1.1 Gas-Freeing and PurgingAs general guide process equipment containing 96% nitrogen by volume shall be considered (hydrocarbon) gas free.

The key steps in the process of gas-freeing and purging with Nitrogen are:

Isolate from all other equipment.

Reduce pressure.

Remove hazardous liquids.

Remove hazardous vapours by depressurisation.

Flush with water if appropriate.

Purge with nitrogen.

Purge with air, if necessary.

Nitrogen for purging purposes shall be supplied in ‘quads’ or for large operations tankers or portable high volume tanks. The nitrogen tanks and associated equipment are normally supplied by a contractor, who will also supply competent personnel to operate the equipment. However, this does not remove the responsibility of the PDO Operations staff for the overall supervision and the safe completion of the work under the PDO PTW System.

1.1.1 Preparation and PrecautionsThe following shall be taken into account in the preparation of gas-freeing and purge procedures:

Hired equipment shall be inspected to confirm compliance with PDO standards, particularly for diesel engines and electrical equipment.

Personnel, whether PDO staff or Contractors, shall be competent in the use of the equipment being used for the purge.

Personnel must be in attendance whenever purging is taking place to monitor system pressure and purging equipment.

Notwithstanding the classification of the hired equipment, it should be located outside and in a non-hazardous area where possible.

All low temperature equipment must be cordoned off and appropriate warning signs fitted. Entry shall be restricted to authorised personnel only.

Any low temperature equipment must be kept clear of decking or other structural members and be provided with drip trays at joints. For details refer to Appendix 1

Where equipment is located such that any spillages of nitrogen liquid may cause structural damage, charged fire hoses must be available to deal with any spillage.

All nitrogen hoses shall have current pressure test certification, be visually checked for condition.

Great care shall be taken with the choice of location to which temporary vent lines are directed, due to the hazardous nature of the vented gas. Vent lines should be vented external of closed modules / areas. Temporary vent lines must also be secured against movement, especially where the gas released may be under pressure.

NOTE: Consideration should be given to installing permanent supply and vent lines for activities that are carried out regularly.

Prior to starting the purging operation, with the exception of the flare system which may be required to complete the purge activity, the equipment being purged shall be isolated from all other equipment.

If venting is into a common flare system, Operations staff should be aware of the possibilities of gas being vented, or leaking across PCV's from another system and thus recontamination the purged vessel.

When discharging nitrogen to the flare system, considerations shall be given to the possibility of extinguishing a lit flare.

If nitrogen is being supplied by cylinders, a two-gauge regulator control, which indicates cylinder and flowing pressure, shall be used, fitted directly onto the manifold.

CAUTION: At no time, irrespective of the relative volumes of the operation and the cylinder(s), shall nitrogen be used without a regulator.

If the nitrogen is being supplied by `quads', the cylinders shall not be removed from the frame and the lifting slings shall not be altered in any way.

To avoid reverse flow of product into the nitrogen supply, the line up shall be equipped with check valves.

CAUTION: Where temporary hoses are used for purging and re-pressurisation these should be secured in position at 3m intervals to prevent ‘whipping’. Where temporary hoses are fitted with ‘crow’s foot’ type connections these must be fitted with ‘R’ clips and whip checks. Temporary hose connections and tie-in points shall be blanked off when not in use and

for all tie-in points an isolation valve shall be fitted. Where the connection is envisaged as being required to operate against a pressure in the vessel, the connection to the vessel shall comprise a hose connection coupling, a bleed valve, a check valve and a block valve.

All temporary connections and vent points shall be controlled under the PTW system. Labels should be fitted to identify all nitrogen purge points and regular checks carried out to identify the valve status. This status should be recorded on an Isolation Certificate (IC).

During periods when large numbers of temporary connections are in use (i.e. during shutdown or startup) regular checks shall be carried out, referring to a checklist, on all nitrogen utility points in service, in order to identify their status.

PVC or leather gloves must always be worn when handling anything that contains, or has been in contact with, cryogenic liquids. These gloves should be loose fitting so that they may be easily removed if liquid is splashed onto or into them.

Where spraying or splashing may occur, a face visor must be worn to protect the face and eyes.

Before the arrival of any temporary nitrogen equipment on site, the dangers of nitrogen in both liquid and gaseous form should be discussed at a safety meeting. Refer to Appendix 1.

It is very difficult to totally gas free a hydrocarbon vessel by nitrogen purging, even if flushing with water is part of the procedure. It is therefore most important that any hotwork in the vicinity of the vessel to be opened up is stopped prior to the removal of any manway doors, access hatches etc.

The possible leakage of nitrogen in confined spaces/modules, whether permanent or temporary (habitats), MUST be considered in the planning of purging operations.