6.3.6 Bled-steam connections
Steam for feedheating is bled from the LP turbines and additionally from either the IP turbine or the boiler feed pump turbine. In current practice, steam is not bled from the HP turbine except at the exhaust, although future designs with higher final feed temperatures may bleed steam from the later HP cylinder stages.
In LP cylinders (Fig 1.96), the steam is bled-off into annular belts within the fabricated inner casing from which it is piped to the feedheaters, bellows being used to take up differential expansions between inner and outer cylinders. The connection to the bled-steam belt is usually made at the bottom; if made elsewhere, a drain connection must be provided at the bottom. The bled-steam connections are identical in each half of the double-flow LP cylinder so that changes in steam flow to the heater do not affect the thrust forces on the rotor. However, the connections may not be the same on all LP cylinders on a unit. If, for instance, bled-steam is required at stage 2, stage 3 and stage 4 conditions, it may be taken from each stage on each cylinder, or it may be taken from stage 2 on one cylinder, stage 3 on another and stage 4 on the third (for a three LP cylinder machine). The latter method involves simpler pipework but has the penalty of less interchange-ability of spares between the cylinders. Usual practice is to favour the arrangement requiring minimum spares holdings.
IP cylinders usually only have one stage of bled-steam from within the cylinder, although it may also be taken from the IP exhaust.
On double-casing cylinders, the steam may be bled between the inner casing and a carrier ring, or between carrier rings, into a belt between the ring and the outer casing. On single-casing IP turbines the bled-steam belt is cast into the casing, with slots cut to allow the steam to bleed from the main steam path into the belt. The bled-steam pipework can then simply be bolted to the outer casing. Many IP cylinders have no bled-steam tappings, as steam is taken from the boiler feed pump turbine instead; however, latest practice with development of variable-speed electric drives, is for electric boiler feed pumps and hence bled-steam is taken from the IP cylinder.
Steam bled from HP turbines in the future is likely to be via a belt cast into the inner cylinder wall, with connections similar to the HP inlet connections.