Koeberg steam turbine retrofit increases power and extends life

Koeberg power station is the only nuclear power station on the African continent. Located in Duynefontein, some 30 km northwest of Cape Town in South Africa on the Atlantic coast, it ensures a reliable supply of electricity to the Western Cape, one of the fastest growing regions in South Africa.

The power station has operated safely and efficiently for more than 25 years and has a further potential active life of 30-40 years.

In March 2009 South African electricity utility Eskom awarded a contract to Alstom Power for a steam turbine retrofit project. This project will make use of the latest steam turbine technology to increase the power output from the plant, extend the plant’s operating life and to improve reliability.

Strategic asset in meeting demand

At 1800 MW, Koeberg is an important asset in the Eskom fleet. The station’s two French supplied 900 MW pressurised water reactors supply about 6% of South Africa’s electricity needs.

Koeberg also helps to save the country’s natural resources. At current economic and population growth rates, South Africa is expected to experience a permanent water shortage from 2020.

Koeberg, by utilising seawater for cooling, saves 22 billion litres of fresh water per annum which would be consumed generating the same power at an inland coal fired plant.

Koeberg has also been an important asset in terms helping South Africa to keep the lights on. The country has experienced above average GDP growth in recent years of about 5-6%, which translated into a growth in electricity demand of about 4% between 2006 and 2008. This resulted in a dwindling reserve margin, which fell from about 20% to less than 10% in the same period. Just prior to the financial crisis, the reserve margin was estimated to be as low as 5-7% and the country experienced severe power shortages in 2007.

Looking to the future, Eskom has developed an integrated electricity plan to boost the country’s installed capacity, currently around 42 GW. The programme includes the building of power lines, the installation of open cycle gas turbine power plants for meeting peak demand, a return to service of mothballed power stations, increase in generation by improving the operational efficiencies and the addition of new capacity.

Overall the programme would see the addition of around 40 GW to the grid by 2025. This would be sufficient to meet power demands and sustain economic growth of around 6% per annum.

Eskom’s capacity expansion budget is R385 billion up to 2013 and is expected to grow to more than a trillion rand by 2026. Ultimately, Eskom plans to double its capacity to 80 000 MW by 2026.

Since the programme started in 2005, an additional 4453.5 MW has already been commissioned, with plans to deliver an additional 16 304 MW in power station capacity by 2017.

Retrofit strategy

The Koeberg retrofit is part of this plan to increase the nation’s installed capacity. Under a R1.5 billion (approximately r135 million) contract Alstom will retrofit the low pressure turbines of the two 900 MWe units to improve power output and increase availability, flexibility and extend operating lifetime.

Specifically, the retrofit will:

• Increase the power output of each unit by more than 32 MW.

• Ensure operation until 2035, at least.

• Increase the flexibility of the plant to improve on the operational start up ramp rates.

• Remove turbine life limiting mechanical issues such as stress corrosion cracking.

Power plant retrofit is part of South Africa’s strategy to increase installed capacity since it is a fast and cost-effective way of adding new capacity.

Eskom and Alstom have been working together on other retrofit projects such as the integrated retrofit of the Arnot coal fired power plant, which covers the boiler, turbine and auxiliaries, and is increasing the capacity of the station by more than 300 MW in total.

Welded rotor technology

The steam turbines at Koeberg are of the CP1 type (as used for a tranche of the French 900 MW PWR programme) and were originally supplied by Alstom. Each turbine has one double-flow high pressure (HP) cylinder with seven stages per flow and three double-flow low pressure (LP) cylinders also with seven stages per flow. The units at Koeberg operate at 1500 rpm.

The steam turbines are supplied with 1500 kg/s of steam from the reactor at a pressure of 55 bar (a) and a temperature of 270ºC. The steam exhausts to the condensers at a pressure of 0.43 bar (a). After the retrofit the turbines will have a maximum design output of 997 MW, compared with 965 MW prior to the retrofit.

The old units have rotors that feature a shrunk-on disk design, which can be at risk of stress corrosion cracking.

Alstom will replace the LP rotors with a welded design. In this design, each LP rotor is made up of four forgings. These four forgings are then welded together and so form the basic rotor shaft ready for machining to the finished rotor.

The rotating and stationary blading will also be replaced with the latest blading design. The rotating blading is fitted onto the new welded rotors, whilst the stationary blading is mounted into 14 stages of diaphragms per LP cylinder arranged in a double flow design of seven stages per flow. Alstom will also perform modifications to the LP hoods to accommodate the new design and to improve the exhaust steam flows.

The retrofit uses similar technology to that already installed and proven on similar machines in France.

Schedule

The Koeberg power station is the backbone of electricity supply to the Western Cape. Shutdowns, whether planned or unplanned, on any of the two units at the power station have in the past increased the strain on the national power grid in the region. The retrofits will therefore, as already noted, be carried out during planned refuelling outages, thus minimising the interruption to generation from the station.

Alstom is installing the retrofit in collaboration with Rotek, the maintenance arm of Eskom. Alstom will be providing the technical and management support for the installation.

Execution of the retrofit project presents many challenges. Apart from the short installation timescales, there is a shortage of local skilled labour. The demands from other maintenance work taking place at the same time is also complicating the planning process. This necessitates a close working arrangement with Eskom. All parties involved have had to make a significant effort to integrate and optimise shared resources such as craneage and lifting gear. Temporary cranes are being bought in to optimise lifting arrangements. Special tools such as diaphragm racks are being procured and manufactured to reduce the number of crane movements on both the old removed diaphragms and the new replacement diaphragms.

The old diaphragms from the three LP stages will be taken out and put into carriers, to enable a single lift for removed diaphragms. At the same time, the new diaphragms will be ready and available on the turbine block also in a special set of racks – clean, set, and ready to fit before the unit is taken off line for the installation of the retrofit. The retrofit installation is due to start in August 2010.

As part of the contract, Alstom is also changing the generator rotor, making modifications to the valve control system, the turbine protection system and the moisture separator reheater temperature control system.

Care also has to be taken in scheduling the transport of what are very large pieces of equipment. For example, the diaphragms will measure 5.5 m in diameter and therefore cannot be transported in standard containers to Cape Town. The transport of the LP rotors to Cape Town will also be a challenge. These are manufactured in Belfort, France, and weigh 150 tonnes. They will travel by road to Strasbourg and then by barge to Rotterdam, before finally being transhipped to a cargo vessel for onward delivery to Cape Town.

Project co-ordination

In the current environment where factories are heavily loaded, the project is being executed under a very tight deadline utilising Alstom’s resources worldwide. The LP rotors and blades are being made in Alstom’s Belfort facility in France. The diaphragms are being made in Alstom’s Morelia factory in Mexico. Engineering is being led from Elblag, Poland with support from Levallois, France. The entire project is being managed from Rugby in the UK.

The manufacturing arrangement will be slightly different for unit 2. The LP rotors will be built in Belfort, the moving blading will be manufactured in Birr, Switzerland, and Elblag, and the stationary blading and diaphragms will be manufactured in Morelia.

Close working relationship

With the pressure on reduction of installation times to maximise generation, Alstom is working closely with Eskom and Rotek to optimise the installation programme.

This co-operation is reflected in the production and supply of special tools such as the diaphragm racks, and temporary and new permanent lifting arrangements and craneage, so that the retrofit can be executed in the required schedule. Shift and work patterns are also being optimised so that progress is always being made on the critical path, whilst maintaining the statutory requirements for working time.

Alstom is also managing interfaces with other teams performing work during the retrofit installation and has the co-ordination contract for these maintenance activities. Under this contract, Alstom is also responsible for planning items such as the overall programme, interfaces between all parties, safety, scaffolding and lagging.

While the retrofit at Koeberg will be a challenge, the close working relationship developed between Alstom and Eskom will ensure that the retrofit is carried out on time and line with Eskom’s requirements.