relative expansion between turbine rotor and the cylinder, called differential expansion.

Rotor expansion bulging is bigger than the differential expansion when the cylinder is expansion is positive differential expansion, expansion of the cylinder is bigger than the differential expansion when the rotor is expansion is negative differential expansion. According to cylinder classification can be divided into the differential height, the poor, low I poor, low II poor. Differential expansion values are very important operating parameters, if the differential expansion overrange, the thermal protection action causes the host trip.