Plastics are increasingly being used in major pipe systems in place of traditional pipe – these materials generally offer flexibility, durability, abrasion and chemical resistance. Underground pipe systems are subject to a range of additional environmental factors including in-ground temperature, primary soil load, secondary loads (such as rock impingement and squeeze off) and exposure to chemicals in soil, for example in contaminated or acid regions. The integrity of pipe located under main transport routes is particularly critical. AASHTO (the American Association of State Highway and Transportation Officials) has published standards relating to buried pipe and installation, as have other standards institutes. Polyethylene and PVC hold the bulk of the market, other materials used in pipe include PP, GRP, ABS, polybutylene, PVDF, CPVC and polyamide.
Methods of pipe installation affect the cost of a pipeline project and the disruption to local business and communities. Plastics, as flexible materials are very suited to the latest trenchless technology and joining systems have developed in parallel.
Drainage is one of the largest markets for plastics in pipe, with some use in sewage. A study of HDPE corrugated drainage pipe by the US Department of the Interior showed that the majority was still fully functional after 29 years of service. KWH has pioneered development of some of the largest diameter pipe for these applications. In 1976 the company produced a 1600 mm (63 inch) polyethylene sewage pipe weighing around 250 kg/meter. The latest pipe is claimed to be the largest in the world (up to 3,000 mm (118 inch) diameter) and is kept lightweight by extruding hollow sections which are wound to form the wall of the pipe. In a European study of sewage pipes, Professor Stein and Partners found that the overall defect rates in flexible sewage pipe systems were around 25% of those in rigid and that many of the problems were caused by poor installation.
In terms of natural gas, in 2005 in the US around 48% of the distribution mains was plastics. In the same year, in a study on municipal pipe systems, Polymer materials have been improving – when thermoplastics were used around 40 years ago in America, they were more susceptible to brittle cracking, partly due to handling during installation and environmental factors. The US Department of Transportation is currently examining the issue of ageing pipelines and the American Gas Association (AGA) is compiling a database of historic pipe manufacturers, materials used, colors of pipe, etc. to facilitate the identification of older systems and to estimate lifetime. Around 97% of plastic gas pipe in the USA is HDPE. Polyamides are being tested for the larger diameter gas high pressure pipes.
PVC has been used in buried pipe for decades. It is not susceptible to biodegradation and has excellent chemical resistance. Chlorinated PVC (CPVC) has even higher performance, hence its use in industrial pipe. The water pipe in the US is dominated by PVC followed by ductile iron pipe – HDPE has made some inroads here where horizontal drilling has been required (for example under roads and bridges). In Europe, PE is used more extensively in water pipes. The German water company OOWV has just published a report on the successful installation of 710 mm (28 inch) diameter HDPE (PE 100) in a 10.5 km stretch of water mains. The savings were estimated at 0.6 million euros.
As pipes age, there is a need for cost-effective rehabilitation methods. Using polymer solutions can assist in cutting down on surface disruption and there are a variety of techniques available including lining or pipe replacement. A spray on lining can be used – epoxy and cement are two types of coating. Epoxy has two advantages – a much quicker curing time and a lining thickness of only 1 mm. Sliplining involves inserting a lining pipe inside an existing pipe. Often the liner has to be secured in place with grouting and the additional pipe inevitably reduces the volume of the pipe – PVC, PE, PP, polybutylene and iron have all been used. Close fit lining is an alternative method: the liner is prepared in a C or U shape by the manufacturer and fed into the pipe where it is heated and pressurised to restore its original shape. Cured-in-place-pipe (CIPP) involves inserting a fibre tube into the pipe and filling it with a thermoset resin to form a pipe. Spiral lining involves winding a ribbed plastic strip to form a spiral pipe as a liner and is useful for sewage and drainage applications. Pipe bursting is a technique used in replacement of water mains – a pipe burster is pulled through the existing pipe to break it up and disperse it in the surrounding soil, while a new pipe is pulled in behind the burster.
Polymers are also used as protective liners and coatings in new pipelines made of traditional materials, particularly in corrosive environments.
AMI LLC is launching a new international conference on the use of thermoplastic and thermoset materials in buried pipe. Plastics in Underground Pipes 2008 will take place at the Westin Galleria hotel in Houston, Texas from 22-23 May 2008.