Understanding Single Stage Impulse Turbines

A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Turbines are broadly classified based on how the pressure energy of the steam is converted into kinetic energy and then work. The two main types are impulse turbines and reaction turbines. In an impulse turbine, the steam's pressure energy is completely converted into kinetic energy in stationary nozzles before entering the moving blades. The pressure of the steam remains constant as it passes through the moving blades. The work is done by the force resulting from the change in momentum of the steam as its velocity changes direction while flowing over the blades. A single-stage impulse turbine is the simplest type of impulse turbine. In this design, all the pressure drop from the inlet pressure to the exhaust pressure occurs in a single set of nozzles. The high-velocity steam then passes through a single row of moving blades. This design is suitable for relatively low power outputs or for driving applications where simplicity is key. The most well-known example and often synonymous name for a single-stage impulse turbine is the De Laval turbine. It was invented by Gustaf de Laval in the late 19th century. The De Laval turbine uses a convergent-divergent nozzle (or a set of such nozzles) to expand the steam from a high pressure to a low pressure, producing a very high velocity jet. This jet then strikes a single row of moving blades mounted on a rotor. Let's look at the other options to understand why they are not single-stage impulse turbines:

• Curtis stage turbine: This is a type of velocity-compounded impulse turbine. It uses one set of nozzles followed by multiple rows of moving and fixed blades to gradually reduce the high velocity of the steam. It is not a single stage.
• Reaction turbine: This type of turbine works on the principle of reaction. Both the stationary blades (guide vanes) and the moving blades act as nozzles. The pressure of the steam drops across both the stationary and moving blades, and work is done by the reaction force created by the accelerating steam as it exits the moving blades. The Parsons turbine is a classic example, and they are typically multi-stage.
• Rateau turbine: This is a type of pressure-compounded impulse turbine. It consists of several simple impulse stages (each with nozzles and a row of moving blades) placed in series. The pressure drops in steps across the nozzles of each stage. It is not a single stage.

Therefore, a single stage impulse turbine is also known as a De Laval turbine.