DN-15e - Commissioning of mains ≤ 355mm diameter at pressure up to 2bar by Direct Purging

Title Page

Site conducted
Conducted on
Prepared by
Location
Operative being assessed

Tools and equipment

Metallic vent pipes, earthing straps (and a flame trap for use on rider vent only)
Gas detection equipment
Pressure gauges
Electrofusion control unit (ECU)
Governed rider (where applicable for MP system)

General requirements

Prior to commencing any work ensure that the following documentation is available and
authorised where appropriate.
1. Job instructions
2. Routine / Non-Routine operation procedures
3. Permit to work

Preparatory work

Purge Velocities In order to minimise the mixture of air and gas at the interface, it is imperative that the purge velocity is always above the minimum shown in TABLE 46, (SGN/WI/ML/2) If the information in the table is complied with, then the minimum velocity will be achieved.
Velocity measurement is not required provided the riders and vents are constructed with full bore fittings to the sizes detailed in TABLE 46, (SGN/WI/ML/2)
Purging operations can be undertaken with the controlled release of the squeeze-offs where squeezeoff units can be set to open at the minimum distance shown in TABLE 46, (SGN/WI/ML/2). Where this cannot be achieved then riders across the squeeze-offs must be used.
Within the NRO/RO procedure the Operational Manager will confirm the rider diameters to ensure minimum purge velocities and purge rates. The Operational Manager will confirm when the commissioning of the main must be carried out by indirect purging.
The purge duration in seconds will be decided by the Operational Manager who will divide the length by the proposed purge velocity, this will be included within the NRO/RO procedures.
- Yes
- No
- N/A

Once a purge has started, it must continue without interruption until completed:

Communications and external liaison For all commissioning operations a system of communications must be organised to enable the Operative to communicate with personnel operating the vents and riders. The system of communications must be tested prior to the commencement of the operation and recorded on the NRO/RO procedures.
Where electronic communication systems are utilised within the work area, these must be intrinsically safe. If the equipment is not intrinsically safe i.e. mobile phones. The communication system should be treated as a potential hazard and used only outside of the designated hazardous area, i.e. at least 5m upstream of any possible source of purging gas.
- Yes
- No
- N/A
For major purging operation, the Operational Manager will inform the necessary authorities and this will be recorded on the NRO procedure. However, for all purging operations you should advise local residents that purging is being undertaken and advise them in respect to noise, smell and fire hazard. Where necessary, request local residents to close windows and doors during purging operations.

Branched system

Branched systems may be directly purged either simultaneously or sequentially.
Any loops must be isolated by valves or stopping off equipment (squeeze-offs) and then treated as branches.
Short stubs, whose length is less than eight diameters, are not classed as branches and will automatically be purged by natural convection.
Sequential purging Sequential purging involves purging each branch in turn, starting with the largest diameter branch. It must be used if the work force is limited or the minimum velocities cannot be achieved in each branch during simultaneous purging.
Simultaneous Purging Simultaneous purging is quicker than sequential purging. It involves opening all the vents together and purging all the branches at the same time.
Where branches are to be purged Simultaneous the riders and vents must be sized for a pipe diameter equivalent to the sum of cross-section area of the entire pipe being purged. The Operational Manager will advise you when Simultaneous purging is to be carried out and will prepare these calculations. Information in respect to these will be given as part of the authorised written procedures. This will be provided as part of the authorised NRO/RO procedures.

Riders

An on-site risk assessment must be carried out to determine the construction of the rider and the following factors must be considered: Potential impact on the rider from mechanical damage Interference or fire hazard
The NRO/RO should detail the material from which the rider is to be constructed. However, if the on-site risk assessment indicates that a different material or method of construction is required, then the Operations Manager must approve this.
For purging medium pressure mains, a governed rider may be used to bring the main up to operating pressure in stages. This will facilitate testing of the final tie-in connections with leak detection fluid to enable any leaks to be identified at a lower pressure. Such equipment must be used by a competent Operative
- Yes
- No
- N/A
Riders must:
Be constructed from metal tube, approved flexible steel pipes. PE or a combination of these.
Be no more than 10m in length.
Have no more than six bends.
Use full bore valves.
Have rider connections to the main that are full bore.
Where metric sizes are given, the equivalent imperial size can be used i.e. 32mm equivalent is 1”
The vent point on the rider must be metallic and have a flame trap and earthing strap. It must be adequately supported and sited a minimum of 2.5m above ground level and a minimum of 5m downwind of any ignition source.

Vents

An on-site risk assessment that identifies the potential impact on the public and the environment of the vent location and the possibility of mechanical damage, interference or fire hazard must be carried out prior to starting work on site.
Wind speed and direction must be taken into account to prevent venting gas coming into with area of public assembly, open fires or other possible ignition sources, major transport routs etc
Vent tubes must be fitted at the far end of the main to be commissioned and must be manned whilst venting.
Vents must be:
Constructed from metallic tube or approved flexible steel pipes.
Include a full-bore control valve and sample test point.
Connected to the pipeline or main with a full-bore fitting.
Vent pipes must be installed:
To discharge vertically into the air no less than 2.5m above ground level.
Be sited 5.0 m downwind of any possible sources of ignition
Be sited so as any vented gas is unlikely to drift into buildings through air intake grills, air conditioning units, open doorways and windows.
Be firmly supported.
Must not include any flame trap.
Be adequately earthed if attached to a PE main to prevent the build-up of static electricity in plastic pipes.
Adequate protection must be given to the general public within the designated hazardous areas i.e. at least 5.0m upwind of any possible source of escaping gas.
Where purging is carried out bellow or near a known flight path then advice must be sought from the Operation Manager and purging operations must only be completed with the agreement of the local aviation authority

Commission operations - Preparation

The new main must be physically connected to the feeder / parent main, however the new main must remain isolated from the gas supply as follows: - + Low-pressure mains not greater than 180mm can be isolated by a single squeeze-off if the isolation is sound and verified be a let by check. + The branch of an under-pressure tee with a maximum branch outlet of up to and including 355mm/12”/300mm on low and medium pressure mains can be isolated by a single face valve if the isolation is sound a verified by a let by check. + Other mains must be isolated by a double block and bleed system. This system may be a valve with a double block and bleed facility or, alternatively, a double squeeze-off system with intermediate vent
Riders can be isolated across the isolated section of main in accordance with the NRO/NO procedure in accordance with TABLE 46, (SGN/WI/ML2).
- Yes
- No
- N/A
Commissioning of mains by the removal of bags or by the use of an ejector must not be carried out.
All rider and vents must be manned throughout all live procedures.
Mains should be commissioned by direct purging in accordance with Figure 96, (SGN/WI/ML2) as
follows
Confirm that the valves/squeeze-offs are closed and the bleed vent pipes on the valve/squeeze-offs are open.
Fit riders and vents in accordance with Figure 96, (SGN/WI/ML2) with valves A, B, C, D, E, and F in the closed position.
Fit pressure points P1 and P2 and ensure that P1 indicates pressure greater than minimum allowable as specified within authorised NRO/RO procedures
If P1 is lower than the minimum allowable pressure, then stop operations and advise your operational Manger
Establish communications between personnel at the rider and vent position(s).
Open rider valve B and purge, by slowly opening valve A, until two successive reading above 90% gas in air (GIA).
Close valve B and pressurise the rider, check all joints for soundness using leak detection fluid.
Fully open valve A.
Open vent valves D and E.
Continually monitor P1 and fully open rider valve C commencing purge: if pressure drops to the minimum pressure at P1 as specified within the written procedure, close rider valve A and report this to your Operational Manager.
Commissioned riders must not be left unattended.
Monitor pressure at P1 during purge to ensure that it does not fall to below the minimum local requirements detailed in the written operational procedure
Note:
If the purge velocity fluctuates at the vent point as the purge is in progress this could indicate a
blockage within the main. Do not stop the purge in such cases. On completion of the purge close valve
D and attach a suitable test gauge to valve F. turn on valve D and monitor the pressure in the main.
Where the pressure oscillates inform
the Operational Manager.
Continue venting until the expected time for purging has passed and then open valve F on the vent and continue to purge until two successive readings of 90%gas in air (GIA) is obtained at the vent.
Close vent valves D, E and F.
In the case of branch systems, close each vent when two successive reading of 90% gas in air (GIA) is obtained at the vent.
Pressurise the main under commission to the correct working pressure, for MP systems the main should be taken up in stages therefore a governed rider is required to pressurise the new main. Check pressure at the extremities by attaching a suitable pressure gauge at valve F then open valves D and F to confirm that the pressure is equalised in the new main with that in the existing main.
Exposed sections must be checked with an approved leak detection fluid and wash off with clean water. Should leakage be detected the pressurisation must be stopped until the cause of the leakage has been rectified.
Once P1 and P2 have equalised open or remove isolation equipment.
Close rider valves A and C and decommission the rider and vent points.
Check all joints for soundness with an approved leak detection fluid and wash off with clean water
Ensure all tapings are plugged off.
Complete NRO/NO procedures, tidy site.