2 Steam chests, valves and loop pipes
2.1 Steam chest arrangements and construction
Steam is admitted to a turbine from the superheater outlets, via the hieh pressure steam mains, to the emergency stop valves and the associated governing valves which are housed in steam chests. In order to reduce the thermal stresses and hence the risk of thermal fatigue, these steam chests are of simple shapes and are manufactured either from closed-die alloy steel forgings welded together, or from alloy steel castings.
Similar steam chests containing emergency stop valves and interceptor valves are used in the hot reheat pipes from the reheater to the intermediate pressure cylinder of the turbine. These steam chests are invariably manufactured from alloy steel castings, which are thinner but larger than the HP steam chests because of the much lower steam pressure conditions.
The steam chests are usually mounted alongside the turbine: on 660 MW machines they are either on swing links or on springs to accommodate movement due to thermal expansion and flexibility of the HP steam mains. The steam chests on 500 MW machines were keyed to the foundations. In some instances, the steam chests are mounted in the steam mains, and thus are able to move freely with the steam piping.
On modern large machines, the usual practice is to have four steam mains together with four emergency stop valves and four governor valves, two on each side of the turbine. On some of the early 660 MW nuclear turbine-generators, however, the two steam mains on each side join into one large steam chest, so that there are only two steam chests in total. These steam chests, one on each side, contain one large emergency stop valve and one governor valve each. Similar arrangements are used for the reheat steam chests at the IP cylinder.
On some current 660 MW machines, the steam chest arrangement is such that there is one steam chest on each side of the machine, which has an emergency stop valve at each end and the two governor valves are connected to the common chamber between the stop valves. A typical arrangement is shown in Fig 2.33.
Another steam chest arrangement employs the reverse flow type of governor valve (see Section 2.6 of this chapter), permitting a more compact arrangement of stop and governor valves. This requires two steam chests each side of the machine, which have sometimes been interconnected before the emergency stop valve.