METHODS OF GAS FREEING TANKS
Tanks must be gas-freed before man entry or hot work is carried out.
Inert gas, air or steam is used to replace the tank atmosphere.
Using Inert Gas:
Cargo tanks can be gs freed using inert gas by 2 methods- Dilution and Displacement. The tank atmosphere can be diluted with inert gas or displaced by purging.
In dilution, the inert gas is introduced at a high velocity into the tank to penetrate to the bottom as CH gas is heavier then inert gas or air.
In displacement, inert gas enters from the top of the tank at a low velocity. The interface of the incoming and outgoing gas must be stable
without any turbulence. The CH gas escapes through suitable piping leading out from the bottom of the tank.
By Air:
Hydraulic, pneumatic or steam driven fans may be used to blow in air or extract the gas. The materials used in the construction of these fans
must be spark-free. The capacity and penetration of the fan should be such as to gas free the tank in the shortest possible time. It takes about 25 – 30 replacements of the atmosphere to bring down the concentration of CH gas in air from 50% to below LFL. Tank lids must not be opened till ventilation starts and should be shut immediately after the ventilation operation when the gas is tested to be safe.
By steam :
Steam was used previously to inert tanks before inert gas became available. Steam introduced at a high velocity causes a static charge to accumulate, which may lead to creation of a spark. Steam velocity must hence be kept low and steam must not be put into a tank having an earthed probe nor should a probe be put into a tank till at least 10 minutes after steam is stopped to allow it to settle and stable distribution of
charge and concentration is achieved.
Ref: Cargo Work by Capt Errol Fernandes, Myseatime website & Marine teacher website
