6.1.1 High pressure casings
Most modern turbines, with steam pressures over 100 bar and ratings greater than 100 MW, have HP casings of double-shell design (Figs 1.92 and 1.93). This has been adopted because of the difficulty of designing a single casing to withstand the thermal and pressure stresses and yet be capable of flexible operation.
With a double-shell casing, the space between the shells is filled with steam at exhaust conditions, allowing each casing to be designed for smaller temperature and pressure differentials. A baffle is formed between the two casings near the exhaust end as part of the inner-casing casting. The baffle extends almost to the outer casing but does not seal against it. The turbulent exhaust steam is directed by the baffle to the exhaust connections and prevented from cooling the inner casing; this reduces the temperature differentials and hence stresses on the inlet end of the inner casing. Steam leaking through the gland between the inner casing and the rotor at the inlet end is piped away to the exhaust connections, so the space between the casings contains steam at exhaust conditions with a gentle flow being maintained by leakage through the outer casing gland. The smaller pressure differential permits a thinner shell which, combined with the larger surface area of a double casing, allows quicker warming of the turbine on start-up. Thinner shells are also easier to cast and are likely to have fewer defects.
On some machines, reversed flow blading has been adopted, where the steam is diverted back between the casings part way through its expansion, continuing through the final stages in the opposite direction. This arrangement gives a higher inter-shell pressure and temperature, reducing the stress on the hot inner shell at the expense of the outer shell. It also permits simple bled-steam tapping at the inter-shell conditions and reduces the net thrust on the HP rotor.
Triple casings have been used on some modern turbines to further reduce the stresses on the hot inner casing, and hence reduce thermal distortion. The inner casing is enclosed in a barrel-like sleeve which has no horizontal joint. The inner casing is only lightly stressed and can be relatively thin with light flanges, whilst the barrel casing which encloses it is more highly stressed. The barrel casing, however, having no flanges and being of uniform thickness, can easily be designed to accommodate the stresses whilst also remaining relatively thin. The pressure between the inner casing and the barrel is controlled by small radial passages through the inner cylinder walls and piston ring seals between the inner and barrel casings.
One disadvantage of this form of triple casing is the difficulty in assembling and dismantling the HP cylinder. On assembly, the rotor has to be fitted into the inner and outer lower half, the inner casing bolted up, and then the rotor and inner casing lifted and mounted in a special jig to allow the barrel to be threaded over them. The assembly can then be lowered back into the bottom half of the outer casing, and the top-half outer casing added.
The steam inlet pipes pass through the outer casing and deliver the steam into the inlet belt of the inner casing. The inlet belt is formed by an extension to the main casing which ensures that the inlet steam cannot come directly into contact with the rotor but must first pass out through the nozzles and the first row of moving blades. The inlet belt is often blanked at the casing joint to reduce the pressure on the joint face, although care must be taken that the admission of steam to the first stage is not interrupted.
With steam temperatures at inlet in excess of 538°C, separate nozzle boxes of heat-resistant steel are sometimes used to protect the casing from the full temperature. These take the place of the inlet belt, delivering steam from the inlet penetrations to the first stage nozzles.
Some overseas units have nozzle governing instead of the throttle governing employed on all large CEGB machines. With nozzle governing, the inlet belt is divided into sections, each controlled by a separate valve opening in sequence, resulting in a more complicated casting and the need for stronger first-stage moving blades.
The stationary blading is carried in diaphragms which are supported and guided in the inner cylinder by keys near the horizontal joint and vertical centreline, permitting concentric expansion. On modern machines the tip seals and shrouding for the moving blades are usually carried on an extension of the adjacent diaphragm. Earlier designs had separate sealing strips supported between the diaphragms.
HP cylinders on wet steam machines, such as those on PWR stations, are different in design, being more like standard IP cylinders. Further details are given in Chapter 2.