By Joachim H. Pott
To extend the remaining service life of the 730-mw Manapouri project in New Zealand, Meridian Energy Ltd. installed new runners and other turbine components. Model testing, independent inspections of manufacturing, and strong working relationships between the utility and the manufacturer and installation contractor contributed to the refurbishment’s success.
Manapouri Power Station, located in Fiordland National Park in the South Island, is New Zealand’s largest hydropower station in terms of installed capacity and annual generation. Water is supplied to the underground station from Lake Manapouri via seven 170-meter-long vertical penstocks. Water is discharged from a common draft tube manifold via a 10-kilometer-long tailrace tunnel system to the sea. The powerhouse contains seven turbine-generator units of the vertical-shaft, single-runner, Francis type. Each turbine originally was rated at an output of 105.9 mw, at 149 meters of net head and 250 revolutions per minute (rpm). Station commissioning occurred from 1969 to 1971.
In 1997, Meridian Energy’s predecessor, the Electricity Corporation of New Zealand, commissioned a project to construct a second 10-meter-diameter tailrace tunnel in parallel with the first tunnel. The purpose of the second tunnel was to increase the net head by 5 to 25 meters, depending on station operational load. The additional head increased annual generation at the station by 12 percent, and overall water- to-wire efficiency was significantly improved. This second tailrace tunnel was successfully commissioned in 2002.
Even before the new tailrace tunnel was complete, Meridian Energy decided to embark upon a NZ$98 million (US$77.3 million) effort to refurbish and upgrade plant equipment to maximize the benefits of more available head. Meridian also wanted to improve station efficiency and reliability, reduce maintenance costs, and enhance environmental performance. The work, referred to as the Manapouri Half-Life Refurbishment Project, consisted of:
- Replacing station excitation and automatic voltage regulators;
- Refurbishing generators and replacing high-voltage transmission cables; and
- Upgrading the turbines.
By March 2003, Meridian Energy had installed brushless excitation systems, and replaced automatic voltage regulators, isolated phase bus bars, unit cooling water pumps, and inlet strainers. These new components allowed Meridian to increase unit generation from 105.9 mw to 121.5 mw.
In mid-2003, Meridian launched an effort to replace the generator stators, refurbish the rotors, and replace the 220-kilovolt (kv) cables, which led to an increase in electrical rating to 135 mva for all seven machines. This work was completed in July 2006.
Also in 2003, Meridian contracted with GE Canada International Inc. to replace the original turbine runners and guide vanes with a new design, including modification of miscellaneous associated turbine components. The new runners – all in place by the end of 2007 – increased turbine efficiency between 2 and 4 percent in the main operating range of the turbine.
This article focuses on the experiences and lessons learned during the turbine upgrade.
Upgrading the turbines: making the decision
The original Manapouri turbines were manufactured by Harland of Scotland to an Allis Chalmers design. Although original turbine design was subjected to contractual model tests in 1962, machine performance on site did not match model predictions. The absolute efficiency of the turbines had always been questionable. Reliability has generally been good, but cavitation-erosion routinely occurred on the suction side of the runner blade inlet near the band. To repair this damage, the runner had to be removed from the turbine pit, which was costly in terms of lost power generation and increased labor.
In 2001, Meridian Energy commissioned EPFL-IMHEF of Lausanne, Switzerland, to build and test a homologous model of one of the Manapouri turbines. The purpose was to unambiguously verify efficiency of the original turbines and to show cavitation performance under the new net head conditions created by the addition of the second tailrace tunnel. The test confirmed a reasonably high efficiency for a turbine designed in the early 1960s; however, peak efficiency occurred at a lower net head than the head available with the new tailrace tunnel.
The model test demonstrated a need for upgrading the turbines to maximize the use of the increased net head and to solve problems with cavitation at the runner blade inlet. Meridian estimated the upgrade would result in an annual energy gain of 70 to 75 gigawatt-hours as well as reductions in runner repair costs and unit downtime, owing to the avoidance of blade inlet cavitation.
Implementing the upgrade
In April 2003, Meridian Energy awarded a contract to GE Canada International Inc. (GE) for design, model testing, manufacture, and supply of seven new turbine runners and seven sets of guide vanes. The contract also called for design of various turbine modifications to accommodate the new replacement components and verification of performance guarantees via independent contractual model testing.
Designing the runner
GE used its proprietary computational fluid dynamics (CFD) design software XMT, which incorporates three- dimensional potential flow analysis, to design the turbine runner blades. The final design selected features 15 blades, one less than the original Manapouri runner design. The design is called “X-blade” owing to a twist in the blade that causes the inlet and outlet edges to form an “X” shape. (See Figure 1.) A key feature of this design is a backward lean of the blade leading edge. This has the effect of opening up the area between blade suction side and band, resulting in more favorable pressure gradients and superior inlet cavitation behavior.
Model testing to verify performance guarantees
During model development tests prior to the contractual tests, it soon became apparent that proposed modifications to the stay vane and stay ring would not yield the anticipated 0.3 to 0.4 percent efficiency gain. That is because the original design of the stay vanes already was optimal, providing maximum efficiency.
GE created a new runner design with longer blades and, hence, a slightly deeper runner band. Testing showed a shift of peak efficiency toward lower flow and lower head, which proved more favorable for meeting the contractual efficiency guarantees. Meridian again engaged IMHEF, this time for the independent contractual model testing. This testing commenced in January 2004. The weighted turbine efficiency was found to be well within the guarantee range, with the X-blade runner design resulting in favorable flat efficiency curves. Areal cavitation guarantees were mostly met; incipient cavitation may only occur at high unit output and very low tailwater levels – an unusual operating condition for Manapouri. Inlet cavitation limits were outside the station operating range with a considerable safety margin against suction-side inlet cavitation. Hydraulic stability of the re-runnered model turbine was well below the guaranteed limits. Guide vane torque measurements confirmed the reduction in torque from original to new guide vanes. Turbine runaway speed was found to be well below GE’s guarantee. Owing to the occurance of considerable hydraulic thrust under runaway conditions, further model runner modifications were necessary. These modifications reduced hydraulic thrust to acceptable limits without excessively compromising turbine efficiency.
Value of model testing
Model testing of the original turbines allowed Meridian to establish a reliable benchmark for justifying the upgrade. Model testing of the replacement components and of the iterations GE and Meridian went through (including runner seal geometry and drainage modifications) resulted in a runner design that maximized turbine performance.
Manufacturing runners and guide vanes
The first runner and first set of guide vanes were manufactured in GE Canada International Inc.’s workshop in Montreal. Runner castings were sourced from Lokomo Steels/Finland and guide vane castings were locally sourced from Canada Alloy Castings Ltd. Runner also guide vane material for all units is stainless steel to ASTM A743 Gr Ca6NM.
To reduce costs and the duration of plant outages, Meridian decided to carry out the first runner replacement during a scheduled station outage for Unit 4 stator refurbishment. However, this resulted in a tight delivery schedule. The runner castings had to be ordered in July 2003, well before the final model test results became available. This meant that any last-minute design changes had to be restricted to the geometry of the slightly oversized casting components.
LVM Fondatec of Montreal, Canada, conducted independent witness inspections on behalf of Meridian Energy. The first runner and first set of guide vanes left the workshop 16 months after the contract award to GE.
Replacing original runners and guide vanes with a new design in the seven units of the Manapouri station in New Zealand increased efficiency as much as 4 percent in the main operating range of the turbine.
The turbine runners and guide vanes for Units 2 through 7 were manufactured by GE Hydro Asia Com- pany Ltd. (GEHA) in China. Runner blade and guide vane castings were sourced from Uttar Pradesh Steels in India; crown and band castings were supplied by Lokomo Steels in Finland. Meridian Energy engaged Intertek Testing Services in Japan – with operations close to the manufacturing facilities in India and China – to witness non-destructive testing and homology inspections and to carry out quality assurance documentation reviews throughout the process.
Modifying the turbines
Modifications to the Manapouri turbines were required to accommodate the new runners. New stationary aluminium-bronze wearing rings were installed in the turbine head cover and the discharge ring. The discharge ring wearing ring was relocated to the bottom of the runner band from the top to decrease hydraulic down-thrust. Consequently, the discharge ring had to be machined on site to provide a new seating surface for the relocated ring. Additionally, the piping used to deliver water for lubrication of the lower wearing ring for tailwater-depressed operation had to be re-routed.
Meridian decided to eliminate the use of greased bushes. To accommodate the greaseless material supplied by Orkot for the guide vane thrust arrangement, a redesign was required. All guide vane bushes and thrust arrangement in the original turbines were greased bronze. Tenmat supplied the greaseless guide vane and link bushes.
In addition, the guide vane end-stop blocks were redesigned so that, in the event of loss of guide vane control, the guide vane would not make contact with the runner blade.
Due to the new guide vane design and the requirement to meet increased output guarantees, the servo motor stroke was increased as well.
Several of the smaller turbine modifications only become apparent during the detailed design phase. While the big modifications, such as the wearing ring relocation and the servo stroke increase, were known at the tender stage and could be factored into the pricing, smaller modifications – many due to existing deficiencies – were unforeseen, but contributed to the overall cost.
Results of the turbine upgrade
In April 2005, Transfield Services Ltd of New Zealand installed the first new runner and first set of guide vanes in Unit 4. Transfield also completed a mechanical overhaul of the unit and converted the guide vane bushes and associated operating mechanism to the greaseless system.
Winter-Kennedy index efficiency tests were carried out prior to disassembly of Unit 4 and repeated following installation of the new runner. Comparing the results of the two tests showed an increase in efficiency of between 2 and 4 percent throughout the main operating range (which is from 75 mw to 130 mw).
The results were somewhat more favorable than the assumptions used to justify the turbine upgrade. While the majority of the efficiency increase is due to the improved hydraulic design of the runner and guide vanes, a small contribution comes from a reduction in surface roughness of the wetted components of the turbine. This reduction was accomplished as part of the mechanical overhaul of the unit during runner installation.
Visual inspection of Unit 4’s runner and guide vanes after 8,000 and 16,000 hours of operation showed no signs of surface degradation. Based on these inspections and on the very good cavitation performance observed in the model tests, cavitation damage is unlikely to be an issue for Meridian in the future.
The Unit 4 site acceptance tests showed that channel vortex cavitaton is permanently present on the prototype for unit output below about 70 mw. While model testing indicated only intermittent channel vortex cavitation at the low output end of the operating range, transposition to the prototype was not possible at that stage.
The resulting effect of this channel vortex cavitation is a high noise level in front of the draft tube hatch and in the turbine pit and high acceleration levels of embedded components such as draft tube cone, tailwater depression pipework, and runner seal cooling water pipework. Provision was made available on the prototype for determining the most effective and most economical of several air admission locations. Air injection significantly dampens the observed noise and vibration effects.
At Manapouri, Meridian Energy installed sufficient compressed air admission capacity to allow concurrent operation of two units partially loaded.
As of December 2007, all seven turbines had been upgraded. All units show similar performance characteristics.
One lesson learned was that the delivery schedule of 16 months from contract award to shipment of equipment to the plant – and including independent contractual model testing – was too tight. The tight schedule resulted in the need to conduct numerous activities in parallel. For example, casting of the runner components had to be done before the runner’s hydraulic profiles were finalized and before the detailed engineering design for various components had been completed. Fortunately, the castings were sufficiently oversized to allow machining of the longer blades and runner band required by the new runner design.
Another example is that final revision and approval of production procedures for the runner and guide vanes had to be done in parallel with the manufacturing process. This resulted in additional work, increased costs, and delays.
Meridian advises others scheduling refurbishment work to carefully consider the consequences – including increased costs – of squeezing delivery time frames.
Preparation of detailed witness specifications and engagement of independent inspectors in Canada, India, and China proved worthwhile, given the importance of homology between model and prototype hydraulic profiles. The inspections resulted in expedient resolution of production issues.
Assessing the performance of the original turbines by means of model testing was a valuable exercise. Meridian used the results of the test to establish a reliable performance baseline. This baseline was used in making a business case for the upgrade. The alternative to model testing – absolute site efficiency tests – would have been difficult to conduct at Manapouri owing to unavailability of a suitable flow measurement section. Furthermore, the accuracy of these tests would have been insufficient to justify the project expenditure.
The Manapouri turbine upgrade project also highlighted that model testing does not always allow accurate prediction of complete prototype behavior. While the reliability of predictions about efficiency, cavitation, and draft tube vortices is widely accepted, transposition of hydraulic thrust and certain hydraulic stability phenomena from model to prototype can lead to surprises.
During the design phase of the turbine upgrade, Meridian’s project engineer met for several days with GE’s engineers to discuss detailed engineering design. For Meridian, this contributed significantly to an in-depth understanding of the design process. It also helped GE better understand the utility’s perspective about the project’s key drivers. The result was an excellent working relationship between engineers on both sides throughout the upgrade.
Dismantling and refurbishing old hydro units often leads to surprises. Having an open and honest working relationship between owner and installation contractor and adopting joint problem-solving approaches to unexpected situations are keys to successful completion of a refurbishment project.
Mr. Pott may be reached at Hydropower Engineering Ltd., 110 Rakau Road, Hataitai, Wellington, New Zealand; (64) 4-3862585; E-mail: joe. [email protected]
Pott, J., “Manapouri Turbine Upgrade – Discussing the Challenges from Inception through to Implementation,” HydroVision 2006 Conference Technical Papers CD-Rom, HCI Publications, Kansas City, Missouri, USA, 2006.
The author acknowledges the contributions to this article of Bernard Jacquet, Thi C. Vu, and Daniel LeBreton of GE. GE designed, manufactured, and supplied the new turbine runners and guide vanes for Manapouri. GE also designed various turbine modifications. Meridian Energy acknowledges Intertek Testing Services and LVM Fondatec (manufacturing witness inspections), Transfield Services (installation and commissioning), and IMHEF (model testing) for their excellent services and productive working relationships. The author thanks Meridian Energy for granting permission to publish this article.
Joe Pott is a partner in Hydropower Engineering Ltd. in New Zealand. Previously, he worked as an asset development engineer for New Zealand utility Meridian Energy Ltd., where he was responsible for the technical aspects of the turbine upgrade at Manapouri.