Runner Casted-Vs-Microguss Wangdi D

Comparison between cast and microguss runners of Chhukha hydropower plant

Dorji Wangdi, Late Ugyen Tshering, T.Tenzin

Center of Excellence in Vibration and Thermographic Analysis (CoEVaTA), Chhukha Hydropower Plant, Druk Green Power Corporation, Chhukha, Kingdom of Bhutan

Abstract This case study compares the MicroGuss runner (forged runners) with the conventional cast runner, installed at Chhukha Hydropower Plant. The comprehensive analysis of both the runners has been carried out in this case study. A brief cost analysis has been carried out and it indicated that the initial cost of investment for MicroGuss runner is nearly 30% more as compared to conventional cast runner. Like in all Himalayan countries, the rivers of Bhutan are also laden with tremendous amount of sediments, which can be very detrimental to the underwater components. Hence a need of analysis with the precise data is very important. Quartz is found to be dominant, followed by feldspar, which are more damaging compared to other silt particles. Comparison of non destructive testing on the two runners is also been mentioned. The detection of fatigue cracks by non destructive testing is found to be more on cast runners whereas the affect of erosion is more prominent on MicroGuss runner. Recommendation is provided for hard coating of MicroGuss runner to withstand erosion. Introduction Chhukha Hydropower Plant (CHP) with an installed capacity of 4X84MW was fully commissioned by September 1988. The plant is a run-of-the-river scheme, located on the Wangchhu River. The Plant has four generating units, propelled by Pelton turbines. Even though, the generating units are rated at 84MW, up to 10% overloading is allowed during the peak generation. The vertical Pelton turbine unit operates at a speed of 300 rpm and utilizes a net head of 435 m. Since this power plant is a run-of-the-river scheme, the issue of sediments is prominent and the problem is aggravated further with maximum percentage contents of quartz, which is found extremely hard. The issue of silt has an impact on the performance and reliability of the runners, so this premature retirement of MicroGuss runner is reported in this paper. Although severe runner erosion was observed on MicroGuss it seems to have a higher resistivity to crack compared to the conventional cast runner. The turbine is an important asset in the power generation; hence it is necessary to perform a timely inspection in order to utilize it productively and efficiently at an appropriate level of risk. So the critical roles of non destructive testing in monitoring both types of runners are also presented in this paper. The advantage of MicroGuss runner over the conventional cast runner based on the cost analysis is also reported. The limited references for the adequate runner selection for this power plant have been the key point for the need of detailed analysis and the main idea of this paper. 1. Comparison of Casting and Forging Process All manufacturing processes involve turning raw materials to finished products to be used for various purposes. This can be either achieved by Casting or Forging. Casting is one of the oldest manufacturing processes. It involves a preparation of a refractory mould cavity which closely resembles the final object to be made. Hot molten material is poured into the mould cavity and is allowed to solidify. Whereas in forging, a metal maybe heated to a temperature below solidus temperature and then large forces are applied such that materials flow and takes the desired shape. As casting involves solidification of hot molten material in the mould, they are highly susceptible to many flaws like porosities and inclusions. Defects such as blow holes and open blows, pinhole porosity, segregation and cracks are also intimately linked with casting process. The moisture inside the mould, due to heat in the molten material, will cause the moisture to evaporate, part of which when entrapped in the casting will end up as blow holes or ends up as open blows when it reaches the surfaces. As molten metal gets solidified it loses the temperature which decreases the solubility of the gases and thereby expelling the dissolved gases. The hydrogen present while trying to escape

will show very small diameter and long pinholes showing path of escape. All these defects may lead to propagation of crack when the material is worked under pressure. Hence it’s very important to ensure a high quality casting. Forging involves heating the material above its solidus temperature and the material is hot worked into desired shape, the defects such as inclusions and porosities and grains are forced to flow in the direction the material is being worked, hence imparting directionality, which will improve the tensile strength of the material. Enormous and extensive pressure are involved in the plastic deformation of the metal that the desired mechanical properties can be attained. 2. MicroGuss Runner MicroGuss is a Pelton runner manufacturing technology adopted by Andritz Hydro, where runners are manufactured out of a forged disk on which the periphery of the bucket roots is milled. The remaining part of the bucket is welded onto with a robot, layer by layer. The bucket is finally machined and grinded to profile to achieve absolute accurate geometry, what is favorable to efficiency and cavitation behavior. The main concept of the MicroGuss runner is: the right material at the right place [1]. The highly stressed region in the runner being the bucket root area, the periphery of the bucket root is milled out of a forged disk. This gives at this location much higher mechanical and structural properties of the material, which means typical casting problems such as geometrical deviation, non-uniform material structure, porosities and voids can be completely avoided in the highly stressed zone, which would cause the formation and propagation of cracks and may lead to rupture of Bucket. From the design point of view, MicroGuss runner has the advantage of having higher resistance against bucket root cracks which are more common in cast runners. Chhukha Hydropower Plant in the year 2008-2009 availed four new MicroGuss Pelton runners. The MicroGuss runner has been installed considering all the advantages and moreover it guarantees high efficiency as compared to the cast runners. However, during the 2010-2011 annual maintenance, after average running hours of 12,400 hours, heavy erosions were observed on all the buckets of three runners. These runners had to be released and replaced with conventional cast runners except for the runner installed in unit no III, where erosion was found to be minimum as compared to the rest, due to less running hours. 3. Cast Runners The manufacturing of integral cast Pelton runners without any fault is nearly impossible because of differing wall thickness [2]. Most of the time, these flaws are located in the areas like the root of the bucket, the inlet or the bottom of the bucket, which are exposed to mechanical and hydraulic strain. During manufacturing, as the wall dimensions of the cast component changes, the structure of the material will also be different at different cross sections. As the heat is removed, the casting solidifies unevenly inwards at different cross section leaving behind traces of internal flaws such as porosities and pinholes. The flaws and irregularities will have to be repaired by welding during initial stage of turbine manufacturing itself, therefore high cost due to welding may likely incur. This may further delay the delivery period of the runner. Cast Runners in Chhukha have been manufactured by many well known Casting Foundries such as Canadian Steel Foundries, Tamaris, Doosan and George Fisher.

Table 1: Cast Runners at CHP

S. # Runner No Make Running Hours

1 4479 CFS Canada 61,681 2 478901 Tamaris France 27,803 3 478904 Tamaris France 35,340

4 20461 Doosan South Korea 33,159

5 53796 George Fisher Swiss 60,720 6 53763 George Fisher Swiss 74,205 7 53834 George Fisher Swiss 83,129 8 14972 George Fisher Swiss 84,321


10 53849 George Fisher Swiss 75,007


4. Pelton Runner Failures The Plant operates at a designed net head of 435m, which means the pressure at the outlet of the penstock, will be approximately 43.5 kg/cm2. The runner buckets are subjected to such a huge pressure during operation which can initiate fatigue cracks. A careful maintenance is therefore essential for a long life expectancy. The most frequent occurrence of damage to Pelton runners are Hydro-abrasive wear, cavitation pitting, droplet impact erosion and the most detrimental damage initiation and propagation of cracks. 4.1 Wear: Abrasion & Erosion Since Bhutan lies amongst the young growing Himalayas, hence geologically it is very fragile and during heavy monsoon seasons, the rivers are laden with sediments. These fresh sediments mainly carry quartz and feldspar having hardness of 7 and 6 respectively in the Moh scale and are considered, irrespective of their shape and size, the most damaging amongst all the silt minerals. These hard particles striking the base material along with the jet can cause severe wear. Wear mechanism involves abrasion and erosion. Abrasion mechanism involves rubbing of the soft material by the hard material. Wear by abrasion can take place as:

Microcutting Grain detachment Fatigue by repeated ploughing Brittle fracture

Mechanism, where wear is due to solid or liquid impingement on the base material is erosion mechanism. Wear by erosion can take place as:

Cutting Erosion Fatigue Erosion Plastic Deformation Erosion by Brittle fracture

4.2 Design & Manufacturing Flaws

A material is strongest when force is evenly distributed over its area, so a reduction in area, caused by a crack or any flaws in the material, results in localized increase in stresses. These localized stresses superimposed with fatigue loading under very high pressure can lead to catastrophic failure such as rupture of the bucket. These localized stresses can also result from inappropriate welding, irregularities present in the material and abrupt change in the structure and cross section of the material [3]. Welded joints are subjected to residual stresses due to non uniform heating of the parts being joined. There are always a possibility that localized thermal stresses may result from uneven heating and cooling resulting in non uniform structural properties of the material. When these non uniform structures are subjected to complex loading, uneven pressure will be distributed over the material and this will cause localized stresses [4]. Therefore it is very important that proper welding procedures are followed and performed by qualified welders only. Proper heat treatment should be carried out after welding. Stress relieving must be carried out after every major weld repair works. But as much as possible, welding on the material should be avoided and damages should be repaired by grinding. Surface irregularities can also cause stress concentration. The irregularities may be the porosities and cavities in the material. Since material flaws cannot be detected with absolute certainty during the manufacturing process, some of the flaws remain entrapped in the material. These flaws can be very detrimental when subjected under very complex loading; hence it is therefore advisable to carryout periodic inspections after first operation throughout the entire life cycle, with the aim of extending the service life as far as possible without incurring any risk of failure [1]. The defects can be repaired by removal of damaged zones simply by grinding.

5. Analysis

5.1 Silt Analysis Most of Bhutan’s hydro electricity potential is accelerated by run-of-the-river scheme from the melting glaciers; Wangchu River also carries substantial quantities of particles, which can have a detrimental effect on the underwater components. Irrespective of shape and size, with such a huge pressure on the base material can cause wear which will lead to distortion of the bucket profile and will affect the efficiency of the runner. Figure 1 show mineralogy and PSD analyses of sediment samples conducted by Hydro Lab, Nepal. The Wangchhu River like in any Himalayan River mainly contains Quartz and Feldspar which are the most damaging amongst the other silt particles. Others include hard particles like Mica, Tourmaline, Hornblende, and few Garnets and soft particles such as Carbonate, Chlorite, Clay and Organic matters.

Figure 1: Average Mineral Content of Wangchhu River

The particle hardness of 5 Moh scale is generally considered sufficient cutoff value for the hydraulic turbines [5], whereas in the case of the particle content for Wangchu River, the maximum particle content is Quartz with the Moh scale of 7 which is in sufficient quantity to propagate the erosion mechanism. There are numerous remedies to eliminate the particle contents entering the generating units, the most effective and efficient method is to build up or provide a large head water reservoir. However incase of this Plant, best methods would be efficient settling chamber with effective and precise flushing facility and to adopt protective coatings on underwater components. Coating will provide protection to the hydraulic profile which will ultimately extend the lifetime. The approximate extended lifetime of a coated runner is twice than that of uncoated [6], which means that the repair works are less and hence maintenance cost is reduced. The return on the investment is also greater.

5.2 Cost Analysis Due to the inevitable flaws related to manufacturing of the Casting, the delivery of the Cast Runner maybe relatively longer as compared to the Forged Runners [1]. The Flaws has to be repaired during the manufacturing process itself. Additional welding and grinding on the base material may be required. Also this will further add to the cost of the component. The cost incurred for the cast runner of Chhukha Hydropower Plant is given in the table 2.

Table 2: Cost of Cast Runner

Runner No. Make Landed Cost

(MBTN)

4479 CSF, Canada 58.773

478901 Tamaris, France 23.107

478904 Tamaris, France 23.145

20461 Doosan, South Korea 25.407



60%19%

14%

2% 5%

Petrograpgic Analysis

Quartz

Feldspar

Mica

Others (A)

Others (B)

The total Cost for the four MicroGuss runners of Chhukha amounted to Rs. 232.32M, each runner costing to amount Rs.58.081M. The cost of the runner does not include the transportation and handling charges. Even though the cost of MicroGuss runner is relatively huge as compared to the cast runner, the inspection interval for MicroGuss runner is relatively longer (up to three times) which means consequently minimized production loses, investment incurred during inspection and savings of man-hours.

5.3 Non Destructive Testing of Hydro Runners Non destructive testing is a technology of accessing the soundness and acceptability of the components without actually affecting the properties of the components [7]. The Non destructive testing methods are employed for preventive maintenance, for the inspection of raw products, half finished and finished products and for in service inspection. It also helps to ensure the integrity, safety and reliability of plant’s underwater components. Non destructive testing allows operation and maintenance team to accurately determine the conditions of the equipments and identify potential defects, if any. Non destructive testing methods have advanced the goals of condition monitoring programs and to meet the objectives to detect flaws at the incipient stage and to extend the scope of machine downtime, non destructive testing are carried out, thus providing an excellent balance between the plant life assessment and cost effectiveness thereby optimizing plant safety and reliability. Non destructive testing has a critical role in assuring that components and systems perform their function in reliable and cost effective way, thus non destructive testing provides an excellent balance between the plant life assessment and cost effectiveness. Non destructive testing techniques like visual inspection, dye penetrant test, magnetic particle testing and ultrasonic testing are used often for monitoring the underwater components (Runners) in power plants under Druk Green. The above mentioned conventional non destructive testing methods are mainly used for macrostructure inspection that is inspection of total detection of Macroscopic defects like shrinkage holes, pores, slag or other inclusions cracks, weld defects, laminations etc [8]. Non destructive testing in power plants under Druk Green mainly aims to locate various flaws on underwater components that are in service and to reduce the machine down time and avoidance of any catastrophic failures. Even in the history of this plant there was an occurrence of Major Cast Runner bucket failure i.e. the Unit III runner bucket No. 19 got sheared off on 11th October 2001. The inspection of runner is emphasized due to the cost implication and the requirement of the manpower, in case of the machine down time. The target inspection mainly includes in-service deterioration, surface crack detection by magnetic particle test and penetrant test or volumetric inspection by ultrasonic testing. Non destructive testing method like magnetic particle testing using prod technique and yoke technique were frequently conducted based on the runner inspection schedule provide by the OEM on both the MicroGuss and Cast Runners. The expectation of fatigue crack was more prominent for the cast runners comparing to the MicroGuss runner. In an attempt to keep the plant operate safely, periodic inspection are conducted for all the runners to find the cracks before it leads to any catastrophic failure. The inspections are performed by shutting down the generating units for around six hours. In a case, magnetic particle inspection was carried out and cracks length of 150 mm was observed between the root of bucket No.1 and 21 and a crack length of 20 mm was observed between the root of bucket No.20 and 21 (Figure 1). However no indications of flaws (cracks) was detected on MicroGuss runner except some major erosion on the runner buckets as shown in Figure 2, which may be contributed by the high velocity solid particles striking on the bucket surfaces.

Figure 2: Crack Observed (Encircled) on the Root of Cast Runner

Based on the detailed non destructive testing analysis and comprehensive studies, the frequency of fatigue crack detection was towards the higher side for the cast runner when compared to the MicroGuss runner depending on the design aspect of the plant.

Figure 3: Extensive Erosion on MicroGuss Runner Buckets

6. Conclusion Irrespective of the manufacturing processes, if the manufacturing procedures are adopted appropriately, manufacturing defects in both the processes can be eliminated to certain extend. MicroGuss manufacturing technology has an enormous advantage over the conventional cast runner such as the risk of fatigue crack on the bucket root which can lead to catastrophic failure. The bucket roots is forged which enhances the strength against the stresses. Even though the initial investment might be more for MicroGuss Runner, the inspection interval is three times less than that of Cast runners, consequently machine downtime and production loses are minimized. The premature retirement of the MicroGuss runner was mainly due to the hydro-abrasive erosion caused by the high concentration of hard particles. To combat this problem, hard coating technology can be adopted. Coating can improve the life of runner up to twice as compared to the uncoated runners. Based on the practical experience and the details of the non destructive testing analysis collected from the plant, the frequency of fatigue crack detection is higher on the Cast Runner when compared to the MicroGuss. However, the case study does not cover the difference of efficiency between the two runners. The MicroGuss Runner has 2-3 % higher efficiency than the Cast runner [9]. A detailed study on the technologies related to enhancing the overall life of the underwater component needs to be viewed. The effect of erosion on the efficiency needs to be studied as well. References

[1] Tala Hydropower Pant “Description of Goods and Performance Characteristic”, Supply of Four MicroGuss Pelton Runner, Tala Hydropower Plant.

[2] R.Maldet, C. Pfurtscheller, “Virtual Bucket and the Welded Pelton Runners are they the Technical Economical Best Solution for the Substitution of the Schmirn Runner”.

[3] V.B Bhandari, “Design against Fluctuating Load” Design of Machine Elements.

[4] V.B Bhandari, “Stress Relieving of Welded Joints” Welded Joints, Design of Machine Elements.

[5] John H.Gumer, “Combating Silt Erosion in Hydraulic Turbines”.

[6] www.vatech-hydro.com, “Benefits of Coating”.

[7] Trinity Institute of NDT Technology” Training Manual”.

[8] C.V Subramanian, M. Thavasimuthu, “Course Material for Ultrasonic Testing Level –I/II “.

[9] Norplan, “Refurbishment Study Chhukha Hydropower Plant, Bhutan”.

The Authors

Dorji Wangdi: He has a Bachelor of Technology in Mechanical Engineering from the National Institute of Technology, Raipur, India. Presently, he is working in capacity building of hydro turbine expertise. Some of his present assignments include finding suitable technologies available in the international market to enhance the overall life of the underwater components, develop overall maintenance strategy framework for Druk Green to bring efficiencies in the power plants. He is also responsible for capacity building in Ultrasonic flow detection and turbine efficiency measurements, conducting research, analysis, promotion and development of tools, techniques and methods that enhances maintenance practices and safety, imparting training and authoring of technical papers. Late Ugyen Tshering: He had a Bachelor of Technology in Mechanical Engineering from the National Institute of Technology, Durgapur, India. He was a Certified Vibration Analyst Cat II as per ISO 18436-2, a NDT Level II as per ASNT, an Infrared Thermographer Level I as per ITC. He made very good contributions towards the establishment of the CoEVaTA and human capacity development. He was a young man full of energy, patience and an intelligent and fast growing professional in the condition monitoring techniques. He left for heavenly abode on the 20th of the December, 2011 after a brief illness. T. Tenzin: He has an MScE in Mechanical Engineering from the University of New Brunswick, Fredericton, NB, Canada. At present, he is working to promote the Center of Excellence in Vibration and Thermographic Analysis (CoEVaTA) with the Druk Green Power Corporation, the young and largest power company in the Kingdom of Bhutan. The setup of the new Center is aimed to develop human resource capacity and usage of modern predictive maintenance tools to reduce maintenance costs and prevent downtime of the hydropower generating units under Druk Green and extend such services to other industries in Bhutan and the global market. He has interests in erection, operation & maintenance activities of hydropower engineering. He is also assigned as the Project Manager for the Hydropower Service Center, which Druk Green is investing for reclamation of hydro turbines using state-of-the-art technology. He has co-authored a technical paper titled “A Hybrid On-Line Vibration Monitoring System and Finite Element Simulation of Vertical Hydro Turbine Generators” with Dr. Robert J. Rogers, Retired Professor at the University of New Brunswick, Canada and presented it at the Canadian Machinery Vibration Association’s annual conference in Quebec City during the year 2010. He has also co-authored a technical paper titled “Comparison Between Cast & MicroGuss Runners: Chhukha Hydropower Plant”, which is being presented at this International Conference. He is also a certified Vibration Analyst Category II (ISO18436-2). [email protected], [email protected]

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Runner Casted-Vs-Microguss Wangdi D