports the maximum oxygen content of inert gas in the cargo tanks may be 5% to meet particular safety requirements, such as the operation of a vapour emission control system. In such cases follow terminal requirements. For special requirements for arrival tank atmosphere conditions and limitations, prior loading, follow applicable local port / terminal requirements.
It is not required to operate the IGS during cargo oil loading operations.
The subject of secondary venting and the capacity of the equipment is occasionally questioned by vetting inspectors.
Vessels fitted with the Full Flow PV / High Velocity Vent Valve(HVVV) comply with the requirements as required by SOLAS Chapter II-2 Regulation 11, Sections 6.3.2, 6.3.4 & 6.4.
The primary venting system on vessels are via. IG main mast riser, which is protected by a full flow P/V Breaker.
Secondary venting is achieved via. Individual cargo tank P/V Valves which have an individual full flow capacity.
PV (Pressure Vacuum) / HV (High Velocity) Valves
The maintenance of cargo tanks PV / HVVV are to be carried out as per the PMS. Such record of maintenance using Cargo Tank PV Valves , along with related correspondence is to be filed onboard.
During Loaded Voyage
Cargo tanks shall be maintained at positive pressure. Under normal conditions it is not necessary to run the IGS during loaded voyage. More so, due to the hydrocarbon generation, the level of oxygen inside the tanks, would be well under the levels before loading (empty tank conditions), provided no ingress of air has occurred.
In case, where vapor segregation is not required, the tank pressures are connected to the common IG line and can be monitored from the central location (Navigation Bridge or Cargo Control Room).
Be aware that during a sudden drop in the ambient temperature air may enter the cargo oil tanks through the P/V Valves on each Cargo Oil Tank, increasing the O2 level.
For the sake of exercising due diligence, on behalf of the carrier, on completion of loading, the IGS Record of Pressure shall be left in constant operation, so as to carry out voyage recording of Cargo Tank IG Pressure.
Where there is a vapor segregation required to be maintained between grades, the Segregated tanks pressure shall be monitored frequently (not greater than 4 hourly intervals).
This interval shall be reduced, depending upon the nature of cargo (tendency to gasify at maintained temperature), percentage of filling and temperature.
Handling Troubles & Failure Of IGS During Discharging / COW Operations
If the Inert Gas System (IGS) is malfunctioning during operations, the oil transfer operations must be suspended immediately, and not resume until the IGS is in good working condition (or) an alternative Inert Gas supply is provided.
Such emergency action in case of Failure of Inert Gas System, is required in order to maintain a positive pressure in the cargo or slop tanks. The inert gas main isolating valve shall be shut. The terminal, must then be informed, as soon as possible.
Related article : Tank cleaning, purging and gas free check items tanker procedures