Tachibana-wan unit 2 takes a supercritical step forward for Japan

In recent years the Japanese have been steadily improving the efficiency of their large steam turbines by progressively increasing steam pressures and temperatures, and can now claim world leadership. An important milestone was reached in mid-December 2000, with the entry into commercial operation of Tachibana-wan unit 2, which MHI claims has the world's most efficient steam turbine, achieving about 49 per cent (gross at generator terminals and rated conditions), with steam conditions of 25MPa/600°C/610°C. In the past few months, this 1050 MWe plant, located at Anan City, Tokushima Prefecture, Shikoku, has operated extremely well, MHI reports. The plant is owned and operated by the Electric Power Development Co Ltd (EPDC) and runs exclusively on imported coal.

A key feature of the new turbine is its use of high strength steel with excellent heat resistance, which can tolerate the onerous conditions. Another feature of the turbine is compactness and optimised configuration.

The high efficiency stems from the advanced steam conditions employed, combined with high performance blades designed using full 3-D flow analysis. MHI estimates that the turbine heat rate is about 4 per cent better compared with a 1000 MW turbine using conventional steam conditions of 24.2MPa/538°C/566°C.

Since 1993, five MHI high temperature (600°C class) steam turbines have entered commercial operation in Japan and Tachibana-wan 2 represents the culmination of this line of development (see diagram, right).

The realisation of steam temperatures in the 600°C class builds on a wide range of development activities, notably verification testing on an ultra-high temperature turbine at the EPDC's Wakamatsu High Temperature Project. Wakamatsu has played a key role in the verification of design and material technologies for high temperature steam turbines.

Chubu Electric's 700 MWe Hekinan 3 was the first commercial application of MHI's high temperature turbine technology, employing technologies verified at the Wakamatsu High Temperature Project. Hekinan 3, with steam conditions of 24.1MPa/538°C/593°C has been running steadily since commercial operation began in April 1993. A 500 MWe machine with steam conditions of 24.1MPa/566°C/593°C went into operation at Hokuriku Electric's Nanao Ohta plant in 1994. Subsequently, 1000 MW class high temperature units successfully entered commercial operation at EPDC's Matsuura 2 (24.1MPa/593°C/593°C), Chugoku Electric's Misumi unit 1 (24.5MPa/600°C/600°C) and, most recently, EPDC's Tachibana-wan unit 2 (25MPa/600°C/610°C).

In these 1000 MW units, as already noted, cycle efficiency has been improved by elevating the main and reheat steam temperatures, and in addition, internal efficiency has been increased by applying the latest flow dynamic analysis methods to reaction blades and the 46in ISB (Integral Shroud Blade) low-pressure end blades.

The turbine in these 1000 MWe units is a cross compound type using both a high pressure and intermediate pressure turbine on the primary shaft and two low-pressure turbines on the secondary shaft. The design and material technology for intermediate pressure turbines of the 600°C class were established in the design of Chubu's Hekinan unit 3.

To fully realise the 600°C main steam temperature class, high temperature technologies are applied in high pressure turbines, including high temperature materials such as new 12 Cr steel rotors.

Furthermore, to enhance the steam turbine internal efficiency, in addition to the improvement in cycle efficiency achieved by increasing main steam and reheat steam temperatures, advanced-technology 1800 rpm 46in ISB low-pressure end blades and ISB reaction blades are utilised. The 46in ISB is the largest low-pressure end blade in use in the 1000 MW class of thermal power plants in Japan. ISB reaction blades are adopted for higher reliability under high temperatures compared with conventional shroud and tenon riveted-type blades. As already noted, the ISB blades are designed using the latest 3-D flow analysis techniques.

HP turbine design features

The high temperature turbines at Matsuura unit 2, Misumi unit 1 and Tachibana-wan unit 2 incorporate a number of advanced concepts in terms of both materials and and design.

The materials used in the 600°C class high pressure turbines are shown in the table below, and compared with those used in a conventional machine (main steam temp 538°C).

In the MHI high pressure turbines, ferritic heat-resistant steel materials are widely used, such as new 12 Cr forged, 12 Cr cast, and 9 Cr forged steels. In high temperature rotating blades, austenitic refractory alloys are used, while 12 Cr forged steel having sufficient creep rupture strength for operation at 600°C class temperatures is used as the rotor material. In the journal and thrust collar sections of the 12 Cr rotor, overlay welds are built up with a low Cr weld material in order to reduce wear by bearing metals. As for the materials for stationary parts, 12 Cr cast steel (MJC-12) having excellent creep rupture strength is used in the nozzle chamber, inner casing, and No.1 blade ring. 9 Cr forged steel is used in the main stop valve, high pressure turbine inlet, governing valve, and connection piping between the valves and casing.

In the design of the high pressure turbines, component parts exposed to high temperature are made as compact as possible. Cooling features are also employed to ensure sufficient reliability against creep and thermal fatigue. Some key features of the high pressure turbine design are shown in the middle diagram on p45, indicating materials which have been newly developed for high temperature application and used in the high pressure turbine.

A double flow type welded nozzle chamber is used, which has made possible a more compact inner casing design. In the rotor axial centre, a cooling hole is provided in the control stage disc, and the rotor is effectively cooled by supplying control stage outlet steam to the space between the nozzle chamber and the rotor. In the rotating blades, reliability at high temperatures is enhanced by use of ISB-type blades.

Some of the main development and verification activities carried out for the 600°C class HP turbine are summarised in the panel on p47.

The MHI lineup

MHI has been conducting a development programme for new advanced low-pressure end blades which adopt the ISB structure. A new standard series of these low pressure end blades has already been completed for 50 Hz and 60 Hz unit applications, including the 3600 rpm 45 in titanium blade and the 3000 rpm 48 in steel blade.

By applying the 3600 rpm 45 in titanium blade or 3000 rpm 48 in steel blade, MHI says it is able to design a 1000 MW class tandem compound type steam turbine (see diagram below), as an alternative to the cross compound configuration used so far in the 1000 MW class units.

Mitsubishi Heavy Industry's current line-up of large capacity steam turbines is shown in the diagram above. The two cylinder, tandem compound turbine design can reach 750 MWe. However, by applying not only the long low pressure end blades, but also advanced technologies for a combined HP-IP turbine, eg ISB reaction blading, triple-pin control stage, and IP cooling, MHI is planning to develop three cylinder designs up to 1000 MW.
Tables

HP turbine materials