Gas Freezing Purging Following Inspection or MaintenanceWhen a vessel or pipework system has been opened up, it shall be purged with nitrogen to remove the oxygen prior to re-introducing hydrocarbons.

As with the purging for removal of hydrocarbons, the purging for removal of oxygen can use any, or a combination, of the methods described in: Purging for Gas Freeing

The main difference is that the sampling is for a different gas and to ensure that the vent lines are not discharging an explosive mixture.

WARNING: THE INTRODUCTION OF HYDROCARBONS INTO A SYSTEM THAT HAS NOT BEEN INERTED SHALL BE RISK ASSESSED. CALCULATIONS SHALL BE CARRIED TO ENSURE THAT IT IS NOT POSSIBLE FOR THE OXYGEN CONTENT TO REMAIN ABOVE 2% AFTER HYDROCARBONS HAVE BEEN REINTRODUCED. WHERE IT IS NOT POSSIBLE TO GUARANTEE THIS THE SYSTEM WILL BE N2 PURGED.

1.1 SamplingWhen purging hydrocarbons using nitrogen, it should be noted that the detector used must be capable of measuring a hydrocarbon gas in an inert atmosphere. The MSA Tankscope and Neotronics Digiflamm are examples of detectors that can be used.

For both flowing and cyclic purges the checks should be made at not only the recognised vents but also at any dead legs in the system.

When purging the nitrogen out of the system with air, it is equally important that representative checks are carried out. If the vessel or system is going to be entered, conditions for the entry are as defined in PR-1148 Confined Space Entry .

The recommended criteria to give safe conditions when purging a mixture of hydrocarbon gases with nitrogen are shown in Table 1.

Table 1 - Recommended Criteria Hydrocarbon / Nitrogen Purge

Mode O2 Level Hydrocarbon Level N2 Level

HC to Inert - <_ 4 % >t 96%

Air to Inert <_ 4% - >t 96%

For specific criteria when purging individual hydrocarbon components with nitrogen, refer to Table2.

Table 2 - Inert Gas End Points for Purging Equipment Out of Service

Purge Medium Nitrogen Nitrogen Nitrogen Nitrogen

Combustible Percentage Inert Gas required to render mixtures non-flammable when air is added in any amount

Purging end points with 20% Safety Factor

Percent of Combustible below which no mixture is flammable when air is added in any amount

Purging end points with 20% Safety Factor

Hydrogen 95 96 5 4

Carbon Monoxide

81 85 19 15

Methane 86 89 14 11

Ethane 93 95 7 5

Propane 94 95 6 5

Butane 95 96 5 4

Iso-butane 95 96 5 4

Pentane 97 98 3 2

Hexane 97 98 3 2

Gasoline 96 97 4 3

Ethylene 94 95 6 5

Propylene 96 97 4 3

Benzene 96 97 4 3