Chemical tankers safety precaution for gas freeing

Chemical tankers safety precaution for gas freeing

A space is considered as “gas free” when the concentration of flammable gases in its atmosphere is less than 0% LEL, the concentration of toxic gases (including IG components) is less than the TLV and the Oxygen concentration is not less than 20.8%. Comply with IBC/BCH code and ICS Chemical Tanker safety Guide requirement for gas freeing for chemical cargoes. (Ref IBC code CH.8)

The Chief Officer is to supervise gas-freeing operations. The gas freeing programme and the progress of operation must always be available to all concerned.

Protective clothing, resuscitation and fire fighting equipment must be ready for immediate use. All doors, ports, windows are to be kept closed.

The inerted space can be purged with fresh air using the inert gas fans or water driven portable fans. Careful readings of the tank atmospheres must be taken with the ship’s portable gas detection equipment throughout gas freeing operations, a log of the readings must be maintained through the earlier stages of the operation, hydrocarbon readings are to be taken with the Tankscope (hydrocarbons by volume) and during the completion of gas freeing with the Explosimeter or Dragger Multigas Detector tubes.

Many vapours are heavier than air, and after they escape from the tank openings or vents they will tend to lie around the decks. With light wind (below 5 knots) flammable or toxic mixtures may not disperse and lie about at some distance from where they arise. These gases may be carried through openings into galleys, accommodation, deck lockers etc. or be drawn into machinery spaces.

It should always be suspected, even after spaces have been cleaned and made gas-free that some cargo liquid or vapour or both may be released whenever pumps, cargo lines, valves, heating coils, etc. are opened up. Precautions must be taken against such releases. Due to the risk of air pollution, gas freeing operations must not be carried out within port limits without the express permission of the appropriate authority.

When the ship is not provided with an inert gas system, the operation shall be such that the flammable vapour is discharged initially through:

Outlets at least 2 m above the cargo tank deck level with a vertical efflux velocity of at least 30 m/s maintained during the gas-freeing operation; or

Outlets at least 2 m above the cargo tank deck level with a vertical efflux velocity of at least 20 m/s and which are protected by suitable devices to prevent the passage of flame.

The above outlets shall be located not less than 10 m, measured horizontally, from the nearest air intakes and openings to enclosed spaces containing a source of ignition and from deck machinery, which may include anchor windlass and chain locker openings, and equipment which may constitute an ignition hazard.

When the flammable vapour concentration at the outlet has been reduced to 30% of the lower flammable limit, gas-freeing may be continued at cargo tank deck level. See SOLAS Ch 2-5 and IBC Code Ch 8 for more details.

Gas freeing precautions

Check what type of vapours are involved; they may be flammable, toxic, corrosive or a combination of the above.

Alert other non-essential crew including the engine room that gas freeing is to take place and that non-essential personnel should stay clear of deck areas.

Wind direction may cause vapours to enter accommodation spaces or engine room and necessitate proper precautions (i.e. at sea the vessel may have to be turned off the wind).

Portable ventilation equipment must be checked and fully operational

Personnel involved must wear personal protective equipment as necessary

Vapour must only be released through openings as stipulated in SOLAS and IBC/BCH code

After gas freeing no tank entry must take place before the Enclosed Space Entry Permit has been issued by the responsible officer and the tank has been “tagged” safe to enter.

All fixed and portable gas detection equipment must be in operation and suitably calibrated throughout the operations

Some vessels are provided with a fixed gas freeing system comprising a fan unit connecting to a pipeline on deck, which might be the cargo lines, the vapour return line or inert gas line. When gas freeing operations are completed, the fan must be completely isolated from the pipeline either by a removable spool piece or a blanking arrangement. This is to ensure flammable or toxic vapours cannot reach the fan after the tank has been loaded.

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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