The steady renewal of the “Auer Ring” long-distance drinking water system in the Western Ore Mountains in Saxony – an interim report

1 Choice of material

The difficult rocky terrain of the Westerzgebirge, or Western Ore Mountains, with large geodetic height differences demand care in the choice of pipe material. After weighing up all the pros and cons, the Westerzgebirge waterworks association opted for the use of DN 400 ductile iron pipes with restrained push-in joints. In a few particular sections it was planned to use pipes with cement mortar coating.

Because of the low buffering capacity of the water and the technically unavoidable stagnation points in a ring pipe system the pipes lined with cement mortar in the factory needed to be conditioned in sections before being put into operation. In this way the possible influences on pH value due to stagnating water are excluded.

2 Introduction

The Zweckverband Wasserwerke Westerzgebirge (ZWW) water supply association located in South-West Saxony was formed on 01 April 1993 by 39 towns and communities of the rural district of Aue and Schwarzenberg and entrusted with the tasks of supplying drinking water and wastewater treatment. Currently around 171,500 residents are served by the ZWW.

The territory covered by the ZWW is divided into three drinking water and two wastewater supervisory areas (Figures 1A and 1B). At the end of 2013 the rate of connection to the drinking water network was a remarkable 98.9 %. The length of the piping network for drinking water totals 1,717 km. Altogether ZWW has an elevated storage tank capacity of approximately 71,000 m³. In 2013 4,554,160 m³ of drinking water was able to be supplied to ZWW customers.

3 Project

The “Auer Ring” long-distance water pipeline system is one of the core elements of the drinking water supply in the Western Ore Mountains region. Ten towns and communities are connected over a total length of around 22 km; they are supplied with purified reservoir water from the Sosa reservoir dam (Figure 2).

The long-distance water system was laid in the context of the construction of the Sosa reservoir dam in around 1950. At that time engineers were predominantly using unprotected metallic pipes (grey cast iron or steel) or concrete pipes in dimensions DN 350 to DN 800. The latter were mainly used in the low pressure ranges, in sections located geodetically higher.

Because of the high operating pressures of up to 22 bars in places and also because of the materials used, there were often a number of pipe bursts recorded each year. The DN 450 concrete pipelines in particular proved to be critical. Usually the sockets were leaking or the concrete was corroded.

3.1 Planning

The clean water reservoir in the Sosa waterworks with its water level of 602 m above sealevel determines the pressure. Three additional elevated tanks are connected to the ring itself, with a storage volume of between 4,000 m³ and 10,000 m³. They are arranged approximately on the same contour at around 565 m above sea-level.

So that the “Auer Ring” can continue to fulfil its function without restriction in the future, in recent years the Westerzgebirge waterworks association has invested about six million euros in the supply system. Extensive hydraulic calculations and simulations in combination with a basic analysis of water requirements resulted in a pipeline which, at DN 400, is ideally designed for the whole ring. This was confirmed by a cost comparison calculation.

Because of the existing dead pressures of up to 22 bar and the sometimes extreme pressure shock amplitudes of up to 33 bar, the Westerzgebirge waterworks association decided in favour of using pipes and fittings in ductile cast iron in accordance with EN 545:2010 [1] with BLS^® positive locking and restrained push-in joints (Figure 3). The wall thickness of the BLS^® pipes used corresponds to wall thickness class K 9 as per EN 545:2006 [2]. Hence it deviates from the minimum wall thicknesses stated in Table 17 of EN 545:2010 [1]. The minimum wall thicknesses and pressures (C class = PFA) stated here only apply for ductile iron pipes with non-restrained push-in joints (e.g. TYTON^® push-in joints).

Pipes and fittings according to EN 545 [1], [2] are lined as standard with cement mortar based on cements to EN 197-1 [3] for application areas according to DIN 2880 [4]; Annex E to EN 545 [1] also permits other types of cement. In this specific case, however, there was no need to choose any alternative types of cement.

According to the manufacturer’s specifications, pipes of nominal size DN 400 with BLS^® push-in joints and a minimum wall thickness of 6.4 mm can be used for a PFA of 30 bars. The safety factor for joint failure according to EN 545 [2] is 1.5 + 5 bar (type test pressure). PFA is to be understood as the maximum hydraulic pressure that can be withstood by the pipeline (EN 805 [5]). The maximum allowable operating pressure that a component is capable of withstanding from time to time due to pressure surges for example (PMA) is calculated as follows:

The maximum allowable test pressure of a component on the installation site is determined as follows:

In the present application, the pipeline material selected has very high safety reserves. The piping system is equipped throughout with BLS^® restrained push-in joints. Therefore there was no need for concrete thrust blocks on bends and branches or at pipeline ends for pressure testing. Also this system makes it possible to dismantle individual components without problem. These properties are advantageous for, among other things, conditioning the cement mortar lining and during pressure testing.

3.2 Installation work

So far six individual sections have been completed. Depending on the degree of difficulty and the location, the lengths of the individual construction stages are between 0.5 km and 2.8 km. Altogether the Westerzgebirge water supply company has managed to replace approximately 10.8 km of DN 400 GGG long-distance pipeline. At the same time it was possible to take advantage of synergies by replacing some 3.1 km of local network pipelines.

For the main part the project was able to use the existing pipeline route. At certain points, however, the pipeline takes alternative routes because it is too close to builtup areas or because of logistic advantages, above all for cross-country sections. It is important, particularly for subsequent operation, that the pipeline remains accessible without restriction.

Where the new DN 400 pipeline was laid along the same route, the old DN 350 to DN 500 steel, grey cast iron or reinforced concrete pipes were first of all dug up and removed (Figures 4 and 5). Depending on the material of the old pipeline, the pipe trench was opened up over a length of up to 12 m or 14 m, the old material was salvaged and then the trench bottom was prepared. After this, the assembly engineers laid the new DN 400 ductile iron pipes in the pipe trench and assembled the BLS^® restrained push-in joint with the V 302 laying tool.

In places where the local PE network pipeline ran parallel with the long-distance pipeline, sand had to be used as a pipe bedding anyway, so the standard “zinc with protective finishing layer” was sufficient coating for the longdistance pipeline. Particularly in more remote sections, site logistics proved to be a major challenge as there are only a few intersecting roads running to the route of the pipeline; otherwise construction roads had to be built at some expense. Therefore it made sense to dispense with sand bedding by using cement mortar coated pipes (Figures 6 and 7).

In order to optimise the hydraulic conditions, intermediary high points with a height difference of up to 5 m were levelled. The enormous volumes of excavated soil which this produced further exacerbated the logistic challenges, which once again were able to be overcome by the use of cement mortar coated pipes. After assembly the push-in joints were covered with protective cement mortar sleeves.

At the remaining high and low points, laterally offset venting and draining valves were installed (Figure 8). All flanged tees with an eccentric branch were best suited for this (Figure 9). The air valves are housed in shafts for ease of maintenance.

Branch pipelines were installed at selected points in the “Auer Ring”. The Westerzgebirge waterworks association favoured butterfly valves over gate valves for this. Bypasses with flushing devices were installed to make commissioning of the pipeline system easier (Figure 10).

If pipe cutting is necessary on site, this involves extra expense for supervision. Only selected DVGW-certified companies are able to produce welding beads for BLS^® push-in joints correctly under construction site conditions (Figure 11). They are welded using a copper gauge. Posttreatment with corrosion protection coating has to be performed according to the manufacturer’s specifications.

3.3 Commissioning

The water to be transported by the “Auer Ring” long-distance piping system is very soft with a low buffering capacity. The acid capacity KS 4,3 is less than 1.0 mmol/l and the pH value is around 8.3.

With the existing ring circuit system stagnation points cannot be excluded. In specific terms, stagnation times of up to six hours must be assumed. The ductile iron pipes to EN 545 [1], [2] used for this project are lined with a mortar based on blast furnace cement. The calcium hydroxide Ca(OH)₂ produced during the hydration of the cement raises the pH value of the pore water in the mortar considerably. Depending on the composition of the water to be transported, its pH value may even rise above the limit value of 9.5 set by the drinking water regulations.

According to DVGW worksheet W 346 [7] the water in question is to be classified as type WKSII. Accordingly, counter-measures may be necessary in order to avoid high pH values. As a guide, therefore, a lab test was conducted by adding a sample of pipe to the specific water in a beaker. After 24 hours the pH value increased to 10.9. Therefore the Westerzgebirge water supply company decided to treat the cement mortar lining of the pipes before commissioning in accordance with DVGW worksheet W 346 [7].

 Four processes were considered here:

• pre-carbonation by flushing with harder water,
• pre-carbonation with soft water modified by additives,
• treating the cement mortar lining on site with CO₂,
• treating the cement mortar lining in the factory.

Options 1 and 2 were eliminated because of the lack of availability of harder water and/or sufficient flushing water containers. As a rule they are only appropriate for small nominal sizes and shorter pipe lengths. Option 4 was not practical either. The Westerzgebirge water supply company opted for using the tried and tested process of treating the lining on-site with gaseous carbon dioxide. For this the lining of the pipes has to be dry and, as far as possible, in its condition as delivered. This meant that the greatest possible care was needed during storage, transport and installation of the pipes.

Once the pipeline had been laid, the ends were sealed to be gas-tight and first tested for tightness with air. The negative pressure testing process as per DWA-A 139 [8], for example, is suitable for this. However, this does not replace the pressure testing before commissioning as required by EN 805 [5]. After tightness testing with air, the pipeline is flushed with CO₂ gas from its lowest point. When the gas escaping from the ends of the pipeline reaches a CO₂ concentration of more than 90 % by volume, the pipeline ends are sealed and a pressure of 4 bars is built up. Ductile iron pipes are equally suitable for gases and water in this pressure range.

The calcium hydroxide Ca(OH)₂ of the cement mortar which is soluble in water as the pH value rises reacts with the gaseous carbon dioxide CO₂ and is converted to insoluble calcium carbonate CaCO₃, whereby the density of the mortar increases and CO₂ is used up (Equation 3).

This causes the gas pressure on the inside of the pipe to drop. So that sufficient carbon dioxide can continue to flow, the connection pipeline remains open to the CO₂ source, which is usually a bundle of cylinders (Figure 12) or vaporiser system (Figure 13). A constant gas pressure must be ensured in the pipeline. If necessary the source of CO₂ may need to be replaced. The pipeline is now the equivalent of a gas container. It is essential that the applicable safety and accident prevention regulations are observed.

During the conditioning measures, the pressure inside the pipe is to be monitored and recorded. If the CO₂ supply is interrupted, then the pressure inside the pipe will fall due to the consumption of CO₂ as a result of the carbonation of the Ca(OH)₂. The conditioning of the pipes is finished when the pressure drop due to the CO₂ consumption is less than 0.1 bar/h. At this point the carbon dioxide can practically no longer find a reaction partner. A lasting, barely soluble protective coating is formed on the surface of the cement mortar. It behaves neutrally with respect to the water to be transported: its pH value is no longer affected, or only insignificantly so.

The treatment of the cement mortar lining as described in accordance with DVGW worksheet W 346 [7] has no effect on the durability and working life of the lining. In fact the natural conditioning process of young cement mortar linings is accelerated. Depending on weather, this treatment takes 4–7 days; in exceptional cases however a longer time may be needed.

After the CO₂ treatment described, tightness testing is carried out as per EN 805 [5], usually accompanied by disinfection. It is important to note here that, according to DVGW worksheet W 346 [7], hydrogen peroxide and sodium hypochlorite (chlorine bleaching agent) only have a moderate disinfecting effect with soft water. With water of this kind, chlorine dioxide and hydrogen peroxide with 1 % phosphoric acid work considerably better as disinfecting agents [9].

4 Final comment

The expectations invested in the selected pipe material in matters of cost effectiveness and technical security were entirely met. In close collaboration between client, planning engineers, construction companies and pipe suppliers, a challenging structure was replaced in sections over short construction times. Working in this way, the Westerzgebirge water supply association will certainly be able to tackle the remaining sections (Figure 14).

Bibliography

[1] EN 545: 2010
[2] EN 545: 2006
[3] EN 197-1: 2014
[4] DIN 2880: 1999-01
[5] EN 805: 2003
[6] Notification of the new version of the German drinking water regulations dated 2 August 2013 Federal Law Gazette Part I no. 46 dated 07.08.2013
[7] DVGW worksheet W 346: 2000-08
[8] DWA worksheet A 139: 2010-01
[9] E-Book 10.2015, chapter 21 download: www.eadips.org/e-book-e/

(Primary publication: GUSS-ROHRSYSTEME - Information of the European Association for Ductile Iron Pipe Systems • EADIPS^®)

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