Prior to opening for cleaning or maintenance and repair, all the Pipe works, tanks, towers and vessels that have contained hydrocarbon or toxic materials shall be removed. Typically decommissioning a pipeline will include the steps of decommissioning
by removing the hydrocarbon from the piping system and flaring any remaining residual product. The oxygen content in the piping system will be reduced to less than 2% by the process of purging with nitrogen gas.
A combination of dilution and cycle purging techniques with nitrogen gas are to be used to reduce hydrocarbon concentration to less than 2 % by volume. Multiple injection points are to be used. Initially nitrogen
will be injected with the vent path open to the atmosphere to displace hydrocarbons from the systems. The vent will then be closed and cycle purging technique used to ensure all potential dead legs are hydrocarbon
free. Cycle purging involves pressurization of the systems to 2-3 bar, a hold period is then applied to allow mixing, and finally the system is depressurized to atmospheric pressure via a suitable vent path. During
purging the oxygen content in the system shall be checked, when the oxygen content is less than 2% by volume, then purging will be stopped.
DE-COMMISSIONING
The construction, upgrading or maintenance of the pipeline, can be performed under necessary circumstances. Under the completion of repairing or maintenance, the piping system is inserted with nitrogen gas. In order to re-commissioning the piping system, the nitrogen must be replaced by the desired product. When using water to gas freeing of tanks, towers and vessels ensure that high level vents or manway/ access hatches are open to prevent drawings and negative pressure and risk collapse.
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