By Ian M. Meredith
In 1995, Mount Ruapehu in New Zealand erupted. The crater of this volcano is 20 kilometers from the intake to the underground headrace tunnel of the 120-mw Rangipo plant. Ash ejected from the volcano entered the Tongariro River, which feeds Rangipo. This ash caused significant wear to the plant’s two Francis turbines and all auxiliary components that came in contact with the water.
A year after the eruption, project owner Genesis Energy shut down the plant to complete repairs to both machines. As part of this repair, Genesis Energy applied a protective coating to the turbine components subject to high erosion wear – runner blades, labyrinth seals, wear rings, band seal, cheek plates, and wicket gates.
Cheek plates for the two turbines at the 120-mw Rangipo plant suffered serious damage due to erosion from volcanic ash. Project owner Genesis Energy treated the new cheek plates with a hard coating to resist erosion.
During this outage, Genesis Energy determined that, for Rangipo to continue to operate effectively in the future, it needed to replace the turbine runners in the two units. Genesis Energy anticipated that Rangipo would be subjected to ash contamination for many years until the ash was completely “washed” off the mountain, so the utility decided to apply protective coatings to the new runners before they were installed.
Rangipo and the Mount Ruapehu eruption
The run-of-river Rangipo project has an underground power station containing two turbines supplied by Escher Wyss. The 14-pole, 63-mw units run at 428 revolutions per minute (rpm).
The Rangipo facility has headrace and tailrace tunnels and surge chambers, as well as a single vertical penstock that bifurcates into two horizontal penstocks, one for each turbine-generator. The net head of the system is 210 meters. The station produces 631 gigawatt-hours (gwh) per year.
On September 23, 1995, eruptions commenced from Mount Ruapehu. The eruptions continued – with varying degrees of activity – until the volcano was downgraded on October 3, 1996, to Level 1 active alert. Activity alert levels range from 5 (the mountain is erupting) to 1 (the mountain is active but not erupting). During this one-year period, about 7.6 million cubic meters of tephra (volcanic ash) were deposited onto the Rangipo catchment. About 40 percent of this ash comprises mineral with angular, sharp, and abrasive particles. On the Moh scale, this hardness ranges from 5 to 6.5, with Moh 6 being the hardness of tool steel in a working file.
Managing the aftermath
Because Genesis Energy had no past experience with such an event, and because the station was operating with full headrace and tailrace tunnels, the utility decided to keep generating. In the seven months between September 1995 and April 1996, about 15 years of normal life was worn from the turbines.
The turbine damage caused by the volcanic ash in the water was not the only problem at Rangipo. Due to a design fault, the runners of these turbines were set too high relative to the tailwater level. From the time the plant was commissioned in 1983, the runners experienced cavitation damage that required annual weld repairs.
In April 1996, Genesis Energy shut down the turbines for a major overhaul. The work undertaken in this overhaul was intended to allow the station to return to service in the shortest time possible. The overhaul consisted of carrying out essential repairs and applying a coating system to protect the runners from future damage.
The utility also determined that the runners at Rangipo were nearing the end of their life owing to the combination of severe blade thinning caused by the erosion and the extensive cavitation weld repairs carried out to date.
In replacing the runners, Genesis Energy wanted to ensure that it incorporated the enhancements necessary to mitigate erosion effects and extend plant life. One such enhancement was to apply some kind of protective coating to the new runners. To determine the best coating systems available, Genesis Energy decided to perform extensive laboratory testing and evaluation.
The utility engaged Materials Performance Laboratories (MPT) and the University of Auckland to carry out research and development of various coating systems. This research and testing regime culminated in a purpose-designed specification for wear-resistant hard coating in a powder form that could be applied using a high-velocity oxygen-fueled (HVOF) or high-velocity air-fueled (HVAF) gun, where the powder is melted in the gas flame and then propelled at high velocity onto the surface in the gas stream. The powder found to be most suitable was tungsten carbide comprising chrome cobalt coarse powder.
Around this same time, turbine manufacturer Voith Siemens Hydro Power Generation was conducting trials of a newly developed soft coating system for runners installed in highly abrasive environments. Additionally, the British company Irathane International had developed a soft coating system successfully applied to small runners.
Consequently, Genesis Energy commissioned MPT to also establish a testing regime for soft coating products and then to study both the hard and soft coating systems in order to recommend the most suitable product for Rangipo.
New runners for Rangipo
In 1999, Genesis Energy invited expressions of interest from runner manufacturers to design and build new runners for Rangipo. Bidders were required to not only design runners that would mitigate past cavitation problems but also to provide evidence of their expertise and ability in applying specialized erosion protection coating systems. After evaluating information supplied by the bidders, Genesis Energy chose Voith Siemens. This company clearly demonstrated its ability to design relatively small (1.9-meter-diameter) runners and had experience with hard and soft coatings.
Voith Siemens’ tender was based on a runner fabricated from low-carbon martensitic stainless steel. The crown and band would be cast and the blades pressed from plate, then welded to the castings.
The original runners were 1.9 meter in diameter, having 14 blades matched against 26 wicket gates. Cavitation repairs were difficult because of restricted gaps between the blades. Genesis Energy’s specification called for new runners of the same diameter, possibly with greater blade length for cavitation mitigation, but ideally to have 11 blades to allow better access for welding and coating repairs. However, Voith Siemens could only design an 11-blade turbine if each unit’s capability was derated from 72 mw to around 65 mw. This derating was unacceptable, so Genesis Energy accepted the 15-blade design.
In addition, Voith Siemens would not supply erosion or performance guarantees for coatings applied by third parties. This stance placed an unacceptable runner performance risk on Genesis Energy, so the utility allowed Voith Siemens to apply both the hard and soft coating systems.
For the hard coating, Voith Siemens chose its tungsten carbide powder, Diaturb. This coating tested successfully in the erosion trials carried out by MPT. It was applied using HVOF spray to a thickness of 300 to 400 microns, with a surface roughness range of 4 to 6 microns (Ra values).
Several runner blades in the two turbines at 120-mw Rangipo cracked because of weakening due to excessive erosion from volcanic ash after an eruption. The replacement runners were coated to resist erosion.
When Voith Siemens quoted the price to apply the hard coating, Genesis Energy realized that using this coating for all surfaces exposed to water flows would be too expensive and difficult to justify. So the utility highlighted critical areas on the blades, crown, and band for hard coating treatment. Again, the relatively small openings between the runner blades restricted Voith Siemens’ ability to apply this coating to the selected areas. The company was not able to procure a HVOF gun small enough to allow post-assembly application of the powder at a near 90-degree angle to the surface in the restricted area between blades. Instead, the company elected to apply the coating before the runner was assembled, meaning the weld areas between blade to band and blade to crown were not treated.
For the soft coating, Voith Siemens chose its polyurethane coating, Softurb, and suggested it be spray applied. However, product developments before work commenced at Rangipo dictated the product be applied using a trowel.
To achieve the desired bond, the metal to be coated needed a surface roughness of 4 to 6 microns (Ra values). Surfaces that had been treated with the hard coating had the required roughness, meaning only the uncoated areas required roughening.
During coating of the new runner blades for the 120-mw Rangipo project, the contractor worked on every third blade. The yellow coating is primer, and the blue coating is the soft coating.
Voith Siemens primed the surfaces and left them for 24 hours before applying the soft coatings in three layers. The first layer was applied with a serrated hand trowel, with serrations set at the required coating depth. When cured, this initial coat was overlaid with a flat trowel to fill the voids. Final smooth coatings were applied to provide the required surface finish and uniformity. After each application, the product was allowed to cure for three days.
When Genesis Energy received the first runner, personnel were disappointed to see that the soft coating was wavy and rough. This was considered undesirable with respect to runner performance. However, Voith Siemens personnel maintained that performance would not be jeopardized.
Coating the runner seals
The challenge for the runner crown seal was to develop a design that could be machined and hard coated and provide sufficient seal length (labyrinth path) in the gap between the existing head cover and new runner crown.
The labyrinth arrangement was designed to ensure that:
- Deflection and stress of the rings was within acceptable limits at machine runaway speed of 745 rpm;
- Any distortion caused by the heat generated during the tungsten carbide application could be accommodated;
- The HVOF/HVAF gun orientation to the treated surfaces be tangential (i.e., at 90 degrees 1/- 40 degrees with a nozzle tolerance of 1/- 12 millimeters from its ideal distance to all sprayed surfaces); and
- Units could be readily assembled without damaging the hard coatings.
For the runner band seal, the design defaulted to match the existing system. This means one straight seal length, although this is longer than the original due to the runner being about 75 millimeters deeper (longer blade length for enhanced cavitation performance) than the original unit.
The crown seal and the static and dynamic wear rings were manufactured from one piece of forged stainless steel, as was the band seal wear ring. The material choice was based on obtaining the hardest stainless steel available that could be readily machined. The hardness of the heat-treated stainless steel is about Brinnel 240.
The tungsten carbide powder chosen for the hard coating of components manufactured in New Zealand was Metco 5848 applied using an HVOF gun, with the heat from the spray application controlled at about 35 to 40 degrees Celsius (C). The labyrinth or leg deflection of the rings was measured at about 0.1 millimeter on diameter, with all rings distorting the same amount and in the same direction, thus presenting no real concern.
Coating the cheek plates
Material selection for the cheek plates was based on the hardest material available that could be readily machined. Both Bisalloy and heat-treated stainless steel have hardness of about Brinnel 240 and can be machined.
Genesis Energy already had a spare set of rough machined Bisalloy 360 cheek plates available from a previous overhaul, so the utility decided to use these for the rebuild of the first turbine. Rusting was not an issue because of the hard coating and sealing of the cheek plate to the scroll case.
A new set of cheek plates for the second machine was manufactured from one piece of forged stainless steel, post heat treated to achieve the desired hardness.
Ideally, all corners to be spray coated should be radiused to prevent the risk of chipping. But this is not desirable for the wicket gate penetrations because a sharp 90-degree corner is necessary to mitigate erosion in this vulnerable region. Diamond grinding of the tungsten carbide was discounted as being difficult and possibly not achieving the required sharp corner. The solution reached was to press insert rings manufactured from Deloro stellite 6 having a hardness of Rockwell C 50 to 55. These rings were pressed into the cheek plate before applying the hard coating, with the upper surface protruding to the equivalent of the hard coating thickness. This allowed the hard coating to butt up to the ring, ensuring that there was no step between the coating and ring.
Coating the wicket gates
Experience from overhauling other turbines showed Genesis Energy that it is more economical to manufacture new wicket gates (guide vanes) for machines with relatively small wicket gates (blade size 400 by 300 millimeters), rather than repairing worn or eroded gates. A&G Price of New Zealand cast and profile machined new gates from series 400 martensitic stainless steel. Genesis Energy chose a martensitic stainless steel because the utility had eliminatedgreased wicket gate bushings on its turbines and installed deva greaseless bearings. Deva bearings require journals manufactured from hard non-rusting materials.
During the temporary overhaul work completed in 1996, the wicket gates were treated with plasma nitride to provide a “glass like” hard surface more capable of withstanding the erosion forces of water contaminated with volcanic ash. When the machines were dismantled during the 2002 rebuild, the blade surfaces were “patchy” and inconsistent and contained large areas of surface rusting, presumably due to the forced depletion of chromium from the stainless steel in the treatment process. However, later impact testing by MPT revealed that even the rusted areas were extremely hard, at about 700 Vickers.
The plasma nitriding process achieved the required surface hardness of about 1,000 Vickers. But the surface was found to be very brittle, comparable to a glass overlay of a soft substrate, and prone to chipping. Further trials on test samples varying the quantity of nitrogen, temperature, and time still did not achieve the desired result.
MPT tested an alternative plasma nitrocarburising process. After considerable experimentation, the approved process consisted of plasma nitrocarburising at 550 C for 30 hours under 25 percent N2 1 1 percent CH4 1 74 percent H2. The hardness achieved was 900 Vickers, with a surface less brittle than would result from plasma nitriding.
Results to date
The surface finish of the soft coating is rough or lumpy, with the first runner being worse than the second unit. Genesis Energy was concerned about this aspect because performance and efficiency is highly dependent on smooth surface finishes. However, the post-commissioning efficiency tests results did not support this concern, with overall performance being near the stated design performance criteria.
The soft coating has failed in areas, where there appears to be peeling possibly resulting from lack of bond. This situation may also be exacerbated by the rough or irregular surface finish of the coating. Efficiency testing after commissioning fell within the specified limits, so no redress was made to the contractor. Genesis Energy reluctantly accepts the roughness of the soft coating and hopes to create a smoother finish with subsequent patch repairs.
Quarterly inspection of both turbines has revealed no apparent damage to the tungsten carbide coatings, stellite inserts to the cheek plates, or plasma nitrocarburising of the wicket gates. The wicket gates do not display the rusting syndrome of the past plasma nitrided gates.
Inspection of the labyrinths is not possible, and it is assumed that the coatings are sound.
Not applying a hard coating to the weld areas means this highly vulnerable erosion zone remains unprotected. Of concern with any hard coating process is its ability to withstand cavitation damage. Therefore, it is essential that runners display excellent cavitation performance. If cavitation repairs are required, the hard coating would have to be ground off.
The Mount Ruapehu ash eruption was an event Genesis Energy had not planned for. This was the first such event that directly affected the 23-year life of the Rangipo station. The decision to keep the machines running while the water was so contaminated with ash wiped away 15 years of machine life in just six months. But shutting the machines down was less attractive because this would have resulted in a tunnel system completely blocked with ash.
The new turbines with the selected coatings have been operating for four years, with minimal wear.
Ian Meredith is a member of the engineering asset management team of Genesis Energy and is the engineering team leader for the renewable energy facilities.
Mr. Meredith may be reached at Genesis Energy, Private Bag 36, Turangi 2751 New Zealand; (64) 7-3847236; E-mail: email@example.com.