Table 1.1 also shows the general adoption of reheat for all units over 100 MW. It has been shown earlier in this chapter that reheat allows a closer approach to the ideal Carnot cycle. Its adoption has cost and design significance.
There is, of course, the requirement of additional piping to and from the boiler, the latter requiring high temperature with larger bore size than the HP steam pipes. The higher volume flow of the reheated steam requires the IP turbine to have a larger rotor diameter and longer blades than the HP turbine. Sometimes there is a choice between increased outlet blade angle and double flow in the IP cylinder, resolved in the case of CEGB 500 and 660 MW units by using double flow.
To reduce creep in the high temperature regions, it is common to admit a supply of lower temperature cooling steam to the space between the inlet fixed blades and rotor. This steam tends to flow through the balance holes in the discs and the diaphragm glands, thus shielding the rotor from the hotter steam at the blading.
The IP turbine casing has to provide openings large enough to pass the exhaust steam on to the LP turbine at moderate velocities without unduly reducing its stiffness. The casing inlet end must be carefully designed to reduce thermal stress.
The pipework to and from the reheater and in the reheater section of the boiler represents a significant storage volume. It is therefore necessary to supply additional valves to protect the turbine from over-speeding in the event of a sudden loss of load. As in the HP turbine, these valves must be in series-pairs to provide back-up in the event of one valve failing to close when required. They operate at the same high temperature but must be considerably larger than the HP valves. To handle the flow, up to four sets of valves may be provided.