There are two basic types of turbines, which are analogous to the centrifugal and axial types of compressors. These two turbines include the radial-inflow turbine and the axial-flow turbine. The radial-inflow turbine is essentially a centrifugal compressor with a reversed flow. Less than 20% of gas turbines use the radial-inflow type of turbine and they are usually used for smaller loads and for smaller operating ranges than axial turbines. Radial turbines are shorter that axial turbines, which can be an advantage depending upon the application.
The two main parts that make up a turbine are the turbine blades and the disk. The disk is often made mostly of alloyed steel, chromium, nickel and cobalt. Blades are attached to the disk with a “fir tree” design fit. The blades are prevented from sliding out of the disk by rivets, locking tabs, or other devices. The turbine must also be carefully balanced. Turbine blades may be shrouded or open at the tips. A shroud helps prevent blade tip losses and excessive vibration, Shrouded blades also tend to resist distortion from high loads. The added rigidity of the shroud prevents the blades from twisting. On the other hand, a turbine with shrouding must be run at cooler temperatures or at reduced speeds. The turbine is designed to distribute the load evenly between all the turbine stages, so each successive stage has larger blades with a larger area.