HDPE taking the place of concrete in some sewer structures

Extruded high density polyethylene pipe is increasingly used as a replacement for traditional materials in infrastructure projects. But now even sewer structures, which traditionally would be built from precast concrete, are being made from extruded polymer. 

England's Anglia Water is the water and sewerage company that supplies the East Anglia region in the United Kingdom. In August, Anglia Water installed a new sewer system, with an HDPE pumping station chamber rather than one produced from concrete.

The pipe — and pumping station — for the project was manufactured in Wales by Asset International Ltd., the UK licensee of the Weholite and WehoPanel systems, developed by Uponor Infra Ltd. of Vaasa, Finland.

The pumping station for the Cambridge Sewerage Treatment Works is 15 meters long, 4.5 meters wide and 5 meters high. It was delivered Aug. 12 in two sections, and was assembled and welded on site. 

Simon Thomas, Asset International's managing director, explained the Weholite system: “The pipe is made from a hollow box section which we extrude using standard extrusion equipment. So, in effect, instead of making a round pipe, we're making a rectangular box. And then we spirally wind [the rectangular box profile] around a drum to make a pipe.”

The WehoPanel process takes away the winding stage. Thomas said.

“In this case, all we're doing is cutting lengths of profile, before they go to the drum. So we end up with straight lengths of profile. Then we have a machine that welds those together to form panels known as WehoPanels.

“We have a team of fabricators that erect the WehoPanels into all sorts of weird and wonderful shapes; in this case it's a big box that's going to be a pumping station.”

The box sections are about 250 millimeters wide and are welded together on a machine into pieces that are 2 meters wide.

“I liken it to putting up plasterboard,” he said. “We turn the profiles we weld on the machine into panels of 2 meters wide. And then the panels are erected in our prefabrication shop and welded by hand.”

The process used to join the extruded panels is heat extrusion welding. Thomas explained. 

Asset started producing Weholite pipes in 1996, with two production lines making pipes up to 2.2 meters in diameter. In 2000 the company added a third production line to produce pipe of a 3 meter diameter.

“Then in 2008 we went one better,” said Thomas, “and put in a fourth line to take us up to 3.5 meter diameter which is, as you can imagine, a hell of a big pipe.

“As opposed to a solid wall pressure pipe, you don't have the weight of material. Consequently a 3.5-meter-diameter pipe only weighs 500 kilograms per meter. But when it's installed it has all the structural qualities that will allow it to take full highways loading.”

The company only started making WehoPanels in 2013. Thomas said. The equipment went into production in September 2013.

 “The interesting thing is that it would originally have been specified from precast concrete. We'll be delivering it in two sections — and that's only because of transport restrictions on height. It will be delivered onto a concrete base, the top dropped on and then — after some welding on site to seal up the joints — it's ready to go.  

“If it had been constructed in concrete it would have arrived in panels. The panels would have to be assembled on-site, which would take them a lot longer, with a lot more plant and equipment. Whereas this takes a few days to put together. It's saving a lot of time and money but also saving greenhouse gas emissions, which is becoming very big news in the water industry right now.”

Asset plans to build even bigger structures in the future and is working on designs for chambers for combined sewer and overflow (CSO) systems. “These are unique to the water industry,” said Thomas. “In this industry there's a lot of combined storm water and sewerage lines that have to have control chambers to separate the solids from the water in the case of a storm.”

The company does its design work internally, using CAD 3-D modelling to ensure that tanks are structurally sound before going to the manufacturing stage. 

Thomas admits that, generally, pipes do not travel very well — particularly larger pipes — because of pressure. But Asset does have export markets. Also delivered in August were pipes of various diameters for use as road culverts in the Falkland Islands — a journey of nearly 13,000 kilometers.

“We also have a nice order for road culverts up in northern Norway,” said Thomas, “and that's for 3.5 -meter pipe because we're the only people that make it in Europe — or in the world, probably, but I'm hedging my bets. We've been getting orders once a year from one of the northern Norway roads authorities because they like to use the big plastic pipes rather than traditional materials that can fail in the extreme weather conditions they have up there in the Arctic circle.”

Asset claims a design life of the pipes of 120 years, based on the testing conducted by the material producers, who use an ageing and extrapolation process.