DN-16-c – Decommission of mains (Decay) ≤ 355mm PE & 12”/300mm metallic ≤ 2bar, direct purge

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Site conducted
Conducted on
Prepared by
Location
Operative being assessed

Preparatory work – mains decay test:

Prior to undertaking any purging, the section of main to be abandoned must be subject to a satisfactory mains
decay test. This is necessary to prove that:
a. There are no consumers still connected to the section being abandoned. b. There is an adequate supply to the surrounding mains system.  c. There are no unknown back feeds into the main to be decommissioned.
Decay tests must be carried out when gas demand is at its highest, i.e. either between 7.00am and 8.30am, 12.00am to 1.00pm or 4.30pm to 6.30pm.
- Yes
- No
- N/A
A record of the decay test must be made utilising two one-hour LP recorders fitted both ends of the main to be de-commissioned. The charts from the recorders must be returned as part of the job completion documentation
- Yes
- No
- N/A

There are two types of decay test:

Where there is a known back feed i.e. the main is two way fed:
The main to be de-commissioned must be isolated with the bypass fully commissioned.
The bypass must be slowly closed whilst pressure on either side of the isolated section and within the section are observed.
If either pressure falls to 5mbar or below the initial operating pressure, the bypass must be immediately reopened and an investigation carried out to identify the cause of the pressure drop.
If both pressures remain steady at less than 5mbar drop, both pressure gauges must be observed for a minimum of 5 minutes to establish if: 1. There is a back feed (indicated by a rise in pressure to the level initially observed) 2, That there are still customers connected or there is leakage from the system (indicated by a steady or rapid decline in pressure) 3, The isolated section has no back feed and there are no customers or leakage (indicated by a maintaining of the pressure within the isolated section(s))
When a satisfactory initial decay test has been undertaken the main must be physically isolated by cutting and capping with the bypass fully opened and the decay test in test described in a-d above repeated with the time for observation in d) increased to 15 minutes.
If after 15 minutes the main has maintained its pressure, proceed with the operation
Pressure decay test indicates that there are no customers attached to
main back feed:
If the pressure in the main to be de-commissioned remains steady the main must be physically isolated
by cutting and capping both section of pipe, ensuring that the bypass is fully open. The following steps
can be taken to de-commission the main:
A vent pipe 2.5m above ground level must be fitted to the main at one end for two way fed mains and at the end furthest away from the disconnection site for single fed mains, and the gas vented slowly until the pressure falls to 5mbar below the operating pressure.
The vent must be closed, and the pressure gauge observed for any build-up of pressure, which would indicate an unknown back feed. Should this occur the section must be vented to 5mbar below the opening pressure and time taken to recover to the district pressure recorded. This will provide guidance on the size of the back feed. Report this information to your Operational Manager immediately.
If a back feed is detected investigations may be required to locate the back feed through the interrogation of the gas network plans., trial hole excavations, camera survey, etc., and your Operational Manager must provide you with guidance
If the pressure is maintained at 5mbar below the operating pressure it must be observed for a further 15 minutes and any decline in pressure would indicate that there are still customers connected or there is leakage from the system.
Under these circumstances the bypass must be reopened and kept open until the cause of the pressure decay has been established.
Under these circumstances the following actions can be taken.
Check records and premises within the confines of the abandonment to ensure that there are no supply connections being fed from the main.
Carry out a survey of the area using a bar hole and gas detection equipment to locate the gas leak from the isolated system.
Following the location of leakage, a decision to repair or abandon the main must be taken.

Riders

Refer to Section H1, (SGN/WI/ML/2)

Vents

Refer to Section H1, (SGN/WI/ML/2)

Decommissioning direct purge operation using a compressor:

NO SMOKING signs must be displayed, and actions must be taken to ensure that no smoking, naked lights or other sources of ignition are present. The Operative must be vigilant to ensure that no members of the public or operatives contravene the instructions. Fire extinguishers must be available on site and positioned for immediate use. The main must be physically isolated in accordance with this procedure after carrying out successful decay tests as detailed in procedures B1 or B2 and B3. the following action must now be undertaken:
Close and decommission all bypasses by closing the valves A and C and opening valve B. Check all joints on live capped ends with leak detection fluid and wash off with clean water.
Vent the main to atmospheric pressure by opening valve G on vent at extremity of the mains system to be decommissioned, close vent and check that the pressure in the main does not increase.
If there is no pressure increase, fully open valve G.
Start compressor and admit compressed air by opening valve D, ensure that valve E is fully opened.
Monitor pressure at P2 if this exceeds the design pressure of the system, 75mbar for LP and 2bar for MP, stop the purge and investigate the reasons causing this excess pressure, i.e. Vent not fully open or undersized.
Continue purge until two consecutive checks confirm, via sample point F, indications of 10%LEL or less at vent.
Turn off compressor and close valve D.
Close all vents and inlet valves remove all bypasses and install plugs.
All openings to the abandoned main must be securely sealed to prevent egress of residual gas or the ingress of water or possible leakage from the mains system. All plugs on the live mains must be checked using leak detection fluid.
Complete written procedures and documentation and tidy site.

De-commissioning branched system using a compressor:

Connect the compressor to the largest diameter pipe in the network.
Where branches are to be purged simultaneously the vents and compressor, must be sized for a pipe diameter equivalent to the sum of the cross-section areas of all the pipes being purged. The Operational Manager will prepare these calculations and information in respect to these will be given as part of the NRO/RO.
If the system is to be sequentially purged, the largest diameter branch must be purged first with the vents sized for that branch and the compressor sized for the largest pipe diameter. The next largest branch must then be purged, and the sequence continued until all the branches have been purged.