13.1 Influence of steam on components - part 3
The valves may be used in a reheat governing mode, as in conventional machines, but in the event of loadshedding to zero load, the interceptor valves close rapidly, preventing the steam present in the separator and reheater, together with any water which might evaporate off hot walls and pipework, from overspeed-ing the turbine to unacceptable limits.
The valves are designed so that steam forces assist closure as soon as a small degree of rotation is imparted. The valves are fitted with bearings which are capable of operating at relatively high temperatures and have shock absorbing characteristics.
The thermodynamically optimum choice of the crossover pressure between HP and LP cylinders, and the universal application of steam reheat, result in inlet conditions to the LP cylinders which are closely similar to those encountered in a high temperature reheat fossil-fuelled cycle. As a result, LP cylinders for water-cooled reactors are subjected throughout all their stages to steam pressures and temperatures virtually identical to those for fossil-fuelled stations. It is therefore possible and desirable to use LP cylinders of identical design to those used in turbines for fossil-fuelled stations. However, exhaust volume flow is 60-70% greater than for a fossil-fired unit of the same rating. If, therefore, exhaust losses are to be kept to a minimum, either the exhaust annulus of each LP turbine must be increased or the number of LP turbines must be increased.
In the USA, half-speed machines have been adopted for use with large output light-water reactors. Power generation is at 60 Hz, which limits the choice of rotational speed to 3600 r/min or 1800 r/min. The stresses in the exhaust blades impose a limit to their length in full-speed machines and hence a limit in the exhaust area per turbine. This, together with the widespread use of 1800 r/min LP turbine elements in conventional power plants, has led naturally to 1800 r/min as the design speed for PWR turbines in the 60 Hz system.
In a 50 Hz system the position is entirely different. Much larger turbine exhaust blade annulus areas are available and still longer blades are being developed; making 3000 r/min turbines economical for large outputs and different condenser pressures. Half-speed machines are invariably more expensive than full-speed machines. The design of the LP turbine elements can be based upon extensive experience of identical proven components for conventional power stations. However, nuclear power generation leads in itself to increases in the size of generating units, and it may be necessary to consider half-speed machines for future very large units requiring large annulus areas, particularly in combination with the low cooling water temperatures likely in UK plants.