DN-16- Metallic Flow Stopping up to & including 12”/300mm

Title Page

Site conducted
Conducted on
Prepared by
Location
Operative being assessed

Tools and equipment

Bag stop drilling machines/bases
Bag stop housings/spindles
Flow stopping bags plus one spare bag tube assembly plus at least one spare bag per bag tube
Drilling base drills
Metallic vent stacks, flame traps and earthing straps
DI/CI pipe cutters & steel cutters
Pressure gauges
Bag inflation equipment
Continuity bond
Expanding pipe stoppers
Nitrogen bottles and regulators (as appropriate)
Timber supports

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 – Mains preparation & drilling:

A position on the main must be selected where there is no large corrosion defects or hard encrustations and the main cleaned by scraping and wire brushing. If necessary, wash the main with clean water to ensure secure seal at machine saddle. For ductile mains ensure that the plastic wrapping is removed. Ensure that the plastic coating is removed when encountered on Steel mains.
The underside of the main must be excavated at points for access to the drilling machines and where the cut is going to be undertaken. Ensure the underside of the main is supported throughout the operation; wood blocks should be used to support it. Ensure sufficient room is made to operate the cutters to achieve a full circumferential cut.
A machine saddle and seal must be selected according to the main size, its condition checked, and fitted to the machine or main as appropriate. The fixing chain nuts should be tightened evenly to a moderate hand torque while ensuring that the machine is level.
The drill tap must be inspected and lubricated with oil or grease.
A parallel threaded drill tap must be fitted to the chuck and inserted in the machine.
An air test at a pressure of 350 mbar must be applied with the gate valve in the OPEN position and the machine seal checked with an approved leak detection fluid solution. No leakage must be visible during the test duration of 5 minutes
If leakage is observed the machine must be inspected for the leakage path if the seal cannot be achieved without tightening the fixing chains beyond a moderate hand torque the base must be removed and the main cleaned further, or the equipment exchanged.
The pressure gauge used for this test must be left connected and monitored whilst the main is being drilled. The pressure gauge used must be suitable to operate within the pressure regime of the main.
Drill and tap the main in preparation for the installation of the foam bags using an approved under pressure-drilling machine using the tapping hole sizes as detailed in Table 37, (SGN/WI/ML/2)
Drill the main using a hand ratchet or air motor. Preferably the main should be tapped using a hand ratchet. If excessive force is needed this should be investigated. The drill tap should be withdrawn and for single spindle machines, the valve closed. The coupon should be retained for inspection.
This process must be repeated at each of the drillings to be made.
Complete all drillings
Ensure an electrical continuity bond is installed across the full section of the main to be isolated and cut out.
Pressure gauges and/or recorders must be installed at either end of the main.

Equipment set up:

The bag stop equipment must be laid out and inspected as per the manufacturer’s instructions.
Ensure all bags are checked for damage and are within their “use by date”. Check the bag inflation pressure will withstand the pressure they will be subjected to (pressures and use by dates are normally written on the side of the bags).
Gauges used to inflate the bags must be checked to ensure that they are within the pressure range and are not damaged.
Each bag must be pressure tested as per Table 27, (SGN/WI/ML/2) and tested for 5 minutes. During this time the rest of the bag connections on the housing should be tested with leakage detection fluid. At least one spare bag must be available per bag tube, these must also be tested prior to commencing insertion of the bags into the main.
Where the mains operating pressure is above 75mbar the bags must be inflated with an inert gas, the Operation Manager on site must ensure that all safety precautions have been taken and an appropriate risk assessment for the procedure completed. The inert gas source must be separate to that used for purging purposes. A nitrogen bottle for each tube assembly must be provided.
Any bags that fail must be checked for damage or defects and then destroyed.
Should oils be suspected within the main the bags must be protected by either covering with an impervious cover or by sealing the stitching with a suitable sealant, such as Bostick 2064. On completion of the bag stop operation all bags that have been subjected to oils, etc. Within the main must be destroyed.
When the equipment has been checked and the bags tested the bags must be withdrawn into the bag tube assemblies, ensuring that the natural bag curvature lines up with the direction indicator marked on top of the bag tube assembly.
The bag tube foot must now be attached, ensuring correct alignment with the direction indicator. The correct foot size can be checked by reference to the manufacturer’s instructions.

Assemble bypass/set up equipment onto main:

Construct bypass in size and material as per the written procedure. The bypass must be protected from any risk of vandalism or mechanical damage and this must include the burying or sleeving of PE materials, using a flexible steel bypass or using all welded steel for constructing the bypass.
For mains abandonment, a governed bypass may be installed at the instruction of the Operational Manager.
- Yes
- No
- N/A
The bypass should be fabricated using the minimum number of joints.
Prior to installing around the main, the bypass must be pressure tested with air to 350mbar for LP or 3 bar MP for 15 minutes. No pressure loss is permitted. On successful completion of this test the bypass can now be installed. Where larger diameter bypasses are used appropriate under pressure drilling arrangements must be undertaken in accordance with Section D1 or D2, (SGN/WI/ML/2). The bypass must be tested to the same requirements as above prior to connecting onto the under-pressure tees/values.
For the correct insertion and withdrawal sequence the direction of gas flow should be determined as stated in Procedure A (f) and gauges or recorders fitted.
It is important that the bypass is confirmed free from blockage by purging both ways.
All bag tube assemblies must be fitted with valved metallic vent stacks with flame traps, the top of which must be a minimum of 2.5m above ground level. All valves must be in the closed position and the vent stacks must be adequately supported.
The bag tube direction indicators, on the bag tube assemblies, must face away from the section to be cut out.

Commission bypass:

Ensure that the pressure gauges or recorders are working and fitted correctly, showing current pressure in the main.
Open the vent value on the downstream side of the bypass
Slowly open the upstream bypass gate valve on the bag stop base and allow gas to vent out of the bypass at the downstream vent position.
Confirm two consecutive readings of greater than 90% G.I.A and close the vent valve.
Close the upstream bypass gate valve, open the upstream bypass vent and open the downstream bypass gate valve.
Confirm two consecutive readings of greater than 90% G.I.A from the vent and close the upstream bypass vent.
Re-open the bypass gate valve. Any joint(s) not included in a pressure test must be checked using leak detection fluid once the bypass has been commissioned.
The bypass is now commissioned.

Bag stop main

The insertion of the bags into the main must be carried out in a controlled manner in the correct sequence.
Ensure that the pressure gauges monitoring the mains pressure are continuously monitored to ensure that the pressure downstream of the operation remains stable. Should the pressure start to fall below the minimum pressure stated on the written procedure the operation must be stopped immediately and the Operational Manager informed.
- Yes
- No
- N/A
Throughout this insertion and inflation process the two pressure gauges at either end of the operation must be monitored.
Ensure all vents are closed and the bypass is commissioned.
Open the gate on bag tube assembly 1, upstream secondary, and insert the bag into the main, ensure that the bag is facing away from the cut-out section. Inflate the bag gradually to required pressure Table 27, (SGN/WI/ML/2)
Prior to inserting the 2 nd bag the bypass must be closed down and the gauges monitored for 5 minutes to ensure that there is no back feed. If a back feed is suspected the Operational manager must be informed prior to commencing with the operation. If there is no back feed re-open the bypass valve
As shown in Figure 64 once the pressure has stabilised on the gauges, insert bag number 2, (downstream secondary) and inflate to the required pressure. Table 27, (SGN/WI/ML/2)
Vent the area between the two bags by opening the valve on the vent stack of bag tube assembly 1 or 2 or both...
Ensure that an adequate seal between the bags has been achieved. If there is still a large volume of gas passing between the two bags, close the vent(s), deflate the bags and then re-inflate, and re-open intermediate vent(s) between the bags.
When the Competent Person, under the Safe Control of Operation procedure is satisfied that the seal between bags 1 and 2 is acceptable, the operation should re-commence by inserting bag number 3, downstream primary and inflating to the required pressure. Opening the vent on bag tube 3 can check the seal between bag number 2 and 3. Should a seal not be achieved the process as described in i. above must be repeated.
Finally bag tube 4, upstream primary can be inserted and inflated, check the seal between bags 1 & 4 by opening the vent on bag tube 4. Should a seal not be achieved the process as described in step i. above must be repeated.
The pressure gauges on the bag tube equipment should be monitored continuously. Should the pressure gauges on the downstream side of the operation start to fall the operation must be stopped immediately and further investigation completed by the Competent person. Should the pressure on a bag tube assembly gauge fall the sequence described in procedures H & I below must be followed.
All bag tube assembly vents must remain in the open position.

Cutting operation

The main can now be cut out and the work carried out. The gauges/recorders upstream and downstream, along with the gauges on the bag tubes, should be observed for changes in pressure throughout the operation. Ensure the pipe sections to be cut are accurately determined and marked.
Ensure that the section of main to be cut is well supported with timber and wedges. These must not impede the installation and assembly of new fittings or pipe
The method of cutting will be determined by the material and diameter of the main and should contain a minimum of three cuts as detailed in the work procedures
Where necessary additional cuts should be made to ensure that the section of pipe is removed with minimum of effort.
The open ends of the pipe must be temporarily sealed with expanding stoppers or cap ends installed, as appropriate.
Before the new section of pipe/fitting is placed into position a check must be made to ensure tools, stoppers or other materials have not been left in the ends of the main.
In some instances, the section of main between bags 1 & 2, (the secondary bags), may be purged with an inert gas, care being taken to ensure that the inert gas pressure does not exceed one third of the pressure in the live main.
Whenever welding is to be carried out on the isolated section of main, the following additional
precautions are required.
Assembly of the bag tube equipment and bypass will probably be in remote excavations from the proposed hot works.
Ensure that the main to be hot worked is the one that has been isolated.
Bag stoppers must be positioned at least 10m from any potential source of ignition.
Bags must be inflated with an inert gas regardless of diameter
The section of main between the two secondary bags must be purged with an inert gas detection instrument.
The atmosphere adjacent to the cut out must be monitored continuously using a gas detection instrument.

Re-commission main (using a purge rider):

Inert gas may be used to purge the section of main between bag tube 1 and 2, secondary bags. It is important that the inert gas purge is disconnected and removed prior to the natural gas purge commencing.
Close the vent valve on bag tube 4 upstream primary and remove vent stack with flame trap.
Remove the vent stack with flame trap from the upstream bypass connection.
A suitably sized rider, as stated in the written procedure, should be connected between the vent assembly on bag tube 4, see Figure 67, upstream primary, and connected to the upstream bypass vent valve. The construction of the rider is detailed within Section H1, (SGN/WI/ML/2)
Test for leakage with leak detection fluid by opening vent valve on upstream bypass connection ensuring that the vent valve on bag tube 4 remains in the closed position.
The vents on bag tubes 1 and 2 (secondary bags) should be closed and bag 2 deflated, followed by bag 1. It is important to leave the deflated bags in the main at this point in case the replaced section is leaking, and the bags need to be re-inflated. Natural gas can now be slowly introduced through the rider by opening the vent valve on bag tube 4, venting out of the vent stack on bag tube 3. When two readings of greater than 90% GIA are observed, the vent valve on bag tube 3 can be closed and the renewed section allowed to pressurise to line pressure.
The renewed section must now be tested with approved leak detection fluid. Should a leak occur the re-commissioning operation must be stopped immediately by closing off the rider to bag tube4, all personnel must exit the excavation and the situation assessed as to the cause of the leak. The leak must be rectified before continuing.
Bags 1 & 2 should be withdrawn into their assemblies, followed by the deflation and removal of bag tube 3 and then 4. Figure 69, (SGN/WI/ML/2). Monitor the pressure in the main at both ends. Isolate the rider supply at the vent assembly on the upstream bypass and on bag tube 4.
The bag tube assemblies may then be removed, and the tapings plugged off.
Test all plugs for soundness with leak detection fluid.

Re-commission main (with-out using a purge rider):

Inert gas may be used to purge the section of main between bag tubes 1 & 2, (secondary bags). It is important that the inert gas purge is Disconnected and removed prior to the natural gas purge commencing
The vent on bag tube 2 must be closed and bag 2, downstream secondary, deflated. Close the vent on bag tube 1 and deflate bag 1, upstream secondary. It is important to leave the deflated bags in the main at this point in case the renewed section is leaking, and the bags need to be re-inflated.
The vent on bag tube assembly 3, downstream primary, must now be closed and 3 slowly deflated. The rate of flow should be controlled to ensure that the downstream gas supply is not affected.
The purged natural gas must be allowed to vent out of the vent stack on bag tub 4 until 2 readings of greater than 90% GIA are observed. At this stage the vent on bag tube 4, upstream primary, must be closed and the section allowed to pressurise to line pressure.
The renewed section must now be tested with leak detection fluid. Should a leak be detected all personnel must exit the excavation and the situation assessed. Where deemed satisfactory reinflate the bags and vent the area between them. The leak must be rectified before continuing.
Once the mains pressure has equalised, bags 1 & 2 may be withdrawn followed with bag 3.
Bag 4 can then be deflated and withdrawn into its assembly. Monitor the pressure at both ends
The bag tube assembly may then be removed, and the tapings plugged off.
Test all plugs for soundness with leak detection fluid.

Removal of bypass:

Once the section of main has been re-commissioned and the bag tube assemblies removed and plugged under pressure, the by-pass and rider valves may be closed down. Before purging the bypass with air, or with inert gas, (specialist technique) the mains pressure either side of the flow stopping operation should be monitored to ensure that the pressure does not fall downstream. If this starts to happen the by-pass must be immediately reopened and the situation should be assessed
When the gas pressure downstream has stabilised, the by-pass and pressure gauges can be removed, and all tapings plugged off and tested for soundness with leak detection fluid.
If a continuity bond has been fitted this can now be removed.
All equipment should be checked for damage, if there is any evidence of damage of over pressurisation or contamination with oils on the bags they must be destroyed.
The seals on the bag tubes and machines must be checked and replaced as necessary.
All gauges and recorders must be checked and recalibrated if necessary.
The equipment must not be returned to storage until it is fully serviced
All equipment must be cleaned prior to storage in order to keep the equipment in well maintained and in good working order