Shipboard generated inert gas systems :Continuous availability of inert gas and its correct use is highly important for chemical tankers. To ensure its availability it must be operated, tested, and maintained at regular intervals, and in accordance with the manufacturer’s instructions and the vessel’s Planned Maintenance System.

The stock of spare parts carried must be maintained at the appropriate level and whenever critical parts are used they must be re-ordered immediately.

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All inert gas safety systems must be checked prior to every operation commencing to ensure that correct operation is possible when required. These tests are described in SOLAS, the Inert Gas Operations and Equipment Manual and the manufacturer’s instructions, and must also include the following:-

1. Oxygen monitoring equipment must be calibrated and alarm points checked, this includes both portable and fixed equipment.
2. Automatic shut-down systems and valves checked.
3. Deck seal alarms tested.
4. Non return valves should be checked.
5. Hydrocarbon gas detectors should be checked and alarms tested.
6. Dryers, dew point meters etc. where fitted are maintained and calibrated. An inert gas maintenance record is to be maintained during all above checks and testing.

Failure of the inert gas system - Countermeasures

In the event of failure of the inert gas system or supply, action must be taken immediately to prevent air from being drawn into the tanks. All cargo operations must be stopped and the deck isolating valves for the inert gas system closed. Immediate action must be taken to repair the system. & Terminal notified.

It is essential that the Company be informed immediately of such a failure together with the reason for failure if known, and of any spares that are required to repair the system.

Masters are reminded that national and local regulations may require the failure of an inert gas system to be reported to the harbour authority, terminal operator and to the port and flag state administrations.

On chemical carriers where a failure cannot be immediately repaired, discharge or tank cleaning may only be resumed if an external supply of inert gas is provided or the following precautions are taken, where permitted by the BCH/IBC Code, local regulations and by the particular products/chemicals carried, following the guidance given in the ICS Chemical Tanker Safety Guide :-

• Flame screens are checked to ensure they are fitted and in good condition.



• No dipping, ullaging, sampling or other equipment is to be introduced into the tank unless essential to the safety of the operation. If it is necessary for such equipment to be introduced into the tank, it is to be carried out only after at least 30 minutes has elapsed since the cargo operation has been stowed.
  
  All metal components of any equipment to be introduced into the tanks must be securely earthed. This restriction is to be applied until a period of 5 hours has elapsed since the cargo operation has ceased.



• If it becomes necessary to clean tanks which have contained flammable cargo while the inert gas system is out of operation, the precautions detailed in the ISGOTT & ICS CTSG must be observed.

Maintenance

Vessels which are fitted with an inert gas system must carry out the routine inspections, tests and maintenance required by the vessel’s “Inert Gas System Operation and Equipment Manual”. To demonstrate that the system is fully operational and in good working order, a record of inspection and maintenance should be maintained onboard.


Deck Seal

Inspection of a deck seal is to include the following:-

• Opening of the unit for internal inspection.
• Checks for blockages and condition in the venturis of semi-dry type seals.
• Checks for corrosion or cracks in the inlet pipe and housing.
• Checks for corrosion of the heating coils.
• Checks for corrosion or damage to any of the internal parts of the seal or of the casing.
• Inspection of Filters/Demisters.

Note: Where block valves are fitted, instead of deck seal, the block valves are to be maintained as per manufacturer’s instruction.

P/V Breaker

If the P/V Breaker is of the liquid type, Inspection of the P/V breaker is to include the following:-

• Ensure gauge glass intact, clean and level visible (If fitted).
• Check correct liquid used and the level is maintained for the density of liquid used. The level to be maintained in static condition is to be conspicuously marked.
• If freezing conditions are expected, check the liquid used is suitable for low temperature use, if necessary anti-freeze is to be added.

Inert gas scrubber

Inspection of the inert gas scrubber may be made through the access cover or manholes. Checks must be made for corrosion, fouling and damage to:-

1. Scrubber shell and bottom.
2. Cooling water pipes and spray nozzles.
3. Float switches and temperature sensors.
4. Other internals such as trays, demister pads and linings.

Non Return Valves

Non return valves must be opened for inspection to check for corrosion and also to check the condition of the valve seat and the free movement of the valve. The operation of the valve is to be tested in service.


Flue Gas and Re-circulating Valves

Flue gas and re-circulating valves are to be “stroked” at regular intervals when the system is not in service to ensure that they remain free to operate over their full range. Maintenance of the Inert Gas System and other components located within the machinery spaces is the responsibility of the Chief Engineer.


Shipboard energy saving practices

In various type of tankers, IG is needed for cleaning, purging and top of the cargo tanks for safety reasons. The IGG (Inert Gas Generation) system produces exhaust gas with minimal O2 concentration for this purpose. The IGG operation resemble that of boilers and consumes fuel thus its management is required for saving energy. The IGG usage needs to be monitored to ensure that it is not used excessively.

Also, optimising of the cargo tank cleaning, gas freeing and inspection intervals will reduce the usage of IGG system. When IGG system is used, the level of discharge to atmosphere (blow off of not needed IG) should be minimised via optimal operation of the system.



Related Info:

Failure prevention and maintenance guideline for inert gas systems onboard

Cargo tanks inerting safety practice

Supply of Nitrogen from shore - safety guideline

Gas freeing safety precautions for chemical tankers

Poisoning and required first aid treatment onboard

How to test a tank environment prior entry ?



Reference Publications :

• IBC/BCH Code
• IMO – Inert Gas Systems
• Ship’s “Inert Gas Systems Instruction Manual” (Approved by Class)
• Ship’s “Procedure and Arrangements Manual” (Approved by Class)
• International Safety Guide for Oil Tankers and Terminals (ISGOTT)
• ICS Chemical Tanker safety Guide
• SOLAS

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