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Die Drawing

Technique

For all close-fit PE lining methods, during the reduction process the outside diameter of the PE100 pipe is reduced to less than the minimum bore diameter of the host pipe. Once the PE100 liner pipe is in the required position it is reverted to a close fit with the host main by the application of internal pressure, thus reversing the deformation process. In practice, there are limits to the amount of diameter reduction that can be applied in concentric reduction processes (this and roller reduction), whilst to achieve a close fit throughout a host pipe of varying diameter there are limits to the amount of diameter variation that can be accommodated, and both of these limitations need to be understood and taken into account when choosing both the method and the liner diameter for the particular application.


Image courtesy: ISTT


Image courtesy: Exova Utilities

Die Drawing is a concentric reduction/expansion process

A PE100 liner pipe which is initially of a larger diameter than the inside diameter of the host pipe is winched through a die, reducing the diameter of the liner pipe to less than the minimum inside diameter of the host pipe, with a proportional increase in length, and with virtually no reduction in pipe wall thickness. The outside diameter of the reduced PE100 liner pipe is maintained during insertion through the constant application of longitudinal tension on the liner pipe as it is pulled into the host pipe. There are two similar but slightly different methods of die drawing: using a static die (Swagelining); and using a roller die (TiteLiner).

Once the PE100 liner pipe is in the required position it is reverted to a tight fit with the host pipe by releasing the longitudinal tension, thus reversing the deformation process and achieving the maximum possible capacity and enhancement against collapse under surge vacuum conditions or external hydraulic pressure due to ground water when empty.. Because the length of the liner pipe is increased as it is drawn through the die, it needs to be pulled beyond the end of the exit pit or manhole to provide sufficient exposed liner pipe to allow for the shortening of this pipe as it reverts to its original diameter and length.

Whilst the operation essentially stretches the liner pipe, the effect of stretching it through the die results in a longitudinal expansion and a proportional diametrical contraction, and in which a constant wall thickness is maintained. The operation is carried out well within the elastic limits of the polyethylene and when the stretching force is released, the liner pipe recovers almost to original length and diameter. During the operation, some of the polymer chains will be re-aligned into a more longitudinal direction and this is not an elastic process, so there will inevitably be a small increase in length and decrease in diameter that is not recoverable.

Applications

Water mains. Gas mains. Sewer rising mains. Rehabilitation.

Close-fit lining is an ideal application for the rehabilitation of pressure pipes that are relatively straight or have only modest bends, and that have largely maintained their circular profile.

Close-fit lining is possible because of the memory properties of PE. PE pipe will change shape when force is applied to the material either through the application of compression or tension but will return to its original shape when the external force is removed or internal pressure is applied. This property allows the PE100 pipe to be temporarily deformed and pulled into the host pipe. When the new pipe has been pulled to the desired position, tension on the pipe is no longer applied or internal pressure is applied and the pipe will return to its original shape. The versatility of PE pipe has spawned the development of a wide range of innovative close-fit lining systems.

Installation Procedure

The PE100 pipe to be inserted is laid out on the surface at the entry excavation and butt fused to the required length. It is preferable to fuse the entire length prior to commencing the pulling-in stage. However, where there is not sufficient space to weld up the entire PE100 liner pipe string into one length, it is quite feasible to stop the insertion whilst a new length is welded on. During such stops it is essential that the pulling load is maintained to prevent any reversion and the welding unit must be free to move if the liner pipe does creep forward. Also the tensile load must be carefully controlled to ensure that the bead up pressure at the butt fusion machine is adequate. When welding on additional pipes sufficient time must be allowed for the butt weld to cool before proceeding with the insertion process. All external weld beads must be removed.

The PE100 pipe must be placed on suitable rollers in order to minimise friction, and therefore running load, and to prevent damage to the pipe which should not be scraped along the ground during installation. During insertion, the pipe wall should be continuously inspected before it reaches the die to confirm that any surface scratches or defects are less than 10% of the pipe wall thickness. Any defect in excess of 10% puts the pipe out of specification, so should be cut out and a new section of pipe welded in to replace it.

The pulling force is generally provided by a winch sited at the exit excavation. The first step is to feed the winch wire or rods into the die and connect it to the nose cone on the front of the PE100 liner pipe. The nose cone is lubricated and the winch operated to pull the nose cone into the die, taking care that nothing touches the die apart from the PE of the nose cone and liner pipe.

Once the nose cone is through the die the pulling load is increased very gradually until the liner pipe itself starts to move through the die, at which point the lubricator is refilled. The pulling load will increase to a little above the predicted initial running load [die load plus string roller friction load plus excess die friction] until the die becomes fully lubricated . The transition geometry of the liner pipe will change as it passes very slowly into the excavation and under the guide roller to approach the host pipe.


Image courtesy: Exova Utilities

When the liner pipe has entered the host pipe and all the adjustments have been made to optimise the geometry, the pulling speed can be slowly increased whilst carefully monitoring the pulling load which should stabilise at around the predicted load within a few minutes of the maximum speed being reached.

The insertion should then proceed smoothly, though it is prudent to slow down as the liner pipe passes through any intermediate excavations to ensure that nothing snags as the liner enters the next section of the host pipe, after which the pull can be accelerated once more. The pipe wall should be inspected to confirm that any surface scratches or defects are less than 10% of the pipe wall thickness. Any defect in excess of 10% must be investigated and, if necessary, replaced. Pulling load should be closely monitored throughout to see that it follows the predictions, particularly during progress round and after any bends. The pulling load must be maintained constantly throughout the insertion phase, including any stops for welding further sections to the liner pipe, as rapid reversion of the PE100 occurs as soon as the load is relaxed, and although the rate of reversion decreases with time, running clearance would soon be lost and the liner pipe would become fixed in place within the host pipe.

The die frame may be located within the trench, anchored by attaching it to the end of the host pipe. This may be satisfactory for small diameters but there is a risk in thrust loading and possibly damaging the end of the host pipe, and the load transferred from the guide roller can increase this risk considerably. For larger diameters, this does not place the guide roller in a suitable position anyway because it becomes impossible to align the liner pipe correctly with the die. This makes it impossible to use the lubricator resulting in increased friction and pulling load and stressing the welds asymmetrically.

On completion of the insertion, recovery is simply a case of releasing the pulling load to allow the liner pipe to contract longitudinally and expand circumferentially, and the time taken to achieve tight fit should be as predicted. In practice, the liner pipe extends longitudinally during the insertion, then contracts when the pulling force is released during recovery, and the installer will predict both the length of that extension and the following contraction as key parameters.

If the reception excavation is long enough to accommodate the contraction length the liner pipe is pulled up to the end of the excavation and released. Typically, a 200 metre long insertion using a 10% die would have a contraction length of some 8 metres. In such circumstances, it is necessary to pull more liner through the die – until the predicted extended length of liner pipe less the length of the entry excavation has been pulled through – and this can be achieved by releasing the pulling load for several minutes, during which time the liner pipe will contract longitudinally and may disappear back up the host pipe, and then pulling it up to the end of the excavation again, repeating this several times if required.


Image courtesy: Exova Utilities

From this diagram, it can be seen that when the pulling load is released for the first time, a rapid recovery of diameter, maybe 2% or even 3% of the original diameter, occurs within a minute or two, and this rapid reaction is why the pulling load must never be released until the insertion is complete. When the pulling load is applied once more, there is a very small decrease in diameter, followed by another increase in diameter when the pulling load is released for the second time, though the increase is both smaller and slower.

There are two basic methods of releasing the liner pipe from the die: a split die system which requires a more expensive die but the time and cost savings that result on site should more than compensate; or cutting the liner pipe behind the die, and then cutting a long ‘V’ longitudinally into the liner pipe towards the back of the die, followed by a repeated process of careful pull and release stages. This latter is a delicate and time-consuming operation and requires a competent and experienced site team.

Ambient temperature has a significant effect upon pulling loads; typically a load at 35°C is some 50% of that at 6°C but, unlike a load increase due to excessive die friction, this would result in no change to the elongation or running diameter because the temperature would also affect the material modulus and yield stress.

The PE100 liner pipe will increase slightly in diameter as it emerges from the die. This is due to ‘die swell’ and is an almost instantaneous elastic recovery when released from the die constriction, generally about 5% of liner pipe initial diameter irrespective of liner pipe diameter or SDR. The effect of this is that if a 10% die is used, the actual diameter reduction achieved will only be 5%.

If the speed of insertion is increased, there is an initial increase in pulling force required, but this quickly falls back to its original value, and speeds of 5 metres per minute or more are quite feasible. This can be important on long insertions, as the longer the liner pipe is under tension, the slower will be the recovery. However higher rates do need to be applied as a continuous process because in a ‘stop-start’ action, as would be the case with a single action rod or wire puller, the liner pipe would be continually subjected to the higher pulling forces incurred during acceleration, and this would make the recovery process slower.

Except in the case of short insertion lengths, there is likely to be a significant difference in the length of time the front end of the liner pipe and the tail end are under tension, as a result of which the tail end will recover more rapidly when the tension is removed, so the liner pipe will usually achieve a tight fit progressively, starting from the tail end.

Typically, in a well designed and executed die draw, the liner pipe should recover to 98% or more of its initial diameter within 24 hours, though recovery will be slower if excessive pulling loads have been applied or if the pulling load is applied for an excessive length of time.

Equipment

At the entry end of the works the main equipment is the die itself. The apparent simplicity of this process is deceptive, and the design and operation of the die is absolutely critical in order to maximise the elastic part of the operation and keep the non-elastic part to a minimum. Failure to use the optimum die geometry and to keep die friction to a minimum will result in a greater pulling force being required, and whilst this will decrease the running diameter, it will also increase the elongation and result in a substantially slower recovery. To this end the die must be finished to a very high standard, generally then being chrome plated, and maintained in a highly polished condition, effectively protected from dirt ingress during operation, and lubricated with a good quality lubricant throughout.

SWAGELINING

Image courtesy: Exova Utilities

Image courtesy: Exova Utilities

Image courtesy: LUDWIG PFEIFFER HOCH- UND TIEFBAU GmbH

TITELINER

The TiteLiner system works in stages, this time each stage consisting of a unit of concentrically distributed rollers, hydraulically driven to minimise axial stress, and producing a symmetrical reduction in diameter.


Image courtesy: AEGION CORPORATION

The machine is made up of a series of these roller units, each one reducing the liner pipe diameter by a few percent, and by ‘nesting’ four or five of these together in tandem diameter reductions of up to 20% can be achieved.

This concentric die reduction machine can accommodate a wide range of host pipe inside diameters, the modular nesting of units enabling changes to be made very quickly on site, and the maximum differential in axial speed between roller and liner is low. Also, because the reduction is truly concentric, the liner pipe will remain truly round throughout.

This machine requires tension to be applied to the reduced diameter pipe to maintain the reduced diameter. Therefore a winch with the required pulling force capacity is located at the exit end of the works.

This view of a TiteLiner machine shows the hydraulic roller motors.

Image courtesy: AEGION CORPORATION

Image courtesy: AEGION CORPORATION

PULLING & TENSION EQUIPMENT

At the exit excavation the equipment is a winch located either in the excavation or at the surface. The capacity of this winch must be calculated in advance. Maximum load on the winch will be a little above the predicted initial running load until the die is fully lubricated and the required insertion speed is attained. After that it will be:

Running load = die load + string roller friction load + drag friction in the host pipe

It is particularly important that the winch is fail safe, in that the pulling load can never be released inadvertently or due to a failure of the drive system, and that if this does fail, the pulling system will be locked thus maintaining the pulling load. Continuous monitoring and recording of the pulling load applied to the PE100 pipe is essential, and the winch should be equipped with a load limiting device that can be set to the maximum permitted load recommended by the pipe manufacturer.


Images courtesy: BAGELA BAUMASCHINEN GmbH &Co KG


Image courtesy: TRACTO-TECHNIK GmbH & Co KG


Image courtesy: HAMMERHEAD TRENCHLESS EQUIPMENT

Rod pullers are much more compact than winches, and can be located within excavations much more easily. However rod removal during insertion needs careful attention, and should be done mechanically and remotely rather than manually to make a safe system of working. Both entry and exit excavations must be ‘no man entry’ when there is tension applied. Because of their stop start operation (unless they are of the double acting design) they are generally much slower than a winch.

PRACTICALITIES

Soil Types and their effects

This is a rehabilitation/replacement technique and is not dependent on the soil type. The impact of soil type and groundwater level will be on excavations for entry and exit of the PE100 liner pipe.

Diameter, Pressure and Length Range

Diameter: 50-1200mm
Length: Up to 500m
Pressure: depends on SDR or interactive design status

EXCAVATIONS

See Excavations, pit sizes, Space and Access

DESIGN, SPECIFICATION & PLANNING

Planning

It is necessary to make decisions regarding liner pipe initial diameter and running diameter. This in turn requires an accurate determination of the internal diameter of the host pipe, so that a sensible combination of master and slave dies can be selected for Swagelining, or roller die units for Titeliner... . As the technique involves the preparation, handling and insertion of often long lengths of liner pipe, site conditions generally apply considerable constraints, and it is necessary both to be fully aware of all such constraints, and to fully understand their impact upon the technique when developing an operational plan for the works.

It is vital to know the internal diameter of the host main very accurately, and to ensure that before insertion the inside of the host main is thoroughly cleaned and free from any debris or geometric intrusions, such as ferrules, plugs or even casting faults. It is recommended that a recorded internal inspection is made using CCTV ; this is mandatory in certain countries, for example UK and USA. The internal diameter must be known to enable the diameter of the liner pipe to be chosen, so that diameter may need to be determined well in advance in order to have a non-standard pipe size manufactured if necessary. It is possible to compensate for some measurement inaccuracies by adjusting the percentage drawdown, and for this eventuality the die can be made up of a master die sized to assume the host is larger than stated, and a pair of slave dies, ideally 2% and 1% smaller, so that an actual host internal diameter of between plus 1% and minus 2% of what has been determined can be accommodated satisfactorily.

The reversion process for die drawing requires the liner pipe to contract longitudinally to allow it to expand diametrically, therefore a satisfactory tight fit is conditional upon the internal diameter of the host pipe being essentially constant throughout its length. If it is not, then it is possible that a tight fit will not be attained in any sections of larger diameter, as the liner pipe will lock up at both ends of a larger diameter section before tight fit within that section has been attained. If the host main internal diameter does vary substantially, consideration should be given to using a more diameter tolerant technique such as Rolldown.


Image courtesy: Exova Utilities

The pulling load must be maintained constantly throughout the insertion phase, as rapid recovery occurs as soon as the load is relaxed, and although the rate of recovery decreases with time, running clearance would soon be lost and the liner pipe would become fixed in place within the host. There is therefore a temptation to work with a large running clearance so that in the event of a failure of the pulling device there would be sufficient time to do something about it. This creates a dilemma, as the greater the running clearance, the longer it will take to achieve a tight fit, and until this is achieved, longitudinal contraction will continue and the liner pipe cannot be pieced up. Because the recovery characteristics of the liner pipe are logarithmic the choice of running clearance is very critical, and for example, increasing the running clearance by just 2.5% can increase time to achieve a tight fit from less than twelve hours to seven or eight days.



Image courtesy: Exova Utilities

This diagram shows the effect of applying different reductions to achieve different running clearances, the blue line at the top showing liner pipe initial diameter, the black line showing the host pipe internal diameter. The 10% reduction recovery (green) achieves tight fit in less than a day, whilst the 12.5% reduction recovery (yellow) takes almost ten days and the 15% more like fifty.

There are limitations on the radius of bends through which a die-drawn pipe can be pulled. Reference should be made to the pipe manufacturer’s recommendations and to IGEM/TD/3.

This method applies tension to the PE100 pipe during installation. The tensile load on the pipe during installation must be calculated to establish whether it exceeds the maximum allowable load. Pipe manufacturers can advise on the allowable loads. If the calculation shows that the maximum permissible load may be exceeded then it may be necessary to increase the capacity of the PE100 pipe by increasing its thickness. This will require a check of ID and OD to ensure that flow capacity and external clearances are still adequate.

HEALTH, SAFETY & ENVIRONMENT

All excavations, pits, etc. which personnel will enter must comply with relevant safety regulations covering support and shoring. Relevant confined space entry procedures must also be followed. Examples of relevant legislation are:

  • Germany: DIN4124 Excavations & Trenches
  • USA: OSHA, State and local regulations
  • UK: HSG47, Avoiding Danger from Underground Services; HSG150, Health & Safety in Construction; L101 Safe work in confined spaces

For safety, reliable communications must be in place between the pipe entry location, any intermediate excavations and the winch operator, so the winch can be stopped immediately if anything goes wrong. This communication is vitally important throughout, as the pull and die ends are often several hundred metres apart, yet it must be possible to stop the pull instantly at any time.

STANDARDS & CODES OF PRACTICE

IGEM/TD/3: Steel and PE pipelines for gas distribution.

See also Standards and Codes of Practice

COMMON/SECONDARY MODULES

Pipe assembly & handling
Installation Manual
Isolation
Host pipe cleaning & inspection
End fittings
Testing & inspection
Piecing up
Excavations & pit sizes

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