POWER

Diagnosing the problem
The postmortem revealed that simultaneous exertions of random vibrations and flashback vibrations during the low load test operation exerted a level of stress on the blade fork that was not expected during design. This excessive stress cracked and eventually severed the blades from their turbine rotor disc.

Exclusively, neither is intensive enough to damage a blade as severely as the one in the Hamaoka incident. Collectively however, the vibrations exerted an uncharacteristically high level of stress on the blade fork that was not anticipated during design. This excessive stress cracked and eventually severed the blades from their turbine rotor disc.

Understanding the challenge
Last stage blades generate 10% of a turbine's entire output, making them key producers in the operation that determine the overall performance and reliability of a steam turbine. Therefore it is imperative that these blades maintain their integrity and peak performance under any circumstance.

Prescribing the solution
Following the events at Hamaoka, Hitachi introduced the Continuous Cover Blade (CCB), built with a revolutionary dampening structure that incorporates an advanced aerodynamic blade profile perfected through a 3D stage flow analysis.

Designed specifically for a steam turbine's lengthier last stage blades, a CCB is built to withstand the tremendous stress brought on by high operating velocity and large centrifugal force, as well as the unexpected superposition of vibrations.

Working in tandem with the CCB to ensure a smoother operation, the L-2 stage nozzle – responsible for facilitating backflow – was upgraded from a single-extraction to double-extraction structure, allowing for a more even flashback flow. This optimises stage flow during low load operation and minimises turbulent flow conditions.

Powering on to success
The CCB excelled in the extensive operation tests that followed and received the seal of approval from the Japanese government. They have since been eagerly implemented to Hitachi's standard ABWR TC6F-52 turbine design, which powers the Haramachi Thermal Power Station, Tachibanawan Thermal Power Station and Tomato-Atsuma No.4 Steam Turbine Generator in Japan, as well as the Genesee No.3/EPCOR Power Development Corporation in Canada.

Building on heritage
Hitachi's 38 years of power generation began in 1972, when Karachi Nuclear Power Station, a CANDU reactor plant owned by the Atomic Energy Commission of Pakistan, became the first to experience Hitachi power with the installation of a 139MW Nuclear Steam Turbine. With that, so too began Hitachi's quest for green energy.

Today, there are 18 Hitachi steam turbines in operation around the world – including one in Shika Unit #2, the most recent Advanced Boiling Water Reactor (ABWR) Nuclear Power Station in the world to be commissioned – generating nearly ten times as much energy as the one in Pakistan.