Renovation using plastics pipes is increasingly used to restore the condition of pressure pipelines. The problem of assuring the quality and long life durability of the renovated pipeline can now be overcome with the aid of soon to be published international standards, which are also described.

Our society depends very much on our infrastructure, such as roads and bridges. Our pipe and cable networks underground however, are also crucial assets in our daily life.

Pressure pipelines cross our cities invisibly and slowly but surely get older and older. They gradually decay because of corrosion and in many cases are a potential threat to our daily life.

Water mains may burst, causing traffic delays. Gas mains may explode, causing an even greater impact. Water and gas companies all over the world realise the hidden problems and have started taking measures to prevent disasters from happening.

Instead of troubleshooting by repairs when it is too late, the companies are increasingly taking proactive measures. Over time pipeline rehabilitation can save a lot of hassle and money.

Trenchless renovation

Unlike other consumptive materials simply replacing the old stuff is no longer considered a viable option. Replacing pipelines via traditional open cut trenching causes enormous inconvenience, in particular in crowded urban environments.

This is why trenchless pipeline renovation, typically by lining, is becoming increasingly popular: installing a new pipeline to take over the function of the old one without having to dig it up. Trenchless renovation techniques use the existing hole in the ground.

As with all civil constructions the quality of the installed pipe is crucial for the durability of the new pipeline. With pipe renovation it must be assured that the pipe can handle the specific installation constraints.

This is the reason why liner pipe, which is pulled or pushed into place, is generally made from abrasion-resistant plastic material, such as polyethylene. Some renovation techniques make use of pipes with a temporarily decreased cross-section. This enables easy installation, and once installed the pipes can be reformed again to come to a close fit with the interior of the old pipe.

Clearly there is need for quality assurance of the installed product.

To enable independent quality assurance, the international standardisation organisation, ISO, is in the process of publishing standards for pipeline renovation techniques, for both pressure and non-pressure applications. These new standards update, on the basis of recent industry experience, the pioneering documents originally developed by the European standards organisation, CEN, that were published between five and seven years ago. Classification of pressure liners

When considering pressure pipe liners, the basic structural aspects must firstly be addressed.

In the forthcoming ISO 11295 standard ‘Classification and information on design of plastics piping systems used for renovation’, a universally applicable classification system is set out. It arose from combining the existing European and American classifications.

In Europe, EN 13689 (to be replaced by EN ISO 11295 once published) already defined ‘independent’ and ‘interactive’ pressure pipe liners as follows:

• Independent pressure pipe liner – liner capable on its own of resisting without failure all applicable internal loads throughout its design life
• Interactive pressure pipe liner – liner which relies on the existing pipeline for some measure of radial support in order to resist without failure all applicable internal loads throughout its design life.

In America, the AWWA Manual M28 makes distinction between liners defined as ‘Non-Structural’, ‘Semi-Structural’ and ‘Fully Structural’. Four classes in total are defined, since the ‘Semi-Structural’ category is also subdivided.

The new classification concept in ISO 11295, introduced by Gumbel et al., 2004, has adopted four structural classes A – D (pictured page 30).

Class A: independent liners, as defined above, do not rely on the existing pipeline for radial support. They may be loose-fitting or close-fitting according to installation technique. Class A liners are capable of surviving future structural failure of the host pipe and will then continue to carry the internal pressure loads over the remaining design life.

Classes B and C: interactive liners are not capable on their own of resisting all applicable internal loads, and therefore rely on the existing pipeline for some measure of radial support. A liner is considered interactive if, when tested independently from the host pipe, the long-term pressure strength is less than the maximum operating pressure of the rehabilitated pipeline. An interactive pressure pipe liner is always a close-fitting installation. Class B liners stand on their own, while Class C liners depend on sticking to the old pipe’s interior. The existing pipeline may contain holes or joint gaps to a certain extent, which can be bridged.

Class D: these liners may improve the condition of the existing pipeline, but from a structural point of view they do not contribute at all.

This new ISO structural classification refers only to the response of the liner pipe to internal pressure. Where applicable, the designer has to take other forces into consideration, such as external loads (groundwater head) or negative pressures.

Renovation technique families

Many different techniques and materials are available for lining pressure pipelines. To enable the drafting of comparable requirements for all these techniques, ISO 11295 provides a grouping into so-called Renovation Technique Families.

The technique families applicable for pressure pipelines are:

• Lining with continuous pipes
• Lining with close-fit pipes
• Lining with cured-in-place pipes
• Lining with adhesive-backed hoses
• Coating.

Lining with continuous pipes (often referred to as sliplining)

Lining with pipe made continuous for the length of the section to be renovated prior to insertion; the cross section of the lining pipe remains unchanged. For pressure pipe applications, continuous pipe liners must be designed structurally as Class A.

Lining with close-fit pipes

Lining with a continuous plastic pipe for which the cross-section is reduced to facilitate installation and reverted after installation to provide a close fit to the existing pipe. Two types of methods can be distinguished here:

• Methods using site-reduced pipes;
• Methods using factory-reduced pipes.

For pressure pipe applications, close-fit pipe liners may fall into structural Class A or Class B. Lining with CIPP

Lining with a flexible tube impregnated with a thermosetting resin which produces a pipe after resin cure.

This family includes a wide range of resin-fibre composite pipes installed and cured by a similarly wide range of techniques. For pressure pipe applications, CIPP liners generally fall under Class A or B, but where thin liners are applied using adhesive resin may also be Class C. Lining with adhesive-backed hoses

This is lining with a reinforced hose that relies on an adhesive bond to the host pipe in order to provide resistance to collapse.

In the original EN standards this family was included with CIPP liners due to similarities in installation and resin curing techniques. In the new ISO standards it has been separated because the structural action of the woven hose component is quite different and requires different quality assurance tests from those applied to resin-fibre composites. Adhesive-backed hoses, by definition, always fall into structural Class C. Coating

Coating refers to spray application of a layer of protective material to the inside of the existing pipeline. Coating has been applied extensively to renovate water lines where the structural condition of the existing pipeline is still sufficient, but problems with water quality have to be tackled. According to water chemistry and durability requirements, coatings of cement mortar, epoxy or polyurethane resin may be applied. Coating generally conforms to structural Class D, although high build variants could in principle also fall under Class C.

Quality assurance of plastics liners

The approach taken by ISO is to provide the tools (material-specific requirements and test methods) to demonstrate and assure the long-term quality of the liners. Drafting system standards for products for renovation of existing pipelines brought along one complexity: many products delivered from the factory undergo a change in shape and/or material state during site installation.

It could be stated that the products as supplied by the manufacturer are half-products only, while the user is only interested in proof of functionality of the product after installation. This complication has been addressed in the ISO renovation product standards by considering the lining system at two distinct stages as follows:

• ‘M’ stage – stage as manufactured, before any subsequent site processing of components associated with the particular renovation technique;
• ‘I’ stage – stage as installed, i.e. in final configuration after any site processing of components associated with the particular renovation technique.

Following this, the system standards for plastics piping systems for renovation of existing pipelines are distinguished from those for conventionally installed plastics piping systems by setting requirements for certain characteristics in the as-installed condition, after site processing. This is in addition to specifying requirements for system components as manufactured. To verify conformity of a liner to ‘I’ stage requirements, testing can be done on a representative sample taken either from the actual installation or from a simulated installation. In a simulated installation ‘as-installed’ samples are produced under conditions incorporating all relevant circumstances which may affect the end-product characteristics.

The feature ‘approval installed products before they have been installed’ is unique in pipeline construction, even with newly installed pipes. Pipes, fittings and assemblies should preferably be produced by the manufacturer under a quality system conforming to ISO 9001, which includes a quality plan.

Installation should likewise be executed under a quality system that includes site procedures documented in an installation manual.

By using the above, technique suppliers are capable of demonstrating the viability of the liners installed with their technique. It can be concluded that quality of installed liners is well assured.

Quality tests

Although specifying the related product tests, the new ISO standards do not directly cover procedures for assessment of conformity. The recommended scheme for such assessment included in informative annexes to European standards EN 14409-1 and EN 14409-3, corresponding respectively to Part 1 ‘General’ and Part 3 ‘Lining with close-fit pipes’ of the new ISO standard 11298 ‘Plastics piping systems for renovation of underground water supply networks’, nevertheless still applies. The general approach is described here, and illustrated by reference to one example of a factory-folded, close-fit PE liner, namely Wavin Compact Pipe.

When applying the product tests prescribed in the ISO standards, distinction is made between type tests (for product qualification), batch release tests (part of ongoing factory process control) and site process verification tests (additional quality checks made after each installation).

Type tests should demonstrate that products conform to all requirements for the characteristics given in the respective standard. Batch release tests should be performed by the manufacturer at the start of each batch of pipes, which has to be satisfactorily completed before the batch can be released. Site process verification tests include, for example, CCTV inspection and pressure testing to confirm complete re-rounding and leak-tightness of the liner pipe once installed.

Following the latest version of ISO 11298-3, the following should be type tested:

• Compound
• Appearance
• Geometrical characteristics
• Mechanical characteristics of pipes: hydrostatic internal pressure strength (three different tests including long-term and high temperature), and stretchability
• Physical characteristics: memory ability, effect on water quality and resistance to weathering
• Mechanical characteristics of assembly: hydrostatic internal pressure strength and other tests, including tensile strength or resistance to pull out, of all applicable types of joint (butt fusion, mechanical, electrofusion and/or socket fusion).

Most of these tests are repeated on each batch of pipes manufactured: the only exceptions are long-term tests for hydrostatic strength and resistance to weathering.

For heat-reverted PE pipes, the ISO standards require a memory test to be carried out. A pipe sample put in an oven with an air temperature of 120°C should come back to at least 65 per cent of its circular diameter (without applying any internal pressure).

For pressure pipe applications, hydrostatic pressure resistance testing is particularly crucial to assure durability of the pipe after installation.

Furthermore, in particular for gas applications, the standards for regular PE pipes contain special requirements to test the ability of the pipe to resist cracks that could lead to catastrophic failure. This is the reason why the slow crack growth (SCG) test and the rapid crack propagation (RCP) test are included in these standards. Of course these tests were taken over in the renovation standards and this is why pipes like compact pipe should be tested for such crack resistance.

Having assured the quality of the liner product both in both the ‘M’ and ‘I’ stage, via type testing and with every batch of pipes produced, the pipe can be safely applied, respecting the installation prescription provided by the technique owner. Once fully installed, the pipe will be camera-inspected to check on the geometric shape and will be tested for leak-tightness to make sure that a leak-free new pipeline is achieved.

Conclusions

• Available lining technologies for pressure pipelines can be structurally rated in classes A-D.
• Pipes used for renovation should be tested both in the as-manufactured (‘M’) stage and the as-installed (‘I’) stage.
• Quality assurance of installed liners is possible using the latest ISO standards.

Please refer to the PDF version of this article for diagrams.