Boxberg achieves world record for efficiency19 October 2001
The steam turbine at VEAG's Boxberg plant, now with over a year of commercial operation to its credit, is setting new standards. Uwe Hoffstadt, Siemens Power Generation, Muelheim an der Ruhr, Germany
The 907 MWe Boxberg plant has successfully completed its first year of operation and is proving to be a showcase for supercritical lignite-burning power plant technology in general and steam turbine innovations in particular. The plant went on line in June 2000 and acceptance tests were finished in October 2000. With a net efficiency of 42.7 per cent for this new power plant, the east German utility VEAG (Vereinigte Energiewerke AG) can claim to have the most modern lignite-fired power plant fleet in the world.
With gross efficiency of 48.5 per cent, the steam turbine-generator in the Boxberg power plant unit can be regarded as the first of a new generation of turbines. Steam conditions are 266bar/545°C and 58bar/581°C.
The internal turbine efficiency levels demonstrated in Boxberg of 94.2 per cent for the high-pressure turbine and 96.1 per cent for the intermediate-pressure turbine represent a new benchmark for steam turbine design.
In 1991, VEAG commissioned a consortium consisting of RWE Energie AG and VEBA Kraftwerke Ruhr AG to draw up a conceptual study for the construction of modern twin-unit power plants at the Boxberg and Schwarze Pumpe sites.
VEAG had inherited a tremendous burden from its communist parents. The power plants and power distribution facilities in the former East Germany had been poorly maintained, and the pollution control equipment that was already standard in Western Europe was virtually unknown in East German plants. The terms of reference for the study took into account not only operational and economic considerations but also the objective of using domestic lignite from local open-cast mines for production of power to save jobs, and of raising the power plant efficiency to over 40 per cent in order to reduce carbon dioxide emissions.
In 1994, when the engineering for the plant was already quite far advanced, VEAG changed its choice of supplier for the turbine, and Siemens Power Generation was awarded the contract for a five-cylinder turbine, the generator, a single-shell condenser, and various ancillary systems. The original plan was to build two identical units with a rating of 907 MW each, but the option for the second unit was never exercised, due to changes that took place in the German power market, resulting in a fall in projected revenues.
The Boxberg plant is equipped with a five-cylinder turbine of the HMN series consisting of a high-pressure, an intermediate-pressure and three low-pressure cylinders. The whole turbine-generator is mounted on a specially tuned, spring-supported reinforced-concrete foundation. The entire set-up is 55 meters long.
The main steam coming from the boiler first flows through two combined stop and control valves into the high-pressure turbine, where it expands to the pressure prevailing in the reheater. From there the steam again flows through two combined stop and control valves into the intermediate-pressure turbine and is passed to the low-pressure turbine cylinders via a crossover line. Finally, the remaining heat from the spent steam is passed to the circulating cooling water via the condenser. The bearings are rigidly mounted on the foundation, separate from the turbine casings. The outer casings of the high-pressure and intermediate-pressure turbines and the inner casings of the low-pressure turbine cylinders rest on the bearing pedestals, on which they are free to slide in the direction of the turbine axis. The common anchor point and the origin for the axial expansion is the bearing between the high-pressure and the intermediate-pressure turbines, which is designed as a combined journal and thrust bearing.
The high-pressure turbine
The high-pressure turbine is of single-flow, two-shell design with guide blade carriers and outer casings in a modified barrel-type design without a bolted horizontal joint. The rotational symmetry of this design avoids major asymmetric deformation due to thermal stressing and thus makes comparatively short start-up times possible. By contrast with earlier designs, however, the outer casing consists of two parts, the steam admission and exhaust casings being bolted together at a radial joint. With this arrangement, the rear part of the guide blade carrier projects a long way into the colder exhaust area, which minimises thermal deformation of the guide blade carriers in steady-state operation and thus leads to smaller radial clearances. Splitting the casing into smaller components also considerably reduces procurement times. The rotor is a monobloc forging with forged-on coupling flanges and is connected to the intermediate-pressure turbine shaft via hydraulically tensioned bolts.
Despite its overall weight of 120 tons and thanks to favourable local transportation conditions, it was possible to deliver the high-pressure turbine to the site fully assembled. The compact design thus considerably shortened the time taken to install the turbine.
The intermediate-pressure turbine
The modified intermediate-pressure turbine is of dual-flow, two-shell design. The upper and lower halves are bolted together along an axial joint. To compensate for the influence of thermal deformation of the inner casing due to the axial split, the upper and lower halves are each reinforced by an extra rib. The exhaust steam leaving the inner casing exits the turbine through an upward port on the outer casings. To counteract the deformation of the inner casing due to the uneven temperature distribution as a result of the unidirectional flow between the upper and the lower half, the inner casing is provided with a heat shield. Besides the positive effect of smaller radial clearances, this design also significantly reduces heat losses and leads to an optimised flow contour between the inner and outer casings.
As in the case of the high-pressure turbine, the steam entering from the reheater first passes through the two stop and control valves, which are bolted to the outer casing below the joint. When the turbine is opened for inspection, the lower halves of the inner and outer casings remain on the foundation, significantly reducing working times.
Because of its overall weight of 260 tons and the local transportation conditions, the components of the intermediate-pressure turbine had to be delivered to the site separately. Pre-assembly including dimensional checks performed at the Siemens Power Generation manufacturing plant ensured that final assembly on site went off without a hitch.
The low-pressure turbine
The three low-pressure turbine cylinders are each designed as a two-flow cylinder with an axially split multiple-shell casing. The outer casing consists of two side walls, two end walls, a reinforcement system, and the upper part. Its entire weight rests on the condenser dome, to which it is permanently welded. The cast inner casing is likewise of two-shell design, with the inner shell centred in the outer casing so as to be free to slide axially in response to thermal expansion. Because of the high droplet content in the steam and the associated heat transfer properties at the tail end of the expansion cycle, the outer shell of the inner casing is provided with a droplet shield. Besides providing direct mechanical protection, the shield allows superheated steam to form between itself and the outer shell of the inner casing. This steam acts as an insulator, reducing heat losses.
Because of its overall weight of 400 tons and its large as-assembled dimensions, the low-pressure turbine was delivered to site in separate parts. Pre-assembly, including dimensional checks performed at the manufacturing plant, ensured that final assembly on site went off without a hitch.
Achieving world records
Several warranty points were agreed with VEAG with allowance for various weighting factors. The turbine was designed for 97.5 per cent of the boiler rating with the aim of achieving a rated output of 887 MW and a gross efficiency of 48.65 per cent. This is equivalent to a gross efficiency of 48.5 per cent, obtained as a weighted mean over all load conditions. Such an efficiency level represents a milestone in modern turbine technology.
These figures are world records. How were they achieved?
A completely new shape of blading was used for the first time in the high-pressure and intermediate-pressure drum blading of the turbine. Apart from featuring consistently refined blade geometry with improved stage efficiency and less susceptibility to deposits on the suction side, the 3 DS blading is primarily the result of three-dimensional, advanced CFD methods. The aerodynamic burden on the profile sections in the blade tip and root areas is eased by inclining the blades in the direction of rotation, thus reducing the secondary losses. As these are significantly high, especially in the turbine admission area, this measure made it possible to improve efficiency by around 2 percentage points. To reduce windage losses, the rotor blades are fully shrouded. Only the last rotor blade row in the low-pressure turbine is free-standing, because of the centrifugal forces acting in that area. Modern CFD computation methods combined with extensive model trials made it possible to optimise blading vibration so effectively as to completely eliminate the need for vibration damping elements that would interfere with the steam flow.
CFD computation methods were also used to upgrade the non-bladed areas of the turbine. For instance, optimisation of the high-pressure admission section led to the development of a helical configuration that yielded a more uniform flow distribution over the entire stationary blade ring area. A new diffuser geometry was developed for the high-pressure and intermediate-pressure exhaust areas specifically to counteract backflow and vortex formation caused by flow separation in the exhaust steam region. This made it possible to further reduce pressure drops due to internal circulation flows. The exhaust steam area of the low-pressure turbine was also subjected to a thorough CFD analysis. Flow in this region is highly prone to vortex formation due to the presence of various condenser dome internals. The findings of this analysis led to baffles being introduced above and below the turbine centreline to minimise the flow losses caused by the pronounced vortices. And finally, the entire outer casing was widened at the level of the joint, in order to reduce the outlet energy and the associated pressure drops.
The Boxberg turbine-generator that can be regarded as a milestone in the history of turbine technology. However, on the basis of experience gained from the project, we have initiated further studies and innovations. Siemens' new 3DV blading design, with its variable stage reaction, the development of 12.5 m2 low-pressure blading stage (to be applied at RWE's Niederaußem plant, due to enter service in 2002) and a redesigned high-pressure turbine mark further steps forward in terms of both the technology and economics of power plant operation.