Bigger Blade25 April 2018
After a decade of development, Mitsubishi Hitachi Power Systems has completed testing of its new 74-inch blade for nuclear steam turbines, Kei Sagimori explains.
In September 2017, mItSubIShI hItachI Power Systems, Ltd (MHPS) successfully performed rotational vibration testing of a 74 inch (1880mm) last-stage rotating blade (74in ISB). The 74in ISB is the world’s longest blade class for nuclear steam turbines.
MHPS began development of the 74in ISB blade type in 2005 and has already completed various verification tests. The rotational vibration test was the final step in the blade’s development process.
The specification of the 74in ISB that completed rotational vibration testing is shown in Table 1.
The testing process demonstrated that:
- the 74in ISB achieved 27m2 exhaust area per one flow,
- the integral shroud blade (ISB) structure achieved excellent vibration resistance,
- the large root and groove achieved reduced stress, ? the full 3D design achieved high efficiency.
A key characteristic of the 74in ISB is its use of the integral shroud blade (ISB) structure. This utilises a “twist” structure that accommodates the large differences in peripheral velocity between the blade tip and the blade root.
During operation, as the centrifugal force increases with the turbine rotation speed, each twist blade reshapes to untwist the blade and locks into contact with the adjoining blade. Together, they then form an integral row of blades. In this configuration, frictional force on the contact faces generates significant mechanical damping, and this reduced stress from vibration, contributing to higher efficiency and reliability. The blade root, blade profile and shroud are designed as a single body, providing the optimum design for turbine blades in the low-pressure section, offering both excellent vibration resistance and superior damping characteristics.
Blade development and testing
Newly developed blades are verified prior to being used in actual plants, to ensure safety and reliability. MHPS has established a verification process for new blades which has been proven through the experience of developing as many as 15 types of blades over more than 20 years.
Steam loading tests are implemented at our in-house actual steam load test facility using a half (or higher) scale test turbine. The facility is one of the world’s largest, with a steam capacity of 450t/hr.
After the steam load test, rotational vibration testing is carried out with a full-scale test rotor, to verify the vibratory characteristics of the blades. Testing for each actual rotor is also carried out in the factory before shipment.
Finally, the vibratory stress of the blade is also verified in actual operating conditions in the field during the commissioning stage.
The 3D FEM (finite element method) is used to analyse vibration and strength, including factors such as centrifugal and bending strength, vibratory strength against steam exciting force and electromagnetic torque, and non-synchronous vibratory strength. The flow pattern analysis helps to improve performance and overall efficiency. Higher efficiency and higher reliability are achieved by state of the art analysis and by testing.
Steam load test
The steam loading test is carried out at our in-house actual steam load test facility, following a 3D FEM simulation and flow pattern analysis. The blade vibration characteristics and performance are verified under conditions that are more severe than actual operating conditions, using a half-scale (or higher) test turbine.
This is the key process in blade development, because it allows vibratory stress to be verified in actual operating conditions and confirms blade performance.
Steam loading testing for the 74in ISB began in 2009, using a half-scale test turbine. The test results confirmed that the vibration stress was reliable, with very low vibration levels across all operating ranges as well as under severe conditions. It was also observed that performance showed clear alignment between predicted efficiency and actual test results.
Rotational vibration test
After the steam load test, the rotational vibration test is the next stage of the process. This test uses a full-scale test rotor, operated at nominal rotational speed, and is designed to verify the vibratory characteristics of the blades.
Full-scale blades are assembled into their disc and the assembled test rotor is set up in the vacuum chamber of the high-speed balance test facility. The test rotor is rotated up to 110% over-speed, and an air jet is used to excite the blades to measure their natural frequencies, centrifugal stress and deformation. The testing is performed at the rated speed and the operating speed, including low and high cycle ranges.
A rotational vibration test with a 74in full scale ISB turbine rotor was performed at our in-house high-speed balance test facility in September 2017 as the final step of the blade development. The test results confirmed that the 74in ISB achieved its targeted vibratory characteristics and can be operated safely.
Steam turbines using the 74in ISB can be used at 1200MW nuclear power plants – currently the most popular type globally – as well as at larger nuclear plants exceeding 1500MW. When used in a 1200MW nuclear plant, the 74in ISB turbines can generate more power than the 54inch (1375mm) ISB turbine, due to the former’s longer blade length. Also, depending on the conditions of the plant’s location, the number of turbine casings can be reduced from three (54in ISB turbine) to just two (74in ISB turbine), saving space and simplifying plant designs.
The 74in ISB turbine has potential applications for both regions using 50Hz supply (including the EU and China) and those supplying at 60Hz (including North America and Saudi Arabia).
Going forward, MHPS will continue to provide safe and dependable high-quality steam turbines for nuclear power plants as its way of contributing to ever more stable energy supplies within the global market, economic development, and easing of environmental burdens.
Kei Sagimori is with Mitsubishi Hitachi Power Systems