3.4.1 Steam conditions
The effects of increasing turbine stop valve pressure and temperature and the effect of reheat have been discussed in regard to cycle efficiency in Section 3.3 of this chapter.
It is also relevant to consider the major influences on turbine efficiency. The most important factor in turbine efficiency is blade length. Generally, an increase in blade lengths implies reduced leakage losses and secondary losses [5,7].
An increase in steam pressure at a given steam flow reduces the volumetric steam flow and hence reduces blade length. Increased steam pressure also requires thicker casing walls and larger horizontal joint flanges and joint bolts. It also implies thicker steam pipe walls which require additional length to obtain the required flexibility. These difficulties are only partly mitigated by the reduction in casing size and pipe diameter made possible by the smaller volume flow.
Another important effect of increased steam pressure is the requirement of increased pumping power to raise the feedwater to the higher pressure.
It follows that increases in steam pressure are only a benefit in practice when associated with an increase in unit size and/or an increase in steam temperature.
An increase in steam temperature has the effect of increasing the specific volume and hence blade length and therefore tends to increase turbine efficiency without any increase in unit size. There are practical disadvantages, which include the required increase in casing size and pipe diameter, and increased thermal stresses especially when starting and loading.
The influence of the combined effects of temperature and pressure changes and turbine efficiency considerations on unit size can be illustrated by a study of the 'standard' steam conditions specified by the BEA and CEGB from 1945 onwards.
Table 1.1 has been simplified by the exclusion of a number of units of close to the 'standard' 60 MW size, some of which employed reheat. It shows the general trend.
There is a progressive increase in unit size accompanied by a series of step increases of pressure and temperature, with a few notable exceptions. The main exceptions are the two 375 MW supercritical units for Drakelow C, commissioned in 1967-68 and the two 550 MW cross-compound units for Thorpe Marsh, commissioned in 1963-65. These units could be said to have been ordered before their time and have not been followed by further development.
The steam temperatures can be seen to reach a plateau of 538°C and 565°C. The 538°C limit applies to oil-fired units to avoid the use of austenitic materials in the boiler. These suffer heavy corrosion due to the sodium and vanadium content of the fuel oil.
The 565°C limit for coal-fired plant is to avoid increasing costs due to the extended use of austenitic materials to avoid creep failure. Creep is the phenomenon of slow progressive yielding which occurs at high temperature. In the turbine, it leads to reduced clearances and increased stress levels over a period of time, and in the boiler (where temperatures are higher) to time-dependent tube failures.
Current worldwide practice for modern large units is to limit steam temperatures to the 540°C level.