FLOWTITE Sets New Strandards In Pungwe-Mutare Water Supply Project

Water Supply Project

Includes first use of 32-bar pressure pipe for water service

With the rushing waters of beautiful and highly popular Victoria Falls at its Western border, problems of water supply might not first come to mind with the mention of Zimbabwe. But with a total land area slightly larger than the US State of Montana, landlocked Zimbabwe faces a shortage of the precious liquid in several parts of the nation.

In the past decade, limited water resources combined with a substantial increase in the demand has led to a shortage of water, which has hindered the nation's development. The growing urban regions, industry, mining and agriculture need more and more water. With severe droughts in 1991 and 1992 and late rains in the following years, many cities were forced to impose water rationing. Although several large dams had been constructed, the water supply remained a problem.

A prime example of the problem was the bustling city of Mutare, situated on Zimbabwe's eastern border with Mozambique. Mutare had an insufficient supply of water and the problem threatened to crimp development and force the relocation of citizens to wetter areas of the country. Therefore, in 1996, the Zimbabwean government decided to implement the Pungwe-to-Mutare Water Supply Project, which will divert a portion of the natural river flow of the Mutare River through a tunnel and pipeline in order to provide water to the city of Mutare.

Mutare is Zimbabwe's third-largest city, with a population of over 150,000. It is surrounded by mountains, and is known as one of the best laid out and prettiest of urban areas in Zimbabwe. The city has a diverse industrial base but serves mainly as a center for activities related to forestry within the Eastern Highlands. Large conifer forests grow in the region and Mutare's industries include sawmilling and the manufacture of paper and furniture.

Located just 62 miles south of Nyanga National Park, Mutare lies in a valley that creates a natural gateway between the Mozambique lowlands and the Zimbabwean interior. Nyanga is not a game park but there are waterbuck, steenbok, leopard, kudu and wildebeest here, and the area is rich in birdlife. Among the first national parks to be declared in Zimbabwe, Nyanga contains such features as Inyangani, at 8,501 feet the country's highest mountain, and the spectacular Pungwe Falls and Gorge.

The Pungwe River moves swiftly through the Eastern Highlands of Zimbabwe and will become a much-needed source of water for the city of Mutare, more than 60 miles to the south.

The Pungwe-Mutare Water Supply Project will enable the withdrawal of 0.7 m³/s of water by gravity from Pungwe River within Nyanga National Park, via a 4.3-km tunnel and more than 70 km of GRP pipeline to augment Mutare's water supply system. The project is being implemented by the City of Mutare and designed/built on a turnkey basis at a contract cost of USD 75 million.

Several environmental issues and cultural values have been considered in the project. Among them: Bio-diversity resources downstream of the intake and erosion along the pipeline. Other environmental concerns included the possibility of taking too much water from the Pungwe River and that the route of the pipeline through the forests would leave scars for a very long period. The finance institutions and the contractor focused upon these issues and special plan was prepared in order to meet all environmental concerns. For example, an environment assessment consultant was appointed to report on all issues listed as concerns and to respond to the owner, the financial institutions the Zimbabwean government.

And early on there was a protest from some of the indigenous residents the area. Before work on the Pungwe project started in early 1997, two ritual ceremonies were performed to appease the spirits of the Manyika people.

The terrain is extreme in this area of Zimbabwe, with lots of hills and deep valleys. This aspect of the project was the greatest challenge for the contractor, Skanska (Sweden).

Hills and valleys along the rugged trace of the pipeline necessitated a great many adjustments in the deflection angle of the couplings as well as the construction of special fittings.

During installation of the pipeline, the soil types encountered were mostly silty sand, with some areas of silty clay. A very short stretch had organic materials as well.

A natural rock channel has been excavated above the tunnel intake to divert water into the tunnel. A compensating weir in the channel will ensure that sufficient water flows in the river are maintained.

In the early construction phases, the first major task to be performed was the construction of a tunnel beginning at the Mutare River and ending here, 4.3 km to the south.

The pipeline includes break pressure tanks, valve chambers, interconnection chambers to existing pipelines and connections to the Odzani water treatment works and two reservoirs near Mutare. As a result, numerous fittings needed to be produced and in order to accomplish that, Skanska set up an on-site production facility.

It helped, however, that the large allowable angular deflection of the Flowtite coupling minimized the need for elbows, as turning radii were made by off-setting the pipe.

The Odzani plant will be kept intact and continue to be used for treatment of water from the Odzani and Smallbridge Dams. Water from the Pungwe River is of very good quality and will be purified separately using a sand filter.

There are 71,508 meters of Flowtite pipe on this project and 7,896 couplings were produced to join the pipe, which was supplied by Flowtite Pipe & Tank A/S, Sandefjord, Norway, in competition with steel pipe from South Africa. Pipe produced for the Pungwe-Mutare project includes a number of standard as well as specially produced diameters.

But perhaps the most important aspect of the project, from the point of view of innovation in GRP pipe design and capabilities, is the introduction of pipe for 32-bar service. Nearly 22,000 meters of this high-pressure pipe was made for the Pungwe-Mutare project, in two different diameters: DN630 and DN760. This was the first project where high-pressure Flowtite pipes were used for in-ground service in a water supply project. As a note of interest, these pipes were tested at the factory to 64 bars and in the field to 48 bars of pressure.

Never heard of DN630 or DN760? Perhaps that would also be true for DN810 and DN860. All of these are made-to-order diameters, which is a unique capability of the Flowtite manufacturing process, with its infinitely adjustable mandrel. The odd diameters are the result of "nesting" requirements for the project: The pipe was shipped from Norway with two smaller-diameter sections inserted in each standard-diameter outside section. The rugged, yet relatively thin walls and light weight of Flowtite pipe are the features that allow this procedure, which saves a significant amount in shipping and handling costs.

The Flowtite pipe for Pungwe-Mutare as shipped from Norway. "Nesting" the pipe saves a significant amount in shipping and is permitted by the high-strength, thin-wall construction of the pipe as well as its light weight. A special nesting package was developed by Flowtite engineers to maximize the stacking of nested pipes for the ocean shipment.

Engineered Pipe Systems manufacturing facilities will produce custom, non-standard diameters for any project where the pipe quantity exceeds 8 km of a given diameter. The lead time needed to supply non-standard diameters will be 6 to 8 weeks longer than standard, as additional equipment is required. To develop the most economical overall solution for a pipeline project, engineers might want to consider such odd diameters when thinking about the total volume of material to be carried in the pipeline, the required pressures and the higher flow factors of GRP pipe.

A close inspection of the chart above also will reveal some unusual figures in the "stiffness" column. Nominal stiffnesses for Flowtite pipe are SN2500, SN5000 and SN10000. In this case, the required pressure ratings resulted in production of unusual stiffnesses: SN4000 and SN9000. Here again, the continuously advancing mandrel manufacturing process and its materials-input variability are highly adjustable in order to accommodate various engineering requirements for Flowtite pipe.

Silty sand soils along the trace of the pipeline were used as bedding and backfill, following new compaction procedures for Flowtite pipe. Screens were used to prevent large boulders from contaminating the pipe zone backfill.

The Pungwe-Mutare project also generated a significant update to Flowtite pipe installation procedures. Because the contractor was faced with a 120-km haul for clean backfill material, Skanska requested the development of new technology that would allow the use of native soils. This resulted in a series of trials conducted on site and coordinated by Flowtite Technology in Granville, Ohio (US) in which pipes were installed in different soils with various levels of compaction. The deflection of the pipes was monitored over several months. The upshot is that completely new installation procedures have been developed that will apply to installations of Flowtite pipe anywhere in the world. These new procedures allow the use of native soil as backfill in most cases where imported backfill previously had been required. In the Pungwe-Mutare project, the native silty sand soils generally are compacted to 80% Standard Proctor Density, thereby eliminating the 120-km haul as well as the cost of clean backfill material.

The Pungwe-Mutare Water Supply project proved challenging in many respects. The engineers and contractors faced myriad challenges, including a difficult tunneling process, rugged terrain all along the trace of the pipeline and the construction of numerous and varied fittings. Throughout it all, however, Flowtite pipe proved equal to the challenge. And the final result—ample new supplies of water for the people of Mutare—represents another example of why rugged, smooth-bore, non-corroding Flowtite pipe is the best alternative for water supply, sewerage and many other types of pipeline projects. And now that its installations can be performed more efficiently than ever, specifiers can count on Flowtite pipe as the material of choice wherever long-term performance with a low initial and life-cycle cost is desired.