Steam turbine development in China

DongFang has successfully developed a new generation of steam turbine, the parameters reach 35MPa/ 615℃/ 630℃/630℃, heat consumption is lower than 6,800 kJ/ kWh and the generating efficiency of power plant exceeds 50%. The goal is to reach 650℃ and 700℃.'

The article contains excerpts from the paper, "The technology development of high efficiency steam turbine" by Dong Fang engineers presented at the 2017 ASME Turbo Expo conference.

The 13^th Five Year Plan has targeted the improvement of the efficiency of the existing coal power plants. As per the plan, the coal consumption of active coal-fired power generating units which would be upgraded must be lower than an average of 310 g/ kWh; the coal consumption of 600MW or more in active service (except air cooling units) which would be transformed is demanded to be lower than an average of 300 g/ kwh after 2020. For the newly built power plant, the power supply coal consumption of 1000MW turbine is not higher than 282(wet cooling), 299 (air cooling) g/ kwh, and for unit of 600MW the parameter is not higher than 285(wet cooling), 302 (air cooling) g/ kwh.

In the next ten years, it is expected that the following units will become the new direction of the development of steam turbine technology --  620℃ level of high-power double-reheat unit; 620℃ level low back pressure power unit; 630℃ level steam turbine technology; 650℃ level steam turbine technology; 700℃ level steam turbine technology.

By strengthening reheat, the efficiency of the coal-fired units can be improved. The efficiency of the double-reheat unit can increase about 2% than single-reheat units.

After the technology of 600℃ supercritical power generation is mature, many countries have started the advanced ultra-supercritical power generation technology research project of 700 ℃. Ni-based materials are envisaged for 650℃, although the 700℃.

The development of steam turbine technology has been closely related to the material. The steam parameters of the unit largely depend on material development. DongFang has the capacity of self casting cylinders and valves with the material ZG12Cr9Mo1Co1NiVNbNB. Up to now DongFang has produced the valves and cylinders more than 200 pieces with total weight up to 1140 ton. The heavy forging use material 9Cr-3Co-3W-B which is developed by JSW (Japan Steel Works) for 630℃.

In the case of certain parameters and boundary conditions, a better thermal system can bring the unit higher efficiency. This unit adopts the T-turbine scheme which is proposed in the 700℃ power plant. The scheme can greatly reduce the temperature of all the heat recovery steam extraction and the manufacturing cost and pipe cost , which improves the safety and reliability of the unit as well.

The system brings the following advantages：  The volume flow of reheating steam was reduced by 35%.The cost of the first/second heating pipeline was obviously declined, and the safety and reliability increases;  The high pressure module of steam turbine has no steam extraction, which increase the efficiency and decrease obviously the stress of rotor.  The system can reduce the initial investment without setting the steam cooler; the inlet temperature of the heater can be reduced significantly; after the reduction of the flow rate, the geometric size of the high pressure module of the steam turbine is decreased. Although the system has many advantages, but also make the system more complex. Control and adjustment of variable working conditions for the system needs further research and actual running test.

Because of the high pressure, DongFang has developed a new type cylinder which was named double-barrel-cylinder. The upper and lower half of the inner casings are set together by seven rings

With ring hoop the cylinder, the inner casing has better air tightness. This structure has been successfully put into operation in the many projects of DongFang, such as Anyuan (31MPa), Wanzhou (28MPa). Because the main steam pressure is increased to 35MPa, the geometric size of the cylinder is smaller and more secure. Inner cylinder shares most of the pressure, about 22MPa, the outer cylinder only to withstand the exhaust pressure of 13MPa. The studs of outer cylinder works in lower temperature and lower stress level, which improves the safety after long-term operation.

DongFang has been using radial-flow stator design to reduce the rotor temperature. With this design, the actual working temperature of the rotor is lower than 620℃.

The loss of the flow path in the steam turbine mainly derives from the loss of type, end loss, steam leakage and other aspects, especially the VHP (HP) module. The secondary loss and leakage of steam loss accounted for the main part. DongFang has developed a new highly after-loaded stator profile. For stages with large blade aspect ratio, DongFang has developed and applied highly frontal loaded stator profile. The secondary loss does not dominate anymore, and the flow pattern is more like a 2D flow. So it is reasonable to use a profile which has lower profile loss, while has larger blade loading to decreases blade number.

In order to further improve the efficiency of the unit, the HP VHP module uses the single path design. The single path has a longer blade than the double path, and the leakage loss is smaller. The total internal efficiency of steam turbine is expected to exceed 92%.

The LP module uses last stage blades of height 1200mm. The blade has been applied in many 1000MW units, such as Zhou Shan, Wan zhou, Liu heng project.

Dong Fang has designed a new steel last blade whose height is 1400mm. The LP model design has been completed. The exhaust area of the 1400mm blade is 14.5m2 , it’s suitable for low back pressure of 1000MW units or a higher power unit. The 1400mm steel blades will be used in some 1000MW units with low back pressure, such as Yun cheng project.

The 700 ℃ unit needs a lot more research and development. The 630 ℃ unit will become the main trend in the next five years. In particular, after the parameters are improved, the economic performance of the unit can be further improved by combining with double reheating technology. The DongFang 1000MW ultrasupercitical steam turbine has parameters of 35MPa/615℃/630℃/630℃. With double-reheat, the heat rate of the steam turbine is lower than 6800kJ/ kWh, The generating efficiency is expected to exceed 50%. This unit being more efficient than the 620 degree unit, when proved safe and reliable, will become a good option in the future.

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