10.3 Turbine blading development
The development of turbine blading can be expected in the future to meet three objectives:
- Reduced manufacturing cost.
- Improved integrity.
- Improved efficiency, including new LP blading of increased exhaust area.
The cost of blade manufacture can be minimised by the application of computer-aided design and manufacture, in which the optimised geometry from performance, vibration, and stress considerations is transferred directly to the numerically-controlled machines.
Improved integrity blading can be obtained by attention to detailed design to reduce stress concentration, by controlling the vibration characteristics to avoid resonances near running speed, and by reducing the use (or improving the quality) of attachments such as shrouding lacing wires, and erosion shields. There is a balance to be made between robust blading with thicker sections and efficient blading which favours a higher aspect ratio (length/chord).
As most modern turbine blading in HP and IP turbines already has an internal efficiency in the range 90% to 95%, the potential improvement is not large. Turbine makers have developed standard blading families of high efficiency which are tolerant to the range of steam flow incidence angles, which arise in different applications and different conditions of operation. Root and tip clearances are as small as practicable, with as many flow constrictions as the design permits. The detailed geometry of these restrictions can allow some reduction of the flow discharge coefficient, so that steam leakage through these clearances is minimised.
The development of LP turbine modules is a costly and lengthy process, yet there is always an incentive to increase the exhaust area of each flow section to reduce the exhaust kinetic energy (the leaving loss). This development may also permit, for particular unit sizes and steam cycles, a reduction in the number of LP cylinders: this is a worthwhile objective to save cost and reduce the size of turbine hall required. The last-stage and penultimate stage blading is always the limiting area of design; in terms of the steam bending stresses on the blades and the large mass of the blades which imposes a large centrifugal stress on the blade fastening to the disc. Last-stage blades for 3000 r/min turbines have been developed up to 1200 mm long: this is believed to be about the limit for conventional 12% Cr blade materials. Indeed, the 1200 mm blades in service in the USSR are made from titanium alloy, and this is likely to be the preferred material for longer blades. In particular, the introduction of longer last-stage blades is likely to permit the use of 3000 r/min machines with the largest nuclear reators in 50 Hz systems, although the 3600 r/min equivalents are much further away.