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Continuous availability of inert gas onboard chemical tankers

Why inerting of cargo tanks ?: The term inerting/purging generally refers to the replacement of air in a cargo tank by an inert gas, in chemical tankers most often by nitrogen, in order to prevent the formation of flammable vapours, oxygenation of the product, reduce humidity in the tank and/or protect the quality of the cargo.

The cost, through additional bunker consumption, and time spent on inerting/blanketing, are usually paid by the Charterer. Vessels should be aware regarding the most efficient and cost effective method of completely purging their vessel.

chemical tanker navigation at sea
Continuous availability of inert gas and its correct use is highly important for chemical tankers. The cargo spaces and slop tanks are to be kept fully inerted where required, in accordance with the requirements BCH/IBC Code where required by the particular products/chemicals carried. If entry is required the space will be thoroughly cleaned whilst using inert gas. After cleaning the tank atmosphere must be purged with inert gas to reduce the hydrocarbon levels to less than 1.4% by volume to prevent the atmosphere from entering the flammable range during venting. The Tankscope is used for this purpose.

The space must then be vented with fresh air ensuring that the space has not less than 20.8% of oxygen, not more than 1% LEL Explosimeter reading and that the concentration of toxic vapours less than the TLV before entry is made.

The Master must ensure that the oxygen content of the inert gas supply in the main line to the tanks is not more than 5% by volume, at any required rate of flow. Code requirements, cargo requirements and local regulations may require the vessel to operate to a lower limit.

If for any reason, the inert gas system cannot be operated, or the oxygen content of the inert gas supply cannot be reduced to below the required O2 level, or the level required by the local port regulation, the Master must stop operations and if the problem cannot be resolved, the Company must be informed without delay.

The Chief Engineer Officer is responsible for the maintenance of the Inert Gas Plant / Inert Gas Generator. It is the responsibility of the Chief Officer to measure the oxygen content of the Inert Gas main supply and the oxygen content in the cargo spaces.

The Chief Officer is also responsible for the setting of valves on deck and in the pumproom (as applicable).

In addition to the shipboard PMP, an inert gas operations and maintenance log is to be maintained for recording of all required checks and tests and operational parameters of the IG plant and related equipment.

Oxygen levels in the inerted gas spaces must be maintained at not more than 8% by volume oxygen unless local regulations require a lower figure. For certain sensitive chemicals, for purity or safety reasons, the use of standard ships generated inert gas is not suitable. In some cases therefore the cargo containment will be pre- purged with N2 and the cargo kept under a N2 blanket.

In other cases charterers may if allowed by IBC/BCH code require tanks to be left without being inerted. In the event of any doubt about product property and whether or not inerting is required the company are to be consulted for guidance and further instructions.

Maintenance of the nitrogen cover during the voyage is usually responsibility of the vessel. The atmosphere requirements will normally be laid down by the BCH/IBC codes or by the cargo shippers. Inerting/purging is normally done by Nitrogen supplied from shore and the procedure should be part of the pre-loading checklist with special emphasis on;
  1. Tanks to be purged shall be tagged-out with a “No Entry” tag as described in
  2. Ship/shore to agree on the inert gas and pressure rate of supply in maximum standard cubic meter per minute (m3/min)
  3. Tanks to be inerted must have sufficient hatches/vents open in order to avoid over-

    i) Pressurising the tank and the tank pressure must be monitored.
    ii) Ensure that all “dead ends” of cargo lines and drains etc are also suitably flushed and that pockets of Oxygen do not occur in the cargo tanks.

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 :

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