By: Robert Colvin

Demand for plastic coating of steel pipes to provide anticorrosion protection reached double-digit growth worldwide from 2004-2007, according to figures from market researchers AMI (Bristol, England). AMI expects the demand of coated steel pipe to reach 240 million square meters by 2010, more than 50 million square meters above the 2007 volume.

For coated pipe, coal tar-based coatings and traditional asphalt are losing ground—principally because of environmental concerns and safety reasons to polyethylene (PE) or polypropylene (PP) for pipe coating where temperatures exceed 70-80 deg. C, and epoxy. PE, mainly medium- and high-density grades, accounted for two-thirds of this market last year. Epoxy formulators are battling polyolefins in these applications by offering standalone epoxy-based dual- and multilayer systems. They point to possible risks of delamination and de-bonding of 3-layer polyolefin systems.

One technical trend introduced from plastics supplier LyondellBasell (Rotterdam, Netherlands) is designed to eliminate such de-bonding problems, reduce installation costs, and, at the same time, boost reliability. Although 3-layer polyolefin coatings can provide good protection, they can also be compromised if the coating is damaged during storage, transport, or backfilling of the pipe trench. Special sand or gravel beddings can be costly, so LyondellBasell offers an alternative approach by building mechanical protection into the coating itself. Developed jointly between the supplier and Mülheim Pipecoatings (Mülheim/Ruhr, Germany), the approach uses a layer of crosslinked polyethylene (PEX) applied onto the conventional 3-layer PE coating.

The silane-grafted HDPE extruded over the main coating, processed on conventional extrusion equipment, provides impact strength and high stress-cracking resistance, says Heinz Vogt, LyondellBasell’s PE product and application development manager. "Process times are a little longer because the pipe sections need to be cooled down more slowly after extrusion to allow the cross-linking reaction to complete," Vogt says. But on the plus side, pipe trenches can be backfilled with excavated limestone rock fragments without need for additional, and costly, rock-shield systems.

Away from the trenches
In many developed markets, pipe renovation projects are driving demand, but these can mean traffic jams, construction noise, and obstruction in urban areas.

City leaders in Phoenix, AZ and surrounding communities were confronted with having to refurbish 25 miles (about 40 km) of 20- to 30-year-old concrete sewer pipe (diameters: 137-229 cm). "Digging trenches to reach the problem would be even more costly and disruptive than the original installation," says Bill Moore, material manager at Insituform Technologies (Chesterfield, MO), a contractor for pipe rehabilitation. The company decided on cured-in-place pipe (CIPP) to handle the problem.

The job was compounded by corrosive soil, high ambient temperatures (average of 89 days/yr at 100 deg. F/38 deg. C), and the development of hydrogen sulfide gas that caused increased formation of sulfuric acid in the headspace of the sewers. Insituform supplied a pipe liner made of Vipel-brand corrosion-resistant isophthalic polyester from AOC (Collerville, TN), which was turned inside out into the host concrete pipe as water pressure moved and navigated the tube-shaped liner through the pipe. Installers circulated 180 deg. F/82 deg. C water through the tube to cure the resin into a crosslinked, solid pipe-within-a-pipe.

Extruded plastics pipes are providing modern solutions to many urban areas, including a replacement for London's porous cast-iron water distribution system from the 19th century. Thames Water has embarked on a program to replace 1603 km of old mains by 2010 at a cost of $1.4 billion. Using a "Whole Life Costs" model, the service provider was able to compare competitive pipe materials, expected service life, and ease of installation. Thames Water selected PE because it offered the best solution for trenchless installation, says David Walton, marketing manager pipe at polyolefins supplier Borouge (Singapore).

In Turkey, local officials were confronted with the need to pipe up to 300,000m3 of potable water from the Ömerli reservoir on the eastern side of Istanbul to the western part of the city. The problem was that the pipeline had to be laid undersea, through the Bosporus, in an earthquake zone. The pipe has to withstand both the high pressures of the drinking water flowing through it as well as the currents surging through the narrow channels connecting the Black Sea with the Mediterranean.

Turkish pipe processor Firat Plastik Kaucuk Sanayi ve Ticaret (Istanbul) took on the job to extrude the 4 km of large-diameter PE100 pipe, with wall thickness of 109.1 mm. Each 13m-long pipe weighed 5 tonnes. Firat needed to churn out the 1200-mm-diameter pipe in a minimum time using two lines, one equipped with a Proton 150-30G single-screw extruder and the second featuring a Monos 120-37G high-speed extruder, both manufactured by Cincinnati Extrusion (Vienna, Austria). Firat says its Monos extruder reached a 30% higher output than the company’s predecessor model from the Proton series.