Turbine technology |
Arabelle turbine applications. For improved performance and higher power output, the turbine design team at Arabelle Solutions needed to increase the last stage blade exhaust area by increasing the last stage blade length from 69 inches to 75 inches. The two turbine trains, as finally designed, can produce 1770 MW each (gross).
To compare the length of steam turbine last stage blades, in terms of the stresses in the blade roots and airfoils, as well as how challenging they are to design, one could scale them geometrically so they operate at the same rotational speed. If this is done, it becomes clear that the last stage blades employed in steam turbines deployed in combined cycle and coal fired plants are significantly longer than those typical in the nuclear market. Combined cycle/coal applications are predominantly full speed and the lengths of steel last stage blades used for large full speed combined cycle/coal plants range up to 48-50in (at 50 Hz). If one were to scale a nuclear half speed blade to full speed (25 Hz to 50 Hz), for example a 69in blade, its length would be around 35in, well within the design space envelope of existing full speed LSB designs.
Because the length of the last stage blade required for Hinkley Point C would be similar to that already developed for full speed last stage blade applications, the design team could have simply taken an LSB developed for full speed applications and scaled it by a geometric scaling factor of two for the new application.
However, whilst the aerodynamics and thermodynamics of the layout would remain similar, given that the rotational speed is also scaled by the inverse of the geometrical scaling factor, other parameters scale differently. The turbine mass flow and machine power scales by a factor of 4 – this is an advantage of the half-speed configuration. The last stage blade weight scales by a factor of 8. This increase in blade weight would require extremely large bearing pedestals to contain the rotor in the event of a last stage blade failure and would potentially result in a large missile if a blade failed and escaped the casing.
The restriction on airfoil weight is one of the biggest challenges in the development of the low-pressure turbine for half speed applications: that is keeping the blade mass to a minimum, whilst maintaining a blade stiffness that avoids aeromechanical excitation, and ensuring the blade can be manufactured in a repeatable and cost-effective manner. The scaling of other turbine components from full speed to half speed can also lead to a sub-optimal solution. There are limits on the physical size of the inner casing and outer casing in terms of transport by road and rail. The bladed rotor also needs to fit inside the “spin pit” bunker (the spin pit being an evacuated chamber used for testing). Other components would also have an excessive weight, if simply scaled from a full-speed application.
A new last stage blade for Arabelle The last stage blade for the LP module of the Arabelle turbine platform consists of
CAD model of the 75in LSB (source Aarabelle Solutions)
three main elements: a fir tree root; an airfoil; and a snubber. The blade airfoil geometry is represented by a series of profile sections and these are initially designed by the aerodynamic designer, then can be further modified in the downstream design processes. Due to the requirement for a very light blade with thin profile sections, a different frequency tuning strategy needed to be developed for this particular design. Many more airfoil design sections were required to enable more local changes, to allow the designer to more easily influence the complex mode shapes of the higher modes and tune their frequencies.
The performance of the LP flowpath is highly sensitive to the shape of the airfoils of the last stages. For this reason, an integrated design system was applied that focuses on achieving high aerodynamic performance, whilst also considering the requirements of other design aspects, such as mechanical integrity and manufacturability. The assessment methods for the non-aerodynamic aspects in the design system are of lower fidelity than used in the mechanical integrity and design disciplines, however it enables a more efficient design development because there are fewer design iterations needed to achieve the product requirements.
Lift of Arabelle LP rotor at Belfort factory (source Aarabelle Solutions) 20 | September 2024|
www.modernpowersystems.com
In order to have a sufficiently stiff last stage blade, the aerodynamics layout was adapted to increase airfoil camber. Local changes in the section area, and profile shape to influence the bending and torsional stiffness of the profile over the blade height have been applied to tune the higher modes. In other designs, the geometry has been modified after the
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