12.2.1 Turbine cylinders
Probably the most significant change in turbine design has been the adoption of multishell cylinders. The advantage of this type of construction is that pressure difference and temperature gradients across each shell is reduced compared with single-shell construction. It is now normal practice in one form or another, in high temperature machines, for HP and most IP cylinders, and is usual for most LP cylinders.
Low pressure cylinders sometimes have a larger temperature range from inlet to exhaust than any other cylinder, and the inner casing may itself be of double-shell construction to reduce thermal stresses and distortion. As ratings have increased, component size has increased and with it the problem of transient temperature gradients within a component and thus thermal stress. The use of multishell cylinders has allowed temperature distribution and gradients to be controlled, thus minimising cyclic thermal stresses. In addition, this design allows the heavy flanges which were necessary on single-shell designs to be reduced in size. In some cases, the use of one-piece shells, with no horizontal split, obviates the need for flanges, thus removing the problem of axial thermal stresses due to uneven heating of flanges and cylinder walls. The use of flange heating has also contributed to matching the rate-of-rise of temperature of the flanges and cylinders more closely.
The adoption of multishell casings and the separation of steam chests from the turbine cylinder, has allowed simplification of the geometry of each component. This means that the design has fewer changes in section and more attention is paid to radii to avoid high stresses in areas where a change in section is unavoidable. Figure 2.94 shows a triple-shell HP cylinder with an unsplit intermediate sleeve. To further reduce temperature gradients, particularly in the first stage of the HP turbine, pure throttling of the governor valves has been adopted for load control. Although this type of control reduces efficiency at part-load, this can be tolerated in two-shifting operation, where the machine tends to be operating at rated load or is shut down. The practice of using nozzle control to maintain part-load efficiency has the disadvantage of reduced efficiency at full-load and leads to temperature gradients because of the circumferential non-uniformity of steam admission. This reduces the rate at which load can be changed without inducing thermal stresses. Rapid starting and loading, which are essential requirements for modern machines, are therefore achieved more securely with throttle control.
During run-ups and rapid loading, the turbine is most at risk from a 'rub' between stationary and rotating components. Apart from localised damage at the area of contact, the resulting heat generated by the 'rub' can cause a bend in the rotor shaft. Continuous asymmetrical rubbing, which bends the rotor even more, can cause so much heating that the heated area of the rotor, expanding and constrained by the surrounding unheated material, yields in compression. As a result, when it is allowed to cool, local tension is applied leading to a permanent bend. Such 'rubs' are usually caused by distortion of gland seal positions from their normal concentric location due to rapid thermal cycling. The adoption of spring-back gland seals allows retraction of the seal should contact with the rotor occur. This limits the damage caused by a rub and reduces the risk of a shaft bend.
Cylinder expansion and contraction during run-up and run-down, requires that cylinder/pedestal sliding surfaces are well maintained, to prevent unnecessary external forces on the cylinders due to sticking.