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How PE Pipe Relining Works: A Guide

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The process of relining or renovating damaged or leaky concrete, GRP or ductile iron pipes with profiled polyethylene (PE) has been around for many years and has proven to be a reliable form of pipe rehabilitation in almost all fields of operation. The adoption of pipe relining technologies allows for the construction of new, self-contained pipe systems using the old pipe system as an empty conduit for the new PE pipe, thereby providing increased stability and a high static load capacity, especially when using profiled pipes in larger diameters.

Primary applications for PE pipe relining include:

  • Renovating corroded drinking water pipelines
  • Sealing a pipe system against leakages or infiltration
  • Changing gas supply systems over to higher operational pressures
  • Renovating inverted siphons
  • Various industrial applications (aggressive media, acids, alkalis, etc.)

The methods typically used to reline pipes with large-diameter PE pipes involve either a pushing (insertion) or pulling-through process depending on the application and environmental conditions. The technique that is decided on will also inform the pipe dimension, pipe material selection and pipe connection, or jointing, considerations.

Please note: The information considered below for the PE pipe relining process represents only a technical background and provides only a fraction of the considerations that need to be taken into account for pipe rehabilitation projects.

Pulling Procedure

In the pulling method, the old pipeline will be subjected to axial tensile load. This results in pipe expansion which is reinforced by bending loads that can emerge in the angles of the old pipeline.

Insertion forces are dependent on the following factors:

  • Length of the pipe to be installed
  • Weight of the pipeline
  • Ring gap
  • Interior surface of the old pipe (friction between old and new pipe)

During this process, the external surface of the pipe is mechanically stressed which causes notches and grooves that can result in weak spot in the new pipe system. This must be considered when making the static analysis and in pipe material selection. Pipe manufacturers should indicate special values that provide a reference point regarding the groove insensitiveness of the material and its resistance to slow crack growth.

The maximum permissible load during the pulling process depends on the selected pipe wall cross-section and on the selected welding method. It is recommended to use a tension head or tension anchors and a measuring box must be integrated in the pulling system to monitor force application.

Angular deflections within the pipe string can lead to an unexpectedly high tensile resistance. For the calculation of permissible tensile force, the expected load duration and the ambient pipe temperature must be taken into account.  Tensile loading lasts a few minutes or several hours depending on the length of the pulled pipe segments, but often within 24 hours in practice.

1

Where:
Fax = axial forces [N]
Aax = axial area [mm2]]
σ = design strength (from creep rupture curves) [N/mm2] for the required time and temperature
CW = Welding factor [-]
Sf = safety factor [-]

The corresponding tensile dimensions can be taken from the creep strength curves, which are listed in the relevant standards or as per the pipe manufactures guidelines.

Welding Factors according to DVS 2203-1/2205

 

Processes

Materials

PE-HD
PP
PVC-U
PVC-HI
PVC-C
PVDF
Heated Tool butt welding HS Fz 0,9 0,9 0,9 0,8 0,9
  Fs 0,8 0,8 0,6 0,6 0,6
Hot gas extrusion welding WE Fz 0,8 0,8 - - -
  Fs 0,6 0,6 - - -
Hot gas welding W Fz 0,8 0,8 0,8 0,7 0,8
  Fs 0,4 0,4 0,4 0,4 0,4

When using electrofusion sockets, the transferable shearing stress should be approximately 50% of the hoop stress.

2

Where:
Fax = axial forces [N]
Aw = welding area (without wire area) [mm²]
σ = design strength [N/mm²] for the required time and temperature
CW = Welding factor [-]
Sf = safety factor [-]

Pushing Procedure

In the insertion method, the pipe is not loaded on tension, but rather on distortion, buckling and compression. Loading duration is similar to the pulling method and depends on the length of pipe segments.

Creep strength curves can be applied for the calculation of the compression stress. Typical pressure resistance values are normally higher, however it is recommended to keep deformation exposed through higher stresses relatively small to prevent the pipe shape formation.

NOTE: The more the pipe deviates from the ideal circular/cylindrical condition, the lower the resistance against distortion and buckling (Stability Consideration).

Calculation of critical length:

3

Where:
Lk = buckling length [mm]
Fax = axial compression forces [N]
Ipipe = Moment of Inertia of pipe ! [mm4]
E = flexural modulus [N/mm²] for the required time and temperature
Sf = safety factor [-] (should be 2 for stability calculations)

With: 

4

Where:
Ipipe = Moment of Inertia of pipe ! [mm4]
OD = outside diameter pipe [mm]
eequ = equivalent wall thickness pipe [mm] (at solid wall pipes = wall thickness)

Injection grout mortar grout is used to fill the space between the old and new pipe to generate a defined load situation. During the injection process, the pipe will experience radial buckling stress which is dependent on the injection pressure and hydrostatic pressure as a result of the grout mortar density and the height between the pipe and grout mortar tank.

Calculation of radial buckling resistance:

5

Where:
E = flexural modulus [N/mm²] for the required time and temperature
ID = inner diameter pipe [mm]
eequ = equivalent wall thickness pipe [mm] (at solid wall pipes = wall thickness)
Sf = safety factor [-] (should be 2 for stability calculations)

In addition, the lifting force during the injection process must also be taken into consideration – this can be reduced by placing a distance holder at the crown between the old and new pipe. The lifting force must be lower than the weight force of the pipe segment that lies between the distance holders.

Lifting force can be calculated as:

6

Where:
Flifting = lifting force [N]
OD = Outer diameter pipe [mm]
Ldh = distance between distance holder [mm] (at solid wall pipes = wall thickness)
γgm = Density grout mortar [kg/dm³]

Contact Marley Pipe Systems

Marley Building Division
t. +27 11 739 8600
f. +27 11 739 8680

Marley Mining & Industrial Division
t. 0861 MARLEY (627539)
t. +27 12 045 0997

e. info@marleypipesystems.co.za

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