7.1.6.12
When it is required to gas free a tank after washing, it should first be purged with inert gas to reduce the hydrocarbon content to 2% or less by volume. This is to ensure that during the subsequent gas freeing operation, no portion of the tank atmosphere is brought within the flammable range.
The hydrocarbon content must be measured with an appropriate meter designed to measure the percentage of hydrocarbon gas in an oxygen deficient atmosphere. The usual flammable gas indicator is not suitable for this purpose.
If the dilution method of purging is used, it should be carried out with the inert gas system set for maximum capacity to give maximum turbulence within the tank. If the displacement method is used, the gas inlet velocity should be lower to prevent undue turbulence.
Gas Freeing
Before starting to gas free, the tank should be isolated from other tanks. When either portable fans or fixed fans connected to the cargo pipeline system are used to introduce air into the tank, the inert gas inlet should be isolated. If the inert gas system fan, drawing fresh air, is employed, both the line back to the inert gas source and the inert gas inlet into each tank being kept inerted, should be isolated.
Preparation for Tank Entry
To ensure the dilution of the toxic components of inert gas to below their Threshold Limit Values (TLV), gas freeing should continue until tests with an oxygen analyser show a steady oxygen reading of 21% by volume and tests with a flammable gas indicator show not more than 1% LFL.
If the presence of a toxic gas, such as benzene or hydrogen sulphide, is suspected, gas freeing should be continued until tests indicate that its concentration is below its TLV.
Positive fresh air ventilation should be maintained throughout the period that personnel are in a tank, and frequent tests should be made of both oxygen and hydrocarbon content of the tank atmosphere.
When other tanks in an inert condition are either adjacent or interconnected (e.g. by a pipeline) to the tank being entered, personnel should be alert to the possibility of inert gas leaking into the gas free tank through, for example, bulkhead fractures or defective valves. The risk of this occurring can be minimised by maintaining a small but positive inert gas pressure. When a gas free tank is re-connected to the inert gas main, it should immediately be re-inerted.
For general advice on entry into enclosed spaces see Chapter 10.
7.1.7.3 Entry into Cargo Tanks
Entry into cargo tanks should be permitted only after they have been gas freed, as described in Sections 7.1.6.10 and 7.1.6.11. The safety precautions set out in Chapter 10 should be observed and consideration given to the carriage of a personal oxygen deficiency alarm. If the hydrocarbon and oxygen levels specified in Section 7.1.6.12 cannot be achieved, entry should be permitted only in exceptional circumstances and when there is no practicable alternative.
Personnel must wear breathing apparatus under such circumstances. (See Section 10.7 for further details).