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(a) through the vent outlets as specified in regulation 23 of the Merchant Shipping (Cargo Ship Construction) Regulations 1997[1]; or

(b) if the ship is one constructed on or after 1st February 1992, through outlets at least 2 metres above the cargo tank deck with a vertical efflux velocity of at least 30 metres per second during the gas-freeing operation; or

(c) through outlets at least 2 metres above the cargo tank deck with a vertical efflux velocity of at least 20 metres per second and through devices (other than flame screens) complying with Schedule 3 of Merchant Shipping Notice MSN 1671 so as to prevent the passage of flame into the cargo tanks;

until the flammable vapour concentration in the outlet has been reduced to 30 per cent of the lower flammable limit. Thereafter, gas-freeing may be continued at the cargo tank deck level.

Cargo pump rooms
     26.  - (1) Except as otherwise provided in paragraph (2) in every tanker of Class VII(T) of under 500 tons, each cargo pump room and each pump room having a similar hazard shall be provided with either a fixed pressure water-spraying system comply with the requirements of Schedule 2 of Merchant Shipping Notice MSN 1666 or a fixed gas fire-extinguishing system complying with the requirements of Schedule 4 of Merchant Shipping Notice MSN 1666, which shall be operated from a readily accessible position outside the pump room, provided that where the fixed extinguishing system is a gas system - 

(a) the alarm referred to in Schedule 4 in Merchant Shipping Notice MSN 1666 shall be safe for use in flammable cargo vapour/air mixture;

(b) a notice shall be exhibited at the controls stating that due to electrostatic ignition hazard, the system is to be used only for fire-extinguishing and not for inerting purposes;

(c) where the extinguishing medium used in the cargo pump room system is also used in systems serving other spaces, the quantity of medium provided or its delivery rate need not be more than the maximum required for the largest compartment.

    (2) In chemical tankers where the fixed fire-extinguishing system referred to in paragraph (1) is a gas system, the concentration shall be as specified in the Code for the Construction and Equipment of Ships carrying Dangerous Chemicals in Bulk.