Basics of Steam Turbine Design

Steam turbines account for more than half of the world’s electricity production in power plants around the world and will continue to be the dominant force in electricity power generation for the foreseeable future. The enhancement of steam turbine efficiency is increasingly important as the urgency to reduce CO2 emissions into the atmosphere is a problem at the forefront of power production. Increasing efficiency in steam turbines, and other components of power plants, will help meet the growing demands for electricity worldwide while reducing harmful greenhouse emissions.

Figure 1 Steam Turbine with Long Last-Stage Blades
Figure 1. Steam Turbine with Long Last-Stage Blades. Source

Steam turbines are used in coal-fired, nuclear, geothermal, natural gas-fired, and solar thermal power plants. Also steam turbines are increasingly needed to stabilize fluctuating power demands from solar and wind power stations as renewable energy sources grow worldwide. The current emphasis on steam turbine development is for increasing efficiency, mainly by increasing steam turbine capacity, as well as increasing operational availability, which translates to rapid start up and shut down procedures. 

What are Steam Turbines?

Steam turbines convert thermal (heat) or kinetic (movement) energy to mechanical energy by using rotating blades. More specifically, these rotating stator blades accelerate high pressure and temperature steam which then provide impulse and reaction forces to the rotating blades. The torque generated by this steam force on the rotating blades is then transferred to the rotor.  Steam turbines can come in all shapes and sizes, ranging from a single stage design to a multi-stage machine. A stage consists of a set of stator blades and a set of rotating blades and every stage and blade row is designed to provide the most efficient flow path and aerodynamic behavior possible. The efficiency of steam turbines increases with increasing inlet temperature and inlet pressure; however, limits are imposed due to the thermal and structural limits of blade shapes and materials.

The Rankine cycle is the basic process behind steam power generation, where expansion of steam in the turbine provides mechanical work to generate electricity. There are many methods to improve the thermal efficiency of a steam cycle, but a balance must be reached between efficiency improvement and economical costs with each design. Increasing steam inlet pressure can lead to improved thermal efficiency, but care must be taken to avoid excessive wetness fraction in the low-pressure turbine that results in wetness loss increases. Therefore, the temperature of the steam is increased alongside the pressure to reduce this loss, but again the increased thermal stress on the blades must be taken into account when increasing temperature. This improved thermal behavior of blades leads to engineering challenges and increased costs, which must also be taken into account during design.

Figure 2 Reheated Rankine cycle Temperature-Entropy chart
Figure 2. Reheated Rankine Cycle Temperature-Entropy Chart. Source

Other methods to improve thermal efficiency are vast and ever growing. A reheat cycle takes exhaust steam from the high-pressure turbine and returns it to the boiler to then expand in the intermediate-pressure turbine. A regenerative cycle extracts steam to the feedwater heater from intermediate stages of the turbine, which is used to heat feedwater to the boiler, resulting in reduced heat loss to the cooling tower from the condenser. Reheat and regenerative cycles can also be combined at the cost of additional system complexity and equipment costs.

Thermodynamic cycle simulation software, such as AxCYCLE, provides the opportunity to quickly and easily design, analyze and optimize the entire Rankine or combined cycle in which the steam turbine will function. This allows modeling at design and off-design conditions and even performing techno-economical studies and optimizations to determine inlet and outlet conditions of all components in the cycle, including the steam turbine. The design criteria can then be seamlessly transferred to the turbomachinery design, analysis and optimization suite AxSTREAM® in order to begin design on the actual steam turbine. The design of a turbine is a very complex and challenging task, but with the help of turbomachinery design software, the complexity can be drastically reduced.

Figure 3 Condensing steam turbine with four regenerative extractions designed in AxCYCLE
Figure 3. Condensing Steam Turbine with Four Regenerative Extractions Designed in AxCYCLE


How do you increase efficiency in steam turbines?

Increasing short blade heights in high-pressure and intermediate-pressure turbines leads to increased capacity and therefore improved efficiency. The largest and second largest losses in typical steam turbines are the low-pressure blade loss and low-pressure exhaust loss respectively. These losses are directly related to the last-stage blades. Developing longer and highly efficient last-stage blades is one of the most important challenges faced by engineers in steam turbine design. A good design can lead to large improvements in efficiency and performance. AxSTREAM allows the designer to perform design and optimization of all blades including long last-stage blades as well as full 3D CFD simulations using AxCFD and structural, modal and harmonic analysis using AxSTRESS. This results in blade designs with optimal length, stacking, material and twist/lean to provide the efficient design possible.

Care needs to be taken when accounting for factors that affect steam turbine operation. These include: deviations in turbine back pressure; excessive moisture that can lead to blade erosion; low load operation leading to last stage windage and exhaust heating; thermal transients resulting in thermal stress and differential expansion; turbine governing to avoid unwanted and dangerous acceleration; as well as axial thrust balancing where off-design conditions can lead to thrust bearing failure and rubbing of inter stage seals. The importance of accounting for all these effects and more cannot be stressed enough during the development of a steam turbine, as overlooking potential disastrous effects will lead to damage and eventual shutdown of the steam turbine and significant economic costs or worse.

Steam Turbine Design
Figure 4. 3D View of a 10MW Steam Turbine Designed in AxSTREAM

The design of a steam turbine is an inherently complex task that requires in depth knowledge from many fields of engineering. The development of such a machine is a big investment that is essential for today’s economy and modern life. Steam turbines will continue to provide the world with the majority of electricity for the foreseeable future. The technological innovations currently being developed will continue to improve the efficiency and longevity of steam turbines.

Examples of Steam Turbine Blade Designs
Figure 5. Examples of Steam Turbine Blade Designs. Source 

The design of steam turbines is more accessible than ever with the software platform AxSTREAM that has an all-encompassing approach to the process of turbomachinery design. These tools will ensure that the technology of steam turbines will continue to improve, and new ideas and innovations will continue to be developed.

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