ThyssenKrupp Steel Europe AG is supplying 17,000 metric tons of high-strength, quenched and tempered plate to the Ingula pumped storage scheme project of the South African power utility Eskom. The steel will be used to manufacture piping up to 5.1 meters in diameter for the scheme. The Ingula power station is currently under construction and is scheduled to come on line at the end of 2012. With the 16.6 billion euro investment, Eskom aims to respond more flexibly to peak loads on the electricity supply grid.
In pumped storage schemes like Ingula, huge water reservoirs are used to store energy which can be converted into electricity quickly when required and fed into the supply grid. Located in the Drakensberg mountains on the border of the Free State and KwaZulu Natal, the power station features two reservoirs each with a capacity of 22 million cubic meters. They are situated around six kilometers apart, with a height difference of 470 meters. Between them is a powerhouse with four 333 megawatt turbines. The turbines can be used both as generators to produce electricity and as pumps. The underground powerhouse is connected to the two reservoirs by underground waterways.
During times of peak electricity demand, water is released from the upper to the lower reservoir, driving turbines to generate electricity which can be fed into the grid. During off-peak periods, the water is pumped back up to the upper reservoir. Although electricity is needed to operate the pumps, the scheme is economical for the operator: The off-peak electricity it consumes is cheaper than the on-peak electricity it produces. In addition, the scheme increases the flexibility of power generation and the existing power plants can be operated at more constant levels and therefore at lower cost.
The 17,000 tons of quenched and tempered plate for the project are the biggest single order since this product line was introduced in the ThyssenKrupp Group over 40 years ago. Plate is the term used by steel producers for material of three millimeter thickness and above. The steel plates supplied for Ingula are 32 to 60 millimeters thick. The material, known as NAXTRA M 700, has a strength of 700 megapascals. It is also extremely tough, which means it can withstand temperatures down to minus 40 degrees without becoming brittle. The steel owes its properties among other things to a heat treatment process in which the material is first heated to over 900 degrees, then cooled with water and tempered.
The Ingula pumped storage scheme's conduit system consists of tunnels and shafts, some of them lined with concrete and others, in zones exposed to particularly high water pressure, with NAXTRA M 700. The strongest pressure builds up immediately in front of the powerhouse. Loads equivalent to the pressure of a 490 meter head of water can occur temporarily here. A high-strength steel lining is also needed to stabilize the shafts on a permanent basis. NAXTRA M 700 will be used to manufacture altogether 4.6 kilometers of penstocks.
ThyssenKrupp Steel Europe won the order not just because of the first-class properties of the material but also because of the quality of its technical advice service. Advice is required in particular for the welding of high-strength steel plate: Because the heat-treated material is heated again during welding, close attention has to be paid to welding temperatures and cooling times to ensure the material maintains its good properties in the region of the weld.