6.6 Casing and diaphragm glands
Glands are used to provide a steam seal between fixed and rotating parts of the turbine; in particular, between the rotor and the diaphragms and between the rotor and the cylinder casing. The glands are of the labyrinth type, consisting of a series of fins on the inside of the diaphragm or casing in close proximity to the rotor surface.
Steam trying to escape between the fins and the rotor suffers a series of expansions which reduces its pressure in stages and restricts the flow through the gland. A detailed explanation of the working of glands and the different types is given in Chapter 2.
The HP cylinder has glands at both ends of the outer casing and the inlet end of the inner casing (Fig 1.92). They are housed in gland carriers, which are supported either on keys to allow differential expansion or simply on spigots in machined recesses in the casing.
The HP glands are longer than those in other cylinders because of the higher pressure drop required across the gland, and are often divided into several sections. The IP glands are similarly supported (Fig 1.94), but the LP glands may differ; if the bearings are separate from the LP outer casing, then the LP gland carriers may be supported from the bearings and connected to the rest of the casing by a bellows piece (Fig 1.96).
The LP glands prevent air entering the cylinder, unlike the HP and IP glands which prevent steam escaping. Thus the LP glands need a continuous supply of higher pressure steam, whereas the HP and IP glands need additional steam only at start-up. This is explained in detail in Chapter 2.
The gland carriers are split, like casings, along the horizontal centreline and bolted together.