Kanzingbach power station (Tyrol) with a high boost in output – increased security provided by “leak-before-break” fracture mechanics design of turbine pipes

1 Introduction

While in the past ductile iron pipes were mainly used for the transport of drinking water and wastewater, these days new areas of use are opening up such as penstock pipelines for hydroelectric power stations. In particular as a result of government incentives for renewable energy, movement can be seen in all areas – wind power, solar energy and not least the use of water power.

With the developments in existence today, a lot of resources are already being exploited in the Alpine region but there are still considerable reserves to be utilised by modernising and increasing the output of existing plants, as has happened with the extension of the TIWAG (Tiroler Wasserkraft AG) installation in the district of Flaurling located in the Tyrolean Kanzing valley.

The latest turbines, generators and control technology are essential conditions for better efficiency. But, with further development, the turbine pipelines can also produce better yields and above all contribute to increased safety. Because of the energy stored in the reservoir or water tower, the failure of a pipe, on a steep slope for example, can have catastrophic effects. Therefore safety considerations take first place for the pressure pipelines of hydropower stations.

Hence early recognition of the beginnings of pipe damage which, particularly with high pressure applications, could be a considerable risk is essential. If a crack appears in the pipe then this has to develop into a through-wall crack before it comes to the pipeline actually bursting. This is referred to as “leak-before-break” behaviour of the pipeline, i.e. the fault goes through the wall and leaks become detectable before the pipe finally fails. Hence the defect can be eliminated by repairing or replacing the pipe in good time before things become critical. Complicated leakage warning equipment only really makes sense under these conditions in areas where safety is critical.

However, particularly in challenging locations such as in the high-Alpine topography in the area of turbine pipelines, the replacement of a defective pipeline is very costly. For this reason attempts are made right from the planning stage to design these pipelines in such a way that an unstable crack development does not happen. However as yet there are no standard specifications or design bases for this and attempts are often made to manage with other known parameters such as elongation at fracture A₅.

Sometimes planners also rely on determining the impact energy consumed, as required in American standards ANSI/AWWA C 151-09/A21.51-09 [1] for example. But impact energy consumed is only an approximation as well and it merely describes behaviour under an impact and hence short-lived load. With a pressure pipe, however, crack formation and crack growth are processes which are many orders of magnitude longer lasting than the force impact of the notched bar impact test.

2 Turbine pipelines – a domain for the ductile iron pipe systems

In 1997 Vorarlberger Kraftwerke AG put a turbine pipeline consisting of DN 1400 ductile iron pipes into operation at the “Klösterle” hydropower station for which the “leak-before-break” principle was implemented in practice for the first time [2].

For the new turbine pipeline at Kanzingbach as well, the owner and operator TIWAG asked for the “leak-before-break” safety concept, which was included in the criteria for selecting the pipe material. This choice is influenced by a number of factors.

2.1 Safety

Ductile cast iron has always been considered as a highly flexible material because of its particular structure. A large number of the properties of ductile iron pipes for the transport of drinking water or wastewater are defined in the familiar standards EN 545 [3] and EN 598 [4] and are routinely checked by pipe manufacturers. When it comes to additional unplanned loads due to violent impacts not caused by operation-related events, such as landslides, rockfalls or earthquakes the ductile iron pipe has great reserves. What is more, with the centrifugal casting process the wall thickness of iron pipes can be almost arbitrarily adapted to suit the pressures occurring.

2.2 Handling ductile iron pipes in Alpine terrain

For one thing, it saves having to transport bedding material to areas which are often difficult to access. The excavation material set aside on site is quite sufficient for this purpose. For another thing, the proven joint technology is an advantage when using ductile iron pipe systems on steep slopes: with the so-called open-close method the pipe trench is only opened up for the length of one pipe and then filled again once the joint has been assembled. This means that the risks of a change in the weather can be kept within limits.

Ductile iron pipes are supplied with a cement mortar coating to EN 15542 [5] (Figure 1). The protection works in two ways: mechanical protection and protection against corrosion. The pipe trench can be backfilled with stony and compressible excavation material.

Tiroler Wasserkraft AG, as the largest operator of hydroelectric power stations in the Austrian Tyrol, also places emphasis on safety and ease of handling. It also defines the toughness of its pipe materials using the criteria of fracture mechanics and sets down certain requirements in the project described.

3 Elastic-plastic fracture mechanics

Elastic-plastic fracture mechanics, also known as yielding fracture mechanics, describes crack propagation under static loading taking account of plastic deformation, as it occurs in all ductile materials. Before a crack which forms during production, installation or operation reaches a critical dimension, it grows during its operating time under dynamic loads (cyclical crack growth, fatigue), corrosion fatigue cracking and/or stress corrosion. The progress of a crack over time is illustrated in Figure 2.

Figure 3 shows a typical “leak-before-break” diagram with the stages of crack formation.

Static crack propagation is determined with the help of crack resistance curves. Fracture toughness J_0.2 here is the energy in the area of the crack tip which describes the propagation of the crack over a length of 0.2 mm. It represents the critical crack size and hence the limit value of the design. This description has been very much simplified; detailed information on this can be found e.g. in [6]. The method of crack resistance measurement and the resulting diagram are shown in Figure 4.

As illustrated in Figure 4, a frequently used test procedure uses the bending load, in this case with a 4-point bend. But a combined tensile and bending load using a CT specimen (CT = compact tension) is also possible. A cubeshaped specimen is given a V-shaped notch in the same way as with the notched bar impact test. In addition a crack is introduced into this specimen in order to record crack resistance curves by dynamic loading. The length of this is determined before the test as it has a random length which can differ from one specimen to the next.

By stressing this specimen in the 4-point bending test (Figure 4, top) the widening of the notch and/or the deflection is determined (Figure 4, bottom left). These results are used to calculate the physical work J required for widening the crack Δ_a (Figure 4, bottom right). The J_0.2 value which is of interest in these investigations can then be read off at Δ_a = 0.2 mm.

When designing pipelines according to the “leak-before-break” criterion, the ratio of the permissible through-crack to the wall thickness still also plays an important role. The length of the through-crack here can be equated with the longest crack length not detectable in production. The greater this ratio is, the higher J_0.2 must also be in order to achieve a stable leak. Figure 5 shows this dependency for different application cases.

4 Investigations carried out

For the turbine pipeline of the Kanzingbach hydroelectric power station, owner and operator TIWAG also asked for the safety concept as presented above, namely “leak-before-break”. This requirement is associated with a further development of the well-known “ductile cast iron” material, the aim of which was yet to be defined.

After some preliminary talks with TIWAG a requirement was finally formulated which made provision for tolerable through-crack lengths to the extent of six times the wall thickness. This results in fracture toughness values of KJ_0.2 = 1,900 N/mm^3/2 and crack resistance energies of J_0.2 = 19.3 kJ/m².

The test centre for material and mechanical engineering in Innsbruck (V.A.M.) determined the fracture toughness values direct on specimens taken from the pipes and fittings. To do this, a defined crack was introduced into the test specimens with a resonance test system (“pre-cracking”). Then the fracture toughness and the crack resistance curve were determined on a 3-point bending test specimen using the partial unloading process (also known as the compliance method) in accordance with ISO 12135 [8].

The required properties were achieved in the course of joint research and development work by the two pipe manufacturers TIROLER ROHRE GmbH – TRM and Duktus Rohrsysteme Wetzlar GmbH together with the client TIWAG and the V.A.M. test centre in Innsbruck. The development included a modification of the material. In practice this means that special types of pig iron and scrap had to be used forthe new composition. In this way it was possible to produce the desired ferritic structure with its extremely fracture-tough properties.

By the end of the research work the material conventionally used for ductile iron pipes according to EN 545 [3] (similar to GJS 450-10 as per EN 1563 [9], [10]) can be compared with the material modified for the requirements of TIWAG on the basis of the different mechanical and technological properties.

Table 1 shows the comparison at room temperature. In addition the impact bending energy used in the temperature range between -40 °C and room temperature was determined. The measurement results are plotted in the graph in Figure 6.

At first glance the large differences between the two materials are apparent. While GJS 450-10 is already exhausted or overstressed, the modified material clearly exceeds the elongation at fracture requirements > 10 %, which however is at the expense of tensile strength and technical yield strength. Also the impact energy consumed is considerably improved with the modified material over the entire temperature range tested.

However the tests also show that an assessment about the impact energy consumed using elastic- plastic fracture mechanics alone is not possible because the material GJS 450-10 would be considerably undervalued here. Hence the impact energy consumed is to be assessed very critically as a measurement parameter for the sustained loading of the pipe.

After completion of the metallurgical and production technology development work, improved material properties could be achieved: the pipes and fittings in ductile cast iron standardised for water pipelines in EN 545 [3] must have breaking elongation values of at least 12 %, if R_p0.2 ≥ 270 MPa. By comparison, the material optimised in terms of fracture mechanics has values of up to 20 % and achieves the required fracture toughness values while observing the mechanical properties required in EN 545 [3] (tensile strength ≥ 420 MPa and 0.2 % yield strength ≥ 270 MPa).

5 Pipeline construction

Before the construction of the powerhouse the 4.5 km long pressure pipeline had to be installed. The DN 600 ductile iron pipes used for this were developed by the traditional Tyrolean manufacturer TRM together with Duktus Rohrsysteme Wetzlar GmbH and delivered by TRM. In the upper area of the route – apart from about 300 m – the pipeline runs along existing pasturing pathways. Here restrained pushin joints (PFA = 25 bar) from the BLS^®/VRS^®-T system were used, meaning that the construction of concrete thrust blocks was not necessary.

In the lower lying section, in which numerous pipe bends were also installed, the production of permanent anchors in the form of concrete thrust blocks was unavoidable. In this area of the route the client used ductile iron pipes with the conventional TYTON^® push-in joints (Figure 7).

The old route was deliberately not chosen for the new pipeline. This meant that the old power station could stay in operation and produce power for the entire construction time of one and a half years. However, because of the difficult topographical conditions this entailed some considerable challenges in some cases. During the summer pasturing period between 15 June and 15 September no pipes could be laid in this area. But because of the very late onset of winter the work could nevertheless be continued on the pipe and cable route until the middle of December 2014.

A further challenge for the installation team commissioned was the rocky subsoil in the higher grazing ground. The advantage of a pipe trench in the rock (Figure 8) lies above all in the fact that the stability of a pressure pipeline installed in it is distinctly increased. However, producing it was anything but simple.

The construction crew used an excavator with a cutter head (Figure 9) as well as other excavators which removed the material cut out and which were used for the installation of the pipes.

In this way it was not unusual to have six to eight excavators in use along the route at the same time in order to keep the installation work within the narrow timeframe set. Altogether the cutting work for the pipe route extended over a length of 2 km.

A not inconsiderable synergy effect was exploited in favour of the community during the installation of the pipes. There was an old, pressureless pipeline which ran from an elevated tank to the drinking water source on the Flaurlinger Alm, which TIWAG would have to have replaced anyway. For this reason a small DN 200 pressure pipeline – also in ductile cast iron – was also installed in the pipe trench (Figure 10), giving the community of Flaurling the option of having a drinking water power plant, which is already being implemented today.

Great value was placed not only on the durability and robustness of the new small power station but also on its implementation being as environmentally compatible as possible: about 150 m² of wetland has been established in the area of the powerhouse. This wetland has been designed according to ecological viewpoints and meanwhile is providing a habitat above all for amphibians but also for other animal and plant species. No protected areas have been disturbed by the implementation of the project. The parts of the old plant are being dismantled and currently a so-called “conveyancing operation” is in preparation.

Added to this is the fact that one of the strict specifications for aquatic ecosystems is being met concerning residual water output into the residual flow stretch of the Kanzingbach. This represents a striking improvement as compared with the previous state of affairs. In concrete terms, 15 % of the naturally occurring water, but at least 100 L/s in low water times, is now being sent to the residual water stretch. Regulation of the inflow of water is fully automatic.

6 Schedule and conclusion

The construction work was started at the beginning of October 2013, whereby the check dam structure for water catchment was still able to be completed in the same year. The construction companies commissioned were able to begin installation of the pressure pipeline in Spring 2014, once nature had thawed out in the Flaurling valley. After some rapid progress with construction in the following months, the topping- out ceremony took place at the beginning of October 2014. With the formal commissioning on 11 June 2015 the construction project came to a worthy conclusion after a construction time of around a year and a half.

With the replacement of the two old power stations with a new one, TIWAG is able to use the water of the Kanzingbach more efficiently and ensure the power supply of at least 4,000 households. The new hydroelectric power station makes a contribution to the ecological, efficient and sustainable expansion of home-grown water power. Small water resources are worth careful handling! 

Bibliography

[1] ANSI/AWWA C 151-09/A21.51-09: 2009
[2] Fussenegger, F., Mathis, R., Titze, E., Rammelsberg, J. and Schütz, M. GUSSROHR-TECHNIK [CAST IRON PIPE TECHNOLOGY] Issue 32 (1997), p. 58 ff. Download: www.eadips.org/jahreshefte-d/
[3] EN 545: 2010
[4] EN 598: 2007+A1:2009
[5] EN 15542: 2008
[6] Homepage: http://bruchmechanik.info/ bruchmechanik-themen/grundlagen/ kennwerte-der-bruchmechanik/
[7] Pusch, G. Konstruieren + Gießen [construction + casting], Issue 33 (2008), no. 4, p. 2 ff.
[8] ISO 12135: 2002-12
[9] EN 1563: 2011
[10] prEN 1563: 2015

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

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