DN-17b – PE Double squeeze-off > 180mm ≤ 355mm LP, & > 63mm ≤ 355mm MP

Title Page

Site conducted
Conducted on
Prepared by
Location
Operative being assessed

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

Tools and equipment

4 No. Squeeze-off units
Bypass.
Pressure gauges
Metallic vent pipes, flame traps, earthing straps and pressure points.
Clean, damp, on-synthetic cloths, paper towelling and water
Expanding pipe stoppers.
PE pipe cutters/saw.
Squeeze-off tape.
Re-rounding clamp.

Pipe & fittings preparation:

Thoroughly clean the main where the squeeze-off and joints are to be made
Check diameter, Standard Dimension Ratio (SDR). Material type – information will be printed along the pipe barrel at 1m intervals on each side of the pipe. Check then pipe for ovalty and re-round as required.
Ensure the correct distance is maintained from the existing fittings, joints, squeeze-off applied locations and any proposed future joints used for connection purposes.
Set out the squeeze-off operation by marking the exposed pipe with all positions for pressure points, by-pass connections, vent points, rider points and location of squeeze-off.

Operational material limits

Limits have been set for squeeze-off operations to prevent long-term damage to PE pipes during squeeze-off operations.
The minimum distance between squeeze-off bars or beams is calculated as a percentage of twice the minimum of the wall thickness. > For up to 250mm pipe the limit is 80% of twice the minimum wall thickness. > For 315mm-355mm pipe the limit is 90% of twice the minimum wall thickness.
The range of PE wall thickness are detailed in pipe specifications but should be confirmed on be the use of an ultrasonic measuring devise.

Establish network pressure:

Install two pressure monitoring points on the PE main either side of the proposed flow stop section of pipe, a PE tapping tee can achieve this
Attach and commission pressure gauges.
Confirm Network pressure expected and that it does not exceed 2bar for medium pressure and 75mbr for low pressure works.

Assemble by-pass & by pass vents:

Construct the bypass in size and material as per written procedures. Where large diameter branch saddles are required refer to module 6-Specialist techniques for construction details.
Where the by-pass is 90mm or below, it must be pressure tested with valves A and B in the open position with the vent pipes capped off above the vent valves C and D, these vent valves must be left in the open position. The bypass must be pressure tested with air to 350mbr for low pressure or 3bar for medium pressure for 15 minutes. No pressure loss is accepted. On successful completion of the air test the vents on valves C and D must be closed and the vent pipes erected, close vales A and B
The main can now be drilled.
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). To provide valves A and B. the bypass must be tested to the same requirements as above prior to connecting on to the under-pressure tees/valves. Any joint not included in a test must be checked using a leak detection fluid once the bypass has been commissioned.

Commission bypass

It is important that the bypass is confirmed free from blockages by purging both ways. Check all valves are closed, check network pressure.
Open vent valve C, then open bypass valve A,
Purge until two independent readings of 90% gas in air have been obtained from vent C, then close vent valve C and bypass valve A.
Open vent valve D then bypass valve B.
Purge until two independent readings of 90% gas in air have been obtained from vent valve D. Close vent valve D.
Open bypass valve A slowly and pressurise to network pressure.
Fittings not tested must be checked for soundness with approved leakage detection fluid, washing off pipe and fittings with clean water afterwards
The bypass is now fully commissioned.

Squeeze-off preparation

Set the stops to the correct pipe size and SDR. If the SDR of the pipe cannot be found, then set the stops to the size of pipe. For 90mm and 125mm SDR 17.6 pipe it is necessary to set the stops to one size below i.e. 125mm to 90mm and 90mm to 63mm. See procedure J below. For larger diameter pipes the squeeze-offs manufacturer’s operating instructions should be consulted for correct use of the equipment
Check squeeze-off units are in good working order by closing and opening them on a scrap piece of PE.
Place the squeeze-off units on the main
- Yes
- No
- N/A
Attach all vent pipes and the purge riders onto main as set out on the pipe. Vent pipes must be constructed as per Section H1, (SGN/WI/ML/2) and protrude 2.5m above ground level. Metallic vent pipes must be earthed using approved electrical earth rods and straps from the metallic vent pipes to earth. Vent pipes must be adequately supported

Squeeze-off PE main:

Re-check Network pressure to confirm mains operating pressure.
Re-check bypass is fully open.
Slowly apply downstream squeeze-off unit 1, until it reaches its maximum position, monitoring the pressures at both mains pressure points to ensure the pressure is not falling. Monitor the pressure gauges for 10 minutes before proceeding. If the pressure starts to fall below that stated on the written procedure release squeeze-off and inform the Operational Manager
Apply squeeze-off safety stops.
Slowly apply upstream squeeze-off 2. Continuously monitor pressure.
Apply squeeze-off safety stops.
Open vent valve F or K and check between the squeeze-off units 1&2 to establish whether there is any gas passing.
If gas is passing, refer to Section E1, (SGN/WI/ML/2)
If no gas is passing continue with the operation, leave vent pipe F and/or K open.
Slowly apply upstream squeeze-off 3, monitoring Network pressure.
Apply squeeze-off safety stops.
Check at the vent between the squeeze-off units 2 & 3 by opening valves G and I, ensuring valve H remains closed, to establish whether there is any gas passing.
If gas is passing, see Section E1, (SGN/WI/ML/2)
If no gas is passing continue with the operation, leave vent pipe between squeeze-offs 2 and 3 open.
Slowly apply downstream squeeze-off 4, monitoring network pressure.
Apply squeeze-off safety stops.
Check at the vent E between the squeeze-off units 1 & 4 to establish whether there is any gas passing.
If gas is passing, see Section E1, (SGN/WI/ML/2)
If no gas is passing continue with the operation, leave vent pipe E open.
At the instruction of the Operational Manager the section of main between squeeze off 1 & 2 may be purged with an inert gas, introduced to the work site through K, and vented through F, with care being taken to ensure that the inert gas pressure does not exceed one third of the pressure in the live main

Cut out PE main:

Drape damp cloths around the pipe and ensure they are in contact with the ground on each side of the cut to prevent a spark from static electricity.
Cut pipe section as specified in the work instruction, continually monitor the network pressure and the atmosphere within the cut-out area. The cutting of PE mains must only be undertaken using approved equipment. Should a guillotine be used a re-rounding clamp should be put on the main prior to joining.
Install expanding stoppers whilst preparing fittings for installation.
Remove expanding stoppers.
Install new section as specified in the work instruction and in accordance with Appendix D, (SGN/WI/ML/2) Electrofusion.

Re-commission main:

Re-commissioning of the PE main can be carried out either by using a rider or the controlled release of squeeze-offs. For medium pressure mains, the preference is to use a rider to enable the pressure in the newly inserted section of main to be raised in stages allowing leakage test to be undertaken using leak detection fluid.

Re-commission PE main by use of a rider:

Connect purge rider through valve J to closed vent point H. A governed purge rider may be required as detailed in the written procedure. The construction of rider’s is detailed within Section E2, (SGN/WI/ML/2)
Close valve G, Open valve J & H and purge rider until two independent readings of 90% gas in air have been obtained at vent.
Close valve I.
Remove safety stops and release downstream squeeze-off 1.
Remove safety stops and release upstream squeeze-off 2.
Close valves F & K (valve K where applicable), open valve G and purge through vent pipe E until two independent readings of 90% gas in air have been obtained at vent pipe E.
When purge is complete turn off valve E, pressurising the section to line pressure.
Test installed fittings with leakage detection fluid. Wash off with clean water.
While carefully monitoring pressure gauges, remove safety stops and slowly release upstream squeeze-off 3.
While carefully monitoring pressure gauges, remove safety stops and slowly release upstream squeeze-off 4
Close valve G
Isolate purge valves at H and J and remove rider.

Re-commission main by controlled release of a squeeze-off:

Close valves G and, ensuring H is in the closed position.
Remove safety stops and release downstream squeeze-off.
Remove safety stops and release upstream squeeze-off.
Close valve(s) F & K (K where applicable).
While carefully monitoring pressure gauges, remove safety stops and slowly release upstream squeeze-off 3 and purge through vent E until independent reading of 90% gas in air (GIA) are obtained.
When purge is complete turn off valve E. pressurising the section to line pressure.
Test installed fittings with leak detection fluid.
While carefully monitoring pressure gauges, remove safety stops and slowly release downstream squeeze-off 4.

By-pass & vent point de-commissioning:

Isolate vent points E, F, G and K (K where applicable)
Remove completion caps from top tees and re-insert pipe cutter onto the main at the vent connection. Check seal using leak detection fluid.
Remove vents; install electrofusion caps on the outlets of the saddle fusion tees. Raise integral PE cutters and fit completion caps. Test electrofusion and completion cap. Wash off with clean water.
Slowly close bypass valve B whilst monitoring network pressure for 10 minutes to ensure renewed section is supplying downstream.
Slowly close bypass valve A and vent bypass by opening vent valve C and checking the pressure reduces to zero.
Close vent valve C.
Connect air supply to valve C, open valve C and D and direct purge bypass to vent D using no more than 1/3 rd. of the mains operating pressure.
Remove bypass pipe work.
Remove squeeze-offs 1and 4 and re-round PE main.
Seal bypass connections at tapping tees/saddles, cut and cap PE.
Test with leakage detection fluid. Wash with clean water.
Apply squeeze-off applied tape to the main at all squeeze-off positions.
Follow Section E1, (SGN/WI/ML/2) for squeeze-offs passing gas and for
other fault finding: