Onsite extrusion of irrigation pipe

The South Island of New Zealand witnessed a revolution in irrigation system installation last year through a $30 million project to extrude pipe up to 1.6-m in diameter onsite.

Historically the irrigation water for the farms in the mid Canterbury area of New Zealand's South Island has been supplied through open channels. This particular scheme, called the Valetta Irrigation project receives its water from the Rangitata Diversion Race, which is a canal constructed in the early part of the 20th century taking water from the Rangitata River. However the open channel system loses a large amount of water due to seepage and evaporation. In this case it was calculated that a piped irrigation system would deliver water savings of 28% and enable 37% more land to be irrigated without increasing the level of water extraction.

As this farming area is a long way from New Zealand's industrial centers the pipes would need to be transported long distances at great cost and environmental disturbance. The solution proposed by Gerard van den Bosch, Managing Director of Bosch Irrigation (Burnham, New Zealand) and founder of sister company Aquaduct NZ (Ashburton, New Zealand) was to produce all the pipes close to the site using a temporary tented extrusion hall housing three extruders. The Aquaduct pipe factory is believed to be a world first, being designed to be relocatable.

Other novel features of van den Bosch's proposal were the use of a large chain trenching machine to efficiently install the pipes and the inclusion of two hydro electric generators to run on water that flowed through the irrigation system and provide additional power.

In all over 84 kilometers of pipe in sizes ranging from 200 mm to 1600 mm diameter were produced in the temporary extrusion factory despite an ambient temperature variation from -7°C to 36°C.

All the PE100 material was supplied by Borouge (Singapore) from the nearest port, which was 128 km away and when running at its peak the factory was consuming 120 tonnes of BorSafe PE100 compound per day. Therefore the deliveries needed to be timed to fit with factory demand. The entire project consumed 5,800 tonnes of HDPE over the course of 60 days.

Extruding onsite meant that welding requirements could be slashed by 80%. The pipes were extruded in lengths of up to 250 meters and transported to site on dollies, to reduce the cost of installation.

For the installation of the pipes, Bosch Irrigation purchased a Trencor 65-ton chain trenching machine which could cut a 2-m deep trench, 1.2-m wide, at the rate of 170 meters per hour. A stone separator was used so that the excavated soil was screened during the removal process and the separated fines were used as pipe bedding material, saving the cost of purchasing and transporting imported bedding material. The trench for the 1600-mm pipe was excavated in the traditional manner and where the quality of the backfill material was substandard, Borouge's High Stress Crack Resistant (HSCR) PE100 material BorSafe HE3490-LS-H was used for the pipe production to ensure that all possible practical steps had been taken to prevent point load or stress crack related failures of the relatively thin walled 1600 mm diameter pipe.

The use of pre-compounded BorSafe PE100 material ensured that good quality pipes could be simply produced in the temporary extrusion hall. The "low sag" material provided excellent wall thickness control thereby reducing start up times and minimizing scrap.

The same 1600-mm PE100 pipe was used as the "penstocks" to feed two hydroelectric generators with a total capacity of 2.2 MWh. These units are located at the head of two storage ponds designed into the scheme and use the natural available head of water to generate the power highlighting again the innovative nature of the scheme.

Note: This article is based on a case study previously published by Borouge.