3.4.5 Turbine designs
It is now proposed to consider these cycles from the viewpoint of turbine cylinder designs. The 660 MW fossil fuel and the 660 MW AGR turbines are quite similar. The use of the same steam conditions and rating clearly implies this.
In fact, the AGR machine has about 15% less HP steam flow and 5% more IP steam flow for similar power output due to its reduced feedheating requirement. It also has about 1% greater LP turbine inlet steam flow.
These differences are accommodated by small blade height or pressure level changes. In the cases chosen, there is an apparent contradiction in that LP inlet pressure for the oil-fired unit is lower, while its re-heater pressure is higher. This arises from the fact that this machine uses a four-flow LP turbine with only five stages while the AGR uses six flows with six stages. The IP turbines are also rather different.
The choice of IP exhaust/LP inlet pressure is entirely a matter for the turbine designer. It is the point where steam passes from the two flows of the IP turbine to the four or six flows of the LP. A low pressure results in large crossover pipework and long last-stage IP blading, while a high pressure results in short LP inlet blading and higher LP inlet temperature. The selected pressure represents a compromise between these considerations and must give heat drops in the IP and LP turbine appropriate to the number of stages and the stage diameters required.
With modern designs, an effort is made to select pressures to permit the use of standard turbine modules. This leads to increased reliability and reduced manufacturing costs and also reduces the number of spares required — notably spare rotors.
Turning now to the new coal-fired proposals, it must be remembered that, although the condition lines are similar, these turbines will be very different from the 660 MW units because they will be designed for something of the order of 30% increase in power output. They will also embody the most recent blading and a whole range of detail design improvements.
It is noticeable how little difference there appears between the subcritical and supercritical condition lines for the IP and LP sections. It should be added that the reheater pressures quoted are approximate and are not fully optmised values.
The IP exhaust/LP inlet 'crossover' pipework merits comment. This pipework handles low pressure steam and is of large diameter to minimise pressure losses. The pipework has separate branches where the IP exhaust steam enters and where the LP inlet steam leaves. It is always provided with a number of flexible sections to allow for pipe/casing expansion differentials. It is sometimes installed above, and sometimes below, the turbine centreline. The 'above centreline' arrangement requires removal of the pipework before the LP turbine can be opened for inspection. The 'below centreline' location has sometimes been thought to be responsible for some turbine misalignment, due to heat transmission to bearing pedestals and casings.
The IP exhaust is considered to be the most suitable point for connection to the de-aerator. This is a direct contact heater which is vented to the condenser and designed to remove air from the feedwater before it enters the main feed pumps. It is always combined with a large capacity storage vessel and elevated to provide a static head to the main feed pump suction.
The CEGB has used reheat cycles for all power station turbines above 100 MW rating. This is not true worldwide. In some countries there is a need for large units operating on a simple поп-reheat cycle for peak-load lopping duty. These machines run for only a few hours a year, so maximum efficiency is not important, but they must be able to run-up to full-load quickly and be inexpensive.
For this class of turbine, the feedheating cycle is kept as simple as possible and the expense and complication of reheating is not economic. A typical machine of this type would deliver 280 MW from a single-casing condensing turbine in double-flow.