Concerns over reliability of fully-cast integral Pelton runners prompted one major manufacturer to explore alternatives that allow fabrication of the runner from stronger, more robust materials. Based on forging and welding practices, these techniques offer an alternative to traditional casting.
By David Appleyard
Pelton turbines are designed for high-head applications and consequently high pressure. Thus, the large forces and rapid load cycling typically found within a Pelton unit during operation present special challenges for both materials and manufacturing.
In particular, high stresses are found in the bucket root area of the Pelton runner and, coupled with the operational characteristics, this can lead to rapid fatigue and failure of the material. Cracks can develop quickly and, if unnoticed, can result in a bucket rupture with potentially disastrous results. The mass of the ruptured bucket combined with the high centrifugal forces can result in a projectile with a kinetic energy capable of destroying the turbine casing and even the area beyond. Furthermore, the sudden unbalancing of the rotor inevitably taxes the capabilities of the bearings and bearing supports in preventing total destruction of the turbine.
Pelton runners are traditionally cast as a single piece, more often than not using stainless steel G-X5 CrNi 13.4. The complex shapes and components of a Pelton runner make their manufacture difficult and time-consuming, and this process typically leads to long delivery times, in particular because of the mold preparation required and finishing and machining of the turbine runner. However, it is also known that the risk of hidden residual defects in the cast material in the high-stress areas of the runner cannot be totally eliminated.
Indeed, the number of reported bucket ruptures or cracks in the bucket root area of cast Pelton runners reveals an area of some significant risk for plant operators.
Using better materials
Recognizing the incidence of this type of failure mode, in the late 1980s Andritz Hydro began a detailed analysis of Pelton runner designs using finite element analysis and fracture mechanics methodologies focusing on eliminating the risk of residual failures in fully cast integral runners. The analysis was conducted with a view to executing the development of a number of alternative runner manufacturing technologies based on materials with better properties in terms of fatigue, crack resistance, and crack propagation, as well as minimizing the likelihood of casting flaws below the surface.
This work coincided with the closure of a number of well-established and experienced steel foundries in the early 1990s, which limited the available supply of high-quality cast materials and made the development of new manufacturing technologies the highest priority for the company at that time.
|A robot welds the bucket of a Pelton runner in a method that is an alternative to traditional fully-cast integral Pelton runners.|
Designed to take advantage of higher material quality and thus to increase reliability and availability, this approach uses welding and forging techniques to increase the strength of the Pelton runner, in particular in the high-stress bucket root area, and reduce the likelihood of flaws leading to failure of the material.
|This photo shows the final results of buckets on a Pelton runner that were welded using a robot.|
After economic and technical analysis, the idea of separating the runner into a central forged disc containing the bucket roots and creating the outer part of the bucket pieces by welding was carried forward.
Here the central disc is machined from a forged block of stainless steel CA6NM (CrNi 13.4), taking advantage of the higher mechanical properties (such as fatigue strength and fracture toughness) of forged versus cast steel. Forged material was favored for use in the high stressed zones compared to both traditional integral castings and a cast donut (runner disc from cast material).
The fact that the highly loaded root section of the buckets is made out of a forged disc in each of the manufacturing processes provides superior fatigue and lifespan characteristics, Andritz Hydro says, noting that corrosion resistance and corrosion fatigue properties are improved by the more homogeneous and fine-grained microstructure. In addition to the improved quality of the microstructure, impurities (e.g. slag flaws and inclusions) and segregations are drastically reduced by the forging process.
These material advantages are also reflected in the calculation of inspection intervals, which is based on material characteristics such as the threshold stress intensity factor ΔK0 and the crack growth rate da/dN by Paris and improved Forman equations.
An intensive check for hidden cracks is performed with ultrasonic testing after the disc is premachined. Because of the favorable accessibility of the premachined disc, the results of an ultrasonic test at this stage are much more reliable and accurate when compared with the ultrasonic results of a cast runner in the critical root zone, which in many cases presents poor accessibility and strong surface curvature.
The specific choice of the bucket separation planes allows the fabrication of relatively narrow discs but ensures that the high-stress areas remain inside the disc.
In early designs, the buckets were comprised of three pieces. The central and two lateral pieces were cast in CrNi 13.4 steel with an overthickness and then milled before welding assembly.
However, this early process is no longer used and, since the late 1990s, each bucket is cast or forged as a single piece that is then machined and given a final grinding before being attached to the forged runner disc by welding.
The buckets are welded onto the bucket roots, following a sequence to continuously correct for heat distortion caused by the welding process, and the assembled runner is then submitted to a post-weld heat treatment. To ensure the highest quality of the weld seams, ultrasonic, magnetic particle and liquid penetration testing is performed.
After the final machining of coupling and cut outs, only a final balancing and polishing of the hydraulic surfaces is necessary. A complete geometrical check of the hydraulic surfaces and the high stressed regions is performed after manufacturing of the runner is completed.
While it is also possible to manufacture the runner buckets from forged steel, the lower cost option of casting of such small and relatively simple pieces is possible at a great number of foundries with a quality higher than for a fully cast runner. In addition, Andritz Hydro claims the manufacturing time is reduced by 30% in comparison with integral fully cast runners.
With the progress of high-performance numerically controlled machining of large pieces, the question of manufacturing the entire Pelton runner out of a full block, preferably of forged stainless steel, arises.
Indeed, in 2003 Andritz Hydro began manufacturing fully forged Pelton runners, but depending on the cost of the forged material this method is mainly applied to smaller sizes of runners. About 60% to 70% of material must be removed from the forged disc during the machining process.
|This 423 MW Pelton runner, installed at the 1,269 MW Bieudron plant in Switzerland, was fabricated using welding of the buckets.|
The technology of fully forged Pelton runners, as well as HiWeld manufacturing, has subsequently been applied by various vendors, all supporting the trend away from integrally cast runners.
In 1992, Andritz Hydro had also initiated another welding process for use in manufacturing of Peltons with its MicroGuss™ technology. Again, the key to the design is the use of forged material in the high-stress bucket root area, in this case followed by the building up of the outer part of the runner bucket using numerically-controlled robot MAG welding, as opposed to manual welding technology used for the HiWeld process.
|A worker welds the runner buckets for a Pelton turbine intended for the 146 MW Bitsch hydroelectric plant in Switzerland.|
As with the HiWeld, resistance against corrosion fatigue is claimed to be significantly higher than conventional casting using this process while, again, the boundary between the forged and welded material is optimized according to the load distribution.
After the build-up process of the buckets, the runner is heat treated, machined and ground to profile. Final steps include non-destructive testing inspection, disc flange machining and balancing.
Ranging up to 30 tons in mass, the largest runner manufactured using the MicroGuss technique was supplied to a 423 MW installation with an 1,869 meter head.
More than 370 MicroGuss runners with a combined more than 9 billion running hours are already in operation, and the company claims a current order book of 430 runners within 20 years.
Application for welded Pelton buckets
Andritz Hydro's policy with respect to runner manufacturing is to apply primarily forged discs and use cast runners only in very specific situations, such as small hydro projects or runners with very low stress levels from time to time. In all projects, an assessment of the stresses is made with the support of combined flow and mechanical simulations, so-called fluid-structure interaction. The unsteady pressure load in the buckets is used as input for an unsteady finite element simulation at prototype scale from which stresses and deformations can be evaluated. The runner design can thus be properly assessed against the project specifications.
With this Pelton runner fabrication process, mechanical properties of material are increased in all the runner areas: in the disc and bucket roots through the forged steel, in the outer parts through high quality of casting or through forged steel pieces, and in the welding areas through an adapted qualified procedure reaching higher mechanical properties than integral cast steel.
Very demanding quality criteria have been defined for the welding procedure, testing methods and acceptability of welding, as well as the constitutive parts of the turbine runner, all this in collaboration with the TVFA (Technische Versuchs- und Forschungsanstalt, Tech-nical University of Vienna, Austria), a neutral institute.
Pelton turbine runners made from forged discs have proven their reliability, whereas integral cast runners or runner discs from cast material are still giving cause for concern in the industry. And, while there are cost differences between the manufacturing technologies, it can be argued that investment in higher quality with increased product reliability and safety should be prioritized for a long-term component with an extremely large number of load cycles, such as a Pelton turbine runner.
David Appleyard is chief editor of HRW-Hydro Review Worldwide. Acknowledgements to Thomas Weiss head of the Center for Excellence's Pelton department, Etienne Parkinson, head of the R&D department and Helmut Keck, vice president and head of technology and R&D with Andritz Hydro in Switzerland.