Compounding (Staging) of Impulse Turbine

In an impulse turbine, the steam expands completely in one stage (i.e., the entire pressure drop from high pressure to low pressure takes place in only one set of nozzles). As a result, the rotor rotates at a very high speed of about 30,000 rpm (K.E. is fully absorbed). This high speed poses number of technical difficulties like destruction of machine by the large centrifugal forces developed, increase in vibrations, quick overheating of bearings, impossibility of direct coupling to other machines, etc. To reduce the speed, reduction gearing is not preferable because of large and bulky construction. Hence a method of reducing the speed of the turbine is adopted. To overcome the above difficulties, the expansion of steam is performed in several stages. A number of successive stages are usually arranged one behind the other such that the expansion of steam would perform in several stages. Utilization of the high pressure energy of the steam by expanding it in successive stages is called Compounding (Staging).