6.1.2 Intermediate pressure casings
In modern reheat machines, the design considerations for IP cylinders are similar to those for HP cylinders, the steam being at a similar temperature but lower pressure, which allows IP casings to be thinner.
In general, machines over 300 MW rating have at least a partial double-casing covering the first few stages, subsequent stages being supported by carrier rings (Figs 1.94 and 1.95). Both the inner casing and the carrier rings reduce pressure and temperature loading on the outer casing and also allow a smooth outer casing which is simple to design and manufacture and has better thermal response. Carrier rings allow greater flexibility in design, as changes in blading do not require changes to the main casings, and one casing design can serve different stage arrangements.
IP cylinders are often of double-flow design and always so on large modern machines. The decision for single or double-flow is primarily made on blad¬ing design and efficiency, but a double-flow casing also has the advantage of eliminating the rotor gland at the high pressure end. As with the HP cylinder, the rotor is protected from the high temperature steam at the inlet; a deflector ring directs inlet steam to the nozzles, whilst a supply of cooler HP exhaust steam is piped into the centre of the deflector ring adjacent to the rotor. The deflector ring is supported either independently on keys in the inner casing, or from the inside of the first-stage nozzles.
A cooling flow of steam around the partial inner casing can be achieved by slight differences in the blading in the two flows producing different pressures either end; this keeps the outside of the casing and the bolts cooler, allowing smaller diameter bolts to be used.