12.3.1 Cooling of turbine
For this technique to be employed, the turbine is designed to be suitable for injection of cooling air into the HP and IP cylinders via special injection points located around the cylinders. Diffusers are normally provided at the injection points to ensure that unacceptable local chilling of the cylinder or other components does not occur.
To prevent the rotor cooling faster than the casing, which would cause unacceptable positive differential expansion, cooling is achieved by injecting the air into the interspace between the inner and outer casings. This ensures that the rate at which the casing cools is higher than that of the rotor, thus allowing better control of the differential expansion.
A typical forced-air cooling system for a 660 MW generator is shown in the diagrammatic arrangement of Fig 2.96. This shows the direction of air flow for the HP and IP turbines. Air entering between the outer cylinder and the inner sleeve of the HP turbine flows in both directions to the cold reheat pipes and reheater, exhausting to atmosphere via the reheater dump valves. Air from the IP turbine exhausts via vents from the IP/LP crossover pipes. The heated air must be discharged in such a way as to present no hazard to either personnel or to other plant.
During cooling, it is essential that close attention is paid to HP and IP turbine rotor eccentricities; HP, IP and LP differential expansions; HP and IP metal temperatures and the barring motor current, which could give indication of contact between rotating and stationary components. The air flow must be controlled to give a cooling rate which allows eccentricities and differential expansions to be maintained within the manufacturer's specified limits.