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Steel Reticulation Design Excels

Publication: IMIESA
Issue: November/December 2018

Serving the growing needs of the City of eThekwini’s industrial and residential sectors is an expanding network of steel pipelines that traverse the hilly terrain common in this region. The challenging topography calls for innovative design and construction methodologies, which had a direct bearing on the pipeline reticulation system adopted.

For the Western Aqueduct Phase 2 component from Inchanga to Hillcrest, the steel pipes selected were manufactured and supplied by Hall Longmore, a market leader in South Africa and internationally since 1924. Both contracts for Phase 2 were completed in 2015.

Contract WS6190 comprised some 7 km of DN 1 600 steel pipe from Inchanga Station to Alverstone Nek Reservoir, with a short section of DN 1 400 steel pipe at Alverstone Nek Reservoir. The maximum static pressure in this stage is 29 bar, requiring 13.5 mm thick X65-grade steel pipe.

The final section (Contract WS6191) comprised approximately 7.5 km of DN 1 400 steel pipe from Alverstone Nek
Reservoir to the Ashley Drive Break Pressure Tank (BPT). The maximum static pressure in this case is 25 bar, requiring 12 mm thick X65-grade steel pipe at high-pressure sections. The Ashley Drive BPT was designed for a through-flow capacity of 400 Mℓ/day.

The Western Aqueduct was conceived as a large-diameter, thin-walled, continuously welded steel pipe. The steel pipe was selected to be both economical and functional. In low-pressure areas, this typically meant using the thinnest available X42-grade steel pipe, subject to typical slenderness limits not being exceeded. The thinnest wall was 10 mm on the DN 1 400 pipeline and 12 mm on the DN 1 600 pipeline, giving respective slenderness ratios of 140 and 133.

When pressure increases necessitated higher-strength pipe, the grade of steel pipe was changed in the first instance before increasing the wall thickness, as this was found to be a more economical solution – e.g. 10 mm wall thickness X42-grade pipe was upgraded to 10 mm thick X52-grade pipe, rather than, say, 12 mm thick X42-grade pipe. The highest grade of pipe specified was a 13.5 mm thick X65-grade steel pipe with a minimum yield strength of 448 MPa.

The pipeline was designed to be continuously welded, which resulted in the pipe wall stresses having to be calculated at every point of intersection. Because concrete thrust blocks were not a practical means of restraining longitudinal forces in such a large-diameter pressurised pipeline, these forces were taken up in the pipe itself.

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