- 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.
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