Dirk Schlichting, GEA Thermal Engineering Division, Germany
South Africa experienced an electricity supply overcapacity in the early 1990s and as a result several power plants were mothballed. Rapid growth since then, however, has placed heavy strain on the country’s power infrastructure, which is increasingly struggling to cope with demand.
Eskom, the national electricity utility, has embarked on an aggressive revised capital expansion programme to cater for this growth in demand. The current installed nominal capacity is in the region of 42 GW, consisting of coal, nuclear, hydropower, gas turbine and pump storage power stations.With this in mind, the main aims of Eskom’s immediate five-year plan to help overcome the current situation are:
- A return to service of mothballed power stations – this will re-activate 3800 MW of installed capacity.
- 2100 MW of new open cycle gas turbine power stations.
- 1200 MW of new pumped storage systems.
- 100 MW of renewable power plants.
- 9600 MW of new coal fired power plants.
One of the new coal fired power plants currently under construction is the 6 x 790 MW Medupi power station located near Lephalale in the country’s Limpopo Province. And the cooling technology employed in this development merits close inspection.
Cooling technology of choice
Water is a scarce commodity in this region, so an air-cooled condenser was the preferred choice for the Medupi power station’s cooling technology.
The large air-cooled condenser of the 3600 MW Matimba plant (left)
GEA Aircooled Systems, based in Germiston, near Johannesburg, received the turnkey order for the complete design, manufacture, supply and erection of the air-cooled condenser. The company, which is part of the German based GEA Group Aktiengesellschaf, was chosen as a supplier for this new project because it had previously supplied the air-cooled condensers for the 3600 MW Matimba and 2000 MW Majuba power plants; plus its air-cooled condensers have a proven track record for reliability – essential for Medupi, which is a critical project for the future power supply of the country. The order was awarded by Alstom S&E Pty. Limited, which is part of leading power OEM, Alstom Power Systems.
Big is beautiful
The new air-cooled condenser will be the largest of its kind installed worldwide once the Medupi project goes online. The air-cooled condenser at the Matimba power station currently holds this record. The sheer dimensions of the Medupi air-cooled condenser are impressive with a footprint of 108 m x 669 m it is equivalent to ten football fields.
and the plans for the even bigger one at the Medupi plant
Before the actual work on the hardware part of the air-cooled condenser could begin, a lot of engineering work was performed. System performance, availability and reliability are key to the success of any project and ultimately to the performance of the complete power plant.
Wind speed and wind directions
In addition to the outside temperature, wind speeds and wind directions will have a huge impact on the performance of the air-cooled condenser, which in turn influences the turbine back pressure at the power plant.
GEA Aircooled Systems has performed computational fluid dynamics (CFD) studies, with the results confirmed by an independent expert, in order to predict the airflow distribution around and inside the air-cooled condenser, as well as the impact of the distribution on the performance of the air-cooled condenser. In addition to the wind speed and direction, the position and dimensions of the boiler house were also taken into consideration when conducting the analysis.
The findings of these studies have resulted in the decision to install a wind cross on the ground level below the air-cooled condenser. This will reduce re-circulation of heated air back into the air-cooled condenser.
It is essential that for a power plant with the large capacity of the Medupi plant the availability rate must be as high as possible.
GEA Aircooled Systems has used the FMECA (Failure Mode Effects and Criticality Analysis) method to calculate the reliability and availability of the system. One deciding factor in the calculation of these parameters is to analyze whether parts are operating in series or in parallel.
Parts running in series imply that a failure of one part will automatically stop the other component from operating. In contrast, when a failure occurs in one component in parts that are running in parallel, the other part will take over the operations of that failed component. Thus, parts running in series will have a lower total availability than the individual components, while parts running parallel will have a higher total availability than the individual parts.
The FMECA calculation was used to calculate the total number of systems that must be available in order to safeguard the availability rating demanded by the customer.
In addition to this, greater system availability will also be supported by an appropriate spare parts stock on site, a service concept with reduced repair times, as well as an increased number of redundant components.
Finally after the system installation, plant performance tests and plant reliability tests will be run. These tests will then be followed by classroom and on-site training for the operators and maintenance staff of the plant.
As part of the turnkey contract the main components of the air-cooled condenser have been sourced from within South Africa, and include the air-cooled steam condenser modules, which are based on GEA’s A tube design. The A tube design has already been installed in the existing Matimba and Majuba projects, as well as in a number of other power plants around the world. In addition to the galvanized air-cooled condenser tubes, the condenser modules also consist of tube sheets and the steam and condensator collection headers.
The other main components include the complete air moving system made up of fans, gearboxes, couplings, electric motors, fan support bridges, as well as all relevant auxiliary components such as condensate tanks, drain pumps, steam ejectors, rupture discs, and the bundle cleaning system.
Utilizing air-cooling technology for power plants located in areas where water is scarce makes sense
In addition to the main components, GEA Aircooled Systems will also supply the entire steel structure including fan rings; the supporting steel structure and fan deck; the necessary wind walls on the fan deck; the wind cross previously mentioned; the steam duct; the condensate, the air and steam piping; and the electrical controls.
And, in addition to the engineering of the total system and its individual parts, GEA Aircooled Systems has also provided the loads for the concrete pillars that will carry the fan deck.
Producing the air-cooled condenser bundles under the current power situation in South Africa presents a special challenge as power outages cannot be allowed to stop the tight production schedule in Germiston. All the air-cooled condenser bundles will be manufactured there between 2008 and 2011; manufacturing began in March of this year. The manufacturing line was also completely overhauled prior to the production beginning.
In order to be independent from the public power supply, generator sets have been installed, which will assure that production continues despite any unforeseen power outages.
Furthermore, to streamline the production processes and the total order execution procedures, GEA Aircooled Systems has been integrated into GEA’s global enterprise resource planning (ERP) system. This system allows standardized processes to be implemented within all GEA’s main sites around the world, as well as a quick data exchanges with the company headquarters in Germany.
Meeting the challenge
Continuing this attention to detail, GEA Aircooled Systems also conducted a study to determine the correct positioning of the cranes, so that the installation of the air-cooled condenser bundles happens as easily and quickly as possible. This involved taking the soil bearing capacity on site into consideration. Furthermore, the transport logistics as well as the temporary storage of the components on-site have also been planned.
Because the project will be erected in a remote area, even accommodation and office space is being built for the site supervisors. The company has already acquired land and will start erecting what will be one of its most remote offices in the world. Although, the office is physically far away from the world’s technical and engineering hotspots its full integration into GEA’s global IT network enables communication with the global engineering force of the company.
For the Medupi project, GEA Aircooled Systems is using all the experience it has gained in managing large-sized power projects in South Africa and other places around the world in order to safeguard the proper order execution and ultimately the performance of the air-cooled condenser system in the field.