13 Wet steam turbine plant
13.1 Influence of steam on components - part 1
The steam produced by a modern light-water reactor, such as a PWR or BWR, is of poor thermodynamic quality. The pressure is typically in the range of 65-70 bar and since the steam is around 3% wet, the temperature corresponds to saturation conditions. It therefore follows that the steam mass flow per unit of generation is about 80% higher than in fossil-fired or AGR power stations.
The very modest steam conditions at the turbine stop valves results in a very large volumetric flow. The turbines consist of one high pressure and two or three low pressure cylinders, with the steam exhausted from the HP cylinder being dried and reheated in external moisture separators and reheaters before being readmitted to the LP cylinders. A typical turbine arrangement is shown in Fig 2.97. Unlike high temperature reheat turbines for fossil-fuelled power stations, where reheat pressure affects boiler design and is therefore not a free choice of the turbine designer, the reheat pressure in turbines for light-water reactors is chosen by the turbine builder to suit the turbine and moisture separator/reheater design, performance and economics. Due to the very large mass flow of steam, it is not necessary to select a low reheat pressure in order to preserve an adequately high volumetric flow at the entry to the LP cylinders, and thereby an adequately high efficiency of blading. Higher reheat pressures help to minimise the size of the moisture separator/reheater, and the mean diameter and length of the HP cylinder exhaust blading.
The inlet steam conditions also dictate the scantlings of the HP stop and governor valves, which are very similar in arrangement, size and thickness to reheat valves on a conventional unit. A typical back-to-back arrangement is shown in Fig 2.98. Because the low temperature (about 280°C) and pressure do not dictate a material with high temperature strength and, since the steam at inlet is insufficiently wet to cause erosion, a low carbon steel is used for the valve chest. The loop pipes supplying steam from the chests to the HP cylinder may also be made from carbon steel for the same reason. The loop pipes are generally arranged to be as short as possible to limit the quantity of steam which might overspeed the machine in the event of load rejection.
The HP turbines of large wet steam machines are usually of double-flow design, similar in appearance to the IP turbines of conventional units. As many as seven stages of moving blades in each flow are carried by a rotor which, in the UK, is normally of a monobloc construction. Diaphragms or fixed reaction blading are supported by carrier rings bolted to the outer casing at the horizontal joint as in conventional machines. This effectively forms a double-shell construction and permits moderate casing thickness, flange and bolt sizes to be employed.