Impulse Type Turbine

An impulse type turbine is a kinetic energy machine which converts part of the heat in the steam into velocity in the fixed blade. There is no heat drop and, thus, there is no decrease in pressure across the moving blades. The mechanical work done in the moving blades is, therefore, only due to the loss of a proportion of the velocity gained in the fixed blades.

Impulse turbine moving blades are carried on discs, which are either integral with, or attached to a small diameter shaft. The axial thrust on the rotor is small because there is no pressure drop across the blades and, thus, there is no pressure forces across the discs. The efficiency with which the transfer of energy to the moving blades is fundamentally dependent on the ratio of moving blade velocity and the steam absolute velocity. Thus, the effect of the velocity ration on efficiency in an impulse stage is somewhat different to that in a reaction stage.

Modern blading designs

Modern turbines are neither purely impulse, nor are they reaction. They are a combination of both. At the beginning impulse type turbine utilized reaction of up to 20% at the root of the moving blades in order to counteract poor efficiency incurred from zero, or even negative reaction. On the other hand, reaction at the root of reaction turbines has decreased to as little as 30%, resulting in the reduction in the number of stages required and the sustaining of 50% reaction at the mid-point. Nowadays, it would be more accurate to describe the two designs as “disc and diaphragm turbine:, using low reaction blading, and “drum-rotor turbine,” using high reaction blading.