13 Wet steam turbine plant
13.1 Influence of steam on components - part 1
For impulse blading and an adopted rotational speed of 3000 r/min, the bladed wheel diameters tend to be relatively small and, with the high specific steam consumption, the blade lengths become relatively large. The double-flow arrangement serves to keep the blade lengths, and hence the blade stresses, to conservative values.
Care must be taken in the design of the HP casing to eliminate crevices and pockets where water may collect, otherwise, during sudden load reduction, pools of standing water could flash-off and overspeed the turbine.
It is normal practice to pack the shaft end glands with live steam which has been throttled and has therefore acquired a small amount of superheat. This also removes the possibility of erosion damage to the rotor and other components in an area of high moisture content at the end of the HP expansion.
The steam leaves the HP turbine with a wetness of between 10% and 15% dependent upon the selected crossover pressure. If the steam were then passed directly into the LP cylinder without any form of mechanical drying or reheat, the terminal wetness at condenser pressure would exceed 20%. This would cause unacceptable LP moving blade erosion, particularly in the last stages, and turbine efficiency would be impaired. Reheating is therefore always employed in saturated steam cycles.
In order to avoid the complication of additional large penetrations into the reactor, steam is passed from the HP exhaust to moisture separators in the turbine hall where the wet steam is dried mechanically before entering a reheater, also situated in the turbine hall. Here the steam is superheated by the use of live steam which results in a performance improvement of about 1.5%. The thermal efficiency can be increased by a further 0.3-0.5% by two-stage superheating, i.e., by using bled-steam from the HP turbine as heating steam in the first stage and live steam in the second stage. The efficiency improvement gained by bled-steam reheating has to be balanced against the increased capital cost, and is dependent on the crossover pressure, bled-steam pressure and the reheater terminal temperature differences. The bled-steam re-heater improves the cycle efficiency by reducing the steam demand of the live steam section, but it also incurs an additional pressure drop of the reheated steam through the tube bundle and requires additional drainage facilities. Despite these factors, bled-steam reheating is becoming increasingly economically viable and is normally offered on present wet steam machines.
Because of the large volumetric flow at LP cylinder inlet, the interceptor valves tend to be very large. This precludes the use of normal plug valves; instead, butterfly valves similar to that shown in Fig 2.99 are normally used for this application. Since there are no right angle bends and no restriction to the flow, they have the advantage of providing very low pressure drop in the normal operating fully-open position. Butterfly valves are used for both stop and governing valves, the two valves being arranged in a single chest welded into each hot reheat pipe.